Senior Member, IEEE Franck Luthon
email: franck.luthon@univ-pau.fr

Benoît Larroque

LaboREM -A Remote Laboratory for Game-Like Training in Electronics

Keywords: E-learning, game-based learning, industrial engineering, labwork, motivation, online education, virtual instrument

The advances in communication networks and web technologies, in conjunction with the improved connectivity of test and measurement devices make it possible to implement e-learning applications that encompass the whole learning process. In the field of electrical engineering, automation or mechatronics, it means not only lectures, tutorials, demos and simulations, but also practical labwork for training with real-world devices that are controlled remotely. To make e-labs attractive, they should be easily implemented and accessed on the web by a client. This keypoint raises technical issues that are recalled in this paper. Nonetheless pedagogical issues are equally important. The benefit of a remote lab must be evaluated and compared to simulation labs or hands-on. Here, to foster student motivation, a game-like scenario embedded in a learning management system is proposed.

INTRODUCTION

T HIS paper deals with a remote laboratory called LaboREM, developed for training in electronics, where students can build their circuits using a remotely controlled robotic arm. It is implemented for e-learning in industrial engineering. Indeed, Bayonne Technological University Institute in Anglet, France, promotes the introduction of distance learning for the education of students in electrical and mechanical engineering, maintenance and computer science, either to learn to do teleworking or for lifelong learning.

As part of the academic curriculum for the Bachelor of Science degree in industrial engineering, the course of Analog Electronics (ENA) addresses the following topics: operational amplifiers, active filters, feedback loops, oscillators, regulators. It is reckoned as 3 ECTS credits (European Credit Transfer System), of which one half corresponds with practical labwork. The ECTS is a system for measuring and recognizing credit for student learning, designed to help mobility of students within the European Union. One standard full course semester corresponds to 30 ECTS credits. The learning objectives of ENA are to implement basic electronic functions (amplification, filtering etc.) with simple systems based on OpAmps, and to use instruments for test and measurement. The practical work assigned to students consists in the characterization of electronic circuits. The course consists of 45 hours per student, of which 15 hours are dedicated to practical labwork (four hands-on sessions lasting 3 to 4 hours each). Part or all of this labwork can be done remotely.

The lab platform is based on the design and control of Virtual Instruments (VIs) for the management of remote experimentation through the web, together with the use of a Learning Management System (LMS) for the management of students and for the collaborative work supervision. The client-server architecture that allows remote control of instruments in real-time, with the technical management of both the list of experiments and the stack of users, is similar to many other works listed in [START_REF] Gomes | Current trends in remote laboratories[END_REF], [START_REF] Garcia-Zubia | Using Remote Labs in Education: two little ducks in remote experimentation[END_REF]. The originality of the proposed work is twofold: (i) a feeling of the lab (immersion) via the use of a video camera and a robotic arm for placement of components, plus the optional use of computer vision and image processing to address user-specific requests to see the equipment in detail (zoom-in for displaying instruments or device under test); this is intended to mimic both the user eye and the user hand; (ii) a game-like scenario (treasure hunt) that is implemented as learning approach, with an attempt to mimic the behaviour of students when they work close together (collaborative work through use of chat, forum, hall of fame, Top 10) and with a learning itinerary that is adapted to each student (difficulty level depending on the evaluated knowledge and motivation).

Some authors (e.g., [START_REF] Pleul | Teleoperated laboratory experiments in engineering education. The uniaxial tensile test for material characterization in forming technology[END_REF], [START_REF] Dormido | UNED labs: a network of virtual and remote laboratories[END_REF], [START_REF] Ordu Ña | Generic integration of remote laboratories in learning and content management systems through federation protocols[END_REF]) already addressed the coupling of remote labs with LMS, or the coupling of virtual labs with games [START_REF] Graven | Computer-based role playing game environment for analogue electronics[END_REF], but to the best of our knowledge, the present work is one of the first attempts to mix three views together in a pedagogical perspective: remote lab, LMS and game.

As regards our previous work, we evolved in the following way: in a first collaboration with the polytechnical university of Bucharest, Romania, an open architecture was proposed for remote lab, based on the concept of plugins of VIs [START_REF] Steriu | Open architecture for signal processing lab distance learning[END_REF]. The deployment on client-side was a desktop application (i.e., executable installed and running on the client computer). Next, a game-like scenario was conceived to boost student motivation [START_REF] Luthon | LaboRem: open lab for remote work[END_REF]. Here, those ideas are put together within an LMS, so that the collaborative dimension is added to the remote labwork, together with the supervision of students' activity. We therefore switch to a web-based client application (accessed via a browser). Moreover, interaction tools for better immersion and motivation are proposed: a Top 10 score is displayed in real-time to motivate students by a challenge, and a robotic arm plays the role of the student hand for choosing and placing components to build the Device Under Test (DUT). The solution proposed here is an alternative to the virtual wiring of a relay switching matrix implemented in [START_REF] Gustavsson | On objectives of instructional laboratories, individual assessment, and use of collaborative remote laboratories[END_REF], [START_REF] Tawfik | Virtual instrument systems in reality (VISIR) for remote wiring and measurement of electronic circuits on breadboard[END_REF].

The paper is organized as follows. In section 2, the motivations of the work are given with respect to the state of the art. Section 3 shows the technical choices for software architecture and hardware setup of the lab, while section 4 describes the game-like pedagogical scenario implemented via an LMS. An evaluation of the proposed application is given in section 5. The discussion in section 6 ends the paper.

MOTIVATION OF THE WORK

Issues and Requisites

Developing a remote lab may involve various issues. First, one can address the sociology of technical knowledge, facing a large public like undergraduate students, poorly educated audience, heterogeneous students that may need to repeat and revise many times, adults with a full-time job and hence heavy constraints and little day-time available, disabled people needing more time to complete an activity, people not affiliated to a university, etc. Here, we focus on first year undergraduate students that are teenagers of about 18-19 years old. For that typical target audience, who sometimes lacks personal motivation in their studies, we believe that both game-based learning and adaptive learning are good solutions to promote, as in [START_REF] Verginis | Enhancing learning in introductory computer science courses through SCALE: An empirical study[END_REF] for computer science. Therefore, a game-like scenario is implemented in order to enhance students' motivation and autonomy.

Second, international cooperation between technological universities is targeted, especially in industrial engineering curricula, where practice on real-world devices is important. Universities that do not have modern instruments at their disposal may use hardware devices available in another university thanks to an Internet connection. This is part of the objective of the democracy of technical knowledge. Bayonne institute of technology already has strong partnerships with Latin America in the fields of management, business, commerce and computer science and aims to expand its partnership in the technical field of industrial engineering, especially with Colombia, El Salvador and Mexico. To this end, some issues are of utmost importance for users: (i) comfort (ease of use both for students and tutors); (ii) extensibility (ability to deploy new experiments on the web); (iii) portability (easy installation on client side).

Third, another major concern is the financial aspect. Indeed, in the present economic climate worldwide, drastic cuts have been made by governments in their education budget. One must reduce the costs of teaching by all means, especially if one wants to face the educational mission in technical fields that are demanding in lab activities. Mutualization of expensive equipment thanks to e-labs is thus a good solution (no need to buy any expensive software at the client side, no need to buy many instruments but one). This is part of the economy of technical knowledge.

A final keypoint is to alleviate the constraints on non financial resources (human, space and time constraints, apart from money) induced by hands-on activities (need of teachers, rooms and dedicated time slots, apart from need of high investments). The sharing of resources and infrastructures [START_REF] Auer | Guest editorial: The many facets of remote laboratories in online engineering education[END_REF] is then part of the ecology of technical knowledge, for a sustainable society. In such an open context, scheduling between users and security are issues to take into account.

Related Works

Practice is known to be an essential part of learning in engineering education [START_REF] Feisel | The role of the laboratory in undergraduate engineering education[END_REF], as long as clear learning objectives are assigned (c.f. ABET criteria where 13 fundamental objectives are listed). As assessed by a study on the efficiency of virtual laboratories [START_REF] Wolf | Assessing student learning in a virtual laboratory environment[END_REF], almost 50% of learning takes place during labwork.

The pros and cons of the three types of labs (handson, virtual and remote) are wellknown [START_REF] Nickerson | A model for evaluating the effectiveness of remote engineering laboratories and simulations in education[END_REF]. Virtual labs are based on simulations only (mathematical models implemented in a computer), whereas remote labs use real-world instruments in a physical lab [START_REF] Ma | Hands-on, simulated, and remote laboratories: A comparative literature review[END_REF]. Virtual and remote labs belong to the generic group of e-labs, made possible thanks to the development of Internet. They are the modern counterpart of traditional hands-on labs that take place in front of real-world devices. Since access to real-world labs is restricted due to high number of students, cost of equipment, supply and maintenance needs, e-labs are great alternatives to alleviate those constraints [START_REF] Gomes | Current trends in remote laboratories[END_REF]. A specific argument in favor of real remote labs (compared to virtual labs) is the fact that nowadays in industry, more and more engineering activities take place remotely, for example the maintenance of systems or control of processes through a computer interface located in a distant supervision room (SCADA: supervisory control and data acquisition). Hence, there is a strong need to educate students to this new way of working.

However, the bottlenecks of remote labs lie in the interaction with instruments and the interaction between people. Therefore, smart software is needed [START_REF] Garcia-Zubia | Addressing software impact in the design of remote laboratories[END_REF] between hardware and humans in order to: (i) design an open (flexible enough) architecture for distant control of hardware devices (technical need) and (ii) develop tools and interfaces to enable and track the cognitive behavior of the user in a distant instrumentation application (human need). Such pluridisciplinary projects require teams with various skills: teachers (tutors) that build labwork activities and evaluate students, cognitive science experts (to conceive scenario models), human-machine interface designers, computer science programmers, network and system managers, technicians for maintenance (service available 24 hours a day).

The complementarity of activities is also well known: a mixed pedagogy including hands-on, simulations and remote control is preferable, which is called blended learning [START_REF] Saighi | Electronics basics e-learning: From lectures to lab[END_REF]. However when distance activities are developed, motivational features are very important for the self-regulation of learning. As an example, a motivation controller based on fuzzy logic is implemented with Java and Moodle in [START_REF] Mendez | Implementing motivational features in reactive blended learning: Application to an introductory control engineering course[END_REF]. This allows to estimate student motivation, and hence to propose the best workload with respect to various parameters (autonomy, performance, participation, anxiety).

To assess the quality of remote labs, there is a strong need for evaluation, both technical and pedagogical. Evaluation of usability of the lab infrastructure (hardware and software) is necessary [START_REF] Kostaras | Evaluating usability in a distance digital systems laboratory class[END_REF]. To get real-time knowledge of the state of work of a student and for automatic assessment procedures, the use of an LMS is mandatory [START_REF] Rapuano | A learning management system including laboratory experiments on measurement instrumentation[END_REF], [START_REF] Trenas | Use of a new Moodle module for improving the teaching of a basic course on computer architecture[END_REF], [START_REF] Llamas-Nistal | Use of e-learning functionalities and standards: The Spanish case[END_REF]. In [START_REF] Augustin | Individualized skill assessment in digital learning games: Basic definitions and mathematical formalism[END_REF] a theoretical model to assess the knowledge and learning progress in an embedded, non-invasive way is proposed in the context of digital learning games, in place of the classical formal tests that students have to pass. It points out the importance of an appropriate balance between gaming and learning, and between challenges through the game and abilities of the learner.

ARCHITECTURE OF THE LAB

Software Model of the Platform

The software should ideally enable a completely connectible application, enable an easy installation by the client, run in real-time, communicate easily with hardware devices, communicate easily with clients, enable extensibility (to plug in new experiments), manage all the resources and users (hardware, humans), implement collaborative work. It should allow a teacher to be able to add easily a new remote lab activity into the LMS course as stated in [START_REF] Sancristobal | Remote labs as learning services in the educational arena[END_REF]. The choice of network technology to build the architecture should therefore be compliant with various issues like accessibility, reusability, portability, security, scalability, interoperability, traceability. The importance of software universality versus software power dis-cussed in [START_REF] Garcia-Zubia | Addressing software impact in the design of remote laboratories[END_REF] leads to the conclusion that a serviceoriented architecture (SOA) should be adopted.

Design and implementation issues for remote labs are discussed in [START_REF] Guimaraes | Design and implementation issues for modern remote laboratories[END_REF]. A formal reference model for weblabs described in UML language identifies the major functions that a remote lab with SOA must support in order to yield the proper quality of service (Fig. 1, where SLA stands for service-level agreement). Purposely, four main entities are outlined in this Fig. 1. A reference model for weblabs from [START_REF] Guimaraes | Design and implementation issues for modern remote laboratories[END_REF].

figure: (i) the client that uses the lab, (ii) the lab that operates the hardware resources for experiments, (iii) the LMS that organizes the learning activity, and (iv) the webserver that gives access to the platform.

LaboREM should be compliant with this model. Compared to other solutions, the main differences are as follows: the collaborative tools are provided by the LMS, not by the remote lab itself, contrary to NetLab [START_REF] Nedic | Enriching student learning experiences in remote laboratories[END_REF], [START_REF] Nedic | Remote laboratory NetLab for effective teaching of 1st year engineering students[END_REF] which offers a booking system and collaborative tools without using any LMS. In our case, the LMS is in charge of almost everything (authentication, trace recording, data storage), apart from the actual task of interaction with hardware. Compared to the iLab shared architecture from the MIT [START_REF] Delong | Online spectrometer experiments using the iLab shared architecture[END_REF], which is based on a service broker, labservers and labclients (with additional storage and scheduling services for interactive solution), our project is less ambitious since a single labserver is in use and most of the services are taken in charge by the LMS: only the queuing management to give access to experiments is handled by the labserver itself (in place of a service broker).

Client Communication Techniques

CGI scripts (Batch Mode)

A first basic solution is low-level programming with http protocol: it means to use CGI scripts (common gateway interface) to transmit the information (PUT, GET and POST instructions). It just implies uploads and downloads of data (input parameters and output measurements) via a desktop-client application [START_REF] Steriu | Open architecture for signal processing lab distance learning[END_REF]. One main drawback is that it only works in batch mode (asynchronous communication), so that realtime interaction is not possible (contrary to the two other solutions described in section 3.2.2 and 3.2.3 that can transmit continuous dataflows, allowing realtime control). Another drawback is that the desktop application has to be updated by the client each time the lab-server application undergoes a software upgrade.

Remote Front Panels

Another solution is to use the concept of remote front panels (RFP) available with NI-LabVIEW software. Starting from an existing VI including drivers to communicate locally with instruments, the web publishing tool, with remote control option activated, allows to create a HTML file so that a VI is visible and controllable from Internet. As soon as the lab application server (LabVIEW webserver) is started, the HTML file is accessible over the Internet through its specific URL address [START_REF] John | Towards virtual laboratories: a survey of LabVIEW-based conduction of science experiments via the internet with an illustrative consideration of remote control of an oscilloscope[END_REF]. Therefore it is both easy for a client to use the lab (simple access via a web browser) and for a tutor to add a new lab activity: there is very little programming overhead needed to publish new VIs on the web. A good point is that the front panel seen by the remote client is exactly the same as the one seen within the local application: the interface may be rich and convivial (Fig. 2). One main drawback is that RFP client-server data communication uses a non-http binary protocol. It is detected by some firewalls, anti-viruses, reverseproxies as non-http communication and many clients and IT group managers have rules for blocking it. A second drawback is that the client has to download and install the shareware LabVIEW RunTime Engine (LVRTE) which is a plugin of size 40Mbytes. Previously, there were strong browser restrictions to use this plugin: only Internet Explorer could display the LabVIEW user interface. Fortunately, since the latest release of LabVIEW, browser restrictions no longer hold: other browsers like Chrome or Firefox work with LabVIEW 2014 plugin. A third point worth mentioning is that the VI is executed on the serverside. Clients who are connected to the remote front panel, and waiting to get the control over the VI, can observe what the current client who has the control is doing. Note however that this can become a good point for student collaboration [START_REF] Dutta | A web-service and interface for remote electronic device characterization[END_REF].

Webservices

A third solution is to design an SOA based on the use of webservices. The advantage of webservices [START_REF] Dutta | A web-service and interface for remote electronic device characterization[END_REF] is to conform to standardized communication protocol (fully compatible with any network and web browser configuration, no restriction due to firewall or antiviruses, mobile device compatibility). The client interface may be thin (light). A specific URL address with command parameters generates dataflows in real-time, containing output data described in XML, TEXT or JSON format. This solution may even be easily deployed starting from a LabVIEW project (that provides a tool to build Restful Webservices) and using Silverlight and WebUI Builder. This alternative yields a remote interface to LabVIEW that avoids the use of the non-http RFP protocol.

Its main drawback is that the remote graphical interface requires dedicated programming and is not highly sophisticated (Fig. 3 to be compared with center panel in Fig. 2). Moreover the inclusion of a

Proposed Technical choice

The three solutions have been implemented by the authors [START_REF] Steriu | Open architecture for signal processing lab distance learning[END_REF], [START_REF] Luthon | LaboRem: open lab for remote work[END_REF], [START_REF] Talmaciu | Système de Formation en Ligne pour les Laboratoires Distants[END_REF]. Notwithstanding, this is the 3rd solution using webservices (section 3.2.3) we clearly advocate, although we describe in the following the richer interface (Fig. 2) based on the second solution using LabVIEW RFP (section 3.2.2). The prospective use of HTML5, merging JavaScript and XML, shall be the best technical choice for the client software. Among others, this will allow to address the question of flexibility and ubiquitous use of the application on mobile devices (smart phone, tablet) [START_REF] Garcia-Zubia | Mobile devices and remote labs in engineering education[END_REF]. This is of course very desirable for gaming purposes. Tab. 1 summarizes the pros and cons of RFP and web service solutions. Here, as the second solution based on RFP does not require extra-coding and provides a richer interface, we have implemented the remote front panel solution in our lab.

LaboREM Prototype Implementation

Network and Software Architecture

The network and software architecture is described in Fig. 4. It is based on the use of two servers. It Fig. 4. Network and software architecture.

is designed to make the LMS (i.e., the middleware including databases for management of users (using Lightweight Directory Access Protocol LDAP), authentication (using Central Authentication Service CAS), administration, learning tools, content packaging, evaluation, communication tools for collaborative work) communicate with the remote lab (i.e., the hardware including instruments with software drivers and VIs). The labwork is accessed through the use of the LMS: this is simply achieved by calling, at some specific steps of the learning sequence, the proper URL address corresponding to the remote VI to pilot. The e-lab availability service is taken in charge by the labserver itself (queue of users). The e-lab control service is for piloting the remote devices (instruments, robot).

Server Side

In addition to the corporate webserver of the university for connection to the LMS via the webcampus, the application server located inside the industrial engineering department laboratory is another dedicated server (lab application server) for connection to the lab equipment (Fig. 5). A typical hardware setup is shown in Fig. 6. It • LaboratoryWorks is the main VI of the remote lab. It enables to select the type of experiment (GPIB-Bode plots or DAQ-Signal spectrum analyses) and the type of filter (passive, active, lowpass, highpass, bandpass etc.). It allows to control the instruments and the robotic arm to build the DUT, to make measurements, to recover data graphs and to participate to the Top 10 ranking.

The client has full control over this VI.

Client Side Interface

The installation on the client machine requires to download and install only one plugin: the LabVIEW RunTime Engine. This plugin allows to execute the remote front panel object code embedded in a HTML page.

To run the lab, the client logs into the webcampus of the university. It is based on the LMS platform Chamilo which is standard-compatible with other LMS thanks to automatic export facility of SCORM data (we intend to use Moodle instead since it offers dedicated modules for plugging weblabs).

The remote labwork is one of the various activities proposed online to the students, among others like content delivery, quiz, test, simulator, demo, tutorial, help, survey, forum etc. It is simply accessed by pointing to a specific URL address, that is physically in the labserver directly connected to the equipment. Fig. 2 shows a typical interface seen by a client, i.e., a remote student, when asking for plotting Bode curves to characterize the transfer function of active filters. The user builds his own circuit by piloting the robotic arm, makes measurements in real-time, exploits graphs thanks to cursors and markers, optionally takes part in the Top 10, has a visual feedback of the lab through a camera, and gets information about the waiting list of users with a time-counter, so that he knows exactly when he will be served. In the meanwhile, he can switch to other activities proposed in the pedagogical scenario described in section 4.3. A videotutorial of the use of the remote lab is available at [34].

Robotic Arm for Component Placement

Instead of virtual wiring, LaboREM gives the opportunity to build the circuit using an old fashioned robotic arm (Fig. 8). The circuits under test are active filters consisting of OpAmps, resistors and capacitors only (no inductance). Thus, a simple solution for easily placing components is to equip them with magnets, so that the right place and good connection contact are achieved, without needing high spatial precision or big mechanical effort. This minimizes the risk of misplacement: tests conducted in 2013 give a reliability higher than 99.5%. Fig. 8 shows the two-step action: first, grasp one component from the bank of components; then, move and drop it down on the receiver board connected to the breadboard where OpAmps are already in place. In addition to Fig. 8. Robot in action for (i) choosing and (ii) placing components equipped with magnets.

the robot-built filters, there are 8 precabled passive and active filters on the breadboard, so that the student can choose either to build his own filter, or to use precabled ones among high-pass, low-pass, band-pass, rejection, plus one extra unknown filter to be uncovered. Choosing the filter is simply done in the client interface by selecting the type of filter and component values (in the minimal version of the prototype, components can be chosen among 8 values: 4 resistors, 4 capacitors). Note that in case of failure of the robot placement, the client can switch to precabled filters instead, that do not require the robot use.

SCENARIO FOR E-LEARNING

Prerequisites and Objectives

The curriculum content for the technological diploma in industrial engineering is composed of various technical modules, ENA being dedicated to analog electronics. Prerequisites are the basics in mathematics and electricity. The objective is to learn active filters. In the remote lab, the student has the choice between various filters: some of them are known (SallenKey, Wien etc.), or can be built by the student himself; others are unknown and should be identified during the lab session.

Game-like strategy

It is well-known that the motivation of students practicing e-learning may not only be modeled and measured, but also fostered, with the proper use of an LMS (ARCS model [START_REF] Mu Ñoz-Organero | Student behaviour and interaction patterns with an LMS as motivation predictors in e-learning settings[END_REF]: attention, relevancy, confidence and satisfaction). In addition, the use of games may contribute in enhancing motivation, yet it is unclear to which extent instructional games directly affect learning outcomes [START_REF] Pivec | Games for learning and learning from games[END_REF], [START_REF] Dziabenko | Remote experiments and online games: How to merge them[END_REF]. Games are characterized by 6 key-dimensions [START_REF] Garris | Games, motivation, and learning: A research and practice model[END_REF]: fantasy, clear rules and goals, sensory stimuli, challenge, mystery and control. The game process involves a repeated loop of three successive steps: user judgment, behavior and feedback. The objective of adopting a game-like approach in learning is to prompt the student to repeat the activity, like when playing a game over and over (addictive process). In the proposed approach, the 6 game dimensions are taken into account as follows: mystery (unknown filter to uncover), sensory stimuli (visual feedback and robotic motion), challenge (Top 10), control (choice of DUT), rules (limited time and number of trials), goals (clear learning objectives with three difficulty levels to choose from), fantasy (freedom to conceive one's own circuit). Depending on the filters chosen for study, and on the exactness of answers, the student accumulates more or less points during his "travel" and may take place in the "hall of fame". If he discovers what the unknown filter is, he has reached the final objective of the "quest". If he fails, he may choose to backtrack and try again (limited repetitions). All the game scenario is managed by the LMS, apart from Top 10 and robot activities that are directly handled by the lab.

Pedagogical Scenario

A polylinear learning path is implemented in the LMS as a "treasure hunt", that is adapted to the competence level and motivation of the student. Competence and motivation are measured with an automatic assessment procedure (quizzes, tests and surveys). Depending on the level, a flag (red, orange or green) is generated to orientate the student towards the best path and with the best workload according to the student's profile and current study mood.

A detailed description of the game-like scenario may be found in [START_REF] Luthon | LaboRem: open lab for remote work[END_REF]. The scenario is based on four concepts, as in e-games: levels, lives, points, and time. Four counters are used for the evaluation of student's activity: a timer, a counter for highest level achieved (difficulty level), a counter for number of lives used (number of trials), a counter for scoring (marks obtained to online quizzes and final exam). This allows to estimate a participation index and a performance index which are parameters taken into account as in [START_REF] Mendez | Implementing motivational features in reactive blended learning: Application to an introductory control engineering course[END_REF] for driving motivation in autoregulated distance e-learning. The scenario outlined in Fig. 9 gives an overview of the global labwork activity with the objective to be achieved along due time. The various colors code the difficulty level.

A typical scenario contains: 10 is a snapshot of the screen seen by a client when asking for the remote lab within the LMS: he can download a preformatted text-file for writing the report to answer the questions (left button), click on "Travaux Pratiques" (middle button) to run real experiment, or run the simulator (right button). The traffic light indicates the best difficulty level to choose from, depending on previous answers to quizzes. The student can also review a tutorial that explains the use of the lab by playing a video (bottom movie). Fig. 10. LMS interface seen by the client when requesting access to remote lab experiment.

Student Motivation

In 2011, a group of 29 undergraduate students in industrial engineering was tested. At first, an a priori survey about their intrinsic motivation1 showed that their main motivation was acquisition of knowlegde (mean score of about 3.8/5), long before their position with respect to others (mean score of about 2/5). Maybe the distance activity explains partly this result. To enhance external motivation, a Top 10 list was added in 2013, that gives in real-time the best score achieved in the measurement activity. Moreover, the robotic arm was also added (with a videotutorial presenting its use). This proved to increase external motivation by a factor of about 20%. Indeed, Tab.2 gives the average values for intrinsic and extrinsic motivation measured from pre-and post-inquiries2 (before and after the lab activity) in 2011, 2012 and 2013. It also shows (in bold) the increments between 2012 and 2013. The LMS delivers all the traces for each student: number of accesses, time spent, number of trials, performance achieved, difficulty level reached, answers given to quiz (Fig. 11). Four observations are taken into account for statistical analysis of pedagogical achievement: score (and number of trials) on a formative test, duration (and number) of document access, duration (and number) of lab activities, score on summative test. During a session, five variables are recorded: the self-perception of competence (Boolean V 1 ), the score to a formative test (integer V 2 ), the choice of proper remedial path (Boolean V 3 ), the duration of remedial activity (time in seconds V 4 that depends statistically on V 3 ), the score to a summative test (integer V 5 ). The student motivation by the game-like scenario is also estimated a posteriori from a satisfaction survey ("Post-Inquiry"). Note that all those variables are recorded from the traces inside the LMS only, but not from the remote lab which is quite independent (since it is not compliant yet with SCORM or IMS standards, as suggested in [START_REF] Sancristobal | Integration view of web labs and learning management systems[END_REF]). Whereas the dependency between V 2 and V 5 is not statistically significant on our test group (Wilcoxon test), the dependency between V 1 and V 5 is statistically significant. This shows that motivation (through self-perception of competence) is highly correlated to the performance achieved at the final test. Namely, 85% of students that feel competent succeed in their activity. It is therefore very important to estimate and control motivation. Indeed, motivation may be increased by many factors like competitiveness, flexibility, autonomy [START_REF] Rengel | Supervised coursework as a way of improving motivation in the learning of digital electronics[END_REF]. Worth to mention is the fact that the students that most participated in the Top 10 questions were not necessarily the "best" of our students (in terms of academic marks).

PLATFORM EVALUATION

To assess the quality and interest of a remote lab, both a technical evaluation and a pedagogical evaluation must be conducted. We conducted four runs of LaboREM: in 2009 (2 groups: 13+19=32 students), 2011 (2 groups: 18+29=47 students), 2012 (22 students) and 2013 (25 students) respectively. The first runs took place in a classroom equipped with PCs at the university (intranet access), whereas the two last ones were done by each student at home (Internet access, possibly with wifi wireless connection).

Each time, two anonymous questionnaires are used for the surveys: a detailed questionnaire with about 70 questions, some of them are given in Tab. 3 (answers on a 5-point Likert scale) and a global satisfaction survey scoring ten main issues reported in Tab. 4 (marks on a 20-point scale). The average satisfaction is about 13.5/20 for the last session (25 students in 2013). The previous sessions (about 100 students between 2009 and 2012) gave mean scores ranging between 12 and 13.5 (20/20 being the best and 0/20 being the worst).

TABLE 3

Sample questions and their scoring from the 2011 student survey to evaluate the application (Likert scale: 5 = "strongly agree"; 1 ="strongly disagree") From the evaluation in Tab.4, the following facts may be assessed. First, passing from the classroom in 2011 (with a tutor) to the home in 2012 (student alone) led to a sharp fall in the marks (student less confident, less motivated). Improving the application in 2013 (mainly by the use of robotic arm and Top 10) clearly increased student motivation and hence satisfaction, leading to the same average mark as in 2011. Second, the improvement each year of the quality of online documentation for help and preparation to prerequisites is rewarding: the students' marks reflect this fact by constantly increasing. Then, two issues are very positive from the students point of view: collaborative learning, autonomous activity. This is particularly true for undergraduate students (mostly teenagers). The issue of pedagogical efficiency must be investigated further: we intend to conduct experiments with two separate groups of students from the same year, one doing hands-on lab, the other doing remote lab and compare their academic results before drawing any firm conclusion. Apart from those quantitative surveys, the students have also the opportunity to leave free comments about the application. From their point of view, two issues are clearly the keys for success: technical quality and prerequisities. Indeed, bugs should never happen in distance e-learning (leaving the student alone with a problem) and the work should be well prepared and documented in order to yield an effective activity (need of clear objectives). "More early explanation" before leaving the student alone in front of the computer may be required as reported in [START_REF] O'leary | An online, interactive renewable energy laboratory[END_REF] that deals with remote control of photovoltaic panels at NASA center in California.

I

DISCUSSION AND FUTURE WORK

If motivation increase was clearly established (section 4.4), we do not claim that better learning outcomes, compared to hands-on, are clearly visible yet. This is in agreement with other research works on weblabs [START_REF] Henry | Engineering controls labs operated remotely[END_REF]. Nevertheless, this platform is one supplementary tool to help students in their studies, especially in an autonomous and student-centered context, as recommended in Europe by the Bologna process. Moreover, this tool is exhibited each year as a demo during the "open doors day" of the university and proves to be very attractive for the visitors which are teenagers coming from high schools. It contributes to increase the number of students willing to undergo a STEM curriculum (science, technology, engineering, mathematics). This is a highly desirable educational impact since there is nowadays a shortage of science and technology students.

Altogether, the results of the e-learning experiments conducted here show that, for 75% of the student group under test, the self perception of efficiency and/or the proper choice of remedial strategy (use of online help) is highly correlated with their success in learning. However, for the remaining 25% of the group under test, it is important to implement tools to enhance students' motivation and hence performance. For that purpose, learning paths with polylinear flowcharts that can adapt to student's proficiency level are relevant. Moreover, game-like scenarios help to counteract the possible demotivation in distance e-learning. The evaluation of the proposed method shows the same phenomenon and leads to the same conclusion as reported in [START_REF] Locke | The application of goal setting to sports[END_REF] which deals with motivation setting in sports: a difficult but still achievable objective enhances the students' motivation. The tutor (real or virtual) acts like a coach in sport games, so that students achieve the best they can (adaptive learning). The need is clear for a platform that estimates automatically the motivation and controls the regulation of workload for each student. Implementing a Top 10 feedback ("hall of fame") to increase motivation while answering the quizzes proved to be very efficient. Indeed, competitiveness increases motivation, as shown in the surveys answered by students themselves.

The perspective of this work is manifold. From a technical point of view, a multi-server solution is of high interest, to give access to experiments in various remote labs, like the iLab shared architecture [START_REF] Harward | The iLab Shared Architecture: A web services infrastructure to build communities of internet accessible laboratories[END_REF]. The network solution based on webservices for more portability [START_REF] Dutta | A web-service and interface for remote electronic device characterization[END_REF] is implemented: it should now be evaluated and compared to the proprietary remote front panel protocol solution currently in use. With our university partner in Bucharest, Romania, we successfully tested 30 simultaneous connections in 2009. Now that the platform is deployed and operative, it can be shared with other universities, namely the 25 other Technological Institutes of industrial engineering in France, and with other interested academic partners in Latin America (the translation from French to Spanish is under work).

As regards pedagogical aspects, namely to improve the process of differenciated learning, a controller based on the Dempster-Shafer theory of evidence could be implemented to improve the estimate of the student state (motivation, proficiency, autonomy etc.) and regulate in real-time the workload for each student.

We intend to switch to Moodle (instead of Chamilo and Dokeos LMS previously in use at our university). Indeed, it will make easier the design of adaptive learning paths and the plug-in of weblabs thanks to dedicated Moodle modules. We intend to make LaboREM fully compliant with SCORM or IMS standards as in [START_REF] Sancristobal | Integration view of web labs and learning management systems[END_REF] that deals with the complete integration of weblabs, based on web services, into an LMS by creating SCORM packages in a graphical way.

As regards image processing use, face expression analysis may be a good help to know the state of motivation of a student, as mentioned in [START_REF] Mu Ñoz-Organero | Student behaviour and interaction patterns with an LMS as motivation predictors in e-learning settings[END_REF]. To give the control of the camera to the client is attractive both for enhancing the student motivation and for addressing student requests for visualizing hardware settings or for closer collaboration through computer vision (cf. videoconferencing with a tutor) or even augmented reality purposes. In addition to the wide-angle camera that supervises all the scene, a minidrone equipped with a micro-camera could be used to fly over the lab at user's request. Audio hints and sound should also be added since sensory stimuli are one of the 6 key dimensions that characterize games [START_REF] Garris | Games, motivation, and learning: A research and practice model[END_REF], and some students are more sensitive to the aural learning channel (cf. VARK cognitive model [START_REF] Marcelino | Virtual 3D worlds and remote experimentation: A methodology proposal applied to engineering students[END_REF]).

Other applications of remote activities for delocalized research laboratories are under investigation, namely to control prototypes of plasma reactors for biomedical applications, and to pilot remotely renewable energy systems on the roof of the university buildings.

Fig. 2 .

 2 Fig. 2. Client rich interface showing a remote front panel: left) learning steps of the pedagogical scenario; center) results of experiment running (here Bode plots of a high-pass filter); right) video feedback of instruments, Top 10 list and queuing list of users.

Fig. 3 .

 3 Fig. 3. Thin client interface based on Flash programming and web service data communication.

Fig. 5 .

 5 Fig. 5. Hardware architecture: DUT means device under test; INSTR means instruments; DAQ means data acquisition.

Fig. 6 .

 6 Fig. 6. Hardware resources on server side: instruments, DAQ board, on-off switch, electronic protoboard with connection matrix and DUT (here active filters), robot, camera (visible in the lower right corner).

  (i) videopresentation of the lab and of experiments; (ii) test on prerequisites; (iii) first remote labwork (Lab1 on passive filters) with three levels of difficulty; (iv) lesson about active filters (online documents); (v) formative test; (vi) second and third remote labworks (Lab2 on Bode plots, Lab3 on signals and spectra analysis); (vii) comparison with simulator (virtual lab); (viii) final summative test with upload of work report; (ix) satisfaction survey.

Fig.

  Fig.10is a snapshot of the screen seen by a client when asking for the remote lab within the LMS: he can download a preformatted text-file for writing the report to answer the questions (left button), click on "Travaux Pratiques" (middle button) to run real experiment, or run the simulator (right button). The traffic light indicates the best difficulty level to choose from, depending on previous answers to quizzes. The student can also review a tutorial that explains the use of the lab by playing a video (bottom movie).

Fig. 9 .

 9 Fig. 9. Learning map as a treasure hunt with various steps (implemented as activities in the LMS): start, lesson (documentation and help), quiz (evaluation and test), remote lab control, finish.

Fig. 11 .

 11 Fig. 11. Typical traces of student activity delivered by the LMS: steps, status, points, time spent, details.

TABLE 1

 1 Comparison between webservice and Remote Front Panel solutions

	Software side	Criteria	Remote Front Panel Pros Cons	Web Service with HTML5 Pros Cons
		Programming	Graphical	Proprietary	Standard	Quite heavy
	Lab Server		langage, easy	langage	langage (XML
					stream)
		Web Access	http Webserver	Proprietary	http Webserver	-
				protocol
		Operating	Good	-	Very good	-
		safety
		Interface	Rich, with	-	Web based	Extra coding
	Client		measurement		interface	required
			tools (cursors)
		Requirements	Free software	Plugin and	None	-
			download	runtime needed
		Compatibility	HTML page	Browser	Very good	-
		(mobile, tablet,		restrictions
		computer...)

•

  Visualization gives a feedback on the remote lab. The client can observe this VI. It includes three functions: VideoFeedback uses an image acquisition toolbox for video capture and image processing in real-time. Each image is compared to some reference sub-images representative of the various instruments in use (front panel of oscilloscope, multimeter etc.

	Simulator is a 3rd VI accessible via the LMS. It is
	the main VI of the virtual lab (as opposed to the
	remote lab) that only offers numerical simulations
	of the various DUTs. It gives students the oppor-
	tunity to make comparisons between simulation
	results and actual measures. Since it does not
	communicate with any instrument, there is no
	waiting queue to manage: many users can run
	simulations in the same time. By selecting "re-
	entrant execution" in the VI options, LabVIEW
	can duplicate the simulation VI to get as many
	VI-clones as needed without conflict.
	• Virtual Manager is a set of VIs in charge of the
	lab management. It ensures 3 main functions.
	(i) Storage: it stores data of each experiment
	(username, timer, experiment selection, Top 10
	answers) into a MySQL database (currently not
	linked with the LMS database);

) A template matching algorithm, applied on color and shape information in each image captured at video-rate, enables the user to focus selectively and zoom on each of the devices. Thus, a remote user can see the instruments front panels evolving with time. TOP10 is a function to display the ranking in a "hall of fame" (best answers to measurement requests). WaitList is a function to display the waiting queue of users that are requesting the activity on instruments. • (ii) Scheduling: it manages the waiting queue of connected clients. It uses a specific tool (Remote Panel Connection Manager) for the management of remote front panel connections. There is no sophisticated scheduling mechanism nor concurrency problem, since a user is allowed 5 minutes only, and a first-in first-out solution is adopted (no time slot reservation). In the meanwhile, a waiting user can switch to other activities proposed inside the LMS as described in section 4.3, or watch what his colleague having current control is doing, and optionally communicate with him if he wants to collaborate in real-time (using LMS tools, chat, forum etc.) (iii) Initialization and Security: it reinitializes the system (interface and robot) to its default state, each time an experiment ends, or if another user takes the control of the LaboratoryWorks VI. Six variables are used for that purpose: user name, VI name requested by user, user status (waiting, viewing or controlling), starting time, ellapsed/remaining time before closure, plus a global variable that is a flag to track any user change (bit for re-initialization).

TABLE 2

 2 Student motivation (on a 5-point Likert scale)

	year (# students)	Intrinsic	Extrinsic
	Pre-Inquiry	mean	incr.	mean	incr.
	2011 (29 stud.)	3.73		2.06
	2012 (22 stud.)	3.89		1.95
	2013 (25 stud.)	4.18	+0.29	2.42	+0.47
	Post-Inquiry	mean	incr.	mean	incr.
	2012 (22 stud.)	3.1		2.55
	2013 (25 stud.)	3.5	+0.4	3.1	+0.55

TABLE 4

 4 Students answers (with marks ranging from 0 to 20)

	Year	2009	2011	2011	2012	2013
	(# students)	(13)	(18)	(29)	(22)	(25)
	Evaluated issue	in classroom	at home
	Comfort of	12.8	11.6	13.9	12.3	13
	interaction
	Technical quality	14.1	12.6	13.7	12.8	13.5
	of platform
	Game like	13.9	14.9	13.5	10.4	13.4
	scenario
	Pedagogical	13.8	13.1	12.9	10.3	12.6
	efficiency
	Remote control	13	12.7	14.2	11.7	14.1
	of devices
	No space-time	12.5	14.2	13.9	13.1	12.4
	constraints
	Collaborative	12.6	14.3	14.5	13.4	15.2
	work
	Autonomy	13.8	13.7	13.7	12.3	14.2
	Preparation,	10.9	12.6	13.4	14	14.5
	Docs and
	Prerequisites
	Overall	12.6	12.5	12.1	10.7	13.3
	satisfaction
	Average Mark	13	13.2	13.6	12.1	13.6

The definition may be found in[START_REF] Carré | De la Motivation à la Formation[END_REF]. Sample assertions for measuring motivation are, among 20 sentences: "It is important for me to learn"; "It is important for me to look like being proficient"; "I am motivated by this remote learning activity"; "I am motivated by the final mark I will obtain for this labwork" etc.

Note that Pre-and Post-inquiry values are not directly comparable since they are based on two different types of questionnaire; moreover, in 2011, there was no Post-inquiry available.

ACKNOWLEDGMENTS

The authors thank the Regional Council of Aquitaine, Bordeaux, France for financial support. They are also thankful to their former master students, Irina Talmaciu with the technical military academy of Bucharest, Romania, and Bernard Cauquil with the university of Toulouse-Le Mirail, France. Finally, they would like to thank two colleagues: Kevin Levron for his skills in programming that helped for network configuration and adaptation of the LMS to the specific needs of remote lab, and Christine Druon for correction of English flaws in the manuscript. They also thank the anonymous reviewers for their fruitful comments that helped to improve the paper.


est destinée au dépôt

Assessment of natural dynamics and anthropogenic impacts on residence times in the urban aquifers of Recife (Brazil) using a multitracer approach (noble gases, CFCs, SF6, 14C) Eliot Chatton, Thierry Labasque, Luc Aquilina, Emmanuelle Petelet-Giraud, Lise Cary, Guillaume Bertrand, and Rébecca Hochreutener Géosciences Rennes, Université de Rennes 1, Rennes, France BRGM USP

The Metropolitan Region of Recife (RMR) is an urban area of the northeastern coast of Brazil located in an estuary zone and overlying a complex multi-layered sedimentary set. Over the last decades, population growth and recurrent droughts have been increasing the pressures on the aquifers of the region (over-exploitation, contamination and salinization). Through a multi-tracer approach (major and noble gases, CFCs, SF6, 14C) and the implementation of an inverse model to infer recharge conditions, the study aims to investigate the past natural dynamics of the aquifer system and to assess the impacts of the increasing withdrawals on the residence times in the aquifers. Noble gas results were implemented in an inverse model allowing the assessment the recharge conditions of the coastal aquifers of Recife. The results allowed to discriminate two types of recharge in terms of temperature and excess air. The overexploited surficial aquifer records recharge characteristics of the modern wet seasons. However, groundwaters sampled in the two underlying aquifers (Cabo and Beberibe) showed recharge temperatures below the regional minima. According to the radiocarbon dating performed in this study and previous paleotemperature studies in tropical Brazil (Stute et al., 1995), it appears that the major component of the current water supply of Recife originates from recharges dating back more than 10,000 years. Furthermore, the analysis of atmospheric tracers (CFCs, SF6) show that the exploitation of these old groundwaters has lead to a mixing with a more recent component (<50 years old) making these aquifers vulnerable to contamination and salinization.


Nicolas Poirier
email: nicolas.poirier1@univ-tlse2.fr

Florent Hautefeuille

Carine Calastrenc

Low altitude thermal survey by means of an automated UAV for the detection of archaeological buried structures

Keywords: UAV, thermal survey, site detection, mapping, aerial archaeology, remote sensing

  L'archive ouverte pluridisciplinaire

State of the art and objectives

Richard Atkinson is usually considered as the first who developed, for the first time in 1946, a ground-based non-invasive method to get a detailed map of the archaeological site by the study of the electrical resistivity of soil [START_REF] Atkinson | Field Archaeology[END_REF]; although earlier experiments have been documented [START_REF] Bevan | An early geophysical survey at Williamsburg, USA[END_REF] It was followed by the development of techniques such as, magnetic mapping and Ground Penetrating Radar. The development of air and space-based techniques, such as multispectral satellite remote-sensing or Airborne Laser Scanning, has diversified the methods and thus increased the number of tools to visualize archaeological remains without direct intervention [START_REF] Devereux | The potential of airborne lidar for detection of archaeological features under woodland canopies[END_REF][START_REF] Bewley | New light on an ancient landscape: lidar survey in the Stonehenge World Heritage Site[END_REF][START_REF] Lasaponara | On the LiDAR contribution for the archaeological and geomorphological study of a deserted medieval village in Southern Italy[END_REF].

Airborne thermal prospecting is one of these non-invasive methods. It is based on the principle that there is a fundamental difference between the thermal characteristics of buried remains from the environment in which they are buried [START_REF] Eppelbaum | Near-surface temperature survey: An independent tool for delineation of buried archaeological targets[END_REF]. Therefore archaeological structures tend to react differently to changes in temperature compared to the surrounding sediment (warming or cooling more or less quickly depending on their nature). For example, structures built of stone and mortar present a greater thermal inertia than the surrounding sediment; conversely, hollow structures (ditches and pits) have a lower thermal inertia, allowing their detection on bare soils (Rowlands and Sarris, 2007). On cultivated soils, the hydric stress of cereals disturbs their thermal regulation, permitting the detection of remains before they are visible on conventional pictures as crop marks [START_REF] Aqdus | The potential of hyperspectral and multispectral imagery to enhance archaeological cropmark detection: a comparative study[END_REF] as visible on figure 3 for example.

This detection method has been used in archeology since the 1970s [START_REF] Bellerby | A Thermal Method for Archaeological Prospection: Preliminary Investigations[END_REF][START_REF] Ben-Dor | Detection of buried ancient walls using airborne thermal video radiometry[END_REF][START_REF] Berlin | Identification of a Sinagua Agricultural Field by Aerial Thermography, Soil Chemistry, Pollen/Plant Analysis, and Archaeology[END_REF][START_REF] Lundén | Aerial Thermography: A Remote Sensing Technique Applied to Detection of Buried Archaeological Remains at a Site in Dalecarlia, Sweden[END_REF][START_REF] Prakash | Estimating the depth of buried hot features from thermal IR remote sensing data: a conceptual approach[END_REF][START_REF] Tabbagh | II Prospections thermiques et électriques[END_REF]. But the use of aircraft or satellite imagery made the method difficult to access due to its cost and lack of organizational flexibility: flights must be scheduled several weeks in advance and the relatively high flight altitude is only able to detect large structures (abandoned roads and parcel boundaries).

The parallel development of Unmanned Aerial Vehicles (UAV) and miniaturization of sensors and data transmission suggests a technological breakthrough in terms of archaeological prospection [START_REF] Bendea | Mapping of archaeological areas using a low-cost UAV. The Augusta Bagiennorum test site[END_REF][START_REF] Chiabrando | UAV and RPV systems for photogrammetric surveys in archaelogical areas: two tests in the Piedmont region (Italy)[END_REF][START_REF] Eisenbeiss | Comparison of DSMs generated from mini UAV imagery and terrestrial laser scanner in a cultural heritage application[END_REF][START_REF] Eisenbeiss | Photogrammetric documentation of an archaeological site (Palpa, Peru) using an autonomous model helicopter[END_REF][START_REF] Everaerts | The use of unmanned aerial vehicles (UAVs) for remote sensing and mapping[END_REF][START_REF] Hendrickx | The use of stereoscopic images taken from a microdrone for the documentation of heritage -An example from the Tuekta burial mounds in the Russian Altay[END_REF][START_REF] Verhoeven | Providing an archaeological bird's-eye view -an overall picture of ground-based means to execute low-altitude aerial photography (LAAP) in Archaeology[END_REF]. However, the recent development of UAVs has so far benefited archaeological survey by the addition of conventional digital cameras, to achieve vertical or oblique aerial photographs, mainly at the scale of a site or excavation.

The French 'Archéodrone' project, developed at the University of Toulouse 2 -Le Mirail (France), aims to combine the flexibility and relative low cost of using an airborne drone with the accuracy of the registration of a thermal digital camera. Indeed, even if the price of acquiring the equipment is quite high (several dozens of thousand Euros), the possibility to repeat as many flights as needed makes the methodology affordable compared to the price of a single plane or helicopter flight. This combination allows the use of thermal survey for archaeological detection at low altitude, resulting in a higher resolution of information, from a micro-regional scale to the site scale, and the possibility to repeat several flights above the same area in different air and soil conditions.

Equipment used

The UAV used is an octoroctor helicopter (Figure 1) with electric propulsion and a wingspan of about 80 cm, with a three-dimensional inertial stabilization system and a GPS for its geolocation. It is radio controlled and allows a payload of 3 kg. It is accompanied by a telemetry system to display real-time flight data: battery consumption, flight time, altitude, distance from the point of takeoff, and GPS status. The same system is used to display a map and manage the GPS' points of interest (waypoints).

The sensor used is a FLIR T620 thermal digital camera with high sensitivity for the detection of temperature changes of about 0.04 °C, on images of 640 x 480 pixels. A radio transmission system captures the camera images in real-time and can track the flight on a LCD screen with return video.

The whole equipment was acquired for about 70,000 €. This is a large investment for a research team, but we consider this as a low cost method given that this material will be usable for several years, several times a day and several days in a row if needed during a survey campaign. This should be compared to expensive "one-shot" data acquisition such as airborne Lidar or thermography.

Intervention process

The intervention process first consists in the definition of flight plans ahead of field data acquisition. For this, the Mikrokopter Tool software allows extraction of geographical views at the plot scale from web portals like Google Earth or Virtual Earth. It is then possible to generate flight plans by creating waypoints to be followed by the UAV in automated flight mode guided by GPS. These waypoints can be automatically generated, including regular grids which the operator defines the equidistance and altitude. This option is particularly useful when it comes to cover an entire plot of land (Figure 2).

In the field, the operator can choose to perform manual takeoff and landing operations, while the flight itself can be automated following the waypoints through GPS guidance. When the UAV is stopped on each waypoint, the operator starts data capture, controlling in real-time quality and recording thanks to the return video system.

Post-processing and first results

In the laboratory, the captured images can be processed to improve the recognition of thermal anomalies (stretching and compression of the histogram of the measured values, application of different ranges of colors, etc). It is possible to restrict the range of representation of the color composition to reduced amplitude of a single degree Celsius, using the FLIR Tools software.

The calibration process of the intervention protocol is still in progress, but the first results have already allowed us to assess the contribution of this technique for the detection of ancient roads and parcel boundaries (Figure 3). But the experiment in progress promises a high potential for detection at the site level of smaller structures, thanks to the low altitude of the flights. For example, the first tests performed over the ancient roman town of Saint-Bertrandde-Comminges (South of France) revealed evidence of correlation between ancient urban plans (streets network, buildings) and linear thermal anomalies (Figure 4).

The potential is also important regarding the detection of underground cavities whose presence is manifested on the surface by different temperature from that of the atmosphere and the surface of the soil. We already experienced this detection process of underground cavities on the two prehistoric sites of Kroomdraï and Sterkfontein in the south-African Cradle of Humankind (Figure 5).

The coupling of the thermal camera and the drone is still new. Ongoing tests aim to define an intervention protocol to determine what is the optimal flight altitude, time, and ground cover for the detection of buried remains based on a small corpus of sites periodically overflown in various flight conditions. The flexibility of the machine makes it possible to take the decision to fly on the day, and repeat it several times a day over the same plot if necessary. It is already obvious that the low altitude of flights permits a gain in resolution to detect archaeological buried remains.

Figure 1 :Figure 3 :

 13 Figure 1 : Drone Octorotor with FLIR T620 thermal camera onboard

Figure 4 :

 4 Figure 4 : On the left hand, the thermal image taken 100 m above the ground shows warm marks (in light).The circles show where these thermal anomalies are related to buried roman walls, which are invisible on the conventional image of the right hand (Saint-Bertrand-de-Comminges, France).

Figure 5 :

 5 Figure 5 : On the left hand, the thermal image taken 50 m above the ground reveals warm marks (in light) indicating warm air outbreaks from underground caves (Sterkfontein cave, South Africa). The circles highlight the warmest anomalies interpreted as paths to hidden cavities. The conventional image (on the right hand) allows no distinction between the different holes.

Acknowledgements

The purchase of the equipment was funded by the European Union through the FEDER Presage 35827 project.


S Fornasier

I N Belskaya

D Perna

The potentially hazardous asteroid (214869) 2007 PA8: an unweathered L chondrite analogue surface

Keywords: Near-Earth objects, Asteroids, Surfaces, Spectroscopy, Polarimetry, Meteorites, Asteroids, composition

, indicate that 2007 PA8 may be a member of the Gefion family recently ejected from the 5:2 resonance and a potential source of L chondrites.

Introduction

Potentially hazardous asteroids (PHA) are near earth asteroids (NEA) whose minimum orbit intersection distance with the Earth is 0.05 AU or less and whose absolute magnitude H v is 22.0 or brighter. The knowledge of their physical properties (size, albedo, density, surface mineralogical composition) is very important to define a successful mitigation strategy, since the technology needed to deflect or disrupt a potential impactor depends on its physical properties [START_REF] Perna | The near-Earth objects and their potential threat to our planet[END_REF].

The entire population of NEAs is supposed to contain about 1000 objects with diameter larger than 1 km and more than twenty of thousands greater than 100 m [START_REF] Mainzer | NEOWISE observations of Near-Earth objects: preliminary results[END_REF]. NEAs are responsible for most meteorite falls and for the occurrence of occasional major catastrophic impact events. Our knowledge of the structure and composition of NEAs is still rather limited, since only 5% of the known NEAs have spectral types determined from observations. The knowledge of the chemical and mineralogical composition of NEAs surfaces gives insight on the materials which formed the protoplanetary nebula at di erent solar distances, and on the processes that governed the formation and the evolution of our planetary system. Asteroid (214869)2007 PA8 is a PHA discovered on 09 August 2007 by the LINEAR sky survey. It is an Apollo Earth-crossing asteroid with a minimum orbit intersection distance (MOID) of 3.59 millions of km (0.024 AU). It is also a target of relatively easy access for a space mission, having a minimum V of 6.8 km s (computed following the method by [START_REF] Shoemaker | Earth-approaching asteroids as targets for exploration[END_REF]. 2007 PA8 has a Tisserand invariant T J 2.95, and it resembles that of a Jupiter-family comet. Its physical and orbital parameters are summarized in Table 1. [START_REF] Hicks | Broadband Photometry of[END_REF] got Bessel B, V, R, and I observations of this PHA finding a long rotational period of 95.1 3.4 hr and visible colors compatible with a C or Xc-type spectral classification following the Tholen or the Bus-Demeo taxonomy [START_REF] Tholen | Asteroids taxonomy[END_REF][START_REF] Demeo | An Extension of the Bus Asteroid Taxonomy into the Near-Infrared[END_REF]. Assuming a geometric albedo of 0.05, typical for C-type asteroids, they compute an e ective diameter of 3km. Extensive radar observing campaign were performed at the Goldstone radar on October-November 2012 .

Radar images reveal an irregular and asymmetric shape with rough dimensions of 1.9 km 1.4 km 1.3 km, the presence of possible craters and boulders, and a very slow rotation (102 2 hr) in retrograde spin state [START_REF] Brozovic | Goldstone radar imaging and shape modeling of near-Earth Asteroid (214869) 2007 PA8[END_REF].

We present in this paper the results of the 2007 PA8's polarimetric and spectroscopic observations obtained during its favorable apparition in October-November 2012, when it approached the Earth at the minimal distance of 0.043 AU (the next close approach of this asteroid to the Earth will be in 2036). Our data allow to constrain the surface composition of this PHA, to obtain the first measurement of its albedo, and thus to determine its size.

Observations and data reduction

Spectroscopy

Spectroscopic observations in the visible and near infrared range of 2007 PA8 were made at the 3.56 m Italian Telescopio Nazionale Galileo (TNG) of the European Northern Observatory (ENO) in la Palma, Spain, on 12-13 October 2012.

For visible spectroscopy we used the Dolores (Device Optimized for the LOw RESolution) instrument equipped with the low resolution red (LRR) and blue (LRB) grisms. The LRR grism covers the 0.52-0.95 m range with a spectral dispersion of 2.9 Å px, while the LRB grism covers the 0.38-0.80 m range with a spectral dispersion of 2.8 Å px. The Dolores detector is a Loral thinned and back-illuminated CCD with 2048 2048 pixels, with a pixel size of 15 m and a pixel scale of 0.275 arcsec px. The 'red' and 'blue' spectra in the visible range were separately reduced and finally combined together to obtain spectral coverage from 0.38 to 0.95 m.

For the infrared spectroscopic investigation we used the near infrared camera and spectrometer (NICS) equipped with an Amici prism disperser. This equipment covers the 0.85-2.40 m range during a single exposure with a spectral resolution of about 35 (Ba a et al. 2001). The detector is a 1024 1024 pixel Rockwell HgCdTe Hawaii array. The spectra were acquired nodding the object along the spatial direction of the slit, in order to obtain alternating pairs (named A and B) of near-simultaneous images for the background removal. For both the visible and near infrared observations we utilized a 2 arcsec wide slit, oriented along the parallactic angle to minimize the e ect of atmospheric di erential refraction.

Visible and near-infrared spectra were reduced using ordinary procedures of data reduction with the software packages Midas as described in [START_REF] Fornasier | Water ice on the surface of the large TNO[END_REF][START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF]. For the visible spectra, the procedure includes the subtraction of the bias from the raw data, flat-field correction, cosmic ray removal, sky subtraction, collapsing the two-dimensional spectra to one dimension, wavelength calibration, and atmospheric extinction correction. The spectra were normalized at 6000 Å.

For observations in the infrared range, spectra were first corrected for flat fielding, then bias and sky subtraction was performed by producing A-B and B-A frames.

The positive spectrum of the B-A frame was shifted and added to the positive spectrum of the A-B frame. The spectrum was then extracted and wavelength calibrated. For both visible and near infrared observations the reflectivity of 2007 PA8 was obtained by dividing its spectrum by that of a solar analog star, observed close in time and in airmass to the object. The infrared and visible spectral ranges were combined by overlapping the spectra, merging the two wavelength regions at the common wavelengths and utilizing the zone of good atmospheric transmission to find the normalization factor between the two spectral parts.

Details on the observing conditions are reported in Table 2.

Polarimetry

Polarimetric observations were carried out at the 2.5 Nordic Optical Telescope (NOT) in La Palma, Spain, during 4 runs between 13 October and 14 November 2012. The observations were done in the Bessel B, V, R and I filters using the Wedged Double Wollaston prism [START_REF] Oliva | Wedged double Wollaston, a device for single shot polarimetric measurements[END_REF] mounted on the ALFOSC instrument. The detector is a back illuminated CCD of 2048 2048 pixel (pixel size of 13.5 m), providing a total field of view of 6.5 6.5 arcmin, with a pixel scale of 0.19 arcsec px. This polarimeter, previously mounted at the 1.82 m Asiago telescope, splits the incoming light into four polarized beams at 0 o , 45 o , 90 o , and 135 o , separated by 20 arcsec [START_REF] Pernechele | Device for optical linear polarization measurements with a single exposure[END_REF]. These four beams are su cient to determine simultaneously the linear polarization parameters I, Q, and U with a single exposure. This instrument was tested successfully for several polarimetric observations of minor bodies as shown in [START_REF] Fornasier | Polarimetric survey of asteroids with the Asiago telescope[END_REF][START_REF] Belskaya | Polarimetry and BVRI photometry of the potentially hazardous near-Earth Asteroid (23187) 2000 PN9[END_REF].

During each run, the observational procedure includes bias and flat field acquisitions, together with the observation of polarized and unpolarized standard stars to calibrate the instrumental polarization. The flat field images were acquired in two di erent sets corresponding to the adapter position of 0 o and 90 o , to average the polarization induced by the reflections on the dome screen of the flat field lamp. These 2 sets were first bias subtracted, then added to produce a master flat.

The asteroid and standard stars images were corrected for bias and master flat, then aperture photometry was applied. The flux for each channel was integrated over a radius corresponding to 3-4 times the average seeing, and the sky was subtracted using a 3-5 pixel wide annulus around the asteroid. Naming I 0 ,I 90 ,I 45 , and I 135 , the flux corresponding to the channels at 0, 90, 45, and 135 o of polarization, the Stokes parameters have been derived as follows:

Q I 90 I 0 I 90 I 0 (1) U I 135 I 45 I 135 I 45 (2)
The degree of polarization P and the position angle of the polarization plane in the instrumental reference system are expressed via the parameters Q and U with the well-known formulae

P U 2 Q 2 P U dU Q dQ P (3) 1 2 arctan U Q 28 65 P P (4)
where dU and dQ are the errors on the Stokes parameters [START_REF] Shakhovskoy | Polarization observations of nonstable stars and extragalactic object. I: Equipment, method of observation and reduction[END_REF].

The position angle of the polarization plane relative to the plane perpendicular to the scattering plane, r , is expressed in terms of the position angle as

r ( 90 o )
where the sign inside the bracket is chosen to assure the condition 0 o ( 90 o ) 180 o . The polarization quantity P r is derived from

P r P cos(2 r )
The observing conditions and the final results on the degree of polarization P, on the position angle of the polarization plane, and on the polarimetric quantities P r and r are reported in Table 3.

Results

Taxonomy and composition

The two visible spectra acquired at the TNG telescope are shown in Fig. 1, together with the albedo values, normalized in the V band, derived from B, V, R and I polarimetry. The two spectra are taken 72 minutes apart and they are very similar, showing that the surface sampled during these observations is homogeneous.

The combined visible and infrared spectra of 2007 PA8 (Fig. 2) clearly show the presence of silicate bands in the 0.9-1 mand2 m region, and an overall behavior consistent with a Q-type asteroid classification, following the Bus & DeMeo classification scheme [START_REF] Demeo | An Extension of the Bus Asteroid Taxonomy into the Near-Infrared[END_REF]. Within the S-complex, Q-type asteroids match ordinary chondrite spectra, and therefore have surfaces that appear young, fresh, and so unweathered, implying they have been recently exposed [START_REF] Demeo | Mars encounters cause fresh surfaces on some near-Earth asteroids[END_REF]).

We analyzed the spectrum following the procedure described by Ga ey et al.

( 1993). We find a BI band center of 0.9578 0.0042 m, with a band depth of 16.5%. This band is due to the presence of both olivine and pyroxene. Beyond 1.8 m the spectrum is quite noisy, so we apply a order 2 polynomial fit to the data in the 1.7-2.3 m region to determine the BII band spectral parameters. The BII band is centered at 1.958 0.015 m and has a band depth of about 3.9%. Nevertheless the upper edge of the 2 micron band is not well defined, so the band II area is probably underestimated.

The derived 2 m 1 m band area ratio is 0.4 0.1. Following the [START_REF] Dunn | A coordinated spectral, mineralogical, and compositional study of ordinary chondrites[END_REF][START_REF] Dunn | Mineralogies and source regions of near-Earth asteroids[END_REF] formulas that relate mineral abundances with band BI center and with 2 m 1 m band area ratio, we find a fayalite abundance (Fa) of 23.1 0.8 mol% in olivine, a ferrosilite abundance (Fs) of 19.7 0.6 mol% in pyroxene, and an olivine on olivine plus pyroxene abundance of 0.63 0.02.

On the basis of these values 2007 PA8 is placed in the space of ordinary chondrites, and, in particular, comparing them with those of L, LL, and H chondrites (Fig. 2 in [START_REF] Dunn | Mineralogies and source regions of near-Earth asteroids[END_REF]), it appears that the NEA has spectral parameters consistent with the L chondrites.

Meteorite analogs

To constrain the possible mineralogy of 2007 PA8 we conducted a search for meteorite spectral analogs using the RELAB spectrum library [START_REF] Pieters | RELAB (Reflectance Experiment Laboratory): A NASA Multiuser Spectroscopy Facility[END_REF], and following the same procedure explained in [START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF][START_REF] Fornasier | Spectroscopic survey of X-type asteroids[END_REF]. For each spectrum in the library, a filter was applied to find relevant wavelengths (0.4 to 2.45 m). Then, a second filter was applied to reject spectra with irrelevant albedo values (i.e. brightness at 0.55 m 0 15 and 0.4). A Chisquared value was calculated relative to the normalized input asteroid spectrum.

We used the asteroid wavelength sampling to resample the laboratory spectra by linear interpolation. This allowed a least-squares calculation with the number of points equal to the wavelength sampling of the asteroid spectrum. The RELAB data files were sorted according to the corresponding Chi-squared values, and finally we visually examined them for dynamic weighting of spectral features.

The two best spectral matches are represented in Fig. 2: both meteorites are Ltype chondrites, in agreement with the spectral analysis of 2007 PA8 that already pointed versus a L-type spectral analogy for this PHA. The best match is given by the Vouille L6 ordinary chondrite, which has an albedo of 0.24, and the second best fit by another L6 chondrite, Denver, which has an albedo of 0.34. Both samples have grain size 150 micron, and are ordinary chondrite with almost all metal removed [START_REF] Jarosewich | Chemical analysis of meteorites: a compilation of stony and iron meteorites analysis[END_REF].

Albedo and polarimetric properties

The NEA (214869) 2007 PA8 was observed in BVRI bands during five nights in October-November 2012 (Table 3), covering both low (12-23 o ) and large phase angles (88-99 o ). The measured polarization phase curve is shown in Fig. 3. We

P( ) b(sin ) c 1 (cos(0 5 )) c 2 sin( 0 )
where we fixed two parameters: the parameter c 2 0.35 and the inversion angle 0 which was estimated from the linear fit. In Fig. 3 we show the fit for the B and R bands (dashed lines) just to demonstrate the predicted behavior at large phase angles. We assumed that our measurements at the phase angle of 99.2 o are close to the polarization maximum. In Table 4 we report our estimations o fP min , inversion angle, polarimetric slope (h), P max , and their uncertainties for each band.

We use the measured polarimetric slope to calculate the asteroid's albedo ( p v ).

Albedo from the polarimetric slope was determined using well-known empirical relationship h-albedo:

log p v C 1 log h C 2
The coe cients C 1 andC 2 are slightly di erent according to the di erent albedo datasets used to calibrate this relationship. 4.

We have also analyzed how the calibration of the relationship h-albedo influence the values of albedos. We found that the available calibrations based on asteroid data (Lupishko and Mohamed, 1996;[START_REF] Cellino | Polarimetric observations of small asteroids: preliminary results[END_REF][START_REF] Cellino | A new calibration of the albedo -polarization relation for the asteroids[END_REF][START_REF] Masiero | A revised asteroid polarization-albedo relationship using WISE NEOWISE data[END_REF] gave consistent results within the uncertainties associated to the polarimetric slope h. The calibration based on meteorite data [START_REF] Zellner | Minor planets and related objects. XX. Polarimetric evidence for the albedos and compositions of 94 asteroids[END_REF] gave smaller values of albedos (0.21 0.03 in the V band), but it must be noted that the meteorites' albedos were measured at the phase angles of 5 o (see e.g. [START_REF] Geake | Planetary surface texture and albedo from parameter plots of optical polarization data[END_REF]) and not at zero phase angle, as normally defined are close to those measured for Icarus and Itokawa, indicating that these bodies have similar surface properties. These asteroids also show similar polarization wavelength dependence (Fig. 5). Moreover the geometric albedo of Itokawa and Icarus, 0.29 [START_REF] Bernardi | Absolute magnitude and slope parameter G calibration of asteroid 25143 Itokawa[END_REF]) and 0.33 [START_REF] Harris | A thermal model for near-Earth asteroids[END_REF], respectively, are very close to the one here determined for 2007 PA8.

To estimate the albedo from the P max value we used the empirical relationship:

log P max C 3 log p v C 4
based on measurements of lunar fines and terrestrial samples of di erent grain sizes [START_REF] Geake | Planetary surface texture and albedo from parameter plots of optical polarization data[END_REF]. The best match of the 2007 PA8 V-band albedos defined from polarimetric slopes and from P max occurred for C 3 -0.73 and C 4 -0.13. These coe cients are intermediate between those found for lunar fines with grain sizes 50 m, and for terrestrial samples with grain sizes 200-340 microns [START_REF] Geake | Planetary surface texture and albedo from parameter plots of optical polarization data[END_REF].

Summary and Discussion

From our studies the PHA 2007 PA8 is a Q-type asteroid with a fresh unweathered surface, a spectrum similar to the L chondrites, and with polarimetric (2014) BII center are similar, the shape and depth of this band is very di erent, our spectrum showing a much fainter absorption compared to them. The spectra were taken at similar phase angles (about 12 o ) and only one month apart, and any problem related to the instrumental and observing conditions or data reduction were found by us or reported by [START_REF] Nedelcu | Evidence for a second parent body of H-chondrite[END_REF]. Considering that 2007 PA8 is a large NEO with a long rotational period, and with an asymmetric shape having probably craters and boulders, as put in evidence by radar images [START_REF] Brozovic | Goldstone radar imaging and shape modeling of near-Earth Asteroid (214869) 2007 PA8[END_REF], we interpret these spectral di erences as due to heterogeneities in the surface composition of this slow rotator asteroid.

2007 PA8 is the Q-type asteroid with the largest semi major axis value in the known Q-type NEO population [START_REF] Demeo | Mars encounters cause fresh surfaces on some near-Earth asteroids[END_REF]. According to Bottke et al.

(2002) NEO model, 2007 PA8 has a probability of 0.534 to have origin from the Jupiter family comet region, and a probability of 0.438 to come from the outer main asteroid belt [START_REF] Nedelcu | Evidence for a second parent body of H-chondrite[END_REF]. [START_REF] Nedelcu | Evidence for a second parent body of H-chondrite[END_REF] also perform some dynamical simulations showing that 2007 PA8 probably originated from a region bordering the 5:2 resonance in the outer belt (2.824 AU), and that it rapidly enter in the NEO space. Indeed, the 5:2 resonance is the one that pumps orbital eccentricities on the shortest time scale, of the order of 10 5 years [START_REF] Yoshikawa | Motions of asteroids at the Kirkwood Gaps. II -On the 5:2, 7:3, and 2:1 resonances with Jupiter[END_REF][START_REF] Ipatov | Evolution of asteroidal orbits at the 5:2 resonance[END_REF][START_REF] Morbidelli | Orbital and temporal distributions of meteorites originating in the asteroid belt[END_REF]. [START_REF] Nedelcu | Evidence for a second parent body of H-chondrite[END_REF] propose that 2007 PA8 was originated from the 5:2 resonance, then its eccentricity was rapidly pumped up, it crossed the main belt up to the Mars-crossing region, and it entered in the NEO space first as an Amor-class, then, five hundreds years ago, it became an Apollo-type. Close to the 5:2 resonance, two large families, Gefion and Koronis, dominated by S-complex asteroids have been identified.

The Gefion family was proposed as the probable source of L-type chondrites by [START_REF] Nesvorny | Asteroidal source of L chondrite meteorites[END_REF] and [START_REF] Bottke | The Gefion Family as the Probable Source of the L Chondrite Meteorites[END_REF] because of its proximity to the 5:2 resonance and because of its age, which is compatible with that of the catastrophic event of L-type meteorites parent body. Indeed, these chondrites represent about 47% of ordinary chondrite falls, and two third of them were heavily shocked near 470 Myr ago, during the Ordovician period, probably as a consequence of the catastrophic disruption of their parent body at that time [START_REF] Korochantseva | L-chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40Ar-39Ar dating. Meteorit[END_REF][START_REF] Haack | Meteoritic, asteroidal, and theoretical constraints on the 500 Ma disruption of the L chondrite parent body[END_REF][START_REF] Heck | A single asteroidal source for extraterrestrial Ordovician chromite grains from Sweden and China: High-precision oxygen three-isotope SIMS analysis[END_REF]. According to [START_REF] Nesvorny | Asteroidal source of L chondrite meteorites[END_REF] studies, the Gefion family, located at 2.7-2.82 AU, has an estimated age of about 485 Myr, while the Koronis family is probably Gys old [START_REF] Bottke | Dynamical spreading of asteroid families by the Yarkovsky e ect[END_REF]. They also show that putative meteorites from the Gefion family may reach the NEO orbit in short time, and that the computed cosmic age exposure (CRE) of these simulated fragments should range from 5 to 100 Myr, with the peak of the dis-tribution around 30-40 Myr, that is closely matching the CRE of OC chondrites [START_REF] Marti | Cosmic-ray exposure history of ordinary chondrites[END_REF]. Faster Gefion meteorites reached the Earth between 50 Kyr and 1-2 Myr after they had been placed in the 5:2 resonance, a time range compatible with the short CRE ages of fossil L chondrites observed in middle Ordovician sediments on Earth [START_REF] Heck | Fast delivery of meteorites to Earth after a major asteroid collision[END_REF].

Considering both the dynamical works of [START_REF] Nesvorny | Asteroidal source of L chondrite meteorites[END_REF] and [START_REF] Nedelcu | Evidence for a second parent body of H-chondrite[END_REF], and the results of this paper indicating for 2007 PA8 a fresh surface, a Q-type classification, and a strong spectral similarity to L chondrites, we suggest that this PHA may be a member of the Gefion family recently ejected from the 5:2 resonance and a potential source of L chondrites.

Tables Table 1: Orbital and physical characteristics of ( 214869

  find a strong dependence of the polarization with wavelength. These data allow us to estimate the polarization maxima and minima, and the polarimetric slopes for B, V, R, and I bands. The polarimetric slope was calculated by a linear fit to the observations at small phase angles. Since there are only three phase angles covered, the uncertainties of slope's values are rather large. The whole range of phase angles were fitted by the Lumme and Muinonen function (Lumme & Muinonen, 1993):

  We utilize C 1 -1.118 0.071 and C 2 -1.779 0.062 given by Cellino et al. (1999), and we derive a geometric albedo of 0.29 0.08 in the V band. The obtained albedo values in the BVRI bands with their errors are given in Table

  The determined value of the asteroid's albedo perfectly matches the mean albedo value for Q-type asteroids(Thomas et al., 2011). The obtained values of albedos in the BVRI bands well reproduce the spectral behavior measured in the visible wavelengths (Fig.1).We estimated the asteroid's absolute magnitude analyzing the photometric observations made by Hicks et al. (2012) in the phase angle range 6.4-37.6 o . We first did a linear fit of these data, then we corrected the linear magnitude for the opposition e ect, estimated to be 0.35 0.04 mag for moderate albedo asteroids (Belskaya & Shevchenko, 2000). The resulting absolute magnitude H v is 16.2 0.1 mag, where the uncertainty takes into account the errors of the linear fit and of the opposition e ect. Using this H v At present the polarization maximum was measured for very few NEAs (e.g. De Luise et al. 2007, Belskaya et al. 2009 and references therein). The NEA (214869) 2007 PA8 is the forth moderate albedo asteroid and the first Q-type forwhich the value of polarization maximum has been determined. In Figure4we plot all the available polarimetric data for NEAs with moderate albedo values. We used only the V band data to avoid di erences due to wavelength dependence of polarization. The value of P max for 2007 PA8 is the lowest one compared to other silicate rich asteroids at the same phase angle of 100 deg.Considering that the P max value strongly anticorrelates with albedo, this means that the surface of 2007 PA8 is brighter compared to those of 2000 PN9, Toutatis and Toro. There are systematic di erences between polarization measured for these asteroids and that of our target, assuming that the surface of 2007 PA8 is brighter. On the other hand, the polarization degrees measured for 2007 PA8

  properties analogues to the NEAs Itokawa and Icarus, both classified as S(IV) Qtype asteroids[START_REF] Demeo | Mars encounters cause fresh surfaces on some near-Earth asteroids[END_REF][START_REF] Binzel | MUSES-C target asteroid (25143) 1998 SF36: A reddened ordinary chondrite[END_REF].[START_REF] Nedelcu | Evidence for a second parent body of H-chondrite[END_REF] got a spectrum of 2007 PA8 in the NIR region (0.8-2.4 m) from the IRTF telescope one month before our observations. Their spectrum is di erent compared to the one here presented, with a deeper 2 micron band, and with band I and II center positions at 0.92 0.02 m and 1.95 0.01 m, respectively. They found the H5 Cangas de Onis chondrite as best match of their NIR spectrum, proposing 2007 PA8 as a second source of H chondrites, beside asteroid 6 Hebe.The results based on our visible and near infrared spectral investigation confirm that 2007 PA8 has a young surface almost unweathered, but point toward a different meteorite analogy than that proposed by[START_REF] Nedelcu | Evidence for a second parent body of H-chondrite[END_REF], i.e. with the L chondrites. Our data cover the full 0.4 to 2.4 m wavelength region, so we can better constrain the BI band spectral parameters and bands BII BI area ratio compared to[START_REF] Nedelcu | Evidence for a second parent body of H-chondrite[END_REF] work. We find a di erent BI center, situated at larger wavelength, in agreement with the mean BI band center reported in[START_REF] Dunn | A coordinated spectral, mineralogical, and compositional study of ordinary chondrites[END_REF] for the L chondrites (0.945-0.967 m). If our and Nedelcu et al.

  ) 2007 PA8. Orbital elements are taken from the JPL Small-Body Database. a : Hicks et al. (2012), b : Brozovic et al. (2013), c : this paper.

Figure 1 :Figure 2 :

 12 Figure 1: Visible spectra of 2007 PA8. The albedo from B, V, R, and I polarimetry, normalized in the V band, is superposed to the spectrum acquired at 01:44. The spectrophotometry from the Hicks et al. (2012) B, V, R, and I data is superposed to the spectrum acquired at 02:56.

Figure 3 :Figure 4 :

 34 Figure 3: Polarization phase angle dependence measured in the BVRI band for the NEA (214869) 2007 PA8. The fit with the Lumme and Muinonen function (Lumme & Muinonen, 1993) is shown for the B and R bands (dashed lines).

  30

Figure 5 :

 5 Figure 5: Wavelength dependence of polarization for NEAs. The phase angle is also shown.

Table 2 :

 2 Observing conditions of (214869) 2007 PA8 during the spectroscopic study in 2012 at the TNG telescope.

	Day	UT start	Exp. (s)	Instrument	Airm	Solar An. (airm)	r (AU)	(AU)	( o )
	13 Oct. 01:44:06	180	DOLORES-LRB 1.23 Land93-101 (1.27) 1.1552 0.1611 11.3
	13 Oct. 01:54:02	240	DOLORES-LRR 1.25 Land93-101 (1.27) 1.1551 0.1610 11.3
	13 Oct. 02:56:02	180	DOLORES-LRB 1.40 Land93-101 (1.27) 1.1546 0.1605 11.3
	13 Oct. 03:03:59	180	DOLORES-LRR 1.44 Land93-101 (1.27) 1.1545 0.1604 11.3
	12 Oct. 23:43:31	240	NICS-Amici	1.26 Land93-101 (1.42) 1.1555 0.1614 11.3
	Figures

Table 4 :

 4 Parameters of the polarization phase curve and estimated values of the polarimetric albedo.

	Band	P min	slope (h)	Inv. angle	P max	Albedo
		(%)	(% o

Preprint submitted to ElsevierDecember 16, 2014

Acknowledgments

The research leading to these results has received funding from the European Union Seventh Framework Program (FP7 2007-2013) under grant agreement No. 312430 (OPTICON). This paper is based on observations made with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association, and with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Centro Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica), both located at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias." S.F. thanks Dr. V. Lorenzi, Dr. T. Augusteins and the TNG and NOT telescopes sta for their help in performing the observations. This project was supported by the French Planetology National Programme (INSU-PNP). D.P. acknowledge financial support from the NEOShield project, funded by the European Commission's Seventh Framework Programme (Contract No. FP7-SPACE-2011-282703). Part of this work 16 was supported by the COST Action MP1104 "Polarization as a tool to study the Solar System and beyond". Taxonomic type results presented in this work were determined, in whole or in part, using a Bus-DeMeo Taxonomy Classification Web tool by Stephen M. Slivan, developed at MIT with the support of National Science Foundation Grant 0506716 and NASA Grant NAG5-12355. We thank R.

Gil-Hutton and an anonymous referee for their comments who help us improving the paper.