Maurizio ; Galetto
email: maurizio.galetto@polito.it

Luca Mastrogiacomo
email: luca.mastrogiacomo@polito.it

Barbara Pralio
email: barbara.pralio@polito.it

A Wireless Sensor Network-Based Approach to Large Scale Dimensional Metrology

Keywords: METROLOGY, SENSORS, CMM user): Dimensional Metrology, Large-Scale Metrology Dimensional Metrology, Large-Scale Metrology, Large-Volume Metrology, Coordinate Measuring Systems, Mobile Measuring System, Wireless Sensor Networks

In many branches of industry, dimensional measurements have become an important part of the production cycle, in order to check product compliance with specifications. This task is not trivial especially when dealing with large scale dimensional measurements: the bigger the measurement dimensions are the harder is to achieve high accuracies. Nowadays, the problem can be handled using many metrological systems, based on different technologies (optical, mechanical, electromagnetic, etc.). Each of these systems is more or less adequate, depending on measuring conditions, user's experience and skill, or other factors such as time, cost, accuracy, portability. This paper focuses on a new possible approach to large scale dimensional metrology based on Wireless Sensor Networks (WSNs). Advantages and drawbacks of such approach are analysed and deeply discussed. Then the paper briefly presents a recent prototype system -MScMS-II (Mobile Spatial coordinate Measuring System) -which has been developed at the Industrial Metrology and Quality Laboratory of DISPEA -Politecnico di Torino. The system results to be suitable for performing dimensional measurements of large-size objects (sizes on the order of several meters). Due to its distributed nature, the system -based on a wireless network of optical devices -is portable, fully scalable with respect to dimensions and shapes, and easily adaptable to different working environments. Preliminary results of experimental tests, aimed at evaluating system performance as well as research perspectives for further improvements, are discussed.

INTRODUCTION

Due to recent advances in integrated circuits and radio technologies, the use of distributed sensor networks is more and more widespread for a variety of applications, such as, environmental monitoring, indoor navigation, people and objects tracking, logistics, surveillance, industrial diagnostics, and other activities. The facts of being typically composed by compact, lightweight and cheap devices make the Wireless Sensor Networks (WSNs) appealing also for other possible uses. Among these, the field of dimensional metrology certainly offers a challenging and interesting scenario especially when the dimensions to be measured are of the order of several metres (largescale/large-volume dimensional metrology). Dimensional metrology is the branch of metrology that deals with measurements of geometrical features. Large-scale metrology (LSM), in particular, can be defined as "the metrology of objects in which the linear dimensions range from tens to hundreds of meters" [START_REF] Puttock | Large-Scale Metrology[END_REF]. LSM includes a wide range of applications, such as dimensional measurements of large structures, monitoring of deformations, alignment procedures in manufacture assembling, in many different industrial sectors, among which aerospace, railway and shipbuilding. The large number of existing metrological solutions can be classified according to different criteria, including measuring technologies, working principles, measurement procedures and system architectures. Although a wide variety of technologies has been implemented for dimensional metrology applications, at present, optical-based systems are largely used due to their advantages over the other approaches as to metrological performance and their potentialities for LSM applications. A taxonomy of main existing LSM solutions is reported in Figure 1, referring to measurement procedures (contact, non-contact instruments), working principles (multiple lengths, one length and two angles, and multiple angles), and system architectures (centralized and distributed). According to measurement procedures, a classification has been proposed in [START_REF] Maisano | Comparison of two distributed large volume measurement systems: MScMS and iGPS, Proceedings of the institution of mechanical engineers[END_REF], distinguishing between contact and non-contact instruments. Contact systems perform measurements by touching the workpiece, either through a mechanical arm or a portable probe. On the contrary, non-contact systems do not need to touch the object to be measured.

INSERT FIG. 1

A further categorization of the available instruments has been reported in [START_REF] Cuypers | Optical measurement techniques for mobile and large-scale dimensional metrology[END_REF], referring to their working principles. Three groups have been identified, depending on whether they perform measurements by using multiple lengths, one length and two angles or multiple angles. The former method, firstly used by the well-known Coordinate Measuring Machines (CMMs) to evaluate the position of its remotely controlled contact probe by knowing the probe geometry and the distances from the three reference axes along which it moves, is common to the systems implementing a multilateration approach as well. More specifically, these techniques use the known locations of three or more reference points, and the measured distance between the point to be localized and each reference point. The unknown coordinates can be found by solving a non linear optimization problem (Franceschini et al., 2009a). This approach is very similar to GPS (Global Positioning System) localisation principle [START_REF] Hofmann-Wellenhof | GPS: Theory and Practice[END_REF]. Multilateration principles are used by the measurement systems based on laser interferometers [START_REF] Cuypers | Optical measurement techniques for mobile and large-scale dimensional metrology[END_REF] as well as by those based on ToF (Time of Flight) or TDoF (Time Difference of Flight) (Franceschini et al., 2009a). On the other hand, many coordinate measuring systems rely on the determination of one length and two angles. These systems, also called Spherical Coordinate Measurement Systems (SCMS), locate a point with reference to a spherical coordinate system. Examples of such systems are laser trackers, laser radars and total stations [START_REF] Cuypers | Optical measurement techniques for mobile and large-scale dimensional metrology[END_REF]. Finally, instead of using two angles and a distance measurement, it is possible to evaluate the position of a point in a three dimensional space using angular information from two or more reference points. This approach relies on the well-known triangulation principle, which uses the known locations of two or more reference points and the relative measured angles between the point to be localized and each reference point [START_REF] Doğançay | Bearings-only target localization using total least squares[END_REF]. All camera-based systems [START_REF] Mikhail | Introduction to modern photogrammetry[END_REF] as well as the indoor-GPS (iGPS) (ARC Second, 2010) rely on this working principle. In the last decade many new large-scale dimensional metrology systems have been designed and proposed [START_REF] Peggs | Recent developments in Large-Scale Dimensional Metrology[END_REF]. In particular, some of them -composed by several distributed components -arouse a certain interest because of their unique features of flexibility and scalability [START_REF] Maisano | Comparison of two distributed large volume measurement systems: MScMS and iGPS, Proceedings of the institution of mechanical engineers[END_REF]. In general, according to their architecture, dimensional metrology systems can be classified into two groups (see Figure 1):

Centralized systems. A centralized system is essentially a standalone unit which works independently from other external devices in order to perform dimensional measurements.

Often the centralized unit is difficult to move and its working volume is inevitably limited by the technology used. Laser trackers (Leica Geosystems, 2009;Faro Technologies, 2009;Automated Precision, 2009), laser radars (Leica Geosystems, 2009), theodolites and optical CMMs (Axios 3D, 2009;Nikon Metrology, 2010;Metronor, 2009;Northern Digital, 2009) represent widely known examples of metrological instruments based on a centralized approach.

Distributed systems. These systems generally consist of a set of remotely distributed devices and a central processing unit, which is in charge of data acquisition and post-processing elaboration to provide measurement results. According to the level of interaction of network devices, a further distinction can be made between the following categories:

o semi-distributed systems: the distributed approach is limited to the spatial location of the devices, which are just remote sensing units, providing reference points in the three-dimensional space. They are unable to communicate with one another and to adaptively reconfigure their sensing task. The iGPS (Nikon Metrology, 2010) and the V-Star system (Geodetic Services, 2010) are representative examples of such a category;

o fully-distributed systems: similar to the semi-distributed ones, these systems are characterized by a distributed hardware architecture. In this case, however, the remote sensing devices are intelligent agents, i.e. autonomous entities which cooperate and coordinate their activities to achieve the common objective of performing the measurement. Due to their nature, these systems may be organized according to a flat structure (each node is linked to the central unit) or a hierarchical structure (clusters of nodes are in charge of more powerful cluster nodes) [START_REF] Cassandras | Sensor Networks and Cooperative Control[END_REF]. It has to be observed that, up to now, even if many noteworthy efforts have been directed toward the realization of distributed systems, a real fully-distributed system has still to come.

Distributed dimensional metrology systems

State of the art

The history of distributed large scale metrology systems is quite short and dates back to the last few years [START_REF] Peggs | Recent developments in Large-Scale Dimensional Metrology[END_REF]. Nevertheless, a few systems -relying on different technologies and working principles -have already been proposed [START_REF] Maisano | Comparison of two distributed large volume measurement systems: MScMS and iGPS, Proceedings of the institution of mechanical engineers[END_REF]. The iGPS (Nikon Metrology, 2010) uses a number of distributed transmitters equipped with two rotating laser beams and an infrared light to determine the relative angles from the transmitters to the photodiodes sensors embedded in a mobile hand-held probe (ARC Second, 2010) (see Figure 2). According to the known location of the transmitters, which is normally obtained in an initial setup phase, the position of the mobile probe can be calculated [START_REF] Ferri | Sources of variability in the set-up of an indoor GPS[END_REF]). An experimental characterization of i-GPS operational performance and a comparison to those of a laser tracker has been reported in [START_REF] Wang | Experimental Deployment of the Indoor GPS Large Volume Metrology System in a Large Scale Production Facility[END_REF], referring to a real working environment. Whereas the distributed system demonstrated high repeatability, measurement uncertainty resulted to be consistently affected by working volume size and environmental factors. A similar system is the HiBall (3rdTech, 2010). It consists of two key integrated components: a mobile probe equipped with lenses and photodiodes and a set of infrared LEDs, generally mounted on the ceiling of the working environment. The mobile probe is able to estimate the angular position of the LEDs with respect to its local reference system. The position of the probe is thus found by triangulation knowing the locations of the LEDs [START_REF] Welch | High-Performance Wide-Area Optical Tracking The HiBall Tracking System[END_REF]. Developments in imaging technology, which afforded the community large area CCD sensors, and improvements of target image location algorithms have lead to an ever-increasing competitiveness of vision-based metrology. As a matter of fact, different well-settled solutions based on photogrammetry are commercially available, providing for accurate, portable, and versatile instruments for three-dimensional coordinate measurement (Geodetic Services, 2010;Optitrack, 2009;Vicon, 2009). The fundamental principle used by photogrammetry is triangulation [START_REF] Mikhail | Introduction to modern photogrammetry[END_REF]. By taking photographs or video images from at least two different positions, it is possible to reconstruct the spatial location of a point and, therefore, the geometry or the main features of an object. Notwithstanding their multiple sensor-based structure, the existing photogrammetric instruments are applied to reduced working volumes and do not exploit the potentialities of a wireless sensor network layout. Generally speaking, although the systems so far described present a physically distributed layout, their distributed components do not possess any kind of "intelligence", computational capability or coordination potentiality.

Appeal and drawbacks

The appeal of distributed systems derives from many features that make them different from conventional centralized systems:

Flexibility. As they consist of multiple remote sensors, distributed systems can be easily arranged in the working volume according to the user needs, the environment geometry, and the measurement task. System flexibility can be further enhanced by implementing preprocessing software tools providing possible configuration layout, aimed at optimizing the metrological performance and/or the measurement volume [START_REF] Franceschini | The problem of distributed wireless sensors positioning in the Mobile Spatial coordinate Measuring System[END_REF][START_REF] Galetto | Optimal Sensor Positioning for Large Scale Metrology Applications[END_REF]. The sensor placement problem, intended as the problem of positioning and orienting a set of sensors in an attempt to cover a measurement region, has been extensively faced referring to different sensing technologies and fields of application (Younis and Akkaya, 2008). Optimal camera placement issues have been treated in (Fraser, 1996;Mason, 1997;Olague and Mohr, 2002) with reference to accurate 3D measurements through photogrammetric systems. The problem of determining sighting and positioning strategies for fringe projection sensors is tackled in [START_REF] Weckenmann | Model and Simulation of Fringe Projection Measurements as Part of an Assistance System for Multi-Component Fringe Projection Sensors[END_REF], to provide an assistance to the operator dealing with a multi-sensors system. The next best view problem is faced in [START_REF] Munkelt | View planning for 3D Reconstruction Using Time-of-Flight Camera Data[END_REF] as the problem of planning sensor positions for a 3D reconstruction task using time-of-flight camera data. On the other hand, the configuration design issue has been approached also in (Laguna et al. 2009) and(Ray andMahajan, 2002) for positioning ultrasonic devices for indoor localization and navigation of autonomous vehicles. Adaptive sensor placement strategies (Bulusu et al. 2001) could represent a viable way to provide capabilities to add or remove sensing units according to the user needs, making these systems extremely flexible as to their implementation for industrial applications.

Redundancy. In typical working conditions, distributed systems are often able to refer to more distributed components than strictly needed. Depending on the localization technique adopted, information redundancy enhances system accuracy and gives the system the possibility of implementing real time verification strategies (Franceschini et al., 2009b).

Reliability. Reliability is the ability of a system to perform and maintain its functions in routine circumstances, as well as hostile or unexpected circumstances. A survey of fault tolerance issues in sensor networks is given in (Koushanfar et al. 2004), including techniques for detection and diagnosis. Research work is focused both on sensor deployment strategies providing fault tolerance against node failures (Bredin et al. 2005;Hao et al., 2004;Han et al. 2007) and fault-tolerant algorithms for detecting and localizing targets in networks with faulty nodes (Ding et al., 2007). Similarly, if one or more remote devices are not working properly, distributed metrology systems, as generally characterized by hardware redundancy, can actually use the "healthy" nodes to compensate the malfunctioning of a part of the network.

Scalability. The major strong point of a distributed system is the capability to easily adapt to large dimensions and unusual shapes. The real working volume of a distributed metrology system is related to the network layout. Changing density and/or position of the remote sensing units, the user can size and shape the working volume, within the network design phase as well as during the experimental campaign. Concurrent measurement capability. Distributed metrology systems generally allow the use of different measurement tools (multiple targets and/or mobile probes) at the same time. Once the system infrastructure has been set up, an unlimited number of tools can actually operate within the workspace, without any additional cost per user. Sensor fusion. A wide variety of sensor data fusion applications have been proposed in literature, considering heterogeneous sensors, spatially distributed sensors, and asynchronously sampled sensors. The use of heterogeneous sensors, i.e. sensors measuring different physical entities, entails accurate models for correlating sensor observations. Moreover, the possibly distributed sensor architecture requires to model how the observations depend on sensor positioning. On the other hand, a modelling of the time evolution of the measured parameters is needed whenever asynchronous sampling of sensors is performed. Therefore, a modular architecture could give the opportunity to integrate the metrological system with other spatially distributed sensors (in order to monitor temperature, humidity, vibrations, light intensity, etc.) [START_REF] Akyildiz | A survey on Wireless Multimedia Sensor Networks[END_REF]. The sensor data fusion, intended as the processing and elaboration of data from heterogeneous sensors, provides capabilities to perform an environmental mapping of the working volume and to monitor the operating conditions of the dimensional measuring devices. Moreover, multi-resolution systems can be integrated to provide different granularity in data acquisition. Line of sight. The distributed nature of the configuration layout and the sensor redundancy better face with "visibility" problems, such as shadowing, line-of-sight obstructions and signal reflections. On the other hand, contrary to centralized systems that are made by a single metrological unit, the distributed nature of the described systems requires the coordination and management of multiple stations. The major disadvantages of these systems are:

Set-up. In order to work properly, every distributed system needs to know several parameters of the local hardware. Some of these parameters may change either because of environmental factors (e.g. vibrations, temperature or humidity changes), or due to unpredictable reasons (such as accidental movements of network nodes). Errors during the set-up phase adversely affect the accuracy of the measurements [START_REF] Mastrogiacomo | Network localization procedures for experimental evaluation of mobile spatial coordinate measuring system (MScMS)[END_REF]. In order to achieve the optimum accuracy, each distributed system generally needs a careful set-up phase.

During this phase, which can be automated to some extent, the system calculates information like positions and orientations of components, local temperatures, humidity, pressure and so on. This information is useful during the measurement.

Expertise. Distributed metrology systems are typically less user friendly than centralized systems. They generally need a more experienced and careful user, especially during the set-up process. Since they consist of multiple stations, particular attention has to be paid to coordinate the data acquisition from different sensing devices (e.g. sensor device synchronization).

Standards. While these new systems are attractive to potential end-users, standards, best practice methods and independent performance verification techniques are usually not available [START_REF] Peggs | Recent developments in Large-Scale Dimensional Metrology[END_REF].

Accuracy. The performances of distributed metrology systems are strongly related to several factors that can affect the accuracy of the system adversely, such as the number of constellation devices, the set-up parameters and the constellation geometry with respect to the spatial distribution of measurement points.

WSN-based approach

WSNs are typically composed by small and lightweight devices that can be easily deployed and arranged in a working environment. Furthermore, each device is generally provided with both communication and computation capabilities given by the embedded electronic components [START_REF] Akyildiz | Wireless Sensor Networks: A Survey[END_REF]. These features certainly increase the appeal of WSNs and make them suitable for the design of a fully-distributed system. Recently the attention has been focused on the integration of video devices with scalar sensors, to setup networks of wirelessly interconnected devices able to fuse multimedia data from heterogeneous sources. Comprehensive surveys on Wireless Multimedia Sensor Networks and their applications and testbeds are available in [START_REF] Akyildiz | A survey on Wireless Multimedia Sensor Networks[END_REF][START_REF] Akyildiz | Wireless Multimedia Sensor Networks: Applications and Testbeds[END_REF].

Besides existing application fields, such as tracking, home automation, environmental monitoring, industrial process control, the distributed network-based layout has been more recently implemented also in dimensional metrology applications (e.g. Nikon Metrology, 2010;3rdTech, 2010). Accordingly, measurement systems based on spatially distributed sensing units demonstrate profitable scalability features. As a matter of fact, their modular architecture makes them suitable for large-scale metrology, overcoming limitations of existing digital photogrammetry-based systems 3D, 2009;Metronor, 2009;Northern Digital, 2009). Real-time image acquisition of different targets, possibly located in different regions of the working volume, is then possible by spreading around the sensor devices, provided that the acquisition task is synchronized and a common reference system is given. These capabilities make the proposed system a feasible solution for tracking mobile objects, even if characterized by fast dynamics. This property is particularly interesting in an attempt to automate the contact measurement procedure. Most of commercially available instruments provide an accessory hand-held probe for touching the reference measurement points (Automated Precision, 2009;Leica Geosystems, 2009;Axios 3D, 2009;Nikon Metrology, 2010), thus involving a direct interaction between the sensor equipment and the human operator besides a strong dependence on his/her skills. An alternative approach, proposed in (Franceschini et al., 2009d), relies on autonomous unmanned platforms for carrying the sensor equipment and moving the contact probe around the working volume. According to this new perspective, the human role should be scaled down to simply manage the task-related issues, such as type of measurement (e.g. dimensional measurement, geometry reconstruction, single point verification) and data acquisition procedures (e.g. point sequences, repeated sampling), and remotely monitor the unmanned platform that should autonomously perform the measurement. This approach clearly shows the need for a flexible system architecture able to provide measurement data for control as well as metrological issues.

As described in Section 2, currently available dimensional metrology systems rely either on distance or angle measurements. Thus the possible use of a WSN-based system for dimensional metrology applications is certainly bounded by its capabilities of performing such kind of measurements. Nowadays there are many approaches to this field, relying on different technologies and sensors. Angular measurements can be achieved -for example -using accelerometers, magnetometers, gyroscopes, CCD sensors, photodiodes or simply measuring the difference in the received phase of a radio signal at each element of an antenna array [START_REF] Kwakkernaat | High-Resolution Angle-of-Arrival Measurements on Physically-Nonstationary Mobile Radio Channels[END_REF]. On the other hand, distance measurements can be obtained -for instance -evaluating the time of flight of a particular signal (such as an ultrasound signal), the time difference of arrival of different signals or the received strength of a radio communication signal (Franceschini et al., 2009c). Whatever the system components and the localization algorithms are, a WSN-based metrology system represents a further step towards hardware and software automation in dimensional measurement applications. Due to its capabilities of sharing the metrology task, each network device could work cooperatively with the aim of determining the geometrical features of an object. In this way, the measurement results to be the synthesis of the information locally gathered and shared by each network node. Communication links among the network nodes also provide capabilities to possibly reconfigure their orientation during the task according to measurement conditions and procedures, aiming at optimizing the overall system performance.

System prototype implementation

The Mobile Spatial coordinate Measuring System (MScMS-II) -developed at the Industrial Metrology and Quality Engineering Laboratory of DISPEA, Politecnico di Torino -is an indoor coordinate measuring system based on InfraRed (IR) optical technology and designed for LSM applications. As a first prototype, it implements both distributed and centralized logics. Its architecture consists of three basic units: a sensor network of optical devices, suitably distributed within the measurement volume; a mobile wireless and armless probe, equipped with two reflective markers, to "touch" the measurement points; a central unit, connected via an antenna linked to the WSN, to acquire and elaborate the data sent by each network node.

The network of spatially distributed optical sensors is aimed at providing reference points for locating the portable probe, by establishing visual links with the markers that are visible in the camera "viewing volumes". The probing point, i.e. the point of the probe tip contacting the workpiece, is then calculated according to the reconstructed positions of markers' centres and the apriori known probe geometry. An earlier prototype of MScMS exploited Ultrasound (US) transceivers in order to communicate and evaluate mutual distances between the distributed sensor nodes and the hand-held probe (Franceschini et al., 2009a). The poor characteristics of US devices (non punctiform dimensions, speed of sound dependence on operating temperature, wave reflection and diffraction, etc.) caused a low accuracy in the measurement results (Franceschini et al., 2009a;Franceschini et al., 2009b). To enhance system performance, current version implements an IRbased optical outside-in system, estimating the position of passive retro-reflective markers from their projections in different camera views [START_REF] Galetto | An Innovative Indoor Coordinate Measuring System for Large-Scale Metrology Based on a Distributed IR Sensor Network[END_REF]).

The system is characterized by a fast acquisition and measurement procedure, being the acquisition task related to camera frame rates, and the measurement task dependent on the number of points and the processor used. System portability strongly depends on modular architecture, which shares sensing and computational capabilities among several remote units of reduced size and weight. The system can then be easily transportable and objects can be measured in place. Setup takes just a few minutes, as no warm-up times are required for the sensing units. The camera self-calibration, based on a collection of images of a single reflective marker, randomly moved in several unknown positions within the working volume, requires a few minutes, including data gathering and elaboration tasks.

Wireless Sensor Network

Currently, the distributed network of the MScMS-II prototype has been set up by using low-cost commercial IR cameras, characterized by an interpolated resolution of 1024x768 pixels (native resolution is 128x96 pixels), a maximum sample rate of 100 Hz, and a Field-of-View (FoV) of approximately 45°×30°. Graphical representations of a single camera viewing volume and the coverage volume of a set of sensors are reported in Figure 3 and Figure 7, respectively. Each camera implements a real-time multi-object tracking engine, allowing to track up to four IR light sources. In order to work with passive markers, each camera is coupled with a near-IR light source (Figure 4), consisting of a 160-chip LED array with a peak wavelength of 940 nm and a viewing half-angle of approximately 80°. The overall sensor set (camera and LED array) weights less than 500 g and is about 13×13×15 cm sized. Since marker dimensions, camera resolution, IR light source power and working volume are strictly related parameters, the IR sensor sensitivity has been experimentally evaluated by testing the visibility distance of differently sized retro-reflective spheres (see Figure 3). Referring to the used IR technology, the system demonstrated to be able to track a 16 mm diameter marker in a range between d min = 50 mm and d max = 3500 mm. On the other hand, by using a 40 mm diameter marker the traceability ranges from 300 mm to 6000 mm. Whereas the upper bound (d max ) of this range represents a limitation in terms of marker visibility in the camera image plane, the lower bound (d min ) represents the distance under which the tracking engine is unable to correctly find the centre of the point projection in its view plane.

Besides the camera, each network device is provided with an accelerometer used for diagnostic purposes (i.e. to detect possible movements or vibrations changing the calibrated positions).

INSERT FIG. 3 INSERT FIG. 4 It has to be noted that part of the data elaboration task is locally performed by the remote sensing units. In detail, wireless network devices are in charge of: Image processing. In order to save computational capabilities and to reduce the radio communication loads, all images acquired by the wireless devices are processed onboard. Each device implements a real-time tracking engine, allowing to track up the brightest IR sources in the camera field of view and thus providing to the central unit their 2D position coordinates. Image filtering. To prevent noisy measurements and undesired reflections, the network devices only track IR sources with a brightness larger than a certain threshold empirically established.

No filtering on the shape of the light sources is performed. As a matter of fact the embedded real-time tracking and filtering capabilities of the distributed remote sensing devices save the computational effort for performing the image analysis and spot coordinates identification by the central unit.

Measuring probe

The mobile hand-held probe (Figure 5) consists of a rod, equipped with two reflective markers at the extremes and a calibrated tip to physically "touch" the measurement points. Reflective markers have been made by wrapping a retro-reflective silver transfer film around polystyrene spheres. Referring to Figure 5, spatial coordinates of point V can be easily determined, knowing the positions of the two marker centres and the geometry of the probe, through a linear equation (Franceschini et al., 2009a;[START_REF] Galetto | An Innovative Indoor Coordinate Measuring System for Large-Scale Metrology Based on a Distributed IR Sensor Network[END_REF]. A correction algorithm, taking into account the probe trajectory when approaching the measurement point, has been implemented. To this end, the central processing unit stores the time history of the reconstructed spatial coordinates of the probe markers. The probe trajectory in a predefined time interval before the measurement is thus reconstructed and used to correct the coordinates of the probe tip.

INSERT FIG. 5

Central unit

The central unit consists of a PC equipped with an Intel Quad Core Q9300 (4 x 2,5GHz) CPU and 4GB DDR 2 RAM. The PC is connected to the WSN devices via a radio link. The central unit is currently used both for data processing and visualization. The centralized unit is in charge for the following tasks: Synchronization. According to the radio communication link, cameras are sequentially sampled. This introduces a delay between the acquisition time of images from different cameras. On the other hand, the onboard image processing lightens the communication load, thus reducing the acquisition delay to a minimum. Camera calibration. A camera calibration procedure has to be carried out in order to provide to the triangulation algorithm the spatial coordinates of the reference points, i.e. the IR sensor devices. The multi-camera calibration problem is faced by using a fully automatic technique of self-calibration [START_REF] Svoboda | A Convenient Multi-camera Self-calibration for Virtual Environments[END_REF]. This method is able to reconstruct camera internal parameters besides its positions and orientations. It requires a minimum of three cameras and a calibrated artefact in order to align and scale the reference system. Localization. According to the 2D coordinates of the IR spot(s) provided by each camera, the central unit reconstructs the 3D position of any marker by applying triangulation algorithms [START_REF] Hartley | Multiple View Geometry in Computer Vision[END_REF]. Preprocessing. The central unit is able to run pre-processing software tools in order to provide an optimal WSN configuration, taking into account the metrology task, the geometry and shape of the measurand, and the working environment layout. Diagnostics. The central unit is responsible for running diagnostic algorithms, reporting possible malfunctions of the sensor devices.

These functions have been implemented into an ad-hoc developed software, providing a comprehensive and user-friendly graphical interface for managing the measurement tasks (Figure 6). Network design and calibration, marker localization as well as dimensional measurement tasks are managed through different applets. As a matter of fact, spatial coordinates of single points, dimensional data as well as geometric features of the measured object are provided for both on-line and off-line analysis.

INSERT FIG. 6

Experimental tests for preliminary metrological characterization

Preliminary experimental tests have been carried out to evaluate prototype performance (in terms of measurement accuracy, repeatability, and reproducibility) as well as system potentialities.

The data herein discussed refers to a network layout consisting of six wireless IR cameras, arranged in a 5.0×6.0×3.0 m working environment according to a grid-based configuration. Cameras were oriented toward the centre of the lab, in order to ensure better volume coverage and measurements redundancy. Figure 7 shows a virtual reconstruction of the experimental set-up.

INSERT FIG. 7

The black wireframe represents the camera "field-of-sensing", whereas the light grey wireframe represents the working volume (interpreted as the volume of intersection of at least two "field-ofsensing"). It has to be noted that the actual working volume was about 2.0×2.0×2.0 m wide. A first evaluation of the measurement accuracy of point coordinates, intended as the "closeness of agreement between a measured quantity value and a true quantity value of a measurand" (VIM Standards, 2008), has been carried out using a 3D aluminium alloy calibrated artefact (see Figure 8). In order to have a set of reference points with known nominal positions, 22 points on the artefact have been calibrated (at nominal temperature T=21°C and relative humidity RH=27%) using a coordinate-measuring machine (DEA IOTA 0101).

INSERT FIG. 8

The reference points have been measured by the MScMS-II system prototype, by moving the artefact in five different positions uniformly distributed within the working volume. Figure 9 shows the histogram of the distances between measured and nominal positions.

INSERT FIG. 9

It is noteworthy how the 50% of the measured points is within a distance of 1.87 mm from the nominal position, while the 94.2% of results is less than 5 mm far from the nominal position [START_REF] Galetto | An Innovative Indoor Coordinate Measuring System for Large-Scale Metrology Based on a Distributed IR Sensor Network[END_REF]. At worst the maximum measured distance is below 6.5 mm. By considering several issues (e.g. geometric distortion of the reconstructed working volume and measurement process) whose effects on measurements strongly depend on the location within the working volume, the severe experimental testing procedure consistently affect the extent of measurement errors as well as their high variability. In a second test, measurement repeatability of point coordinates, intended as "closeness of the agreement between the results of successive measurements of the same measurand carried out under the same conditions of measurement" (VIM Standards, 2008), has been tested on five different points, uniformly distributed within the working volume, by repositioning the probe in the same positions for k = 30 times. It is noteworthy that repeatability characteristics are related to the sensor device performance as well as to the operator skills. Human skills actually represent an external factor related to capabilities in holding the probe in a fixed position. The sample standard deviation of repeatability tests was found to be smaller than 1.25 mm [START_REF] Galetto | An Innovative Indoor Coordinate Measuring System for Large-Scale Metrology Based on a Distributed IR Sensor Network[END_REF]. Finally, measurement reproducibility of point coordinates, intended as "closeness of the agreement between the results of successive measurements of the same measurand carried out under changed conditions of measurement" (VIM Standards, 2008), has been tested with reference to five points, repeating the measurement k = 30 times with different mobile probe orientations. Reproducibility tests stress the importance on measurement quality of network and probe relative position and orientation. A sample standard deviation smaller than 3.45 mm has been obtained [START_REF] Galetto | An Innovative Indoor Coordinate Measuring System for Large-Scale Metrology Based on a Distributed IR Sensor Network[END_REF]. According to the results emerging from these tests, MScMS-II prototype do not appear to be very competitive if compared to commercial systems such as CMMs, laser trackers, iGPS etc. With those technologies, in the same working volume, accuracy deviation may range from few micrometers up to one millimetre at worst, depending on the system and the working conditions (Franceschini et al., 2009;[START_REF] Maisano | Comparison of two distributed large volume measurement systems: MScMS and iGPS, Proceedings of the institution of mechanical engineers[END_REF]. However, these results become particularly interesting if cost and potentiality of MScMS-II are considered. While ensuring scalability and flexibility that existing commercial systems cannot guarantee, the prototype still has significant room for enhancement mainly related to the improvement of the employed technology. Since the state-of-the-art of IR cameras actually provides a wide choice of resolution (from less than 1 megapixel up to 16 megapixels), current CCD sensors (128x96 pixels of native resolution) could be easily replaced with higher performance ones. Commercially available solutions generally enable intelligent features such as on-board 2D image analysis and processing, making the computational workload almost independent of the IR sensor resolution. Nonetheless a trade-off between the target system performance and the economic impact of the entire system has to be found. Furthermore, due to its ease-of-use and fast data acquisition characteristics, the MScMS-II can be applied also for geometry reconstruction and reverse engineering tasks by untrained operators. As an example, the chassis of a toy car model has been geometrically reconstructed through a trianglebased linear interpolation relying on 690 measured points gathered by an unskilled operator in about one hour (see Figure 10).

INSERT FIG. 10 4 Conclusions

This paper discusses the concept of distributed systems for large scale dimensional metrology tasks. These systems -due to their nature -are more flexible and suitable than centralized systems. Furthermore, if they are composed by "intelligent" sensing units, they can implement network logics (such as auto-diagnostics, compensation, correction, substitution etc.) that can improve the overall performance of the system itself. The proposed approach, based on WSNs, demonstrates to have the potentialities of being fully-distributed and combining flexibility and network logics. A prototype implementation, based on a WSN of infrared cameras, has been used to test system feasibility and demonstrated satisfactory metrological performance and appealing flexibility, and scalability properties. The prototype represents a step toward fully-distributed systems, since it implements a fully distributed approach for measurement data acquisition and both centralized and distributed logics for data elaboration and sensor management. Future research efforts will go in the direction of a self-coordination of the remote sensor devices as to diagnostics and compensation. Although accuracy resulted to be relatively good in view of the technology used, further studies are intended to increase the resolution of the optical devices, in order to enhance the accuracy and working volume coverage.

Figure 1 :Figure 2 :Figure 3 :

 123 Figure 1: Taxonomy of available solutions for metrology applications.

Figure 4 :Figure 5 :

 45 Figure 4: Main components of the IR-based sensor network: an IR camera is coupled with an IR LED array to locate passive retro-reflective targets.

Figure 6 :Figure 7 :

 67 Figure 6: A screenshot of the graphical user interface for managing the dimensional measurement task (example of a plane reconstruction task).

Figure 8 :

 8 Figure 8: The reference artefact used for the accuracy test.

Figure 9 :

 9 Figure 9: Accuracy in distance measurement. Histogram of the distances between measured and nominal positions.

Figure 10 :

 10 Figure 10: (a) Toy car model. (b) Measured points and reconstructed shape.


Jesús Duarte

Miguel Ángel Farfán

J Mario Vargas

New data on mortality, home range, and dispersal of red-legged partridges (Alectoris rufa) released in a mountain range

Keywords: Partridge, Restocking action, Success, Survival, Territory size

published or not. The documents may come

New data on mortality, home range, and dispersal of red-legged partridges () released in a mountain range

Introduction

Release and restocking programs using farm-reared animals are among the most commonly used management tools employed with game species in Europe [START_REF] Gortázar | Las repoblaciones con perdiz roja[END_REF][START_REF] Gortázar | Disease risks and overabundance of game species[END_REF]. In Spain, the red-legged partridge (Alectoris rufa) is one of the most popular small-game species and is of important socioeconomic value in several rural areas [START_REF] Vargas | Alerta cinegética. Reflexiones sobre el futuro de la caza en España[END_REF]. Most of the European red-legged partridge population is found in the Iberian Peninsula [START_REF] Aebischer | The red-legged partridge[END_REF], where the species occurs in a large variety of Mediterranean habitats up to a height of 1,500 m; however, the species has suffered a notable decline during the last two decades and thus there has been a significant reduction in hunting yields [START_REF] Blanco-Aguiar | ) Atlas de las aves reproductoras de España[END_REF]. Restocking with farm-reared birds has been widely practiced to alleviate this decline [START_REF] Vargas | Dos modelos discrepantes de gestión de la perdiz roja en España[END_REF]. It is worth noting that in Spain between 3.5 and 5 million farm-reared partridges are released every year [START_REF] Lucio | Caza y conservación de aves en España[END_REF][START_REF] Vargas | Alerta cinegética. Reflexiones sobre el futuro de la caza en España[END_REF].

There is a rich literature on the release and success of this species as well as well-documented scientific data; however, there are wide variations in the type of release, with or without acclimatization; variations in the age of released birds; releases conducted in different seasons; releasing birds at different points; or implementing habitat improvements around the release points [see [START_REF] Gortázar | Success of traditional restocking of red-legged partridge for hunting purposes in areas of low density of Northeast Spain Aragón[END_REF]; [START_REF] Alonso | Study of survival, dispersal and home range of autumn-released red-legged partridges (Alectoris rufa)[END_REF] or [START_REF] Pérez | Use of radiotracking techniques to study a summer repopulation with red-legged patridge (Alectoris rufa) chicks[END_REF] in Spain; and [START_REF] Capelo | Sobrevivência e disperso de perdizes (Alectoris rufa) largadas em duas operaçoes de repovamento cinegético[END_REF] and Castro-Pereira et al. (1998) in Portugal]. One of the crucial variations is the habitat type into which the birds are released and its effect. Most of the available studies report releases on plains or mixed farmland areas.

In the case of Andalusia, partridge release areas have a temperate Mediterranean climate with a diversity of landscapes that range from the agrosystems of the Guadalquivir valley to low coastal mountain ranges with Mediterranean scrubland, or to high inland mountains and forests [START_REF] Vargas | Land use and environmental factors affecting red-legged partridge (Alectoris rufa) hunting yields in southern Spain[END_REF]. In this study, we present data on the survival, dispersal, and home range of radio-tagged farmreared red-legged partridge released in a low mountain range where the main features are managed forests and scrubland.

Material and methods

The fieldwork was conducted in the private La Algaida hunting estate (surface area, 6.4 km 2 ), located in the southern extreme of Málaga province (Andalusia, southern Spain; 5°0.3′W 36°31′N). This area is on a coastal mountain (10 km from the sea) where the main rock and soil type is peridotitic. Mixed limestones and gneisses are also present in marginal areas of the estate. Elevation ranges from 165 m to 760 m. It has a temperate subtropical Mediterranean climate, with average temperatures of 11.7°C in January and 25°C in July. Annual average rainfall ranges from 0.0 mm to 1,068 mm [START_REF] Capel-Molina | Los climas de España[END_REF].

The area consists of Mediterranean scrubland, with a scattering of pines (Pinus pinaster), carob trees (Ceratonia siliqua), wild olive trees (Olea europaea), gum rockroses (Cistus ladanifer), European fan palm (Chamaerops humilis), mastic tree (Pistacia lentiscus), and short grassland (79% of the surface area). Cork oaks (Quercus suber), oaks (Quercus faginea), and Strawberry tree (Arbutus unedo) are only found on low elevations with gentle slopes (18% of the surface area). A well-established forest, mainly consisting of Salix spp, grows along and close to the river (3% of the surface area). Less than 1% of the estate is used for livestock (sheep and cows) or farming. Forestry and hunting are the main uses. The game species harvested consists of wild rabbits, red-legged partridges, wood pigeons, thrushes, and wild boar. In this area, the potential predators of partridges are Bonelli's eagle (Aquila fasciata), golden eagle (Aquila chrysaetos), goshawk (Accipiter gentilis), red fox (Vulpes vulpes), common genet (Genetta genetta), and rock marten (Martes foina).

At the end of January 2000, we radio-tagged a total of 20 partridges (ten females and ten males, farm-reared, between 9 months and 10 months old) with 11-g TW 3 necklaces (Biotrack, UK) equipped with activity sensors and placed five males and five females in each of two release areas separated by 1 km. Each area had identical release points equipped with an acclimatization pen composed of a circular area (10-m radius by 2 m high) enclosed by 2-cm wire mesh. The pens were provided with water and feeding points as well as natural vegetation (from the scrubland) and herbaceous plants. The partridges were kept in the release areas for 4 weeks for acclimatization (i.e., in February). The aim was to release the birds at the start of the breeding season to improve the wild partridge breeding population. In early March, all the partridges were released by leaving the pen door open. No partridges died during the period they were in the pens. We located the partridges on a daily basis during the first 2 weeks and then between two and three times weekly to estimate their survival rate [START_REF] Kenward | Techniques for radio-tagging pheasant chicks[END_REF]. To locate the radio-tagged birds, we used a GPS eTrex Vista Cx (Garmin, USA), a portable Yagi antenna, and a Yaesu FT-290 receiver (Wagener Telemetrie, Germany). We used LOCATE II software (Nams 2000) for the triangulation procedure in fixed locations. To avoid bias caused by bird inactivity, locations were performed at dawn and dusk, which are the main times partridges are active. We used the homing-in technique [START_REF] White | Analysis of wildlife radio-tracking data[END_REF] as the standard procedure in all locations. We directly estimated the survival rate by recovering transmitters from partridge carcasses and estimating the Kaplan-Meier product limit. The consumption of partridges by mammals and raptors was distinguished by the presence of feces, feathers, and footprints around the carcasses or by inspecting tooth marks or other relevant signs on the partridge feathers, carcasses, and transmitters. We used RANGES 6 software for Windows [START_REF] Kenward | Ranges VI: for the analysis of tracking and location data[END_REF] to calculate bird dispersion after release and the home range of each bird, applying the minimum convex polygon criterion (i.e., 95% of locations excluding the release point) to estimate the home ranges and the 60% Kernel contours to identify activity centers and core areas [see [START_REF] Buenestado | Habitat selection and home range size of red-legged partridges in Spain[END_REF] for a similar procedure]. Means are given with their standard errors. All the experiments complied with current laws on wildlife care, protection, and conservation in Spain and the community of Andalucía.

Results

Between March and September, a total of 318 radio locations were conducted over 68 days comprising 306 field-effort man hours. The overall survival rate was 20.6%; mean dispersal distance, 832.5±139.5 m (ranging from 362 to 2,688 m); home range (MPC 95%), 16.6±2.7 ha (range, 5.6-77.1); and activity centers (Kernel 60%), 15.1±4.9 ha (range, 3.3-27.8). Mortality was 25% during the first 10 days, 31.2% at the end of the first month, 51.9% at the end of the second month, and 65.6% at the end of the third month. By the 14th week, mortality stopped at 79.4% (see Fig. 1). Terrestrial carnivores (mainly red foxes) were responsible for 40% of deaths and raptors for 60%. We found one buried partridge that had been killed by a fox. Two released female partridges paired with wild males and one nested successfully.

Discussion

This study focuses on releases in a low mountain area, a habitat that has been little studied from the point of view of partridge releases; however, the release success was similar to that reported in previous release experiments in other habitats [START_REF] Alonso | Study of survival, dispersal and home range of autumn-released red-legged partridges (Alectoris rufa)[END_REF][START_REF] Castro-Pereira | Monitorizaçao de repovoamentos de perdiz-vermelha: Sobrevivência e efeito na populaçao reprodutora Silva[END_REF][START_REF] Pérez | Use of radiotracking techniques to study a summer repopulation with red-legged patridge (Alectoris rufa) chicks[END_REF]). These studies have consistently reported low survival and high mortality rates during the first 2 weeks (ranging from 34-90%) with only a few partridges surviving beyond the first month; however, [START_REF] Capelo | Sobrevivência e disperso de perdizes (Alectoris rufa) largadas em duas operaçoes de repovamento cinegético[END_REF] suggested that partridges were likely to survive for more than 2 months following release, and Gortázar et al. ( 2000) described better survival rates (25% at 120 days). Our results agree with those reported by the latter authors; we observed a survival rate of 48.1% at the beginning of the second month and 20.6% after the fourth month.

All the authors cited above also report average dispersal distances ranging from 450 to 890 m and maximum dispersal distances ranging from 1,600 to 2,590 m. Our results on dispersal also agree with previously reported data.

Previous studies have reported home ranges of 6 to 100 ha for wild partridges [START_REF] Buenestado | Habitat selection and home range size of red-legged partridges in Spain[END_REF]) and up to 29 ha for released partridges, but with very low survival rates [START_REF] Alonso | Study of survival, dispersal and home range of autumn-released red-legged partridges (Alectoris rufa)[END_REF][START_REF] Pérez | Use of radiotracking techniques to study a summer repopulation with red-legged patridge (Alectoris rufa) chicks[END_REF]). Our results were also within this interval. The home ranges found in this study were even smaller than those reported for wild partridges in autumn in other areas, but are similar to those expected in the period from the end of winter to the start of spring [START_REF] Berger | Domaine vital, occupation du milieu et comportament social chez la perdrix rouge (Alectoris rufa L.) en automne au nord de son aire de repartition en France[END_REF][START_REF] Ricci | Influence de l'organisation sociale et de la densité sur les relations spatiales chez la Perdrix rouge. Conséquences démographiques et adaptatives[END_REF].

The percentages of partridges consumed by raptors or terrestrial carnivores were similar to those found in previous experiments [START_REF] Gortázar | Success of traditional restocking of red-legged partridge for hunting purposes in areas of low density of Northeast Spain Aragón[END_REF]. Most release studies define a postrelease period during which the species adapts to the habitat. This period is used to identify when mortality is no longer due to release-related problems and when it is only due to natural mortality or real predation. During this period, predators mainly consume a surplus of partridges instead of predating on them.

All these results (low survival rates, low dispersal distances, limited home range, and consumption by predators instead of real predation) indicate that the released species faces great difficulties in its attempts to adapt to the territory. This conclusion seems apply to a wide variety of habitats, areas, and different types of release operations. Thus, it is of great importance to discourage this type of wildlife management operation.

Partridge release is widely used in Spain and has at least three serious consequences: (1) the economic value of the partridges undergoes a marked decrease during release. Considering the mean mortality rate reported by many studies, the cost of each surviving partridge that is of use for hunting purposes undergoes a fourfold to fivefold increase. Thus, the real economic cost of the release operation and its profitability has to be taken into account;

(2) a general outcome of these operations is that the opportunistic predator community takes advantage of the situation, consuming a surplus of partridges and likely posing an additional risk to the long-term survival of natural partridges in the release area; (3) given the fact that some farm partridges can mate and also breed with wild ones, the normal reproductive activity of the natural population could be impacted. Therefore, releases are not only barely profitable but may cause greater damage among the natural population than in the original situation that the operation attempts to rectify.

Finally, although hunters assume that hand-reared redlegged partridges breed when released in the wild, this has only been confirmed by a few studies [START_REF] Duarte | Field interbreeding of released farmreared red-legged partridges (Alectoris rufa) with wild ones[END_REF][START_REF] Casas | Gestión agraria y cinegética: efectos sobre la perdiz roja (Alectoris rufa) y aves esteparias protegidas[END_REF]. Direct observation has normally been prevented by the high mortality during the first 2 weeks, the low survival rate over time, and the fact that some studies have been conducted outside the breeding season; however, [START_REF] Borralho | Demographics trends of redlegged partridge (Alectoris rufa) in southern Portugal after implementation of management actions[END_REF] and [START_REF] Carvalho | Red-legged partridge restocking programs: their success and implications on the breeding population[END_REF] found significant increases in partridge breeding populations after restocking. It is worth considering that if the number of released birds is increased then there may be more breeding attempts by the released partridges. Although the percentages of released breeding females are always low, large sample sizes may lead to hybrid genotypes in the wild, thereby compromising natural reproduction activity. In conclusion, although the habitat and the predator community described in this study may differ from those reported in previous release studies, the overall success of this experiment was the same. Low partridge survival rates are always reported. Partridge releases can introduce new parasites into the field [START_REF] Villanúa | Sanitary risks of red-legged partridge releases: introduction of parasites[END_REF] and are also of great value to the predator community, since they rapidly take advantage of the situation (also see [START_REF] Millán | Effects of a fiber-rich diet on physiology and survival of farm-reared red-legged partridges (Alectoris rufa)[END_REF]. In terms of population restocking and improvement, the advantages to hunters may be few; however, no advantage is gained regarding partridge conservation. Furthermore, the existence of gene admixture and widespread hybridization between farm-bred partridge and wild population lineages, as reported by [START_REF] Blanco-Aguiar | Variación espacial en la biología de la perdiz roja (Alectoris rufa): una aproximación multidisciplinar[END_REF], suggests that a combination of time and millions of farm partridge releases may have affected the Iberian wild partridge genotype.

Fig. 1

 1 Fig. 1 Kaplan-Meier survival function for red-legged partridge in the study area

Eur J Wildl Res (2011) 57:675-678