domain: spi phys

text:
Carmelo Arcidiacono
email: carmelo.arcidiacono@inaf.it

Simone Di Filippo
email: simone.difilippo@inaf.it

Davide Greggio

Kalyan Kumar

Radhakrishnan Santakumari

Elisa Portaluri

Maria Bergomi

Valentina Viotto

Demetrio Magrin

Roberto Ragazzoni

Luca Marafatto

Radhakrishnan Santhakumari

Marco Dima

Jacopo Farinato

Pierre Janin-Potiron

Thierry Fusco

Benoit Neichel

Olivier Fauvarque

Lauren Schatz

Ingot wavefront sensor: from the Fourier End2End numerical simulation to the LOOPS test bench

Keywords: Adaptive Optics, Wavefront Sensing, Ingot WFS, Numerical Simulations, LGS, Sodium Laser Guide Star, ELT

INTRODUCTION

The Sodium Laser Guide Star 1 (LGS) device is one of the key components of any advanced telescope providing adaptive optics [START_REF] Rabien | ARGOS: the laser guide star system for the LBT[END_REF][START_REF] Van Dam | Observatory Laser Guide Star Adaptive Optics System: Performance Characterization[END_REF][START_REF] D'orgeville | Gemini South multi-conjugate adaptive optics (GeMS) laser guide star facility on-sky performance results[END_REF][START_REF] Bonaccini Calia | Laser Development for Sodium Laser Guide Stars at ESO[END_REF] instruments. LGSs give to the telescope the capability to enlarge the sky coverage creating an artificial reference star, to compensate for the absence of a sufficiently bright star for wavefront sensing. On the other hand, LGSs are not point-like sources, but elongated objects in the three-dimensional space. This is due to the intrinsic nature of the Sodium layer, which is located at about 90 km in the atmosphere and extended for approximately 20-25km. [START_REF] Butler | Measuring the absolute height and profile of the mesospheric sodium layer using a continuous wave laser[END_REF] This layer, not only has a specific thickness, but also a particular vertical density distribution, both varying spatially and temporally. [START_REF] Pfrommer | High resolution mesospheric sodium properties for adaptive optics applications[END_REF] The Ingot Wavefront Sensor (I-WFS), [START_REF] Ragazzoni | Ingot Laser Guide Stars Wavefront Sensing[END_REF] has been designed to cope with this typical elongation of the LGS and presented as novel pupil-plane wavefront sensor in the framework of the Extremely Large Telescope 9 (ELT) project. The LGS produces a significant elongation on many of the Shack-Hartmann wavefront sensor (SH-WFS) spots, especially for large telescopes. The effect is even more significant for the ELT instruments that use SH-WFSs for the wavefront sensing and benefit of LGSs fired from launching telescopes placed at the side of the primary mirror. We developed an end-to-end numerical tool to perform analysis of a closed-loop system. We considered the generation of the input turbulence, deformable mirror definition and control, and, of course, details of the I-WFS. To fully take into account the elongation effect across the vertical (propagation) axis the I-WFS has to consider a three-dimensional description. The wavefront propagation in the three-dimensional volume requires some ray tracing. [START_REF] Portaluri | Evaluating the performance of an Ingot wavefront sensor for the ELT: good news from simulations[END_REF] Ray tracing is heavy in terms of required computing resources, and it is not convenient for control optimization studies. Such a solution would limit the kind of analysis we want to perform: using the numerical tool we aim to get insights about noise limit, optimal gain, best pupil geometry and performing slope calculation algorithm. Both in the numerical simulation and the laboratory framework, we considered a two-dimensional description of the I-WFS. It has been tested at the LAM-ONERA On-sky Pyramid Sensor LOOPS 11 bench at Laboratoire d'Astrophysique de Marseille (LAM), in a quasi-real Adaptive Optics (AO) system using two Spatial Light Modulators (SLM) for the generation of the two-dimensional phase mask representing the I-WFS faces and, separately, the phase screen and the corrector (working as a deformable mirror). For more and further details about the I-WFS, we direct the reader to the original papers [START_REF] Ragazzoni | Ingot Laser Guide Stars Wavefront Sensing[END_REF][START_REF] Ragazzoni | Extending the pyramid WFS to LGSs: the INGOT WFS[END_REF][START_REF] Viotto | Dealing with the cigar: preliminary performance estimation of an INGOT WFS[END_REF] and further developments. [START_REF] Ragazzoni | Pupil plane wavefront sensing for extended and 3d sources[END_REF][START_REF] Viotto | Ingot wavefront sensor: Simulation of pupil images[END_REF][START_REF] Portaluri | Ingot wfs for lgss: First results from simulations[END_REF][START_REF] Di Filippo | Ingot wavefront sensor: from the optical design to a preliminary laboratory test[END_REF][START_REF] Santhakumari | Aligning and Testing the Ingot Wavefront Sensor in the Lab[END_REF] In section 2 we describe the I-WFS, in section 4 the LOOPS bench setup, in section 3 we introduce the numerical tool used to produce the simulation presented in section 5 and discussed in section 6.

THE INGOT WAVEFRONT SENSOR

The actual design of the I-WFS is based on an evolution of the four faces Pyramid Wavefront Sensor 19 (P-WFS), based on a refractive prism. The I-WFS, on the contrary, is composed of a reflecting roof, which allows splitting the incoming LGS light into three beams, two reflected and the third is passing unperturbed towards the following obejective. These three separated beams are re-imaged in as many pupils at the conjugated plane, as shown in Figure 1. The I-WFS has a reflective roof with its edge positioned along the elongation axis of the LGS image. The roof is placed on the LGS image such that it can intercept only a portion of the LGS (left side of Figure 1), while the remaining portion of the LGS does not intercept the prism and produces the third pupil (right side of Figure 1). One can linearly combine the intensity map on the three pupils to calculate signals proportional to the first derivative of the incoming wavefront, like for the P-WFS. The signals are defined using the following relations:

S x = B -C A + B + C - B ref -C ref A ref + B ref + C ref , S y = A A + B + C - A ref A ref + B ref + C ref .
(

Where x and y describe respectively the movements orthogonal and along the elongation direction. A is the intensity on the transmitted pupil, and B and C are the intensities on the two reflected pupils.

SIMULATION TOOL DESCRIPTION

In the simulations we modelled the I-WFS similarly to the P-WFS, [START_REF] Vérinaud | On the nature of the measurements provided by a pyramid wave-front sensor[END_REF] as a combination of Foucault knife-edge sensors. We simplified the computation through a far-field configuration involving only a sequence of Fourier transforms. The complex electromagnetic field on the I-WFS edges is computed as the Fourier transform of the phase in the aperture. We represent the ingot faces as a phase-mask that splits (tilting) the light. The image of the I-WFS focal plane (pupil images) is simulated through the Fourier transform of the complex array in output from the previous step. The P-WFS is typically modulated; 19 however here we do not consider modulation because of the computational workload and especially because a similar algorithm is already exploited to generate the extension of the LGS source. The two effects, modulation and extension, of the reference source, are almost equivalent: the extended source can be modelled as an elaborated form of modulation. We approximate the elongated spot to be two-dimensional, as it was also simulated on the LOOPS bench, and divided it into pointlike sources, to cover the angular extension observed from the ground.

We adapted the already available structure of the MAO code [START_REF] Arcidiacono | End to end numerical simulations of the MAORY multiconjugate adaptive optics system[END_REF][START_REF] Arcidiacono | Numerical simulations of MAORY MCAO module for the ELT[END_REF] to build the algorithm for the closed-loop system. The module is approximated into its main components: input turbulence, WFS, deformable mirror, and control loop. For the input turbulence, we foresee two options: the turbulence is injected using the deformable mirror, or the phase is generated by an external input (e.g. moving phase screen or spatial light modulator). In the numerical simulation, the phase is ultimately a moving random phase screen or a combination of mirror modes with a dynamics following Kolmogorov [START_REF] Kolmogorov | The Local Structure of Turbulence in Incompressible Viscous Fluid for Very Large Reynolds' Numbers[END_REF][START_REF] Kolmogorov | Dissipation of Energy in Locally Isotropic Turbulence[END_REF] or Von Karman 25 power spectrum. The deformable mirror can be computed as a linear combination of the vectors composing a Zernike or actuators influence function modal base. The control loop is a pure integrator with user-defined frequency expressed in frame delay and bandwidth modulated by a gain value. The comparison of the laboratory results with the outcomes of the simulation analysis provides a better insight into the ingot behaviour in the closed-loop.

THE INGOT WAVEFRONT SENSOR AT THE LOOPS BENCH

The LOOPS 11 is an adaptive optics facility hosted at LAM and focusing on the study of Fourier-filtering wavefront sensors. The LOOPS bench allows testing the I-WFS in a quasi-real AO closed-loop context. The optical design of the LOOPS test bench is shown in Figure 2:

The bench is divided into four blocks: (I) a common path, (II) a metrology path, (III) a classic PWFS path, and (IV) a Fourier-based WFS path, as described in previous work. [START_REF] Janin-Potiron | Adaptive optics with programmable Fourier-based wavefront sensors: a spatial light modulator approach to the LAM/ONERA on-sky pyramid sensor testbed[END_REF] The peculiarity of this facility is the presence of a SLM, shown in block (IV). This device is a Hamamatsu SLM (LCOS-SLM X13138) with 1024 × 1280 pixels (with a 12.5 μm pitch) that can produce arbitrary phase masks modifying its shape. It uses liquid crystal technology and exploits the birefringence properties of the crystals to produce configurable phase masks. It is located in the focal plane to reproduce a two-dimensional phase mask of the wavefront sensor. Other components of this facility, used to test the I-WFS are described in the following:

• Monochromatic light source (S in Figure 2) generated from a laser diode and feeding an optical fiber at the wavelength of λ = 660nm

• PI piezoelectric tip-tilt mirror (TTM in Figure 2). It produces the modulation of the point-like source. It is controlled in an open-loop mode and operated at 500 Hz. The I-WFS collapses different pupil viewpoints in one single device, see Figure 3. As seen from one single subaperture, the inclination of the I-WFS prism changes with the distance from the edge. The approximation of the three-dimensional geometry, cannot be reproduced in a two-dimensional system in a straightforward way. We opted for the realization in the model of three different configurations (perspectives), corresponding to three different shapes of the wavefront sensor phase masks. To realize the LGS reference on the bench, we used a fast tip-tilt mirror (TTM) to simulate the extension of the source by introducing an elliptical modulation to the available point source. We calculated the length of the extended source considering the ELT scenario and system parameters and ≈ 20km sodium layer thickness: different portions of the pupil have correspondingly different geometry. We take into account three cases corresponding to different sub-apertures over the telescope pupil, starting from the closest to the laser launcher position (considering a single laser launcher located aside from the telescope), and ending on the one on the opposite edge of the pupil, considering, in addition, one in the middle.

In such configuration, we found three different phase masks, which differ for their projection of the prism roof. These differences correspond in the two-dimensional picture to as many shapes, that we draw starting from the angle shown in Figure 4. In the following, we refer to these respectively as: T30, T15, and T0. On the bench we coupled these sub-apertures to as many different aspect ratios of the spot elongation in these cases of 1:1, 1:7 and 1:15. However, on the ELT the aspect ratio will be larger, respectively 1:1, 1:15 and 1:30: the longest elongation was not feasible using the tip-tilt mirror modulation. In this paper we report only about the test performed using a source aspect ratio of 1:7. Other elongations, but using as turbulence input device a deformable mirror instead of the SLM, have been already tested, and the results are available in a previous work: 17 in 2019, the LOOPS bench facility has been upgraded with the SLM. [START_REF] Janin-Potiron | Seeking for the ultimate wavefront sensor -Probing the free-parameters space of Fourierbased wavefront sensors with the LOOPS v2.0 testbed[END_REF] This new device is able to serve at the same time as a high-resolution deformable mirror and a turbulent phase screen generator, with a much higher resolution with respect to the previous configuration [START_REF] Di Filippo | Ingot wavefront sensor: from the optical design to a preliminary laboratory test[END_REF] using the deformable mirror. The spatial resolution of the input phase increased from the 65 Zernike modes to the 230 Karhunen-Loéve modes used for our test. The latest optical design of the bench is the one shown in Figure 2.

In this new framework, we started testing the I-WFS for several combinations of source aspect ratios and wavefront sensor phase masks: here we report about test using the 1:7 aspect ratio and the T15 mask. We also performed the comparison with the P-WFS case by making the SLM produce a four-faces phase mask. In this work the SLM on a intermediate pupil plane applied a disturbance with a resolution of 320 pixels in the pupil diameter. Figure 5 shows, for the same frame, on the left the shape of the input turbulence, and on the right the complementary correction applied by the SLM.

Here, we report the characteristics of the input turbulence that has been created using the OOMAO 28 code:

• r 0 = 0.9m

• Equivalent Telescope Diameter = 8m • Wind speed = 10 m/s • Bandwidth = 200Hz
• Seeing = 0.15"

• Calibration and correction made with 230 KL modes • Pupil dimension = 318x318 px • Turbulence history, length = 1000 iterations Note that, to make the calibration and calculate the interaction matrix, we used the signals relations shown in formula eq. 1. According to the input parameters, we report in this work the results for the configuration of the T15 phase mask for the 1:7 source aspect ratio for three cases and the P-WFS: case 1 12.5% of the total flux (1/8th) in the transmitted pupil, using the signals calculated following eq. 1 case 2 12.5% of the total flux (1/8th) in the transmitted pupil, using the fluxes measured from the pupils case 3 25% of the total flux (2/8th) in the transmitted pupil, using the signals calculated following eq. 1 case 4 P-WFS using the fluxes measured from the pupils Figure (6) shows the Standard Deviation of the Wavefront Error map during the iterations for open and closed loop for the cases 1, 3, and 4. As the plot in Figure 6 shows, both I-WFS and P-WFS well perform in the closedloop scenario. Apparently the two I-WFS curves superimpose on the open-loop line more than the P-WFS does. This effect is due to some delay in the activation of the loop. While if we look and the residual's mean values of the three cases, we can easily note that the I-WFS shows a lower order of residual aberrations (i.e., a lower mean value), even if the different is negligible.

Besides, we report a comparison between "case 1" and "case 2". For both of them, the flux in the transmitted pupil is equal to the 12.5% of the total, as shown in Figure 7 Here, we can notice from the plot that "case 2" shows a residual mean value lower than "case 1". This very preliminary result could suggest to us to consider as alternative to the eq. 1, the possibility to use as signals the normalized fluxes of the three pupils. We need to take into account, that this is a single result, that needs to be provided with more statistics.

DETAILING THE SIMULATED CASE

We performed several simulations using in input the phase map used also as input for the SLM on the LOOPS bench. The modal base used for the modelling of the deformable mirror in the SLM, was not exported into file. We substituted in the numerical simulations the Karhunen-Loéve modal base used in LOOPS, with a 200 Zernike polynomials modal base, defined on a full circular pupil. The pupil amplitude map and the complex phase array were zero-padded in order to get a resolution on the I-WFS roof of 4λ/D per pixel (λ = wavelength, D is telescope diameter). In this test, we did not include nor photon noise neither read noise of the detector. We also decided to remove from the turbulent phase in input and from the modal base the tip and tilt terms. One simple reason is that LGS should not compensate for atmospheric tip-tilt. Moreover, we already noticed in previous simulations 17 performed using smaller Field of View (FoV) (1") that the circular wrapping of the numerical FFT may cause instabilities: since we noticed suspicious instabilities spikes we preferred to eliminate this possible source of problem. Another possible issue is related to light spread out by diffraction. In the numerical simulation, we didn't address it properly and some light from the two reflected pupils in the top part of the detector pollutes the measurement on the transmitted single one. In the forthcoming simulations we are appropriately correcting the pupil position to overcome the issue. In Figure 8 we present the results in terms of wavefront error standard deviation as measured for each iteration of the loop. We provide at first the same configurations used in the laboratory setup "case 1", "case 2" and "case 3", and in Figure 9 similar configurations but using a different ratio of the light split between transmitted and reflected pupils. These simulations were not converging as expected, most probably because of the extra noise introduced by diffracted light on the refracted pupil. In order to counterbalance this effect we decided to change the illumination ratio, forcing more light on the refracted pupil and reducing in this way the relative intensity of the diffracted light. The cases shown in Figure 9 correspond to the following configurations: case 1b 50% of the total flux in the transmitted pupil, using the signals calculated following eq. 1.

case 2b 50% of the total flux in the transmitted pupil, using the fluxes measured from the pupils. case 5 a functional case using a point source, no modulation, 50% of the total flux in the transmitted pupil, using the fluxes measured from the pupils.

Table 1. Input parameters used for the numerical simulations.

Pupil Geometry D wavelength FFT resolution FoV

Full Circular 8m or 318px λ = 660nm 4λ/D 5arcsec

Figure 8. The plots show the closed loop residual phase standard deviation of the configurations corresponding to "case 1", "case 2" and "case 3" (in blue the open loop). For a better readability: in red using the slope computation of eq. 1 and 1:8 pupil illumination ratio, in green using normalized pupils instead than slopes, and finally in black the eq. 1 slopes and ratio 1:4.

Comments on the results in the next section. Referring to the "case 1b", "case 2b" and "case 5" described in Section 5. In short: in red the case with slope computation of eq. 1, in green using normalized pupils, in black the functional test using a point-like source.

TESTS AND SIMULATION COMPARISON

LOOPS bench and numerical simulations provides insight about the behaviour of I-WFS in closed loop. A crucial point is the linearity of I-WFS response that we are addressing. Strictly related to this point is the optimal illumination of the refracted pupil versus the reflected two: both simulation and experiment show the need to close the loop with a non-zero reference slope vector. However, large amplitudes of the slope vector may introduce non-linearity issues (as the case of the P-WFS).

In terms of result the I-WFS performs slightly better of the P-WFS having an extended reference source. Or being less optimistic the performance is comparable. We need to explore different level of illumination and turbulence strength. The numerical simulation provided similar values for the standard deviation of the residual phase to the LOOPS experiment (both in the range of 40-60nm). In the simulations we excluded the tip-tilt both from the input and the controlled modes: in the case of the laboratory we can assume that the contribute of the very low order modes is negligible since the input aberration is a pure atmospheric disturbance that shows a slowly evolving bent of the wavefront, and also vibration-free. Theoretically, we expect using the parameters of the turbulence statistics, the number of modes used for correction and the bandwidth of the system (in section:4) a noise-free correction residual with standard deviation of 37nm for the LOOPS setup and 39nm for the simulation. Both values are in good agreement with the measured values (i.e., 45nm and 60nm) that include also other error sources related to the bench realization and the I-WFS sensitivity. For a cross-check, the values in open loop are respectively 650nm (assuming pure Kolmogorov turbulence) and 260nm in the tip-tilt-free case: the turbulence statistic in open loop fits well with a turbulence spectrum with a finite outerscale. The experiment and the simulations suggest to further investigate the optimal definition of the I-WFS measurement output formula, since the case using the normalized pupil intensities as vector for the multiplication for the control matrix in both cases provides marginally more stable results.

CONCLUSION

The Ingot Wavefront Sensor has been proposed as a novel pupil plane wavefront sensor, to cope with the intrinsic elongation effects of the Sodium Laser Guide Stars. In this work we review the status of both the numerical simulation tool and the laboratory tests we are performing on the LOOPS bench at LAM about the I-WFS. In this configuration the LOOPS bench has been upgraded with a second SLM to increase the spatial resolution of the turbulent phase and the effect of high spatial frequencies both on the PSF and for the I-WFS measurements. We performed tests for a model of three different configurations (perspectives), corresponding to three shapes of the wavefront sensor phase masks, which differ from their projection of the prism roof. These phase masks have been identified considering three extreme cases: ingot seen from the farther sub-aperture of the telescope pupils with respect to the laser launcher. Ingot seen from a middle positioned sub-aperture, and ingot seen from the closest sub-aperture to the laser launcher. They have been called, respectively: T0, T15, T30. On the other hand, we coupled these sub-apertures to as many different aspect ratios of the source spot elongation, to reproduce the LGS real world extensions, for three cases: 1:1, 1:7, and 1:15. We reported, as example, the analysis made for the configuration 1:7 source aspect ratio and T15 phase mask. We showed which is the behaviour of the I-WFS in closed-loop, making in addition a comparison with the 4 Faces Pyramid Wavefront Sensor. We similarly updated the numerical simulation tool, derived from the MAO code developed for MAORY, to harmonize the model of the I-WFS to what was applied on the LOOPS bench. We performed numerical simulations matching the I-WFS geometry and mask configuration used on the trials on the LOOPS bench. The results are in good agreement, showing the ability of the I-WFS to close the adaptive loop using more than 200 modes on the corrector. In the next months we foresee to solve the issues already identified (i.e., the diffracted light polluting the refracted pupil) and perform more extensive test about control strategies focusing on the achievable performance.

LOOPS Pupils

MAO Simulated Pupils

Figure 1 .

 1 Figure 1. Conceptual design of the 3-faces I-WFS

Figure 2 .Figure 3 .

 23 Figure 2. Optical Design of the upgraded LOOPS bench facility. (I) is the common path optics, (II) a metrology path, (III) the classic P-WFS path, and (IV) the Fourier-based WFS path including a Spatial Light Modulator (SLM).

Figure 4 .

 4 Figure 4. Ingot phase masks used to reproduce the three different and extreme cases used in the test. Left: ingot seen from the farther sub-aperture with respect to the laser launcher, (T0). Center: ingot seen from a middle positioned sub-aperture, (T15). Right: ingot seen from the closest sub-aperture to the laser launcher, (T30).

Figure 5 .

 5 Figure 5. The two pictures freeze an instant of the loop evolution. On the left the phase map of the turbulence in input as processed by the SLM. On the right the complementary correction applied by the SLM as a phase map.

Figure 6 .

 6 Figure 6. Standard Deviation Wavefront Errors Open-Closed loop comparison between I-WFS cases (1-3) and P-WFS.

Figure 7 .

 7 Figure 7. Standard Deviation Wavefront Errors Open-Closed loop comparison between cases (1-2).

Figure 9 .

 9 Figure 9. Referring to the "case 1b", "case 2b" and "case 5" described in Section 5. In short: in red the case with slope computation of eq. 1, in green using normalized pupils, in black the functional test using a point-like source.

Figure 10 .

 10 Figure 10. The figure shows the three pupil images arrangement for the laboratory and in the numerical simulations, respectively on the left and the right part of the image. 25% illumination vs 50% illumination of the transmitted pupil respectively.

ACKNOWLEDGMENTS

This work is supported by the INAF Progetto Premiale "Ottica Adattiva Made in Italy per i grandi telescopi del futuro". The INAF team thanks the Laboratoire d'Astrophysique de Marseille (LAM) for their kind hospitality.


text:
Donald Erbschloe

Adam Antczak

Dennis Carter

Gary Dale

Carsten Döll

Robert Luckner

Tobias Marks

Michael Niestroy

Operationalizing Flight Formations for Aerodynamic Benefits

Nature abounds with examples of animals which decrease their energy expenditure by gliding off the bow waves of large moving objects, riding thermals, or flying in efficient configurations. The past twenty years has seen significant advances in the scientific understanding and feasibility of large aircraft saving energy by flying in advantageous pair or triple geometries. Several recent flight tests demonstrated reductions in fuel burn for trailing aircraft of 10% or more. The energy savings can be converted into a variety of beneficial operational capabilities: extended range, increased loiter time, larger payload, cost savings, etc. The next looming challenge is operationalizing this practice. The discussion areas include: benefits, penalties, and trade-offs; operational modes; maneuvers; routing and scheduling; regulations; and the process of a deliberate, evolutionary approach in fielding this capability into existing aviation enterprises.

I. Introduction

There are many factors which contribute to the feasibility of incorporating a new, potentially disruptive procedure such as efficient formation flight (or Air Wake Surfing for Efficiency-AWSE) into wellestablished and proven operational techniques and maneuvers. Basic operational challenges and opportunities include benefits from reduced fuel burn in the trailing aircraft, penalties incurred from pairing and tripling up aircraft, potential configurations, maneuvers, routing, scheduling, partnering, interoperability, authorities, permissions from air traffic control, and training.

II. Fundamentals of AWSE

A. History

Taking inspiration from nature is basic to powered flight-influencing shapes of wings, stabilizers, control surfaces, etc. In 1914 Carl Wieselsberger was the first who explained the energy advantage trailing birds in formation can exploit when flying in the upwash field created by leading bird 1 . Aerodynamic theory in this efficient maneuver was refined during the ensuing decades by Schlichting 2 and Hummel 3 . Hummel was also instrumental, along with Beukenberg, at the Technical University of Braunschweig, to conduct flight tests in Dornier aircraft (starting in 1984) to validate not just the theory, but to validate in experiments and to initiate the development of automatic controls 4 .

Following the Braunschweig test flights, NASA conducted a series of efficient formation flight tests with fighters and transport aircraft [START_REF] Morgan | A Study in Drag Reduction of Close Formation Flight Accounting for Flight Control Trim Positions and Dissimilar Formations[END_REF][START_REF] Wagner | Flight Test Results of Close Formation Flight for Fuel Savings[END_REF][START_REF] Anonymous | NASA Sky Surfing for Fuel Economy[END_REF] . A Defense Advanced Research Project Agency (DARPA) and Air Force Research Laboratory (AFRL) sponsorship, NASA teamed with Boeing to explore AWSE. Test flights in 2012 with this platform helped substantiate and quantify theoretical predictions and finetune automatic systems [START_REF] Bienawski | Summary of Flight Testing and Results for the Formation Flight for Aerodynamic Benefit Program[END_REF] . A significant flight test conducted in 2013 focused on maneuvers, procedures, human factors, safety, and compatibility with normal operations and air traffic control. NASA continued flight testing, most recently in the Gulfstream III, validating the use of Automatic Dependent Surveillance-Broadcast (ADS-B) as a datalink [START_REF] Hanson | Experimental Measurements of Fuel Savings During Aircraft Wake Surfing[END_REF] . Commercial companies such as Lockheed-Martin and Airbus, academia, and government research laboratories (e.g., the Deutsches Zentrum für Luft-und Raumfahrt, DLR) have conducted extensive modeling and simulation and flights for AWSE. The flights demonstrated a fairly consistent fuel savings for the trail of 5-12% for homogenous (same platform) and 18-30+% savings for heterogeneous (e.g., a fighter aircraft following a large tanker or transport).

B. The basics of aviation operational fuel efficiency

Not surprisingly, interest in AWSE grows when jet fuel prices are high. Figure 2Error! Reference source not found. shows the past 3 decades of jet fuel prices. When there is a spike or high plateau in fuel costs, R&D funds follow. When the fuel returns to low values, funding (and customer interest) wanes. Over the past two decades many studies have explored improving aviation system efficiency. [START_REF]Technology Options for Improved Air Vehicle Fuel Efficiency[END_REF][START_REF]Improving the Efficiency of Engines for Large Nonfighter Aircraft[END_REF][START_REF]More Fight, Less Fuel[END_REF]15,16 In these studies, three broad approaches have been identified:

1. Boost the aircraft performance by increasing thrust specific fuel consumption (propulsion), improving Lift/Drag (aerodynamics), and/or reduce aircraft structural or payload weight 2. Improve the use of the aircraft through streamlined operations (e.g., advanced flight simulators, enroute modernization, best practices such as continuous descent approaches, etc.) [START_REF]Going Virtual to Prepare for a New Era of Defense[END_REF]18,[START_REF]Continuous Descent Final Approach[END_REF][START_REF] Boeing | Fueling Efficiency[END_REF]21 3. Harvest energy from external sources (e.g., solar power, optimized use of favorable winds, and air wake surfing) 22

C. Decision making considerations: tradeoffs and penalties

Whether or not to perform AWSE as a standard practice for a fleet requires a rigorous calculus, weighing benefits against penalties. According to Roberto I. Guerrero, Deputy Assistant Secretary of the Air Force for Operational Energy, "My office is interested in AWSE as an intriguing approach to tap into a resource of otherwise 'lost energy'. The energy gains have the potential for conversion into real operational benefits-extended range, longer loiter time, bigger payloads, or cost savings, to name a few." 23

1) Benefits

Increased flight endurance

Flight endurance is the maximum time an aircraft can stay airborne (at cruise). 24 This is important in mission profiles which include loiter (cruising over a small area for an amount of time) such as reconnaissance, air refueling ovals, hunter/seeker, or command and control. The fuel saved by flying in AWSE can be used for greater time aloft.

Extended range

Fuel saved in efficient formation flight can be applied to fly longer distances. Models show this would be for missions like fighter drags, in which a large tanker aircraft ferries several smaller fighter aircraft long distances. The tanker generates a strong vortex with a magnified benefit on the smaller receiving aircraft.

Additional payload

Anticipated fuel savings could allow for the decrement of the fuel load prior to launch. The fuel weight saved could be substituted for payload.

Fuel replenishment

In special circumstances, such as humanitarian operations in disaster area, fuel is often limited at the destination airport. In this case, AWSE can save fuel, which reduces the requirement for onload for the return mission. Alternatively, the aircraft can leave excess fuel at the destination, replenishing the resources for further use.

Tighter spacing

The grouping of several aircraft for AWSE formations has the potential to increase the airspace capacity and reduce air traffic controller workload. This so-called "loose formation" can be beneficial to air traffic flow even if no aerodynamic benefit is achieved.

Reduction of climate impact

Fuel savings from AWSE can be part of a holistic approach for reducing greenhouse gases generated by aircraft (approximately 2% of human-produced CO2 annually). [START_REF]Intergovernmental Panel on Climate Change (IPCC)[END_REF] New standards and guidelines for emissions will impose restrictions and require technological and/or operational advances to meet the limits.

Reduction in number and strength of vortices

A two-ship formation generates four wingtip vortices. In AWSE, the inside vortices-one produced by lead and the other on the opposite wing by trail-are counter-rotating and in proximity. Although all four vortices interact dynamically, the two close vortices can react in a chaotic manner, but the expectation is an overall diminishment in strength, or even cancellation.

2) Penalties and inefficiencies

Suboptimal operating point

Unless aircraft are perfectly matched in aircraft type and weight, they cannot both be flying at optimal conditions (e.g., speed and altitude). A compromise is required to maximize common benefits.

Detours

Aircraft originating from different airports need to fly to an agreed rendezvous point to create the AWSE formation. This generally adds distance to solo flight routes for one or both of the aircraft.

Additional holdings

If a rendezvous is necessary to pair up, precise timing is required to prevent one aircraft holding while awaiting the arrival of the other. This issue can be ameliorated to a degree if air traffic control allows for a flexible rendezvous point or the delay of the aircraft can be compensated prior to takeoff.

Trim drag

The flight within the wake vortex of another aircraft leads to additional aerodynamic drag induced by the compensation of the additional rolling moments (e.g. by aileron and rudder deflection) and must be included in the aerodynamic benefit model to properly predict the fuel savings. [START_REF] Okolo | Application of Sweet Spot Determination to a Conventional Pair of Aircraft[END_REF][START_REF] Kaden | Modeling Wake Vortex Roll-Up and Vortex-Induced Forces and Moments for Tight Formation Flight[END_REF]

Station Keeping Inaccuracy

The benefit area in the (lateral/vertical) vortex plane is not symmetric about the optimum and drops off rapidly inside the optimum point. Significant induced rolling moments exist and station keeping systems have variance in position. The best strategy is to favor the outside of the optimum point. Furthermore, it can be assumed that the encounter of turbulence and gust during the flight will lead to a substantial loss of the benefits and therefore cannot be neglected. 28

III. Basic requirements to execute AWSE

There are a handful of fundamental necessities to justify and be able to perform AWSE at the enterprise level. These are described next.

A. Capable aircraft and systems

To execute AWSE, an aircraft must position itself in the vortex updraft and maintain station. During the test flights in the 1980s and early 2000s, this task was relegated largely or entirely to the pilot. Many of the flights were flown in tight fingertip formation-with 1-2 wing spans separation-which some pilots called "white knuckle flights". Because the vortex persists for long distances behind the generating aircraft, the recommended method now is to maintain a large longitudinal separation (10-40 wingspans), greatly improving safety. A drawback is that the visual cues (e.g., parallax) that the trail depends on to maintain position are restricted at a distance. The aircraft must be able to predict or detect the location of the vortex, from which it must sustain a relative displacement. Automated systems, station-keeping equipment, datalinks, and proper displays are necessary.

1) Automated systems

Automated equipment in many military aircraft reduces pilot workload, keeps the aircraft on course, and maintains set headings, altitudes, and speeds. The automated systems and information which are required or desired to perform AWSE are:

• Autopilot: Lead (desired), Trail (required)

• Autothrottle: Lead (desired), Trail (highly desired)

• Measurement of winds aloft: Lead (required), Trail (required)

• Station-keeping equipment (SKE): Lead (required), Trail (required)

• Flight computer capable of AWSE calculations: Lead (desired), Trail (required)

2) Datalink Aircraft performing AWSE need to communicate and share vital information about each aircraft, intentions, and connectivity status. The respective AWSE element information to be sent is:

• Report position, velocity (> 2

B. Trained and qualified aircrews

A source for trained and qualified aircrews is the military. Military pilots undergo comprehensive and rigorous and training across a broad spectrum of procedures, and techniques. They also must be qualified and certified for specific maneuvers.

Boeing and AFRL developed procedures and aircrew training modules for employment of AWSE. Several instructor, examiner, and test pilots were involved in the development and demonstration phases. The comments from this group provided useful insight and suggestions, including that airwake surfing procedures would be easy to train in the simulator. A current and proficient tactical formation qualified pilot could be certified with a single simulator flight, while non-formation qualified pilots might require one simulator and one flight.

Commercial pilot training and certification would be more extensive, although much of that could be accomplished in a simulator. Bieniawski, et al., [START_REF] Bieniawski | Summary of Flight Testing and Results for the Formation Flight for Aerodynamic Benefit Program[END_REF] and Blake [START_REF] Blake | Surfing Aircraft Vortices for Energy[END_REF] confirmed that significant fuel savings for airline fleets in general using AWSE. [START_REF] Pahle | An Initial Flight Investigation of Formation Flight for Drag Reduction on the C-17 Aircraft[END_REF] . A Stanford study 37 demonstrated for real air traffic networks the significant potential fuel saving. It was maximal if airplanes of different airlines that fly along the same flight corridor would fly in formation, e.g. a fuel saving of 6.8% would be feasible for the transatlantic network of the Star Alliance.

C. Operational compatibility

AWSE operations must conform to normal air operations to be accepted and adopted by fleets. Simply put, this is a new set of maneuvers, but they should not be unfamiliar. Challenges to implementing the practice of AWSE must be considered and overcome.

1) Longitudinal Displacement

A vortex is typically fully formed and stable typically within 1.5 wingspans downstream from the generating aircraft and begins to break up after a few minutes-several miles--behind the aircraft. The trail aircraft therefore needs to be placed in the stable part of the vortex (test flights were usually flown. 3000-6000 feet behind lead). Other constraints influencing the longitudinal displacement include those imposed by Air Traffic Control.

2) Lateral displacement A distinguishing feature of AWSE is that the trailing aircraft must position itself relative to the vortex core rather than relative to the lead aircraft (see Figure 4). Due to the nonuniform inflow on the followers wing an additional rolling moment is induced that needs to be compensated for by aileron and rudder deflections. The magnitude of the rolling moment depends on the lateral and vertical displacement. Areas where the rolling moment exceeds the compensation capabilities of the trail or where a change of sign occurs need to be avoided. The desired lateral displacement will also depend on the flight conditions. At night or in IMC (Instrumented Meteorological Conditions), the crew in the trailing aircraft may decide to fly a loose geometry with greater separation from the vortex. Automated formation systems are IFR (Instrument Flight Rules) certified; without such systems, AWSE may be limited to VMC (Visual Meteorological Conditions).

Crosswinds will push the vortex away from the earth-referenced flight path of the lead aircraft. In order to compensate for these effects, detailed models of the vortex movement and an exchange of meteorological data between the aircraft are necessary.

3) Vertical displacement

The lead aircraft generates two counter-rotating vortices. These do not act independently, rather they form a dynamic system where each vortex pushes down the other. The result is vortices which descend in time.

The greater the longitudinal distance behind lead the drop increases. The descent rate is modest, for example the vortex descent behind a medium size transport at cruise is roughly 7 ft/s. Flying at M0.75, the expected descent of the vortex is roughly 50 feet at one nautical mile in trail. Since the recommended longitudinal separation is a few thousand feet to 2 nm, the trail aircraft will still be within 100 feet of lead vertically. This is important for Air Traffic Control, who must keep aircraft separated by safe vertical distances.

3-Ship Table 1: Formation Configurations 40

A fundamental question concerning the operation of aircraft in formation is the number of participants as it influences the technical systems as well as the operational procedures. Formations with large number of aircraft are more difficult to maintain and control but yield the highest benefits. Vortices may overlap, cancel themselves or behave in a chaotic way. Small formations are easy to establish and control.

++ +

...

5) Airport usage

Another important operational question that needs to be defined is whether the participants take off from the same airports or join together from different airports. Using the same airport can maximize the benefits as the aircraft can fly together as long as possible. Using different airports yields more opportunities for pairing up aircraft but can lead to significant detours that reduce the benefits. [START_REF] Erbschloe | AVT-279 original work product, Different operational scenarios for formation flight 42[END_REF] Table 2 shows the various possibilities arising from different departure and arrival airports. Pros and cons of the different options are:

Same-Different

Departing at the same airport reduces the coordination of the formation members significantly. In this case a sequential departure can be conducted with one formation member performing a holding pattern or flying at speeds which allow a smooth join to the formation. .

Different-Same

Arriving at the same destinations yields similar operational problems as in the case of same origins, as after the separation the formation members need to land sequentially if no independent runways are available. This requires additional coordination among the formation members.

Same-Same

The same origin and destination of the formation members yields the highest benefits in terms of fuel savings as the length of the formation route can be maximized. However, the take-off and landing procedure needs to be adapted if no independent runways are available at the origin and destination airport.

Different-Different

This case allows the highest flexibility in joining up the aircraft, as a formation can be established among all suitable flights.

6) Maneuvers

A recommendation of this article is to initially limit the application of AWSE to extended, non-maneuvering phases of flight. In Figure 6, this would be during cruise and possibly the region in climb or descent close to level flight. As confidence in AWSE grows, more of the climb and descent might be included, however efficient formation flight in the takeoff, departure, approach (instrument or visual), and landing phases will likely remain forbidden. Additional useful maneuvers are described below.

Positional changes

Positional changes can be observed in the formation flight of birds. It occurs if the leading bird gets tired, falls back, and is replaced by a fresh flock member. In powered flight with similar platforms, the trailing aircraft burns less fuel than the leader. It is reasonable to consider a change of positions during the formation in order to share benefits. Such a decision must balance the savings against the cost of aircrew training and currency to perform this maneuver.

Course adaptations

Minor course adaptations should be permissible while in formation. If larger course adaptations need to be performed the formation needs to break up and rejoin after the adaptation is finished. In order to synchronize the movement, all aircraft in the formation need a stable datalink.

Separation

It will be routine for the trailing aircraft in AWSE to separate either for short periods of time (e.g., when it is necessary for one of the aircraft to perform some necessary flight task or when forecast or reported weather issues, such as IFR conditions or increased turbulence) or permanently (e.g., when the formation leg of a different destination AWSE is complete or in failure cases). In either case, the trailing aircraft should announce the separation to lead and move to a safe lateral displacement (250-500 feet is adequate and gets the trail into a position where the updraft is minimal). If the separation is temporary, trail should request permission to reassume AWSE position and receive confirmation from lead before moving back.

Breakaway

A breakaway can be called by any aircraft in the formation when a safety of flight issue arises (e.g. unexpected moderate or greater turbulence, unintentional wake/vortex crossing, failure conditions, etc.). An immediate separation should be initiated. A rejoin is possible after the causal condition is mediated.

Novel maneuvers

As AWSE is a new, albeit familiar, way of flying we anticipate that aircrews will discover new, unforeseen procedures and maneuvers to improve safety and effectiveness. Two novel maneuver examples which have been previously discussed are presented to spur innovative juices.

Upwash cradle:

A 1976 paper by Donaldson proposed an intriguing configuration, which is almost the inverse of a staggered V formation 43 . As shown in Figure 7, the outermost aircraft assume lead positions, in a staggered manner (although for safety, the recommendation is to have the leads longitudinally displaced by 1000-2000 feet).

In this configuration the trail benefits from the upwash of both lead aircraft. With precise placement, the superposition of these two could result in an induced thrust (vice an induced drag) for the trailing aircraft. Leapfrogging: Leapfrog is a game in which one person uses the back of a hunched over leader to propel himself or herself ahead of lead. In aviation, a similar boost can be achieved when a trailing aircraft joins up on a lead, receiving a fuel savings benefit, and can then reach a destination which would have been unavailable to it flying solo.

7) Routing / Operating point

Flight paths can be altered due to constraints given by air traffic control, mission requirements (including avoiding threat areas) or optimization (e.g. favorable winds or more direct routing). In order to realize the maximum absolute fuel savings in a formation, it is essential to optimize the routes of the participating members and the group altogether. Optimization also includes the locations of the rendezvous and separation points; the formation track as well as the altitude and speed of the formation.

The need for optimization is strongly defined by the selected CONOPS. Airport usage determines whether the rendezvous or separation points are subject to optimization. The highest benefits can be expected if the formation members depart from the same airports and go to the same destination. If the loading and aircraft types are identical the formation can fly at an optimal operating point.

Mission type

Depending of the type of mission that needs to be accomplished (e. g. surveillance, cargo transfer, fighter drag) different optimization criteria apply. For example, an optimal operating point and formation track will differ if a maximum flight time or a cost saving should be realized.

Optimization of rendezvous and separation points

The geographic location of the rendezvous and separation points strongly influences the benefits depending on the airport scenarios. Detours lead to a decreased benefit; however, one formation member may need a large detour in order to join an optimized formation. A seasonal variation in winds can affect the location of optimal rendezvous and separation points. Fixed rendezvous and separation points can lead to a decrease in efficiency that can reach up to 1% of total burned fuel. 44

Optimization of operating point

The compromise to fly at the same altitude and at the same speed necessitates the optimization of both factors in order to achieve the highest benefits for the whole formation. Simultaneous step climbs can be an option to achieve even higher benefits.

Daily changes

Daily changes like wind patterns strongly influence the optimization and can lead to a decrease in benefit. In order to simplify the strategic optimization (e.g. fuel planning) statistical information about typical wind situations can be used. For the tactical planning weather forecast data needs to be considered to determine the optimal flight tracks.

Flight plan management

Airlines in order to create flight plan for every operation are using dedicated systems (e.g. Netline, Sabre).

Those software systems are designed to build the flight plan for every known conditions (e.g. weather, tracks, aircraft limits, number of passengers) and business rules (fastest, cheapest etc.). If the formation flight module would be available, all the changes and optimization would be done here.

8)

Scheduling/Partner Allocation Some operators may hesitate to adopt AWSE from a perception that pairing is too difficult to manage in a large enterprise or that the benefit generated from a low number of potential pairs does not make a compelling business case. An initial glance at a daily flight schedule seems to suggest this. This is by design. The historic scheduling process purposely deconflicts most aircraft flying similar routes at similar times. For example, positioning/depositioning aircraft transporting equipment and/or troops for an exercise is typically executed by staggering takeoff/landing times.

Two modifications to building schedules and matching aircraft dramatically increases the number of potential pairs: 1) allow flexibility in takeoff times and 2) pair up aircraft taking off and/or landing at different airfields if the fuel savings outweigh the cost of rendezvous and route change. Boeing analyzed a year's worth of mobility transport flights in the North American region, allowing for pairing of C-17 aircraft originating and/or terminating at different airports (up to 1,000 nm apart) and up to a 12 hour shift in takeoff time. [START_REF] Flanzer | Operational Analysis for the Formation Flight for Aerodynamic Benefit Program[END_REF] Allowing a 1-hour shift in the scheduled departure times produced a 6-fold increase in possible pairs over the number with an inflexible schedule. Even a modest 2-hour shift in take-off time would have achieved 2/3s of the maximum benefit. A 4-hour shift results in 90% of the maximum benefit.

If the operator can generate, and manage the flying schedule, it can be creative. In the exercise positioning/depositioning scenario, pairing up adjacent launches could result in fuel savings and, potentially, allow for quicker closure. Another target is a schedule in which there are daily flights between airbases. A more efficient approach would be to pair up aircraft every other day. The (near-) simultaneous take off/landing should not overwhelm ground support.

Originating aircraft from different bases presents a complication. If this is not coordinated well or there is a delay and one aircraft has to hold at the rendezvous point, then any projected fuel savings will rapidly dwindle away. This can be ameliorated by adopting commercial practices (i.e., coordinating engine starts), adjusting relative speeds inflight to achieve the best closure and creating a dynamic rendezvous calculator that builds in time delays.

9) Proper authorities

Authorities and allowances for joining up of aircraft already exist in guidance and directive documents from regulatory agencies for aviation. Relevant language on formation flights include:

FAA JO 7110.65W, Air Traffic Control [START_REF]FAA Order JO 7110.65W Air Traffic Control[END_REF] Formation flights are treated and controlled as a single aircraft and will continue until separation is established. Separation procedures are detailed.

FAA Part 91 Sec. 91.111 Operating near other aircraft 47 Pilots in command of all aircraft in a formation must agree to the formation and coordinate actions and roles.

ICAO Annex 2 Sec. 3.1.8, Rules of the Air [START_REF]ICAO, Rules of the Air[END_REF] ICAO rules are similar to the FAA rules. These add that formation coordination extends to periods of transition (e.g., join up and separation). In addition, separations to not exceed 1 km laterally and longitudinally and 30 m vertically from flight lead are established.

IV. Target applications (near-term, mid-term, far-term)

AWSE is being pursued by several different research groups (e.g., DLR, NASA, AFRL, etc.), operators and aircraft companies (e.g., Airbus, Boeing, Lockheed-Martin, etc.). The applications vary from nearterm (can be adopted in less than 5 years), the mid-term (5-10 years), or far-term (greater than 10 years). Three representative use-cases, each with a unique perspective on the employment of efficient formation flight are suggested. These cases can start with rigorous modeling & simulation and wargaming and culminate in operational flight tests.

A. Near-term: Homogeneous mix. Rigorous modeling & simulation and wargaming, looking at a variety of similar platform pairs. These should culminate in operational flight tests.

B.

Mid-term: Heterogeneous mix. A fighter aircraft in formation with a large aircraft (normally an air refueling tanker) is useful construct. Indeed, roughly 1 in 20 sorties for USAF air refueling aircraft involves the movement (or "ferry") of fighter aircraft long distances (CORONET missions). The USAF has looked at the formation flight efficiencies in CORONETs and one study determined that incorporating AWSE-like arrangements could allow the tankers to increase the number of fighters carried, or, alternatively, reduce the number of required tankers by one (Reference 43).

C. Far-term: Autonomous aircraft (paired with other autonomous or human-crewed aircraft).

Development in both autonomy and understanding of a heterogeneous mix must be accomplished before this is considered feasible. However, an autonomous aircraft offers multiple advantages as a partner. Many of the challenges facing the adoption of AWSE relate directly to aircrews or passengers, such as training/currency requirements and ride quality issues. These issues disappear if the receiving aircraft is unoccupied. Military examples include autonomous aerial refuelling 49 , aircraft companions 50 , drones, and swarms of drones. A common theme of these military programs is that the requirements and systems to make these work (e.g., automated controls, datalinks) will also enable AWSE. As imagined in the aircraft companion program, the synergy of a piloted lead aircraft with autonomous companion(s) is compelling. In many scenarios the robotic partner is a smaller aircraft carrying additional sensors, communications equipment, armaments, etc. As NASA observed in the combination of fighters and transport aircraft, the expectation is that the smaller, lighter aircraft could see fuel savings on the order of 10-20%. The fuel savings can be traded for operational benefit: extended range, longer loiter time, additional payload, etc.

V. Conclusions

Introducing and adopting AWSE as a standard practice will initially be disruptive and there will likely be some scepticism and resistance. This is understandable and suggests a gradual accession into operations. Two related guiding principles can help:

• Start with use of efficient formation flight where and when aircraft fly for extended periods of time in close proximity-for example, tactical formations enroute or returning from areas of operation; positioning or depositioning of aircraft and equipment for exercises; tanker cells (air refuelers often pair up enroute to/from orbits); fighter drags (when multiple fighter aircraft are ferried long distances with one or two air refuelers-known as CORONET missions in the USAF). • Limit efficient formation flight initially to non-maneuvering phases of flight.

Minimizing energy expenditure is a common practice in nature: dolphins (and human surfers) ride the bow waves off ships, hawks (and gliders) circle in thermals to gain altitude and potential energy, and birds like geese fly in v-shaped formations to reduce their exertions during long migrations. It was inevitable when humans took to the skies in the early days of powered flight to take cues from nature. There have been many attempts over the decades to fly in formations, like birds, for energy efficiency. These trials demonstrated fuel savings-often an impressive amount-but came with a huge workload cost. In recent times, improved automation, station keeping, ability to pass data between aircraft, and enhanced tools for situational awareness make a procedure like AWSE possible and practical. The fuel saved by a trailing aircraft can be traded for a variety of operational benefits: increased range, longer endurance, increased payload, cost savings, and reduction in emissions and carbon footprint. This is true for military as well as civil airline operations.

Figure 1 :

 1 Figure 1: Overhead view of experimental test of formation theory using Dornier Do28 aircraft (1984) 5

Figure 2 :

 2 Figure 2: Jet Fuel Price ($/gal) 1990-2019. Note: Arrows are periods with flight tests, grey bars represent recessions. 11

  Hz)• Heading angle and warning if about to maneuver • Meteorological message, including winds (desired) • Weight (occasionally) • Acknowledgement of information received (desired) Available datalinks are incorporated in dedicated flight formation systems and SKE (e.g., Leonardo AN/AP-243 Station Keeping Equipment 29 and Honeywell MILACAS-FR Formation-flying systems 30 ). However, more affordable solutions exist-NASA Armstrong recently completed a successful test of transmitting and receiving adequate information using ADS-B out (a mandatory capability for aircraft transiting Mode C transponder airspace by 2020).31 3) Sensors/Instrumentation Physical models can predict the location and strength of the vortex, however direct measurement using systems like Doppler LIDAR or radiometers would be very useful. Standard cockpit instrumentation can display behaviour indicative of aerodynamic benefit and proximity to the vortex: fuel flow and angle of attack should decrease while aileron trim should increase. Such cues could be used for peak seeking and optimization. Sensors placed along the wing can help pinpoint the wake position.[START_REF] Fischenberg | Online Wake Identification Algorithms Using Forward Looking LIDAR Sensor Measurements[END_REF] 4) Displays Pilots executing AWSE want good situational awareness. They want to know their position relative to the vortex (and where they can and should not fly) and some sense of fuel benefits could be an app on an Electronic Flight Bag.[START_REF] Kaden | Maneuvers during Automatic Formation Flight of Transport Aircraft for Fuel Savings[END_REF]

Figure 3 :

 3 Figure 3: Notional Display for AWSE

Figure 4 :

 4 Figure 4: Fuel savings benefit map off vortex core[START_REF] Koloschin | Towards Automatic Control of Fuel-Saving Formation Flight[END_REF]

Figure 5 :

 5 Figure 5: Counter-rotating vortices and vertical displacement of trail[START_REF] Faa | Advisory Circular 90-23G, Aircraft Wake Turbulence[END_REF]

•

  Depending on the number and types of aircraft different arrangements are possible (Table1: Formation Configurations). Operators should look at missions in which aircraft are sufficiently close proximity for extended periods of non-maneuvering flight and exploit those first.

  A ' -) . . ~+~--0 --. origin A' -) . . ~ ++~X destination A

Figure 6 :

 6 Figure 6: Phases of flight 42

Figure 7 :

 7 Figure 7: Configuration for upwash cradle-the rearmost aircraft flies in the upwash of each of the two aircraft ahead (from Reference 43)

  VERTICAL COMPONENT OF VELOCITY FIELD INDUCED BY WAKE VORTICES F ROM 2 P-3' S -------h • 20,000 fi (6096 111) V • 220 kll (112 ffllHC) b • IOOft (30.5111)

Table 2 : Different operational scenarios for formation flight

 2


C Granata

M Chetouani

A Tapus

P Bidaud

V Dupourqué

Voice and graphical-based interfaces for interaction with a robot dedicated to elderly and people with cognitive disorders

Human-robot interaction (HRI) takes place especially through interfaces. The design of such interfaces is a very delicate and crucial phase because it influences the robot accessibility and usability by the user. In this paper, we describe and analyze the results of 2 tests conducted so as to understand some of the optimal features that should characterize the robot voice and graphical-based user interfaces. Our test platform is an assistive robot developed for the elderly with mild cognitive impairments. Therefore, the user interfaces must be clear and simple. The ambiguities must be eliminated so as to facilitate the use of the robot and hence not to discourage the elderly population to use new technologies.

I. INTRODUCTION

The world's population is growing older. Most of the elderly population suffers from the effects of social isolation and age related cognitive decline (deterioration in memory, attention, concentration, etc) [START_REF] Gusmano | The Elderly and Social Isolation[END_REF], [START_REF] Victor | Being Alone in Later Life: Loneliness, Social Isolation and Living Alone[END_REF]. The new trend in assistive technologies is promoting aging of elderly persons at home. It is still necessary to define the skills and the characteristics which a social robot should have. The authors in [START_REF] Bartneck | A Design-Centred Framework for Social Human-Robot Interaction[END_REF] describe a social robot capable of interacting and communicating with humans by following the behavioral norms expected by the people. Hence, having a social robot capable of moving and acting independently and in the same time capable of assisting people in their own home environments is still a challenge.

An important question which it is necessary to be addressed is to know if an elderly and/or a non-expert user would be able to interact naturally and intuitively with social robots. Voice interactions are the most common way people express their needs. Voice based interfaces are more powerful than graphical based ones. However, the population to whom the robot is dedicated has cognitive, memory, and/or understanding problems. Gödde et al. [START_REF] Gödde | Study of a speech-based smart home system with older users[END_REF] have shown that profiling user input is particularly important for the elderly population. The authors have compared the interactions of older and younger users with a speech-based smart home The mobile robot Kompa¨ı (by Robosoft-France) used for interaction.

system. The results illustrated that older users were less likely to speak to the system in a way that was easy for the system to understand, and achieved lower task success. Agerelated changes affect many interrelated aspects of cognition, such as information processing speed, mental flexibility, fluid intelligence, and memory [START_REF] Salthouse | What and when of cognitive aging[END_REF]. The memory deficits in the elderly impair language comprehension and production. The interfaces used should be able to handle potential problems of human speech such as sentence fragments, false starts and interruption.

To allow the elderly with cognitive impairments to interact comfortably with the machine, we propose a multimodal approach: a combination of voice and graphical support. A classical voice-based interface is composed of an Automatic Speech Recognizer (ASR) that interprets the human voice, a Dialogue Manager that selects the appropriate response, and a Text-To-Speech (TTS) module that synthesizes the selected response. As showed in Fig 2 , we propose a multimodal approach adding the fusion process to combine voice and graphical inputs and the fission process to combine voice and graphical output.

Our aim is to create a graphical support that accompanies the voice interaction. To facilitate the use of the robot, the system of fission must be designed according to some well founded criteria concerning the language, the vocabulary (words have to be chosen with respect to their frequency and familiarity), the syntax (the rules by which words are organized to form a correct sentence should be simple), and semantics (the meaning of a word or of a sentence should Multimodal approach using the fusion and fission process to combine voice and graphical input/output. avoid any ambiguities). However, if for the linguistic part there are some solid rules that can be employed so as to define a model, for the graphical-based interface, substantiated criteria are still not very well known. In this paper, we posit that the concomitant use of voice and graphical support can increase the usability of the robot by the elderly and people with mild cognitive impairments.

II. RELATED WORKS

Different kinds of interfaces have been tested over the past few years to allow a non-expert human to interact with a robot. Some tried to address this challenge by transposing human/human modes of interaction based on voice recognition, gesture recognition and gaze tracking [START_REF] Haasch | Biron -the bielefeld robot companion[END_REF], [START_REF] Nickel | Real-time recognition of 3d-pointing gestures for human-machine-interaction[END_REF], [START_REF] Atienza | Active Gaze Tracking for Human-Robot Interaction[END_REF]. Other interfaces have been developed using some handheld devices such as the Wiimote [START_REF] Guo | Exploring the use of tangible user interfaces for human-robot interaction: a comparative study[END_REF], the Personal Digital Assistant (PDA) [START_REF] Severinson-Eklundh | Social and collaborative aspects of interaction with a service robot[END_REF] and the I-Phone [START_REF] Rouanet | A comparison of three interfaces using handheld devices to intuitively drive and show objects to a social robot: the impact of underlying metaphors[END_REF]. In order to enhance the flexibility and the naturalness of the user interfaces, others researchers have developed multimodal systems. Reichman [START_REF] Reichman | Communication Paradigms for a Window System[END_REF], [START_REF] Reichman | Integrated Interfaces Based on a Theory of Context and Goal Tracking[END_REF] proposed the conversation metaphor, a hybrid interaction style, which integrates graphical and user's natural language. In [START_REF] Perzawnoski | Building a Multimodal Human-Robot Interface[END_REF], the authors explain a multimodal interface combining speech recognition, hand gesture recognition, and inputs from a PDA to drive mobiles robots. Concerning language in the elderly, relatively little has been published on fission process in HRI interaction. Wolters et al. [START_REF] Wolters | Reducing working memory load in spoken dialogue systems[END_REF] assessed two approaches to accommodate users with memory troubles: on one hand they wanted to reduce the number of options proposed by the machine and on the other hand the system was supposed to provide confirmations (if the system confirms each aspect of the interaction, users will find it easier to remember it). At the cognitive level, we know that there is a relation between an image and the linguistic representation of an object (or of a scene) represented in an image. In recent years, many researches concerning the graphical representation of objects have been conducted. Snodgrass and Vanderwart [START_REF] Snodgrass | A standardized set of 260 pictures: norms for name agreement, image agreement, familiarity, and visual complexity[END_REF] were the pioneers towards a phase of standardization. Indeed, these authors have established an extended set of pictures (black-and-white line drawings) that have been standardized on four variables (name agreement, image agreement, familiarity, and visual complexity). In our study, we made reference to the studies of Alario and Ferrand [START_REF] Alario | A set of 400 pictures standardized for French: Norms for name agreement, image agreement, familiarity, visual complexity, image variability, and age of acquisition[END_REF] (see Fig 3), Bonin and Peereman [START_REF] Bonin | A new set of 299 pictures for psycholinguistic studies: French norms for name agreement, image agreement, conceptual familiarity, visual complexity, image variability, age of acquisition, and naming latencies[END_REF], and to some welldefined databases such as Picture Communication Symbols

III. ROBOT TEST BED

The tests proposed in this paper are based on a static interaction between the user and the robot. They can be made also with a simply computer rather than a mobile robotics platform. However, we have chosen the same to using our robot in order to not condition the participant's response. In fact the approach of people with a simple computer is totally different from the approach with a mobile robot, especially for elderly people.

A. Hardware

The robot used in this work is a Kompaï robot (Fig 1 ), a newly developed companion robot. The robot embeds all necessary sensors for autonomous navigation:

• A laser sensor for localization and obstacle detection • Ultrasound and Infrared sensors for obstacle detection • 2 Cameras The low-level control is managed by the embedded Microcontroller (Emtrion SH7780 SBC). For high-level control and user interfaces, the Kompaï uses a Tablet PC running Windows Vista.

B. Software

The robot's functionalities currently accessible by the voice-based interface are:

• Hear management (start/stop robot listening) • Shopping list, appointment, and drugs management • General information (time and date)

• Wake up • Robot movement • Send e-mails • Medical diagnostic
The management of shopping list, agenda, table of drugs and e-mails sending is made using Google API developed for .NET application. The robot movements exploit a SLAM (Simultaneous Localization and Mapping) software. The SLAM software used is based on the Karto R robotic navigation software under license from SRI International R . Our dialogue system consists of 3 different components: 1) ASR which converts audio signals of human speech into text strings and matches the results of the analysis with the known vocabulary and syntax; 2) Dialogue Manager which maps the meanings and intentions of the recognized utterances, and selects the appropriate response; 3) TTS which converts the system utterances into speech output. The Dialogue application runs under Windows Vista OS and is programmed in MRDS (Microsoft Robotics Dev Studio) platform.

IV. METHODOLOGY

A. Voice-based interface

Concerning the voice-based interface an important research question must be answered: "Which words and which syntax the robot has to use to be easily understood by the elderly?" We posit that in order to answer to this question we have to study the speaking habits of the elderly and of people suffering of mild cognitive troubles. If we understand some of these speaking patterns, we will be able to appropriately model the robot way to speak to the users. In order to build our voice-based interface, we first analyze the type of vocabulary to be employed by the users, the frequency of the words, and the syntax and semantics of sentences used. The choice of the words and of the sentence structure will be made considering the results of this work. Our current system contains a vocabulary of about 170 french words. We have selected these words with respect to the French robot capabilities and functionalities exposed in Section III. Additionally to the robot functionalities developed for our application, we have also built different syntactic models. Such an example is described below: in order to ask to the robot the date, the user can say different sentences ("Quel jour sommes-nous aujourd'hui?" 1 , "S'il vous plait, quelle est la date d'aujourd'hui?" 2 ). In the table on Fig 5 the number of words and syntactic models defined for the different robot functionalities are reported. In this table not all the words of the defined vocabulary are enumerated (only the keywords). Taking as an example the Shopping function, some defined keywords are "Ajouter", "Ecrire", "Supprimer", and "Effacer" 3 . The words as "Pomme", "Fromage", and "Tomate" 4 are not considered as keywords. The idea is that the user can also add to the system the names of products if they are not defined in the grammar, using the graphical-based interface. In order to help users to understand the sentences pronounced by the robot, we worked closely with our colleagues from Pitie-Salpetriere Hospital, specialists in psychology and cognitive neurosciences. We have learnt that the robot sentences do not have to be too long or too complex. The syntactic structure must be simple in order to avoid to the elderly a cognitive overload. One way to categorize sentences is by the clauses they contain. A simple sentence contains a single, independent clause (ex: Today you have an appointment with the doctor"); a compound contains two independent clauses that are joined by a coordinating conjunction (ex: "Today you have to meet the doctor and to call your son"); a complex sentence contains an independent clause plus one dependent clause (ex: "Today you have to meet the doctor who called you yesterday"). The language of the robot has been chosen on the basis of simple syntactic models.

B. Graphical-based interface

When we started focusing on the graphical-based interface an important research question arose: "Which type of image has to be used to make the graphical-based robot interface easily understandable and, in the same time, motivating for the elderly?". Moreover, we wanted to understand the style of image to use in the graphical-based interface for robots dedicated to the elderly: simple or rather complex, userfriendly, drawings or photos, image with or without text, colored or black-and-white images. We couldn't find the answers to these questions in the literature, considering that our image selection is specifically oriented to service robotic applications for elderly. We have consulted two existing 1 " What day is it today? " 2 " Please, what's today's date? " 3 Add, Write, Delete, Erase 4 Apple, Cheese, Tomato databases ([22], [23]) in order to select the desired images. Each image of these databases is considered by the literature as the most representative for a specific word for the majority of people. After defining the robot functionalities and services accessible from the graphical-based interface (the same functions also accessible from the voice-based interface), we have collected the related pictures. Moreover, we have also searched in other image databases (i.e., BoardMaker and Fotolia) some very similar and some very different pictures, including both drawings and photos, both colored and blackand-white images, both simple and complex images (see an example in Fig 7). We seek the best match image-word; this means that we tried to understand which type of image the patient associated as easy as possible to a given word.

V. EXPERIMENTAL DESIGN

A. Subject Pool

In order to test and validate our methodology, 11 patients were recruited from our project partner the Memory Clinic of Broca geriatric hospital in Paris (France). 6 patients participated to the tests concerning the voice-based interface: 5 females and 1 male; 3 MCI 5 , and 3 normal people all aged between 66 and 79 years. 5 patients participated to the tests concerning the graphical-based interface: 3 females and 2 males; 2 MCI and 3 normal people all aged between 67 and 74 years. The patients have signed the Informed Consent and the entire intervention has been recorded on video tape.

B. Voice-based interface -Experimental Design

First of all, we have explained to the patients the role of the robot in their everyday life. Furthermore, each patient had to achieve the following tasks / functions with the robot:

• Ask for date and time • Ask the content of the shopping list • Add something on the shopping list • Remove something from the shopping list • Request to wake up • Request the appointments of the day • Make an appointment with somebody • Ask for the next appointment with somebody With this test session, we wanted to know the syntax models and the words used frequently by the targeted patients to interact with the robot. This test was done using the "Wizard of Oz" framework.

C. Graphical-based interface -Experimental Design

For each set of proposed images, the patients had to express their preference from different points of view:

1) Which is the most consistent image to represent the object? 2) Which is the most convivial (user-friendly) image? 3) Which is the image that they would like on the robot graphical-based interface? 5 MCI=Mild Cognitive Impairment We have also proposed to the patients to write 3 words ("Bonjour" 6 , "Soleil"7 , and "Village" 8 ) with a keyboard displayed on the touch screen. We have repeated this test twice. The first time, we have proposed an azerty keyboard layout and, the second time we have proposed an alphabetical keyboard layout (i.e., the letters are placed in alphabetical order). The main goal was the comparison of the usability of the two different keywords.

VI. EXPERIMENTAL RESULTS

A. Voice-based interface

In order to understand the syntactic models and the vocabulary used by patients, we have annotated and analyzed all the sentences uttered. After analysis (annotation from videos recorded during the tests), we realized that the syntactic models and vocabulary used by elderly are totally different from those used by non-elderly adults (e.g. the use of courtesy words and the modes of expression differ considerably). For example, the most part of the young/adult French population, ask the scheduling of the day as follows:"Est-ce que j'ai des rendez-vous aujourd'hui?" 9 . The elderly say most formal sentences as "Pourriez-vous m'indiquer si j'ai des rendezvous aujourd'hui?" 10 . Between all the sentences uttered by all patients, only 44.5% coincides exactly with the sentences and with the words known by the robot, and hence used by the robot, but only 13.4% of sentences differ from the syntactic structure known by the Dialogue Manager. So, as it was predictable, the patients use very linear and simple syntax, close to the model showed in Fig 6 . Concerning the vocabulary, 42.1% of uttered sentences contain some words unknown to the robot. This means that the robot language and the elderly language don't coincide. We can conclude that in order to facilitate the use of our system to the elderly, we have to enrich the vocabulary with words employed by the target population. Moreover, in order to ameliorate further the voice-based interface we also need to add others simple syntactic models to our existing model.

B. Graphical-based interface

For our tests, we have presented 24 different sets of images to the users. Each set represents a service or an object concerning the robot. For each of these sets, each patient has chosen: 1. the most consistent image; 2. the most convivial (user-friendly) image; 3. the most adequate image for the robot graphical-based interface. Of course, for each patient the 3 choices can agree or be different. We have 120 images chosen as the most adapted to be on the robot interface (24 sets of images for 5 patients). From these 120 images, the following results have been obtained: 30 images have been chosen by the majority of people as the most consistent, 17 images have been selected as the most convivial, and 26 images have been considered at the same time the most consistent and the most convivial. The other 47 images chosen as the most adapted for the graphical-based interface have been considered neither the most consistent, nor the most convivial. This means that neither the consistency nor the conviviality is the adequate criteria to select good images for the interface (Fig 8).

Analyzing the images preferred for the graphical-based interface, we can conclude that the images have to be colored drawings (see Fig 11), with a shape close to the real object, but not necessarily similar to the database image (see Fig 10). Moreover, 75% of the users preferred a big icon without text to a smaller icon with the object name (see Fig 9). They affirmed that "A picture is worth a thousand words" and this was especially for the elderly suffering of visual impairment.

Concerning the consistency of the proposed images, we obtained a very strange result: only 21% of interviewed people have chosen the database of images as the most consistent (strange because the images of database have been considered the best representation of the objects). Much more expected is the fact that only 4.2% of people have chosen these images as the most convivial and only 10% of people would have these images on the robot graphical-based interface. In fact, the database of images is very simple and composed of black-and-white drawings. 51.33% of people have chosen a drawing as the most consistent image (48.67% have chosen a photo). For some robot functions, we have proposed the choice between an image (drawing or photo) and an icon type button. The following results have been obtained:

• Only 16% of users think that the image type button is more consistent than a photo or a drawing

• 32% believe that a button is more convivial • And 16% prefer an image type button to a photo or a drawing for the robot interface

Regarding the usability of the 2 proposed keyboards layouts (azerty and alphabetical), the performances of all patients were much better with the azerty keyboard layout. This is explained by the fact that all patients who participated to the test have a computer at home and use it regularly. The average time to write 3 words ("Bonjour" 11 , "Soleil" 12 , and "Village" 13 ) on the robot touch screen gave an average of 6s (average per word) with the azerty keyboard layout and 13.4s with the alphabetical keyboard layout. Patients were very destabilized by the alphabetical keyboard layout (see Fig 13).

We have also learnt from the medical staff that for the patients who do not have the habit of using computers, it is much better to use alphabetical keyboard layouts. For these patients (especially in the cases where the patients suffer from cognitive impairments) the classic azerty keyboard layout is very difficult and dissuading. For these reasons, we conclude that our system should offer to the user the possibility to choose the keyboard layout according to their preferences and habits.

VII. CONCLUSIONS AND FUTURE WORKS

In this paper, we proposed the study of two humanrobot interfaces -the voice-based and the graphical-based interfaces -to interact with an assistive robot designed for the elderly. Our work mainly focuses on the fission process of these two interfaces. We showed that the choice of the vocabulary, of the syntactic models, and of the images is very delicate and important so as to improve the interaction between the user and the robot. The words and the sentences used by the elderly don't usually match with the language of non-elderly adults people. The images preferred for the graphical-based interface are not the images known in the literature as the most consistent. This means that the robot has to adapt its communication channel to the elderly and the interfaces have to be customized for this type of users. Future work will first focus on the modification of the existing voice-based interface with the syntactic structures and vocabulary based on the results obtained from these tests and described in this paper. We are also developing a graphical-based interface using the images selected by patients (see an example in Fig 12). These improvements will help us to demonstrate that the processing of a target word or of a target picture is facilitated if the sound stimulus (voice) and the visual stimulus (images) are presented at the same time. In other words, if the interface between the robot and the patient is implemented on two different modalities (i.e., voice and graphical-based interfaces) the accessibility of the machine increases considerably.

  Curie-Paris6-UPMC) and Robosoft (Bidart) consuelo.granata@robosoft.com Mohamed Chetouani Institut des Systmes Intelligents et Robotique (ISIR), Université Pierre et Marie Curie-Paris6 (UPMC), 4 Place Jussieu, 75252 Paris Cedex, France mohamed.chetouani@upmc.fr Adriana Tapus, Computer Science Department Ecole Nationale Superieure de Techniques Avancees (ENSTA-ParisTech) 32 Blvd Victor, 75015, Paris, France adriana.tapus@ensta.fr Philippe Bidaud Institut des Systmes Intelligents et Robotique (ISIR), Université Pierre et Marie Curie-Paris6 (UPMC), 4 Place Jussieu, 75252 Paris Cedex, France philippe.bidaud@upmc.fr Vincent Dupourqu, Robosoft, Technopole d'Izarbel, 64210 Bidart, France vincent.doupourque@robosoft.com

Fig. 1 .

 1 Fig. 1.The mobile robot Kompa¨ı (by Robosoft-France) used for interaction.

Fig. 2 .

 2 Fig. 2.

Fig. 3 .

 3 Fig. 3. Some images from the Alario&Ferrand and Lexique databases [22], [23].

Fig. 4 .

 4 Fig. 4. 1,6 images from the BoardMaker database; 2, 3, 4, 5 images from the Fotolia database.

Fig. 5 .

 5 Fig. 5. Number of keywords and syntactic models defined in our vocalbased interface for each robot function. In the third column, number of images and buttons proposed for graphical-based interface.

Fig. 6 .

 6 Fig. 6. An example of simple syntactic model in French.

  Fig 6 is depicting a detailed example of a simple french sentence.

Fig. 7 .

 7 Fig. 7. For the function Agenda 1. Image of the Alario-Ferrand database; 2, 3, 4, 5. Chosen images from Fotolia; 6. Image selected from BoardMaker.

Fig. 8 .

 8 Fig. 8. The set of images preferred for the robot interface. The majority of these images are considered neither as the most consistent nor as the most convivial.

Fig. 9 .

 9 Fig. 9. For the function Shopping 1. Big icon without text; 2. Smaller icon including the object name.

Fig. 10 .

 10 Fig. 10. For the function WakeUp 1. Database image; 2. Preferred image for the robot graphical-based interface.

Fig. 11 .

 11 Fig. 11. Comparison between photos and drawings chose as most adequate for the interface, most consistent and convivial.

Fig. 12 .

 12 Fig. 12. Example of robot interface that we are developing following the results of the graphical-based interface.

Fig. 13 .

 13 Fig.[START_REF] Reichman | Integrated Interfaces Based on a Theory of Context and Goal Tracking[END_REF]. The average time to write the three French words "Bonjour", "Soleil", and "Village" with the two types of keyboards for each participant.

"Good morning"

"Sun"

"Village"

" Do I have any appointment today? "

" Can you tell me if I have any appointment today? "

" Good morning "

" Sun "

" Village "