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The study proposes and tests Multi-purpose Earth Orbit Navigation System (MEONS) architecture aimed at providing a proper GNSS based orbit sequential estimation strategy to override rising scenario criticalities. The activity is focused on a complete feasibility and performance assessment of the developed filtering scheme by testing it on a high fidelity LEO-MEO autonomous transfer simulation. Stability and navigation accuracy of the proposed MEONS architecture is thus verified.

GPS relative navigation filters could benefit notably from an accurate modeling of the ionospheric delays, especially over large baselines (>100 km) where double difference delays can be higher than several carrier wavelengths. This paper analyzes the capability of ionospheric path delay models proposed for spaceborne GPS receivers in predicting both zero-difference and double difference ionospheric delays. We specifically refer to relatively simple ionospheric models, which are suitable for real-time filtering schemes. Specifically, two ionospheric delay models are evaluated, one assuming an isotropic electron density and the other considering the effect on the electron density of the Sun aspect angle. The prediction capability of these models is investigated by comparing predicted ionospheric delays with measured ones on real flight data from the Gravity Recovery and Climate Experiment mission, in which two satellites fly separated of more than 200 km. Results demonstrate that both models exhibit a correlation in the excess of 80% between predicted and measured double-difference ionospheric delays. Despite its higher simplicity, the isotropic model performs better than the model including the Sun effect, being able to predict double differenced delays with accuracy smaller than the carrier wavelength in most cases. The model is thus fit for supporting integer ambiguity fixing in real-time filters for relative navigation over large baselines. Concerning zero-difference ionospheric delays, results demonstrate that delays predicted by the isotropic model are highly correlated (around 90%) with those estimated using GPS measurements. However, the difference between predicted and measured delays has a root mean square error in the excess of 30 cm. Thus, the zero-difference ionospheric delays model is not likely to be an alternative to methods exploiting carrier-phase observables for cancelling out the ionosphere contribution in single-frequency absolute navigation filters.

This paper aims to investigate the performance of stereoradargrammetric methods applied to spaceborne monostatic-bistatic synthetic aperture radar (SAR) data for digital elevation model (DEM) generation. Stereoradargrammetric techniques for robust DEM generation were successfully experienced on monostatic repeat-pass SIR-A, SIR-B, SIR-C/X-SAR, ERS1/2, JERS-1, and Radarsat data. However, novel configurations achievable by modern spacecraft flying in formation will allow for the attainment of very large baselines between the antennas in a single-pass bistatic geometry so that the height determination accuracy can benefit from both stereo effect and simultaneous acquisition. Five models for relief reconstruction by monostatic-bistatic SAR stereoradargrammetry are presented, and an error budget is assessed for each of them. Results of the sensitivity analysis exhibit metric accuracy, and therefore, the technique could be applied for height reconstruction as a methodology complementary to SAR interferometry.

This paper discusses the performance of a novel millimeter wave radar sensor for UAS autonomous indoor operations. A sensor based on the interferometric Synthetic Aperture Radar (SAR) principle should be able to satisfy the stringent requirements set by indoor operations in terms of high-resolution three-dimensional mapping and autonomous navigation. A solution based on a Frequency Modulated Continuous Wave (FMCW) technology is proposed. The outlined nominal configuration serves as a basis to analyze the performance of the sensor in possible operational scenarios by means of a software simulator. Four different focusing algorithms are implemented and tested and capabilities of the novel sensor are finally discussed, demonstrating that high-resolution, high-quality observation of an assigned control volume can be achieved.

The paper focuses on similarities and difference between adaptive threshold and sub-look processing in ship detection by Synthetic Aperture Radar (SAR). The main goal is to formalize the cascade application of both methods to achieve an overall reduction of the false alarm rate. The methods are tested on TerraSAR-X stripmap SAR data collected over the Gulf of Naples, Italy. Input parameters are set considering vessel traffic statistics in the area. Preliminary results confirm the capability of both the methods to attain very high detection rate (with zero missed detections in the considered case), and show that the synergic use of the methods reduces the number of false alarms, with respect to application of a single method.

The paper investigates different solutions for ionospheric delay handling in high accuracy long baseline relative positioning by Carrier-Phase Differential GPS (CDGPS). Standard literature approaches are reviewed and the relevant limitations are discussed. Hence, a completely ionosphere-free approach is proposed, in which the differential ionospheric delays are cancelled out by combination of dual frequency GPS measurements. The performance of this approach is quantified over real-world spaceborne GPS data made available by the Gravity Recovery and Climate Experiment (GRACE) mission and compared to the standard solution.

This paper presents a novel millimeter wave radar sensor for UAS applications, in particular autonomous navigation in indoor GPS-denied environment. The sensor is aimed at both navigation with obstacle detection and highresolution 3D mapping with moving target detection. Poor visibility due to dust, fog, smoke or flames often causes failure of state-of-the-art sensors for UAS, which are very sensitive to environmental conditions. On the contrary, a sensor based on the Interferometric Synthetic Aperture Radar (InSAR) principle has been identified as potential candidate to satisfy stringent requirements set by indoor autonomous operations. Main features of the architectural solution based on frequency-modulated continuous wave (FMCW) scheme and millimeter-wave technology are discussed. New procedures for system design are outlined and a set of nominal values for the system are provided. Finally, a software simulator, developed in order both to demonstrate that high-resolution, high-quality observation of an assigned control volume can be achieved and to assess mapping capabilities, is presented.

Long gravity waves or swell dominating the sea surface is known to be very useful to estimate seabed morphology in coastal areas. The paper reviews the main phenomena related to swell waves propagation that allow seabed morphology to be sensed. The linear dispersion is analysed and an error budget model is developed to assess the achievable depth accuracy when Synthetic Aperture Radar (SAR) data are used. The relevant issues and potentials of swell-based bathymetry by SAR are identified and discussed. This technique is of particular interest for characteristic regions of the Mediterranean Sea, such as in gulfs and relatively close areas, where traditional SAR-based bathymetric techniques, relying on strong tidal currents, are of limited practical utility.

Small autonomous unmanned aerial systems (UAS) could be used for indoor inspection in emergency missions, such as damage assessment or the search for survivors in dangerous environments, e.g., power plants, underground railways, mines and industrial warehouses. Two basic functions are required to carry out these tasks, that is autonomous GPS-denied navigation with obstacle detection and high-resolution 3D mapping with moving target detection. State-of-the-art sensors for UAS are very sensitive to environmental conditions and often fail in the case of poor visibility caused by dust, fog, smoke, flames or other factors that are met as nominal mission scenarios when operating indoors. This paper is a preliminary study concerning an innovative radar sensor based on the interferometric Synthetic Aperture Radar (SAR) principle, which has the potential to satisfy stringent requirements set by indoor autonomous operation. An architectural solution based on a frequency-modulated continuous wave (FMCW) scheme is proposed after a detailed analysis of existing compact and lightweight SAR. A preliminary system design is obtained, and the main imaging peculiarities of the novel sensor are discussed, demonstrating that high-resolution, high-quality observation of an assigned control volume can be achieved.

The ionosphere is commonly divided into the portion below (bottomside) and above (topside) the region at which peak values of electron density occur. Topside ionospheric modeling is a challenging problem because of the limited data available. Indeed, the more intense peak ionization region, or bottomside ionosphere, dominates the effects observable from ground stations. High-altitude ionosondes, such as sounding rockets, have been traditionally used for direct sounding only of the higher ionospheric layers. Nowadays, signals of opportunity exist for sounding the ionosphere with no dedicated ionosondes. With the continuous deployment of GPS receivers on board spacecraft for positioning, indirect sounding of the topside ionosphere using navigation signals can be performed. This paper reviews geometric-based models allowing to infer the total electron content of the topside ionosphere from spacecraft GPS measurements.

The present paper reports on some insights in the use of Synthetic Aperture Radars (SAR) for bathymetric data retrieval by exploiting variation in swell wave parameters approaching the shoreline. Since SAR signals are unable to penetrate sea surface and to reach seabed, echoed signals from sea surface are used to investigate underwater bathymetry in coastal area. A suitable data processing methodology is proposed to properly detect swell shoaling and refraction phenomena, and to accurately measure swell parameters. The described methodology is tested by using ALOS L-band SAR images over the Gulf of Naples, Italy. A Digital Elevation Model (DEM) of the investigated area is thus obtained. Results are consistent with the values reported in the Official Nautical Chart provided by the Italian Navy Hydrographic Institute.

This paper deals with the relative navigation of a formation of two spacecrafts separated by hundreds of kilometers based on processing dual-frequency differential carrier-phase GPS measurements. Specific requirements of the considered application are high relative positioning accuracy and real-time on board implementation. These can be conflicting requirements. Indeed, if on one hand high accuracy can be achieved by exploiting the integer nature of double-difference carrier-phase ambiguities, on the other hand the presence of large ephemeris errors and differential ionospheric delays makes the integer ambiguities determination challenging. Closed-loop schemes, which update the relative position estimates of a dynamic filter with feedback from integer ambiguities fixing algorithms, are customarily employed in these cases. This paper further elaborates such approaches, proposing novel closed loop techniques aimed at overcoming some of the limitations of traditional algorithms. They extend techniques developed for spaceborne long baseline relative positioning by making use of an on-the-fly ambiguity resolution technique especially developed for the applications of interest. Such techniques blend together ionospheric delay compensation techniques, nonlinear models of relative spacecraft dynamics, and partial integer validation techniques. The approaches are validated using flight data from the Gravity Recovery and Climate Experiment (GRACE) mission. Performance is compared to that of the traditional closed-loop scheme analyzing the capability of each scheme to maximize the percentage of correctly fixed integer ambiguities as well as the relative positioning accuracy. Results show that the proposed approach substantially improves performance of the traditional approaches. More specifically, centimeter-level root-mean square relative positioning is feasible for spacecraft separations of more than 260 km, and an integer ambiguity fixing performance as high as 98% is achieved in a 1-day long dataset. Results also show that approaches exploiting ionospheric delay models are more robust and precise of approaches relying on ionospheric-delay removal techniques.

The paper presents a method to discriminate ship targets from sea background in focused Single-Look Complex (SLC) Synthetic Aperture Radar (SAR) images. The method is based on Doppler Centroid analysis and it is able to generate an estimate ship velocity. Experimental results are presented showing the application of the method to TerraSAR-X data of the Gulf of Naples, Italy.

Surface current variations generated by underwater bottom topography produce intensity modulations in SAR images. The direct inversion from image intensity to bathymetry is difficult, in general, because of the theoretical complexity of the involved physical mechanisms and the practical consideration that most of the required parameters cannot be derived from SAR data. The conventional approach for SAR bathymetry relies on the forward mechanism, i.e. the simulation of SAR images from (partially) known bottom topography and, then, the adjustment of the bathymetry through iterative comparison between simulated and collected images. The present paper deals with the development of a bathymetric SAR algorithm able to perform the direct inversion limiting the need for a-priori information or in-situ measurements, and for human intervention in the processing chain. The proposed approach is tested on COSMO-SkyMed data collected over coastal regions in the Gulf of Naples, showing that dense coverage and metric accuracy can be achieved even when the current is not enough strong to dominate SAR response.

This paper analyzes relative orbit design for multi-satellite radar missions aimed at multistatic SAR tomography. To this end, formation requirements and performance parameters are derived by adapting existing models for SAR tomography to single pass techniques. Then, relative trajectory design is carried out on the basis of an analytical relative motion model including secular J 2 effects. By properly scaling the differences in orbital parameters, different formation geometries enable uniform sampling of the effective baseline along the whole orbit. The difference among the possible choices lies in latitude coverage, formation stability, and collision avoidance aspects. A numerical example of relative trajectory design is discussed considering L-band as operating frequency. In particular, achievable height resolution and unambiguous height along the orbit are pointed out for a multi-cartwheel, a multi-pendulum, and a multi-helix formation. In view of future implementation of a multi-satellite SAR tomography mission, new concepts aimed at the reduction of required satellites, and long term evolution of designed formations, are also discussed.

This paper presents a technique for on-the-fly rejection of GPS data outliers. It is particularly suited to applications where a high accuracy navigation solution is needed in real time, as for the relative positioning of satellites in Low Earth Orbit. The proposed technique relies on two metrics which screen the residual ranging errors in the Zero-Difference and Single-Difference pseudorange measurements with respect to a prefixed threshold. Preliminary tests carried out on flight data from the Gravity Recovery and Climate Experiment mission demonstrate the effectiveness of the proposed approach.

This paper deals with the problem of real-time onboard relative positioning of low-Earth-orbit spacecraft over long baselines using the Global Positioning System. Large intersatellite separations, up to hundreds of kilometers, are of interest to multistatic and bistatic synthetic-aperture radar applications, in which highly accurate relative positioning may be required in spite of the long baseline. To compute the baseline with high accuracy, the integer nature of dualfrequency, double-difference carrier-phase ambiguities can be exploited. However, the large intersatellite separation complicates the integer-ambiguities determination task due to the presence of significant differential ionospheric delays and broadcast ephemeris errors. To overcome this problem, an original approach is proposed, combining an extended Kalman filter with an integer least-square estimator in a closed-loop scheme, capable of fast on-the-fly integer-ambiguities resolution. These integer solutions are then used to compute the relative positions with a singleepoch kinematic least-square algorithm that processes ionospheric-free combinations of debiased carrier-phase measurements. Approach performance and robustness are assessed by using the flight data of the Gravity Recovery and Climate Experiment mission. Results show that the baseline can be computed in real time with decimeter-level accuracy in different operating conditions.

Remote sensing application of Galileo upcoming constellation in the field of civil security is preliminarily analyzed, defining low resolution (25 m) and high resolution (7.5 m) working modes for a bistatic Synthetic Aperture Radar system utilizing Galileo satellites as transmitters and Unmanned Aerial Systems as receivers. Simulations offshore Somali coast and in a South Mediterranean Sea region showed that both low and high resolution acquisitions are feasible. For the low resolution mode a probability of successful image formation no matter the azimuth position of the receiving UAS has been estimated at nearly 100%. Whereas, for the high resolution mode it decreases to about 90%, which, if deemed unsatisfactory for the application, leads to the need for UAS to adapt its route to the actual Galileo satellite coverage.

A simplified algorithm for SAR-based bathymetry is presented able to measure surface current variations generated by the bottom topography from SAR intensity images and to reduce the need for both a-priori information and human-inthe- loop operations. The algorithm is first analyzed from a theoretical point of view and an error budget model is developed to estimate the achievable depth accuracy as a function of the uncertainty in the input parameters. Preliminary experimental results are also presented in which the algorithm is applied to both COSMO-SkyMed and ALOS images of the Gulf of Naples. The results show that the technique has the potential to generate depth measures that are significantly denser than those commonly reported in the nautical charts.

This paper deals with star tracker algorithms validation based on star field scene simulation and hardware-in-the-loop test configuration. A laboratory facility for indoor tests, based on the simulation of star field scenes, is presented. Attainable performance is analyzed theoretically for both static and dynamic simulations. Also, a test campaign is presented, in which a star sensor prototype with real-time, fully autonomous capability is exploited. Results that assess star field scene simulation performance and show the achievable validation for the sensor algorithms and performance in different operating modes (autonomous attitude acquisition, attitude tracking, and angular rate-only) and different aspects (coverage, reliability, and measurement performance) are discussed.

This paper presents the idea of a hybrid bistatic Synthetic Aperture Radar (SAR) formed by a constellation of spaceborne illuminators and receiving-only airborne SARs. The study aims at identifying and developing new remote sensing techniques, able to produce forward-looking, real-time, any-weather, day and night, high geometric and radiometric resolution data for Unmanned Airborne Systems (UAS) autonomous navigation, namely obstacle detection and landing. Special emphasis is devoted to potentials and limitations of using Galileo or a Galileo improved constellation as illuminator. Preliminary feasibility assessment and definition of strawman concepts conducted under ESA contract in the framework of future integrated aerospace systems are reported, consisting in performance and trade-off studies of two configurations of the space segment and of two operating modes of the space–airborne bistatic SAR.

This paper describes a carrier-phase differential GPS approach for real-time relative navigation of LEO satellites flying in formation with large separations. These applications are characterized indeed by a highly varying number of GPS satellites in common view and large ionospheric differential errors, which significantly impact relative navigation performance and robustness. To achieve high relative positioning accuracy a navigation algorithm is proposed which processes double-difference code and carrier measurements on two frequencies, to fully exploit the integer nature of the related ambiguities. Specifically, a closed-loop scheme is proposed in which fixed estimates of the baseline and integer ambiguities produced by means of a partial integer fixing step are fed back to an Extended Kalman Filter for improving the float estimate at successive time instants. The approach also benefits from the inclusion in the filter state of the differential ionospheric delay in terms of the Vertical Total Electron Content of each satellite. The navigation algorithm performance is tested on actual flight data from GRACE mission. Results demonstrate the effectiveness of the proposed approach in managing integer unknowns in conjunction with Extended Kalman Filtering, and that centimeter-level accuracy can be achieved in real-time also with large separations.

This paper focuses on a hardware-in-the-loop facility aimed at real-time testing of architectures and algorithms of multisensor sense and avoid systems. It was developed within a research project aimed at flight demonstration of autonomous non-cooperative collision avoidance for Unmanned Aircraft Systems. In this framework, an optionally piloted Very Light Aircraft was used as experimental platform. The flight system is based on multiple-sensor data integration and it includes a Ka-band radar, four electro-optical sensors, and two dedicated processing units. The laboratory test system was developed with the primary aim of prototype validation before multi-sensor tracking and collision avoidance flight tests. System concept, hardware/software components, and operating modes are described in the paper. The facility has been built with a modular approach including both flight hardware and simulated systems and can work on the basis of experimentally tested or synthetically generated scenarios. Indeed, hybrid operating modes are also foreseen which enable performance assessment also in the case of alternative sensing architectures and flight scenarios that are hardly reproducible during flight tests. Real-time multisensor tracking results based on flight data are reported, which demonstrate reliability of the laboratory simulation while also showing the effectiveness of radar/electro-optical fusion in a non-cooperative collision avoidance architecture.

Bistatic Synthetic Aperture Radar represents an active research and development area in radar technology. In addition, Bistatic and Multistatic SAR concepts are tightly related to formation flying and distributed space missions that also represent the new space-based remote sensing and surveillance frontiers. This chapter introduces Bistatic SAR, in particular by comparing its peculiarities, operation and performance with respect to conventional monostatic SAR. Some basic concepts of bistatic SAR image formation and the main elements of bistatic SAR geometry are preliminary presented. Performance parameters are then analyzed, including geometric resolution, radiometric resolution and bistatic radar equation. Special emphasis is placed on analytical methods to evaluate the effects of bistatic SAR geometry on image resolution. Further implementation issues, such as footprint, time and phase synchronization are also pointed out. The analysis of past bistatic radar and bistatic SAR experiments and proposed spaceborne bistatic SAR missions supplies essential information to understand how these issues have been faced and can be potentially solved in ongoing and future operational systems. Finally, several scientific applications of bistatic SAR are outlined taking advantages of different techniques and methods.

Microwave signals emitted by SAR are not able to penetrate sea surface and to reach seabed. Nonetheless bathymetric measurements can be retrieved from SAR images and potentially used to support the monitoring of geomorphological risks in coastal areas and marine pollution. This paper presents the preliminary results of SAR4BAT project, aimed at the development of SAR-based products for coastal area bathymetry. (6 pages)

Maritime surveillance problems are drawing the attention of multiple institutional actors. National and international security agencies are interested in matters like maritime traffic security, maritime pollution control, monitoring migration flows and detection of illegal fishing activities. Satellite imaging is a good way to identify ships but, characterized by large swaths, it is likely that the imaged scenes contain a large number of ships, with the vast majority, hopefully, performing legal activities. Therefore, the imaging system needs a supporting system which identifies legal ships and limits the number of potential alarms to be further monitored by patrol boats or aircrafts. In this framework, spaceborne Synthetic Aperture Radar (SAR) sensors, terrestrial AIS and the ongoing satellite AIS systems can represent a great potential synergy for maritime security. Starting from this idea the paper develops different designs for an AIS constellation able to reduce the time lag between SAR image and AIS data acquisition. An analysis of SAR-based ship detection algorithms is also reported and candidate algorithms identified.

This paper focuses on a prototypical test system aimed at estimating field performance of integrated aircraft navigation systems. Assumed design requirements for the navigation system are cm-level positioning accuracy and absolute attitude estimation error in dynamic conditions of the order of 0.1° at most. Since performance estimation does not require real time operation, the test system is based on offline data processing and analysis. It implements the concept of GPS-aided inertial navigation making use of carrier phase differential GPS (with a fixed ground antenna as base station) and a multiple antenna configuration onboard the moving vehicle. Accuracy and availability of the GPS differential solution benefit from the fact that the solution is not required in real time as in the case of Real Time Kinematic implementations. System architecture and the adopted algorithms are described in the paper, and the results from first experimental tests are discussed. Though obtained in ground tests by means of a partial test setup, they show the potential of the proposed approach.

Starting from the reborn international interest in lunar exploration and the widely documented need for hyper-accurate measurements of Earth crustal dynamics, at a high revisit frequency and on a global scale, this paper presents the idea of a Moon-based interferometric synthetic aperture radar. After introducing models to describe synthetic antenna formation and discussing key issues of a Moon-based radar for Earth remote sensing, a preliminary system design and performance analysis are conducted. Quantitative results are presented in terms of achievable resolutions and needed radar parameters. Although the Moon-based observatory requires technical solutions and a budget that make likely its realization only in the far term, a comparison with currently planned spaceborne interferometric systems shows that it offers height measurement accuracies at a level and with a frequency not achievable otherwise.

This paper deals with the GPS-based relative navigation of LEO formations. Specifically, we consider applications characterized by two co-flying satellites with a large and highly variable separation, which are relevant to next generation monostatic/bistatic Synthetic Aperture Radar missions. In these applications, both scientific goals and control needs require the determination of the relative state with high accuracy. To this end, an Extended Kalman Filter is developed that processes double-difference pseudorange and carrier phase measurements on L1 and L2 frequencies. To preserve accuracy and robustness of the integer solution problem against large variations of the baseline, an original approach is developed in which, for each receiver, the Vertical Total Electron Content is included in the filter state. In addition, the double-difference ambiguities are re-estimated by the filter at each time step. A major technical problem of a filter processing double-differences is re-organizing the filter state when the pivot satellite changes. This is solved by an original and effective procedure that speeds up the filter convergence. Once the floating point estimates of the double-difference ambiguities have been produced by the dynamic filter, their integer values are extracted with the Least-Square Ambiguity Decorrelation Adjustment method and processed within a kinematic filter to estimate the relative position with high accuracy. Filter robustness and performance are evaluated by means of Monte Carlo simulations performed on the reference orbital scenario identified within the Italian SABRINA mission study. Results show that the integer ambiguities are always resolved, allowing to achieve a centimeter-level accuracy in all the simulated conditions.

Performance analysis of Bistatic Synthetic Aperture Radar (SAR) characterized by arbitrary geometric configurations is usually complex and time-consuming since system impulse response has to be evaluated by bistatic SAR processing. This approach does not allow derivation of general equations regulating the behaviour of image resolutions with varying the observation geometry. It is well known that for an arbitrary configuration of bistatic SAR there are not perpendicular range and azimuth directions, but the capability to produce an image is not prevented as it depends only on the possibility to generate image pixels from time delay and Doppler measurements. However, even if separately range and Doppler resolutions are good, bistatic SAR geometries can exist in which imaging capabilities are very poor when range and Doppler directions become locally parallel. The present paper aims to derive analytical tools for calculating the geometric resolutions of arbitrary configuration of bistatic SAR. The method has been applied to a hybrid bistatic Synthetic Aperture Radar formed by a spaceborne illuminator and a receiving-only airborne forward-looking Synthetic Aperture Radar (F-SAR). It can take advantage of the spaceborne illuminator to dodge the limitations of monostatic FSAR. Basic modeling and best illumination conditions have been detailed in the paper.

This paper addresses the problem of determining the baseline of formation flying satellites using carrier-based differential GPS. Specifically, a dynamic-filtering scheme relying on an Extended Kalman Filter is proposed to determine the baseline with high accuracy in space applications characterized by a high variability of the intersatellite distance. These applications cover a broad range of space missions, from in-orbit servicing to Earth remote sensing. In the paper, a filtering scheme is proposed constituting a unique environment in which either single or dual frequency double-differenced pseudorange and carrier-phase measurements can be filtered out to determine in real time the relative position. Filter performance with single and dual frequency GPS receivers is numerically analysed considering orbital scenarios relevant to the considered applications. Results demonstrate filter robustness and capability of providing real-time baseline determination at the sub-centimetre level.

This paper deals with the real-time onboard accurate relative positioning by carrier-phase differential GPS (CDGPS) of LEO formations with baselines of hundreds of kilometres. On long baselines, high accuracy can be achieved only using dual-frequency measurements and exploiting the integer nature of double difference (DD) carrier-phase ambiguities. However, large differential ionospheric delays and broadcast ephemeris errors complicate the integer resolution task. This paper is concerned with analysing possible approaches to DD ionospheric delays compensation in such applications. The first formulation models differential ionospheric delays as a function of the vertical electron content above the receivers, whereas the second one is based on combining DD measurements for removing ionospheric delays from the observation model. The effectiveness of the developed solutions is assessed by comparing the relative positioning accuracy that can be obtained on flight data. Results show that modelling the delays is advantageous for relative positioning only in mild ionospheric conditions.

A dynamic filtering scheme for carrier-based differential GPS relative navigation of formation flying satellites is described. It relies on an augmented-state Extended Kalman Filter processing double-difference GPS observables and using a non linear model of the relative dynamics. Filter performance is specifically evaluated for formations where the inter-satellite distance largely varies during the mission. The filter provides estimation accuracy ranging from the millimetre to the centimetre level as the baseline grows from a few kilometres to 200 km.

This paper presents and approach for real-time onboard relative positioning of two spacecraft separated of hundreds of kilometers with centimeter-level accuracy. The approach extends techniques developed for long baseline terrestrial relative positioning to spaceborne applications, making use of an on-the-fly ambiguity resolution technique especially developed for the applications of interest. Such technique blends together parametric models of the ionospheric delay, nonlinear models of relative spacecraft dynamics, and partial integer validation techniques, while being capable to comply with typical requirements for real-time on board operations. The approach is validated on flight data from the Gravity Recovery and Climate Experiment (GRACE) mission. Centimeter-level root-meansquare relative positioning is proved to be feasible for spacecraft separations exceeding 260 kilometers. Results show that in a one-day-long dataset 98% of integer ambiguities are fixed almost instantaneously, and less than 2% of these ambiguities are erroneous.

This paper presents the idea of a hybrid bistatic synthetic aperture radar (SAR) formed by a constellation of spaceborne illuminators and receiving-only airborne forward-looking SARs. The study aims at identifying and developing new imaging techniques for avoidance, landing, and observation, able to guarantee forward-looking real-time, all-weather, day and night, high geometric and radiometric resolution images for Unmanned Airborne Systems (UAS). Nowadays, UAS Navigation is mainly performed with satellite navigation systems (e.g. GPS, Galileo) which offer accurate and reliable navigation data, but no information about the surrounding environment. A vision-based navigation system can greatly improve UAS autonomy with an additional beneficial impact on obstacle avoidance capability. SAR systems have been already experienced on-board aircrafts of different classes. However, such sensors have been only used as remote sensing payloads in side-looking geometry. For navigation purposes a forward looking geometry is preferable, but it has been rarely experienced due to major limitations (left/right ambiguity and poor Doppler resolution) which can be only partially mitigated at the cost of strong complexities. These limitations can be overcome by the proposed space airborne bistatic system provided that specific requirements on the acquisition geometry are satisfied. In addition, the airborne receiver can be much more compact and lightweight, with a reduced power request thus complying with typical UAS requirements. Preliminary feasibility assessment and definition of strawman system concept have been conducted under ESA contract (22499/09/F/MOS) considering UAS vision-based navigation as main application even if further uses, e.g. remote sensing or surveillance, have been envisaged.

This paper analyzes relative orbit design for multi-satellite radar missions aimed at multistatic SAR tomography. To this end, formation requirements and performance parameters are derived by adapting existing models for SAR tomography to single pass techniques. Then, relative trajectory designed is carried out on the basis of an analytical relative motion model including secular J2 effects. By properly scaling the differences in orbital parameters, different formation geometries enable uniform sampling of the effective baseline along the whole orbit. The difference among the possible choices lies in latitude coverage, formation stability, and collision avoidance aspects. A numerical example of relative trajectory design is discussed considering L-band as operating frequency. In particular, achievable height resolution and unambiguous height along the orbit are pointed out for a multi-cartwheel, a multi-pendulum, and a multi-helix formation.

This paper focuses on the autonomous real-time relative navigation of LEO satellite formations. Specifically, a novel closed loop approach which integrates an Extended Kalman Filter with an Integer Least Squares estimator is presented in which double differenced code and carrier measurements on two frequencies are processed to get accurate relative positioning. Real-world GPS measurements from the GRACE mission are used for assessing the positioning algorithm performance. Results demonstrate that the approach is suitable for real-time relative positioning with a centimeter-level accuracy.