Stephen Hobbs, Cathryn Mitchell, Biagio Forte, Rachel Holley, Member, IEEE, Boris Snapir, and Philip Whittaker
Abstract—Geosynchronoussyntheticapertureradar (GEO SAR) has been studied for several decades but has not yet been implemented. This paper provides an overview of mission design, describing significant constraints (atmosphere, orbit, temporal stability of the surface and atmosphere, measurement physics, and radar performance) and then uses these to propose an approach to initial systemdesign. The methodology encompasses all GEO SAR mission concepts proposed to date. Important classifications of missions are: 1) those that require atmospheric phase com- pensation to achieve their design spatial resolution; and 2) those that achieve full spatial resolution without phase compensation. Means of estimating the atmospheric phase screen are noted, including a novel measurement of the mean rate of change of the atmospheric phase delay, which GEO SAR enables. Candidate mission concepts are described. It seems likely that GEO SAR will be feasible in a wide range of situations, although extreme weather and unstable surfaces (e.g., water, tall vegetation) prevent 100% coverage. GEO SAR offers an exciting imaging capability that powerfully complements existing systems.
The paper addresses this issue by suggesting a SAR sys- tem utilizing a reflector in conjunction with a digital feed array. Keeping future follow-up systems for the German TerraSAR-X and TanDEM-X SAR satellites in mind, the reflector system will be designed for X-band operation with performance requirement possibly exceeding those of HRWS . In this paper empha- sis will be given to the various operation modes and the perfor- mance; the antenna design is detailed in , while  elab- orates on the performance improvement using dedicated DBF techniques and  addresses the issue of imaging gap removal by varying the pulse repetition frequency (PRF ).
II. MULTIDIMENSIONAL WAVEFORM ENCODING The HRWS system concept assumes a wide area illu- mination by a separate transmit antenna. This enables an independent electrical design and optimization of the transmit and receive paths, but it requires also the accom- modation of an additional antenna on the spacecraft and reduces the flexibility to operate the radarsystem in differ- ent SAR imaging modes like ultra-wide-swath ScanSAR, high SNR spotlight, or new hybrid modes to be discussed later. It is hence worth to consider also the application of digital beamforming techniques in radar systems that use the same antenna array for both the transmission and re- ception of radar pulses, thereby taking advantage of al- ready existing space-qualified T/R module technology. Since the high-resolution wide-swath SAR imaging capa- bility is essentially based on a large antenna array, this poses in turn the question of how to distribute the signal energy on the ground. The trivial solution would be ampli- tude tapering, or as an extreme case, the use of only a part of the antenna for signal transmission, but this causes a significant loss of efficiency. Another possible solution is phase tapering, but the derivation of appropriate phase coefficients is an intricate task which requires in general complicated numerical optimization techniques.
A big challenge for future spaceborne remote sensing missions is now turning to the estimation and long-term monitoring of dynamic processes in the Earth’s environmental system, such as deformation events, forest and biomass change, and ocean surface cur- rents. The German Aerospace Center (DLR) is investigating an innovative single-pass interferometric and fully polarimetric L-band radar mission, named Tandem-L, which ex- ploits innovative high-resolution wide swath SAR modes, together with the use of large bandwidths, high pulse repetition frequencies, and multiple acquisition channels, result- ing in an achievable swath width of about 350 km on ground. Such an increase in term of coverage has as a main drawback the generation of a huge amount of onboard data, which is of around 8 Terabytes per day. One of the proposed solutions to reduce the resulting onboard data reduction suggests to perform a complex onboard processing (i.e. an onboard interpolation, low-pass filtering and decimation) and allows a data reduction up to 50% . On the other hand, the onboard computational memory required for the data reduction processing is at the limit of the hardware components, leading to high energy consumption. Moreover, the practical realization of the technique is very complex, including many specific coefficients which must be correctly selected during acquisition. The research of alternative solutions is therefore of great interest in order to have differ- ent options to choose for the mission development, which motivates the present master thesis. In this work, a data reduction strategy based on Linear Predictive Coding (LPC) is investigated in the context of Tandem-L. The method has been designed to reduce the complexity as much as possible while achieving a certain data reduction. A mathemati- cal formulation for the novel technique is an interesting goal for understanding in which situation the present method can be more or less efficient. The resulting performance has been verified through Monte Carlo simulations in order to evaluate the solution under different aspects (i.e. final performance versus resulting system complexity). Moreover, other complications introduced by the Tandem-L system such as the presence of missing samples (so-called gaps) during the acquisition, have been investigated and successfully solved through novel coding strategies.
The HRWS system concept assumes a wide area illu- mination by a separate transmit antenna. This enables an independent electrical design and optimization of the transmit and receive paths, but it requires also the accom- modation of an additional antenna on the spacecraft and reduces the flexibility to operate the radarsystem in differ- ent SAR imaging modes like ultra-wide-swath ScanSAR, high SNR spotlight, or new hybrid modes to be discussed later. It is hence worth to consider also the application of digital beamforming techniques in radar systems that use the same antenna array for both the transmission and re- ception of radar pulses, thereby taking advantage of al- ready existing space-qualified T/R module technology. Since the high-resolution wide-swath SAR imaging capa- bility is essentially based on a large antenna array, this poses in turn the question of how to distribute the signal energy on the ground. The trivial solution would be ampli- tude tapering, or as an extreme case, the use of only a part of the antenna for signal transmission, but this causes a significant loss of efficiency. Another possible solution is phase tapering, but the derivation of appropriate phase coefficients is an intricate task which requires in general complicated numerical optimization techniques.
the receive array height. In this system operation, a severe nonuniform sampling will not occur due to the narrow PRF range. In the case that a wide range of PRF is considered, the degree of nonuniform sampling will be significant and reach the maximum at the highest PRF. In the designed system, it is assumed that all receive channels have the identical noise figure of 3.75 dB and a system loss of 3 dB. The required peak and average power of this system are higher than those of the TerraSAR-X system (2-kW peak) in order to achieve the desired SNR over the 100-km swath width. As a rule, the sampling frequency of 275 MHz includes a 10% guard band, and then, the pulse length of 150 μs leads to 41 250 subcar- riers with the 6.67-kHz subcarrier spacing. In this example, we assume that the ICI due to instantaneous Doppler shift is compensated by the method introduced in . Regarding the undersampling for HRWS SAR imaging in the MIMO SAR, the reconstruction algorithm  recovers the original Doppler spectrum prior to the Doppler compensation. This approach has been used for frequency-modulated continuous-wave (FMCW) SAR with DBF in  and also for the very high resolution SAR data processing . Therefore, the ICI issue is not included in the performance estimation in Section VI-A. To improve the computation speed of DFT/inverse DFT, one can select a number of subcarriers that are equal to a power of two. This MIMO SAR antenna is composed of six panels in azimuth and 42 receive subarrays in elevation in each panel. The receive subarray consists of three X-band radiators in elevation and azimuth, respectively. Fig. 12 depicts the MIMO SAR antenna configuration and geometric parameters in this example design.
The relativistic phase and time offsets from this paper are not only of high importance for DEM generation with a formation flying SAR cross-track interferometer. Formations with multiple satellites have also been suggested for a wide range of further remote sensing applications, ranging from along-track interferometry for moving object and ocean current measurements over sparse aperture ambiguity suppression and super resolution for enhanced high-resolution wide-swath SAR imaging up to single-pass SAR tomography for vertical struc- ture measurements –. Due consideration of relativistic effects from varying along-track baselines is again of essential importance for these advanced bistatic and multistatic SAR systems to avoid mutual range and phase offsets between the received SAR signals. The phase accuracy requirements for the combination of the different receiver signals are typically in the order of 1° or a few degrees. For comparison, an along-track baseline of 100 m causes in an X-band system a relativistic phase shift in the order of several tens of degrees. Future multistatic SAR satellite missions should therefore take into account relativistic effects in the design of the radar synchronization system and/or the SAR processor to avoid a possible performance loss.
Stephen HOBBS 1 * & Joan Pau SANCHEZ 1
1 Cranfield University, Cranfield, Bedford MK43 0AL, UK
Received January 1, 2016; accepted January 1, 2016; published online January 1, 2016
Abstract This study is inspired by the Laplace orbit plane property of requiring minimal station-keeping and therefore its potential use for long-term geosynchronoussyntheticapertureradar (GEOSAR) imaging. A set of GEOSAR user requirements is presented and analysed to identify significant mission requirements. Imaging geometry and power demand are assessed as a function of relative satellite speed (which is determined largely by choice of orbit inclination). Estimates of the cost of station-keeping as a function of orbit inclination and right ascension are presented to compare the benefits of different orbit choices. The conclusion is that the Laplace plane (and more generally, orbits with inclinations up to 15 ◦ ) are attractive choices for GEOSAR.
The G-CLASS orbit inclination and eccentricity are defined so that the satellite remains outside a box 400 km in size radially and north-south. The satellite thus remains about 300 km away from the geostationary ring at all times (Fig. 1). The inclination and eccentricity must be phased correctly to achieve this: the resulting projection of the relative orbit on Earth's surface is an almost perfect diagonal line (Fig. 2). Such an orbit is good for imaging perpendicular to this line since the azimuth speed component is high, but viewed ‘end-on’ the apparent satellite speed is very low and integration times would be too long to form useful synthetic apertures. Table 3 lists orbital elements for the baseline G-CLASS Table 1 Science Objectives
The most relevant system and performance parameters for the L-band SAR considered in this paper are shown in Fig. 1. Here the system parameters basically describe the instrument both in terms of quantities fixed by the systemdesign such as antenna dimensions, and parameters which can be altered during operation (e.g. the Pulse Repetition Frequency: PRF). The system parameters will be altered for the various im- plementations considered later, however, to allow for a “fair” comparison (and to limit the trade space dimension), three system parameters will be fixed. As shown in Fig. 1 these are the center frequency, total average Tx power, and orbit height (resulting, for the chosen swath width of 400 km in an incidence angle range from 25 ◦ to 45 ◦ ).
The HRWS system concept assumes a wide area illumination by a separate transmit antenna. This enables an independent electrical design and optimization of the transmit and receive paths, but it requires also the accommodation of an additional antenna on the spacecraft and reduces the flexibility to operate the radarsystem in different SAR imaging modes like ultra- wide-swath ScanSAR, high SNR spotlight, or new hybrid modes to be discussed later. It is hence worth to consider also the application of digital beamforming techniques in radar sys- tems that use the same antenna array for both the transmission and reception of radar pulses, thereby taking advantage of al- ready existing space-qualified T/R module technology. Since the high-resolution wide-swath SAR imaging capability is es- sentially based on a large antenna array, this poses in turn the question of how to distribute the signal energy on the ground. The trivial solution would be amplitude tapering, or as an ex- treme case, the use of only a part of the antenna for signal transmission, but this causes a significant loss of efficiency. Another possible solution is phase tapering, but the derivation of appropriate phase coefficients is an intricate task which re- quires in general complicated numerical optimization tech- niques.
The work focuses on the analysis of influences of motion errors on near-range SAR applications and design of specific motion measuring and compensation algo- rithms. First, an improved metric to determine the op- timum antenna beamwidth is proposed specifically for the near-range SAR applications with potential ultra- wide beamwidth. Then, a comprehensive investiga- tion of influences of motion errors on the SAR system with wide beamwidth is provided. On this ground, the octave division motion compensation algorithm is designed to deal with the near-range specific artificial motion errors. Furthermore, by exploiting the features of near-range SAR geometry, motion measuring algo- rithms using microelectromechanical system (MEMS) inertial measurement unit (IMU) of only one degree of freedom (DoF) and 3 DoF are proposed. In the end, these investigations and algorithms are verified through SAR measurements with 3 different setups. Huaming Wu, born in 1981 in Xiamen, obtained the ti- tle of M.Sc. from Beihang University in 2006. Since 2007 he worked towards his doctoral degree in the field of syntheticapertureradar signal processing at the Insti- tut für Hochfrequenztechnik und Elektronik (IHE) at the Karlsruher Institut für Technologie (KIT), which he successfully obtained in August 2012.
The advanced mapping capabilities of Tandem-L provide a unique database for new Earth observation applications. One example is the quasi-tomographic mapping of internal 3-D structure changes of semitrans- parent volume scatterers via the repeated acquisition of single-pass SAR interferograms . This will provide important information about structural processes in vegetation, ice, permafrost soils, etc., and we expect a range of novel applications emerging from the advanced interferometric measurement capabilities. Tandem-L can be regarded as a first step towards a global monitoring systemfor the quasi-continuous observation of natural and anthropogenic processes that continuously restructure the Earth surface. Radar is the optimum sensor for continuous Earth system monitoring since it provides high-resolution images independent of weather conditions at day and night. Together with its unique ability to measure subtle changes with millimetric accuracy and its even more unique ability to obtain quasi-tomographic images from space, radar will likely become the most important sensor for a huge amount of remote sensing applications, most of which we are currently even not thinking about. It is our responsibility to develop the best tools and techniques to be able to deal with the upcoming challenges in a rapidly changing world and environment. The missions and concepts outlined in this paper are first steps in this direction. h
In order to estimate the performance achievable in realistic weather conditions on real landscapes by these low azimuth speed missions, a method has been developed . During the development of this method, the review of the available literature – has shown the need for a new clutter model, due to the different system geometric conditions. Indeed, the Billingsley model has been developed for ground- based radar to model trees windblown clutter and even if it has been extended to other landcovers, the model is valid for a grazing angle that is less than 10 ◦ . Conversely, in the GeoSAR
Typically, EO satellites are placed in Low Earth Orbit (LEO), to take advantage of the shorter slant range. The key limitation of LEO for remote sensing is the difficulty of providing frequent timely images of an area. A single satellite typically images a swath 100-200 km wide steerable within a field of view 500 km across. LEO orbit periods are approximately 100 minutes, and in this time, the Earth has turned almost 3000 km at the equator: it thus takes ~3000/500=6 days to obtain complete access. Envisat actually uses a 35-day repeat orbit, which means a repeat period of 5 weeks for interferometry, and 1-2 weeks (at mid-latitudes) for imaging. Processes with timescales shorter than these periods are difficult to measure usefully. Satellites in geosynchronous orbit, on the other hand, have a permanent view of 1/3 of Earth’s surface so that, as soon as one image is complete, the next can be started. Allowing for the time required to acquire the image this means that a geosynchronous satellite could provide in principle several images each day of any location in the field of view. An inconvenience caused by the geosynchronous orbit is its fixed longitude. This implies that only a constellation of at least three satellites can guarantee a nearly global coverage. Even so, polar zones cannot be imaged with nominal resolution. However, global coverage is not always an essential requisite for a space system. The problem that many satellite service providers have to tackle is that the request for satellite products is concentrated in the most industrialised regions of the globe therefore a system that is able to provide nearly continuous imaging of a certain region may prove to be advantageous for certain customers.
Index Terms— GEO SAR clutter, Parametric land clutter model, SAR clutter, wheat clutter model
GeosynchronousSyntheticApertureRadar (GEO SAR) has attracted increasing interest in the last two decades. The concept is now widely accepted, but there are some concerns on the performance achievable on non-static target scenes. The movement of a target in a SAR system causes target signal to be smeared in the azimuth direction. This smeared signal is a form of clutter. For some GEO SAR mission con- cepts, the azimuth spread of the power scattered from clutter is a potentially important constraint on imaging performance because it smears a noise-like power across the image. Mod- els of GEO SAR imaging therefore need to include clutter spread [1, 2].
However, even though optical sensors can be easily inte- grated in cube sats, the use of syntheticaperture radars (SARs) has been typically precluded to small satellite platforms due to the inherent volume and mass limitations. Recently, in , it has been demonstrated how high-resolution wide-swath SAR imaging sensors could be designed employing digital beamforming (DBF) techniques in a receive-only multistatic satellite constellation. The introduction of these architectures allows the development of new SAR systems where a single master satellite illuminates the surface of the earth and a constellation of small satellites equipped with compact DBF SAR sensors scans the scattered field following the echo on ground . The implementation of this new architecture relies on the possibility to exploit highly integrated DBF SAR sen- sors. However, existing DBF SAR spaceborne instruments do not match these compactness and modularity requirements . Several concurrent research activities are tackling the devel- opment of compact DBF SAR sensors proposing different solutions. For example, in , it is presented a three-channel DBF SAR antenna providing dual-polarized L-band opera- tion. This system uses a hybrid analog beamforming/DBF scheme where groups of four subarrays share the same digital channel. Another possible implementation is being developed within the framework of the EU Project “DIFFERENT” . The challenge of this project is to implement a receive-only SAR sensor capable to acquire data in two bands, namely, in the X-band and Ka-band, in the single-pass full polarimetric mode. The adopted center-band frequencies in the X-band and Ka-band are 9.6 and 35.75 GHz, respectively. In the proposed architecture, a DBF feed array with 60 digital channels is used to illuminate a deployable reflector , , as shown in Fig. 1(a). High resolution is achieved combining the benefits of the multistatic constellation with the multichannel capability enabled by the DBF technique and with the dual-band single- pass full polarimetric acquisitions . Moreover, the flexibility of this SAR receiver platform will support the realization of a variety of spaceborne SAR missions .
The focus of this work is to formulate a model for computing the location and form of the paired echoes produced by vibrating targets within SAR imagery. The generalised model should operate for both the SAR near and far-ﬁeld regimes. This paper provides an overview of the developed numerical model describing the precise form of vibration artefacts in SAR images. Validation results for the model from both simulated and experimental data are then presented.
The spaceborne active remote sensing have the unique capability of observing sea surface through cloud, which has been playing an important role of monitor- ing response of sea surface under extreme weather situations. Scatterometers on board the European Remote Sensing (ERS), the Quick Scatterometer (QuikSCAT), and the Meteorological Operational (MetOp) satellites are particularly suitable for measurements of sea surface wind field, as both wind direction and wind speed can be derived without needing external information. Another active remote sensing instrument, spaceborne SyntheticApertureRadar (SAR), e.g., the ERS-1/2 SAR, ENVISAT/ASAR, RADARSAT-1/2, TS-X/TD-X and Cosmo-Skymed, can not only provide sea surface backscatter intensity like scatterometer, but also image the sea sur- face in two-dimension with large spatial coverage and high spatial resolution, which provides abundant oceanic and atmospheric information of TCs, such as hurricane- generated long swell waves in small scales [ 3 , 4 ], hurricane/typhoon eye morphology [ 5 ] and roll vortices occurred in marine boundary layer [ 6 ] in meso-scale.
Based on the given distribution of signal samples, reflectivity maps are simulated in the azimuth - ground range plane. To this end, the sampling is adapted to the real SAR image shown in figure 50. The simulated reflectivity map for bounce levels 1-3 is shown in figure 52a. Within each resolution cell, the signal contributions have been added coherently. The simulated map has been clipped at 8.1% of its maximum amplitude and is displayed using 8-bit grayscale. Despite the low clipping level, almost no signatures are distinguishable. The signal response of the railway station is characterized by a pattern of point signatures corresponding to the facades of building parts 1 and 2. Following the ground range axis top-down, the first point pattern represents building part 1. As building parts 1 and 2 are connected by a roof, the effective height of the facade of building part 2, i.e. the facade area visible to the SAR sensor, is smaller than for building part 1. Hence, the corresponding pattern of point signatures is of smaller size. On the real SAR image, the tower is represented by a pattern of bright points. As expected, these signatures are not confirmed by the simulation as the tower model is composed by flat surfaces. Hence, only diffuse signal components are detected which are weak compared to the point signatures. As the corresponding pixels obtain gray value 0, the diffuse backscattering from the tower is not distinguishable on the reflectivity map. No multiple reflected signal is derived from the ground and the building roof, as no corresponding geometrical information is provided by the 3D model scene. The direct backscattering from the ground is negligible compared to the signal response of the point signatures. To conclude, the visible part of the railway station is reduced to a low number of point signatures. As the reflection behavior of surfaces is adapted to the SPM model, the increase of the number of signatures heavily depends on the increase of the level of detail of the 3D model scene, i.e. the representation of facade/roof details or objects on the ground.