Research progress on geosynchronous synthetic aperture radar

Based on its ability to obtain two-dimensional (2D) high-resolution images in all-time and all-weather conditions, spaceborne synthetic aperture radar (SAR) has become an important remote sensing technique and the study of such systems has entered a period of vigorous development. Advanced imaging modes such as radar interferometry, tomography, and multi-static imaging, have been demonstrated. However, current in-orbit spaceborne SARs, which all operate in low Earth orbits, have relatively long revisit times ranging from several days to dozens of days, restricting their temporal sampling rate. Geosynchronous SAR (GEO SAR) is an active research area because it provides significant new capability, especially its much-improved temporal sampling. This paper reviews the research progress of GEO SAR technologies in detail. Two typical orbit schemes are presented, followed by the corresponding key issues, including system design, echo focusing, main disturbance factors, repeat-track interferometry, etc, inherent to these schemes. Both analysis and solution research of the above key issues are described. GEO SAR concepts involving multiple platforms are described, including the GEO SAR constellation, GEO-LEO/airborne/unmanned aerial vehicle bistatic SAR, and formation flying GEO SAR (FF-GEO SAR). Due to the high potential of FF-GEO SAR for three-dimensional (3D) deformation retrieval and coherence-based SAR tomography (TomoSAR), we have recently carried out some research related to FF-GEO SAR. This research, which is also discussed in this paper, includes developing a formation design method and an improved TomoSAR processing algorithm. It is found that GEO SAR will continue to be an active topic in the aspect of data processing and multi-platform concept in the near future

Three Dimensional Bistatic Tomography Using HDTV

The thesis begins with a review of the principles of diffraction and reflection tomography; starting with the analytic solution to the inhomogeneous Helmholtz equation, after linearization by the Born approximation (the weak scatterer solution), and arriving at the Filtered Back Projection (Propagation) method of reconstruction. This is followed by a heuristic derivation more directly couched in the radar imaging context, without the rigor of the general inverse problem solution and more closely resembling an imaging turntable or inverse synthetic aperture radar. The heuristic derivation leads into the concept of the line integral and projections (the Radon Transform), followed by more general geometries where the plane wave approximation is invalid. We proceed next to study of the dependency of reconstruction on the space-frequency trajectory, combining the spatial aperture and waveform. Two and three dimensional apertures, monostatic and bistatic, fully and sparsely sampled and including partial apertures, with controlled waveforms (CW and pulsed, with and without modulation) define the filling of k-space and concomitant reconstruction performance. Theoretical developments in the first half of the thesis are applied to the specific example of bistatic tomographic imaging using High Definition Television (HDTV); the United States version of DVB-T. Modeling of the HDTV waveform using pseudonoise modulation to represent the hybrid 8VSB HDTV scheme and the move-stop-move approximation established the imaging potential, employing an idealized, isotropic 18 scatterer. As the move-stop-move approximation places a limitation on integration time (in cross correlation/pulse compression) due to transmitter/receiver motion, an exact solution for compensation of Doppler distortion is derived. The concept is tested with the assembly and flight test of a bistatic radar system employing software-defined radios (SDR). A three dimensional, bistatic collection aperture, exploiting an elevated commercial HDTV transmitter, is focused to demonstrate the principle. This work, to the best of our knowledge, represents a first in the formation of three dimensional images using bistatically-exploited television transmitters

Contributions to GNSS-R earth remote sensing from nano-satellites

Premi extraordinari doctorat UPC curs 2015-2016, àmbit de CiènciesGlobal Navigation Satellite Systems Reflectometry (GNSS-R) is a multi-static radar using navigation signals as signals of opportunity. It provides wide-swath and improved spatio-temporal sampling over current space-borne missions. The lack of experimental datasets from space covering signals from multiple constellations (GPS, GLONASS, Galileo, Beidou) at dual-band (L1 and L2) and dual-polarization (Right Hand Left Hand Circular Polarization: RHCP and LHCP), over the ocean, land and cryosphere remains a bottleneck to further develop these techniques. 3Cat-2 is a 6 units (3 x 2 elementary blocks of 10 x 10 x 10 cm3) CubeSat mission ayming to explore fundamentals issues towards an improvement in the understanding of the bistatic scattering properties of different targets. Since geolocalization of specific reflections points is determined by the geometry only, a moderate pointing accuracy is still required to correct for the antena pattern in scatterometry measurements. 3Cat-2 launch is foreseen for the first quarter 2016 into a Sun-Synchronous orbit of 510 km height using a Long March II D rocket. This Ph.D. Thesis represents the main contributions to the development of the 3Cat-2 GNSS-R Earth observation mission (6U CubeSat) including a novel type of GNSS-R technique: the reconstructed one. The desing, development of the platform, and a number of ground-based, airborne and stratospheric balloon experiments to validate the technique and to optimize the instrument. In particular, the main contributions of this Ph.D. thesis are: 1) A novel dual-band Global Navigation Satellite Systems Reflectometer that uses the P(Y) and C/A signals scattered over the sea surface to perform highly precise altimetric measurements (PYCARO). 2) The first proof-of-concept of PYCARO was performed during two different ground-based field experiments over a dam and over the sea under different surface roughness conditions. 3) The scattering of GNSS signals over a water surface has been studied when the receiver is at low height, as for GNSS-R coastal altimetry applications. The precise determination of the local sea level and wave state from the coast can provide useful altimetry and wave information as "dry" tide and wave gauges. In order to test this concept an experiment has been conducted at the Canal d'Investigació i Experimentació Marítima (CIEM) wave channel for two synthetic "sea" states. 4) Two ESA-sponsored airborne experiments were perfomed to test the precision and the relative accuracy of the conventional GNSS-R. 5) The empirical results of a GNSS-R experiment on-board the ESA-sponsored BAXUS 17 stratospheric balloon campaign performed North of Sweden over boreal forests showed that the power of the reflected signals is nearly independent of the platform height for a high coherent integration time. 6) An improved version of the PYCARO payload was tested in Octover 2014 for the second time during the ESA-sposored BEXUS-19,. This work achieved the first ever dual-frequency, multi-constellation GNSS-R observations over boreal forests and lakes using GPS, GLONASS and Galileo signals. 7) The first-ever dual-frequency multi-constellation GNSS-R dual-polarization measurements over boreal forests and lakes were obtained from the stratosphere during the BEXUS 19 using the PYCARO reflectometer operated in closed-loop mode.Global Navigation Satellite Systems Reflectometry (GNSS-R) es una técnica de radar multi-estático que usa señales de radio-navegación como señales de oportunidad. Esta técnica proporciona "wide-swath" y un mejor sampleado espacio-temporal en comparación con las misiones espaciales actuales. La falta de datos desde el espacio proporcionando señales de múltiples constelaciones (GPS, GLONASS, Galileo, Beidou) en doble banda (L1 y L2) y en doble polarización (RHCP y LHCP) sobre océano, tierra y criosfera continua siendo un problema por solucionar. 3Cat-2 es un cubesat de 6 unidades con el objetivo de explorar elementos fundamentales para mejorar el conocimiento sobre el scattering bi-estático sobre diferentes medios dispersores. Dado que la geolocalización de puntos de reflexión específicos está determinada solo por geometría, es necesario un requisito moderado de apuntamiento para corregir el diagrama de antena en aplicaciones de dispersometría. El lanzamiento del 3Cat-2 será en Q2 2016 en una órbitra heliosíncrona usando un cohete Long March II D. Esta tesis representa las contribuciones principales al desarrollo del satélite 3Cat2 para realizar observación de la tierra con GNSS-R incluyendo una nueva técnica: "the reconstructed-code GNSS-R". El diseño, desarrollo de la plataforma y un número de experimentos en tierra, desde avión y desde globo estratosférico para validar la técnica y optimizar el instrumento han sido realizados. En particular, las contribuciones de esta Ph.D. son: 1) un novedoso Global Navigation Satellite Systems Reflectometer que usa las señales P(Y) y C/A después de ser dispersadas sobre la superficie del mar para realizar medidas altimétricas muy precisas. (PYCARO). 2) La primera prueba de concepto de PYCARO se hizo en dos experimentos sobre un pantano y sobre el mar bajo diferentes condiciones de rugosidad. 3) La disperión de las señales GNSS sobre una superfice de agua ha sido estudiada para bajas altitudes para aplicaciones GNSS-R altimétricas de costa. La determinación precisa del nivel local del mar y el estado de las olas desde la costa puede proporcionar información útil de altimetría e información de olas. Para hacer un test de este concepto un experimento en el Canal d'Investigació i Experimentació Marítima (CIEM) fue realizado para dos estados sintéticos de rugosidad. 4) Dos experimentos en avión con esponsor de la ESA se realizaron para estudiar la preción y la exactitud relativa de cGNSS-R. 5) Los resultados empíricos del experimento GNSS-R en BEXUS 17 con esponsor de la ESA realizado en el norte de Suecia sobre bosques boreales mostró que la potencia reflejada de las señales es independiente de la altitud de la plataforma para un tiempo de integración coherente muy alto. 6) Una versión mejorada del PYCARO fue testeada en octubre del 2014 por segunda vez durante el BEXUS 19 que también fue patrocidado por la ESA. Este trabajo proporcionó las primeras medidas GNSS-R sobre bosques boreales en doble frecuencia usando varias constelaciones GNSS. 7) Las primeras medidas polarimétricas (RHCP y LHCP) de GNSS-R sobre bosques boreales también fueron conseguidas durante el experimento BEXUS 19.Award-winningPostprint (published version

Retrieval of Ocean Surface Currents and Winds Using Satellite SAR backscatter and Doppler frequency shift

Ocean surface winds and currents play an important role for weather, climate, marine life, ship navigation, oil spill drift and search and rescue. In-situ observations of the ocean are sparse and costly. Satellites provide a useful complement to these observations. Synthetic aperture radar (SAR) is particularly attractive due to its high spatial resolution and its capability to extract both sea surface winds and currents day and night and almost independent of weather.The work in this thesis involves processing of along-track interferometric SAR (ATI-SAR) data, analysis of the backscatter and Doppler frequency shift, and development of wind and current retrieval algorithms. Analysis of the Doppler frequency shift showed a systematic bias. A calibration method was proposed and implemented to correct for this bias. Doppler analysis also showed that the wave contribution to the SAR Doppler centroid often dominates over the current contribution. This wave contribution is estimated using existing theoretical and empirical Doppler models. For wind and current retrieval, two methods were developed and implemented.The first method, called the direct method, consists of retrieval of the wind speed from SAR backscatter using an empirical backscatter model. In order to retrieve the radial current, the retrieved wind speed is used to correct for the wave contribution. The current retrieval was assessed using two different (theoretical and empirical) Doppler models and wind inputs (model and SAR-derived). It was found that the results obtained by combining the Doppler empirical model with the SAR-derived wind speed were more consistent with ocean models.The second method, called Bayesian method, consists of blending the SAR observables (backscatter and Doppler shift) with an atmospheric and an oceanic model to retrieve the total wind and current vector fields. It was shown that this method yields more accurate estimates, i.e. reduces the models biases against in-situ measurements. Moreover, the method introduces small scale features, e.g. fronts and meandering, which are weakly resolved by the models.The correlation between the surface wind vectors and the SAR Doppler shift was demonstrated empirically using the Doppler shift estimated from over 300 TanDEM-X interferograms and ECMWF reanalysis wind vectors. Analysis of polarimetric data showed that theoretical models such as Bragg and composite surface models over-estimate the backscatter polarization ratio and Doppler shift polarization difference. A combination of a theoretical Doppler model and an empirical modulation transfer function was proposed. It was found that this model is more consistent with the analyzed data than the pure theoretical models.The results of this thesis will be useful for integrating SAR retrievals in ocean current products and assimilating SAR observables in the atmospheric, oceanic or coupled models. The results are also relevant for preparation studies of future satellite missions

Using digital television signals for radar imaging

This thesis presents the first systematic theoretical and experimental study of passive bistatic Synthetic Aperture Radar (SAR) imaging utilising Digital Television Broadcasting - Terrestrial (DVB-T) as an illuminator of opportunity. It presents the theoretical background for radar, SAR, and DVB-T transmissions, and studies the waveform on the basis of its ambiguity function. It considers the theoretical constraints of DVB-T SAR, including the impact of multiple transmitters and the properties of the DVB-T signal on image formation. It tracks the development and evolution of a receiver system and image formation algorithm, which is then validated through a series of ground-based and airborne experimental campaigns. From these campaigns, a catalogue of images are produced and analysed in order to establish the informational potential of DVB-T SAR imagery in both quasi-monostatic and bistatic environments. Specifically, the quality of images formed from a range of bistatic angles from 4 to 140 degrees is considered

GNSS transpolar earth reflectometry exploriNg system (G-TERN): mission concept

The global navigation satellite system (GNSS) Transpolar Earth Reflectometry exploriNg system (G-TERN) was proposed in response to ESA's Earth Explorer 9 revised call by a team of 33 multi-disciplinary scientists. The primary objective of the mission is to quantify at high spatio-temporal resolution crucial characteristics, processes and interactions between sea ice, and other Earth system components in order to advance the understanding and prediction of climate change and its impacts on the environment and society. The objective is articulated through three key questions. 1) In a rapidly changing Arctic regime and under the resilient Antarctic sea ice trend, how will highly dynamic forcings and couplings between the various components of the ocean, atmosphere, and cryosphere modify or influence the processes governing the characteristics of the sea ice cover (ice production, growth, deformation, and melt)? 2) What are the impacts of extreme events and feedback mechanisms on sea ice evolution? 3) What are the effects of the cryosphere behaviors, either rapidly changing or resiliently stable, on the global oceanic and atmospheric circulation and mid-latitude extreme events? To contribute answering these questions, G-TERN will measure key parameters of the sea ice, the oceans, and the atmosphere with frequent and dense coverage over polar areas, becoming a “dynamic mapper”of the ice conditions, the ice production, and the loss in multiple time and space scales, and surrounding environment. Over polar areas, the G-TERN will measure sea ice surface elevation (<;10 cm precision), roughness, and polarimetry aspects at 30-km resolution and 3-days full coverage. G-TERN will implement the interferometric GNSS reflectometry concept, from a single satellite in near-polar orbit with capability for 12 simultaneous observations. Unlike currently orbiting GNSS reflectometry missions, the G-TERN uses the full GNSS available bandwidth to improve its ranging measurements. The lifetime would be 2025-2030 or optimally 2025-2035, covering key stages of the transition toward a nearly ice-free Arctic Ocean in summer. This paper describes the mission objectives, it reviews its measurement techniques, summarizes the suggested implementation, and finally, it estimates the expected performance.Peer ReviewedPostprint (published version

Wide-Angle Multistatic Synthetic Aperture Radar: Focused Image Formation and Aliasing Artifact Mitigation

Traditional monostatic Synthetic Aperture Radar (SAR) platforms force the user to choose between two image types: larger, low resolution images or smaller, high resolution images. Switching to a Wide-Angle Multistatic Synthetic Aperture Radar (WAM-SAR) approach allows formation of large high-resolution images. Unfortunately, WAM-SAR suffers from two significant implementation problems. First, wavefront curvature effects, non-linear flight paths, and warped ground planes lead to image defocusing with traditional SAR processing methods. A new 3-D monostatic/bistatic image formation routine solves the defocusing problem, correcting for all relevant wide-angle effects. Inverse SAR (ISAR) imagery from a Radar Cross Section (RCS) chamber validates this approach. The second implementation problem stems from the large Doppler spread in the wide-angle scene, leading to severe aliasing problems. This research effort develops a new anti-aliasing technique using randomized Stepped-Frequency (SF) waveforms to form Doppler filter nulls coinciding with aliasing artifact locations. Both simulation and laboratory results demonstrate effective performance, eliminating more than 99% of the aliased energy

ARRAY PROCESSING TECHNIQUES FOR ESTIMATION AND TRACKING OF AN ICE-SHEET BOTTOM

Ice bottom topography layers are an important boundary condition required to model the flow dynamics of an ice sheet. In this work, using low frequency multichannel radar data, we locate the ice bottom using two types of automatic trackers. First, we use the multiple signal classification (MUSIC) beamformer to determine the pseudo-spectrum of the targets at each range-bin. The result is passed into a sequential tree-reweighted message passing belief-propagation algorithm to track the bottom of the ice in the 3D image. This technique is successfully applied to process data collected over the Canadian Arctic Archipelago ice caps in 2014, and produce digital elevation models (DEMs) for 102 data frames. We perform crossover analysis to self-assess the generated DEMs, where flight paths cross over each other and two measurements are made at the same location. Also, the tracked results are compared before and after manual corrections. We found that there is a good match between the overlapping DEMs, where the mean error of the crossover DEMs is 38±7 m, which is small relative to the average ice-thickness, while the average absolute mean error of the automatically tracked ice-bottom, relative to the manually corrected ice-bottom, is 10 range-bins. Second, a direction of arrival (DOA)-based tracker is used to estimate the DOA of the backscatter signals sequentially from range bin to range bin using two methods: a sequential maximum a posterior probability (S-MAP) estimator and one based on the particle filter (PF). A dynamic flat earth transition model is used to model the flow of information between range bins. A simulation study is performed to evaluate the performance of these two DOA trackers. The results show that the PF-based tracker can handle low-quality data better than S-MAP, but, unlike S-MAP, it saturates quickly with increasing numbers of snapshots. Also, S-MAP is successfully applied to track the ice-bottom of several data frames collected from over Russell glacier in 2011, and the results are compared against those generated by the beamformer-based tracker. The results of the DOA-based techniques are the final tracked surfaces, so there is no need for an additional tracking stage as there is with the beamformer technique