This letter reports the sensitivity of X-band interferometric synthetic aperture radar (InSAR) data from the first dual-spacecraft radar interferometer, TanDEM-X, to variations in tropical-forest aboveground biomass (AGB). It also reports the first tropical-forest AGB estimates from TanDEM-X data. Tropical forests account for about 50% of the world's forested biomass and play critical roles in the control of atmospheric carbon dioxide by emission through deforestation and uptake through forest growth. The TanDEM-X InSAR data used in this analysis were taken over the Tapajós National Forest, Pará, Brazil, where field measurements from 30 stands were acquired. The magnitude of the InSAR normalized complex correlation, which is called coherence, decreases by about 25% as AGB increases from 2 to 430 Mg-ha-1, suggesting more vertically distributed return-power profiles with increasing biomass. Comparison of InSAR coherences to those of small-spot (15 cm) lidar suggests that lidar penetrates deeper into the canopies than InSAR. Modeling InSAR profiles from InSAR coherence and lidar profiles yields an estimate of 0.29 dB/m for the X-band extinction coefficient relative to that of lidar. Forest AGB estimated from InSAR observations on 0.25-ha plots shows RMS scatters about the field-estimated AGB between 52 and 62 Mg-ha-1, which is between 29% and 35% of the average AGB of 179 Mg-ha-1, depending on the data analysis mode. The sensitivity and biomass-estimation performance suggest the potential of TanDEM-X observations to contribute to global tropical-forest biomass monitoring.

Observational architectures for allowing the eventual forecasting of earthquake are discussed. Current science requirements suggest that L-band InSAR systems with short repeat periods would be best suited to such measurements. Constellations of such sensors in orbits around 2000-5000 km altitude might provide optimal Earth coverage for interferometry, while higher orbits around 10,000 - 40,000 km might approach the goal of around-the-clock for disaster-response applications.

Fine temporal sampling is essential for disaster management, e.g. of flooding, fires, landslides, hurricanes, and earthquakes. A powerful technique for mapping such natural hazards is synthetic aperture radar (SAR) interferometry, providing displacement measurements at the subwavelength scale and decorrelation estimates. Pre-seismic deformation, one of the most elusive aspects of earthquakes, will require much finer temporal sampling than present InSAR capabilities provide. Observations taken every few hours could provide time series data of rapidly evolving phenomena, such as pre-eruptive volcano dynamics, leading to major advances in predictive capability, improving the potential for modeling as well as for civil protection. Such radical performance improvements could be attained through large constellations of conventional low Earth orbit (LEO) satellites or small constellations of geosynchronous SARs. The unique capability of a geosynchronous SAR in terms of instantaneously accessible area is contrasted with the requirements for huge electronically steered array (ESA) antennas. The optimal approach is very much dependant on technological developments, in particular geosynchronous SAR depends on the development of affordable very large ESA antennas, but also other technological developments will be required.

Understanding the earthquake cycle and assessing earthquake hazards is a topic of both increasing potential for scientific advancement and social urgency. A large portion of the world's population inhabits seismically-active regions, including the megacities of Los Angeles and Mexico City, and heavily populated regions in Asia. Population growth will exacerbate the potential for huge earthquake-related casualties. However, powerful new tools to observe tectonic deformation have been developed and are being deployed with encouraging results for improving knowledge of fault system behavior and earthquake hazards. In the future, the coupling of complex numerical models and orders of magnitude increase in observing power promises to lead to accurate targeted, short-term earthquake forecasting. Dynamic earthquake hazard assessments resolved for a range of spatial scales (large and small fault systems) and time scales (months to decades) will allow a more systematic approach to prioritizing the retrofitting of vulnerable structures, relocating populations at risk, protecting lifelines, preparing for disasters, and educating the public. The suite of spaceborne observations needed to achieve this vision has been studied, and the derived requirements have defined a set of mission architectures and enabling technologies that will accelerate progress in achieving the goal of improved earthquake hazard assessments.

Geometric calibration of the European Remote Sensing (ERS) Satellite synthetic aperture radar (SAR) slant range images is important in relation to mapping areas without ground reference points and also in relation to automated processing. The relevant SAR system parameters are discussed and calibrated by using the European Space Agency (ESA) transponders at Flevoland. The resulting accuracy of the slant range images corresponds to 10 m horizontally on the ground. The results are verified by using runway intersections and corner reflectors surveyed with differential GPS techniques. Based on a seven-year ERS-1 and a four-year ERS-2 time series, the long term stability is found to be sufficient to allow a single calibration covering the entire mission period. A descending and an ascending orbit tandem pair of the ESA calibration site on Flevoland, suitable for calibration of ERS SAR processors, is described to allow other researchers to geometrically calibrate their processing systems.

Synthetic aperture radar interferometry is an imaging technique for measuring the topography of a surface, its changes over time, and other changes in the detailed characteristic of the surface. By exploiting the phase of the coherent radar signal, interferometry has transformed radar remote sensing from a largely interpretive science to a quantitative tool, with applications in cartography, geodesy, land cover characterization, and natural hazards. This paper reviews the techniques of interferometry, systems and limitations, and applications in a rapidly growing area of science and engineering.

EMISAR is a high-resolution (2/spl times/2 m), fully polarimetric, dual-frequency (L- and C-band) synthetic aperture radar (SAR) system designed for remote-sensing applications. The SAR is operated at high altitudes on a Gulfstream G-3 jet aircraft. The system is very well calibrated and has low sidelobes and low cross-polar contamination. Digital technology has been utilized to realize a flexible and highly stable radar with variable resolution, swath width, and imaging geometry. Thermal control and several calibration loops have been built into the system to ensure system stability and absolute calibration. Accurately measured antenna gains and radiation patterns are included in the calibration. The processing system is developed to support data calibration, which is the key to most of the current applications. Recent interferometric enhancements are important for many scientific applications.

We have evaluated the accuracy of digital elevation models (DEM's) generated by the JPWNASA TOPSAR synthetic aperture radar interferometer instrument by acquiring topographic radar data in the summer of 1992 over the National Training Center, near Ft. Irwin, California, and comparing the measurements to a very accurate digital elevation model derived for this area by the US. Army Topographic Engineering Center (TEC). Fiducial corner reflectors were deployed in the area, and their locations were determined to cm accuracy by the Defense Mapping Agency (DMA). DEM’s generated from the acquired radar data were rotated and translated to precisely overlay the reference DEM, allowing an analysis of the achieved height accuracy. We present here a detailed description of horizontal and vertical errors and their characteristics. The standard deviation measured over a 5.6 x 7 km area was approximately 2 m, the corresponding figures for relatively flat areas were in the 1-2 meter range and for mountainous areas in the 2-3 meter range, consistent with theoretical expectations. We also discuss key factors that presently limit the system performance

A new processing algorithm for the NASA JPL TOPSAR topographic radar mapper is described. It incorporates extensive motion compensation features as well as accurate three-dimensional target location algorithm. The processor applies an algorithm to resolving the absolute phase ambiguity. This allows rectified height maps to be generated without any use of ground reference points. The processor was tested using data acquired with extreme aircraft motion so that performance could be evaluated under adverse conditions. The topographic maps generated by the radar were compared to digital elevation models (DEMs) derived using conventional optical stereo techniques. In one region, the RMS elevation deviations measured were less than the specified DEM accuracy, and, in the region covered by the more accurate DEM, errors varied from 2.2 m RMS in relatively flat terrain up to 5.0 m in mountainous area. The RMS difference between radar and DEM elevation over the 6.5-km by 22-km area covered by the more accurate DEM was 3.6 m.>

The authors have augmented the NASA DC-8 AIRSAR instrument with a pair of C-band antennas displaced across track to form an interferometer sensitive to topographic variations of the Earth's surface. During the 1991 DC-8 flight campaign, data were acquired over several sites in the US and Europe, and topographic maps were produced from several of these flight lines. Analysis of the results indicate that statistical errors are in the 2-4-m range, while systematic effects due to aircraft motion are in the 10-20-m range. The initial results from development of a second-generation processor show that aircraft motion compensation algorithms reduce the systematic variations to 2 m, while the statistical errors are reduced to 2-3 m.>

In January 1986, the design of a high-resolution airborne C-band synthetic aperture radar (SAR) started at the Electromagnetics Institute of the Technical University of Denmark. The initial system test flights took place in November and December 1989. The authors describe the design of the system, its implementation, and its performance. They show how digital technology has been utilized to realize a very flexible radar with variable resolution, swath-width, and imaging geometry. The motion-compensation algorithms implemented to obtain the high resolution and the special features built into the system to ensure proper internal calibration are outlined. The data processing system, developed for image generation and quality assurance, is sketched, with special emphasis on the flexibility of the system.>