Soren N. Madsen

Absolute phase determination techniques in SAR interferometry

Cross-track interferometric SAR can provide 3D radar images. The technique relies on determining the target elevation from the difference in slant range observed by two antennas having a cross-track separation. The range difference is estimated very precisely using the phase difference observed in an interferogram obtained from the two complex images. A key problem is that the range difference can only be determined to within a multiple of the wavelength, as the phase difference is measured modulo 2(pi) . This paper discusses two different methods to determine the unknown multiple of 2(pi) : 1) the split-spectrum algorithm, and 2) the residual delay estimation algorithm. The split-spectrum algorithm utilizes the carrier frequency dependence of the interferometric phase, as subdividing the available range bandwidth into two bands provides two slightly different interferograms. The phase difference of the interferograms corresponds to an interferogram obtained with a system having a carrier frequency which is the difference between the two band centers. THe residual delay estimation method is based on the full bandwidth, one-look images used to form the interferogram and involves precision interpolation and coregistration steps. Principles are presented, along with possible implementations of the algorithms. Principle error sourses, as well as advantages and disadvantages from a processor design and implementation point of view are also discussed.

Concepts and technologies for synthetic aperture radar from MEO and geosynchronous orbits

The area accessible from a spaceborne imaging radar, e.g. a synthetic aperture radar (SAR), generally increases with the elevation of the satellite while the map coverage rate is a more complicated function of platform velocity and beam agility. The coverage of a low Earth orbit (LEO) satellite is basically given by the ground velocity times the relatively narrow swath width. The instantaneously accessible area will be limited to some hundreds of kilometers away from the sub-satellite point. In the other extreme, the sub-satellite point of a SAR in geosynchronous orbit will move relatively slowly, while the area which can be accessed at any given time is very large, reaching thousands of kilometers from the sub-satellite point. To effectively use the accessibility provided by a high vantage point, very large antennas with electronically steered beams are required. Interestingly, medium Earth orbits (MEO) will enable powerful observational systems which provide large instantaneous reach and high mapping rates, while pushing technology less than alternative systems at higher altitudes. Using interferometric SAR techniques which can reveal centimeter-level (potentially sub-centimeter) surface displacements, frequent and targeted observations might be key to developing such elusive applications as earthquake forecasting. This paper discusses the basic characteristics of a SAR observational system as a function of the platform altitude and the technologies being developed to make such systems feasible.

First P-band results using the GeoSAR mapping system

GeoSAR is a program to develop a dual frequency airborne radar interferometric mapping instrument designed to meet the mapping needs of a variety of users in government and private industry. Program participants are the Jet Propulsion Laboratory (JPL), Calgis, Inc., and the California Department of Conservation with funding provided initially by DARPA and currently by the National Imagery and Mapping Agency. Begun to address the critical mapping needs of the California Department of Conservation to map seismic and landslide hazards throughout the state, GeoSAR is currently undergoing tests of the X-band and P-band radars designed to measure the terrain elevation at the top and bottom of the vegetation canopy. Maps created with the GeoSAR data will be used to assess potential geologic/seismic hazard (such as landslides), classify land cover, map farmlands and urbanization, and manage forest harvests. This system is expected to be fully operational in 2002. In this paper we describe an experiment conducted at Californias Latour State Demonstration Forest located near the city of Redding. This experiment marks the first operation of the-P-band radar in a vegetated area.

UAV-based L-band SAR with precision flight path control

NASAs Jet Propulsion Laboratory is currently implementing a reconfigurable polarimetric L-band synthetic aperture radar (SAR), specifically designed to acquire airborne repeat track interferometric (RTI) SAR data, also know as differential interferometric measurements. Differential interferometry can provide key displacement measurements, important for the scientific studies of Earthquakes and volcanoes1. Using precision real-time GPS and a sensor controlled flight management system, the system will be able to fly predefined paths with great precision. The radar will be designed to operate on a UAV (Unmanned Arial Vehicle) but will initially be demonstrated on a minimally piloted vehicle (MPV), such as the Proteus build by Scaled Composites. The application requires control of the flight path to within a 10 m tube to support repeat track and formation flying measurements. The design is fully polarimetric with an 80 MHz bandwidth (2 m range resolution) and 16 km range swath. The antenna is an electronically steered array to assure that the actual antenna pointing can be controlled independent of the wind direction and speed. The system will nominally operate at 45,000 ft. The program started out as a Instrument Incubator Project (IIP) funded by NASA Earth Science and Technology Office (ESTO).

Determining aboveground biomass of the forest successional chronosequence in a test-site of Brazilian Amazon through X- and L-band data analysis

Secondary succession is an important process in the Amazonian region with implications for the global carbon cycle and for the sustainable regional agricultural and pasture activities. In order to better discriminate the secondary succession and to characterize and estimate the aboveground biomass (AGB), backscatter and interferometric SAR data generally have been analyzed through empirical-based statistical modeling. The objective of this study is to verify the capability of the full polarimetric PALSAR/ALOS (L-band) attributes, when combined with the interferometric (InSAR) coherence from the TanDEM-X (X-band), to improve the AGB estimates of the succession chronosequence located in the Brazilian Tapajós region. In order to perform this study, we carried out multivariate regression using radar attributes and biophysical parameters acquired during a field inventory. A previous floristic-structural analysis was performed to establish the chronosequence in three stages: initial vegetation regrowth, intermediate, and advanced regrowth. The relationship between PALSAR data and AGB was significant (p<0.001) and results suggested that the “volumetric scattering” (Pv) and “anisotropy” (A) attributes were important to explain the biomass content of the successional chronosequence (R2adjusted = 0.67; RMSE = 32.29 Mg.ha-1). By adding the TanDEM-derived interferometric coherence (Υi) into the regression modeling, better results were obtained (R2adjusted = 0.75; RMSE = 28.78Mg.ha-1). When we used both the L- and X-band attributes, the stock density prediction improved to 10.8 % for the secondary succession stands.

Radar TopoMapper concept for planetary exploration

Topographic information is key to interpreting the geology and geophysics of planetary bodies such as the icy Galilean satellites. Traditionally elevation information has been derived from stereo-photogrammetry, but the last couple of decades have offered new techniques, including radar interferometry, photoclinometry (shape from shading) and laser altimetry. Combining synthetic aperture radar (SAR) technology with interferometry (InSAR) enables high resolution imaging with elevation information at each image point. With two appropriately spaced antennas on a spacecraft, single-pass imaging radar interferometry can provide wide swath topographic data, independent of solar illumination, as was recently demonstrated on Earth by the Shuttle Topographic Radar Mission (SRTM; www.jpl.nasa.gov/srtm). We will present the science requirements, measurement principle, a straw-man’s design, and the predicted performance of a “compact SRTM” which could be flown on NASA missions such as the proposed Jupiter Icy Moons Orbiter (JIMO). In this paper we discuss challenges, including the calibration strategy and critical technology elements such as the high power RF-amplifier. We expect that the performance, both in terms of elevation accuracy and mapping rate would suffice to 1) determine topography on local and regional scales; 2) search for active geological change on the time scale of JIMO’s orbit around, e.g., Europa (30-60 days); and 3) determine the global tidal amplitude at Europa, Callisto, and Ganymede, which would constitute direct proof of the existence of oceans in all three icy moons.