A. Freeman

The effects of noise on polarimetric SAR data

Polarimetric SAR data can provide a great deal of information about the scattering behavior of the surface under observation. Polarimetric SAR systems often measure the scattering matrices of the areas under observation in linear polarizations (H and V). From the scattering matrix commonly used forms such as the covariance matrix and the Stokes matrix can be easily derived. Other measures derived from polarimetric SAR data include correlation coefficients between scattering matrix terms and the mode and variance of phase differences between scattering matrix terms. The effects of additive system noise on these measurements is not often considered in the literature on this subject. In this paper, the effects of additive system noise on measurements derived from polarimetric SAR data are examined. It is shown how first-order noise effects can be removed and how second-order noise effects can be reduced for some measurements. Some commonly occurring characteristics of polarimetric SAR data which may be attributed to noise, such as the pedestal on a polarization signature, or a broadening in the distribution of the HH-VV phase difference over an area, or a reduction in the magnitude of a correlation coefficient, are identified. The appearance of azimuth ambiguities in polarimetric SAR data is also addressed.<<ETX>>

Calibration of NASA/JPL DC-8 Sar Data

The paper describes a number of improvements in calibration of NASA/JPL DC-8 SAR (synthetic aperture radar) data, intended to make calibrated data from the system available to the majority of the investigators. The improvements include a move to a better site for calibration of the system at the start of each campaign season, the release of software to allow users to calibrate their data, the supply of trihedral corner reflectors and instructions on how to deploy them, and the setting up of an archive of calibrated data. Analysis of the calibration performance of the DC-8 SAR over a number of sites during 1990 suggests that the system is sufficiently stable to allow calibration of the standard data products without the presence of calibration devices within the scene. The limiting factor on DC-8 SAR data calibration would now appear to be the knowledge of the altitude of the aircraft, which determines the relationship between the measured slant range coordinates and the antenna pattern in elevation.

Mapping Sub-Tropical Vegetation Using Multi-Frequency, Multi-Polarization Sar Data

In 1990 the NASA/JPL airborne synthetic aperture radar DC-8 (Airsar) was flown over an area in northern Belize and the surrounding countries of Guatemala and Mexico. The three-frequency polarimetric radar signatures of a variety of natural areas have been extracted, and many have a unique radar signature. Scattering mechanisms which may explain these signatures and results of an image classification technique are presented.

Topographic effects on the antenna gain pattern correction

Analyses and quantifies the topographic effects on the antenna gain pattern: correction of existing spaceborne synthetic aperture radar systems, namely ERS-1, JERS-1, SIR-C, and X-SAR. Simulations and real SAR data of a test site are used. The corrections are carried out taking into account the local surface topography and compared with the standard method based on a reference ellipsoid. Results show that elevation variations in the ERS-1 and JERS-1 cases do not affect significantly the antenna gain pattern correction. For extreme topographic differences, greater than 3000 m, a reference altitude or the radiometric calibration is suggested. On the other hand, for the low-orbit SRL-1/2 terrain information is strongly recommended, particularly, if relief differences within the image are significant, namely greater than 1000 m. Furthermore, it is shown that in the SIR-C case, even if the polarizations of the antenna gain patterns are slightly different, polarimetric calibration errors due to relief variations can be neglected. Finally, implications for forthcoming spaceborne SAR systems, i.e. ERS-2 and RADARSAT, are discussed.

Fitting a two-component scattering model to polarimetric SAR data

Classification, decomposition and modeling of polarimetric SAR data has received a great deal of attention in the literature. The objective behind these efforts is to better understand the scattering mechanisms which give rise to the polarimetric signatures seen in SAR image data. The author describes an approach which involves the fit of a combination of two simple scattering mechanisms to polarimetric SAR observations. The mechanisms are canopy scatter from a cloud of randomly oriented oblate spheroids, and a ground scatter term, which can represent double-bounce scatter from a pair of orthogonal surfaces with different dielectric constants or Bragg scatter from a moderately rough surface, seen through a layer of vertically oriented scatterers. An advantage of this model fit approach is that the scattering contributions from the two basic scattering mechanisms can be estimated for clusters of pixels in polarimetric SAR images. The solution involves the estimation of four parameters from four separate equations. The model fit can be applied to polarimetric AIRSAR data at C-, L- and P-band.

First results from SIR-C calibration

The SIR-C/X-SAR imaging radar took its first flight on the Space Shuttle Endeavour in April 1994. This multi-frequency radar has fully polarimetric capability- at L- and C-band, and a single polarization at X-band (X-SAR). Calibration of polarimetric L- and C-band data for all the different modes SIR-C offers is an especially complicated problem. The solution involves extensive analysis of pre-flight test data to come up with a model of the system, analysis of in-flight test data to determine the actual antenna pattern and gains of the system during operation, and analysis of data from over ten calibration sites distributed around the SIR-C/X SAR orbit track. The SIR-C mission is the first time a multi-frequency polarimetric imaging radar employing phased array antenna has been flown in space. The effort put into the calibration of SIR-C data products has been considerable and is also unique in that this is the first time anyone has attempted to calibrate a spaceborne radar of this complexity. This work was performed by the let Propulsion Laboratory, California Institute of Technology, under contract from the National Aeronautics and Space Administration.<<ETX>>

Calibration results for J-ERS-1 SAR data produced by the Alaska SAR Facility

The Alaska SAR Facility has been receiving and processing SAR data from the J-ERS-1 satellite since Spring 1992. Corner reflectors have been set up for J-ERS-1 SAR calibration at a site near Delta Junction, in central Alaska. Image quality and calibration analysis results from the Delta Junction site and others are presented. The impact of the 3-bit analog-to-digital converter and the automatic stepping of the gain as a function of range in the J-ERS-1 radar receiver on calibration performance has been assessed. Preliminary observations on J-ERS-1 SAR data are that the average signal-to-noise ratio is generally fairly low, in the range 5-6dB. Azimuth ambiguity levels are higher than preflight analysis indicated. Over land, the dynamic range in the backscatter at L-band for /spl sim/36 degree incidence angle is often fairly high. Thus example J-ERS-1 SAR images of vegetated areas, such as tropical rain forests or boreal forests show greater contrast than their counterparts from the European ERS-1, which images at C-band with /spl sim/23 degree incidence angle. Part of the research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the NASA.<<ETX>>

Classification of the Amazon rain forest using JERS-1 SAR data

Imaging radar offers potential for regular monitoring of changes in this region. In particular, the JERS-1 satellite carries an L-band HH SAR system which, via an on-board type recorder, can collect data from almost anywhere on the globe, at any time of year. The authors show how JERS-1 radar images can be used to accurately classify different forest types (i.e. flood plain vs. upland forest) and river courses in the Amazon basin. JERS-1 data has also shown significant differences between data obtained during the dry season and the wet season, indicating a strong potential for monitoring seasonal change. The algorithms used to classify JERS-1 data is a standard maximum-likelihood classifier, using the radar image local mean and standard deviation of texture as input. Rivers are detected using an edge detection and edge-following algorithm. This work was performed by the Jet Propulsion Laboratory, California Institute of Technology, under contract from NASA.<<ETX>>

Validation and calibration of SAR imagery of Manu National Park, Peru

A joint calibration experiment between the synthetic aperture radars (SAR) onboard NASA/JPLs AIRSAR and Japans NASDA J-ERS-1 satellite was performed by cotemporally imaging Manu National Park in Peru on June 7, 1993. The data from both instruments were subsequently processed and calibrated at their respective institutions. In situ data at the site in Peru consisting of ground and aerial photographs, GPS coordinates, dielectric measurements, ecological characterization, rainforest structural parameters, and vegetation species identification were collected in early September 1993. Some of the objectives of this experiment were to quantify the relative calibration of AIRSAR and J-ERS-1 data, identify ecological habitats from radar backscatter, and to classify backscatter response due to structural characteristics of the rainforest. Since temporal, global coverage of the J-ERS-1 L-band HH polarization SAR exists, data from this instrument may provide a means for both remote ecological habitat identification and the ability to monitor the extent and conversion of rainforests as man encroaches on ever more remote regions of the world. This paper includes a discussion of the calibration of the SAR instruments.<<ETX>>

Calibration of SIR-C data products

The SIR-C/X-SAR imaging radar flew on the Space Shuttle Endeavour in April and October of 1994. This multi-frequency radar has fully polarimetric capability at L- and C-band, and a single polarization at X-band (XSAR). Analysis of data collected during the two missions reveals that the SIR-C system performed better than expected in terms of image quality and calibration. The cross-talk and phase stability of both systems has been exceptionally good. Calibration of polarimetric L- and C-band data for all the different modes SIR-C offers has been achieved by analysis of pre-flight test data to come up with a model of the system, analysis of in-flight test data to determine the actual antenna pattern and gains of the system during operation, and analysis of data from calibration sites distributed around the SIR-C/X-SAR orbit track. The SIR-C missions are the first multi-frequency polarimetric imaging radar employing a phased array antenna flown in space. The objectives of flying two missions in one year included the study of seasonal changes in the Earths environment. In this paper, calibration results from both SIR-C/X-SAR missions are presented.