R.K. Hawkins

Ship detection and characterization using polarimetric SAR

Polarimetric information is investigated for ship detection and characterization at operational satellite synthetic aperture radar (SAR) incidence angles (20°‐60°). It is shown that among the conventional single-channel polarizations (HH, VV, or HV), HV provides the best ship‐sea contrast at incidence angles smaller than 50°. Furthermore, HH polarization permits the best ship‐sea contrast at near-grazing incidence angles. The wave polarization anisotropy is used for optimal information extraction from polarimetric SAR data. It is shown that fully polarimetric information permits a significant improvement in the ship‐sea contrast for relatively calm wind conditions, in comparison with conventional (i.e., scalar) single-channel polarizations (i.e., HH, VV, or HV). For rougher sea conditions, the effectiveness of polarimetric tools may be significantly degraded. Ship characterization is also investigated using the symmetric scattering characterization method (SSCM). Identification of ship targets with significant symmetric scattering can provide a useful ship pitch angle estimate under certain conditions.

Simultaneous scatterometer and radiometer measurements of sea-ice microwave signatures

This paper discusses a combined data set obtained in the Canadian Arctic which partially covers the variation with season of the microwave signatures of sea ice. Results draw together airborne collection periods which include: March 1979, April 1979, and June 1980. The data set consists of RC-10 aerial photograph; HH - and HV -polarized backscattering coefficients from a fanbeam scatterometer operating at 13.3 GHz over incidence angles extending from nadir to 60\deg ; 19.4 GHz, H -polarized radiometer brightness temperatures at a fixed incidence angle of 45\deg ; and overlapping X - and L -band SAR imagery. A precision radiation thermometer (PRT-5) was also included in the sensor complement in the 1980 missions. Accumulated statistics for each microwave measurement were determined by segmenting the time histories of the profiling sensors by (WMO) World Meteorological Organization sea-ice class and then recombining the information for each ice category. Eigenvector analysis was applied to specify the radiometer and scatterometer hyperspace boundaries of each sea-ice class and examine the classification utility of this microwave information.

The Influence of Surface Oil on C-and Ku-Band Ocean Backscatter

A comparative study of ocean backscatter depression due to surface oil has been carried out using Ku-and C-band scatterometers supported by some X-and C-band synthetic aperture radar (SAR) imagery. The depression of radar backscatter for both C-and Ku-band HH-polarized radiation has been measured for the incidence angle range from 20° to 50° on two days, September 16 and 17, 1983 on which the average wind and wave height conditions were 3-6 m/s, 0.3 m and ~10-14 m/s, 1.2 m, respectively. Results show that the depression in Ku-band backscatter increases from approximately 3-5 dB at 20° to ~10 dB or more at 30°-40° and then decreases at the larger incidence angles. Generally, the angular dependence of C-band backscatter depression was similar to that at Ku-band, but the peak depression was shifted to slightly larger angles and usually exceeded Ku peak 1-2 dB. The Ku-band results are in good agreement with previous work [9] that tentatively explained the angular results of backscatter depression on the basis of a selective damping of the first-order Bragg resonant waves by the oil film. This explanation, however, is inconsistent with the results of the present work when both C-and Ku-band are considered. The significance of the data is discussed in the context of present and future radar systems (e. g., the ESA ERS-1 SAR and the Radarsat SAR).

Absolute radiometric calibration of the CCRS SAR

Determining the radar scattering coefficients from SAR (synthetic aperture radar) image data requires absolute radiometric calibration of the SAR system. The authors describe an internal calibration methodology for the airborne Canada Centre for Remote Sensing (CCRS) SAR system, based on radar theory, a detailed model of the radar system, and measurements of system parameters. The methodology is verified by analyzing external calibration data acquired over a six-month period in 1988 by the C-band radar using HH polarization. The results indicate that the overall error is +or-0.8 dB (1 sigma ) for incidence angles +or-20 degrees from antenna boresight. The dominant error contributions are due to the antenna radome and uncertainties in the elevation angle relative to the antenna boresight. >

Ship-sea contrast optimization when using polarimetric SARs

The polarization information was investigated in Touzi (2000) for ship detection using calibrated polarimetric Convair-580 SAR data that were collected within the incidence angle range of 45/spl deg/ to 70/spl deg/. It was shown that at operational satellite SAR incidence angles (lower than 60/spl deg/), there is a significant improvement of ship-sea contrast when the full polarimetric information is used instead of the information provided by the (scalar) one channel polarization (HH, VV, or HV). In this paper, the investigation is extended to lower incidence angles, and the robustness of the polarimetric discriminators used for ship enhancement, is assessed at various wind conditions (7, 14 and 20 knots). Polarization channel phase information is also investigated. It is shown that the information contained in the HH-VV channel phase difference, looks to be very promizing for ship enhancement, mainly when a dual-polarized SAR is used.

Determination of antenna elevation pattern for airborne SAR using the rough target approach

Data from a forested region of northern Ontario are analyzed to yield an estimate of the antenna elevation pattern for the Canada Center for Remote Sensing airborne synthetic aperture radar (SAR). The extended uniform area was imaged as a series of short flight segments in which the antenna depression angle was systematically stepped, keeping all other acquisition parameters of the aircraft and SAR essentially fixed. Subsequent analysis of the real-time imagery was then performed, dividing average image powers for discrete bands of pixels across the swath to yield the relative gain of the antenna corresponding to the antenna angles for the center of these bands. Combining the total set of these measurements generates the entire elevation pattern. Results are given for the C-band, HH-pattern over an angular range of approximately 50 degrees and dynamic range of over 30 dB. >

RADARSAT image quality and calibration — Update

Abstract This paper reviews image quality and radiometric calibration aspects of the first two years of operation of RADARSAT. This includes the calibration of almost all beams (a total of more than 25 beams when considering shifted positions of each of the Fine beams), and the stability and calibration accuracies achieved during the mission to date. The measurements show that the SAR performance is better than the specifications. In September 1997, RADARSAT underwent a major configuration change to accommodate the Antarctic Mapping Mission for a period of about five weeks. To achieve this, the spacecraft was rotated to allow imaging from a left-looking geometry. The image quality and calibration results for the left-looking mode are also discussed.

Plans for RADARSAT image data calibration

Abstract RADARSAT, the first Canadian earth observation satellite, is scheduled for launch in 1995. The mission is to provide to users calibrated C-band HH polarized Synthetic Aperture Radar (SAR) data products for five years, at different incidence angles, resolution and area coverage. The SAR is also designed to operate in the ScanSAR mode by combining different elevation beams. Emphasis in the RADARSAT Program is provision of operationally calibrated products which insures targets can be compared radiometrically within each scence and between scences. In order to maintain end-to-end radiometric quality for the mission life, plans are to perform external calibration routinely using precision transponders and homogeneous targets. The calibration data will be analyzed off-line in the calibration workstation and resulting calibration parameters update will be provided to the ground processor to generate calibrated products. The short term variations in the gain of the sensor electronics will be tracked by the on board internal calibration system and these calibration data will be transmitted to the ground with the radar signal data. This allows the processor to automatically compensate for radar gain variations in between the external calibration periods and thus maintain the required radiometric accuracy. This paper describes the calibration requirements for the mission and plans for internal and external calibrations. An outline of the image quality management is also presented.

RADARSAT-1 image quality and calibration — a continuing success

Abstract The Canadian earth observation satellite, RADARSAT-1 was launched on November 4, 1995 with the first image acquired on November 28 of that year. After commissioning it was put into routine operation on April 1, 1996. Since then more than four years of successful operation have been completed, utilizing data for their intended applications. Significant effort has been extended in the provision of radiometrically and geometrically calibrated products to users by the Canadian Data Processing Facility (CDPF). Particular emphasis has been on the maintenance of image quality and calibration as monitored using images of the Amazon Rainforest and of the RADARSAT-1 Precision Transponders (RPT). This first part of the paper will review the image quality and calibration evolution of RADARSAT-1, complementing previously presented reviews on this subject. Data will be given on various image quality parameters related to impulse response, location error, antenna pattern and noise equivalent measures, and on calibration accuracy as achieved to date. Recent work on calibration and image quality improvements for ScanSAR products are also presented. The latter part of the paper describes methodologies developed for maintenance of radiometric calibration performance of RADARSAT-1, including a statistical technique useful for early detection of radiometric problems associated with single calibrated beams.

RADARSAT precision transponder

Abstract This paper describes the set of four RADARSAT Precision Transponders (RPTs) developed for the Canadian Space Agency for the calibration and qualification of the spaceborne Synthetic Aperture Radar (SAR) carried on the Canadian remote sensing satellite known as RADARSAT, launched in November, 1995. The transponder system block diagram, RF diagram, and specification development are described, as well as the overall program which gives the transponder function in the calibration program for RADARSAT. The transponders are deployed at four strategically situated sites across Canada: Fredericton, NB; Ottawa, Ont.; Prince Albert, Sask.; and Resolute, NWT. Some details of the sites and their sensor visitation characteristics are given. Also provided are some early results showing the use and performance of the transponders using ERS - 1 2 and RADARSAT.