S. Cote

High-Precision Assessment and Calibration of Polarimetric RADARSAT-2 SAR Using Transponder Measurements

Independent assessment and calibration of polarimetric RADARSAT-2 (RS2) synthetic aperture radar (SAR) are conducted using “uncalibrated” data collected at various incidence angles (from 20° to 40°). Analysis of the response of a transponder deployed at HV and VH configurations permits high-precision measurement of RS2 antenna crosstalks. It is shown that the RS2 antenna is highly isolated (better than -32 dB) with crosstalks stable with incidence angle. A new calibration method based on transponder measurements is introduced. It is shown that the transponder calibration method removes almost completely the low antenna crosstalk with a residual crosstalk lower than -43 dB. Only one transmitter-receiver distortion matrix measured at a given incidence angle is required for accurate calibration of the 20 polarimetric modes of the RS2 SAR from 20° to 40 °. Uncalibrated RS2 single-look complex (SLC) data with real and imaginary parts provided in 32-bit floating point are required for an effective application of the transponder calibration method. RS2 “calibrated” data are also considered for the assessment of the actual RS2 polarimetric calibration. It is shown that RS2 calibration meets comfortably the CEOS Cal-Val requirements with a residual crosstalk lower than -32 dB. The RS2 calibration accuracy does depend on the mode (i.e., incidence angle) with residual crosstalk that varies between - 32 and -43 dB. To assess the impact of the residual crosstalk on polarimetric applications, the RS2 data are recalibrated using transponder measurements. Each data set was processed four times with a different lookup table (LUT) to reconstruct the 32-bit floating-point data prior to the application of the transponder calibration method. It is shown that the recalibration may not be required for natural targets of relatively high HV backscattering (higher than - 26 dB), such as forests. Data recalibration improves significantly the accuracy of targets of low HV backscattering measurements, whereas like polarization (HH and VV) does not seem to be affected by the residual antenna crosstalk (-32 dB). Urban targets that manifest significant helicity scattering may have their like polarization affected and require data recalibration. However, RS2 data recalibration requires the deployment of a transponder (at HV and VH polarization) for each mode (i.e., incidence angle). Data symmetrization is introduced as a more convenient way to improve polarimetric RS2 data quality without the need to deploy transponders. It is shown that the symmetrization of the modes with the highest residual crosstalk significantly improves the calibration accuracy with a residual crosstalk lower than -37 dB. The latter is negligible as the RS2 noise floor of β° lies between -30 and -34 dB.

RADARSAT-1 image quality evolution to the extended mission

RADARSAT-1, the first Canadian SAR remote sensing satellite has successfully completed its design lifetime of five and a quarter years. It is currently in extended mission operation meeting customer demands. The Image Quality Control program is kept fully operational, and measured results indicate image quality performance is maintained better than system specification. This paper briefly describes image quality results and recalibration work

Maintaining RADARSAT-1 image quality performance in extended mission

This paper discusses the synthetic aperture radar (SAR) calibration and image quality monitoring of RADARSAT-1 in its extended mission, as evolved from the earlier phases of the calibration plan, after its launch in 1995 and the start its routine operation in 1996. Since the early qualification period of the mission, both single beams and ScanSAR operating modes are monitored routinely, based on Amazon Rainforest images for radiometric calibration performance, and on images of Precision Transponders for image quality performance. After an initial calibration phase, radiometric monitoring showed changes in the characteristics of several previously calibrated elevation antenna patterns and compensation for these changes were, and are still made in the image processor. In addition, a major upgrade of the ScanSAR processor completed at the Canadian Data Processing Facility (CDPF) in 2002 provided significant improvements in image quality and radiometry. Through the five-year nominal mission which ended in 2001 and the four years of the current extended mission, radiometrically and geometrically calibrated imagery products were continuously provided to worldwide users. In late October 2000 however, concerns began to rise of the possibility of failure of the Horizon Scanner 1 (HS1), which would result in operating the spacecraft in a degraded attitude control mode, compared to the current primary operation. Experiments were conducted to better understand the impact on image quality when operating in backup attitude control mode. In mid 2002, aging considerations for the On-Board Recorder also led to survey natural sites within Canadian data reception masks for their potential to support radiometric analyses, as an alternative to the Amazon Rainforest, where images are recorded.

RADARSAT-1 AND -2 government calibration activities

This paper examines the calibration activities assumed by the Canadian government within the RADARSAT program, from RADARSAT-1 commissioning in 1996 to the current period, more than one year into the RADARSAT-2 mission. Concepts, operations and results of the RADARSAT-1 calibration plan are reviewed, including the eleven-year image quality measurement history. Government calibration monitoring activities and results for RADARSAT-2 are also presented, indicating that image quality and calibration measurements are better that the specifications.

The RADARSAT-1 imaging performance, 14 years after launch, and independent report on RADARSAT-2 image quality

This paper summarizes the calibration monitoring activities of the Canadian Space Agency (CSA) executed under the RADARSAT Program. The performance history of the RADARSAT-1 SAR since its commissioning in 1996 is reviewed, along with the calibration systems and methodologies used, in the context of the missions thirteen-year calibration history. Independent image quality measurements for the privately-owned RADARSAT-2, launched in 2008, are also presented. It is shown that the calibration parameters of the RADARSAT-1 SAR have consistently been maintained within the missions design goals and specifications, mostly thanks to payload stability and timely recalibrations performed using the calibration ground equipment. For RADARSAT-2, CSAs measurements report outstanding image quality levels.

Radiometric measurements of the Canadian boreal forest using RADARSAT-1 beam patterns

This paper describes exploratory work in evaluating the use of the Canadian boreal forest in potential support to radiometric calibration of the RADARSAT-1 Synthetic Aperture Radar (SAR) sensor. The primary site for SAR calibration performance and monitoring, in the Amazon rain forest, requires the use of the spacecrafts On-Board Recorder (OBR) to store the images until they can be downloaded to a Canadian data reception facility. In mid 2002, aging considerations for the OBR led to the survey of natural sites within data reception mask of Canadian ground stations. Several boreal forest and mixed Tundra-Taiga sites were tested for their ability to support radiometric analyses in case of an OBR failure. A boreal forest-type area, near Hearst, in the province of Ontario, was chosen for a more comprehensive study. Radiometric measurements covering the entire incidence angle range of RADARSAT-1 were performed using an elevation pattern measurement method adapted from the existing methodology used for Amazon image analyses. Radiometric measurements over the chosen area are reported, examining temporal and seasonal variations. Antenna gain pattern shape determination is also attempted, based on seasonal backscattering profiles of the region. The differences between extracted pattern shapes and their corresponding calibrated patterns then provide indications on the mean term, across swath backscattering behavior of the boreal forest, confirming its potential suitability for radiometric calibration monitoring.

On the use of transponder measurements for high precision assessment and calibration of polarimetric Radarsat-2

An independent assessment and calibration of polarimetric RADARSAT-2 (RS2) are conducted using transponder measurements extracted from uncalibrated data collected at various incidence angles between 20° and 40°. It is shown that RS2 antenna is highly isolated (better than −32 dB), and only crosschannel relative phase correction is required to provide calibrated data that meet comfortably the CEOS Cal-Val requirements. To take full advantage of the excellent RS2 performance in terms of low noise floor (−38 dB), transponder measurements are used for high precision assessment of RS2 calibration parameters. It is shown that the antenna cross-talks are stable with incidence angle, and only one transmitter-receiver distortion matrix is required for accurate calibration of the 20 modes of RS2 (Right looking) from 20° to 40°. A new method based on high precision transponder measurements is introduced for polarimetric RS2 calibration. Data collected at various incidence angles are used to validate the calibration. Transponder measurements using calibrated data indicate a residual cross-talk lower than −43 dB. The residual cross-channel error is lower than 0.3 dB in radiometry, and 5° in phase.

Twelve Years of Radarsat-1 Calibration: Operations Experience and Lessons Learned

This paper examines the calibration performance of the RADARSAT-1 products since spacecraft commissioning in 1996, and briefly reassesses the calibration ground systems and methodologies in the light of the missions twelve-year calibration experience and data record. Choices made in the design, deployment and implementation of the RADARSAT-1 calibration plan are retrospectively explored in the context of successor missions, and more specifically RADARSAT-2, which was declared operational on April 25 2008. Preparations at the Canadian Space Agency calibration monitoring activities for RADARSAT-2 are also presented.

Calibration Maintenance Operations and Activities of the Canadian Space Agency within the RADARSAT Program

The calibration operations and activities of the Canadian Space Agency (CSA) within the RADARSAT Program are covered in this paper. The long-term performance of the RADARSAT-1 Synthetic Aperture Radar (SAR) since spacecraft commissioning in 1996 is first reviewed in the light of the missions fifteen-year calibration history. Choices made in the development and implementation of the RADARSAT-1 calibration plan are covered in the context of its successor missions, more specifically RADARSAT-2, launched in December 2007. Independent RADARSAT-2 SAR performance monitoring activities performed at the Canadian Space Agency are then reported, which also include inter-satellite comparison in support of RADARSAT-1 SAR stability measurements. The RADARSAT Program also pursues the validation of new potential natural calibration sites in Africa and Antarctica, to optimize calibration maintenance operations of future RADARSAT missions.

RADARSAT-1 image quality excellence in the extended mission

RADARSAT-1, the first Canadian SAR remote sensing satellite, continues to provide calibrated data to worldwide users since the start of its routine operation on April 1, 1996, more than 9 years after its launch on November 4, 1995. Both single beams and ScanSAR imagery are still monitored routinely for radiometric calibration performance based on images of the Amazon Rainforest, and for image quality performance using imagery of the RADARSAT-1 precision transponders. This paper briefly describes image quality results and recalibration work.