Charles E. Livingstone

Synthetic aperture radar calibration using reference reflectors

A simple expression for the terrain backscatter coefficient is derived in terms of the integrated power of an adjacent known radar reflector in a synthetic aperture radar (SAR) image. It is shown that this technique for SAR image calibration is independent of the radar system focus or partial coherence and thereby possesses an important advantage over the usual technique, which relies on an estimate of the peak of the reflector impulse response. Results from airborne SAR overflights of corner reflectors and active radar calibrators are used to demonstrate the validity and consistency of the method and to show that the method is robust under defocus caused by an incorrect FM rate or inadequate motion compensation of data collected during turbulence. It is also shown that the fading errors associated with the integral method are comparable to or slightly worse than those associated with the peak estimation method. However, this small disadvantage is outweighed by the fact that the integral method is independent of actual resolution. >

Microwave Sea-Ice Signatures near the Onset of Melt

On June 22, 1982, the Canada Centre for Remote Sensings Convair 580 aircraft (CCRS CV-580) made X-band SAR, Ku-band scatterometer, and K-band Radiometer measurements of the sea ice in Crozier Channel. Measurements of the physical properties of the ice and snow cover were in progress at a site in the southern portion of the CV-580 measurement area at the time of overflight. The CV-580 X-band SAR and Ku-band scatterometer were cross calibrated with the University of Kansas Heloscat to examine the frequency dependence of surface signatures. Analysis of the combined airborne and surface characterization data set shows that the microwave signatures of the surface, under the conditions present, were dominated by the snow cover and, in bare ice areas, by surface moisture. At frequencies above 9.35 GHz no scattering cross section/brightness temperature signatures could be uniquely related to ice type over the entire experiment area.

CCRS C/X-airborne synthetic aperture radar: An R and D tool for the ERS-1 time frame

The airborne synthetic-aperture radar (SAR) system developed for the Canada centre for remote sensing is discussed. The SAR consists of two radars at C- and X-bands. Each radar incorporates dual-channel receivers and dual-polarized antennas, a high quality, seven-look, real-time processor; a sensitivity time control for range-dependent gain control; a motion time control for range-dependent gain control; a motion compensation system for antenna steering in azimuth and elevation; and baseband I- and Q-signal phase rotation. The system features a high-power transmitter with a low-power backup and can map to either side of the aircraft, at high or low resolution, at incidence angles which in high resolution span 0 degrees to 80 degrees . The radar operating parameters, data products, key specifications and the motion-compensation scheme used are given. Properties of the real-time imagery are discussed and examples of C-band SAR data in the three operating modes are presented.<<ETX>>The airborne synthetic-aperture radar (SAR) system developed for the Canada Centre for Remote Sensing (CCRS) is described. It consists of two radars, at C-band and X-band. Each radar incorporates the following features: dual-channel receivers and dual-polarized antennas; a high quality, 7-look, real-time processor; a sensitivity time control for range-dependent gain control; a motion-compensation system for antenna steering in azimuth and elevation; and baseband I and Q signal phase rotation. The system also uses a high-power transmitter with a low-power back-up. The SAR maps to either side of the aircraft, at high or low resolution, at incidence angles which in high resolution span 0 degrees to 80 degrees . Radar operating parameters, data products, key specifications and the motion compensation scheme used are presented. Properties of the real-time imagery are discussed and examples of C-band SAR data in the three operating modes are given.<<ETX>>

Consideration of antenna gain and phase patterns for calibration of polarimetric SAR data

A general polarimetric model for orbital and Earth synthetic aperture radar (SAR) systems that explicitly includes key radar architecture elements and is not dependent on the reciprocity assumption is developed. The model includes systems whose receiving configuration is independent of the transmitted polarization (one configuration), as well as systems with two distinct receiving configurations, depending on the commanded transmitted polarization (H or V). Parameters that are independent of target illumination angle and those with illumination angle dependence are considered separately, allowing calibration approaches which are valid for targets at different illumination angles. The calibration methods presented make use of the model linearity to provide tests for the radar model accuracy and for SAR data quality. X-band polarimetric SAR are used to validate the theory and illustrate the calibration approach. The extension of the model and calibration method to other radar systems is discussed. >

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. >

Airborne SAR imagery of ocean surface waves obtained during LEWEX: some initial results

The potential and the difficulties of using airborne synthetic-aperture radar (SAR) to study ocean surface waves are presented. Airborne C-band SAR imagery obtained with the real-time SAR processor on the Canada Centre for Remote Sensing (CCRS) Convair 580 during the Labrador Extreme Waves Experiment (LEWEX) shows waves propagating through the marginal ice zone as well as through open water regions. Examples are chosen to illustrate both coherent and noncoherent scene motion effects. Coherent scene motion effects include velocity bunching. acceleration defocus, and coherence time limitations. Noncoherent scene motion effects include scanning distortion and look misregistration. The slow platform velocity of airborne SAR precludes replication of spaceborne observations, largely due to coherent motion effects. >

Airborne C-band SAR measurements of wet snow-covered areas

Using polarimetric synthetic aperture radar (SAR) data at C-band, the optimum polarization and range incidence angles for the classification of land covers found in the James Bay area, P.Q., Canada-open areas, lakes ice, and forests, all covered with wet snow-have been examined. The separability between classes shows that they can be classified by a single polarization. For /spl theta/ 30/spl deg/. However, the polarimetric parameters were not suitable for classification.

A Calibration Algorithm for Circular Polarimetric Radars

Since circular polarimetric radars can be used to assess the relative contribution of odd-bounce and even-bounce scattering to the response of targets and to implement novel schemes for adaptive rain clutter suppression and radar pulse compression, they are increasingly used in applications such as radar meteorology, biophysical and geophysical remote sensing, and radar navigation. Although methods for calibrating linear polarimetric radars are widely known, they cannot be used to calibrate circular polarimetric radars until account is taken for the different form of the response of calibration targets when measured with respect to a circularly polarized basis. The response of three standard targets used to calibrate linear polarimetric radars (an odd-bounce-reflector and two even-bounce reflectors with different angles of rotation about the radar line-of-sight) does not yield a unique solution to the calibration equations. If, however, the odd-bounce reflector is replaced by either a circularly polarizing or linear polarization selective target, the calibration equations can be solved.

RADARSAT 2 antenna modeling and synthesis using the genetic algorithms

RADARSAT 2 antenna is a phased array antenna, which consists of 512 T/R modules organized in 32 rows in elevation and 16 columns in azimuth. Several instrument modes (ScanSAR mode for example) require a steering on the elevation antenna pattern. In order to come up with RF performance predictions for the various antenna beams, it is necessary to develop an efficient method to synthesize and analyze the beams. In this study, a mathematical model is developed for RADARSAT2 antenna and implemented in a workstation named, the RADARSAT2 antenna pattern synthesis workstation (APSW). The APSW performs two major tasks: antenna pattern modeling and antenna pattern synthesis. The Genetic Algorithm is used for an optimum antenna synthesis. The APSW is described in this paper. Examples of antenna pattern generated by the APSW in the presence of T/R failures, as well as the optimum T/R excitation law to be applied for minimizing the T/R failure impact are also presented.

Symmetry Properties of the Circular Polarization Covariance Matrix

The circular polarization covariance matrix is a convenient method for expressing partially polarized response data with respect to a circularly polarized basis. However, little concerning either the properties of the circular polarization covariance matrix or methods for transforming data expressed in this format has been previously reported in the literature. Here we show (1) how to recover both the diagonal and off-diagonal elements of the circular polarization covariance matrix from response data stored in either Stokes matrix or linear polarization covariance matrix format, (2) how the contribution of physical scattering mechanisms such as odd-bounce, even-bounce, and diffuse or volume scattering are expressed in circular polarization covariance matrix format, and (3) the form of the response after rotation of the target about the radar line-of-sight. Next, we derive the constraints on the matrix elements (and thereby determine the dimensionality of the response) when a target exhibits reflection, rotation, azimuthal, or centrical symmetry. Because the circular polarimetric rotation operator has a particularly simple form, referring the polarization covariance matrix to a circularly polarized basis rather than a linearly polarized basis simplifies the formulation considerably. In many applications, circular polarimetric features are synthesized from data collected using a linear polarization diversity radar. We show that residual amplitude and phase imbalance between channels under a linear polarized basis transforms to cross-talk under the circularly polarized basis.