AIRSAR and POLARSCAT cross-calibration using point and distributed targets

It has become common practice over the past few years to use point calibration targets with known scattering properties as a means for calibrating imaging SAR systems. The calibration accuracy associated with this approach is vulnerable to two sources of error. The first is related to the linearity of the system transfer function. To meet the required high signal-to-clutter ratio, the point calibration targets are designed to have a very large radar cross section, which places the return from such targets in the extreme upper part of the system´s dynamic range. In contrast most terrain targets produce returns in the lower part of the system´s dynamic range. Nonlinearities between the lower and upper segments of the system transfer function will translate into a calibration bias error. The second error source is related to the calibration algorithm itself, namely the fact that “point” targets are used to calibrate radar intended for measuring “distributed” targets. To evaluate the SAR calibration accuracy, a cross-calibration experiment was conducted involving the JPL AIRSAR and the University of Michigan´s truck-mounted polarimetric scatterometer (POLARSCAT). The experiment was conducted at a test site near Pellston, Michigan. Two calibration algorithms were used with the SAR. The first was based on the responses from trihedral corner reflectors that had been deployed in the field. The second algorithm was based on the responses from distributed targets (bare soil surfaces, short grass, and tall grass fields) whose Mueller matrices were accurately measured by the calibrated polarimetric scatterometer. The results of this study demonstrate that calibrating a SAR using distributed targets is inherently a more accurate process