When not being used to acquire data, the AIRSAR system is maintained in a laboratory at JPL. Approximately two weeks before the start of a campaign, the AIRSAR system is shipped to NASA’s Dryden Flight Research Center in Edwards Air Force Base, California for installation on a DC-8 aircraft.
The first mode implemented in the AIRSAR system (operational since 1988) was the three-frequency polarimetric mode, where fully polarimetric data are acquired simultaneously at C-band, L-mode and P-band. This provided prototype data for the Shuttle Image Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) science team. Indeed , many of the algorithms implemented for SIR-C data calibration and analysis were developed using AIRSAR data. At the same time, the addition of L-band and C-band antennas in front of the left wing, separated from the aft antennas by 20 meter, made it possible to acquire along-track interferometric (ATI) data at L- and C-band, while simultaneously acquiring polarimetric data at P-band. We do not currently have an operational ATI processor capable of providing data for customers on a standard basis, however we will have this completed by the middle of 1999.
In 1990 NASA, in collaboration with an Italian consortium(CORISTA), approved the addition of another set of C-band antennas to implement a single-pass, fixed baseline cross-track interferometric (XTI) for topographic mapping. The C-band antennas were provided by CORISTA, while NASA sponsored the system modifications and processor development. This mode of the AIRSAR system became known as TOPSAR and data have been acquired since 1991.
In 1995 TOPSAR was extended to acquire XTI data simultaneously at C-band and L-band. All TOPSAR interferometers can be operated in single or dual baseline modes. For single baseline operation signals are transmitted out of one antenna only, and the received signals are measured simultaneously through two antennas. In the dual baseline mode, signals are alternately transmitted out of the antennas at either end of the baseline, while the received signals are measured simultaneously through both antennas.
In 1998, we began testing two new additions to the AIRSAR instrument adding an 80 MHz bandwidth option to the existing 20 MHz and 40 MHz bandwidth, and a new mode called Polarimetric Inteferometry (POLTOP). The 80 MHz upgrade was initially for L-band VV antenna but has been expanded to add a fully polarimetric L-band 80 MHz antenna. POLTOP is being added to the C-band system and will collect fully polarimetric interferometry data, useful for studying how different polarizations interact with vegetated areas. In June 1998, test data were collected with these two new features and the data are currently being evaluated and calibrated.