Abstract
Inverse Synthetic Aperture Radar (ISAR) is a technique for forming high-resolution images by utilizing the information inherent in the differential target Doppler that results from rotation of an object (the target) relative to the radar. Its applications include imaging and discrimination of ships and aircraft with small airborne or land based radar systems and imaging of planets and earth satellites with land based radar systems that utilize high gain antennas. In the past, satellite-imaging radar has primarily used microwave frequencies in order to provide high resolution with a relatively small synthetic aperture. Microwaves also avoid the defocusing effects of ionospheric group delay and dispersion that become significant below about 1 GHz. This work extends the ability to image satellites from land based radar systems down into the Very High Frequency (VHF) range and includes testing of the techniques on data collected with radar systems that operate between 150 and 400 MHz.

In order to form high-resolution (< 1 meter) images of satellites at these frequencies, very large synthetic apertures (> ~30 degrees) and ultra-wide bandwidths (relative BW > ~50%) are needed. In addition to these challenges, the dispersive effects of the ionosphere can limit range and cross-range resolution to anywhere from 10 to 100 meters if they are not estimated and removed, even for the relatively benign conditions typically found at mid-latitudes. In this work, the defocusing effect of the ionosphere is fully mitigated by using the radar data in conjunction with image focus quality metrics to estimate the Total Electron Content (TEC) and to then remove the ionospheric dispersion from the radar data. A byproduct of the procedure is a high precision (~1e15 el/m^2) estimate of TEC along the radar-target ray path along with its variation across the synthesized aperture.

The imaging procedure is described and an analysis is given of its expected performance in terms of image resolution and TEC measurement accuracy. The results are illustrated with sub-meter resolution satellite images generated from data taken at the Stanford "Big Dish" and at an SRI facility on Ascension Island. The two sites test the performance of the technique in a benign mid-latitude ionosphere (Stanford) and in a more severe equatorial ionosphere (Ascension), and for apertures of over 60 degrees and relative bandwidths of over 65%.