Foliage transmission measurements using a ground-based ultrawide band (300-1300 MHz) SAR system

The attenuation of a forest clearly impacts the ability of airborne SAR systems to image objects within the forest. The level of this attenuation is a function of tree characteristics over the frequency band used in the radar. To experimentally measure the transmission properties of foliage, a bistatic (line-of-site) wide-band system has been built by the Environmental Research Institute of Michigan (ERIM), sponsored by the Air Force´s Wright Laboratory, Avionics directorate. This system is polarimetric and can operate coherently over the band from 300 to 1300 MHz. The variation in foliage transmission over the frequency band is important because an imaging radar typically operates coherently over a bandwidth. The system can scan foliage in angle to determine spatial variations in the foliage attenuation. This angular variation in foliage attenuation is quite important because imaging radars typically synthesize an aperture by scanning over a range of angles. The ERIM Wide-Band System is ground-based, with one antenna attached to a carriage which can move 10 m horizontally along an elevated rigid track and the other antenna attached to a fixed tripod. Measurements with the system were conducted during July 1991 at the University of Michigan Biological Field Station in Pellston, MI. The measurements of the mean attenuation as a function of depression angle (15-45°) and frequency (300-1300 MHz) of four different forest types are presented. In addition to mean attenuation, the variance in attenuation and the autocorrelation of the attenuation (in angle) are presented. These results imply that the variation of the foliage properties over the bandwidth and scan geometry (or angular variation) will degrade the ability of a radar to focus a foliage obscured object