A study of calibration technique for side looking Airborne Radar
Duan Lei Liang
Radar and Remote Sensing Research Lab Shangshai Jiao
Tong University
Shanghai, China
Abstract
With the development and wide application of microwave remote sensing technique, Specially imaging radar is used in estimation of crop, survey of natural resources, using of land, forecast and monitoring of flood, classification of plants and tactical target, etc. These applications are all determined by the image that converts magnitude of scattering cross-section into difference of brightness. For this, one expects to establish quantitative corresponding relationship between target and its image so that one can develop qualitative remote sensing technique. While the former is mainly determinate by the place, shape, veins and relative variance of brightness of target that the are provided by radar image. To realize this, it is necessary to calibrate the imaging radar first.
This paper is a experimental study of calibration technique on side looking airborne radar.
Introduction to calibration technique
The Calibration of technique are of two types, that is relative calibration used in precise measurement and absolute calibration used in accurate measurement and the latter is not only repeatable, but also the accurate absolute value. Internal calibration permits determination of relative scattering cross-section. While, external calibration permits determination of absolute scattering cross section because side looking airborne radar is an active microwave imaging radar.
External calibration is obtained by return power of known SLAR scattering cross-section. It is often desirable to know scattering cross-section with homogeneous extended target. The azimuth and range dimension of the extended target should be much larger than the corresponding spatial resolution of the SLAR, so that the sufficient average can be performed to reduce the effects of signal fading, dielectric properties and surface roughness. It needed remaining constant over a measurement period of time and known scattering cross-section. All of above mentioned should.
be a slowly varying function of theta over the antenna elevation beam width. And the most difficult thing is to account the inherent biases and absolute gain is to account the inherent biases and absolute gain of the antenna and receiver. Therefore, there is a certain restriction in the actual application. Generally, two different methods for internal calibration can be used, the is for the methods of independent or separate calibration in different parts of the system and ratio methods. The latter is a superior approach because that is less error and can process at very frequent intervals. The separate calibration requires interrupting the measurement series. Furthermore, the calibration of different parts results in many errors, which add up to decrease the precision of the overall calibration.
For the SLAR, a system error is same in all time and spatial because the same system is used for all the measurement. Thus, good relative calibration still allows precise distinction to be made between the characteristic of target at different time and place. Sometimes, the output needs to express in terms of the volts, watts, indication on oscilloscopes and film density rather than scattering cross-section.
