In this mode, a series of N characterisation pulses are fed through the complete front-end and routed back via a special calibration loop. In each TRM, a unique code sequence of length n is applied. The characteristics of individual TRM can then be extracted by cross-correlating the recorded sum signal with the code sequences. As it was originally planned to use PN codes the method was called PN-Gating and the special module characterisation mode on TerraSAR-X is termed the PN-Gating mode. Unlike the PN codes, the cross-correlation of optimised orthogonal codes, whose length is an even number of bits, is zero; consequently the inherent code error disappears. Therefore, the PN-Gating mode was implemented using orthogonal Walsh codes.
In Figure 1, results for the TerraSAR-X antenna are plotted for two different types of codes, a PN code and a Walsh code. The estimation error of the amplitude and phase are plotted as a function of SNR, for different phase distributions of the modules and across the antenna array. The results show, that for the TerraSAR-X instrument with 384 modules individual TRM can be characterised with a phase and amplitude accuracy of better than 2.5° and 0.2 dB, respectively. Hence, PN-gating is a promising method for precisely characterizing individual TRM during the lifetime of an active SAR antenna.