Figure 1: Simulated results of the PN-Gating method. Standard deviation of the amplitude and the phase error as a function of SNR for a pseudo noise code and a Walsh code and for different phase distributions across the antenna array: red - constant, blue - random and green - linear phase settings of the TRM

Monitoring the gain and phase variations in the transmit and receive paths of each of the 384 TRM in the TerraSAR-X front-end could be performed module by module, the so-called Module Stepping mode on ASAR. Greater accuracy can be achieved using coding techniques, because measurements can be made with the antenna, or parts of the antenna, operating under realistic conditions, e.g. power supply loading.

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.