(b) Non-ideal Trajectory Error
Because of the airplane control, wind and other non-ideal conditions, the airplane cannot maintain a straight line in practical flights, and the instantaneous slant range will be changed. The changes of instantaneous slant range will induce two effects: first, an additional phase error will exist in the azimuth echo; second, the echo envelop from each azimuth gate will not be aligned. Therefore, phase error compensation and envelop alignment are the two steps in non-ideal trajectory error compensation.
Substitute Eq. 6.2.3 into Eq. 2.2.5, the additional azimuth phase error can be expressed as
Fig. 6.4 Non-ideal trajectory error
It can be noted that the phase error can be corrected by multiplying a conjugate filter. The envelop dis-alignment induced by the non-ideal trajectory error is illustrated in Fig. 6.4. Assuming the RCM is corrected, the echo of the target is a straight after the range compression in the ideal condition, while the echo of the target with trajectory error is not straight. Before azimuth compression, the envelop alignment must be operated by data shift or interpolation.
Phase compensation and envelop alignment both need the instantaneous slant range differences on each sampling bin. There are mainly two way to acquire the data: first, use the INS data; second, estimate from the echo data.