M.Yu. Nesterov

FMCW radar altimeter extracts information of altitude from beat signal. Beat signal PSD is asymmetrical and lengthen out to the right due to extent of reflecting surface. The wider surface backscattering or antenna directivity pattern the more extended to domain of higher altitudes is PSD curve. Nonetheless the left slope of spectrum is of much fewer variability. Common altimeter estimates beat signal frequency by up-zero counter. This estimate correspond to root-mean-square value and thus implies bias of current altitude estimate. The paper presents a new method of current altitude estimation via analysis of the beat signal spectrum shape. At first algorithm of numerical calculation of beat signal PSD is described. Results of calculation and in-situ measurements indicate that true altitude corresponds asymptotically to the middle of PSD rising edge. Method of estimation of PSD shape parameters is further deduced. Required point of rising edge middle is evaluated as inflection point left to the maximum. Flight test data are processed in accordance with the method described. Results of processing compared with common altimeter readings give evidence of much smaller bias in altitude estimation in comparison with traditional approach based on evaluation of PSD center of gravity.