Formation of the Sounding Signal of an Orbital Radio Altimeter

The accuracy of the formation of sounding signals of quasi-continuous radars is determined by the requirements for the permissible instrumental component of the error in measuring of the distance and Doppler frequency shift. High-precision orbital radio altimeters, as a rule, use precision signals with linear frequency modulation (chirp signals) as sounding signals, the linearity characteristics of which are quite demanding, since deviations from the linear law of change in the frequency of the radio altimeter sounding signal cause additional measurement errors. To reduce the instrumental error of the radio altimeter, it is necessary to improve the quality of the formation of the probing chirp signal, which can be ensured by forming them digitally. To assess the possibility of using a synthesizer based on the 1508PL8T microcircuit as a generator of the probing signal of a radio altimeter, measurements of the frequency modulation linearity were performed. The linearity of the modulation was estimated by the deviation of the instantaneous phase of the synthesized signal from the quadratic dependence. Studies have shown that fast fluctuations of the chirp signal frequency are significantly influenced by the time it takes the synthesizer to change the rate of rise of the signal phase. The selection of the parameters of the synthesized signals and the characteristics of the filters at the output of the digital-to-analog converters made it possible to reduce the formation error to 2.9·10−3 rad (RMS). When choosing the type of integrated synthesizer, it is necessary to focus on microcircuits with a high DAC clock frequency (above 1 GHz) and a short time spent on changing the output signal frequency (no more than 4 clock periods).