The fluctuating output of a radar altimeter is coupled to the roughness of the reflecting surface below the altimeter. In particular, when the altimeter is used as a profiler this interaction becomes of primary interest. In this report the interrelations between the roughness charac¬teristics and the properties of the output signal are investigated for two altimeters of FMCW-type. The influence of interfering noise signals is also discussed. The results show that the noise level of the altitude out¬put is mainly determined by the range spread of the different reflectors on the ground, if interfering signals are negligible. However, a consider¬able reduction of the noise level is obtained if the bandwidth of the output is much smaller than the bandwidth of the doppler spectrum. Also the design of the signal processing has a significant effect. A comparison with the lower bound of Cramer-Rao Inequality shows that the noise level of both an amplitude and phase measuring altimeter approaches this lower bound at hard filtering and optimum design. The relationships between the altitude noise, the profiling error, the smoothing effect of the sensitivity lobe and the spectral characteristics of the surface roughness are also investigated in detail. The results indicate that sensitive rougness measure¬ments from a satellite are feasible, for instance sea-state monitoring.
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