Search algorithm for the reflected signal of a satellite radio altimeter

Abstract

Satellite altimeters are installed on spacecraft designed to solve problems of Earth exploration, and in addition to altitude, they are able to measure the degree of sea surface roughness and some other parameters. A rough determination of the delay time of the probing signal of the altimeter is the signal search procedure, the quality of which in real conditions can be degraded by the presence of an unknown rate of change in the height of the spacecraft. The need for a search procedure arises every time the altimeter loses the reflected signal, for example, after the satellite crosses the border of the land with the water surface. The presence of the rate of change in the height of the apparatus during the search leads to a slip of the signal relative to the time scale of the altimeter, reducing the efficiency of the search. The decrease in the influence of this factor is facilitated by a reduction in the duration of coherent signal accumulation. The aim of the work is to develop an algorithm for searching for the echo signal of a space-based altimeter, less sensitive to possible changes in the height of the spacecraft while reducing the duration of the search. The paper analyzes and compares the characteristics of two echo signal search algorithms. The first algorithm is the usual one-step procedure that checks the time uncertainty zone step by step using a bank of correlators of a given size. As an alternative, a two-stage search is proposed, the first stage of which selects the points of the uncertainty zone in which the presence of a signal is suspected. At the second stage, the results of the first are retested in order to identify a single point that should be recognized as containing a signal, while the coherent accumulation time is determined by the longest stage. It is shown that the two-stage algorithm is less sensitive to changes in the altitude of the spacecraft, provides a reduction in the duration of the search with the same probability of its correct outcome, and can be recommended for use in a space radio altimeter.