The present invention relates to a method of determining the rank of ambiguity in distance of radar echoes. It can be applied notably to surveillance radars and also possibly to tracking radars working with low, mean and high frequencies of recurrence for example.
A radar generally sends out bursts of pulses constituting sequences of recurrences, each recurrence being constituted by a pulse followed by a listening period, with one sequence comprising, for example, some recurrences to some tens of recurrences. For a radar working up to a given distance D, the general procedure is to determine the energy required to reach the smallest given target of interest at this given distance D. This distance D defines the duration T of a recurrence by the relationship D=c T/2 where c represents the velocity of the microwaves. Certain targets larger than the given smallest target mentioned here above, located at distances greater than the given distance D, may send back an echo whose energy sufficient is to be detected by the radar. Now, this echo then does not appear in the first recurrence of a sequence for the to-and-fro propagation time of the radar signal exceeds the duration T of a sequence but, from the second recurrence onwards, or in an even higher-ranking recurrence. The radar processing system may then wrongly assume that the target is located within the above-mentioned given distance D. This is the known phenomenon of the ambiguity in distance of radar echoes.
In the working of a radar, the echoes that are ambiguous in distance need special processing. In the case of undesirable fixed echoes, pulses are added to the radar sequencing so as to initialize the input of the Doppler filtering accurately up to the greatest assumed distance of the unwanted echoes, the first recurrences being not processed. These additional pulses take up radar time without providing any advantage to the balance of detection of the useful targets present in the first non-processed recurrence or recurrences.
In the case of moving echoes, whether their detection is desired or not, this method is inefficient and it is necessary to implement a criterion for removing the ambiguity in distance, this criterion being generally based on the use of several bursts of pulses sent out with different periods of recurrence. The echoes then corresponding to the targets located at a distance greater than the given distance D are seen from one sequence to another at a different distance unlike the targets at a distance smaller than the given distance D, which makes it possible to remove the ambiguity. However, this second approach is again costly in terms of radar time.
The aim of the invention is to overcome the above-mentioned drawbacks by making it possible to remove the ambiguity in distance of the radar echoes without making it necessary to have additional radar time.