Method in a tracking radar to attain a large unambiguous range for detected targets by means of radar pulses with high repetition frequency

A method in a tracking radar for achieving a large unambiguous range for a detected target in which radar pulses are transmitted in series with a certain high pulse repetition frequency (f.sub.PRF), a certain pulse length (.tau.) and a certain listening interval (t-.tau.) between two subsequent pulses, the carrier frequency varying from one pulse to the next following in one and the same series, but reappearing in the same order from one series to the next so that for a certain series a total of M frequencies appear, where the number M is chosen so that M.t.sub.m 1/f.sub.FRF and where t.sub.m is the smallest value of the listening interval (t-.tau.) and f.sub.FRF is the frequency with which the pulse series are repeated, comprising (a) measuring the detected target range with an accuracy corresponding to at least half of a listening interval (t-.tau.) to obtain a certain initial value R.sub.o ;

• (b) choosing a certain value of the frequency repetition frequency f.sub.FRF so that the condition c/2 f.sub.FRF >R.sub.o is fulfilled where C is the speed of light;
• (c) after one or more pulse series have been transmitted and the corresonding target echo pulses belonging to a certain series have been received, making a new measurement of the target range so that a new value R.sub.1 is obtained;
• (d) if the measured distance R1<Rk where Rk is the highest value for which c/2f.sub.FRF >Rk being valid, a new series with the same number M frequencies and the same order as previously being transmitted, while if R.sub.1 >Rk, a new pulse series with a greater number M.sub.1 >M frequencies corresponding to a lower value on the frequency f.sub.FRF being transmitted.