Communications: radio wave antennas – Antennas – Balanced doublet - centerfed
Patent
1980-12-23
1983-03-01
Tubbesing, T. H.
Communications: radio wave antennas
Antennas
Balanced doublet - centerfed
G01S 1324
Patent
active
043756417
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method used in a tracking radar for obtaining a large unambiguous range. More specifically the invention relates to a method in which the transmitted radar pulses have a high PRF (Pulse repetition frequency) and where the carrier frequency varies from one pulse to another without the range of the radar being limited to a certain unambigous range determined by the PRF.
DESCRIPTION OF PRIOR ART
The most important task of a tracking radar is, after the position of a certain moving target is established, to follow the target, for example, regarding its distance and give information about the continued movement of the target, see for example Barton "Radar System Analysis" Prentice Hall Electrical Engineering Series 1964 Chap. 9, page 263. In known tracking radar systems, radar pulses are transmitted with a certain determined carrier frequency and with such a value of the pulse repetition frequency (PRF) that a pulse reflected from the target returns within a time interval before the next pulse is transmitted, the so called listening interval. The condition that the reflected target echo pulse has to arrive within the listening interval is necessary, since otherwise the position of the target could not be detected unambigously.
It is furthermore previously known to vary the carrier frequency from pulse to pulse of the radar signals transmitted towards the target, see for example the U.S. Pat. Nos. 3,413,634 and 3,372,391. The purpose of this frequency change is to prevent disturbing eches from influencing the reception. In particular, the local oscillator of the radar receiver is tuned concurrently with the transmitted pulses and to the frequency of each of these, so that only desired echo signals are detected and the radar is uninfluenced by undesired disturbing echoes. Known tracking radar stations which use this principle are, however, still bound to the condition that the pulse repetition frequency (PRF) has to be such that the echo pulse returns within the listening interval.
SUMMARY OF THE INVENTION
As briefly mentioned above, the PRF of the radar, the repetition frequency of the transmitted pulses, is crucial for unambiguous determination of the target the radar has to follow, for example, concerning the range, the angle position or the speed. If the target is situated within a certain short range from the antenna, a relatively high value can be chosen for the PRF of the radar without the risk that an ambiguity arises, while if the target range is long, a sufficently low value of the PRF has to be chosen. More specifically it is valid that the so called unambiguity range Ro=c/2f.sub.PRF, where f.sub.PRF is the pulse repetition frequency for the transmitted radar pulses and c is the speed of light. Thus the condition f.sub.PRF .ltoreq.c/2Ro has to be fulfilled for unambiguous detection of the target range.
In a tracking radar it is, however, desirable that the value of the f.sub.PRF be kept at such a high value as possible so that the target can be better followed. A high value means that the information about the target position more often is received than in the case when pulses with a low value of the f.sub.PRF are transmitted. On the other hand a high PRF-value implies, according to the above, a small unambiguous range which can mean that a reflected pulse cannot be detected and that ambiguity about the target range can arise. These two conflicting demands constitute the basic problem for the invention which is intended to be solved by means of the invention. One condition is that the target distance from the beginning is known with an accuracy equal to .+-.1/2 (ct/2), where t is the length of the listening interval.
According to the invention, radar pulses are transmitted whose carrier frequency in a manner known per se varies from pulse to pulse and in accordance with a certain randomly chosen pattern. The number of the chosen frequencies M of a totality N forms a pulse series with a certain time length corresponding to a certain actual initial
REFERENCES:
patent: 3383686 (1968-05-01), Davis et al.
patent: 3883871 (1975-05-01), Moore
Josefsson Lars G.
Oderland Karl-Erik I.
Winnberg Jan-Olov
Telefonaktiebolaget L M Ericsson
Tubbesing T. H.
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