GB2088667A

GB2088667A - A radar system employing pulses of different types

Info

Publication number: GB2088667A
Application number: GB8031288A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1980-09-27
Filing date: 1980-09-27
Publication date: 1982-06-09
Also published as: GB2085252A; GB2085251B; GB2085251A; GB2085252B; GB2088667B

Abstract

A radar system generates groups of pulses, each group containing at least two pulses of different types, for example, different lengths and frequencies, and in alternate groups the order of the pulses is reversed. This means that the transmitter does not need to change pulsetype between adjacent groups. Also, if different pulses have different lengths, a staggered pulse repetition frequency is automatically obtained, this being desirable in order to avoid blind spots in the radar. <IMAGE>

Description

1 GB2088667A 1

SPECIFICATION

A radar system employing pulses of differenttypes This invention relates to a radar system employing pulses of at least two different types, for example, to detect targets at different ranges as described in out U.K. patent specifi- cation 1,424,026. In such a system, it is necessary for the receiving part of the radar to distinguish between the different types of pulse. This can be done, as described by way of example in patent specification 1,424,026, on a time basis by first transmitting a pulse of one type and waiting for all reflections of that pulse from a given range to be received before transmitting a pulse of the other type. Another possibility, not particularly described in the aforementioned specification, is for the receiver to sense the different characteristics of the different pulses (e.g. frequency and/or pulse length) thereby distinguishing between them. A system in which filters are used to distinguish between pulses of different frequency is described in U.K. patent specification 1,552,877.

In systems where the receiver distinguishes between the different pulses by sensing their different characteristics, there is no need to wait for all wanted reflections of a first pulse (of one type) to be received before transmitting another pulse (of the other type). Accordingly, the two pulses of different types can be transmitted one immediately after the other, with no time spacing. Hitherto, such systems, having a pair of different pulses with no time spacing between them, have been designed so that the two different pulses were always transmitted in the same order, this meaning that a change in pulse-type is required between each group of pulses.

According to this invention there is provided a radar system comprising a transmitter for transmitting groups of pulses, each group including pulses of different types, the pulse at the end of each group, except the last group, being of the same type as the pulse at the beginning of the next group.

Thus, by employing the invention, it is possible to reduce the frequency with which the pulse-type is changed. It is believed that this can be an advantage from the point of view of transmitter design.

According to another aspect of the inven tion, there is provided a radar system compris ing a transmitter for transmitting groups of pulses, alternate groups including a pulse of a first type followed without any time spacing by a pulse of a second type and the other alternate groups including a pulse of the see ond type followed without any time spacing by a pulse of the first type.

Preferably the order of all the pulses in alternate groups is reversed thereby ensuring 130 that a pulse at the end of each group is the same as the pulse at the begining of the next.

If, as will normally be the case, each group of pulses contains pulses of different lengths, the reversal of the order of the pulses can automatically produce a stagggered pulse repetition frequency. A staggered repetition frequency is. sometimes required in order to eliminate blind spots, i.e. target velocities and target ranges to which the radar would otherwise be blind.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying drawings in which:Figure 1 is a schematic block diagram of a radar constructed in accordance with the invention; and Figure 2 shows at A to J waveforms ap- pearing at points denoted by respective reference letters on Fig. 1.

Referring first to Fig. 1, the radar comprises a timer 1, a transmitter 2, a receiver 3, a T/R cell 4 and an antenna 5.

The transmitter receives timing signals A from the timer 1 and these timing signals are passed to a monostable circuit 6, which produce pulses B as shown on Fig. 2. The pulses B are passed to the drive circuit of a multipac- tor magnetron 7 which is caused to generate a microwave frequency output during the period of each pulse B. Pulses B are also fed to a selector 8 which passes alternate pulses directly to a monosta- ble circuit 9 whose period is slightly less than that of the monostable 6. The other alternate pulses are fed indirectly to the monostable circuit 9 via a delay line 10. Thus the waveform at point C is as shown on Fig. 2 where alternate pulses are slightly delayed with respect to the timing pulses A.

The pulses D are passed to a ramp generator 11, which produce pulses E whose timing is similar to that of pulses D, but whose amplitude increases with time. The pulses E are passed to a control input of the multipactor magnetron 7 and serve to increase its frequency by an amount depending on the volatage at the control input.Thus the output frequency of the magnetron 7 is, during the periods of pulses E, swept through a range of frequencies slightly above the basic frequency of the magnetron i.e. the frequency at which it operates when no control pulse is present at its control input. The output of the magnetron is thus as shown at F on Fig. 2. It should be noted that waveforms F to J on Fig. 2 are plots of frequency against time in contrast to waveforms A to E which are plots of ampli- tude against time.

Referring to the waveform F it will be seen that this consists of spaced pairs of pulses, each pair comprising a relatively long frequency swept pulse for a period defined by a monostable 9, immediately adjacent, without 2 GB2088667A 2 any time spacing, a relatively short, unswept pulse whose length is defined by the difference in periods of monostables 6 and 9. Furthermore it will be noted that alternate pairs of pulses are reversed which means that the multiplicator magnetron is not required to undergo a frequency change between pulse pairs. The pulses generated by the magnetron 7 are passed through the T/R cell 4 and are transmitted as electromagnetic waves into the surrounding medium by an antenna 5.

Reflections of the transmitted pulses are received by the same antenna 5 and passed through the T/R cell 4 to the receiver 3.

Waveform G on Fig. 2 shows signals received from two targets at different ranges. These are separated by a range gate 12 into two channels 13 and 14 for short range and long range returns respectively. The short range channel 13 incorporates a filter 15 which filters out the frequency swept pulses to leave the short unswept pulses as shown at H. These are detected by circuit 16 and then passed through a moving target indicator circuit 17, which can operate according to any known principle, for example, using the Dopper effect or using a delay line canceller. The effect of the circuit 17 is to remove all pulses derived from stationary targets which are as- sumed not to be of interest. The circuit 17 receives a reference timing signal from the timer 1. The output of the moving target indicator circuit 17 is passed to a video display system 18, which also receives timing signals from the timer 1, to control the timing 100 of successive scans.

The channel 14 has a filter 19 which removes the short unswept pulses leaving only the swept pulses as shown at 1 on Fig. 2.

These swept pulses are subjected to pulse compression in a circuit 20, the output of which is shown at J. The output J is subjected to a treatment similar to that of pulses H, by a detector 21, a moving target indicator circuit 22 and a video display 23.

It will readily be seen from Fig. 2F that, because of the reversal of the order of the long and short pulses in successive pulsepairs, the period between successive short -50 pulses and successive long pulses can have one of two values. There are thus two pulse repetition frequencies. This is known as p.r.f. staggering and is introduced deliberately in known radar systems to prevent blind spots in a manner known per se. The timing signals supplied to the video display and the MTI circuits need to take this staggered p.r.f. into consideration.

It will be appreciated that the described embodiment of the invention is only one of many different forms that the invention can take. The invention is, for example, applicable to sonar radar systems. Also, whereas in the illustrated system the different types of pulse 0,5 differ in frequency and in length, it would be possible in an alternative system for them to differ only in frequency or only in length or to have some other distinguishing characteristic. Another possible modification would be for each group of pulses to incorporate three or more pulses some or all of which are of different types. Another modification would be to use a single video system instead of the two systems shown at 18 in Fig. 3 so that targets of all ranges are displayed on one screen. In such an arrangement, it would be possible for the range gate 12 to be positioned at the ends of the channels 13 and 14 instead of at the beginning. In some radar applications, it is not necessary to have a video display and the video system shown at systems 18 and 23 could be replaced by some form of digital processing system.

Claims

1. A radar system comprising a transmit- ter for transmitting groups of pulses, each group including pulses of different types, the pulse at the end of each group, except the last group, being of the same type as the pulse at the beginning of the next group.

2. A radar system comprising a transmitter for transmitting groups of pulses, alternate groups including a pulse of a first type fol- lowed without any time spacing by a pulse of a second type and the other alternate groups including a pulse of the second type followed without any time spacing by a pulse of the first type.

3. A radar system according to claims 1 or 2 in which the pulses of different types have different lengths.

4. A radar system according to claims 1, 2 or 3 in which the pulses of different types have different frequency characteristics.

5. A radar system according to claim 4 in which the pulses of one type are of constant frequency and the pulses of another type are frequency swept.

Printed for Her Majesty's Stationery Office by Burgess Et Son (Abingdon) Ltd-1 982. Pu blished at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.

CLAIMS (6 Oct 1981) 1. A radar system comprising a transmitter designed to transmit groups of pulses, each including pulses of a first and a second type and a receiver adapted to distinguish between the pulses of different types and to use them for the detection of targets within respective different range zones, characterised in that the transmitter is adapted to transmit, in some groups, a pulse of the first type followed by a pulse of the second type and, in other groups, a pulse of the second type followed by a pulse of the first type.

2. A radar system comprising a transmit- ter designed to transmit groups of pulses, each including pulses of a first and a second type and a receiver adapted to distinguish between the pulses of different types and to use them for the detection of targets within different range zones, alternate groups inc!ud- 3 GB 2 088 667A 3 ing a pulse of a first type followed without any time spacing by a pulse of a second type and the other alternate groups including a pulse of the second type followed without any 5 time spacing by a pulse of the first type.