RADAR, SONAR AND SIMILAR SYSTEMS

Abstract: A transmitter (2) generating a train of pulses arranged in groups of two, each group including a long pulse and a short pulse of different frequencies. These long and short pulses are separated in a receiver (3) by a range gate (12) which passes the pulses through the upper receiving channel when reflected from long ranges and the lower receiving channel when re­ ceived from short ranges. The upper channel includes a filter (19) which passes only the long pulses (which are frequency swept). These pulses are compressed in a compressor net­ work (20). The lower channel includes a filter (15) which passes only the short pulses which, in this particular embodiment, are not frequency-swept. The transmitter (2) reverses the order of the pulses in adjacent groups. Alternatively, both the long and short pulses may all be frequency-swept. In either event there are adjacent pairs of swept pulses in contrast to known systems in which each swept pulse has immediately been pre­ ceded or followed by an unswept pulse.

Description: This invention relates to a radar system which is defined for the purpose of this specification as apparatus for deriving information about the position or movement of a feature in a medium surrounding the apparatus by transmitting energy, through the medium, to a feature in said medium and observing the energy travelling to the said appratus from the aforesaid feature. Whilst the invention is particularly applicable to apparatus which transmits and receives microwave radiation it is also applicable to acoustic or optical systems. Also, whilst the invention is particularly applicable to radar systems which sense the direction and time of arrival of the energy it is also applicable to systems which sense just the direction or just the time of arrival. It is also applicable to equipment which observes a Doppler shift between the transmitted and received signals. The term "radar" thus includes range finders, direction finders, and speed measuring equipment. The invention particularly relates to radar systems employing pulse compression. In such a system a relatively long pulse is modulated before transmission, e.g., by sweeping its frequency from one value to another between the beginning and end of the pulse. On reception, the modulation of the pulse allows it to be compressed into a short time period. The use of pulse compression allows the transmission of a pulse which, because of its relatively long duration, has a relatively high energy whilst avoiding a loss of range resolution which would ordinarily be expected with an increase in pulse length. When a pulse is received after reflection from two features at slightly different ranges, the received echoes of this pulse may overlap in time. The pulse compression system is normally able to separate such overlapping echo signals However, if the aforementioned features are at close range the received power may be so strong as to saturate initial stages of the receiver: causing it to behave non-linearly. This prevents the pulse compression system from operating properly and thus a close range target adjacent to another larger target or a large source of clutter may not be detected. This problem was discussed by Glasgow in US. Patent Specification 3945011 at the time when pulse compression was first being considered for use in civil aviation radars. The Glasgow Patent proposed a solution to the problem which involved the transmission of a short unswept pulse after each long swept pulse. The long pulses were used in the receiver after reflection from long range targets and were subjected to pulse compression whilst the short pulses were used when reflected from short range targets and were not subjected to pulse compression. The aforementioned problem was thus overcome. GB-A-1552877 describes a similar proposal, except that the pulse repetition frequency of the short pulses is greater than that of the long pulses, in order to increase clutter suppression. It is a principal object of this invention to provide an alternative solution to the same problem as that dealt with by Glasgow and described above. The invention provides radar apparatus comprising: a transmitter adapted to transmit during groups of periods, the spacing between the groups being larger than that between the periods of a group, each group containing periods of first and second different types of transmission, the transmitter being also adapted to modulate or code the transmission during a period of each group in such a way that the signal transmitted during these periods can be pulse compressed on reception and a receiver adapted to distinguish between received signals of the said different types and between signals received after reflection from features in first and second range zones, the receiver, when in operation, using received signals of the first type to give an indicatin of the positions of features in the first range zone and received signals of the second type to give an indication of the positions of features in the second range zone, the receiver including pulse compression means for compressing received signals which are modulated or coded as aforesaid; characterised in that said periods of first and second different types of transmission in each group are contiguous and in that the transmitter is adapted to modulate or code as aforesaid the transmission during the last period of one group and the first period of the next group. The aforementioned modulation, which allows pulse compression on reception, is preferably a continuous frequency sweep but alternative possiblities are feasible For example a discontinuous frequency sweep (sometimes called a "staircase"); or phase changes; or even amplitude modulation can be used to achieve the same effect. The aforementioned first and last periods of transmission (pulses) may be modulated (e.g. frequency swept) in the same way or in different ways. Modulation in different ways (e.g. by frequency sweeping in different directions or at different rates) facilitates separation of the pulses in the receiver. The transmitter is preferably adapted to transmit, in each alternate group of pulses, a long modulated pulse followed by a short pulse; and in each other group of pulses a short pulse, followed by a long modulated pulse. Certain advantages can be obtained if each group of pulses comprises a long, frequency swept, pulse preceded and followed by short pulses, also frequency swept. In this case it is preferable for the short pulses to be swept in opposite directions or with different rates of sweep to facilitate separation thereof in the receiver. One way in which the invention may be performed will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic block diagram of one particular radar constructed in accordance with the invention; and Figure 2 shows, at A to J, waveforms appearing at points denoted by respective reference letters on Figure 1. Referring first to Figure 1, the radar comprises a timer 1, a transmitter 2, a receiver 3, a T/R cell 4 and an antenna 5. The transmiter receives timing signals A from the timer 1 and these timing signals are passed to a monostable circuit 6, which produces pulses B as shown on Figure 2. The pulses B are passed to the drive circuit of a microwave pulse generator 7 which is caused to generate a microwave freqency output during the period of each pulse B. Pulses B are also fed to a selector 8 which passes alternative pulses directly to a monostable 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 Figure 2 where alernate 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 microwave generator 7 and serve to increase its frequency by an amount depending on the voltage at the control input. Thus the output frequency of the generator 7 is, during the periods of pulses E, swept through a range of frequencies slightly above the basic frequency of the microwave generator 7, i.e., the frequency at which it operates when no control pulse is present at its control input. The output of the microwave generator is thus as shown at F on Figure 2. It should be noted that waveforms F to J on Figure 2 are plots of frequency against time in contrast to waveforms A to E which are plots of amplitude against time. The microwave generator 7 is a voltage controlled oscillator followed by an amplifier. The voltage controlled oscillator can be a PIN diode. Referring to waveform F it will be seen that this consists of spaced pairs of groups of pulses, each pair comprising a relatively long frequency swept pulse for a period defined by a monostable 9, immediately adjacent, without any time spacing, a relatively short, unswept pulse whose period or length is defined by the difference in periods of monostables 6 and 9. Furthermore it will be noted that in this particular embodiment of the invention the order of the long and short pulses is reversed in alternate pairs of pulses wich means that the frequency or mean frequency of consecutive pusses does not change between pulse pairs. The pulses generated by the microwave generator 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 where they pass through a filter 3A and a mixer 3B where they are mixed with a frequency from a local oscillator 3C. Waveform G on Figure 2 shows signals received from two targets at different ranges. These are separated by a range gate 12 into 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 additional range gates and Doppler filters or using a delay line canceller. The effect of the circuit 17 is to remove all pulses derived from stationary targets which are assumed 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 of successive scans. The channel 14 has a filter 19 which removes the short unswept pulses leaving only the swept pulses as shown at I on Figure 2. These swept pulses are subjected to pulse compression in a circuit 20, the output of which is shown at J. The ouptut 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 Figure 2F that, because of the reversal of the order of the long and short pulses in successive pulse-pairs, the period between successive short 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 , e.g., as explained in the 1962 edition of Skolnik's Book "Introduction to radar systems" on page 131. The timing signals supplied to the video display and the MTI circuits need to take this staggered p.r.f into consideration. Another advantage accrues from the use of the illustrated radar when there is a large source of clutter at a slightly closer or further range than a small close-range target. The return from such a source of clutter may saturate the common parts 3A, 3B and 3C of the receiver during periods when sone of the short return pulses are being received from the target. These short pulse returns are thus lost in the receiver. However, since some of the long, swept pulses precede, and others follow the associated short pulse, it is assured that not all of the short pulse returns are lost. 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 systems. Also, whereas in the illustrated system the different types of pulse 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 world 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 for the transmitter to produce groups of pulses similar to those shown in Figure 2F but with the frequency sweep in opposite directions in adjacent groups. Another modification would be to use a single video system instead of the two systems shown at 18 and 23 in Figure 1 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 17 and 23 could be replaced by some form of digital processing system.

Claims: 1. Radar apparatus comprising: a transmitter (2) adapted to transmit during groups of periods the spacing between the groups being larger than that between the periods of a group, each group containing periods of first and second different types of transmission, the transmitter being also adapted to modulate or code the transmission during a period of each group in such a way that the signal transmitted during these periods can be pulse compressed on reception and a receiver (3) adapted to distinguish between received signals of the said different types and between signals received after reflection from features in first and second range zones, the receiver (3), when in operation, using received signals of the first type to give an indication of the positions of features in the first range zone and received signals of the second type to give an indication of the positions of features in the second range zone, the receiver (3) including pulse compression means (20) for compressing received signals which are modulated or coded as aforesaid; characterised in that said periods of first and second different types of transmission in each group are contiguous and in that the transmitter is adapted to modulate or code as aforesaid the transmission during the last period of one group and the first period of the next group. 2. Radar apparatus according to claim 1 characterised in that the transmitter (2) is adapted to apply said modulation in the form of a frequency sweep. 3. Radar apparatus according to claim 2 characterised in that the said first and last periods are of the same length and the transmitter (2) is adapted to frequency sweep the said last and first pulses in the same direction. 4. Radar apparatus according to any of claims 1 to 3 characterised in that the transmitter (2) is adapted to transmit, in each alternate group of periods, a modulated period of a first type of transmission followed by a period of a second type and, in each other group of periods, a period of the second type followed by a modulated period of the first type. 5. Radar apparatus according to any preceding claim characterised in that it is adapted to operate at microwave wavelengths. 6. Radar apparatus according to any preceding claim characterised in that the periods of different types have different lengths. 7. Radar apparatus according to claim 6 characterised in that the receiver (3) has means (15, 16, 17, 18, 20, 21, 22, 23) for using the long periods when received from long ranges and the short periods when received from short ranges.

Inventors: ROUSE, DAVID GEORGE
Applicants: THE MARCONI COMPANY LIMITED
Application year: 1981
Main classification: G01S13
IPC codes: G01S13