GB1594604A

GB1594604A - Radar arrangements

Info

Publication number: GB1594604A
Application number: GB3469776A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1977-08-22
Filing date: 1977-08-22
Publication date: 1981-08-05

Description

(54) IMPROVEMENTS IN OR RELATING TO RADAR ARRANGEMENTS (71) We, THE PLESSEY COMPANY LIMITED, a British Company of Vicarage Lane, Ilford, Essex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a radar arrangements and relates more particularly to so-called secondary surveillance radar (SSR) arrangements.

One well known method of achieving IFF (identification friend or foe) interrogation of an aircrafts transponder using SSR techniques is to use a sum and difference interrogation technique and it is common to use the same aerial arrangements and sum and difference circuits as are used for interrogation for receiving and processing the transponder replies. However, this system of interrogation requires very accurately defined phase characteristics to be provided and maintained which can lead to installation and maintenance problems. The present invention is concerned with an interrogation system of this general form but in which the phase requirements are much less stringent.

According to the present invention there is provided a secondary surveillance radar transmit/receive system comprising aerial means divided into two parts sum and difference means arranged to be fed with aerial output signals from the two aerial parts; processing means arranged to be fed with output signals provided by the sum and difference means for affording an indication of the bearing of a received signal, and means for affording a differential phase shift between aerial output signals applied to the sum and difference means, the said differential phase shift being substantially 900 for receiving purposes and substantially zero for transmit purposes.

The differential phase shift may conveniently be afforded by 900 phase shift means connected in the output of one of the aerial parts, switch means being provided for switching the phase shift out of circuit for transmit purposes.

In a preferred transmit/receive system which operates on different frequencies for transmit and receive the 90" phase shift means may be afforded by a passive delay which is connected in the output of one of the aerial parts and which affords a substantially 900 differential phase shift for receive purposes and substantially zero differential delay for transmit purposes.

An exemplary embodiment of the invention will now be described, reference being made to the drawings accompanying the Provisional Specification in which: Figure 1, is a block schematic diagram of a known form of SSR interrogator; Figure 2, depicts the sum and difference patterns afforded by the interrogator of Figure 1; Figure 3, is a block schematic diagram of a known form of sum/difference receiving system for use in conjunction with the SSR interrogator of Figure 1; Figure 4, is a block schematic diagram of a SSR receiving system according to the present invention suitable for use in conjunction with the SSR interrogator of Figure 1, and; Figure 5, depicts the receiving patterns afforded by the SSR receiving system of Figure 4.

In Figure 1 of the drawings, there is shown a known form of SSR interrogator which consists of a linear array of aerial elements 1, half of which are fed from one output port of a hybrid circuit 2 and the other half of which are fed from the other output port of the hybrid circuit 2. The hybrid circuit 1 is provided with a sum input port and a difference input port both of which are fed with interrogate pulses from a transmitter (not shown) via a SLS (side lobe suppression) switch 3 under the control of a control input 4. The interrogate pulses afforded by the transmitter consist of the usual pulse pairs commonly referred to as the P1 and P3 pulses and the suppression pulse commonly referred to as the P2 pulse.In operation of the interrogator of Figure 1 the P, and P3 pulses are normally fed to the sum port of the hybrid circuit 2 thus producing the 'SUM' polar diagram pattern as depicted in Figure 2 and by making use of the control input 4 the P2 pulses are fed to the difference pattern of the hybrid circuit 2 thus producing the 'DIFFERENCE' polar diagram pattern as depicted in Figure 2. An airborne transponder receiving the transmitted pulses then compares the relative amplitude of the P1, P3 interrogation with that of the P2 pulse and only replies when the former is larger than the latter by a fixed ratio. By this means the replies from the transponder only take place over a narrow range of angles about the boresight of the interrogator aerial.

For reception of the replies from the transponder it is becoming increasingly common to use monopulse techniques for determining the arrival angle of the signals relative to the aerial array boresight, rather than to rely on the position of the arc of reply for this purpose. The monopulse receiving system has to make use of the same aerial arrangements as are used for transmission and Figure 3 shows a generally recognised monopulse receiving system. Here one uses the same aerial elements 1 and the same sum and difference patterns for determining the magnitude of the angular deviation from boresight, the sum and difference outputs from the hybrid circuit 2 being applied to respective logarithmic receivers 5 and 6 which are fed with a local oscillator frequency derived from a local oscillator 7.The intermediate frequency (IF) outputs from the logarithmic receivers 5 and 6 are applied to respective detectors 8 and 9 the outputs from which are compared in a comparator 10 to afford an output 11 indicative of the magnitude of the angular deviation from boresight. This indication however does not provide any information as to whether the deviation is to one side or the other and this information is derived from a phase detector 12 which compares the phases of the IF outputs from the logarithmic receivers 5 and 6 to afford a sense output 13. It is found that the difference signal changes its phase by 1800 when the bearing changes from left of boresight to right and the phase detector will then give a positive or negative output signal depending on the sense. This method of sensing leads to a number of practical problems.Firstly it may worsen the signal/noise performance of the system. Secondly it requires good phase stability in the two separate log receivers 5 and 6 and the two interconnecting cables between the hybrid circuit 2 and the receivers proper. This latter effect can lead to instaliation and maintenance problems.

In Figure 4 of the drawings there is shown a SSR receiver according to the present invention in which all problems of phase measurement are overcome and in which the need to provide the sense circuitry is obviated.

The basic circuit of the receiver of Figure 4 is the same as that of Figure 3 except that the phase detector 12 is omitted and an additional 90" phase displacement 14 is introduced in series with one of the feeds from the aerial element 1. This latter has the effect of changing the sum/difference polar diagram of Figure 2 to a left-right polar diagram as shown in Figure 5. The ratio of the sum and difference signals afforded by the hybrid circuit 2 now defines the deviation from boresight and the polarity of the ratio defines the sense of the deviation i.e.

whether it is left or right.

The 90" phase displacement 14 of Figure 4 is only required for reception purposes and is not required for transmission purposes and it would be quite possible to disconnect this by means of a PIN diode switch. However, using such a switch has disadvantages and it is proposed that a more suitable solution is to provide a passive delay cable and to make use of the fact that the frequencies of transmission and reception, which are typically 1030 MHz and 1090 MHz respectively, are different in order to provide for a differential phase shift of 90" on reception.

To do this the difference in feeder lengths between the aerials 1 and the hybrid circuit 2 must be 90 electrical degrees longer at 1090 MHz than at 1030 MHz.

If the difference in lengths is d then 7r D(1090-1030)=- 2 where ,Og0 is the phase shift per unit length at 1090 MHz and p1030 is the phase shift per unit length at 1030 MHz now

where Er iS the relative permittivity iS the relative permeability c is the velocity of light For microwave co-axial feeders ur is 1 and Er is approximately 2.1.

Ld at 1030 MHz------4.29 A at 1090 MHz 4.54 Now the difference in cable length must be equal to a whole number of wavelengths at 1030 MHz to obtain the 0/180 degree SLS phase shift. Assuming the electrical difference in length to be 4R at 1030 MHz then at 1090 MHz the electrical difference in length will be 4.233A. Thus changing from 1030 to 1090 MHz changes the phase by 83.9 degrees.

If the electrical difference in length is 4.25R at 1090 MHz then the difference at 1030 MHz is 4.016A. This will introduce a phase offset of 5.78 degrees to the SLS 0/180 degree function. In practice it is found that such a phase offset is acceptable.

Thus by providing a piece of cable in series with one of the aerial feeders that is 4.25R at 1090 MHz, the whole of the connecting cable and receiving system is made independent of any phase requirements; so that it is only necessary to maintain an accurate amplitude balance which is required anyway, whilst enabling the same sum and difference circuits etc. to be used for transmission purposes. Obviously the difference in feeder length can be chosen to optimise the performance at either 1030 or 1090 MHz, or a length which compromises the performance at both frequencies can be found. Any of these solutions would offer acceptable system performance.

Although in the arrangement described with reference to Figure 4, the required 90" differential phase shift between the aerial outputs is obtained by connecting a suitable delay in one of the aerial outputs, it should be appreciated that a suitable differential delay may be obtained by connecting a phase shift network in both aerial feeds, the combined phase shift corresponding to the required 900 phase shift.

WHAT WE CLAIM IS:- 1. A secondary surveillance radar transmit/receive system comprising aerial means divided into two parts sum and difference means arranged to be fed with aerial output signals from the two aerial parts; processing means arranged to be fed

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

If the difference in lengths is d then 7r D(1090-1030)=-

2 where ,Og0 is the phase shift per unit length at 1090 MHz and p1030 is the phase shift per unit length at 1030 MHz now

where Er iS the relative permittivity iS the relative permeability c is the velocity of light For microwave co-axial feeders ur is 1 and Er is approximately 2.1.

Ld at 1030 MHz------4.29 A at 1090 MHz 4.54 Now the difference in cable length must be equal to a whole number of wavelengths at 1030 MHz to obtain the 0/180 degree SLS phase shift. Assuming the electrical difference in length to be 4R at 1030 MHz then at 1090 MHz the electrical difference in length will be 4.233A. Thus changing from 1030 to 1090 MHz changes the phase by 83.9 degrees.

If the electrical difference in length is 4.25R at 1090 MHz then the difference at 1030 MHz is 4.016A. This will introduce a phase offset of 5.78 degrees to the SLS 0/180 degree function. In practice it is found that such a phase offset is acceptable.

Thus by providing a piece of cable in series with one of the aerial feeders that is 4.25R at 1090 MHz, the whole of the connecting cable and receiving system is made independent of any phase requirements; so that it is only necessary to maintain an accurate amplitude balance which is required anyway, whilst enabling the same sum and difference circuits etc. to be used for transmission purposes. Obviously the difference in feeder length can be chosen to optimise the performance at either 1030 or 1090 MHz, or a length which compromises the performance at both frequencies can be found. Any of these solutions would offer acceptable system performance.

Although in the arrangement described with reference to Figure 4, the required 90" differential phase shift between the aerial outputs is obtained by connecting a suitable delay in one of the aerial outputs, it should be appreciated that a suitable differential delay may be obtained by connecting a phase shift network in both aerial feeds, the combined phase shift corresponding to the required 900 phase shift.

WHAT WE CLAIM IS:- 1. A secondary surveillance radar transmit/receive system comprising aerial means divided into two parts sum and difference means arranged to be fed with aerial output signals from the two aerial parts; processing means arranged to be fed

with output signals provided by the sum and difference means for affording an indication of the bearing of a received signal, and means for affording a differential phase shift between aerial output signals applied to the sum and difference means, the said differential phase shift being substantially 90" for receiving purposes and substantially zero for transmit purposes.
2. A secondary surveillance radar receiving system as claimed in claim I in which the 90" phase shift is afforded by 90" phase shift means connected in the output of one of the aerial parts, switch means being provided for switching the phase shift out of circuit for transmit purposes.
3. A secondary survellance radar receiving system as claimed in claim I in which the transmit/receive system operates on different frequencies for transmit and receive and the said phase shift means is afforded by a passive delay connected in the output of one of the aerial parts said delay affording substantially 90" differential phase shift for receiving purposes and substantially zero differential delay for transmit purposes.
4. A secondary surveillance radar system as claimed in claim 1 and substantially as herein described with reference to and as illustrated in Figure 4 of the drawings accompanying the Provisional Specification.