GB1602954A - Radio identification systems - Google Patents

Description

(54) RADIO IDENTIFICATION SYSTEMS (71) I, MINISTER OF TECHNOLOGY, (formerly Aviation), London, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to radio identification systems.

In a known type of radio identification system for the identification of a movable craft, such as an aircraft, an identifying station emits an interrogating signal. The craft may carry a responder transmitter called a transponder which on receipt of the interrogating signal may respond by transmitting a coded signal indicating the nature of the craft. Alternatively the craft may carry an interrogator in order to ascertain the relative position of the transponder, for example for navigation or homing.

For example the craft may be aircraft and the system the SSR (Secondary Surveillance Radar) system of Air Traffic Control. In this system a ground station emits an interrogating signal which is directed towards an aircraft. The interrogating signal may be one of several different types called modes. Different modes of interrogation call for different information from the aircraft. The interrogating signal consists of two or three pulses the spacing of which determines the mode selected.

However, if two craft are within the interrogator beam at the same time they may be interrogated simultaneously and therefore their transponders may respond simultaneously. The result may be that the identifying station receives a signal which is a combination of the two responses irretrievably garbled. Moreover, unwanted signals may interrogate the transponder and in a noisy environment the transponder may respond to noise to the exclusion of genuine interrogating signals.

According to the present invention there is provided a transponder for use in a radio identification system including meeans for detecting the magnitude of received signals, a plurality of signal channels, separated channels corresponding to separate input signal magnitudes and means for at least partially suppressing an input channel when the rate of signal appearing on it is greater than a given rate.

According to a further aspect of the invention there is provided an electrical circuit including a plurality of input signal channels, means for modulating signals in at least one input channel, a signal processing circuit common to the input channels, a plurality of output channels corresponding to the input channels, means for identifying to which output channel or channels a processed signal belongs by identifying the modulation or absence thereof and means for separating processed signals into channels to which they belong.

The means for modulating signals in the above circuit may include means for differently modulating signals in different input channels in order to identify to which output channel a processed signal belongs.

Clearly the invention is not limited to aircraft radio identification systems: any craft (such as ships, submarines or land vehicles) may be equipped with the system.

An embodiment of the invention will be described by way of example with reference to the drawing filed with the Provisional Specification, which is a block diagram of part of a transponder in a radio identification system.

In the drawing the interrogating signal (which is presumed to be received, detected and amplified in stages of the transponder prior to those shown) enters on a channel 1 and is simultaneously applied to eleven amplitude filters 3, 5, 7, 9,11, 13, 15, 17, 19, 21 and 23. The outputs of these eleven amplitude filters are applied to six pulse rate discriminators 25, 27, 29, 31, 33 and 35. The limits (in volts) of the amplitude range accepted by each amplitude filter (such as 3) are shown in the Figure by numerals placed inside the symbol for the filter. The amplitude ranges accepted by the eleven amplitude filters and the pulse rate discriminators fed by them are also shown in Table 1.

TABLE 1 amplitude range pulse rate amplitude filter accepted discriminators reference volts fed 3 W1 25 5 r1w 25, 27 7 1-2 27 9 1r2+ 27, 29 11 2-3 29 13 2F32 29,31 15 34 31 17 3+ 4+ 31, 33 19 " 5 33 21 4+5+ 33,35 23 5-6 35 By this means the ranges of amplitudes accepted by the six pulse rate discriminators are as follows.

TABLE 2 pulse rate amplitude range discriminator accepted 25 W1+ volts 27 H2+ volts 29 1F3ivolts 31 2-4 volts 33 3H5+ volts 35 4t6 volts The outputs of the pulse rate discriminators 25, 27, 29, 31, 33 and 35 are separately applied to six pulse amplitude modulators 37, 39, 41, 43,45 and 47 respectively. The six pulse amplitude modulators feed a common channel 49 which forms the input to the transponder system described below.

The action of this part of the circuit is as follows. Incoming signals on the channel 1 are sorted by amplitude by means of the amplitude filters 3-23, and applied to the pulse rate discriminators 25-35.

Noise, if present, in the form of pulses having a fairly constant amplitude (for example, interference from other radio transmitters) will tend to be confined to one or two pulse rate discriminators (such as 25). The action of each pulse rate discriminator (such as 25) is to prevent pulses from being applied to the corresponding pulse amplitude modulator (such as 37) when the rate of pulses applied set is greater, than a given rate.

The pulse rate discriminator will function normally (letting signals through) when the rate of noise pulses falls. The remaining channels are unaffected, so a genuine interrogating pulse, which has a chance of 5/6 of being in one of the channels not affected by noise, is more likely to be processed than otherwise.

Furthermore, individual channels may be inhibited manually by means not shown by perfectly familiar to those versed in the art, and in this way the transponder may be made to reply only to signals of a particular amplitude, for example distant weak signals, so that at least some garbling may be avoided.

All the outputs of the pulse rate discriminators 25-35 are processed identically in much of the rest of the circuit (as described below); however they are used separately in one part of the circuit after processing. One way of achieving this would be to arrange six circuits completely in parallel. A preferred method is shown in which one circuit is used to process all the outputs via the channel 49.

First of all the separate outputs are 'labelled' in the pulse amplitude modulators 37, 39, 41, 43, 45 and 47 where frequencies designated fA, fB, fC, fD, fE and fF are superimposed on them. By this means the outputs may be separated after processing.

The channel 49 leads, via an inhibiting gate 51 and a pulse clipper 53, to a delay line 55 which is used both to decode the incoming signal and to encode the outgoing signal. The output of the inhibiting gate 51 is also applied to three andgates 57, 59 and 61, the other inputs of which are taken from separate parts of the delay line 55. The outputs of the gates 57, 59 and 61 are applied to a common orgate 63 the output of which is applied to the inhibiting input of the inhibiting gate 51 via a pulse clipper 65 and a 25 microsecond monostable circuit 67 and to a second input of the delay line 55 via the pulse clipper 65.

Outputs of the delay line 55 are applied to a matrix 69 to which the outputs of the gates 57, 59 and 61 also are separately applied. The output of the matrix 69 is applied, via a modulator 71, to a transmitter oscillator 73.

The action of this part of the circuit is as follows. The pulse clipper 53 removes the amplitude modulation "label" superimposed by one of the pulse amplitude modulators 31 47. The signals entering the delay line 55 emerge to open the and- gates 57, 59 and 61 after different delays corresponding to the pulse spacings of different modes of interrogation. The pulses applied to the gates 57, 59 and 61 directly from the inhibiting gate 51 will still possess their amplitude modulation.

The result is that the gate 57, 59 or 61 will emit an amplitude modulated pulse according to the mode of interrogation.

Whichever gate, 57, 59 or 61 emits a pulse, it will be applied to the pulse clipper 65 via the or-gate 63. The output of the pulse clipper puts on the monostable circuit 67 which closes the inhibiting gate 51 for 25 microseconds, which is the length of time the circuit takes to generate a response. Further interrogating circuits are by this means prevented from causing the transponder to transmit two overlapping replies. The output of the clipper 65 is also applied to the delay line 55 to energise the matrix 69 in a manner shown below.

The function of the matrix 69 is to generate the coded reply. This is done in a conventional manner in which one of the three rows of the matrix (according to the mode called for by the interrogating signal) is energised from the corresponding gate 57, 59 or 61 and individual columns of the matrix are separately energised in turn by outputs from the delay line 55. The connections between the rows and columns are made according to the reply required so that the output of the matrix applied to the modulator 71 is the correct reply to the mode selected by the interrogating signal. This reply signal is applied to the modulator 71 pulsing the transmitter oscillator 73.

The outputs of the gates 57, 59 and 61 are also applied as separate inputs to an or-gate 75 the output of which is applied to a modulation frequency discriminator 77. The modulation frequency discriminator 77 has six outputs corresponding to the frequencies fA, fB, fc, fD, fE and fF and which are separately applied, via six monostable circuist 79, 81, 83, 85, 87 and 89 respectively, six and-gates 91, 93, 95, 97, 99 and 101 respectively and six inhibiting gates 103, 105, 107, 109, 111 and 113 respectively, to the pulse rate discriminators 25, 27, 29, 31, 33 and 35 respectively.

The output of the matrix 69 is applied to all the and-gates 91-101 and an output 115 of the modulator 71 is applied to the inhibiting inputs of all the inhibiting gates 103-113.

The action of this part of the circuit is as follows. The interrogating signal, consisting at this stage of a single pulse from one of the gates 57, 59 or 61 carrying amplitude modulation from one of the pulse amplitude modulators 3747 is applied, via the gate 75, to the modulation frequency discriminator 77. The function of the modulation frequency discriminator 77 is to determine through which input channel the interrogating signal passed and this it does by detecting its frequency of amplitude modulation and applying it to the monostable circuit 79, 81, 83, 85, 87, or 89 when the frequency of amplitude modulation is fA, fB, fcs fD, fE or fF respectively.

The relevant monostable circuit (such as 79) effectively prolongs the pulse for 25 microseconds and applies it to the relevant gate (such as 91), opening it and allowing through the reply signal emerging from the matrix 69. The reply signal is prevented from proceeding further by being inhibited at the relevant inhibiting gate (such as 103). However, the voltage of the output 115 of the modulator 71 is arranged (in a conventional manner) to be dependent on the rate of interrogating signals received.If this rate is higher than a set rate then the voltage of the output 115 d the modulator 71 falls, letting the reply signals through the inhibiting gates 103-113 to the pulse rate discriminators 25-35. If the interrogating signals received are predominantly of a similar amplitude then the reply signals let through the inhibiting gates 103-113 will be largely confined to one channel (or a few of the channels). The relevant pulse rate discriminators (such as 25) will therefore have a greater number of signals applied to them. Signals applied in this way to a particular pulse rate discriminator (such as 25) have the effect of making it more sensitive, i.e.

the rate at which the pulse rate discriminator prevents signals passing through is lowered. Thus if the transponder is over-interrogated by signals having a fairly constant amplitude it will tend to exclude such signals and reply mainly to signals at other amplitudes.

WHAT WE CLAIM IS: 1. A transponder for use in a radio identification system including means for detecting the magnitude of received signals, a plurality of signal channels, separate channels corresponding to separate input signal magnitudes and means for at least partially suppressing an input channel when the rate of signals appearing on it is greater than a given rate.

2. A transponder as claimed in claim 1 and in which the means for suppressing an input channel includes a pulse rate discriminator.

3. A transponder as claimed in claim 1 or 2 and in which the means for suppressing, an input channel is under the control of the output of that channel.

4. A transponder as claimed in claim 3, and including means responsive to the rate of detection of the transponder for regulating the degree of control between the output of a channel and the means for suppressing the input thereof.

5. A transponder as claimed in any one of the preceding claims and including a plurality of input signal channels, means for modulating signals in at least one input channel, a signal processing circuit common to the input channels, a plurality of output channels corresponding to the input channels, means for identifying to which output channel or channels a processed signal belongs by identifying the modulation or absence thereof and means for separating processed signals into channels to which they belong.

6. A transponder as claimed in claim 5 and including means of differently modulating signals in different input channels in order to identify to which output channel a processed signal belongs.

7. A transponder substantially as hereinbefore described with reference to the drawing filed with the Provisional Specification.

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

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. greater number of signals applied to them. Signals applied in this way to a particular pulse rate discriminator (such as 25) have the effect of making it more sensitive, i.e. the rate at which the pulse rate discriminator prevents signals passing through is lowered. Thus if the transponder is over-interrogated by signals having a fairly constant amplitude it will tend to exclude such signals and reply mainly to signals at other amplitudes. WHAT WE CLAIM IS:

1. A transponder for use in a radio identification system including means for detecting the magnitude of received signals, a plurality of signal channels, separate channels corresponding to separate input signal magnitudes and means for at least partially suppressing an input channel when the rate of signals appearing on it is greater than a given rate.

2. A transponder as claimed in claim 1 and in which the means for suppressing an input channel includes a pulse rate discriminator.

3. A transponder as claimed in claim 1 or 2 and in which the means for suppressing, an input channel is under the control of the output of that channel.

4. A transponder as claimed in claim 3, and including means responsive to the rate of detection of the transponder for regulating the degree of control between the output of a channel and the means for suppressing the input thereof.

5. A transponder as claimed in any one of the preceding claims and including a plurality of input signal channels, means for modulating signals in at least one input channel, a signal processing circuit common to the input channels, a plurality of output channels corresponding to the input channels, means for identifying to which output channel or channels a processed signal belongs by identifying the modulation or absence thereof and means for separating processed signals into channels to which they belong.

6. A transponder as claimed in claim 5 and including means of differently modulating signals in different input channels in order to identify to which output channel a processed signal belongs.

7. A transponder substantially as hereinbefore described with reference to the drawing filed with the Provisional Specification.