GB2164822A - A communication system resistant to jamming - Google Patents

Abstract

In a multifunction integrated system for identification, navigation, traffic control, collision avoidance and communications which transmits messages each containing an introduction used for acquisition of the message followed by a text containing the data to be transmitted, the pulses in the introduction and text are modulated pseudo-randomly in different ways, the introduction pulses being modulated at a high bit rate to give a very wide band spectrum spread and transmitted at fixed frequencies, the text pulses being modulated at a lower bit rate to give a more limited band spectrum spread but being transmitted with frequency jumps distributed over the whole band used by the system.

Description

SPECIFICATION A multifunction integrated system for identification navigation traffic control collision avoidance and communications The present invention covers a multifunction integrated system for identification, navigation, traffic control, collision avoidance and communications.

The necessity of a multifunction integrated system appeared some years ago as it offered a relatively economic solution and an increased processing capacity for the problem presented by the various functions called for by the control of a moving object, whether it be air-borne, land-based or maritime, from its departure to its arrival points and the carrying out of the task assigned to it. An air-borne object is taken in hand as soon as it leaves its parking position and control maintained until its return to another parking position, the system intervening in succession for all the functions which are required between these two instants. All these functions, the most important of which are mentioned in the title of the patent application, are carried out by systems which are already existant and operational.Primary radars, which allow the detection of a moving object in the air space covered by their antennas, secondary radars, which interrogate under a given code an aircraft fitted with a responder concerning its altitude and identity, interferometer bases, which enable the position of moving air-borne objects to be determined, and all navigation and landing aid systems may be mentioned in passing. All these specialized systems are evidently liable to progress and have been improved but their grouping together for the performance of the various functions mentioned has been found to be a more expensive solution than a multifunction integrated system.

A multifonction integrated system thus supplies an advantageous solution to the complex problem posed, which makes easier both communications and the design of interfaces and also the interchangeability of equipments and functions. It allows the sequential organization of operation to use equipments fully and rearrange the system as a function of its state by the grading of its operations for the mission or the flight phase or missions in the general sense.

Generally speaking, in multifunction integrated systems, data exchanges, taken in their widest sense, between the ground and moving objects or between moving objects, are carried out by coded signals of the same type, no matter what the function considered, and the messages transmitted are distributed in time, code and frequency, the transmission frequencies being distributed over a wide band. Means are evidently provided, both on the ground and on board the moving vehicles to ensure perfect synchronization of the various system elements in the setting up of the various functions carried out.

Under prior practice, the system called "Sintal" which was designed by the applicant may be mentioned.

This is a multifunction integrated system which carries out the functions of identification, navigation, traffic control, collision avoidance and communications in which the transmissions are arranged by division in time, code and frequency in recurrences of fixed length, such a recurrence being called a base recurrence, or in recurrences whose length is a submultiple of the base recurrence. The messages transmitted are all of the same structure and contain mainly two parts : an introduction, which is used for message acquisition and synchronizing and a text containing the data in the message to be transmitted.

The introduction is formed by a certain number of pulses, each one of which is coded in accordance-with a law,-which may be pseudo-random and-whose distribution in time may also follow a pseudo-random law. By correlation of the pulses on the one hand and their succession in the introduction on the other, the acquisition of the introduction and the synchronizing are determined.

In fact, the introduction arrival decision and hence its acquisition cannot be obtained from a single pulse. The introduction must therefor contain a certain number of pulses at one or several frequencies, which are known in advance and variable from one recur, rence to another in code, frequency and distribution in time, these being also known. This variation in frequency, code and relative position in time of the pulses complies with a pseudo-random law, as has already been indicated, a law which precisely enables the introduction to be correlated without ambiguity.This pseudo-random modulation has a relatively small spectrum spread and, in an example of a variant of the prior practice cited, known under the name of "Sintac 2", which was also designed by the applicant, this code has 32 bits, each bit lasting 0.2 gzsec with a dispersion band of the order of 5MHz.

The text which follows the introduction uses pulses of the same type as before with a code and frequency law which is also variable pseudo-randomly. These pulses are transmitted with frequency jumps in each elementary transmission interval in a band of the order of 250 MMz as an example. These pulses are modulated like those of the introduction with a relatively small spectrum spread.

The transmission of the introduction on several frequencies distributed over the whole of the system frequency dispersion band, to a certain extent, makes it possible to attenuate the disadvantage of jamming which may be done in the case of a fixed frequency transmission after this frequency has been detected.

On the introduction reception side however, it is necessary to have as many receivers as there are introduction transmission frequencies, 8, 4, 2 or 1 depending on the system performance and corresponding reception security.

When the introduction and hence thg synchronizing have been acquired, reception of the text may be undertaken with a single receiver.

This integrated system has disadvantages which are too many receivers in the reception channel and relatively poor protection against selective jamming.

The purpose of the invention is to correct these disadvantages.

In accordance with the invention, the multifunction integrated system for identification, navigation, traffic control, collision avoidance and communications, which transmits messages containing an introduction used for acquisition of the message synchronizing and a text following the introduction, which contains the data to be transmitted, is characterized by the fact that the pulses in the introduction and the text are modulated differently, the introduction pulses being modulated with a very wide band spectrum spread and transmitted at fixed frequencies, the text pulses being modulated with a more limited band spectrum spread and transmitted with frequency jumps distributed over the whole operating band of the system.

Other characteristics of the invention will appear in the description of an example of production of the reception part of the multifunction integrated system which is given with the help of figures which show - in figure 1 the form of the message in accordance with the invention, - in figure 2 a configuration of simultaneous reception of an introduction and two texts interlaced, - in figure 3 the division of a base recurrence into three sub-recurrences, - in figure 4 a receiving channel in the multifunction system in accordance with the invention, - in figure 5 a variant of a receiving channel which allows simultaneous reception of two introductions in two different channels, distributed in two sub-bands, - in figure 6 a variant of figure 5 with two antennas, - in figure 7 a variant of figure 5 with acquisition in four channels distributed in two subbands and simultaneous reception of ten texts, - in figure 8 a receiving station of little importance in accordance with the invention, - in figure 9 a simplified reception station in accordance with the invention, - in figure 10 a very simplified station limited to the reception of the introduction only.

As was noted in the introduction, the multifunction integrated system in accordance with the invention is characterized by different modulations for the introduction and text pulses for the purpose of using a reduced number of receivers for the introduction mainly, the number of text receivers being determined by the capacity of the station where the system is set up.

Figure 1 shows schematically the form of the message in the integrated system in accordance with the invention. This message contains an introduction 1 with coarse synchronizing and a text 2 with fine synchronizing 3. Introduction 1 consists of pulses modulated with a very-wide band spectrum spread, which are transmitted at a fixed frequency with a pseudo-random law and distributed in time, the distribution being defined by a transmission sub-recurrence. Transmissions, in accordance with prior practice, are by recurrences which define the pseudo-random laws used with the difference that, in accordance with the invention, only the reception of the introduction determines the length of the transmission base recurrence.Hence, in accordance with the invention, shorter recurrences than those of prior practice can be and are used, these being called sub-recurrences, with the advantage that time is used more rationally and, because of this, the system capacity is increased. The length of the sub-recurrences depends on the system equipment category, which is determined, among other things, by the range. As well as on the range, the equipment category depends on the processing capacity and on the carrier used. As a result, in accordance with the invention, several sub-recurrences can be considered depending on the circumstances and for the range considered at present the sub-recurrence is about a third of the recurrence of prior practice.

In the example cited, the fine synchronizing 3 and the text 2 are also formed by pulses modulated with spectrum spread but with a more limited band with frequency jumps at each segment. However, it is possible to transmit the fine synchronizing with the introduction and hence with dispersion in a wide band. In a preferred example, the introduction is formed by 12 pulses 6.4,usec long, a band width of 100 MHz, the elementary pulses or bits being 10 nsec long. They are divided into two groups, 4 and 5, of six pulses giving a total spread of 200 MHz with a filling rate of 1/7 in all.

Text 2 consists of pulses of the same length, 6.4 ,sex, as those of the introduction but with a more limited spectrum spread, about 5 MHz, the elementary pulses or bits being 0.200 itsec long with frequency jumps dispersed throughout the whole system band.

The pulse filling rate is 1/4. The standard next consists of "head" words using the 16/7 code and data words using Reed Salomon 31/15 codes.

The fine synchronizing 3 consists of eight pulses.

The introduction and text filling rate is fixed to reduce as much as possible internal jamming in the system between introductions and between the introduction and texts received simultaneously, as this enables them to be interlaced to ensure maximum capacity for the system.

Figure 2 shows a configuration of simultaneous reception of an introduction 6 with a 1/7 filling rate and two interlaced texts 7 and 8 with a 1/4 filling rate.

This simultaneous reception is made possible by the introduction acquisition criterion accepted, i.e. reception of a given number of pulses of the introduction in the whole and text redundancy. In these conditions, in accordance with the invention, only one introduction receiver is required and as many text receivers as there are messages to be received simultaneously.

However, it is possible, with a lower message filling rate, 1/8 instead of 1/4, to consider receiving two texts with interlacing on the same receiver.

It is also possible to receive simultaneously introductions in different channels defined by pseudorandom laws. It is then necessary to have as many pulse and introduction correlation circuits as channels considered.

To prevent transmissions of introductions at the start of recurrence and their selective jamming and to use the time as much as possible, the recurrence is divided into sub-recurrences of sufficient length to allow reception of introductions up to the system range limit.

The shorter the range, the shorter the subrecurrence. The sub-recurrence determines the codes of the introduction in accordance with the channel and of the text which follows the introduction. The text channel is triggered by the synchronizing acquired by the introduction and it is retained until the end of the text indicated in the head word.

Figure 3 shows the division of base recurrence 9 identical to that of prior practice into three sub-recurrences 91, 92 and 93 with the reception of an introduction 1 in each subrecurrence. This causes the reception of interlaced messages with as many text receivers as there are interlaced messages to be received, i.e. three in the example shown. This is not limiting and it is possible to receive more than one introduction in a sub-recurrence and also to receive a higher number of interlaced messages, provided that the number of text receivers is increased in the same proportion. In 10 has been shown the reception capacity of a reception channel of prior practice which contains eight receivers although there is only one message per base recurrence 9.

It can also be seen that the reception capacity of the integrated system in accordance with the invention is much greater than that of prior practice for the same message redundancies. It can be seen too that the transmission by sub-recurrence eliminates the possibility of selective jamming in time and improves the protection of the system against hostile jamming.

Figure 4 shows the design of a reception channel in the multifunction integrated system in accordance with the invention. Starting from the input E, there is the acquisition receiver 11 with a very wide band which processes the message introduction, which may be followed by a selective frequency limiter circuit 12, a pulse correlator 13, an introduction correlator 14 and an acquisition decision circuit 15 whose output feeds a circuit 16 called the data processing circuit. Input E, is also connected to a receiver 17, called the text receiver, whose pass band is smaller than of introduction receiver 11 but equal to that of prior practice, followed by a limiter 18, a pulse corrector 19 whose output is connected to processing circuit 16. S is the reception channel output. At E2 is shown a control input for circuit 16.

The operation of such a reception channel does not cause special problems, the functions of the various items in the channel being clearly defined. The fact that selective frequency limiter circuit 12 is optional is due to a possible desire to cut out selective jamming.

Under these conditions, circuit 12 analyses the signal in a known way from the frequency point of view, using the Fourrier transform and separating the jammed frequencies. The connection 20 between the pulse correlator 13 and processing circuit 16 indicates the possibility of correlating the introduction in circuit 16. The connection 200 shows the possibility of modifying the codes in accordance with the sub-multiples and allowing the processing of the various introductions. Connection 21 shows modifications made to the text codes and connection 210 modifications to be made to receiver 1 7 to allow for frequency jumps.

Figure 5 shows a reception channel in the integrated system in accordance with the invention which allows simultaneous reception of introductions on two different channels in the full band of the order of 200 MHz and of four texts on one, the other or both channels.

In accordance with the invention, there is one receiver per introduction and four text receivers, one for each text.

After the antenna 22, there is a duplexer 23 which ensures separation of the transmission coming from transmitter 24, which is connected to two modulators, 25 and 26 respectively, of introduction and text from the reception part connected to the output of the very wide band preamplifier 27, which covers the whole system frequency dispersion band (about 250 MMz). At the output of preamplifier 27 there are, first of all, two acquisition reception channels in parallel similar to the channel described in figure 4 each containing an acquisition receiver 28-280 and a selective frequency limiter 29-290.

To each limiter 29-290 are connected two channels in parallel, one per channel each of which contains a wide band, programmable acquisition correlator 30-31 and 300-310 followed by an introduction correlator 32-33 and 320-330 and a peak correlation adder or decision circuit 34-35 and 340-350 whose respective outputs are connected to data processing circuit 16.

The output of preamplifier 27 is also connected to four text receivers, 36 to 39 in parallel, each of which is followed by a programmable correlator with a small pass band of the order of 5 MMz, 40 to 43.

It can be seen that, in the case of acquisition on several channels, only the correction circuits are multiplied whereas, in prior practice, the complete reception channel from wide band preamplifier 27 would have been multiplied. In accordance with the invention, there is an advantage in that it is possible to use a smaller number of circuits The text receivers are used independantly of the channels, their choice being determined by circuit 16, which receives the introduction.

When required, this facility enables a certain priority to be applied when all the text receivers are busy.

Figure 6 shows a variant of figure 5 in which two antennas are used, a high one and a low one. In this case, acquisition is divided between the two antennas, 220 and 221, each only operating for the introduction for half the full band, i.e. 100 MHz in the example considered. There are therefore two preamplifiers, 27 and 270, each feeding ong of the reception-acquisition channels, which are identical to those in figure 5. The text reception part is not modified with respect to figure 5.

However, a reception switch 52 enables the texts, which are received within the full band of each antenna, to be switched to the antenna giving the better reception or to be received simultaneously on both antennas, a switch 54, for transmission, enabling the transmitter to be switched to one or other of the antennas 220-221 or to transmit simultaneously on both antennas.

Figure 7 shows a reception station of the integrated system in accordance with the invention, which enables the four channels to be acquired in the full band and 10 texts to be received simultaneously. This requires, at the output of each selective. frequency limiter, 29 and 290, four correlation channels for acquisition, 401-404 and 4010-4040, and ten text receiver channels in parallel, 500 to 510, which are similar to those in figure 5 for example. 250 represents all the modulators for the transmission of introductions and texts. It can be seen that the correlation and text reception channel circuits are not given in detail as they are similar to those in figures 4, 5 and 6.

As has been seen, all the reception stations of the system in accordance with the invention which have been described have a separation between the acquisition receivers and the text receivers in which the texts are decoded. This arrangement allows adaptation to what is necessary. The reception capacity can thus be increased in one place and reduced in another while the protection desired against hostile jamming is retained.

Thus, if figure 7 shows a big station, figure 8 shows a much smaller one such as exists in an armoured vehicle for example. Acquisition is obtained with a single receiver 28 covering the 100 MMz band for example and on two channels, an unsynchronized one for identification by question and response, in which the variation period for the pseudo random codes is long, containing a pulse correlator 55, an introduction correlator 56 and a correlation peak adder 57 and a multifunction channel also containing a. pulse correlator 58, an introduction correlator 59 and a correlation peak adder 60. A single text receiver is provided, 61, with a correlator 62. The transmitter 24 transmits at reduced power, for a short range, and is fed by two modulators, an acquisition one 25 and a text and fine synchronizing one 26.

Figure 9 shows an even more simplified station in which only an introduction is transmitted and received, which is used for identification by question and response. The circuit references are the same as in figure 4.

Figure 10, even more simplified, shows a station in which reception of the introduction only takes place. It is used mainly for identification by question and response for a carrier using for interrogation a special device in the high band or of the laser type. Its use simplifies the responders which are identical for all identification configurations while retaining a high level of protection against hostile jamming.

A multifunction integrated system has thus been described which offers a certain number of big advantages with respect to systems of prior practice while retaining interoperability with systems of prior practice which are present developed. This interoperability may be ensured either from the side of the system in accordance with the invention or from the side of the system of prior practice. In the first case, it is necessary to fit out the system text receivers in accordance with prior practice ; in the second, it is necessary to equip the terminals in accordance with prior practice with acquisition circuits in accordance with the invention.

Claims (2)

1. A multifunction integrated system for identification, navigation, traffic control, collision avoidance and communications which transmits message con-taining an introduction used for acquisition of the message synchronizing and a text following the introduction with the data to be transmitted, characterized by the fact that the pulses in the introduction (1) and text (2) are modulated pseudo randomly in different ways, the introduction pulses (1) being modulated with a very wide band spectrum spread and transmitted at fixed frequencies, the text pulses being modulated with a more limited band spectrum spread and transmitted with frequency jumps distributed over the whole band used by the system.

2. A system as in claim 1 characterised by the fact that the introduction contains coarse synchronization while the fine synchronization and text are transmitted with the second type of modulation in a limited band.

2. A system as in claim 1, characterized by the fact that the introduction (1) contains coarse synchronization while the fine synchronization (3) and text (2) are transmitted with the second type of modulation in a limited band.

3. A system as in claim 1, characterized by the fact that the fine synchronizing is transmitted with the introduction.

4. A system as in claim 1, characterized by the fact that the introduction pulses are transmitted at a fixed frequency with a code that obeys a pseudo-random law and that they are distributed in time, this pseudo-random distribution being defined by a transmission sub-recurrence whose length is fixed to allow reception of the introduction alone.

5. A system as in claim 4 in which the class of equipment used is determined by the range, processing capacity and carrier wehicle, characterized by the fact that the length of a sub-recurrence depends on the range of the class of equipment in the system.

6. A system as in claim 5, characterized by the fact that, for a maximum range, the length of a sub-recurrence is about a third that of a recurrence.

7. A system as in claim 1, characterized by the fact that the introduction pulses can be transmitted on several frequencies, these frequencies being determined pseudo-randomly in the various recurrences.

8. A system as in claim 7, characterized by the fact that only one introduction receiver is used in the introduction- pulse modulation spectrum spread band, the centre frequency of this receiver being determined by the pseudo-random law considered.

9. A system as in claim 1, characterized by thefact that, in the case of simultaneous reception of introduction on two different carrier waves in the introduction pulse modulation spectrum spread overall band, one receiver per carrier is provided.

10. A system as in claim 1, characterized by the fact that a reception channel contains an acquisition receiver (11) for a very wide band introduction, a pulse correlation circuit (13), an introduction correlation circuit (14), an acquisition decision circuit (15) and a processing circuit (16).

11. A system as in claim 1, characterized by the fact that a reception channel contains a text receiver (17) with a relatively narrow pass band compared with that of the introduction receiver, a limiter (18), a text pulse correlation circuit (19) and a processing circuit (16).

12. A system as in claim 10, characterized by the fact that a selective frequency limiter circuit (12) is fitted between the introduction acquisition receiver (11) and the pulse correlation circuit (13).

13. A system as in one of claims 1, 9, 10 and 11, characterized by the fact that, if the introduction pulses are divided into two groups covering the whole introduction modulation spectrum spread band on two different carrier waves and there are a certain number of texts, the reception channel contains a single receiver covering the whole system band (27), which feeds two introduction acquisition channels in parallel and as many text reception channels as there are texts to be received.

14. A system as is claim 13, characterized by the fact that each of the reception channels for an introduction contains an acquisition receiver (28, 280) and, possibiy, a selective frequency limiter (29, 290) and two channels in parallel, one per carrier, each containing an introduction pulse correlation circuit (30, 31, 300, 310), an introduction correlation circuit (32, 33, 320, 330) and an acquisition decision circuit (34, 35, 340, 350).

15. A system as in claim 13, characterized by the fact that, in the case of acquisition in several channels, only the correlation circuits concerning the introduction are multiplied.

16. A system as in claim 13, characterized by the fact that the text receivers (36 to 39) are chosen by the processing circuit after acquisition of an introduction.

17. A system as in claim 10, characterized by the fact that the introduction correlation is done directly in the processing circuit (16).

18. A system as in one of claims 1 to 17, characterized by the fact that it is interoperable with systems of prior practice.

19. A multifunction integrated system for identification, navigation, traffic control, collision avoidance and communications substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

CLAIMS Amendments to the claims have been filed, and have the following effect: Claims 1 and 2 above have been textually amended as follows:

1. A multifunction integrated system for identification, navigation, traffic control, colli sion avoidance and communications which transmits messages, each message comprising a pulse sequence comprising an introduction used to obtain message synchronization and a text following the introduction with the data to be transmitted, characterized by the fact that the pulses in the introduction and text are modulated pseudo-randomly in different ways, the introduction pulses being modulated with a very wide band spectrum spread and transmitted at fixed frequencies, the text pulses being modulated with a more limited band spectrum spread and transmitted with frequency jumps distributed over the whole band used by the system.