FR2573554A1 - Time multiplexed carrier frequency signals appts. for flying object - Google Patents

Abstract

For the transmission of instructions sent between a ground station and a flying object the instructions are encoded using a number of carrier frequencies. These frequencies become the signal operating frequencies. The frequency signals are time multiplexed, and each frequency is transmitted for the same, preset, time period. In the receiver there are circuits which respond only to particular carrier frequencies, and further circuits pass the encoded messages to the executing circuit when the signal has been received for a longer time than a given minimum period. The transmission time is pref. 10 milliseconds and the received signal validation time 7 milliseconds. Any frequency at or near the particular carrier frequency whose received duration is less than the limiting time is thus ignored.

Description

 The present invention relates to a remote control system between a machine, for example of military type, and ground stations and more particularly the backup remote control to destroy this machine in certain special circumstances.

 Indeed, at the beginning, this safeguard was mainly oriented towards the protection of the goods and the populations it was a protection that one could describe as civil. Since the 1960s, missiles fired on the national territory must be equipped with security systems to neutralize them if necessary.

 The new remote control systems must offer the same guarantees for their civil protection function but also present new guarantees for military backup. Indeed, it is necessary nowadays to make the remote control link less vulnerable to industrial and hostile interference.

This remote control must essentially provide three functions that are
1. the function called Dead Man (HM),
2. the Commanded Destruction (DC) function,
3. the function called "Off".

 The first function HM, so named in analogy to the pilot systems, is to ensure permanently that the connection is properly provided with the machine and thus allows its eventual destruction if this connection is interrupted for a period of time determined. A coded order is issued in perpetuity to ensure that the link is well established; it's called Maintenance.

 The second DC function called Destruction Commanded consists in ncutralizing the machine by controlling its destruction also when this order is maintained for a certain time.

 The third "Off" function, when it is issued, inhibits the "Dead Man" function so as not to unduly destroy the craft when the link is lost at the limit of range for example during a ballistic fire.

 These three functions are usually performed using systems using six frequency-modulated command channels; each channel corresponds to a subcarrier frequency.

The same function is ensured by the presence of three subcarriers and the absence of the other three. In these known systems, so as to avoid the non-execution of an order as a result of a slight hardware failure on transmission or reception, or during transmission, the programming is such that a error and only one can be accepted in the composition of the six channels of the subcarriers.

 These known systems have long presented sufficient guarantees of good operation but no longer meet the new requirements.

 Indeed, these remote control systems must be able to operate (case 1) in the presence of parasitic signals received in the operating band with a power slightly greater than the useful signal (for example: interference by pure frequency, industrial jamming, etc ...) .

 Then, these systems must be able to operate (case 2) in the presence of non-hostile emission but high noise density, such as for example radio-taxi or remote control for model aircraft, etc.

 Finally, the hypothesis of hostile jammers that was not retained a few years ago can no longer be considered improbable and the new systems must be protected (case 3) against any attempts at voluntary interference. In particular, a hostile could record the order Hold (which is issued in the majority of the time and reissue it with a slightly higher power so as to block the system with a semblance of good functioning.A hostile could still cause untimely destruction.

 In addition, these new remote control systems must work (case 4) correctly in the presence of two transmitters working on the same frequency and emitting the same order (which could occur during horizontal reconnaissance flights for example).

 Prior systems using frequency modulation can not operate in the cases 1, 2, 3 described above because of the well-known capture phenomenon of this type of modulation. They are also very delicate to use in case 4.

 Other prior systems, based on Spread Spectrum technique or Frequency Shopping can provide an improvement for cases 1, 2, 4 but do not offer any protection for Case 3.

 The present invention overcomes all these drawbacks of known systems and furthermore has many other advantages.

 The invention essentially consists of coding the commands by means of p useful frequencies corresponding to p carriers, each useful frequency being transmitted during a predetermined time interval Δt, said frequencies being temporally multiplexed, p being a natural integer.

 According to a first characteristic, each order is coded by means of at least three useful frequencies, said frequencies being taken from among the frequencies and being different from those of the other orders.

 According to another characteristic, each order occupies a number n of time intervals, the time intervals not occupied by the useful frequencies being occupied by decoys, n being a natural integer greater than p.

 According to another characteristic, the decoys have the same characteristics as the useful frequencies but distinct values.

 In addition, the number n of time intervals characterized by the same order is advantageously greater than or equal to 2p.

 According to another characteristic, the messages are received by means of a high frequency receiver horned per se, the medium frequency output of said receiver having a rigorously controlled level by means of an automatic gain control (AGC), said medium frequency output followed by p receivers R. p useful frequencies placed in parallel, said receivers R. being connected to means 10 for recognizing messages.

 According to another characteristic, each receiver R.

consists of a band-pass filter (20) centered on the wanted frequency, followed by an envelope detector (21) and a threshold detector (22), then an integrator 23 and another detector threshold (24).

 According to another characteristic, the integrator (23) performs integration of the frequency fi detected for a time DC shorter than said time interval and during which said frequency f. is issued.

According to another characteristic of the invention, the message recognition means 10 essentially consist of comparing each sequence of frequencies detected by said p receivers R. to the commands stored in memory
Several sequences of the same order must be recognized by the recognition means 10 to validate this order.

In addition, the threshold detector (22) of each receiver R. of useful frequency is adjusted so that it
1 only takes into account the frequencies having a level equal to or less than 7dB of the nominal level, this in order to avoid the power of parasitic noise with high density.

 According to another characteristic, the useful frequencies transmitted as well as the channel filters of the receiver can be modified before each remote control operation.

 It is advantageous to transmit two messages to transmit the order Maintenance, each of said messages being encoded by means of different useful frequencies.

 According to another characteristic of the invention, the two messages Hold 1 and Hold 2 are transmitted in a pseudo-random rhythm.

 Other advantages and features will become apparent upon reading the following description illustrated by drawings.

 Figure 1 shows a general view of the remote control system of the invention.

Figure 2 shows a detailed view of the receivers
Ri of Figure 1.

 FIG. 3 represents the flowchart of the decoding logic device of FIG. 1.

With reference to this figure, the system of the invention consists essentially of a transmission system and a reception system. The transmission system uses a pure carrier whose different frequencies are encoded and temporally multiplexed. These different useful frequencies are therefore appropriately coded to form messages. In one exemplary embodiment, six frequencies A, B, C, D, E, F, which are transmitted each during a time interval ht, for example of duration equal to 10 ms, are selected; it is then sufficient to choose a combination of these different frequencies A, B, C, D, E, F to form a message, DC for example, or OFF, or Hold. In addition, it is appropriate, to form a message, to occupy a time greater than six time slots. For example, twelve time slots may be occupied by emitting decoys during the unoccupied time intervals. According to the invention, the decoys are also chosen such that they have the same characteristics as the useful frequencies but different values of the useful frequencies. According to the invention, a second sequence Maintain (M) after
2 the first sequence Maintain (M), this second sequence being composed of frequencies different from those of the first sequence.

Such a message coding system has an infinity of solutions. An example of coding is shown in the table below

#t <SEP>#<SEP> 10 <SEP> ms <SEP> t1 <SEP> t2 <SEP> t3 <SEP> t4 <SEP> t5 <SEP> t6 <SEP> t7 <SEP> t8 <SEP> t9 <SEP> t10 <SEP> t11 <SEP> t12
<tb><SEP> DC <SEP> A <SEP> B <SEP> C <SEP> D <SEP> E
<tb> Hold <SEP> 1 <SEP> A <SEP> D <SEP> E
<tb> Hold <SEP> 2 <SEP> B <SEP> F <SEP> C
<tb><SEP> OFF <SEP> D <SEP> F <SEP> C <SEP> E
<Tb>
In addition, the sequences M1 and M2 are transmitted in a pseudo-random rhythm in order to avoid cyclic non-recognitions. In order to transmit a Hold 1 sequence then a Hold 2 sequence, A 1 D 2 3 4 5 6 7 E is then successively transmitted. 89 10 11 12 B 13 14 15 F 16 17 18 C where A to F are the useful frequencies and 1 to 18 are the decoys. The emission of decoys in the unused time slots makes it possible, on the one hand, to make the receiver less sensitive to parasitic emissions as will be seen during the description of the receiver and, on the other hand, to conceal the frequencies useful messages sent.

 These messages are therefore transmitted by means of a transmitter 1 connected to an emission antenna 2.

 In reception, an antenna 3 provides a signal at the input of a high frequency amplifier 4. A local oscillator 6 generates a wave and transposes the signal from the amplifier 4 through the mixer 5. The signal from the circuit 5 is applied to the input of an amplifier 7 and a filter 8 medium frequency. An automatic gain controller (AGC) 9 receives the output level of the filter 8 (called point x) and regulates the gain of the signals of the amplifiers 4 and 7.

 Indeed, the essential function of the CAG 9 is, on the one hand, to strictly control the output level of the filter 9 in the operating range, and, on the other hand, for the low levels (when the signal-to-noise ratio is less than 10 dB at point x), to voluntarily interrupt the link by bringing the point x to a value close to zero.

The signal regulated at x is applied to the input of p devices Ri for receiving the messages themselves. There are as many devices R as useful frequencies provided for emission. Thus, if six useful frequencies are used to code the messages, six receivers will be available.
A. that we will write later. These six receivers R i each output a signal representative of a recognition or non-useful frequency recognition.

The signals from the receivers R. are applied to the input of a microprocessor 10 which processes them and completes the decoding of the messages.

 Each receiver Ri consists essentially of a bandpass filter 20 selecting the useful frequency f. of the carrier followed by an envelope detector 21 and a threshold detector 22. The threshold of this detector 22 is adjusted, for example, so as to take into account the signals having a loss up to 7dB below the nominal operating threshold. The minimum threshold of operation is determined by the output value of the detector 2, when the signal received on the antenna is only composed of a significant carrier.

 In the exemplary embodiment developed in the present descrption, this makes it possible to take into account, as useful information, a parasitic or hostile signal received with a power of less than 7dB, but, in part, has the advantage of receiving the information useful, in the presence of a parasitic or hostile signal that would be received with a power greater than 7dB to the useful signal. The signal from the detector 22 representative of an item of information is integrated, according to the invention, by means of an integrator 23 for a period t suitably chosen.

 If we refer to the previous embodiment for which the useful frequencies are each transmitted for 10 ms, by integrating the nominal value from the detector 22 for 7 ms, the value detected is used only during 3 ms . This value, after integration, is applied to a threshold detector 24 and then to the microprocessor 10. This integration thus makes it possible to eliminate all sorts of cyclic jammers that would have rates greater than 7 ms.

 The microprocessor 10 completes the processing of the decoding of the messages. It therefore receives the information from the different receivers Ri and stores them in a memory. As shown in FIG. 3, the microprocessor 10 compares each received frequency sequence with the words Maintenance 1, Hold 2, DC and OFF stored in memory. . it firstly seeks to recognize the DC order then the OFF command and finally the sequences Hold 1, Hold 2. As soon as the microprocessor 10 is turned on, a non-represented COnM counter said to be Dead Man is triggered which, when the programmed time has elapsed, delivers the HM order. Several DG sequences must be recognized successively for a predetermined time to deliver the DC order. OFF sequences recognized successively for a pre-determined time definitively set COH at zero.

 If there is no DC or OFF sequence recognition during the same predetermined time, the presence of a Hold sequence is sought. At each recognition of a hold sequence, the counter C0HM is reset. The use of two sequences Hold allows to momentarily admit a fault on one of the useful frequencies.

 The system of the invention provides the remote control with satisfactory protection against a great deal of interference.

Indeed, if it is an industrial jamming, if a jammer emits two parasitic carrier frequencies continuously or cyclically, these two channels are received as constituting a sequence; it is necessary, however, that the other frequencies constituting a sequence are recognized and this in the time slots provided during the coding so that a sequence is actually recognized by the system of the invention.

 Another type of interference can come from a high density of noise such as for example radio-taxis or cibists. For an approximately uniform spectral distribution in the medium-frequency band, the noise power taken into account by each detector will be reduced in the band-to-band medium-frequency band ratio, ie around -15 dB of the nominal level and thus in excess of the threshold. minimum consideration that is set at -7dB. This jammer will therefore not affect the system of the inPention.

 Another type of interference can come from a hostile.

The decryption of the sequences of Maintenain by this hostile seems easily conceivable. The recognition of significant frequencies seems very difficult because for this function there are 18- decoy frequency organizations, ie a total of 24 similar frequencies without any recognition criteria. On the other hand, the sequences Maintain can be captured and reemitted by a Hostile by superimposing itself on the useful signal, without affecting the system of the invention.

 Indeed, we saw previously in the embodiment that, for a Hostile signal received on the antenna with a power of t 7dB of the useful signal, no information was eliminated, the interference can only be superimposed on the information helpful. So, if the hostile re-emits, for example the sequences of maintenance, this is not a problem the order will simply be repeated a greater number of times by the decoding device 10. By cons, when the useful transmitter will want to transmit DC order, it will not be hampered by this interference since DC order is recognized primarily by the device as shown in the flowchart of Figure 3 It is the same for the OFF order, even in presence of a Hostile Maintenance sequence.

 In the case of a permanent transmission of several useful frequencies, the power of the received signal is distributed in the spectrum and each channel receiver Ri no longer receives a reduced power in the inverse ratio of the spread spectrum.

 The DC order can only be received from a hostile wrongly if the hile knows the useful frequencies, the temporal distribution, ie the code, the transmission power, and that this hostile has a more powerful transmitter than the system sender.

 The invention provides a last palliative to this type of interference. It is indeed possible to modify the code by changing, either the organization of the frequencies composing an order, or their temporal distribution. Such a change is possible almost instantaneously and assumes a simple modification of the instructions to the decoding logic 10, a change which is very easy.

Claims (14)

 1 - Remote control system intended to provide a link between a vehicle and the ground, said link comprising the transmission and reception of at least two orders, r order HM consisting in ensuring perm.v-nce the link, a DC order as.tllart the destruction of said machine, characterized in that the orders are encoded by p useful frequencies corresponding to p carriers, each useful frequency being transmitted during a predetermined time interval, said frequencies being temporally multiplexed, p being a natural integer.  2 - System according to claim 1 characterized in that each order is encoded by means of at least three useful frequencies, said frequencies being taken from the p frequencies and being different from those of other orders.  3 - System according to one of claims 1 to 2 characterized in that each order occupies a number n of time intervals, the time slots not occupied by the useful frequencies being occupied by decoys, n natural integer being greater at p.  4 - System according to one of revendicatiors 1 to 3 characterized in that the decoys have the same characteristics as the useful frequencies but distinct values.  5 - System according to one of claims 1 to 4 characterized in that the number n of time intervals At of the same order is greater than or equal to 2p.  6 - Remote control system according to one of the revecdications 1 to 5 characterized in that said messages are received by means of a high frequency receiver known per se, the medium frequency output of said receiver having a level rigorously controlled by means an automatic gain control (AGC), said medium frequency output being followed by p receivers R. p useful frequencies placed in parallel, said receivers R. being connected to means 10 for recognizing messages.  7 - System according to claim 6 characterized in that each receiver Ri consists of a filter (20) bandpass centered on the frequency f. useful, followed by an envelope detector (21) and a threshold detector (22) and then an integrator 23 and another threshold detector 24.  8 - System according to claim 7 characterized in that said integrator 23 performs an integration of the carrier frequency f. detected for a time t chosen less than said time interval tt during which said carrier frequency f i is transmitted.  9 - System according to one of claims 6 to 8 characterized in that said means 10 for message recognition consist essentially in comparing each frequency sequence detected by said p receivers Ri to orders stored.  10 - System according to claim 9 characterized in that several sequences of the same order must be recognized by said recognition means 10 to validate this order.  11 - System according to one of claims 6 to 10 characterized in that the threshold detector 22 of each frequency receiver Ri -en useful is set so that it takes into account only the frequencies having an equal level or less than 7dB of the nominal level, in order to avoid the power of high density spurious noise.  12 - System according to one of claims 6 to it characterized in that the useful frequencies transmitted and the channel filters of the receiver can be modified before each remote control operation.  13 - System according to one of claims 1 to 12 characterized in that one of the orders issued Maintenance is provided by means of two messages, each of said messages being encoded by means of different useful frequencies.  14 - System according to claim 13 characterized in that the messages Hold 1 and Hold 2 are transmitted in a pseudo-random rhythm.