GB2327018A

GB2327018A - Control system synchronisation

Info

Publication number: GB2327018A
Application number: GB8716061A
Authority: GB
Inventors: John Douglas Outram
Original assignee: GE Healthcare UK Ltd; Plessey Co Ltd; Siemens Plessey Electronic Systems Ltd
Current assignee: GE Healthcare UK Ltd; Plessey Co Ltd; BAE Systems Defence Systems Ltd
Priority date: 1987-07-08
Filing date: 1987-07-08
Publication date: 1999-01-06
Anticipated expiration: 2007-07-08
Also published as: GB2327018B; GB8716061D0

Abstract

A control system is described for establishing synchronised control of a vehicle, such as an unmanned aircraft, which is either out of contact with, or has lost synchronised control with, a ground control station. The control system is provided with a communication system at the vehicle for determining when the vehicle is out of synchronised control with the control station and for transmitting distress bleats in the form of a short spread spectrum sequence to a receiver at the control station. Synchronised communication with the vehicle is achieved by a process of repeated rapid two-way communication employing pattems of super-imposed phase modulation of the short spread spectrum sequences of signals, the modulation patterns increasing in length with each repetition to achieve a gradual decrease in ambiguity. Thereafter a long spread spectrum code sequence is employed for transmitting an appropriate operation schedule to the vehicle.

Description

CONTROL SYSTEMS The present invention relates to control systems and more particularly to a control system for establishing synchronised control with a vehicle, such as an unmanned aircraft, which is either out of contact with, or has lost synchronised control with, a control station.

A known control system for controlling an unmanned aircraft operates from a ground station and is arranged to track, follow and communicate with the aircraft after launch. The unmanned aircraft is provided with a computer controlled system which is synchronised with a ground station control system so that communication between the aircraft and the ground station exists in bursts of predetermined duration beginning at preset times.

In the known control system it is assumed that contact will be possible between the ground station and the unmanned aircraft at the preset times. A problem arises therefore if such contact is not possible due to unpredictable effects, for example, air turbulence, or as a result of the aircraft flying behind a hill. These problems are worse when the unmanned aircraft is flying at low altitudes.

In the case of covert systems, in which spread spectrum techniques are used to reduce the power spectral density, the knowledge of the preset times for communication between the ground station and the unmanned aircraft become of paramount importance in order to effectively use the advantages of spread spectrum techniques. Loss of contact with an unmanned aircraft, or lost synchronised control between the unmanned aircraft and the control station, in such a covert system can give rise to major difficulties if the time available for re-establishing synchronised control is short. For example, if a "lost" unmanned aircraft is operating at maximum range and in hostile electro-magnetic environments the synchronised control must be re-established very rapidly before the unmanned aircraft is irretrievably lost.

One of the objectives of the present invention is to provide a control system for speadily establishing synchronised control with a vehicle which is not in full synchronised control with the control station.

According to the present invention there is provided a control system for establishing synchronised control of a vehicle which is either out of contact with, or has lost synchronised control with, a control station, the control system comprising a communication system at the vehicle for determining when the vehicle is out of synchronised control with the control station and for transmitting a short spread sequence of distress signals to a receiver at the control station, a communication system at the control station for detecting the distress signals and for synchronising communication with the vehicle by a process of repeated communication with the vehicle, the repeated communication employing a geometrically increasing pattern of super-imposed modulation to the short spread spectrum sequence of signals until ambiguities in synchronised timing are eliminated.

In a preferred embodiment the communication system at the control station has means for transmitting signals to other synchronised vehicles after the distress signals from the vehicle have been detected to delay the next scheduled communication with the other synchronised vehicles.

In one embodiment the vehicle is an unmanned aircraft.

According to another aspect of the present invention there is provided a method of establishing synchronised control between a control station and a vehicle comprising transmitting a short spread sprectrum sequence of distress signals from the vehicle to the control station, detecting the distress signals at the control station, providing two-way communication between the vehicle and the control station, the communication employing a geometrically increasing pattern of super-imposed modulation to the short spread spectrum sequence of signals until ambiguities in synchronised timing are eliminated.

The present invention will be described further, by way of example, with reference to the accompanying drawings in which: Figure 1 A illustrates a timing diagram of a distress sequence of multiple cycles of a short spread spectrum sequence received by a ground station, Figure 1B illustrates one manner in which the communication system at the ground station applies phase supermodulation to the signals and Figure 2 is a block diagram of a control system embodying the present invention.

Embodiments of the present invention operate on the principle that when a vehicle, for example a UMA, is either out of contact with, or has lost synchronised control with the control station action is initiated to achieve first a coarse synchronisation and then a precise synchronisation between the control station and the UMA. The latter stage during which precise synchronisation is established takes place very quickly and without excessive and expensive hardware in the control system.

The UMA is provided with an onboard detector circuit (not shown) which detects when the UMA is lost in the sense that the UMA has inadequate synchronisation with a controlling ground station.

A communication system onboard the UMA is then initiated by the detector circuit to enter a "bleat" mode in which the onboard communication system transmits a distress sequence of multiple cycles, each length T (see Figure 1A), of relatively short spread spectrum sequence. This distress signal transmission is terminated with a unique Barker Pattern, superimposed on the spread spectrum sequence, and is followed by a period during which a receiver in the onboard communication system of the UMA is enabled to receive a transmission from the controlling ground station. If no transmission is received from the controlling ground station the distress signal transmission is repeated.

After the controlling ground station has received the "bleat" distress signals from the "lost" UMA a communication system at the ground station transmits signals to other synchronised UMAs, if any, advising them that an unsynchronised UMA may be transmitting, thereby alerting the other UMAs that their next scheduled communication may be delayed while synchronisation with the lost UMA takes place. The communication system at the ground station then attempts to pick up again the "bleat" distress transmission from the "lost" UMA. The process of receiving the "bleat" signal is known to the person skilled in the art and will not therefore be described.

After the "bleat" distress transmission has been successfully received by the ground station, the communication system at ground station transmits a signal to the "lost" UMA effectively acknowledging receipt of the "bleat" distress transmission thereby preparing the onboard communication system for the process of establishing synchronisation with the communication system at the ground station. The process of preparing the onboard communication system for establishing synchronisation is known to the person skilled in the art.

Referring to Figure 1A it can seen that there is a time uncertainty as the ground station communication system does not know which of the bleat signals received corresponds to a reference time defined in the onboard communication system of the 'lost' UMA.

For the sake of simplified illustration the reference time at the UMA is shown in Figure IA to occur a period 3T from the start of the bleat transmission.

An important aspect of the present invention is that of acquiring precise synchronisation with the 'lost' UMA by rapidly reducing the ambiguity in timing associated with the multiples of T, such that no ambiguity exists so that the UMA may then be fed a transmission schedule enabling the UMA to conform to an overall transmission frame in which all the UMAs take part.

The method of resolving the ambiguity in time includes applying a geometrically increasing pattern of super-imposed modulation to the basic short spread spectrum sequence which repeats in T. In the embodiment illustrated in Figure 1B the ground station communication system applies three phase super-imposed modulation to the spread spectrum sequence and transmits each of the three phased resulting sequences to the onboard communication system which carries out a comparison to determine which phased sequence most nearly resolves the timing ambiguity. In the described embodiment it is the third phase modulated sequence which will most nearly resolve the ambiguity and this information is transmitted back to the ground station as a short spread spectrum sequence corresponding to the third phase modulated sequence. The super-imposed modulation process is repeated a number of times as required. On each two way communication the ambiguity is reduced by a factor of N,N corresponding to three in the described embodiment, so that by repeating the process twice the ambiguity is reduced by N x N =9.

Repeating the process three times reduces the ambiguity by NxNxN=27. In other embodiments longer super-imposed modulation frames could be employed.

For operation in a given signal/noise environment, the period spent on each pass through the process is approximately proportional to N, since adequate integration time (narrow filtering) must be allocated to the trial time for each phase of which there are N. If each of the N phases are tested in series then N units of time are required to reduce the ambiguity N fold. Alternatively the N phases may be tested in parallel provided the necessary receiving hardware is duplicated N times. Conventional processors take N units of time to reduce the ambiguity N-fold. By keeping N very short, and repeating the process several times, a significant improvement in ambiguity reduction rate is obtained. The super-imposed modulation employed should reflect the reducing ambiguity at each stage.

In one embodiment of our invention the above process is used five times in each of which N is set at three, thus taking fifteen units of time (ie 5x3) to reduce the ambiguity by 243 times (35). Once this process is achieved in both transmission directions, a long spread spectrum code sequence can be employed with which to transmit an appropriate operation schedule to the UMA, to complete the acquisition process.

One embodiment of the basic hardware components of a control system for carrying out the invention is illustrated in Figure 2. Both the communication system at the ground station and the onboard communication system of the UMA are provided with a transmitter and a receiver. The transmitter has means whereby a short spread spectrum sequence of signals imposed on a carrier can be supermodulated. In Figure 2 the super-imposed modulation is fed from an ambiguity resolver 2 and imposed on the short spread spectrum sequence at a mixer 4, the resulting super-imposed modulated signal being transmitted from an antenna 6. The ambiguity resolver 2 is controlled by a counter 8 which serves to sequence the ambiguity resolver 2 from one stage of ambiguity reduction to the next. Increasing the count of the counter selects an ever slower supermodulation cycle. The counter 8 is controlled via a timer/processor 10: The receiver at the onboard communication system of the UMA has a means whereby each of the three phases received is compared with the short spread spectrum signal originally transmitted as a distress signal and the best phase decision determined in a comparator 12.

Information ascertaining to the best phase decision is employed by a controller timer/processor 14 to make phase adjustments to an ambiguity resolver 16 which imposes the best phase supermodulation on the short spread spectrum signal for transmission back to the communication system at the ground station ready for the next stage of ambiguity reduction to take place. A counter 18 serves to sequence the ambiguity resolver 16 from one stage of ambiguity to the next. As with the counter 2 in the transmitter increasing the count of the counter 18 selects an ever slower supermodulation cycle. The counter 18 is controlled via the timer/processor 14.

It will also be appreciated that the present invention not only provides a control system enabling the reacquisition of a ground station's "lost" UMA, but it also enables the initial acquisition by the ground station of a newly launched UMA, and the acquisition of a neighbouring ground station's UMA if for example the neighbouring ground station goes off the air for any reason whilst its UMA is airborne.

The specific embodiment described above may be modified without departing from the scope of the invention. For example, the vehicle may be a land vehicle or the actual configurations of the transmitter and receiver may be modified. Furthermore it will be appreciated that the geometrical factor N may have values other then 3, the value chosen for N reflecting the reducing time ambiguity at each stage.

It will be appreciated that whereas the control systems in the above described specific embodiment operate in a Time Division Multiplex (TDM) mode, other embodiments of the present invention may alternatively employ a Frequency Division Multiplex (FDM) or an FDMITDM combination mode of operation. Furthermore, whereas in the above described embodiment the ground station listens for distress signals after it has been alerted to do so by the lost UMA, the present invention also includes embodiments in which the communication system at the ground station itself detects that a UMA is lost by failure of the UMA to report on schedule. Thereafter the communication system starts to listen for distress signals and if the total time frame is busy alerts other UMAs, if any, that their next scheduled communication may be delayed.

In a further embodiment the same N phase super-imposed spread spectrum sequence is transmitted sequentially a number of times to the onboard communication system, the best phase decision being used to correct the phase of the receiving system ambiguity resolver. Thereafter a series of short spread spectrum sequences corresponding to the correct phase modulated sequence is transmitted to the receiving system ambiguity resolver at the ground station. By processing the changes that were required between the "to" and "fro" signals, the ground station is able to deduce the round trip delay.

Claims

1. A control system for establishing synchronised control of a vehicle which is either out of contact with, or has lost synchronised control with, a control station, the control system comprising a communication system at the vehicle for determining when the vehicle is out of synchronised control with the control station and for transmitting a short spread spectrum sequence of distress signals to a receiver at the control station, a communication system at the control station for detecting the distress signals and for synchronising communication with the vehicle by a process of repeated communication with the vehicle, the repeated communication employing a geometrically increasing pattern of super-imposed modulation to the short spread spectrum sequence of signals until ambiguities in synchronised timing are eliminated.

2. A control system as claimed in claim 1 wherein the communication system at the control station has means for transmitting signals to other synchronised vehicles after the distress signals from the unsynchronised vehicle have been detected whereby a delay in the next scheduled communication with the other synchronised vehicles may be initiated.

3. A method for synchronising an unmanned vehicle with a control station substantially as hereinbefore described with reference to the accompanying drawings.

3. A control system as claimed in claim 1 or claim 2 wherein the vehicle is an unmanned aircraft.

4. A control system for establishing synchronised control of a vehicle, the control system being substantially as hereinbefore described with reference to the accompanying drawings.

5. A method of establishing synchronised control between a control station and a vehicle comprising transmitting a short spread spectrum sequence of distress signals from the vehicle to the control station, detecting the distress signals at the control station, providing two-way communication between the vehicle and the control station, the communication employing a geometrically increasing pattern of super-imposed modulation to the short spread spectrum sequence of signals, until ambiguities in synchronised timing are eliminated.

6. A method of establishing synchronised control between a control system and a vehicle the method being substantially as hereinbefore described.

7. A control system for establishing synchronised control of a vehicle which is either out of contact with, or has lost synchronised control with, a control station, the control system being substantially as hereinbefore described with reference to the accompanying drawings.

Amendments to the claims have been flled as follows

1. A method for synchronising an unmanned vehicle with a control station, wherein the control station includes a timer/processor means, a transmitter means, and a receiver means, and wherein the unmanned vehicle includes a timer/processor means, a transmitter means, and a receiver means, the method comprising the steps of: (a) the transmitter means of the vehicle transmitting at intervals having a repetition rate (T) a predetermined signal code sequence of which a record is held by the control station means, and the control station comparing the received signal code sequence with the recorded version in order to determine the time of reception of a sequence so that the timer/processors are synchronised relative to the repetition interval (T); (b) a transmitter means transmitting a plurality (N) of signal sequences in a plurality of stages, each stage having a different phase modulation superimposed on the plurality of signal sequences, such signals being received by a receiving means in order to assess the phase modulated signals to determine the best match, whereby to synchronise the first and second timer/processors relative to the interval NT; (c) repeating step (b) as many times (M) as desired in order with increasing lengths of signal sequences in order to synchronise the first and second timer/processors relative to an interval (N)MT.

2 A method as claimed in claim 1 wherein the control station has means for transmitting signals to other synchronised vehicles after the signals from the unsynchronised vehicle have been detected whereby a delay in the next scheduled communication with the other synchronised vehicles may be initiated.