GB2268016A - Optical communication system - Google Patents

Abstract

An optical communications system for enabling short- range communication between two stations, which may be relatively movable stations such as helicopters, comprises on each helicopter 10 (Fig. 2) a directional optical transmitter 11 coupled to a sight 10' steerable in azimuth by an operator and an array of (e.g. eight) optical receivers (12) arrayed about the helicopter in housings 15-18 and having fields of view defining communication sectors, the adjacent sectors being contiguous or slightly overlapping in azimuth. The receivers are connected to sector selection means 26 (Fig. 4) for determining on a continuous basis the sector containing the strongest signal, identifying the sector to the sight operator and passing the signal from that sector to a speech (or other) decoder. The communication link is established by a first helicopter transmitting a coded signal to a second identified by the sight operator. The receiving helicopter determines in which of its communication sectors the signal is received and a verification code is transmitted into that sector, the first helicopter on receipt of this verifying the connection and permitting messages to be passed. The transmitter may alternatively take the form of a plurality of fixed transmitters housed with the receivers enabling transmission into one or more sectors by choice of transmitter rather than physical rotation of a single transmitter. <IMAGE>

Description

OPTICAL COMMUNICATION SYSTEM This invention relates to optical communications systems and in particular to establishing communications paths between stations which may be fixed, suc as ground stations or which may vary in position relativly to each other, such as two vehicles or a vehicle and ground station.

It is known to use helicopters as launch platforms for guided missiles against targets which are 'spotted' from smaller, more manoeuvrable helicopters, or from advance ground observation posts. A problem exists in transferring the target information in that radio communications are susceptible to jamming, may be detected to indicate presence in an area, which presence may be pin-pointed by direction finding, and intercepted unscrambled speech messages may be used to advantage by the enemy.

An alternative operational possibility is radio silence, whereby the, or each helicopter has to land for the target information to be transferred, which also has disadvantages in terms of time taken and vulnerability of the helicopters and crews.

Other forms of communication over the short ranges involved are line-of-sight methods such as optical communications but whereas once a communications path is established such a system is relatively secure by virtue of the ability to use narrow transmission beams and low radiating powers it is difficult with stations moving relatively freely in relation to each other to establish the line-of-sight communication path initially.

In this specification the term optical radiation is intended to include electromagnetic radiation in the infra-red, visible and ultraviolet parts of the spectrum.

It is an object of the present invention to provide an optical communications system between at least two stations which mitigates the problems of establishing the optical communications path between the stations.

According to the present invention an optical communications system for establishing a communication path between at least two stations comprises at each station a plurality of directional optical receivers, each having a field of view limited in elevation and azimuth, defining a plurality of communication sectors extending from the station and arrayed about it in azimuth, communication sector selection means operable to determine from which communication sector a communication is received and optical transmitting means operable in response to the reception of communications signals in one of said communications sectors to transmit a communication signal in at least said one communication sector.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is plan schematic view of the optical communication system of the present invention as applied to two helicopter stations for the purpose of illustrating the principle of operation, Figure 2 is a plan view of a helicopter showing the disposition of the transmission and reception elements, Figure 3 is a sectional elevation through the transmitting portion along the lines III-III of Figure 2, and Figure 4(a) is a block diagram of the receiving and communication sector selection means associated with each station, and Figure 4(b) is a similar block diagram of the transmitting section.

Referring to Figure 1 helicopters 10 and 110 each carry identical optical communications equipment, that for the helicopter 10 being described and the corresponding items on helicopter 110 being identified by corresponding response numerals increased by 100.

The helicopter 10 carries a roof mounted sight 10' which is rotatable about a vertical axis to be rotated or scanned in azimuth to enable an operator to observe in any direction.

The sight 10' carries an optical transmitter 11 so that a narrow beam of optical radiation suitably modulated by a communication message can be transmitted in a direction determined by the operator to another station observed through the sight.

The helicopter 10 also carries on its roof a plurality of optical receivers 12 each including a photodetector 13 responsive to radiation at the wavelength of the transmitter and an optical system 14 delimiting the field of view of the receiver in elevation and azimuth by a predetermined amount.

As shown, the field of view is limited in azimuth to 45" (or slightly greater) and eight receivers are arrayed around a point on the helicopter such that their fields of view are contiguous (or slightly overlapping) and together cover 3600 in azimuth.

To establish communication between the helicopters an operator on board helicopter 10, say, directs the sight towards helicopter 110 and causes the transmitter to produce a beam of optical radiation modulated by a coded path verification signal which may also be indicative of the transmitting helicopter.

The beam is detected at the helicopter 110 by one of the receivers 112 and the receiving sectcr is indicated to the operator in helicopter 110 by visual or audible means. The operator then directs the sight 110' into that sector where the helicopter 10 becomes visible and directs the sight onto the helicopter. An answering transmission is then made by optical beam including a verification signal which, when received by an appropriate one of the receivers 12 and decoded informs the operator in 10 that a two-way communications path is established and that messages, either speech or data can be transferred.

Such a path establishing procedure may take place at the beginning of each communication session before the operator transmits further messages or may be performed automatically prefacing each signal transmitted.

It will be appreciated that once such line-of-sight communication is established the transmission direction is maintained by operator control of the sight and reception by appropriate one of the receivers. Furthermore as receiver selection is achieved automatically there is no limitation on the relative movements between the stations during transmissions.

Referring now to Figure 2 a more practicable disposition of the receivers 12 is shown in relation to the roof of a helicopter 10. The receivers are located in housings 15-18 housed in pairs disposed about the main rotor shaft 19.

Each housing has a window and the contiguous fields of view of the communications sectors are shown delineated by broken lines 20. Preferably where the receivers are separated the communication sectors are slightly greater than the notional 450 in order to overlap slightly and leave no dead spaces between.

The sight 10' carrying the transmitter 11 is of the periscope type whicb may be gyro-stabilised, such as the type AF532 manufactured by the applicant company, and is shown in elevation in Figure 3. An operator 21 looks through a monocular eyepiece and by way of optical elements, moved by gyro to compensate for helicopter motion, views through an output port 22. The sight is covered by a housing 23 which includes a window 24 which is maintained essentially stabilised for yaw motion by a followup system in front of the sight output port.

The optical transmitter 11 comprises a laser diode emitter and optics may be mounted on (or in) the housing 23 or within the helicopter to emit radiation by way of the sight.

The emitted beam is preferably limited to a few degrees in azimuth but extends in elevation to, say, 200 to allow for different altitudes of operating stations. Similarly the fields of view of each receiver extends in elevation by, say, - 25".

Referring to Figure 4(a) the receiving system is shown in block diagram form. The eight optical systems 14 and photodetectors 22 are shown, each photodetector being part of an amplifier 25, this combination comprising a receiver 12. Each receiver 12 provides signals to communication sector selection means 26 containing for each receiver sector signal level detection and switching means 27 and common threshold setting means 28. Considering one detection and switching means, the signal from receiver 12 is passed by way of an a.g.c. circuit 29 in which all signal levels are maintained within the operating range of the circuit and then to switching means 30. The signals produced by the a.g.c. circuit 29 are applied to a peak signal detector 31 and a signal related to the level reached by the received signal is applied to one input of a fixed threshold comparator 32.The other comparator input is produced by a fixed threshold generator 33 and the level is selected to exclude signal levels below those expected in use and serves to exclude responses to spurious low level signals and noise.

Signals of a level above the fixed threshold cause an output of threshold comparator 32 which is applied to one input a variable threshold comparator 34, the other comparator input being derived from variable threshold generator 35. An output of the variable threshold generator is applied to open the switch 30 and also applied to common gating means 36, the output of which is connected to the variable threshold generator 35. The gating means 36 is arranged to be opened when more than one input is received, that is, when more than one variable threshold comparator produces an output, and causes the generator 35 to increase the variable threshold until a level is reached at which only one comparator, associated with the sector receiving the strongest signal continues to give an output.

Thus only one switch 30 is opened to received signals. The output of the variable threshold comparators are also applied to sector decoding means 37 which produces a visual (and/or audible) identification of the communication sector from which received signals are being used. A convenient form of identification is a display in the form of a mimic plan showing the directional disposition of the communication sectors in relation to the station.

The outputs of the sector selection means 26 are applied to a decoder circuit 38 wherein the signals from the appropriate sector are decoded and, output signal as speech signals on line 39. The decoder circuit also has a second output 40 producing signals of the path verification procedure which are applied to communication path vertification means 41.

Figure 4(b) shows in schematic block form the transmitter which is conventional in that it includes a speech encoding/modulator circuit 42, transmitter drive circuit 43 emitting diode 44, conveniently a laser diode, and optics 45 arranged, as mentioned above, to confine the beam into a wedge-shape, for example, the order of 20 elevation and 30 azimuth. The encoding circuit provides a signal, when a communication signal is applied, on line 46 connected to a second input of path verification means 41, the verification means having an output 47 connected to the coder 42.

Operation of the verification means is dependent upon whether the station originates a transmission or responds to another station. When a transmission is initiated and a signal applied to coder 42, a signal is provided on line 46 which casues the verification means to inhibit transmission of the signal whilst causing a coded verification signal, stored in the verification means to be transmitted. When the receiving station answers and returns a verification signal, directed to the verification means on line 40, the inhibition is removed and transmission of the message may continue.

At the other station the receipt of the coded verification signal without having transmitted one causes the operator to be alerted, who upon directing the sight causes an answer code to be generated and transmitted before opening the coder to receive communication signals as at the originating station.

It will be appreciated that if a two way communication path is not to be set up, for example if the station merely wants to send a message without verifying its reception the verification means may be inhibited.

Verification may further be controlled by relating the sector in which an answer is received (in sector decoding means 37) to that in which transmission is made in verification means 41.

The verification coded signals may include an identification of the station so that certain stations can be included or excluded from setting up a communication link.

The optical communciation conveniently takes place in the near infra-red region of the spectrum for which transmitting and receiving components are readily available although it will be understood that it is not limited to such wavelengths. The form of transmission may conveniently but not essentially be continuously variable slope delta-modulation. Also the system may conveniently allow two way full duplex communication using transmission blanking of the receiver associated with any one system by a blanking circuit 48 connected between the transmitter 43 and receivers 12.

It will be appreciated that the system described above and operated as set forth with reference to Figure 1 offers communication with a high signal to noise ratio in that the transmitted beam is confined and the receiver (whichever it is) has a field of view also limited to that of the communication sector.

The above described communication sector selector 26 is exemplary only and variations may be made to the component parts providing the functions are fulfilled of determiming which communication sector receives the strongest signal and thereby limiting reception to that sector and identifying the sector to the transmitter operator and director. Additional features are also possible. For instance the decoded verification signal on line 40 may be used to gate operation of the sector decoding means 37 so that sector information is only displayed as a result of the receipt of a transmission from an associated station.

As an alternative to making that decision based on signals received in all sectors simultaneously, the receivers may be scanned one at a time for comparision of the levels at each to determine which sector is receiving the strongest signal.

There is of course no reason why the other receivers should not operate in a receive-only mode whilst a communication path is in operation and signals received on such receivers are preferably distinguished in some way to avoid application to, and disruption of, the sector selecting circuit 26.

As described above, the transmission takes place in a highly directional narrow beam under the control of the sight operator.

In an alternative embodiment which in plan view is similar to Figures 1 and 2, and may be described with reference thereto, each receiver 12 (or 112) is mounted coincidentally with a transmitter having a beam angle conveniently coextensive with the field of view of the receiver, that is, filling the whole comnunication sector.

In this way signals can be distributed to many other stations by transmission in some or all sectors, either simultaneously or sequentially.

Furthermore, when a two-way communications link is set up with another station and the communication sector defined by reception of the strongest identification signal therefrom, subsequent transmission to the other station is readily limited to the transmitter associated with that commur.ication sector only, thereby optimising the signal strength in relation to the transmitted power.

Whilst enabling a plurality of such communication links to be operated simultaneously a system such enables the sight and its operator to be freed for other purposes whilst maintaining automatically communication in the appropriate sector.

The receiver sector selector 26 may be as described above in relation to Figure 4(a) or the alternatives thereto mentioned, the transmitter section being substantially as described in relation to Figure 4(b) but including a plurality of transmitters (not shown) corresponding to 43, 44, 45 and a transmitter selector 49 which switches the encoded signals to the appropriate transmitter as under control of the sector decoder 37 or manually to initiate a transmission. In the latter case instead of manual selection of one transmitter, all of them may be employed to make an omnidirectional transmission until a response is received from one sector.

It will also be appreciated that the number of communications sectors employed may be varied, in reception or transmission a larger number enabling smaller fields of view to be employed with improved sensitivity. Also the sectors covered by fixed transmitters, when employed, may be other than coextensive with the communication sectors defined by the receivers. It will be appreciated that it is not essential to cover 360" in azimuth with the communication sectors. The communication sectors may be limited also by having gaps between them so that they are not contiguous.

The above description is directed primarily to stations which are relatively movable during communication, that is, between two helicopters. It will be appreciated that the stations may be other than helicopters e.g. ground vehicles, and that one or both of each pair of stations may be fixed in the sense of maintaining a position for longer than the period of a communication link. The station may be fixed temporarily, for example, a vehicle or an infantry unit stationary during communications but relatively movable between communication links, or may comprise a more permanent structure.

Whether such a communication system is intended for ground or air-borne use will determine the elevation spread and inclination of the transmitted beams and the fields of view of the receivers.

Claims (18)

Claims

1. An optical communications system for establishing a communication path between at least two stations comprising at each station a plurality of directional optical receivers, each having a field of view limited in elevation and azimuth, defining a plurality of communication sectors extending from the station and arrayed about it in azimuth, communication sector selection means operable to determine from which communication sector a communication is received and optical transmitting means operable in response to the reception of communications signals in one of said ccmmunications sectors to transmit a communication signal in at least said one communication sector.

2. A system as claimed in Claim 1 in which the communication sectors defined by adjacent receivers are contiguous or overlap.

3. A system as claimed in Claim 2 in which the total azimuth extent of the communication sectors is substantially 3600.

4. A system as claimed in any one of Claims 1 to 3 in which the communication sector selection means includes signal level detection means operable to determine the receiver from which the strongest signal is received, switching means to pass the received signal from that receiver, and sector decoding means operable to provide an indication of the communication sector served by said receiver.

5. A system as claimed in Claim 4 in which the indication is a visual indication.

6. A system as claimed in Claim 5 in which the visual indication is in the form of a mimic plan showing the directional disposition of the communication sectors.

7. A system as claimed in any one of Claims 4 to 6 in which the sector decoding means is further operable to limit transmission, made in response to signals received in one communication sector, to said communication sector.

8. A system as claimed in any one of the preceding claims in which the transmitting means comprises an optical transmitter operable to produce a beam of optical radiation limited in azimuth divergence and steering means operable to rotate the transmitter in azimuth to fix the beam direction.

9. A system as claimed in Claim 10 in which the beam of radiation is limited in azimuth divergence to less than the angle subtended by a communication sector.

10. A system as claimed in Claim 8 or Claim 9 in which the steering means comprises a steerable sight carried by said station.

11. A system as claimed in any one of Claims 1 to 7 in which the transmitting means comprises a plurality of optical transmitters each having a fixed beam path limited in elevation and azimuth the region defined by said beam paths being at least as extensive with the region defined by the communication sectors.

12. A system as claimed in Claim 11 in which the optical transmitters are arrayed in azimuth about the station and each beam path is substantially coextensive in azimuth with an individual communication sector.

13. A system as claimed in Claim 11 or Clam 12 in which the optical transmission means includes transmitter selector means responsive to determination of the communication of a received signal to cause transmission of a subsequent signal by a transmitter associated with that communication sector only.

14. A system as claimed in any one of the preceding claims including communication path verifying means comprising means for producing a verification code, detectable by the other station and in response to detection of a verification code from another station to produce a verification code for transmission to that station.

15. A system as claimed in Claim 14 in which the verification means is operable to preface each transmission with a verification code and responsive to the transmission of a verification code by said other station, indicative of reception of the transmission to continue the transmission.

16. A system as claimed in any one of the preceding claims including blanking means operable to disable the optical receivers of a station for the duration of optical emission from the station.

17. A system as claimed in any one of the preceding claims in which the communication takes place continuously variable slope delta modulation of pulsed optical radiation.

18. An optical communication system for establishing a communication link between at least two stations substantially as herein described with reference to, and as shown in, the accompanying drawings.