GB2320151A

GB2320151A - Covert infra-red communications system

Info

Publication number: GB2320151A
Application number: GB9623499A
Authority: GB
Inventors: Leonard Fleck
Original assignee: LFD Ltd
Current assignee: LFD Ltd
Priority date: 1996-11-12
Filing date: 1996-11-12
Publication date: 1998-06-10
Also published as: GB9623499D0

Abstract

To prevent an infra-red communications transmitter being observed by someone wearing night vision goggles, the transmitter is operated at a frequency outside the operating range of the goggles, eg > 1100nm. The transmitter forms part of a transceiver, which may be fitted to an aircraft in the vicinity of its strobe light. Preferably the transmitter emits pulses whose repetition frequency is modulated to convey speech information; statistical data can be superimposed in almost imperceptible data packets.

Description

COMMUNICATION SYSTEM The present invention relates to a communication system.

It is known for aircraft to be fitted with infra-red anti-collision lights, which whilst not usually visible to the naked eye, are readily visible with night vision goggles.

It is also known to communicate speech via modulation of transmitted infra-red signals. Such a system is used for roadcasting in conference halls. By its very nature, this is a public broadcasting system.

Despite this, my invention utilises infra-red transmission for covert communication.

Short wavelength infra-red transmission is visible, at least to night vision goggles. In my invention, I use longer wavelength infra-red, which is invisible to night vision goggles, for covert data transmission.

According to my invention there is provided a method of covert communication consisting in the steps of modulating with information and transmitting invisible infra-red signals from one transceiver and receiving and demodulating the signals with another transceiver.

According to another aspect of the invention, there is provided a transceiver adapted to modulate with information, transmit, receive and demodulate invisible infrared signals.

Whilst an infra-red wavelength as short as 950nm - or possibly 920nm - would be usable in my invention, I prefer to use a wavelength of at least ii 00nm.

It is possible for the modulation to be amplitude modulation, however I prefer to pulse the transmission and modulate the pulse frequency.

The information communicated can be speech, in which case the transmission will be at least substantially continuous, at least whilst the speech being sent continues.

Also the information can be data, particularly data relating to the position of the transmitting transceiver. In this case, the position data may be transmitted periodically in scarcely perceptible short bursts within the speech.

In accordance with a preferred feature of the invention, the transmitting elements of the transceivers are light emitting diodes arranged in anti-collision lights of an aircraft. The receiving elements may also be arranged within the anti-collision lights. Nevertheless either or both of the transmitting and receiving elements may be fitted remote from the anti-collision lights.

To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a helicopter equipped with anti-collision lights; Figure 2 is a diagrammatic view of two helicopters in communication by means of the invention; Figure 3 is a block diagram of one transceiver of the invention; Figure 4 is a side view of an anti-collision light adapted for the invention.

Referring first to Figure 1, the helicopter 1 there shown has a front underside anti-collision light 2 and a rear tail-top anti-collision light 3. These lights each comprise a conventional filament lamp or flash tube unit 4 with infra-red emitters mounted in a thin housing 6 spacing the unit 4 from the aircraft's fuselage.

Within the housing 6 are arranged a plurality of night-vision-goggle visible infra-red light emitting diodes 11, a further plurality of night-vision-goggle invisible infra-red light emitting diodes 12 and a plurality of infra-red receptors 13. The NVG visible diodes 11 are powered to flash intermittently in a conventional manner as an anti-collision light.

The NVG invisible diodes 12 are powered by a circuit 14, which pulses them at 120kHz. This frequency is modulated in a super heterodyne manner by an audio circuit 15 for transmission of the aircrew's speech, via a headset microphone 16.

The infra-red receptors 13 are connected to a super heterodyne demodulator 17, whose output controls a loudspeaker drive circuit 18 connected to a headset speaker 19.

When speech transmission is to be made from one aircraft to another, the diodes 12 are powered. Since these are invisible, even to night vision goggles, the transmission is secure and cannot be detected, except by apparatus equipped with suitable receptors and a demodulator.

The transmission can be made independently of operation of the anti-collision light, either with this operating or not. When it is not operating, the other aircraft cannot see the first. So that they can know where each other is, each has a GPS receiver 21 connected to an interface circuit 22, which encodes data indicative of the latitude, longitude, speed and altitude of the aircraft. This data is transmitted by switching 23 the audio circuit 15 from the transmission circuit 14 and switching in the interface circuit 22 to modulate the transmission during brief periods. The apparatus in the receiving aircraft similarly has a decoder 24 switched 25 in in place of the loudspeaker drive circuit 18. The decoder computes the relative position, velocity and height of the aircraft and drives a display 26, which displays this information, in a manner akin to a radar display.

In practice, I have determined that the data can be transmitted in a burst of 50 to 100 milliseconds duration every half second without seriously degrading the intelligibility of the speech being transmitted.

The embodiment of the invention as described is particularly useful for communication between aircraft in close formation, since the invisible infra-red signals have a limited range and cannot be detected by satellite.

Claims

Claims.

1. A method of covert communication between two stations each having a transceiver, the method consisting in the steps of modulating with information and transmitting invisible (to the human eye) infra-red signals from the transceiver of a transmitting one of the stations and receiving and demodulating the signals with the transceiver of the other one of the stations.

2. A method of covert communication as claimed in claim 1, wherein the modulation is amplitude modulation.

3. A method of covert communication as claimed in claim 1, wherein the transmission is pulsed, and the pulse frequency is modulated.

4. A method of covert communication as claimed in any preceding claim, wherein the transmission is of statistical data.

5. A method of covert communication as claimed in any preceding claim, wherein the transmission is of at least substantially continuous speech.

6. A method of covert communication as claimed in claim 5 and claim 6 in combination, wherein the statistical data is superimposed on the speech data in substantially imperceptible periodic packets.

7. A method of covert communication as claimed in claim 6, wherein the statistical data relates to the position and movement of the stations whose transceiver is transmitting.

8. A transceiver for covert communication, the transceiver being adapted to modulate with information, transmit, receive and demodulate invisible (to the human eye) infra-red signals.

9. A transceiver as claimed in claim 8, wherein the transceiver includes a modulator adapted to modulate the amplitude of its transmission.

10. A transceiver as claimed in claim 8, wherein the transceiver includes a modulator adapted to modulate the frequency of its transmission.

11. A transceiver as claimed in claim 10, wherein the transceiver is adapted to pulse the transmission and the modulator is adapted to modulate the frequency of the pulses.

12. A transceiver as claimed in any one of claims 8 to 11, wherein the unmodulated pulse frequency is substantially 120 kHz.

13. A transceiver as claimed any one of claims 8 to 12, wherein the transceiver is adapted to operate at a wavelength such that the infra-red signal is invisible through night vision goggles.

14. A transceiver as claimed in claim 13, wherein the transceiver is adapted to operate at a wavelength of at least 1100 nm.

15. A transceiver as claimed in any one of claims 8 to 14, wherein the transceiver is adapted to drive infra-red light emitting diodes for the transmission of the signals.

16. A transceiver as claimed in claim 15, in combination with anti-collision lights including the infra-red light emitting diodes for the transceiver

17. A transceiver as claimed in claim 16, wherein the anti-collision lights include elements for reception of transmission from a like transceiver.

18. A transceiver as claimed in any one of claim 8 to 17, wherein the transceiver includes an interface adapted to accept transceiver position and movement statistical data for transmission.

19. A transceiver as claimed in claim 18, wherein the transceiver includes a decoder to compute the relative position, velocity and height of the transmitting transceiver, and drive a display to display this information.

20. A transceiver as claimed in any one of claims 8 to 19, wherein the transceiver includes an audio circuit adapted to modulate the frequency in a super heterodyne manner for transmission of speech.

21. A transceiver as claimed in claim 20, wherein the transceiver includes a super heterodyne demodulator adapted to demodulate the received signals, and drive a loud speaker drive circuit connected to a headset speaker.

22. A transceiver as claimed in any one of claims 8 to 21, wherein the transceiver includes a switch to switch between the audio circuit and the interface circuit for transmission during brief periods.

23. A method of covert communication substantially as hereinbefore described.

24. A transceiver for covert communication substantially as hereinbefore described with reference to the accompanying drawings.