This invention relates to relative position indicating means.

It is known to use ground or airborne radar to ascertain the position and relative position of a plurality of land or airborne objects such as ground vehicles and aircraft. A disadvantage is the capital cost of the equipment required and also that during radar operation there is exposure in a military situation, to hostiles homing on the radar emission. it is known to provide present position indication in an aircraft or other vehicle or ship by radio signals from known beacons but this does not allow for one vehicle, aircraft or ship or other station knowing the relative positions of a plurality. it is known that VHF/UHF radio transmissions require relatively low power and are capable of travers¬ ing long distances such that, for example, they are capable of being used for satellite communication cr through drone or helicopter to relay messages even further distances.

It is also known that data can be readily scra ^m bled or coded in VHF/UHF radio communication such that access to the data by a hostile can be rendered difficult. It is an object of the invention to provide improved means whereby any one of a plurality of land, sea or air vehicles may establish relative positions of each of the plurality.

According to the present invention means whereby the object can be met comprises a VHF/UHF or other frequency range radio tranεreceiver operatively ccupled to means for displaying data received by the trans- receiver on a display and for transmitting data relevant

to the vehicle aircraft or station in which the means whereby the object can be met is mounted said radio transreceiver having a plurality of radio channels whereby each channel may be allocated to a respective one of a plurality of vehicles or stations each of which carries means according to the invention and each vehicle ir.ay receive data relevant to each of the plurality and display it on its own display to indicate the relative positions of the plurality. - Suitably the means for displaying data is arranged to be driven by present position indication signals cf the vehicle in which it is carried. Such signals may be generated by a PPI on the vehicle or by a remote PPI transmitting to the transreceiver. In the former case the transreceiver and the PPI are coupled to the means eg area navigation system, for display such that P?I data is transmitted from the transreceiver.

Suitably the tranεreceiver is adapted such that i- may be operated from any one of the plurality ar.α identifier means are provided so that data is transmitted only when an enquiry is from an acceptable identity.

Suitably means according to the invention is provided with scrambler code-decode means whereby transmitted data may be in scrambled form, and further means may be provided whereby scramble codes may be varied according to a programmed control. Likewise means may be provided whereby operating frequency cf each of the plurality of channels is varied at intervals according to a programme.

The invention provides the advantages of presently used radar relative position indicators but has the advantage of substantially reduced costs, low power operation, for example up to 5 watts, reduced weight of equipment, extended range potential, and reduced exposure to homing devices .

In an example means according to the invention may be mounted in a plurality of search aircraft or helicopters, a control station such as a base ship and on sonar buoys adapted to be dropped from the air vehicles at a sea location where an underwater intruder is thought to be located. Each of the air¬ craft or helicopters and the .control station may on enquiry ascertain the relative position of each other which is advantageous, particularly in low visibility and adverse weather conditions, and also ascertain sonar signals from the buoys whereby the underwater intruder may be located. Suitably the buoys include present position indicators driven by satellite signal receiver whereby drift can be taken into account and the buoy transreceivers transmit data relevant to their position and the sonar signal relevant to the intruder position, and the buoy can be designed to last for predictable hours. A particular advantage of the invention is that existing avionics position indicating equipment may be used with add-on adaptation to meet the object of the invention.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a block diagram of means according to the invention for aircraft mounting and connection to aircraft sensors and to a radio transreceiver for communication with and interrogation of other aircraft or gound stations, and

Figures 1 A to H are illustrations the means of Figure 1 and of typical displays which may be developed on the display unit.

In the embodiment illustrated in the figures which is suited for civil aircraft use, the means to be mounted in an aircraft comprises as seen in Figure 2A and the block diagram of Figure 1 a conventional display unit 1 coupled to a data processor or computer 2 for example an RS2 32C microprocessor through a mode switch 3 arranged to couple different programmes to the processor so that different displays can be selected as required.

The computer 2 is coupled to sensors 4 on the aircraft for the input of digital data 5 relative to position, navigational information and altitude and to a radio transreceiver adapted to transmit and receive such data between it and compatible units on other aircraft: or static stations.

The computer 2 is programmed to compute aircraft speed as received data from sensors 4 is updated at clocked intervals, and to compare data related to its own 4, 5 and to received signals 6, 7 such that the

relative data may be presented in different forms of display by use of the mode switch 3, of Figure 2B and is suitably programmed to provide a switch-on check and all satisfactory display, Figure 2C, to indicate that the unit and associated inputs are functioning when the unit is switched on. xhe computer 2 is programmed to develop relative position display in a forward locking mode as in Figure 2D with the interrogating aircraft 10 at centre bottom and its course indicated by vector 11, or in a 360° display with interrogating aircraft in centre as at 10 in Figures 2E and 2F, and relative course by a forwardly extending vectors 11 and 13 as shown in Figures 2D and 2F for the interrogating aircraft .10 and interrogated aircraft 12. The mode switch and the programmes being such that the nature of the display can be manually selected.

The computer 2 is suitably programmed to compare altitude data and to generate a different colour display signal for the contacted aircraft according to whether the altitude separation is greater or less th n a predetermined or selectable amount, e.g. 500 ft. In a suitable system red (orange) is used if the altitude separation is less than 500 ft. and green if greater than 500 ft.

The computer 2 is suitably programmed to present by appropriate selection on the mode switch 3 a display of transmitted and received data in alphanumeric manner as shown in Figures 2G so that by use of the mode switch 3 the alphanumeric display may be selected when

required.

As the transmitted 8 and received 7 data is clocked the displays are continuously updated and the computer is programmed to compute speed and relative speed from the changing data.

The display may be superimposed on a radar screen so that it may, for example, be displayed above a ground picture.

The computer 2 may be programmed to present the displays in other than head up relation to the air¬ craft carrying the display, for example as in Figure 2G where the display is North up and the vector courses 11, 13 of both aircraft 10, 11 are both inclined to the North up direction. Also arrangements may be embodied to embody an overlay display, for example imposed by a control station, to display limitations on or require¬ ments of the aircraft. This could be useful in dis¬ playing ground control flight path requirements.

In a potential military application illustrated in Figure 21 the angled vectors 14, 15 around the flight path of each aircraft or carrying vehicle might be represented by a permitted arc of fire determined by a control aircraft or ground station. In the embodiment described the mode switch has a reset button and 6 two position switches arranged to select display as follows:-

1. Overlay i.e. the overlay of predetermined data on the screen.

2. North up or head up display 3. Centre or bottom position of interrogating aircraft.

4. Arc on or Arc-off i.e. the presence or absen of arcs of fire as indicated in Fig. 2H.

5. Graphic or text display i.e. the displays according to Figures 2C-F and 2H as graphic display or that of Fig. 2G as text.

6. vector on and vector off i.e. the presence or absence in a graphic display of vector representation of course and speed as in Figures 2D, 2F, 2H. The text display is arranged as in Figure 2G to display data in two lines, the first line comprising longitude, lattitude and altitude, and the second line comprising speed in knots, course made good, arc-left and right of the field of fire, as seen in Fig. 2H, range of field of fire.

Thus, in Figure 2G, the upper row of heading cod indicates the following:-

ID - Identification KTS - Speed in knots CMG - Course made good LFT - Left RGT - Right RUG - Range ALT - Altitude The first set of data adjacent the ID FP- interrogating aircraft, represents in the upper row Longitude 56.59 N Lattitude 001.59 W Altitude 2080 ft. and in the second line speed 105 knots, course made good 115 Arc left 100 Arc Right 130 Range 25 miles. Corresponding data for the interrogated aircraft is

indicated in the two rows of data adjacent the ident¬ ification FN. The nature of the data displayed is exemplary but typical of the data requirements for the pilot of an aircraft requiring relative position information.