GB2047039A - Radio direction finding equipment - Google Patents

Abstract

Radio direction finding equipment comprises a directional aerial 1, a receiver 3, an interpreter 6 for interpreting the bearing of a received signal, a morse decoder 15 for identifying the received signal, a memory 18 for memorizing the bearings and identities of a number of signals and a computing unit 22 for computing the position of the equipment from the memorized bearings. <IMAGE>

Description

SPECIFICATION Radio direction finding equipment The present invention relates to radio direction finding equipment.

Presently available radio direction finding equipment, at least for yachts, typically comprises a hand-rotatable ferrite rod aerial attached to a compass for determining the orientation of the aerial and hence the bearing of the transmitting RDF beacon whose signal is being received. Receiving circuitry is provided either in the hand held aerial unit or at a remote installation which is cable-connected to the aerial unit. Such equipment relies upon the yachtsman-operator's direct observation of the beacon bearing from the compass.

The present invention has been conceived with a view to reducing the degree of operator skill involved in the use of the above described prior equipment.

According to the present invention there is provided radio direction finding equipment comprising a receiver, a directional aerial, means for accepting and interpreting information received from the directional aerial to give an output indicative of the relative bearing between a known direction and the direction of a radio beacon from which the receiver is receiving transmission via the directional aerial, means for identifying the radio beacon from which transmission is being received, a memory for memorizing the output indicative of the bearing for each beacon whose signal is successively received, and a computing unit for computing the position of the equipment from the memorized bearings.

The position may be given in the-form of longitude and latitude and/or in the form of a distance and bearing to prominent a geographic feature. Preferably an indication of the degree of accuracy of the position will also be given.

Although if the bearing of a sufficient number of radio beacons can be measured by the equipment, it is not necessary to provide a datum direction, it will generally be convenient to provide a datum against which to measure each bearing. Such datum might be the ship's head. This is unlikely to be particularly satisfactory since it must remain constant over the period during which all the relevant beacons transmit. Typically the beacons are arranged in sets of six each transmitting for one minute. Thus the ship's heading if it is to be the datum should remain entirely constant over six minutes. A more convenient datum is a magnetic compass. A datum may also be provided by using a gyroscope or by observing the direction from which some constantly transmitted signal, possibly a BBC signal, is being transmitted.For example where a rotating aerial is used, the time interval between the null, or any other readily identifiable characteristic, of received beacon signal and the null of received constant signal will be interpretable by comparison with the time taken for a complete revolution as the relative bearing between the direction of transmission of the two signals. It is most unlikely that the absolute bearing of the constant signal will change markedly during the six minute cycle. If however the ship's heading changes markedly during even the one minute period of transmission of the single beacon, this will not markedly affect the constant signal datum or indeed the datum provided by a gyroscope or a compass. The preferred datum is that provided by a magnetic compass.

Preferably the equipment is capable of discriminating between signals whose information content appears reliable and signals whose content appears unreliable. Such unreliable signals may be weak or have a bearing inconsistent with the computed yacht's position having regard to the other signals.

These unreliable signals may come from distant beacons.

There may be circumstances under which certain beacon signals are intrinsically unreliable, for instance where the transmitted signal has been diffracted on passing over a coastline. Furtherthere may be circumstances where navigational corrections, such as for convergency may require to be made. The processor may be programmed to take account of these circumstances or alternatively to alert the user of the equipment to the existence of the circumstances. In certain areas of potential danger the yachtsman-user may be ill-advised to place too much reliance on the position given by the equipment, for instance in the vicinity of a reef. The equipment may be programmed to alert the user to such a position.

Although it is envisaged that information relating to the transmission data of the beacons, that is to say frequency and time of transmission, call sign and beacon position, may be made available to the equipment via a manual keyboard, it is preferred that the information be made available on memory cards appropriate to the area of use. Typically one card would contain the data relating to one set of six beacons. Alternatively fewer or more beacons may be included as appropriate. Data relating to the expectation of intrinsically unreliable received signals and the areas of potential danger may also be included on the memory cards.

To help understanding of the invention, two specific embodiments thereof will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows diagrammatically the first embodiment, Figure 2 shows the computing method employed by the first embodiment, and Figure 3 shows diagrammatically the second embodiment.

Referring first to Figure 1 the radio direction finding equipment shown therein has a rotatable loop aerial 1 which is driven at a constant speed by electric motor 2. A receiver 3 receives signals from the aerial via line 4 and passes via line 5 information relating to the amptitude of the received signal to an interpreter 6. The interpreter also receives a pulse via line 7 from an aerial/compass correlator 8 each time the aerial rotates past a position aligned with magnetic north. The correlator contains a detector servo-driven to an angular position aligned with the azimuth of magnetic north and passing through the axis of the aerial. A compass unit 10 drives the detector into position. Alternatively the compass may be incorporated in the aerial unit and the detector may be mounted directly on the compass card.

The interpreter compares the time of the nulls or other thresholds of amplitude of the received signal with the time of the pulses on the line 7 and the period of rotation of the aerial to obtain the bearing of the beacon with respect to magnetic north. To improve the accuracy of the bearing, the value obtained is obtained a discrete number of times and is averaged over the time of transmission of each beacon in an averaging unit 11 and the average is displayed on display 12, to which it is passed by line 13.

The equipment is provided with the facility for the user to identify the beacon from its audio signal using headphones 14 connected to the receiver 3.

The equipment has a morse decoding unit 15 which is supplied on line 16 with the received morse signal by the receiver 3. Once the decoder has identified the beacon being received, it activates via line 17 the averaging unit 11 which averages for a given length of time during which reception from identified beacon can be expected. The decoder identifies the received beacon by reference to its memory 18, which can be manually programmed with the call signs of the expected beacons from console 23. Once the decoder knows which beacon is being received, it passes the information via line 19 tothe display 12 which displays the identification of the received beacon and its bearing.

The information relating to the beacon is also stored in the memory 18 whence it is passed to the computing unit 22 on demand. Once information relating to at least two beacons has been memorized, the computing unit can calculate the yacht's position. This it may do as follows (see Figure 2): 1) From a knowledge of the actual positions A, B of the two beacons the computing unit calculates the length and direction of the positional vector V connecting the positions A, B; 2) The computing unit then moves the positional vector a convenient unit distance at right angles to its direction to new location V'; 3) The moved vector V' intersects the known bearings a, ss of the beacons at new positions A', B' which are calculated. The length of the vector V' is calculated.

4) The length of V and V' are compared to ensure that the initial step was in the right direction i.e.

towards the yacht's position P.

5) The process of moving the vector is continued until P is reached. The computing unit will recognize this as occurring when the length of the vector has effectiveiy become zero and the positions of its ends have effectively coincided.

The thus iteratively computed position is displayed. The positions obtained by carrying out the iterative process with each pair of beacons whose position is known can be averaged. Alternatively they may be displayed individually to enable the yachtsman to plot them as the corners of a "cocked hat" whereby he may assess the accuracy of the computed position.

The interpreter 6, averaging unit 11, morse decoding unit 15, and computing unit22 may all be included in a hard-wired microprocessor.

Referring next to Figure 3, the radio direction finding equipment shown therein has a rotatable aerial 201 driven by a stepper servo-motor 201. The aerial is connected via a line 204 to a receiver 203 which passes information concerning the amplitude of the received signal into a processor unit 220 via line 205. Within the processor unit there is a servo control unit 226, which via line 221 controls the servo-motor 202 for the aerial 201 to hunt for and locate the bearing of the received beacon. An interpreter 206 coupled to the servo control unit 226 interprets the signals on line 205 with reference to the position of the aerial and information on line 209 from a compass 210 to pass information relating to the bearing of the received beacon to an averaging unit 211.The unit 206 is pre-set to automatically start the aerial hunting again when the bearing has been found and the information passed to the averaging unit 211.

The received signal is also passed on the line 205 to a morse decoder 215. A memory reading unit 218, reads information stored on memory cards 218a and 218b ... corresponding to various localities and feeds the information to the computing unit 222 whereby the computing unit can recognise the beacon from its decoded call sign on line 219.

In an alternative arrangement, an omni-directional aerial 201a passes the received signal via the receiver 204 and line 205 to the decoder 215 to enable the beacon to be recognised independently of the servo-driven aerial 201. This omni-directional aerial may also be employed for removing the 1800 ambiguity from the bearing. Once the beacon is identified the servo motor control unit 226 is initiated to start the aerial 201 hunting for the bearing of the beacon.

Once the computing unit 222 has recognised the beacon, it initiates the averaging unit 211 which averages the bearing of the beacon until the beacon's signal ceases to be received. The averaged bearing is then displayed on the display 212 simultaneously with the identity of the beacon. As the various beacons are received the computing unit memorizes their bearings and proceeds to compute the yacht's position, which is also displayed. A mode control 223 is provided for the operator to control the mode of operation of the equipment in various respects. He can control the form in which the position is given i.e. latitude and longitude or range and bearing of one beacon or other chosen geog raphicfeature. He can also command the computing unit to disregard various beacons which he judges to be unreliable. The equipment may be able to identify the bearing of a beacon but not its call sign.

Accordingly a clock 224 is provided for time recognition of such a beacon. Where provided, the omni-directional aerial 201a may be arranged only to pass on a signal having a certain threshold strength.

Thus very weak and unreliable beacons are ignored since the servo motor unit 226 is not initiated.

It is envisaged that normally the processor will cope with the 1800 ambiguity in received signal bearing by reference to the previously computed position. On start-up when there is no previously computed position, the processor will cope by choosing those bearings which provide a convergent pattern of bearings. Alternatively the omnidirectional aerial 201a may be employed.

A possible modification is that where the yacht is fast moving and the processor is provided with information about its course and speed, the processor will take account of the change n position, during the six minute cycle of the beacon's transmission.

The invention is not intended to be restricted to the details of the above described embodiments. In place of the rotating loop aerial, a rotating ferrite rod aerial may be used. Alternatively use may be made of an aerial comprising two crossed-at-right-angles ferrite rods units and such aerial is connected to the receiver, which in turn passes to the interpreter information proportional to the amplitude of received signal detected in the two crossed units. The interpreter compares the two amplitudes and takes their ratio which is equal to the tangent of the received signal's incidence angle measured from the longitudinal axis of that rod unit whose amplitude is the numerator of the ratio. The interpreter then calculates the angle whose tangent is equal to the ratio.

Claims (11)

1. Radio direction finding equipment comprising a receiver, a directional aerial, means for accepting and interpreting information received from the directional aerial to give an output indicative of the relative bearing between a known direction and the direction of a radio beacon from which the receiver is receiving transmission via the directional aerial, means for identifying the radio beacon from which transmission is being received, a memory for memorizing the output indicative of the bearing for each beacon whose signal is successively received, and a computing unit for computing the position of the equipment from the memorized bearings.

2. Radio direction finding equipment as claimed in claim 1 wherein the aerial is a constantly rotating aerial and including a magnetic compass.

3. Radio direction finding equipment as claimed in claim 1 wherein the accepting and interpreting means detects the times at which the received signal reaches a null orotherthreshold of signal strength and correlates these times with the times at which the aerial is aligned with magnetic north to the said output, and the period of rotation of the aerial to obtain the bearings of the beacon.

4. Radio direction finding equipment as claimed in claim 1, claim 2 or claim 3 wherein the identifying means is a morse decoder recognizing the beacons according to their call signs.

5. Radio direction finding equipment as claimed in claim 1, claim 2 or claim 3 wherein the identifying means recognises the beacons by their times of transmission.

6. Radio direction finding equipment as claimed in any preceding claim wherein the processor computes the position of the equipment by performing an iterative process consisting in the steps of computing the positional vector connecting the positions of a pair of beacons whose bearings have been memorized, moving the vector unit distance at right angles to its direction, calculating the positions at which the moved vector intercepts the memorized bearing directions through the positions of the pair of beacons, checking that the positional vector has moved in the correct direction to reduce its length, and repeating the process of moving the vector and cialculating the positions of interception until the vector reaches substantial zero length and the positions of interception coincide substantially at the position of the equipment.

7. Radio direction finding equipment as claimed in any preceding claim wherein the computing unit is included in a hard-wired microprocessor.

8. Radio direction finding equipment as claimed in any preceding claim wherein information relating to the transmission data and positions of the beacons to be received is made available to the processor via a memory card appropriate to the area of use.

9. Radio direction finding equipment as claimed in claim 8 wherein the memory card contains additional information relating to the computed position being unreliable in certain parts of the area of use.

10. Radio direction finding equipment as claimed in any one of claims 1 to 8 wherein the equipment is adapted to identify weak signals as unreliable.

11. Radio direction finding equipment substantially as hereinbefore described with reference to Figures 1 and 2 or Figure 3 of the accompanying drawings.