GB2159361A - Detecting drifting racons - Google Patents

Abstract

A marine RACON comprises a Radar receiver 1 and transmitter 2. The RACON buoy further includes a VH F receiver 10 via which bearing signals from two Radio Lighthouses 20, 21 can be received and bearings calculated in microprocessor 15. The actual position derived from these bearings is compared in microprocessor 16 with the buoy's designated position. If the buoy is drifting a signal to the RACON inactivates the transmitter 2 and activates the VHF transmitter 11 to send an alarm signal to the Radio Lighthouse control 20a. <IMAGE>

Description

SPECIFICATION Detecting drifting racons This invention relates an arrangement for detecting automatically drifting of RACONS and for inactivating such drifting beacons.

A RACON is a marine navigation aid comprising a radar responsive radio beacon carried by a buoy anchored in a fixed known position. Obviously a RACON is only useful if it remains in its predetermined position. Conversely a RACON which has drifted from its predetermined position can provide misleading information and cause danger to shipping.

It is desirable, therefore, to have some means of periodically checking the position of a RA CON buoy. For example, it can be interrogated by a shore based radar.

The present invention makes use of another navigation aid known as the Radio Lighthouse. This is a shore based radio transmitter equipment which radiates in space a rotating radio vector receivable on an ordinary VHF receiver. The transmission is such that the receiver can determine, from timing information included in the transmission and the time of reception of a predetermined characteristic of the transmission, its bearing relative to the transmitter. Where two Radio Lighthouses provide intersecting radio vectors the receiver can calculate its position relative to the two transmitters. Such arrangements are disclosed in our co-pending patent application 8131583.

According to the present invention there is provided a RACON including additional transmitting and receiving means, means responsive to two different Radio Lighthouse transmissions received via the additional receiving means to determine the RACON position, means for comparing said determined position with a predetermined position for the RACON and for generating a control signal when said determined position and said predetermined position differ by more than a certain amount, said control signal being applied to inactivate the RACON and to activate the additional transmitter to transmit a supervisory signal indicating the inactivation of the RACON.

In a preferred embodiment of the invention the RACON includes means for responding to an override signal whereby the inactivated RACON can be temporarily re-activated.

Embodiments of the invention will now be described with reference to the accompanying drawing depicting in block diagrams form a RACON according to the invention.

A conventional RACON buoy comprises a radar receiver 1 and transmitter 2. When the antenna 3 is illuminated by radiation from a marine radar the receiver 1 detects the radar signal and activates the transmitter 2 to transmit a response signal for use by the marine radar. The RACON buoy is normally anchored in a known position. To detect any significant drifting of the buoy from its proper position the RACON includes a VHF receiver 10, a VHF transmitter 11, and a data processing unit 1 2. The VHF receiver and transmitter share a common aerial 1 3 via a circulator 14.

The VHF receiver 10 is set to receive transmissions from two Radio Lighthouses 20, 21. The principle of the Radio Lighthouse is disclosed in British Patent Application No.

8136889, from which it will be noted that the Radio Lighthouse transmission are of two types, one for normal audible use by mariners, the other for use by dedicated automatic navigation equipment. By utilising the information contained in the transmissions, bearings can be established relative to the Radio Lighthouses. Where two nearby Radio Lighthouses have overlapping transmissions, two intersecting bearings can be used to fix a position. The RACON illustrated includes a first microprocessor 1 5 to which the VHF receiver output is fed, and which determines the bearings of the two Radio Lighthouses 20, 21. From these two bearings the RACON can automatically determine its actuai position.

This information is fed to a second microprocessor 1 6 to which is also input the proper position of the RACON. Microprocessor 1 6 compares the two positions and, if a significant difference is detected, generates a control signal to inactivate the RACON receiver and transmitter 1 8 2. The RACON is thus prevented from responding to a marine radar and thereby giving misleading position information to a navigator. At the same time the control signal activates the VHF transmitter 11 to send a signal to a shore control station, which could be the relevant Radio Lighthouse transmitter, using the other half of the duplex channel (VHF Channel 88) not used for the normal Radio Lighthouse transmissions, or on some other frequency if the RLH were to transmit on a surplex channel.The Radio Lighthouse control centre 20a is thus alerted to the fact that the RACON is drifting and has consequently been inactivated.

As with every other bearing determination system, the lateral displacement accuracy with the Radio Lighthouse will be range dependent. Sea trials have shown the Radio Lighth ouse error to be 3. 15 at the 95% probability level. Thus for every mile range there is a 1:20 chance of having a lateral error greater than 335 feet, or about one cable every two miles. In addition, it must cope with the normal buoy movement resulting from winds and tidal currents. If this accuracy proved insufficient, improvement could be provided in two ways: (a) The microprocessor could be allowed to build up its own history of the bearing variations at that point under normal conditions and test subsequent variations against the resulting standard.

(b) The system could utilise an enhanced accuracy mode at the transmitter.

In any case the system would need a bearing line from two stations (as would be the case with any bearing/bearing system) and the normal criteria for good fixes would need to be observed. An alternative would be a Radio Lighthouse bearing combined with a DME (distance measurement equipment) fitted to the same transmitter.

To assist the marine authority to locate and reposition the drifting RACON it is preferable to include an override means in the RACON, whereby the repair ship can send a signal momentarily re-activating the RACON. This enables the repair ship's radar to locate the out-of-position RACON more easily. This signal can be constituted by the repair ship transmitting in the Radio Lighthouse format and making use of a telemetry bit in the format. This then requires only a small amount of additional software in the RACON microprocessor to negate the first control signal, no extra hardware being needed on the buoy.

Claims (3)

1. A RACON including additional transmitting and receiving means, means responsive to two different Radio Lighthouse transmissions received via the additional receiving means to determine the RACON position, means for comparing said determined position with a predetermined position for the RACON and for generating a control signal when said determined position and said predetermined position differ by more than a certain amount, said control signal being applied to inactivate the RACON and to activate the additional transmitter to transmit a supervisory signal indicating the inactivation of the RACON.

2. A RACON according to claim 1 wherein the RACON includes means for responding to an override signal whereby the inactivated RACON can be temporarily re-activated.

3. A RACON navigation beacon substantially as described with reference to the accompanying drawing.