The present invention relates to the monitoring of the positions of buoys, such as navigationbuoys, or watercraft that are moored or anchored, for example ships in a harbour or yachts in a marina.

An essential feature of any navigation buoy is that its exact position is known and maintained. At present, buoys must be checked physically at regular intervals to check that they have not drifted. In roughweather it canbeimpossible tocheck thebuoysand, asthat is the time that they are most likely to drift, dangerous situations can develop.

Drifting of moored or anchored watercraft can also causeproblans. There are thousands ofcraftmooredor anchoredat anyonetimeandthelosses caused by drifting of craft are considerable. The checking of craft in harbours, marinas, etc relies on visual checks and it is difficult for comprehensive checks to be carried out regularly.

The object of the invention is to provide a means of automatically checking the position of a buoy or craft and providing a warning if it has drifted.

The invention provides a drift warning system for a remote unit comprising: a) means to determine the position of the remote unit; b) means to store an initial position of the remote object; c) means tocompare thepresentpositionoftheremoteunitwiththe initial position; and d) means to activate a warning signal if the present position is outside a predetermined error range from the initial position.

Theremote unit itselfmaybeabuoyor anytypeofwatercraftsuchas a ship or a yacht.

Preferably themeans to determine the position includes a receiver.

mounted at the remote unit, responsive to global positioning information transmitted by earth satellites.

Advantageously the receiver is responsive to global positioning informationf omaGlobalPositioningSystemusedintheDifferentialModeto _ ^' detect changes in the position of the remote unit, as this providesgreater positional accuracy.

Preferablyacentralmonitoring unit is provided atwhich awarning signal is activatedandwhichcanmonitor several remoteunitsconcurrently.

The receiver may include its own processing capability but advantageously it acts as a relaysuch that all the information receivedfrom satellites is passed via the relay and processed at the central monitoring unit.

Preferably the means to store the initial position and the means to compare the present and initial positions of the remote unit comprise processing means, such as a programmed microprocessor.

Preferably the initial position of the remote unit is entered into the processingmeanswhenthe remote unitis set inplace, mooredor anchored as the case maybe. Preferably allowable errors from the initial position are programmmed into the processing means.

If the processing means detects an error in the remote station position it triggers awarning signal. Theprocessingmeansmaybe at the central monitoring unit or at the remote unit.

If the processing means is at the central unit the central unit preferably includes a transmitter to transmit interrogation signals to the remote unit and a receivertoreceivepositional informationfromthe remote unit. The remote unit includes a transponder responsive to the interrogation signals from the central unit and a transmitter to transmit data relatingto its position to the central unit, which is activatedbythe

transponder in response to an interrogation signal from the central unit. The remotestationunitcanbeinterrogatedondemandorbyrepeatedautoma tic interrogations.

If the processing means is at the remote unit the remote unit preferably includesmeans to enter the initialposition into the processing meanswhen the remote station is set inpositionormoored etc. The remote unit also includes a transmitter totransmit informationtothecentral unit and advantageouslyalsoincludeswarningsignalmeans at the remote station. If the processing means detects an error in the remote unit position the processing means activates the transmitter to send a signal to the central unit to activate the warning signal. Advantageously the transmitter also transmits data identifying the remote unit and detailing the positional error.

The remote unit processor automatically informs thecentral unit if there is an error in position. Alternatively the remote unit may also include transponder means responsive to interrogation signals from the central unit which activates the transmitter to transmit positional data to the central unit in response to an interrogation signal. This enables the remote unit positions to be checked on demand or scanned automatically.

If the remote unit is a navigation buoy a warning signal at thebuoy can be effected by the processing means causing the visual signalling to be altered, to alert shipping that the buoy is off station.

The warning signal at the central unit may beofanyconvenient form such as an audio or a visual alarm.

Advantageously displaymeans maybeprovided at the central unit to display positional data relating to a plurality of remote units.

The invention will now be described, by way of example only, with reference to the drawings of which:

Figure 1 shows a schematic diagram of one form of a buoy drift

warning system according to the invention; and Figure2 shows aschematic abuoy drift warning system.

As shown in Figure 1 a bucy is positioned remotely from a'central monitoring unit 2. Mounted in the buoy 1 is a global positioning system (GPS) receiver (or receiver acting as a relay) 3 which receives global, positioning information from earth satellites. Apresetter 4 enables the desiredla itudeandlongitudeco-ordinatesofthebuoy1 tobeset. Adata link5 links thebuoy1withthecentral unit2. Thecentralunit2 includes a receiver/transmitter 6 for receiving and transmitting data over the data link5. Aprocessor7 isprogrammedtostorethepositional datarelatingto the buoy 1 when it is set in its initial position. The processor 7 is prograπmed to activate an interrogator 8 at regular intervals. The interrogator 8 causes an interrogation signal tobe sent over thedata link 5 to the buoy 1. The interrogation signal is input to a transponder 9 which activates a transmitter 10 to transmit GPS positional data relating to the buoy1back to the centralunit2. TheGPSdata isreceivedbythe receiver6 and input to the processor 7. The processor 7 then compares the present position of the bucy 1 with the stored initial position and if the present position is outside a preprogrammed error range the processor displays a warning signal on a display 11. The system uses GES inwhat is called the Differential Mode. The buoy 1 also includes a battery power supply 12.

Figure 2 shows an alternative buoy drift warning system that is particularly suitable for major navigation buoys. A buoy 21 is located remotely from a central monitoring unit 22. The buoy 21 contains a GPS receiver 23 for receiving information fromearth satellites. Apresetter 24 enables the latitude and longitude co-ordinates of the buoy's initial position to be input to a processor 25. The GPS data received by the receiver 23 is input into the processor 25 and compared with the initial position. Ifthepresentpositionofthebuoy21 isoutsideapreprogrammed error range from the initial position the processor 25 activates a transmitter 26 which transmits information identifying the buoy and detailing the positional error via a data link 27 to the central unit 22.

The processor 25 also instructs a flash control unit 28 to alter the visual signal produced by a flashing light 29 to indicate that the bucy 21 is off station.

The information transmitted to the central unit 22 is received bya receiver 30 and input to a processor 31. The processor 31 displays the informationonadisplay32 anddisplaysawarningthatapositionalerrorhas been detected.

Thebuoy equipment is poweredbyabatterypower supply (not shown).

The invention has been descrϋjed in relationto abuoydriftwarning system but is equally applicable to drift warning systems for all types of watercraft such as ships, yachts etc in harbours, rivers, marinas and soon.

It has many advantages including its flexibility and its ability to allowautomaticwarning tobegiven of driftingbeforepotentiallydangerous situations can develop.