GB2488354A

GB2488354A - Measuring the shape of a boom using single and dual frequency GPS receivers

Info

Publication number: GB2488354A
Application number: GB1103183.8A
Authority: GB
Inventors: Vincent William Lynch; Samer Tufail
Original assignee: SALFORD ELECTRONIC CONSULTANTS Ltd
Current assignee: SALFORD ELECTRONIC CONSULTANTS Ltd
Priority date: 2011-02-24
Filing date: 2011-02-24
Publication date: 2012-08-29
Also published as: GB201103183D0

Abstract

Apparatus for monitoring the shape of a boom 2 connected between two vessels, 3 and 4, to allow collection of oil from a sea surface oil spillage. The apparatus has a dual-frequency GPS receiver 11 mountable relative to the boom and capable of determining its position using multiple GPS frequencies and capable of determining its position using only a single GPS frequency. The apparatus also has single frequency GPS receivers 12 mountable to, or mountable relative to, the boom and capable of determining its position using only a single GPS frequency. The apparatus can determine the difference between a position measurement of the multi-frequency GPS receiver using both frequencies and a position measurement of the multi-frequency GPS receiver determined using only the frequency of the single frequency GPS receiver to provide a correction value to allow improved accuracy of the position of the single frequency GPS receiver. The single frequency GPS receivers may be mountable at spaced apart intervals along the boom. The shape of the boom may be approximately determined by measuring the position of the single frequency GPS receivers on.

Description

Apparatus and Method for Monitoringth Shape gftBm The present invention relates to apparatus and a method for monitoring the shape of a boom.

Such is the demand for oil in modem society that oil rigs have been erected to tap into oil fields located on the sea and ocean bed. In addition, oil is commonly transported over the seas and oceans by water based vessels to distribute oil from oil rich areas of the world to oil poor areas. An unfortunate side effect of oil rigs and oil transportation by sea and ocean is that oil spills can occur. For example, an explosion on an oil rig can result in large quantities of oil being spilled into the ocean. The environmental and economic consequences of such an oil spill can be catastrophic.

A number of methods for recovering oil from the sea and ocean have been developed to try and reduce the consequences of an oil spill. Since oil is less dense than water, it tends to float to the surface of the sea and ocean. One such method is therefore to gather the oil floating on the ocean surface and extract the oil from a concentrated area. With reference to Fig. 1, this gathering process can involve the use of a boom 2 which is connected between a tug boat 3 and a larger ocean going vessel 4 as depicted in Fig. 1. By arranging the tug boat 3 such that it leads the vessel 4 by a certain distance, the boom 2 can be arranged to take a so called F formation. This approximate J-formation is maintained by adjusting the speed and distance of the tug 3 relative to the vessel 4. The J-formation is also maintained by tethering the boom 2 to the vessel 4 using a brace rope 5 and arranging the brace rope 5 such that it extends substantially perpendicularly between the side of the vessel 4 and the opposite side of the boom 2. This adjustment process is done by visually monitoring the boom and adjusting the relative vessel speeds, distances and brace rope angle accordingly.

The region of curvature of the boom 2, which is generally adjacent and behind the vessel 4, is referred to as the lens area 6. The lens area 6 is the part of the J-formation in which the oil is collected.

In use> infra-red and. radar imaging techniques are employed to locate an oil spill on the ocean surface. Once located, the tug 3 and vessel 4 adjust their course accordingly to capture the oil in the lens area 6 formed by the boom 2. Once captured, the oil is sucked up from the water surface by skimmers into ocean going oil tankers.

This procedure is very complicated and labour intensive since a number of factors must be taken into account such as speed and direction in order to try and maintain the to required fonttation, such as a J-formation, for oil recovery which, as discussed, is done by visual inspection of the boom. A number of other ocean going vessels as well as helicopters and airplanes are involved in the process with so called D-formations following behind the J-formation in order to capture any missed oil. This further complicates the oil recovery procedure. As a consequence of the general level of activity the boom can often be ignored during the recovery process which causes it to deviate from its preferred formation. Neglecting the boom can result in damage to the boom, the escape of oil and substantial delays in the oil recovery process.

An object of the present invention is to mitigate the difficulties associated with maintaining the shape of the boom.

According to a first aspect of the present invention, there is provided apparatus for monitoring the shape of a boom connected between two vessels comprising a multi-frequency GPS receiver mountable to or mountable relative to the boom and capable of determining its position using multiple OPS frequencies and capable of determining its position using only a single GPS frequency, the apparatus further comprising a single frequency UPS receiver mountable to or mountable relative to the boom and capable of determining its position using only a single UPS frequency, the apparatus further comprising processing means operable to determine the difference between a position measurement of the multi-frequency UPS receiver determined using multiple UPS frequencies and a position measurement of the multi-frequency UPS receiver determined using only the frequency of the single frequency UPS receiver to provide a correction value so that the accuracy of the position measurement of the single frequency UPS receiver can be improved.

A multi frequency UPS receiver such as a dual frequency receiver is significantly more expensive than a single frequency UPS receiver and is typically capable of measuring its position to within approximately 1cm3. Comparing the accurate dual frequency pseudorange and carrier phase measurement of the UPS receiver with its less accurate single frequency pseudorange and carrier phase measurement enables the approximate error in the single frequency pseudorange and carrier phase measurement to be obtained. If the error is measured for the same frequency as the single frequency receiver, this error measurement can be used to correct the measured pseudorange and carrier phase of the single frequency UPS receiver on the boom. Advantageously, an accurate measurement of the boom shape can be obtained using cheaper single frequency UPS receivers in combination with a single more costly dual or multi frequency UPS receiver thereby giving rise to economic savings. The use of cheaper single frequency receivers on the boom also eliminates the risk of losing one or more expensive multi frequency receivers in the event that the boom becomes damaged.

There may be a plurality of single frequency UPS receivers mountable to or mountable relative to the boom. There may be a transmission means associated with the multi-frequency UPS receiver or the single frequency UPS receiver for transmitting data obtained by the multi-frequency or single frequency GPS receiver.

The transmission means may be operable to transmit a wideband signal. The transmission means may comprise a radio frequency transceiver. Alternatively, the transmission means may comprise a radio frequency transmitter and an antenna.

There may be a receiving means associated with the multi-frequency UPS receiver or the single frequency UPS receiver for receiving data. The receiving means may comprise a radio frequency transceiver. The receiving means may comprise a radio frequency receiver and an antenna.

The single frequency UPS receivers may be mountable to a brace rope that extends between at least one of the vessels and the boom. One or more tension sensors may be associated with one or more single frequency UPS receivers. The tension sensors may be adapted to measure the tension of the boom. The tension sensor may be adapted to measure the tension of the brace rope.

There may be two or more transceivers mountable to a vessel and adapted to be separated by a predetermined distance. The transceivers may be adapted to transmit a data packet. The transmission means associated with a single frequency UPS receiver may be adapted to transmit an acknowledgement data packet back to the transceivers in response to the data packet transmission. There may be a calculation engine operable to calculate the position of the transmission means associated with the single frequency UPS receiver based upon travel time of the data packets.

The multi-frequency UPS receiver may be a dual frequency receiver capable of taking measurements based upon the Li and L2 UPS frequencies. The multi-frequency UPS receiver may be mounted to a vessel and one or more single frequency UPS receivers may be mounted to the boom.

There May be display means to display the shape of the boom using the measured data of the UPS multi and single frequency receivers. There may be an alarm for indicating an undesirable measured boom shape.

According to a second aspect of the present invention, there is provided a method of measuring the shape of a boom connected between two vessels comprising the steps of: providing apparatus according to the first aspect; mounting the multi frequency GPS receiver to or relative to the boom; mounting one or more single frequency UPS receivers to or relative to the boom; determining the position of the multi-frequency UPS receiver using multiple frequencies; determining the position of the multi-frequency UPS receiver using only the frequency used by the single UPS receiver; determining the difference between the two positions to provide a correction value; obtaining measurements from one or more single frequency UPS receivers; and using the correction value to improve the accuracy of the measurements of one or more single frequency UPS receivers.

The method may thrther comprise the step of displaying the measurements on a display means to show the boom shape.

According to a third aspect of the present invention, there is provided an oil recovery system comprising a command vessel, a tug boat and a boom connected S between the command vessel and the tug boat, a multi-frequency UPS receiver mounted on or mounted relative to the boom, at least one single frequency UPS receiver mounted to or mounted relative to the boom, processing means operable to determine the difference between a position measurement of the multi-frequency UPS receiver determined using multiple UPS frequencies and a position measurement of the multi-frequency UPS receiver determined using only the frequency of the single frequency UPS receiver to provide a correction value, wherein the correction value is used to improve the accuracy of measurements by the single frequency UPS receiver.

There may be a plurality of single frequency UPS receivers mounted on the boom. A brace rope may be connected between the command vessel and a region of the boom and a single frequency UPS receiver may be mounted to the brace rope.

According to a fourth aspect of the present invention there is provid.e a method of correcting error in position determined by a single frequency UPS receiver comprising the steps of: placing a multi-frequency UPS receiver capable of receiving and processing multiple UPS frequencies including the frequency of the single frequency UPS receiver within a predetermined distance of the single frequency UPS receiver; determining the position of the multi-frequency UPS receiver using multiple UPS frequencies; determining the position of the multi-frequency UPS receiver using only the frequency of the single frequency UPS receiver; determining the difference between the two positions to provide a correction value; and applying the correction value to the position determined by the single frequency UPS receiver to provide a corrected position.

In order that the invention may be more clearly understood an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings of which: Fig I is a schematic plan view of an oil recovery system; Fig. 2 is a schematic plan view of an oil recovery system comprising apparatus according to the present invention; Fig. 3 is an enlarged sectional view of the boom shown in Figs I and 2; Fig. 4 is an enlarged schematic plan view of a vessel shown in Fig. 1; Fig. S is a schematic representation of a technique of radio based positioning using time of flight measurements; Fig. 6 is a schematic representation of a base station node; and Fig. 7 is a schematic representation of a remote node.

Referring to the drawings there is shown a tug boat 3 and a ship 4 between which is connected a flexible boom 2 of substantially circular cross-section and length approximately one hundred metres. The boom 2 is approximately two metres in diameter and is less dense than water so that it floats on top of and sits partially proud of the water surface. A skirt 8 of length approximately one metre extends away from and beneath the boom 2 to capture any surface oil that might inadvertently pass beneath the boom 2. The boom 2 and skirt 8 are anchored to a degree by chains 10 which are connected to the extreme edge of the skirt 8 to ensure that the boom 2 is not thrown significantly from the water surface by a stray wave, for example.

A brace rope 5 links the side of the ship 4 to a region of the boom 2 that extends from the tug boat 3. The brace rope 5 helps to maintain the boom 2 in a roughly J shaped configuration when the tug boat 3 leads the ship 4 by an appropriate distance, other desirable configurations are, however, possible. As discusse& the brace rope 5 extends substantially perpendicularly from the ship 4 so as to create the lens area 6 defined by the curvature of the boom 2.

A base station node 11 comprising a substantially water tight housing 22 and containing a dual frequency UPS receiver 24 capable of utilising the signal from the LI and L2 UPS frequencies is mounted on the ship 4. The UPS receiver 24 is chosen so that it can acquire the carrier phase data and pseudorange data at a rate of ten times per second so that the position of the ship can be determined sufficiently quickly to take into account ship movement. Since the carrier frequency of the Li and L2 signals are 1,575 MHz and 1,227 MHz, respectively, the dual frequency UPS receiver can determine its position much more accurately than by using only the C/A (Coarse/Acquisition) code which is on a much higher frequency. This is because the wavelength of the carrier signal is 0.19m for the Li frequency and 0,24m for the L2 signal as opposed to approximately 300m for the C/A code. Since the dual frequency UPS receiver 24 compares its own internally generated signal with the signal of a UPS satellite and adjusts its clock accordingly and since the UPS receiver is capable of measuring to within one or two percent of the signal cycle width, a measurement based upon the carrier phase data is accurate to approximately 2mm unlike measurements based upon the C/A code which are accurate to only 3m.

The accuracy of the UPS measurement is further improved by comparing the difference between the carrier phase of the LI and L2 signals in order to determine the travel delay caused by the ionosphere which is different for signals of different frequency. A processing means 26 is provided to carry out the various comparison measurements to determine the error in measurements of the UPS receiver 24 based upon a single frequency signal. The base station node 11 also comprises a radio transceiver 28 for receiving and transmitting data via a wideband high frequency radio signal so that data can be transmitted quickly over a good range of at least one hundred metres.

A plurality of nodes 12 are connected to the upper surface of the boom 2. The number and spacing of the nodes 12 along the boom are chosen so as to provide a reasonable resolution of an image of the boom shape, length and curvature. In this embodiment, eighteen nodes 12 are mounted to the boom 2 and are spaced at intervals of five metres with the two extreme nodes arranged five metres from either end of the boom 2. Each node 12 comprises a substantially water tight housing 30 enclosing a single frequency UPS receiver 32 capable of receiving the Li frequency signal (1575.42 MHz) and the C/A code (I 023 MHz) of one or more UPS satellites. Each node 12 is capable of determining the pseudorange and carrier phase between the receiver 32 and a UPS satellite as well as the phase data of the LI frequency signal from any UPS satellite within range. The UPS receiver 32 of each node 12 is chosen so that it can acquire pseudorange and carrier phase data at a high enough rate to take account of movement of the boom over the water surface. In this embodiment, the

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UPS receiver 32 is capable of acquiring this data at a rate of five times per second. A suitable UPS module for this purpose is the U-Blox LEA-6T UPS receiver which is capable of achieving an accuracy of up to I Sns.

Each boom node 12 also comprises a transmitter 34 that is electrically coupled to the UPS receiver 32 for generating a radio frequency signal derived from the measured data of the UPS receiver 32 and an antenna 36 for transmitting a wideband high frequency radio signal so that data can be transmitted quickly over a range sufficient for cross communication between elements of the oil recovery system. Each boom node 12 additionally comprises a differential receiver 38 for receiving radio frequency signals from the ship 4 and other nodes and a tension sensor 40 which is operable to measure the tension of the boom 2 at various points along its length.

A tug boat node 13, having the same features and capabilities as the boom nodes 12, is mounted to the tug boat 3 and adapted to transmit and receive data wirelessly to the base station node 11 and boom nodes 12. Likewise, a brace rope node 16 having the same features and capabilities as the boom nodes 12 is attached to the approximate centre point of the brace rope 5 between the ship 4 and the boom 2.

The brace rope node 16 further comprises a tension sensor adapted to measure the tension of the brace rope 5 and transmit the tension data to the base station node 11.

Two digitally controlled radio transceivers 20 fitted with real time clock devices are mounted on the ship 4 and are spaced apart by a predetermined distance, in this example five metres. Each radio transceiver 20 is capable of transmitting an outbound data packet for receipt by a corresponding radio receiver 38 on each of the boom nodes 12, tug boat node 13 and brace rope node 16. Any suitable standard may be used, such as IEEE 802.1 5.4a baseline or 802.1 lg protocol which works in the 2,4 0Hz band. The antennas of the boom nodes 12, tug boat node 13 and brace rope node 16 are capable of transmitting a data packet acknowledgement back to the ship based transceivers 20. The transceivers enable data to be transmitted between the ship and other elements of the oil recovery system and, in addition, as described below, transmission and receipt of a data packet between the ship based transceivers 20 and boom, tug boat and brace rope nodes 12, 13, 16 enables the position of the different nodes relative to the ship 4 to be determined to within an accuracy of approximately 30cm by performing time of flight distance measurement.

The oil recovery system also comprises a computer system (not shown) which is adapted to receive, via a communications link from the transceivers 20, record and process data from the various nodes 11, 12, 13, 16. Using the live reahtime data, the computer system is able to generate a reasonably accurate graphic representation of the boom shape, position and speed on a display monitor. The computer system is provided with a number of pre-loaded templates for the boom shape which may be selected by a user as desired. Such a template may comprise upper and lower boundaries for the chosen boom shape which, when breached by the measured boom shape, activates an alarm to alert the boom operator. The computer system is also provided with a manual input facility to enable a user to input alterations to the boom display data in the event that the visual look of the physical boom is different from that displayed on screen.

The graphic display may be broadcast to other vessels, in order to facilitate co-ordination of the vessels in the cleanup operation.

The graphic display may be provided with an indicator showing a direction the vessel should move in, in order to better achieve the desired boom shape.

-12 -The computer system is also provided with a learning facility so that the operator can log what are considered to be good boom shapes and dimensions from real time data that that do not otherwise appear in the database.

A Doppler speed log (not shown) comprising two transducers is mounted to the fore and aft ends of the ship 4 respectively. The Doppler log determines the speed and direction of the vessel relative to the water. This information may then be compared with gps data to compare the motion of the ship and boom system relative the earth and relative to the water and thus enable currents in the water to be accounted for. For example ships (tug and towing vessel) might be stationary relative to the earth, but because of a strong sea current the motion of the boom through the sea water could exceed safe working limits. In such a case the ship's captain might reduce power to the engines and effectively start drifting backwards relative to the earth's surface but still be collecting oil as the ship and boom system is still moving through the water but at a safe speed for the equipment.

The Doppler log provides a useful comparison tool for determining whether or not the speed and trajectory of the boom as measured by the UPS receivers is accurate.

In use, the dual frequency UPS receiver on the base station node 11 obtains pseudorange and carrier phase measurements based upon the LI and L2 frequencies to determine the absolute position of the receiver to within an accuracy of approximately 1cm3. The dual frequency receiver also takes pseudorange and carrier phase measurements using only the LI frequency to determine the receiver position to within an accuracy of approximately 3m3. The processing means compares the dual frequency pseudorange and carrier phase measurements (Li and L2) with the single -13 frequency pseudorange and carrier phase measurements (LI) which enables the single frequency measurement errors to be determined, at least to within the accuracy of the dual frequency measurements.

Once obtained, a correction signal is transmitted, via the base station node 11 transceiver 28, to the boom nodes 12, tug boat node 13 and brace rope node 16 which receive the signal via their respective differential receiver 38. Since the various nodes 12, 13 and 16 utilise the Ll frequency to obtain pseudorange and carrier phase measurements, the correction signal is used to greatly improve the accuracy of the measured distances of the single frequency OPS receivers. The crnTected data is then transmitted to the ship to be stored and processed on the ship 4 computer system.

Alternatively, the correction signal is transmitted via the base station node 11 transceiver 28 to the ship to be stored on the ship computer system. Data relating to position measurements taken by the single frequency GPS receivers is also transmitted to the ship computer system via the communication link so that the computer system can apply the correction value to the measurements to improve their accuracy.

Using the accurately known positions of the various nodes 11, 12, 13, 16, the shape of the boom can be displayed graphically on the computer system monitor. In addition, since the position of the centrally located brace rope node 13 is known relative to the ship 4 it is possible to determine the brace rope angle which, as discussed for a J -formation, should preferably be maintained at approximately 90° to the ship 4.

As the absolute positions of the various nodes 11, 12, 13, 16 are known to an accurate degree, repeating their respective positional measurements at a rate of 5 -14 -times per second enables each node's speed and heading relative to one another to be obtained. This data can be used to calculate the boom speed and trajectory. Knowing the boom shape, speed and trajectory as well as the brace rope angle enables the ship 4 and tug boat 3 speed and trajectory to be altered accordingly to maintain the J-formation or other desired formation thereby ensuring efficient oil collection and minimising the risk of damage to the boom 2 and brace rope 5. The speed and trajectory changes of the ship 4 and tug boat 3 can be made on the advice of the boom operator but it is envisaged that these changes might also be made automatically by the two vessels' control apparatus on thebasis of the boom measurements.

The radio transceivers provide an additional way of measuring the relative boom position and shape using a time of flight technique. When required, the two radio transceivers 20 on the ship 4 transmit an outbound data packet 40 to each of the boom, tug boat and brace rope nodes 12, 13, 16. Each of the various nodes 12, 13, 16 transmits a data acknowledgement packet 42 in response to the outbound data packet which is received by the two transceivers 20 on the ship 4. The time required for each data packet to travel from each transceiver 20 to each node 12, 13, 16 and back constitutes the round trip time (RTT). The measured RTT enables the determination of the approximate distance (d) between each transceiver and each node 12, 13, 16 using the following formula: d = __ where c is the speed of light. Since the distance between the two ship based transceivers 20 is known, it is possible to determine the relative position of each node 12, 13, 16 using the measured distances between each node and the two separate

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transceivers 20. A calculation engine is provided to use the time of flight data to determine the positions of the various nodes. It is therefore possible to construct a reasonably accurate image of the boom shape and position relative to the ship on the basis of time of flight data using radio frequency signals. This data provides a useful second point of reference for comparing with the UPS based boom image and provides a useful backup when UPS is unavailable, such as for example in Arctic locations where the satellite signal is relatively poor.

GPS and time of flight positional information may then also be combined with speed information provided by the Doppler log.

In all, the apparatus enables the position and shape and trajectory of a boom to be accurately determined virtually in real time, This enables operatives to better maintain desirable and efficient boom configurations, such as.J and D shapes. The processing system may be configured to trigger an alarm signal when the boom shape deviates from a desired shape or range of shapes.

It is of course to be understood that the above embodiment has been described by way of example only and that many variations are possible without departing from the scope of the invention as defined by the appended claims.

For example, one or more nodes having the same features as the boom node 12, may be mounted to other elements of the oil recovery management system such as the skimmer (not shown).

Claims

-16 -CLAIMS1. Apparatus for monitoring the shape of a boom connected between two vessels comprising a multi-frequency UPS receiver mountable to or mountable relative to the boom and capable of determining its position using multiple UPS frequencies and capable of determining its position using only a single UPS frequency the apparatus further comprising a single frequency UPS receiver mountable to or mountable relative to the boom and capable of determining its position using only a single UPS frequency, the apparatus further comprising processing means operable to determine the difference between a position measurement of the multi-frequency UPS receiver determined using multiple UPS frequencies and a position measurement of the multi-frequency UPS receiver determined using only the frequency of the single frequency UPS receiver to provide a correction value so that the accuracy of the position measurement of the single frequency UPS receiver can be improved.
2. Apparatus as claimed in claim 1, comprising a plurality of single frequency UPS receivers mountable to or mountable relative to the boom.
3. Apparatus as claimed in any preceding claim, comprising a transmission means associated with the multi-frequency UPS receiver or the single frequency UPS receiver for transmitting data obtained by the multi4'requency or single frequency UPS receiver.

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4. Apparatus as claimed in claim 3, wherein the transmission means is operable to transmit a wideband signal.
5. Apparatus as claimed in claim 3 or claim 4, wherein the transmission means comprises a radio frequency transceiver.
6. Apparatus as claimed in any of claims 3 or claim 4, wherein the transmission means comprises a radio frequency transmitter and an antenna.
7. Apparatus as claimed in any of claims 3 to 6, comprising a receiving means associated with the multi-frequency UPS receiver or the single frequency UPS receiver for receiving data.
8. Apparatus as claimed in claim 7, wherein the receiving means comprises a radio frequency transceiver.
9. Apparatus as claimed in claim 7, wherein the receiving means comprises a radio frequency receiver and an antenna.
10. Apparatus as claimed in any preceding claim, wherein the single frequency GPS receivers are mountable to a brace rope that extends between at least one of the vessels and the boom.
11. Apparatus as claimed in any preceding claim, further comprising one or more tension sensors associated with one or more single frequency UPS receivers.
12. Apparatus as claimed in claim 11, wherein the tension sensors arc adapted to measure the tension of the boom.
13. Apparatus as claimed in claim 11 or claim 12 when dependent directly or indirectly upon claim 10, wherein the tension sensor is adapted to measure the tension of the brace rope.

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14. Apparatus as claimed in any preceding claim, comprising two or more transceivers mountable to a vessel and adapted to be separated by a predetermined distance and arranged to measure the location of a transmitter on the boom using time of flight data.
15. Apparatus as claimed in claim 14, wherein the transceivers are adapted to transmit a data packet.
16. Apparatus as claimed in claim 15 when dependent directly or indirectly upon claim 3, wherein the transmission means associated with a single frequency UPS receiver is adapted to transmit an acknowledgement data packet back to the transceivers in response to the data packet transmission.
17. Apparatus as claimed in claim 16, comprising a calculation engine to calculate the position of the transmission means associated with the single frequency UPS receiver based upon travel time of the data packets.
18. Apparatus as claimed in any preceding claim, wherein the multi-frequency UPS receiver is a dual frequency receiver capable of taking measurements based upon the Ll and L2 GIl'S frequencies at the same time.
19. Apparatus as claimed in any preceding claim, wherein the multi-frequency UPS receiver is mounted to a vessel and one or more single frequency UPS receivers is mounted to the boom.
20. Apparatus as claimed in any preceding claim, comprising display means to display the shape of the boom using the measured data of the UPS multi and single frequency receivers.
21. Apparatus as claimed in any preceding claim, comprising an alarm for indicating an undesirable measured boom shape.
22. Apparatus as substantially hereinbefore described with reference to Figs. 2 to 7.
23. A method of measuring the shape of a boom connected between two vessels comprising the steps of: providing apparatus according to any of claims I to 22; mounting the multi-frequency UPS receiver to or relative to the boom; mounting onc or more single frequency UPS receivers to or relative to the boom; determining the position of the multi-frequency UPS receiver using multiple frequencies; determining the position of the multi-frequency UPS receiver using only the frequency used by the single UPS receiver; determining the difference between the two positions to provide a correction value; IS obtaining measurements from one or more single frequency UPS receivers; and using the correction value to improve the accuracy of the measurements of one or more single frequency UPS receivers.
24. The method claimed in claim 23, further comprising the step of displaying the measurements on a display means to show the boom shape.
25. An oil recovery system comprising a command vessel, a tug boat and a boom connected between the command vessel and the tug boat, a multi-frequency UPS receiver mounted on or mounted relative to the boom, at least one single frequency UPS receiver mounted to or mounted relative to the boom, -20 -processing means operable to determine the difference between a position measurement of the multi-frequency UPS receiver determined using multiple UPS frequencies and a position measurement of the multi-frequency UPS receiver determined using only the frequency of the single frequency UPS receiver to provide a correction value, wherein the correction value is used to improve the accuracy of measurements by the single frequency UPS receiver.
26. An oil recovery system as claimed in claim 25, comprising a plurality of single frequency UPS receivers mounted on the boom.
27. An oil recovery system as claimed in claim 25 or claim 26, wherein a brace rope is connected between the command vessel and a region of the boom and a single frequency UPS receiver is mounted to the brace rope.
28. A method of correcting error in position determined by a single frequency UPS receiver comprising the steps of: placing a multi-frequency UPS receiver capable of receiving and processing multiple UPS frequencies including the frequency of the single frequency UPS receiver within a predetermined distance of the single frequency UPS receiver; determining the position of the multi-frequency UPS receiver using multiple UPS frequencies; determining the position of the multi-frequency UPS receiver using only the frequency of the single frequency UPS receiver; determining the difference between the two positions to provide a correction value; and applying the correction value to the position determined by the single frequency UPS receiver to provide a corrected position.