Other PRS which may be used in conjunction with a DP system

• Syledis

Syledis is a proprietary UHF radio positioning system developed by Sercel of France. It relies on a network of shore based transponders or becons to provide positioning over defined areas. Many areas exist all over the world which have Syledis cover. It is a propagation time measurment system.

Typical positioning is up to 100 Kms with an accuracy of 1 metre within line of sight. The accuracy depends upon the beacon height, atmospheric conditions and the network geometry relative to the vessel. There are two types of Syledis, Range- Range Mode and Hyperbolic Mode.

• Hyperbolic or Passive Mode

With hyperbolic mode, the vessel receives a pair of signals from fixed stations. The signals are place din synchronisation by a master beacon, The pair of signals define a hyperbola upon which the vessel lies. With three hyperbola defined, the vessel can be positioned. There is obviously no limit to the number of vessels that can use this mode.

Figure 51. Syledis

• Microfix

Microfix is a short range, 50 Km, microwave positioning and survey system. Arrays of transponders are placed in fixed locations or on platforms. The system uses range-range mode interrogation. It has a multiuser capability of up to 16 users in an array. Accuracy is about 1 metres. As with all microwave systems, they are limited to line of sight and atpospheric conditions.

• Trisponder

Trisponder is similar to Microfix, but offers both microwave anf UHF capability. The microwave version offers a single beacon interrogation providing range and bearing. For line of sight microwave, the accuracy is 1 metre.

• Argo

Argo is an HF multiuser positioning system which provides cover with an array of fixed and mobile beacons. The array is controlled by a fixedmaster station which provides the synchronisation pulse. As expected with HF radio. The range varies between day and night, varying between 300 and 700 Kms. Accuracy is about 5 metres.

• Other Satellite Systems

In 1996 the Russian equivalent of GPS, GLONASS became available in the West. GLONAS also uses a pseudo range system, but is different in its use of frequency (FDMA as opposed to CDMA) and it its choice of geodetic system PZ90 as opposed to WGS84). Translation between the systems is computationally possible and combined receivers are available.

Real Time Kinematic GPS (RTK GPS) is a differential system which uses the pseudo range and carrier phases to improved accuracy to within 5cms. The system is processor intensive, but with improvements in computer speed, acceptable update delays are becoming possible.

• The GLONASS system

GLONASS (the Global Navigation Sateilite System) is the Russian counterpart to the American GPS, being similar in design and operation. The system was initiated with the first sateilite launches in 1982, and by 1996, 24 operationaal satellites were in orbit. However, this number has not been maintained and the number available has, at times, been unadequate for good positioning.

The principles and practice of position determination with GLONASS are identical to that of GPS, using pseudo-range mesaurement from time and ephemeris data transmitted from the satellites.

The higher orbital inclination of GLONASS satellites (65˚), compared to the GPS constellation (55°), results in better satellit e availability in higher latitudes. The limited sateilite availability precludes the use of GLONASS as a continuous position reference for DP. A number of combined GPS/GLONASS receivers are available. These have the effect of increasing the number of usable satellites within view of the observer.

• DARPS (Differential and Relative Positioning)

In its most basic form only 2 GPS receivers are needed, usually between something like a FPSO , and a shuttle tanker. The FPSO broadcasts its GPS fix over UHF to the shuttle tanker, which then calculates a range and bearing to the FPSO. As the 2 are relatively close

together if there are any errors they should be common and the relative range and bearing should be accurate.

• Dual Frequency Systems

If both L1 and the L2 frequencies are used it is possible to calculate actual ray bending. At the Reference station both signals are received, by measuring the difference in rate of bending of the two signals the actual bending at the reference station can be calculated. This can then be applied to the SV signals to remove the effect, and then the fix is calculated and residual errors calculated, and transmitted.

At the user end both frequencies are received, the signal bending at the user is calculated and its effect removed. The corrections from the reference station are applied to give a fix that has no ionospheric-tropospheric-scintillation errors.

Figure 52. Single and dual frequency with scintillation

• Carrier Phase Difference System

40KM range needs a local reference station which the user sets up, gives position, Heading, VRU, and height of tide information.

• Error Segmentation

The errors are divided in 2 part those at the satellite and those at the user. The user errors are calculated using Dual frequency technology, the satellite errors (clock and orbit) are measured at land based reference stations and transmitted to the user. The satellite errors are valid worldwide. The local errors are calculated so with this combination of correction, the user GPS signal can be corrected for anywhere in the world.

• Galileo

A European stand alone GPS system called Galileo has just been given the go ahead, with multiple frequencies, and better atomic clocks, it is expected to exceed GPS standards. This is expected to be in operation between 2008-2010.

• GPS - INS Combination

A combination of ships inertial navigation system (SINS) and GPS, should the GPS dropout the SINS component can be used to provide a reference signal, that will hopefully not degrade before either GPS is regained or the operation is safely terminated.

4.23. The principle of Inertial Navigation, the methods of using INS to