As a worldwide DGPS service provider, Fugro's Marine Survey Divisions continually strive for ways to meet customer expectations for higher performance. Current marine DGPS systems provide meter and sub-meter levels of position accuracy and repeatability. This paper describes a new, Fugro developed, Starfix High Performance system that provides decimeter level accuracy and it's application in the offshore oil exploration and development market.
In the offshore environment, the approach to improving accuracy must be different to the approach on land. One cannot always add local reference stations or establish an RTK network. On the other hand, the open seas offer a near ideal environment for GPS signal tracking. Signal blockage due to tree foliage or buildings is non-existent. The antenna generally has a clear line-of-sight to all satellites above the horizon and GPS receivers on board a ship experience few cycle-slips. Using the existing Starfix DGPS satellite data networks and it's multiple reference stations, the high precision of dual frequency carrier phase measurements and the ideal offshore environment, has allowed Fugro to provide an advanced system with demonstrated accuracy that is better than 10 centimeters horizontally and 20 centimeters vertically.
The applications of the high accuracy, Starfix High Performance system to the marine market are significant and important both in cost savings and improved results. Applications include, reduced noise for DP vessel positioning, improved navigation for the high resolution and deep seismic markets, improved position data for gravity surveys and real-time tide correction to list just a few.
This paper presents technologies for high accuracy positioning offshore. An innovative solution developed by Fugro is discussed, test results are presented and current applications are described. The emphasis is on the accuracy, the integrity, and applications of this system.
In recent years various applications have emerged in offshore and land environments requiring decimeter accuracy real-time positioning. This level of accuracy has either been impossible or prohibitively expensive to achieve over wide area with the existing real-time GPS positioning techniques.
Differential GPS is an established technique that provides meter level positioning accuracy in real-time. Current differential GPS systems typically use single frequency L1 code pseudoranges differenced between the rover and reference station. The accuracy of single baseline DGPS decreases as the distance of reference stations increases because of the special decorrelation of the tropospheric, ionospheric and satellite orbit errors. Various techniques have been developed to improve the accuracy of single baseline DGPS. The use of multiple reference stations in a network configuration has been shown to improve the accuracy of DGPS over the single baseline approach [Lapucha, Huff, 1993]. Nevertheless, the fundamental limitations of GPS, which include pseudorange measurement noise and signal multipath limit the positioning accuracy of DGPS to the submeter level.
Real-Time Kinematic (RTK) is another relative positioning technique that can potentially provide centimeter level positioning accuracy. This level of accuracy is possible to achieve only after resolving integer carrier phase ambiguities.