The Kongsberg Seatex DAPRS (Differential Absolute and Relative Positioning Sensors) for positioning of two vessels such as a shuttle tanker and a FPSO is based on differential GPS for model DARPS 100, dual frequency GPS for model DARPS 102 and differential GPS and GLONASS for model DARPS 200.
For more information on the Kongsberg Seatex DARPS follow the "Full Details" link to the Kongsberg web site