28.2 DGPS

Differential GPS uses a local (within 500km) fixed GPS receiver as a reference station. The fixed station observes its position over a much longer period and knows its own coordinates very well. When a new signal

is received, it observes the position and calculates a local correction which is then transmitted to all other DGPS receivers in the vicinity. In this way, our dynamic unit can observe its position as above but then listen

for a correction signal from the fixed station. Once the latitude and longitude are corrected, the position is usually good to about 0.1m. This is usually adequate accuracy for oilfield positioning applications. If greater

accuracy is required, a more expensive system known as Real Time Kinematic (RTK) can be used which also makes use of the observed frequencies of the incoming signals whish are affected by Doppler shift

based on the relative velocity of the satellite orbit to the receiver. This is used in the mix to improve the observation accuracy and often results in position uncertainties of the order of 0.01m.