Reliable estimation of phase biases of GPS satellites with a local reference network

Precise Point Positioning with GPS and/or Galileo is becoming increasingly popular as it does not need any measurements from a reference station. However, the resolution of the integer ambiguities of the periodic carrier phases requires precise and accurate estimates of satellite phase biases. This paper describes a new method for the estimation of the receiver and satellites phase biases on all frequencies. It uses a geometry-free approach with a Kalman filter and sequentially fixes the undifferenced integer ambiguities. Several steps are performed to improve the reliability of ambiguity fixing, e.g. the fixing decision over a time-window and the use of both statistical information from the Kalman filter and the actual deviation between the float and nearest integer numbers. The proposed method is applied to dual-frequency L1/L2 GPS measurements from 11 SAPOS stations in Bavaria to analyze the stability of the satellite phase biases. The long time span of 24 hours involves several rises and settings of satellites and, thus, requires parameter mappings and trackings within the estimation. The observed satellite biases vary by only 3 cm over 5 hours, and the receiver phase biases are even more stable.