Research on the Incomplete Constellation GPS Positioning Algorithm by a Combined Altitude and Clock Bias Model

GPS positioning is normally required that four GPS satellites are in view at least. However, due to the frequent blockage of GPS signals in worse environmental condition, such as canyons, forests and high buildings and so on, it is difficult to receive the signals of four satellites, that is GPS incomplete constellation. Therefore, the operation of the GPS receiver may be interrupted. Aiming at the poor positioning feature under GPS incomplete constellation environment, the altitude estimating improved algorithm aiding the receiver to positioning is put forward. Moreover, an auxiliary method for GPS receiver positioning using clock bias forecast model is developed. The combined altitude and clock bias model algorithm was introduced into the pseudorange equations, which ensured the service of GPS receiver could be continuous even in incomplete constellation. Even the GPS receiver can only receive the signals of two satellites. The validation and analysis based on the real GPS data show that this proposed method increases the availability of GPS positioning.