Troposphere induced GPS navigation error, its effect on GPS-INS integrated system performance and mitigation strategies

Abstract

Conventional Global Positioning System (GPS) receivers track GPS satellites above 5 degree horizon to avoid multipath effects. In order to improve vertical dilution of precision (VDOP), it is an attractive prospect to utilize GPS satellites below the horizon in addition to those above, for navigation computations in GPS receivers used by vehicles operating at high altitude trajectories. This paper, however, exposes a possible risk of navigation output degradation in adopting this scheme due to unintentional occultation of GPS signals from satellites below the earth tangent horizon. The error on GPS range and range rate measurements induced by troposphere are simulated using a model based approach. Results indicate position errors up to 150 m and velocity errors up to 14 ms^-1 over a period of 15 s under practical conditions. Such slowly varying errors being uncharacteristic of GPS receivers are not rejected by the usual protection features in GPS-Inertial Navigation System (INS) integration schemes. Thereby, the integrated system performance is degraded. In the uncoupled GPS-INS scheme studied, GPS error of the above order resulted in integrated system output error of 100 m in position and 5 ms^-1 in velocity, respectively. An altitude based elevation cut off scheme that can be incorporated in the GPS receiver for retaining the advantages of using negative elevation satellites, while avoiding occulted measurements, is presented in this paper. A method for strengthening integrated scheme protection, using innovation residuals, for rejecting such slow GPS errors is also presented.