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This paper discusses the influence of travel directions on GPS dynamic accuracy for vehicle tracking in two sections. The first section investigates the influence of travel directions on GPS accuracy due to the GPS satellite sky distribution. GPS dilutions of positions (DOP) were calculated based on GPS satellites geometry at a variety of locations and different mask angle settings. Results show a significant difference between North DOP and East DOP in mid-latitude area. A clear trend of the 24 hour average ratio of the North DOP to the East DOP was found related to latitudes and mask angle settings. A cross-track dilution of position (XDOP) is defined as the GPS dilution of position perpendicular to the travel direction. The influence of the GPS satellite geometry on GPS accuracy was mapped into the vehicle platform frame to derive the XDOP, and accordingly to derive the influence of travel directions on the GPS dynamic accuracy. Results show that the XDOP increases as the Course over Ground (COG) changes from o 0 to o 90.

Considering that the regression line fitting through GPS data may be used as the referenced true path for calculating GPS errors, the second section reviewed methods for fitting linear models. The most commonly used approach of least square regression generates the line by minimizing the sum square of vertical offsets, instead of perpendicular offsets, which can result in a potential model fitting error. This fitting error depends on the travel directions and the tested GPS receiver's dynamic accuracy. Results show that the fitting error reaches its maximum when the tested vehicle is traveling in the N-S (or S-N) direction and decreases when the travel direction moves away from the N-S direction.