Precise positioning using gps for category-III aircraft operations using smoothed pseudorange measurements

Currently, standard single frequency Global Positioning System (GPS) receivers provide a positioning accuracy of approximately 4--20 m. This precision can be further enhanced with dual frequency receivers which are able to provide accuracy around 1--12 m. However, these errors are quite large when it comes to safety of life applications such as aircraft landings. Differential GPS (D-GPS) allows for precise positioning using information from reference stations on the ground. Carrier phase tracking is one such D-GPS approach which allows range determination with centimeter level accuracy. However, carrier phase measurements require estimation of unknown fixed integer ambiguities before the receiver can start determining its position. Using single difference smoothed pseudorange measurements the integer ambiguities can be estimated with reasonable accuracy. This methodology brings the position error down to centimeter level which can meet the Federal Aviation Authority (FAA) regulations for Category-III (CAT-III) precision approaches. This paper examines single differenced smoothed pseudorange measurements for integer ambiguity resolution for precise positioning using carrier phase tracking. Simulation of GPS receiver performance using this methodology has been carried out and demonstrates positioning with centimeter level accuracy for the approach phase of flight. Positioning errors are also compared for dual frequency and carrier phase tracking modes.