Bulletin G6odbsique (1993) 67:185-192 Bulletin G6od6sique © Springer-Verlag1993 A comparison of P code and high performance C/A code GPS receivers for on the fly ambigmty resolution G. Lachapelle, M.E. Cannon, and G. Lu Department of Geomatics Engineering, The University of Calgary, 2500 University Ds. NW Calgary Alta T2N 1N4, Canada Received January 15, 1991; Accepted April 14, 1993 ABSTRACT. Carrier phase ambiguity resolution on the fly is investigated using two receiver technologies, namely dual-frequency P code and high performance, single frequency, C/A code receivers. Both receiver types were used simultaneously in a series of land kinematic trials. A least-squares search technique is used to find the correct double difference carrier phase ambiguities. Both C/A and single frequency P code technologies are found to be equivalent and capable of resolving the integer ambiguities on the fly using some 30 to 200 seconds of data under benign multipath conditions. Successful ambiguity resolution on the fly results in cm-level accuracy kinematic positioning. The ambiguity resolution time required and success rate are however found to be strongly dependent on the level of carrier phase multipath and, as a consequence, on the error variance assigned to the carrier phase measurements. The use of widelaning with the dual frequency P code results in ambiguity resolution in seconds. The performance of widelaning is also superior in a comparatively high carrier phase multipath environment. INTRODUCTION Carrier phase ambiguity resolution on the fly, i.e., without static initialization, is the solution to sub-decimetre kinematic positioning without the operational constraint of static initialization as used in semi-kinematic or stop and go positioning (e.g., Cannon 1990). Various methods are available to resolve the ambiguities, e.g., the ambiguity function method (Mader 1990), the least-squares search approach (Hatch 1991, Remondi 1991), and the Fast Ambiguity Resolution Approach (Frei & Beutler 1990). While some of these methods are mathematically equivalent, e.g., the ambiguity function method and the least-squares search approach (Lachapelle et al 1991, 1992), different implementation schemes may result in specific advantages and/or disadvantages (e.g., Erickson 1992). For instance, the sequential square root information filtering method proposed by Landau and Euler (1992) results in substantially lower computational requirements. The level of success of any technique is a function of several parameters, namely the type and quality of observables used, the multipath environment, the distance between the monitor and mobile receivers, the number of satellites available and their geometry. A larger number of satellites and a shorter distance between file monitor and mobile will result in relatively faster ambiguity resolution due to the higher measurement redundancy and the lower differential atmospheric and orbital errors. The objective of this paper is to compare the performance of two receiver technologies for ambiguity resolution on the fly for land kinematic applications. These two technologies consist of high performance single frequency C/A code and dual frequency P code receiver technologies. High performance C/A code receivers deliver C/A code measurements with a level of accuracy similar to that of P code L1 measurements. EQUIPMENT SELECTION The high performance C/A code receiver technology selected for the test is the GPSCard TM, designed and built by NovAtel Communications Ltd. The P code receiver selected is the 36-channel Ashtech P-XII receiver. The GPSCard TM is a high performance 10-channel C/A code unit which has two unique characteristics, namely a 10-cm code noise and a narrow correlator spacing option to reduce code multipath interference (Fenton et al 1991, Erickson et al 1991, Cannon & Lachapelle 1992a, Van Dierendonck et al 1992). Earlier ambiguity resolution on the fly tests with the GPSCard TM corffirmed its capability (Cannon et al 1992). The P-XII is a 36 channel unit which measures carrier phase and C/A, P(L1), and P(L2) code simultaneously. The C/A code noise of the P-XII is 100 cm while the corresponding P code noise is 10 cm. The phase noise is stated as 0.2 nun for the P-XII and 0.4 mm (diff. channel) for the GPSCard TM.