Estimation of P2-C2 Bias by Means of Precise Point Positioning

Rodrigo F. Leandro, Richard B. Langley, and Marcelo C. Santos

Abstract:	The United States has started a modernization program to provide better service to Global Positioning System (GPS) users, with launches of modernized GPS satellites. The sub-group of these new satellites is called Block IIRM, where R" stands for replenishment and "M" for modernized. In this modernization process, GPS has gained a new open civil code (called L2C), centered at the L2 frequency. The first modernized satellite, for which PRN17 was assigned, was launched on 25 September 2005 and the new L2C signal from this satellite has been fully available from 15 December 2005. Even before PRN17 was placed in orbit, the L2C signal became an issue of worldwide interest to the GPS research communities. Currently L2C is being transmitted by three satellites: PRN17, PRN31 and PRN12. Enhanced receivers capable of tracking the modernized GPS signal have been developed and provided by a number of manufacturers. The IGS (International GNSS Service) has organized a network of L2C signal tracking stations which have been established in different places around the world. The role of projects involving the new signal is to analyze its quality, as well as the impact of its use for positioning and navigation. One of the issues to be considered when dealing with the new signal is the impact of mixing L2C-capable and legacy receivers within a network, or processing data from an L2C-capable receiver with satellite clock values generated using a legacy receiver network. Because hardware delays of receivers and satellites for L2C measurements (called C2 in the RINEX 2.11 standard) might not be necessarily the same of those for P2 (pseudoranges based on semi-codeless L2 P(Y)-code tracking), a bias between P2 and C2 code measurements must be considered when mixing observations from different receiver types. This bias will be called here the P2-C2 bias, using the same standard nomenclature used for P1-C1 biases. Code biases are present in the receiver and the satellite hardware, but in a positioning scenario, receiver code biases are usually absorbed by the receiver clock parameter and do not need to be separately accounted for. In a scenario of network clock (receivers and satellites) estimation using mixed receiver types (e.g., legacy and L2C-capable), both receiver and satellite biases have to be considered as parameters in the observational model. The main goal of this work is to determine and analyze values for the IIR-M satellite P2-C2 biases. Knowing these values allows us to begin using L2C as an observable for positioning, applying satellite clock values computed using P2 as the observable on L2, as in the case of IGS clock products. The dataset used is the data observed by the IGS L2C Test Network, which consisted of 12 receivers at the time the data was collected for this research. Observation functional and stochastic models have been realized in a precise point positioning package developed at the University of New Brunswick. Inside this package, called GAPS (GPS Analysis and Positioning Software), tools for data analysis were implemented, allowing, among other things, the estimation of code biases. The approach used treats the observations in the same way as a user would do for positioning, thus the impact of satellite biases is the same as for point positioning using IGS products. In this work our approach is validated by comparing our satellite P1-C1 bias estimates with values determined by IGS analysis centers. This validation is required because up until now, no P2-C2 bias estimates have been published with which we could make comparisons. The two scientific contributions of our work are: (1) an approach for PPP-based bias determination, and (2) the first (to our knowledge) publicly available determination of differential P2-C2 bias, which can be used in the future as a reference for further investigations related to P2-C2 satellite biases. Our results show that the differential P2-C2 satellite biases for the three modernized satellites currently in orbit are very similar, with the value likely between plus or minus 0 and 20 cm."
Published in:	Proceedings of the 63rd Annual Meeting of The Institute of Navigation (2007) April 23 - 25, 2007 Royal Sonesta Hotel Cambridge, MA
Pages:	225 - 231
Cite this article:	Leandro, Rodrigo F., Langley, Richard B., Santos, Marcelo C., "Estimation of P2-C2 Bias by Means of Precise Point Positioning," Proceedings of the 63rd Annual Meeting of The Institute of Navigation (2007), Cambridge, MA, April 2007, pp. 225-231.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In