Transmission of RTK Corrections and Measurements using Optimal Coding

The provision of data services by wireless telecommunications providers has spurred interest in using these data links to transmit differential GPS corrections. Technologies such as General Packet Radio Service (GPRS) are able to satisfy the bandwidth requirements for distributing corrections in a network RTK system, however many mobile service providers charge a per-byte cost for the quantity of data transmitted. This contributes significantly to the total cost of operation for a GPRS based RTK system. Previous research by the author has shown that a significant reduction in data volume requirements can be achieved through differential entropy coding of CA code differential GPS corrections. The technique has been expanded to include carrier phase and P code pseudorange information. The paper presents the design and implementation of a data compression scheme based on differential entropy coding. The bandwidth requirement of this data format is compared with that of several other popular RTK data protocols. A study of the information content of dual frequency RTK corrections was undertaken in order to estimate the maximum reduction in data size that may be achieved through the use of optimal codes. Several polynomial prediction strategies are compared. Results show that the volume of transmitted data for carrier phase and pseudorange measurements can be reduced by as much as 90 percent when compared to RTCM 2 messages, without any reduction in precision or decimation of the sample rate.