Real-Time Dual-Frequency (L1/L5) GPS/WAAS Software Receiver

This paper demonstrates a real-time software receiver supporting GPS/WAAS dual-frequency (L1/L5) processing. The software receiver is implemented on a widely-available recent generation multi-core processor and is capable of post-processing datasets from collection hardware. Key contributions of this software receiver are that 1) it is perhaps the first receiver capable positioning solely on L5 ranging despite there being only one operational GPS L5 signal and 2) it estimates ionosphere delay using L1-L5 measurements The architecture of such a software receiver needs to be carefully developed so that it can process signals from each individual frequency. Multitasking and synchronization mechanisms were developed to support the tracking of multiple channels in real time. To achieve real-time capability, parallel operations are necessary to reduce computation complexity. Bit-wise operations are exploited and implemented by Single Instruction Multiple Data (SIMD) instructions in the software correlator. An assistance mechanism between L1 and L5 is built for shortening the acquisition time and increasing the sensitivity of tracking L5 data-free channel. Currently, positioning using GPS/WAAS L5 signal needs assistance from L1 signal because the ephemeris on GPS L5 is currently not provided. However, using the L1 GPS ephemeris from L1 should consider the time offset or interfrequency bias (IFB) between L1 and L5 which results from the difference of antenna phase center and other hardware on the satellite. Besides, a time offset exists between WAAS and GPS time. These time offsets are solved by adding another unknown into position solution. The solutions of the time offset are used to correct the pseudorange of L5 and then perform positioning by three methods 1) L1 only 2) L5 only 3) iono-free combination. The results of positioning by the dual-frequency software receiver are illustrated and the positioning accuracy is analyzed in this paper.