Analysis of the GNSS Code-Carrier Hardware Delay Difference in RTK Process: Effect, Measurement and Calibration

In satellite navigation receivers, the hardware delays of code and carrier are different because of their different tracking processes, which is called as the code-carrier hardware delay difference. This difference depends on the condition of receivers and varies from one to another. Therefore, in the carrier phase RTK (real-time kinematic) position process, the bias of code-carrier hardware delay difference between the base receiver and the moving receiver will cause the baseline resolution error. In this article, we analyze the code-carrier hardware delay difference, and the theoretical derivation shows that the baseline resolution error increases with the bias as well as the signal Doppler frequency. For a static RTK user on the ground, a microsecond level bias leads to a baseline resolution error in millimeter level, compared to a centimeter level error caused by the LEO satellite whose speed can reach several kilometers per second. In that case, the bias needs to be calibrated for those high precision RTK applications. However, there are few researches to calibrate the receiver carrier hardware delay because the carrier phase measurement integral ambiguity is difficult to be measured. In this article, we propose a method to measure and calibrate the bias of code-carrier hardware delay difference between different receivers. Real data test results show that the proposed method can effectively eliminate the RTK baseline resolution error caused by the bias of code-carrier hardware delay differences between the base receiver and the moving receiver.