Correction Parameters for timing Group Delays

j a n u a r y / f e b r u a r y 2 0 1 1 www.insidegnss.com In satellite navigation, a GNSS receiver must account for several sources of error such as relativistic effects, atmospheric propagation delay, offset of satellite clocks from system time and satellite ephemeris. In order to accurately compute user position, velocity, and time (PVT), these errors need to be predicted/estimated precisely. The navigation signals transmitted on each carrier frequency are imperfectly synchronized due to different hardware paths corresponding to each signal. Each satellite’s navigation message contains parameters describing the timing bias. A user receiver uses these parameters to compute the clock correction for each observation. Dual-frequency receivers directly employ such corrections. However, before a single frequency receiver can use the computed offset, it must be adjusted to account for the differential group delay between the principal signal and the signal on the other frequency. This timing group delay, annotated as TGD, results from hardware differences in the onboard signal paths and will vary among satellites. The dual-frequency signal timing difference is used to infer the line-ofsight delay caused by the ionosphere, subject to the bias difference between the satellite transmissions at the two Indian regional navigation Satellite System