Calibration of geodetic (dual frequency) GPS receivers Implications for TAI and for the IGS

1. Calibration of geodetic (dual frequency) GPS receiversImplications for TAI and for the IGS G. Petit

2. Rationale for TAI/IGS links • TAI is based on clocks in about 65 time laboratories worldwide, to be compared. • GPS is the most widely used time transfer technique • About half the time labs have dual frequency geodetic receivers (which keep synch at power on) • About half of these participate to the IGS network (see Table below, expanded from K. Senior)

3. Receiver calibration • Receiver calibration provides accurate time transfer. Two types of calibration procedures are used: absolute and differential. • Absolute calibration: Laboratory measurement of delays incurred by simulated signals. • Complex and not widely used • History: NRL (White et al.2001, Petit et al. 2001; Plumb et al.2005) • Some new developments, facilitated by wider availability of signal simulators • CNES (Cibiel et al. 2008) • DLR (Grunert et al. 2008) • Antenna calibration still most complex • Must be done (and re-done) for each equipment • Differential calibration: One reference equipment travels to be compared to all equipments to be calibrated.

4. Uncertainty of a differential calibration • Most significant part for a geodetic receiver: Measurement of the phase relation of the input frequency vs. the Lab reference (1PPS-in) • Statistical uncertainty of differential measurements well below 0.1 ns for a few hours averaging time at each frequency: no problem. • Cable measurements also a few 0.1 ns contribution. • Global uncertainty expected of order 2 ns at P1/P2 • Should be reflected in the observed (long-term) repeatability of calibration results

5. Geodetic calibrations results: repeatability (1) • Regular comparisons of Z12-T and Javad receiver kept at the BIPM: 10 measurements in 2001-2002, each lasting several days (each measurement corresponds to a complete re-installation of one system). • DP1 dispersion of results: 2.7 ns p/p (1.0 ns RMS) • DP2 dispersion of results: 3.1 ns p/p (1.2 ns RMS) • D(P1-P2) dispersion of results: 1.4 ns p/p (0.5 ns RMS)

6. Geodetic calibrations results: repeatability (2) • Regular comparisons of two Z12-T kept at the BIPM: 11 measurements over 2004-2008, each lasting several days (each measurement corresponds to a complete re-installation of one system). • DP1 dispersion of results: 3.3 ns p/p (1.0 ns RMS) • DP2 dispersion of results: 3.5 ns p/p (0.9 ns RMS) • D(P1-P2) dispersion of results: 2.1 ns p/p (0.7 ns RMS)

7. Geodetic calibrations results: repeatability (3) • A few years of history of repeated calibrations with Z12-T traveling receiver (a total of ~ 50 calibration measurements) • Several receivers have been measured > 2 times over the years: 1-2 ns consistency is possible, but there are few very conclusive results, mostly because the operational set-up changes with time. Some examples for two receivers participating to the IGS are: • PTBB (Z12-T at PTB): 4 calibrations over 2002-2008 show RMS of 0.5 ns for D(P1) or D(P2) and 0.4 ns for D(P1-P2) . • OPMT (Z12-T at OP): 6 calibrations over 2001-2008 show RMS of about 1.5 ns for D(P1) or D(P2) (with different operational set-ups) but 0.3 ns for D(P1-P2) .

8. Calibration repeatability: conclusions • Calibration of geodetic systems providing P3 measurements has been operational for some years. • Differential P1 and P2 delays can change by up to several ns; a stability at 1 ns RMS is observed only in the best and most controlled cases. • Differential (P1-P2) delays are generally stable at the 0.5 ns RMS level, except when the set-up changes. • (P1-P2) delay is equivalent to the DCB value solved for by the IGS: [P1-P2](i) = Cte + 1.55*DCB(i) for any receiver i. • Therefore receiver DCB values could be expected to be constant at the level of 0.2 ns RMS for a given receiver, even on the long term.

9. DCB stability • DCB should have long-term stability of typically 0.2 ns RMS, presumably short-term stability should be below this level. • While this is verified for many receivers, this is not always the case (TWTF below). • Long-term behavior of IGS DCB probably dominated by the influence of the “DCB reference” (Sum of the satellite DCBs to be zero). • DCBs from a set of calibrated receivers could be used to constrain IGS solutions.

10. Other satellite systems • GLONASS data have been available for years, but were never operationally used for time transfer, mostly because of satellite and receiver delays depend on the transmitted frequency within each band. • GLONASS common-view time transfer may provide results equivalent to GPS common-view, when the two GLONASS receivers have a similar frequency response, because common-view cancels SV biases. • However, common-view has limitations and is not much used any more (for GPS), thanks to the IGS SV ephemeris and clocks products. All-in-view or PPP are now used for GPS. • Similar IGS products for GLONASS SV clocks are not (yet) available because of the bias problem, so that All-in-view and PPP cannot be used for GLONASS. • When such problems are solved, the next problem will be to ensure the consistency of receiver calibration between the different systems.

11. Conclusions • Time transfer accuracy is sensitive to the absolute value of receiver biases. Therefore there is an on-going effort to calibrate these biases and to estimate their long-term stability. This is labor intensive and can cover only part of the needs. • IGS products may not be sensitive to absolute values of delays, but are sensitive to relative value, e.g. inter-frequency biases, or variation with time. IGS results are nearly continuous and “automated”. • IGS results can help to monitor the stability of receiver delays (Senior et al. 2004). • Conversely, receiver calibration results could provide a reference for the IGS solved-for receiver and satellite biases.

12. Issues and possible recommendations for IGS • Differential code biases are typically treated as solved-for parameters with one equation to remove rank deficiency • To ensure solutions that make most physical sense (and may be avoid leaks of some signal into the solved for parameters) • Physical constraints should be introduced (may be they are already??) in the adjustment, e.g. based on the DCBs expected stability • Reference for DCBs should also be based on physical considerations e.g. could be from the measured values for an ensemble of receivers • IGS should (continue to) monitor the stability of receiver delays • This approach demands more work => Check if it is worth the effort? • To be discussed in the Bias and Calibration WG (S. Schaer)