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