# Presentation: 265853

## 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
- 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).
* *P1 dispersion of results: 2.7 ns p/p  (1.0 ns RMS)
* *P2 dispersion of results: 3.1 ns p/p  (1.2 ns RMS)
* *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).
* *P1 dispersion of results: 3.3 ns p/p  (1.0 ns RMS)
* *P2 dispersion of results: 3.5 ns p/p  (0.9 ns RMS)
* *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 P1) or P2) and 0.4 ns for P1-P2) .
- OPMT (Z12-T at OP): 6 calibrations over 2001-2008 show RMS of about 1.5 ns for P1) or P2) (with different operational set-ups) but 0.3 ns for 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)