The analysis strategies developed for this study provide frame results of com- parable quality with reference to the products delivered by the official ACs of the dedicated IAG Services. IGS and ILRS solutions computed in the framework of the reprocessing campaign for the realization of the ITRF2014 have been used as refer- ence term for the validation.
Concerning the analysis of GNSS observations, the inclusion of a high per- centage of GLONASS data in a rather sparse network still limits the accuracy of the computed solution. In this study, it has been necessary to implement an or- bit parametrization relying on three-day arcs in order to obtain station coordinate estimates comparing to IGS official products within a few mm in the horizontal com- ponents and 4 − 5 mm in the vertical. Improvements on shorter time-spans might result, instead, from the exploitation of the newECOModel [Arnold et al., 2015] or of a box-wing approach in the description of the forces acting on the spacecrafts.
A remarkable agreement is found between the results presented in this study and the repro2 CODE’s solution (which also encompasses GLONASS data and was obtained with the Bernese Software). Average translation and scale offsets com- puted between these two solutions and ITRF2008 compare at the mm level, with the only exception of Tz. The discrepancy observed for this component can still be considered reasonable, given the noise level of the time series and marked spurious
oscillations. The behavior shown by Tz is likely to reflect just modeling deficiencies and observation errors rather than actual geophysical signals. Besides being the- oretically explained by collinearity issues [Rebischung, 2014], such insensitivity is also reflected in the direct comparison with the equivalent time series derived from LAGEOS observations, see Fig. 2.20. The other components, on the other hand, are characterized by a seasonal signal in phase with the one sensed by SLR and even the amplitude is nicely reproduced in case of Tx. In the interpretation of the results, however, it should always be considered that the network shift approach allows studying the non linear part of the geocenter motion only up to the possible time variations of the CF-CN vector. Spurious signals depending on the selected obser- vation geometry, network effect, are likely to be aliased in the estimated parameters.
155
5
15
Tx [mm]
155
5
15
Ty [mm]
2011
2012
2013
2014
2015
40
200
20
40
Tz [mm]
GNSS SLRFigure 2.20 – Comparison of translation offsets computed during the long term stacking of GNSS (blue) and SLRtoLAGEOS (red) solutions with MC imposed with reference to ITRF2008. To facilitate the comparison, both time series have been detrended and smoothed with a Vondrak filter with a 7 cpy cutoff frequency [Vondrak, 1969].
Concerning the scale factor, only a small average offset (∼ 0.2 ppb) is observed with reference to ITRF2008. The adoption of IGS official calibrations for satellite P COs conventionally force the GNSS-realized scale into the ITRF2008 one, but Rebischung et al. [2016] pointed out that modeling updates applied in the repro2 campaign might explain the observed variation.
The analysis of SLR observations to LAGEOS satellites has shown that, even in the most recent years, tracking performances are strongly station-dependent. Coor- dinates computed for core stations show a higher week-to-week consistency and a
better agreement with the ILRS official solution, as demonstrated by the reduction of the relevant WRMSs displayed in Fig. 2.14 and 2.15.
The long term frame computed from LAGEOS data collected during the period 2011 − 2014 shows non negligible translation offsets with reference to ITRF2008 in the y− and z−direction. If it should be considered that Tz is the noisiest and worst determined component, the discrepancy observed for Ty is consistent with the discontinuity affecting the relevant historical time series in 2010. An analogous jump, also characterizes the historical series of the scale factor. In this case, however, successive observations tend to restore pre-offset values and, according to the ILRS combined solution, data collected in the period 2011 − 2014 show the expected half a ppb offset with reference to ITRF08. Further investigations are, anyway, required to understand the nature of the detected discontinuities in order to reliably assess the realization of the frame defining quantities. As a side note, it can be pointed out that the scale parameter is the one associated with the largest variability in the estimates provided by the different ACs.
Finally, SLR observations to GNSS satellites have been taken into account. Also in this case, tracking performances are dramatically station-dependent and it has been shown that the four best performing stations acquire more than one half of the total volume of data. For poorly performing stations, it is even impossible to compute a fully parametrized solution over the three-day time span imposed by the selected orbit estimation process. A partial validation of the developed analysis strategy was then performed computing a trial solution encompassing just station positions and range biases. On average, the agreement with the SLR official solution is found at the level of a couple of cm.
Chapter 3
Results
GNSS and SLR solutions computed according to the analysis strategies detailed in Chapter 2 can be employed for the derivation of combined TRFs relying exclu- sively on the space ties on-board GNSS satellites. In this chapter, the results of both long-term and quasi-instantaneous computations are presented and discussed. Future perspectives are also investigated by means of simulations addressing the en- hancement of frame parameter precision resulting from technological improvements and/or strengthening of the ground network.
The following questions are confronted in this chapter:
1. Can the space tie on-board GNSS satellites provide a reliable and effective alternative to the use of terrestrial ties in the derivation of Terrestrial Reference Frames?
2. Can this link transfer the origin and scale information from one technique-specific frame to the other?
3. How does the frame realization based on the space tie approach com- pare with the traditional one?
4. What gain in the precision of frame parameters can be expected in consequence of technological improvements?
5. Should GNSS providers be encouraged to add laser retro-reflectors on all GNSS satellites ?