Tuesday, January 24, 2012

Other limitations of geodetic methods

Since the post “The problem in the campaign data for studying geodynamic processes”, I explained some limitations of geodetic methods for studying geodynamic processes, especially in the North American plate. Further to the listed problems in the sequential posts, there are still some more limitations in geodetic models and methods. Among them, sensitivity of GIA models to the parameters, possible limitations in the velocity solutions, and difficulties to apply new improvements have higher importance and are explained here.

The location of the hinge line as well as the amplitudes and spatial gradient of the predicted vertical rates are sensitive to the GIA model parameters. Different viscosity structures for the same ice model show substantial differences in the predicted uplift rate (e.g., Peltier 1998) and different de-glaciation models for the same viscosity structure also show differences (e.g., Peltier 2002).

The choice of the Earth tide model may influence derived station velocities as shown by Watson et al. (2006). Further, a specific study on aliased tidal signatures in GPS time series are presented in Penna and Stewart (2003), and the propagation of unmodelled systematic errors into coordinate time series is investigated in Stewart et al. (2005).

New advances have been made in the recent years to improve precision of the results. For example, new tropospheric mapping functions (e.g., Tesmer et al. 2007), or absolute calibration values for GPS satellites and receivers antenna phase centre variations (PCVs) are now available. Some of the possible improvements may be, however, difficult to implement in practice. As an example, including corrections for atmospheric loading at the observation level will improve the analysis (Tregoning and van Dam, 2005), but it is hard to apply, or as another example, the PCV determined in absolute antenna calibration may be valid for an isolated antenna, but these properties may change due to electro-magnetic coupling and scattering effects when the antenna is attached to its foundation (Granström, 2006). Consequently, special attention should be directed to the mix of perturbations originating in the succession of GPS satellite block types, antenna PCV models (relative or absolute), limitations in tropospheric mapping functions, and sites at high latitudes where satellites is no more visible up to zenith elevation. This complicated budget of systematic errors with its non-stationary character is likely to result in systematic changes of estimated vertical position over time.

Granström, C. (2006). Site-dependent effects in high-accuracy applications of GNSS (Technical report No. 13L). Sweden.

Peltier, W. R. (1998). Postglacial variations in the level of the sea: Implications for climate dynamics and solid-Earth geophysics. Reviews of Geophysics, 36(4), 603. American Geophysical Union. doi:10.1029/98RG02638.

Peltier, W. R. (2002). Global glacial isostatic adjustment: palaeogeodetic and space-geodetic tests of the ICE-4G (VM2) model. Journal of Quaternary Science, 17(5-6), 491-510. Wiley Online Library. doi:10.1002/jqs.713.

Penna, N. T., and Stewart, M. P. (2003). Aliased tidal signatures in continuous GPS height time series. Geophysical Research Letters, 30(23), 2184. American Geophysical Union. doi:10.1029/2003GL018828.

Stewart, M. P., Penna, N. T., and Lichti, D. D. (2005). Investigating the propagation mechanism of unmodelled systematic errors on coordinate time series estimated using least squares. Journal of Geodesy, 79(8), 479-489. Springer. doi:10.1007/s00190-005-0478-6.

Tesmer, V., Boehm, J., Heinkelmann, R., and Schuh, H. (2007). Effect of different tropospheric mapping functions on the TRF, CRF and position time-series estimated from VLBI. Journal of Geodesy, 81(6-8), 409-421. Springer. doi:10.1007/s00190-006-0126-9.

Tregoning, P., and van Dam, T. (2005). Effects of atmospheric pressure loading and seven-parameter transformations on estimates of geocenter motion and station heights from space geodetic observations. Journal of Geophysical Research, 110(B3), B03408. doi:10.1029/2004JB003334.

Watson, C., Tregoning, P., and Coleman, R. (2006). Impact of solid Earth tide models on GPS coordinate and tropospheric time series. Geophysical Research Letters, 33(8), L08306. doi:10.1029/2005GL025538.

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