The GNSS solution named ’NGL08’ was produced by the Nevada Geodetic Laboratory (NGL) using a Precise Point Positioning (PPP) data analysis strategy. The GIPSY/OASIS-II software (version 6.1.1) from the Jet Propulsion Laboratory (JPL) was employed to produce daily position time series for over 17,000 globally distributed stations using JPL satellite orbits and clock products expressed in the IGS08 reference frame (Blewitt et al. 2018). The data set covers the period January 1996 to May 2018.
This GNSS data analysis strategy complies with the standards (models, corrections,...) of the International GNSS Service (IGS) reanalysis (reprocessing) campaigns ; hence the Sixteenth session of the Group of Experts for the Global Sea Level Observing System (GLOSS), held in Busan (Republic of Korea), 11-13 April 2019, tasked the SONEL data assembly centre to consider including the NGL solution (position time series and velocities) for the subset of GNSS stations nearby tide gauges. Among the more than 17,000 stations processed by NGL, as many as 989 were indeed identified as co-located, or nearby, tide gauges.
The GNSS velocities, provided here for 818 stations with a minimum record length of 3 years and data gaps not exceeding 30%, were produced by NGL using the MIDAS estimator (Blewitt et al. 2016).
Vertical velocities table
The NGL08_Vertical-Velocities_Table provides the vertical GPS velocities and uncertainties from the 818 stations fulfilling the criteria of 3 years of minimum length and data gaps not exceeding 30%, estimated by the NGL group using MIDAS (Blewitt et al. 2016).
Daily time series
The NGL08.zip file contains individual station data files of daily position time series in ITRF2008 with respect to the position at the mid time series epoch. These positions are expressed in meters in the local frame (North, East, and Up). The reference position and the 3D velocity (obtained using MIDAS) in the local coordinate system (East, North, Up) are provided in the header of each file.
The NGL08_discontinuities_Table.txt file provides the position offsets that can affect the station position and velocity estimates, as indicated by the NGL group. Note that MIDAS should be robust regarding the velocity estimates in presence of offsets, but not the parametric methods (e.g. least squares adjustment).
More downloads and information can be found at the Nevada Geodetic Laboratory website.
|Estimated (robust) velocities:||818|
|Average of formal errors:||1.15 mm/yr|
|Median of formal errors:||0.92 mm/yr|
For the time series:
Blewitt G., W. C. Hammond, C. Kreemer (2018). Harnessing the GPS data explosion for interdisciplinary science. Eos, 99, doi:10.1029/2018EO104623.
For the velocities:
Blewitt G., C. Kreemer, W.C. Hammond, J. Gazeaux (2016). MIDAS robust trend estimator for accurate GPS station velocities without step detection. Journal of Geophysical Research, 121, 2054-2068, doi:10.1002/2015JB012552.