No abstract

No abstract

No abstract

A national-scale crustal velocity model has been developed for Canada as part of the current realisation of NAD83(CSRS), delivered as a set of 3 national grids, for each of the North, East and Up (N, E and U) components. It is used to propagate coordinates to different reference epochs, and to support scientific studies such as natural hazards, climate change, and groundwater change. The previous velocity model was based on continuous and campaign GPS data between 1994 and 2011.3. The new model includes new stations in key areas, six more years of data (to the end of 2017), and newly reprocessed historical data using the latest software and GPS products. We include data from continuous GPS sites in Canada, the northern portions of the US, all of Greenland, and a set of globally distributed sites used to define the reference frame; and from repeated high accuracy campaign surveys in Canada. A new type of model is introduced for the vertical grid. It incorporates GPS observations with the crustal uplift predictions of Glacial Isostatic Adjustment (GIA) and elastic rebound models, which are especially important in areas with sparse coverage. Gridded uncertainty estimates are provided for each component of NAD83v70VG.

A working group of the International GPS Service (IGS) was created to look after Reference Frame (RF) issues and contribute to the densification and improvement of the International Terrestrial Reference Frame (ITRF). One important objective of the Reference Frame Working Group is to generate consistent IGS station coordinates and velocities, Earth Rotation Parameters (ERP) and geocenter estimates along with the appropriate covariance information. These parameters have a direct impact on other IGS products such as the estimation of GPS satellite ephemerides, as well as satellite and station clocks. The information required is available weekly from the Analysis Centers (AC) (cod, emr, esa, gfz, jpl, ngs, sio) and from the Global Network Associate Analysis Centers (GNAAC) (JPL, mit, ncl) using a "Software Independent Exchange Format" (SINEX). The AC are also contributing daily ERPs as part of their weekly submission. The procedure in place simultaneously combines the weekly station coordinates, geocenter and daily ERP estimates. A cumulative solution containing station coordinates and velocity is also updated with each weekly combination. This provides a convenient way to closely monitor the quality of the estimated station coordinates and to have an up to date cumulative solution available at all times. To provide some necessary redundancy, the weekly station coordinates solution is compared against the GNAAC solutions. Each of the 3 GNAAC uses its own software, allowing independent verification of the combination process. The RMS of the coordinate differences in the north, east and up components between the AC/GNAAC and the ITRF97 Reference Frame Stations are 4-10 mm, 5-20 mm and 6-25 mm. The station velocities within continental plates are compared to the NNR-NUVEL1A plate motion model (DeMets et al., 1994). The north, east and up velocity RMS are 2 mm/y, 3 mm/y and 8 mm/y. Note that NNR-NUVEL1A assumes a zero vertical velocity. Reference Frame StationsA set of so called RF stations is used to align the weekly and cumulative solutions. For the IGS realization of ITRF97, the RF station set is composed of 51 carefully selected stations. Criterion such as monumentation, data quality, data latency, collocation with other techniques, accurate velocity estimate, geographical distribution etc. were used for the station selection (Kouba et al., 1998). Since GPS week 1021 (August 1, 1999), the weekly and cumulative solutions are aligned to ITRF97 (Boucher et al., 1999, http://lareg.ensg. ign.fr/ITRF). The IGS (http://igscb.jpl.nasa. gov) realization of ITRF96 used 47 stations; most are in the current realization. The earlier realizations used only 13 stations; most of those are still being used in the current realization. The limited number of stations in the early realizations was occasionally causing instabilities, especially when several stations at crucial locations were missing. The larger number of stations in the more recent realizations greatly reduces those potential instabilities. The weekly and the cumulativ...