Analysis methodology and recent results of the IGS network combination

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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.