In the last decade the launch of several satellites able to probe the atmosphere with the GPS RO technique like GPS/MET, SAC-C, CHAMP, GRACE, Formosat-3/COSMIC, TerraSAR-X, MetOp-A etc., have made available a huge amount of data. These data currently are used to reconstruct the mapping function (hereafter MF). The MF is helpful to assess in a proper way the atmosphere delay, which disturbs the estimated coordinates of ground GPS station. The tropospheric delay is given by the following relationship: ZTD ■ m(E) where ZTD stands for the zenith total delay, which is estimated together with the coordinates. The MF has the form
Fig. 5 The solid line represents the MF derived from GPS RO data while the dotted gray line is the Niell MF. The plot is achieved for a site altitude of 300 m
Site Altitude: 300 m.
_ MF-Dry Niell
Elevation (minimum: 3°)
E stands for the elevation angle of the GPS satellite from the horizon. The coefficients a, b, c are indeed functions of latitude and height of the site of which we want to compute the coordinates and the day of the year. Niell (1996) used the same MF estimating the coefficients by using atmospheric profiles retrieved with balloons. We are re-computing these coefficients using the profiles coming from the GPS RO space missions. In Fig. 5 the MF derived from GPS RO data is compared with Niell one.
The next step will be the validation of our MF. For this purpose we will implement it in BERNESE software to estimate the coordinates of the nodes of a GPS ground network. Then the results will be validated against the same solutions achieved applying different MFs (namely Niell). The job is currently performed by the INNOVA company, which is part of the ROSA project.
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