Absolute phase center corrections of satellite and receiver antennas

Abstract

Results of the estimation of azimuth-dependent phase center variations (PCVs) of GPS satellite antennas using global GPS data are presented. Significant variations of up to ±3-4 mm that are demonstrated show excellent repeatability over eight years. The application of the azimuthal PCVs besides the nadir-dependent ones will lead to a further reduction in systematic antenna effects. In addition, the paper focuses on the benefit of a possible transition from relative to absolute PCVs. Apart from systematic changes in the global station coordinates, one can expect the GPS results to be less dependent on the elevation cutoff angle. This, together with the significant reduction of tropospheric zenith delay biases between GPS and VLBI, stands for an important step toward more consistency between different space geodetic techniques.

Realizing a terrestrial reference frame using the Global Positioning System

“…These GPS realizations, however, are not fully independent: the models for the transmitter antennas are developed from ground tracking data, wherein the station locations are fixed to values from the prevailing ITRF [Schmid et al, 2005]. This implies that the GPS-derived terrestrial scale, in particular, is inherited from the ITRF and thus dependent on VLBI and SLR [e.g., Ray et al, 2004;Rülke et al, 2008;.…”

“…Accordingly, significant attention has been devoted to improved modeling of the GPS transmit antenna phase centers and variations. Some of these efforts have focused on ground calibration of the GPS satellite antenna panel shown in Figure 1 [Mader andCzopek, 2001, 2002;Wübbena et al, 2007], while others have used on-orbit data [e.g., Bar-Sever, 1998;Schmid and Rothacher, 2003;Schmid et al, 2005;Haines et al, 2005]. Regardless of the approach, the main focus has been on calibrating the antennas for the dual-frequency (ionosphere-free) combination of the GPS carrier phase (LC ≈ 2.54(L1)−1.54(L2)).…”

“…In order to overcome a near singularity in the solution, the scale of the ground network must also be fixed (e.g., to the value determined from SLR/VLBI as manifest in the current ITRF) [Springer, 2000]. If a sufficiently large number of globally dispersed stations are used, Schmid et al [2005] reason that local effects such as multipath and unmodeled troposphere variation should have only minor influence on the results.…”

“…In contrast, antenna calibrations that rely on terrestrial GPS tracking data [e.g., Bar-Sever, 1998;Schmid and Rothacher, 2003;Schmid et al, 2005] can access this gravitational constraint only on the transmitter side, since ground receivers are not in free fall. Additional information on this important gravitational constraint and its implications on our conclusions can be found in the Appendix.…”

“…The blue and red lines show block-averaged estimates derived from LEO data using TOPEX/Poseidon and GRACE as the reference antenna, respectively. The light blues lines depict the current International GNSS Service standards (IGS08_1793), which are satellite specific and based on the reduction of terrestrial data [Schmid et al, 2005]. The green line (Block II/IIA only) shows measurements from the robot calibration technique [Wübbena et al, 2007].…”