systematic errors. Third, the offsets between different coordinate frames may be detected by some common stations with different satellite positioning coordinates. Fourth, the satellite orbit parameters may reflect the reference coordinate, which can be monitored by multi-GNSS receivers. Fifth, the reference coordinate system should include the dynamic deformation information, and the update of the reference frame should have standards, otherwise additional satellite orbit errors may be introduced in the satellite navigation systems.
Foundation of BDC
BDC is consistent with CGCS 2000, which in turn is consistent with the International Terrestrial Reference System (ITRS), and is a global, three-dimensional, right-handed, orthogonal, geocentric reference system. CGCS is realized by China Terrestrial Reference Frame 2000 (CTRF2000), which is referred to ITRF97 with the epoch of 2000.0 (Yang, 2009). The main parameters of the reference ellipsoid are nearly the same as those defined by ITRS with the semi major axis (a), second degree harmonic coefficient (J2), and the mean angular velocity of the Earth (ω) the same as those of the ellipsoid of GRS1980. The gravitational mass (GM) constant adopts the value of WGS-84 (Wei, 2003).
It should be pointed out that there exists little difference between the BDC coordinate system and the GPS geodetic system (GGS-84). The flattening of the ellipsoid of BDC is f = 1/298.257222101, which is different from that of GPS with f = 1/298.257223563. In theory, the biggest offset introduced by the two flattening ratios is within 1 millimeter in the equator. This kind of difference may be neglected, that is, the coordinate references of BeiDou system and GPS are consistent.
BDC is realized by BeiDou Terrestrial Reference Frame (BTRF), which is connected to CTRF 2000. The CTRF 2000 adopts the same standards as those of the ITRF. There are 47 International GNSS Service (IGS) stations evenly distributed over the world (Figure 1), including those IGS stations in China: BJFS, CHAN, KUNM, LHAS, SHAO, URUM, WUHN, and XIAN (Figure 2) that were used in the integrated adjustment of CTRF. Consistency in definition and diversity in realization will be beneficial for compensating the systematic errors.
Three levels of CTRF are included: the first level is the IGS and continuously operating reference stations (CORS) (28 stations) at the 3 mm level; the second level is the 2000 National GPS Network with about 2,500 stations at 3 cm level (Yang et al., 2007, 2009); and the third level is the combined adjusted network of astro-geodetic network and 2000 National GPS Network with 50,000 stations at 3 dm level (Yang et al., 2005).