VLBI geodetic precision with different models

Erhu WEI, Jingnan LIU, Peijing PAN

Very Long Baseline Interferometry (VLBI) is the unique space geodetic technique which can provide the Celestial Reference Frame (CRF), the Terrestrial Reference Frame (TRF) and the relationship between the two frames — Earth Orientation Parameters (EOP) at the same time. VLBI has a widely usage in space geodetic, ground geodetic, geophysical fields and so on. Presently, it can determine the position of the radio source outside the galaxy with 1mas precision, and determine several kilometers length of baseline on the earth’s surface with 1cm precision. Due to its high stability and high precision character, the Celestial Reference Frame outside the galaxy based on VLBI has been the best realization of the quasi-inertial referenceframe since 1980s. VLBI stations are the most important benchmarks in the International Terrestrial Reference Frame (ITRF), and VLBI is one main supporting technique which determines EOP. Till now, space and ground VLBI have accumulated more than 20 years’ data. They provide continuous and long-term data guarantee for space geodetic, ground geodetic and geographical research.

Though there are several kinds of common ocean tide and nutation models, there have no article giving us the best model combination through special comparison and analysis. This paper firstly introduces several common ocean tide models and nutation models, and then computes the global VLBI data during 2001-2007 using the OCCAM 5.0 software platform. It compares and analyzes the precision of geodetic parameter’s result using these models, and gives us the best ocean tide and nutation model combination.

Model introduction

OCCAM is a common software in processing space VLBI and ground VLBI data. It is used in describing the physical model of time delay and time delay rate, computing and adjusting all parameters’ partial derivative. Generally speaking, there are three physical models in this software[1]: 1) Computing model and setting up Standard Data File (SDF) model: the preparation of the data (DTAU0), precession and nutation and (PN), the correction of station’s displacement and partial derivative (STATION), geometry model (GEOMET), all five parts.
2) Adjusting scheme model: Kalman filter method, Least square method and least square collocate method, all three parts. 3) Application program model: obtaining information from the standard data files (SDF) in OCCAM, obtaining information from Obs-Calc files, all two parts.

OCCAM 5.0 software is made up of several executing programs. Only operating in definite sequence, we can get the accurate VLBI computing results. In this paper, we will compute with multi-baseline Kalman filter method, the detailed processing please refers to [2].

EANES model (CSR4.0 ocean loading model)

The EANES model[3] used in this paper is the CSR 4.0 ocean tide model, which is computed on the basis of the Orthotide model of Eanes et al., Center of Space Research, University of Austin, Texas, kindly made available to the world in April 1999. It is a further development of CSR 3.0 and contains 239 circles (about 6.4 years) of TOPEX/POSEIDON altimetry. Like the CSR3.0 ocean tide model, CSR4.0’s grid resolution is 0.5º×0.5º. It has the same orthotide frequency model; and it comprises the diurnal and semidiurnal bands.

For the computation of the loading effects, a land-ocean mask was constructed from ETOPO5. The same mask was used in CSR3.0. The reason for this is that many coastal model nodes of CSR3.0 inundate ETOPO5 land. Since the altimetry solution does not impose local water mass continuity conditions, the masking does not degrade global mass conservation (which is a more important error source in loading effects than in the tide elevation itself), but improves the realism of the load distribution. This model still uses concentrated load circling method and computes the close load through gradual reduction discrete points in the integral.

RAY model (GOT99.2b ocean loading model)

RAY Ocean tide model[4] is a long wavelength adjustment of FES94.1 using TOPEX/POSEIDON data and is given on a 0.5 by 0.5 degree grid.
FES94.1 Ocean tide model is a pure hydrodynamic tide model tuned to fit tide gauges globally. It has been calculated on a finite element grid with very fine resolution near the coast but the version used here is given on a 0.5 by 0.5 degree grid. GOT99.2b also becomes equal to FES94.1 outside the 66 degree latitudes.