The Potential of VSOP2

WEI ERHU, LIU JINGNAN, YAN WEI

Orbit determination accuracy requirements

In the report prepared by the RADIOASTRON Navigation Astrometry and Geodesy (NAG) Working Group about the precise navigation of the SVLBI satellite, the following orbit determination accuracy requirements have been specified (NAG, 1989):
1) Standard Orbit: Required accuracy better than 1000 m for satellite control, orbit prediction, tracking and data communication.
2) Precise Orbit: Required accuracy better than 50m for processing ground-to-space VLBI data, and most astrometric applications.
3) Highly Precise Orbit: Required accuracy better than 1 m for geodetic and some astrometric.

Required accuracy better than 0.1m for geodynamic applications.
The accuracy of standard orbit and precise orbit has been achieved with the development of tracking technology. Even the OD accuracy of VSOP can be achieved 3-10 m which satisfies the requirement of above two accuracies. But It is not suitable for geodetic and geodynamic studies because it can’t achieve the accuracy of highly precise orbit, better than 1 m, because of its irregular shape, big ratio of area/mass, the limit of the accuracy obtainable using Doppler data only and so on. VSOP2 makes some improvements in its structure. Its higher science goals request higher accuracy of OD. Some studies indicate that OD accuracy can achieve cm level which is suitable for geodetic applications by using GPS and accelerometer (ISAS, 2003).

As reported (ISAS, 2003), the satellite in relatively low Earth orbit can achieve 5 cm level OD with GPS and accelerometer. But there is no signal from GPS near VSOP apogee. Using GALILEO system as a supplement of GPS is a best way to improve OD. Pre-literature has been discussed the coverage instance of SVLBI satellite by GPS and GALILEO system (WEI Erhu, 2006). As some papers (IAIN/GNSS 2006) have studied, it will be possible to use GPS and GALILEO as one system. In this paper, the OD accuracy of VSOP2 by above situation is discussed.

Simulation Study

Simulation Condition

GALILEO constellation in simulation is composed of 27 satellites. They spread equably in 3 orbits whose inclination is 56°. The satellite’s altitude is 23062 km and orbit period is 14.0 hs. The ratio of coverage of VSOP2 near apogee, which is advantaged for OD, will be improved because of higher altitude of GALILEO. The parameters of GPS are from NASA website (ftp://cddis.gsfc.nasa.gov/gps/data/daily/2005/). There are 24 satellites in the simulation.

On the other hand, considering the antenna is not globose to receive signal from all directions, and the influence of ionosphere and troposphere, the antenna is designed to point to space. It is designed as a cone with an apex angle of 80° to let the satellite to receive as more signals as possible. The coverage instance of VSOP2 by GPS and GALILEO system and its DOPs (PDOP which will be chose the best 6 satellites to calculate is mainly considered) is analyzed to achieve high accuracy OD. VSOP’s Simulant conclusion in the same time is compared.

Results of Simulation

There is not enough number of satellites to calculate the coverage time and PDOP during the simulation because of the design of the antenna. For example, the coverage near the apogee is too bad for OD. The coverage instance of VSOP and VSOP2 by GPS and GALILEO system is shown in Table 2.

According to the Table above, VSOP2’s coverage time by GNSS is shorter than VSOP in the simulation because its apogee height is higher. But during the natural tracking time, VSOP2’s mean PDOP is appreciably smaller than VSOP. And VSOP2’s PDOP is steadier than VSOP by analyzing their max and min PDOPs. That means the geometric structure of VSOP2 and GNSS system is better. It ascribes to the superiority of VSOP2’s orbit design.

As reported (ISAS, 2003), because of the combination of GPS and GALILEO system, the mean square error of OD accuracy can achieve 3 cm, so the actual OD accuracy can achieve 0.073 m by calculating with the mean PDOP—2.454. During the simulation, the time which OD accuracy under 1 m accounts for total time 100%, excluding the epoch which doesn’t satisfy the OD requirement. The OD accuracy of VSOP2 and VSOP by GNSS is shown in Fig 2. This conclusion is suitable for geodetic and geodynamic studies.

Improvement

Although PDOP for VSOP2 is better in the simulation, coverage time is an important factor in OD. So if the pointing of GNSS antenna can be set to more directions, the OD accuracy will be better. For example, if another GNSS antenna points to the Earth and its apex angle is also 80°, the percentage of coverage time will increase to 100%. Its mean PDOP is 4.196. But the influence of atmosphere in this kind of situation should be considered. The details are shown in Table 3 and Fig 3

The simulation above shows that VSOP2’s OD accuracy is suitable for geodetic applications by using GNSS system.