Antenna for Precise Orbit Determination
- Paper number
IAC-08.B2.2.2
- Author
Dr. Johan Wettergren, Saab Space AB, Sweden
- Coauthor
Dr. Per Ingvarson, Saab Space AB, Sweden
- Coauthor
Dr. Magnus Bonnedal, SAAB Ericsson Space AB, Sweden
- Coauthor
Dr. Bo Wästberg, Efield AB, Sweden
- Year
2008
- Abstract
The ESA SWARM system will consist of three satellites that will measure the Earth magnetic field. It will be the best ever characterization of the geomagnetic field. The objective is to increase understanding of the Earth interior and climate. Along with the accurate magnetometer, the system also needs metre accuracy knowledge of the measurement locations. To achieve this a GPS receiver is providing measurements for Precise Orbit Determination, POD, processing on ground. The GPS receiver uses the GPS satellites for Real Time Navigation, RTN. At least four GPS signals are clocked to determine the code range, and the receiver position can be determined. The two GPS frequencies are tracked in order to compensate for the iononosphere impact. The RTN accuracy is in the order of several m, where the largest error contributions are the GPS satellite positional knowledge and geometrical strength of the GPS constellation. The accuracy can be improved by using more than the minimum four GPS satellites and by averaging over many measurements. The latter is achieved in the ground processing by using a thorough model-based orbit prediction for each SWARM satellite. This includes advanced Earth gravity models along with influence from the Sun, the Moon, tidal effects and from friction against the (thin) atmosphere. The model is fitted to the GPS code and carrier phase measurements, resulting in cm accuracy positional knowledge. The main error contributions in the POD processing are often systematic measurement errors due to satellite multipath effects on the antenna radiation pattern. The multipath effects are characterized by measuring the antenna on a 1.5 m mock-up, representing the 9 m long satellite. In order to verify that the mock-up is representative, extensive electromagnetic simulations were made. The simulations included the antenna and the complete satellite and were then reduced to the antenna and a section of the satellite. Comparing amplitude and phase variations in the antenna pattern, we could find out how large a mock-up was needed for enough inclusion of multipath. The simulations were very large-scale in terms of computer resources. We used a state-of-the art Multi-Level Fast Multipole Method (MLFMM) program called EFIELD. The actual design of the antenna, taking both the dual-frequency matching at 1.575 GHz (L1) and 1.227 GHz (L2) and the almost hemispheric coverage into account, was performed with several levels of software. First, a fast bodies-of-revolution (BOR) simulation using AKBOR found an approximate geometry with the right coverage. Then, a finite element method (FEM) simulation using HFSS allowed us to match the antenna at L1 and L2 simultaneously. HFSS was also used to include the nearby parts of the satellite.
- Abstract document
- Manuscript document
IAC-08.B2.2.2.pdf (🔒 authorized access only).
To get the manuscript, please contact IAF Secretariat.