The increasing number of telecommunication satellites in geosynchronous orbits has caused their orbital positions and radio frequency channels to become overcrowded. Satellites sometimes come too close to each other in orbit, causing worry in satellite control operations.
To ease this problem, we have developed a radio interferometer that receives microwaves from satellites and tracks precise orbital positions and motions.
Satellite-tracking interferometer
(larger)
Our interferometer, which has two receiving antennas and a rotary baseline (with length B, see opposite) is designed for satellite direction finding. When the baseline goes through one revolution, it becomes equivalent to a parabolic antenna with radius B in regard to the resolution power of direction finding. Precise monitoring of satellite positions thus becomes possible without needing large antennas.
Rotary baseline: underlying concept
Mounting the antennas on the arm causes the RF cables to twist at the pivots, causing phase errors. Further, the weight of the antennas reduces mechanical precision.
Therefore, the antenna positions are fixed, and the baseline is made rotary by using movable planar mirrors. Downlink microwaves are reflected by the rotary mirrors and guide mirrors onto the fixed antennas. When the 13-m arm rotates, the mirror angles are automatically altered so the received beams are guided correctly to the antennas. The fixed antennas (1.8-m diameter) are separate for the Ku and C bands, and the band is selected by tilting the guide mirrors.
Reflecting interferometer
(larger)
The interferometer operates in different modes. In L and X modes, the satellite positions are estimated in real time to the millidegree using Kalman filtering. We use the R mode as a starter to the L and X modes. Errors in the arm's length, horizontality, and zero-azimuth point are corrected using C mode.
Any signal, beacon or band-pass, can be tracked; so our monitoring requires no satellite information beforehand.
R mode
| Determines azimuth and elevation; baseline goes through one revolution
| L mode
| Monitors longitude history; baseline is fixed at near-EW orientation
| X mode
| Monitors latitude-longitude history; baseline goes back and forth between
orientations of 135 and 225 degrees
| C mode
| Corrects mechanical errors
| |
In overcrowded orbits, longitude occupancy has a strong influence on how efficiently a satellite uses its orbit.
Using the L mode enables us to monitor a satellite's long-term longitude history. Station-keeping patterns are clearly observed, as orbit corrections are monitored as they are. This is helpful when two or more satellites operate in proximity and need continuous monitoring.
♦ Show Room
You can see for yourself how the arm and mirrors actually move in a scale model, and learn how the orbit is crowded with satellites from all over the world with an orbit model. These are displayed in our show room.
Longitude history, C band (..larger)
Longitude history, Ku and C bands (..larger)
Sept 2007
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