Geoscience Reference
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Fig. 2.15 Space VLBI
system
ground-based VLBI net extending into space. It has the same function as a ground
antenna, i.e., to receive signals from a radio source. Then, the required observa-
tional data for scientific research can be acquired through correlation processing of
the signals received by both the space VLBI station and the ground antenna.
However, space VBLI is different to ground-based VLBI in technical realization
since the former places the antenna in space. The system of space VLBI is shown in
Fig.
2.15
.
Space VLBI is mainly distinguished by:
1. The phase of the space station's local oscillator is locked onto the hydrogen
frequency standard of the ground tracking station. This frequency standard is
sent to the space station from the tracking station through a radio channel
(upwards).
2. The radio signal and other data received by the space station are sent back to the
ground tracking station through a radio channel (downwards).
3. The space station must be equipped with highly precise systems for the attitude
adjustment of antennae and for orbit control and detection.
4. The space station produces its own source of energy from received solar energy.
5. A ground support system of global coverage that can maintain uninterrupted
communication with the space station is required.
The most significant technical advantage of the application of space VLBI to
geodesy is to turn the geometric measurement of ground-based VLBI into dynamic
measurement. It has been mentioned that the measurement completed by forming a
baseline between two ground VLBI observations is a geometric one from the
perspective of geodesy. Such measurement alone can only determine the relative
position of the two stations but not their geocentric coordinates. Since the orbit of
the space VLBI is described in a geocentric coordinate system and its movement is
affected by various geodynamic factors, when adopting space VLBI a dynamic
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