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1. Necessity for Development of Space GNSS Receiver
Global Navigation Satellite System (GNSS), mainly known as Global Positioning System (GPS), is widely utilized for main navigation sensor. It can be used not only automobiles, pedestrians, aircraft and vessels, but space applications such as guided weapons, satellites and launching vehicles. And it already proves its utility in space environment according to certain reports.
The main advantage of GNSS receiver for space application is its cost-effectiveness. Without GNSS receiver, real-time positioning and orbit information are depend on external data such as NORAD TLE (US SPACE COMMAND TWO LINE ELEMENT) or high-cost Satellite Laser Ranging and Radar Tracking equipment with km-level errors. GNSS receiver makes it possible for independent positioning with low-cost and high-accuracy (tens of meter level error).
The weak point using GNSS receiver in space is its availability. For satellite application, its speed is very high and its body is also rotated, so that the visibility of GNSS satellites is changed rapidly. Therefore additional considerations compared to the ground application is required.
Nowadays, not only GPS, but also GLONASS from Russia, Galileo from Europe, and COMPASS from China are fully or partially serviced. And other local-based satellites systems are prepared their services. Therefore, availability in space applications can be expanded using various satellite based navigation systems.
According to its altitude, space application of GNSS signals are varied as below figure.
Figure 1. GNSS signal in the space
2. GNSS Receiver for Low Earth Orbit (LEO)
LEO satellite has its speed of 8 km/s with rapid change of its attitude, so that its visibility of GNSS satellites are changed at any time, and its doppler rate is high. Therefore space GNSS receiver different from ground application is need to be developed separately. The GNSS receiver for space needs to expand doppler search area for high dopper rate. And to reduce its calculation load, A-GPS concept can be utilized. Robust carrier phase tracking loop is also required as of its high speed.
In SNU GNSS Laboratory, GNSS software receiver for LEO application is being developed with considerations mentioned above. This receiver is designed using L1, L2C and L5 from GPS, and E1B and E5a from Galileo together with integrated navigation algorithm for multi-band and multi-system use.
Below figure describes the structure of LEO integrated receiver.
Figure 2. Multi-Band Software GNSS Receiver Structure for LEO
3. GNSS Receiver for Medium/High Earth Orbit (MEO/HEO)
The speed of MEO/HEO satellites are not high as much as LEO's. But receiving GNSS signal itself is not easy because of its altitude higher than GNSS satellites. MEO/HEO satellites utilized the part of main lobe and side love of GNSS signals broadcasting to the center of Earth, and these signals come from ten thousands of kilometer with weakening its strength. And it is also hard to ensure visibility of GNSS satellites . Therefore, weak signal process technologies are essentially required, and additional orbit filter is need to ensure its continuity.
Special features for MEO/LEO satellites as mentioned above are considered for the development of space receiver in SNU GNSS Laboratory. This receiver increased its integration time of signal correlators up to hundreds of milliseconds to process the weak signals. Special algorithms about doppler of code and estimating of start and sequence of navigation bit are need to be considered.
Next figures show the result of processing weak signals (23dB) using developing MEO/HEO software receiver. Each parameter shows each convergence tendency with successful signal tracking.
Figure 3. Signal Tracking Results for Weak Signal
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