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As was mentioned in the Communication section, the worst-case Receiver Noise Error (Ranging Error) of this design was 4.47m. Because this value was so close to the design requirement (5m), it was necessary to reduce the contributions of the other sources of error. In the earth-based system, there are several methods used to accomplish this task - two of the most effective methods being the Wide Area Augmentation System (WAAS) and Differential GPS (DPGS). In both systems, communication nodes are statically positioned in various locations across a desired region of coverage. In the case of DGPS, the unit acts much like a GPS satellite by broadcasting location data to mobile GPS receivers. This reduces error because the DGPS messages are assumed to have no atmospheric delay or ephemeris error. To have a decent range, these units are typically mounted on towers in highly populated areas of interest.
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On the other hand, WAAS is only able to calculate discrepancies in the GPS data, and communicate correction messages to WAAS enable receivers via satellite. In this project, it was necessary to deploy one of these systems, and the WAAS seemed more appropriate, since there would be no way of mounting DGPS units several hundred feet above the surface of Mars. Our solution to deployment of this system was to piggyback the WAAS units with the GPS units when “colonies” are established. This way, the WAAS units are packaged together with probes, thus cutting down on weight and overall cost. In having these WAAS units located close to the “colonies”, more accurate corrections for atmospheric error and multipath error can be obtained when compared to a system that spaces the WAAS units uniformly across the planet surface.
The Mars-based WAAS system will work in the following way:
- GPS signal is received by WAAS system and corrections are determined
- Correction messages are sent to the nearest GPS satellite
- Message is relayed to remaining satellites
- Corrections are sent to terrestrial GPS receivers as part of the GPS message.
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