The term "microGPS" denotes a Global Positioning System (GPS) receiver design concept that combines relatively simple, lightweight, low-power-consumption hardware with portable, efficient software. The power demand of a microGPS receiver can be made low because it is designed to sample sparsely; that is, to "awaken" from a "sleep" mode only occasionally to sample GPS signals during short intervals. MicroGPS was conceived for navigational use aboard some small Earth-orbiting satellites for which full-fledged GPS receivers would be too complex, massive, and power hungry, and for which positioning errors as large as a few hundred meters would be acceptable as part of the price of low mass and low power consumption. The microGPS concept may also prove attractive for terrestrial applications that involve similar design tradeoffs.
The hardware of a prototype microGPS receiver aboard the Student Nitric Oxide Explorer (SNOE) spacecraft consists of a lightweight patch antenna and receiver circuits that include an inexpensive oscillator, a down-converter/ sampler, and a memory chip. The receiver circuits consume a peak power of 875 mW during acquisition of samples of GPS signals and a power of 75 mW during standby (with the oscillator running, ready for commands). For the SNOE mission, GPS signal data are acquired during sampling periods 20 ms long, once every 15 minutes (about 6 times per orbit). The volume of data is about 450 kilobytes per day. The GPS samples are stored in the memory chip for subsequent processing.
In order to offer maximum flexibility in satellite design, the microGPS orbit-determination software is designed for execution either aboard the spacecraft or on the ground. In the latter case, the sparse GPS samples are telemetered to the ground and processed in post real time to compute spacecraft orbits that can be uploaded to the satellite and projected ahead for use in real time. Aboard the spacecraft, the software could be executed either in the flight computer of the spacecraft or in a special-purpose processor within the microGPS hardware unit (with slight increases in mass and power consumption).
The mass of the receiver, including the patch antenna, is only 595 g. The aspect of the receiver design that makes it possible to achieve such a low mass is a modified architecture in which all of the GPS-specific signal processing typically implemented in parallel on special-purpose hardware in other GPS receivers is, instead, implemented serially in software. Moreover, whereas the amount of computation to acquire signals in other GPS receivers is approximately proportional to N2 (where N is the number of discrete time samples), the amount of computation is proportional to Nln(N) in this receiver due to the use of frequency-domain processing.
The sequential time-tagged GPS signal samples are down-converted, filtered, and quantized to a single bit. The receiver samples the signal at a rate of about 20 MB/s but can be programmed to perform a sum-and-dump filtering and decimation function to reduce the data rate to about 2 MB/s. The sampled data are searched in Doppler shift (over a range of ±45 kHz) and in delay [over 1 repeat cycle (1 ms) of the Coarse-Acquisition (C/A) code]. The search for GPS signals and the extraction of observables (Doppler shifts and pseudoranges) is effected by use of a Fourier-transform-based technique of time-domain correlation.
There is an important distinction between the usual GPS pseudorange and the pseudorange in microGPS: Whereas the usual GPS pseudorange represents absolute, unambiguous range (plus transmitter and receiver clock offsets), the reliability of the pseudorange in microGPS is limited by an ambiguity of 1 ms (300 km); this is because the 20-ms sampling period is not long enough for either decoding the GPS navigation-data message (and thus GPS time is unavailable) or reliably determining the times of the bit transitions of the message.
The range ambiguity is resolved by further processing in three steps. First, a crude (accurate to within 50 km) orbit is computed by use of the Doppler data from multiple GPS satellites. Next, a set of unambiguous range data is computed from a combination of this crude orbit and the GPS-satellite orbits, which are known to far better accuracy. Finally, the ambiguity is resolved by direct comparison of this unambiguous set of computed range data with the ambiguous microGPS pseudorange data.
This work was done by Stephen Lichten, Thomas Meehan, Jeffrey Srinavasan, Sien-Chong Wu, and Lawrence Young of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.
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