HOTSTART EVERY TIME - Compute the Ephemeris on the Mobile

Philip G. Mattos

Abstract:	The research objective of the study was to achieve orbit prediction without actual modeling of the irregular earth gravitation field and moon/sun forces, in order to minimize the amount of CPU resources required (MIPs). The production objective was to enable the consumer with an unconnected PND or mobile to achieve sub-ten-second TTFF’s even for the first fix of the day, and to achieve fixes in deep indoor scenarios without external assistance. The results demonstrated so far have achieved fast pseudo-hot-start fixes meeting the target TTFFs, with low-MIPs prediction of ephemerides, and the accuracy achieved without a server is as good as server based systems. The prediction method has two steps. The first is a curve fit for the perturbations to the orbits for all satellites over all time since IGS data became available, suitably adapted, automatically, for discontinuities such as satellite manoeuvres. The first step is run in the laboratory, on a fast PC, with no limitations to MIP’s. It is establishing the cross dependencies between the perturbations to the orbit, and the actual parameters of the orbit, starting from the minimum set of just Orbit plane (Right Ascension) and Orbit Slot. The second is the automatic generation of the coefficients for these polynomial curve-fits. The second step is critically MIP’s dependent, as it may need to run in the same few seconds as the satellites are being acquired after powerup, although best attempts are made to pre-calculate future ephemerides during the previous power-on cycle. Each step contributes its own errors, the final result being the combination of these, together with the geometric effects of the error direction, of the satellites individual location, and of the fix geometry. So far the error analysis gives satellite position error, and range error. The final step is to run the model in real world scenarios over all locations on earth, with all possible satellite constellations (and obstructions), and generate statistics of the resultant errors. The grid size of these tests is adjusted to ensure a manageable simulation time. Running this suite of tests allows us to compare performance not only with the customer’s requirements, which fundamentally are to permit rapid but correct map matching, but also to compare the accuracy performance with existing server based assistance systems equipped with communication links. The perturbations to the MEO orbits over time have been characterized, their dependencies analysed and appropriate curvefits generated. The dependencies have been collected into the required coefficients that can be calculated at runtime on the mobile unit based on the current values of the orbit parameters, together with simple known constants such as moon period etc. The standard hot-start TTFF has been achieved for all fixes, even the first of the day, or when obstructions prevent the downloading of real ephemerides, as in indoor use. The accuracy meets the requirements for map-matching (outdoors), and indoors is dominated by the poor signal quality, rather than ephemeris accuracy. For the Kepler parameter prediction method, the processor resources required are minimal in terms of MIPs, and can usually be computed in advance, so not contributing to TTFF. The memory requirements for program are small, for back-up memory consist of about 10kbytes for expired ephemeris data, plus 10kbytes per day for optional pre-calculated future ephemerides, if post-calculation is to be avoided for TTFF reasons. (all for a 32 sat constellation). This method is designed for stand-alone GPS engines as found in modules, such as Teseo[2], where resources are limited. For machines with plenty of MIPs and memory available, such as PNDs, the full mathematics of force based orbit simulation has been implemented on the Cartesio[1] family of GPS chipsets.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	204 - 211
Cite this article:	Mattos, Philip G., "HOTSTART EVERY TIME - Compute the Ephemeris on the Mobile," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 204-211.
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