One is satellite dynamic model and the other is satellite initial condition. The performance of orbit prediction depends on two major components. Motivated by rapidly increasing location demands of mobile device users in GPS signal challenging conditions, more and more researches are made on long-term GPS orbit prediction – the key part of ephemeris extension. Considering the wide variety of mobile users under some extreme cases who may have limited access to wireless network for a long time or cost and power concerns etc, a feature of ephemeris extension with sufficient precision is especially of value to location-based service provider and mobile users.
This demands prediction of the GPS ephemeris, which, if available in receivers can be used to bridge the ephemeris outage period to reduce the first-time-to-fix (TTFF) and also improve positioning accuracy in challenging environments. As a result, the positioning may be impossible or dramatically degraded with limited available satellites, which is not acceptable for mobile applications such as E911. Fast TTFF then becomes challenging in some tough environments such as urban canyon and indoor, where GPS signals suffer from multipath effect, signal attenuation and building blockage etc, because of the difficulty in decoding satellite ephemeris data from the broadcast navigation messages. Navigation Performance Using Long-Term Ephemeris Extension for Mobile Deviceīroadcast ephemeris is required for a position fix while decoding broadcast ephemeris in open sky normally takes 18s, but valid just for 2 hours.
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