ION GNSS 2012
Session F5: Consumer Products & Services

Title: Enhanced Broadcast Ephemeris for High Accuracy Assisted GPS Positioning
Author(s): Y. Bar-Sever, B. Iijima, L. Romans, J. Weiss, Jet Propulsion Laboratory
Room: 206 (NCC)

Enhanced broadcast ephemeris (e-BCE) is a novel A-GNSS product that uses the conventional RINEX broadcast ephemeris format to compensate for all key error sources in user positioning. It enables mobile wireless devices to obtain meter-level positioning without altering existing protocols or data interfaces anywhere along the A-GNSS communications chain.
Broadcast ephemeris files in RINEX format are a staple of current A-GNSS services. They enable faster time to first fix (TTFF) by eliminating the need for the GNSS chipset to acquire and decode the broadcast ephemeris. They support positioning accuracy of about 4 m RMS for a typical single-frequency, P-code tracking receiver in the mid-latitudes. This error is driven primarily by the low accuracy 8-parameter ionospheric model (known as the Klobuchar model) employed by GPS, and by errors in the broadcast clock state.

Using data derived from JPL´s dense Global Differential GPS (GDGPS) tracking network using advanced orbit and environmental models, the e-BCE files are tailored to incorporate precise ionospheric delays as viewed by users in any specific region of the world. They also contain more accurate clock states based on short-term (up to 2 hours) prediction of precise real-time clock estimates. Additional enhancements include updated inter-signal bias (Tgd), precise orbit ephemeris, and predicted signal delay due to the neutral atmosphere, as experience by users in the targeted geographical area. The combined effects of all these enhancements typically result in a factor of 2 improvement in User Range Error, leading to a similar improvements in positioning accuracy. As we approach solar maximum, the relative improvement from the enhanced broadcast ephemeris will increase further. The improvement is also more significant in the low and high latitudes, where ionospheric activity is more pronounced.

We will also describe how a similar enhancement approach can be applied to other types of assisted data and augmentation services, resulting in significantly improved user positioning.

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