Extended propagation GNSS ephemerides employing measurement fusion

Michael A Walker, Purdue University

Abstract

The use of Global Navigation Satellite Systems (GNSS) has proliferated in recent years in civilian, military and space sectors. This increased use has also increased the need for GNSS to work in situations for which it was not initially designed. For cellular telephone users, the FCC has mandated positioning requirements for all emergency calls, including situations in which the phone will experience poor GNSS signal conditions, notably indoors. Location Based Service (LBS) applications of GNSS also require that a receiver be able to calculate a position estimate rapidly in any scenario. Furthermore, spacecraft have begun using GNSS as a navigation technique for high altitude orbits above the GNSS constellation. Production of a GNSS navigation solution requires an estimate of the propagation delay in the transmitted signal, and knowledge of the orbits of the GNSS satellites. The first problem, that of tracking the signal delay, has been demonstrated for signal strengths as low as 15 dB-Hz (30–35 dB below the specified signal strength outdoors). Fundamental communications theory, however, limits the ability to extract the transmitted satellite orbit model, also known as the ephemeris, to signal strengths above 27 dB-Hz. In environments in which the signal strength is below 27 dB-Hz, the receiver may not be able to reliably download the GNSS satellite positions from the navigation signal and thus not be able to produce a navigation signal, even if the signal delays can be obtained through weak-signal tracking techniques. Methods to predict the GNSS satellite motion, valid for time periods longer than those of the broadcast ephemeris (4–6 hours), are thus necessary to enable the full capabilities of recent advances in weak signal tracking. In this research, an Extended Propagation Ephemeris (EPE) for GNSS satellites is tested for a large number of epochs. Furthermore, the calculation of the EPE parameters using weighted measurements is explored and compared to a more standard approach. Measurement Fusion, in which the EPE coefficients are updated at the receiver, whenever a new broadcast ephemeris becomes available, is also proposed. Measurement Fusion is tested over a large number of epochs and compared to the performance of the previously described techniques for producing the EPE calculation strategies. Infrastructure is proposed to implement Measurement Fusion in both civil and space applications. Finally, several conceptual designs will be presented for the infrastructure, transmission of orbital data defining EPE and sparse, high precision measurements over third party channels to the GNSS receiver, and for the internal receiver processing necessary to extract satellite position estimates from the fusion of the extended broadcast, and sparse measurements. Data requirements for transmission of these measurements is investigated for a number of civilian and space communications platforms. These are shown to be quite small, making the implementation of any of these methods into a low-cost receiver feasible using the existing communication architecture.

Degree

M.S.A.A.

Advisors

Garrison, Purdue University.

Subject Area

Aerospace engineering

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