Performance of Stand-Alone GPS - Understanding GPS: Principles and Applications - page 301

CHAPTER 7
Performance of Stand-Alone GPS

Rob Conley
Overlook Systems
Ronald Cosentino
Consultant
Christopher J. Hegarty, Elliott D. Kaplan, and Joseph L. Leva
The MITRE Corporation
Maarten Uijt de Haag
Ohio University
Karen Van Dyke
Volpe Center

7.1 Introduction

The accuracy with which a user receiver can determine its position or velocity, or
synchronize to GPS system time, depends on a complicated interaction of various
factors. In general, GPS accuracy performance depends on the quality of the
pseudorange and carrier phase measurements as well as the broadcast navigation
data. In addition, the fidelity of the underlying physical model that relates these
parameters is relevant. For example, the accuracy to which the satellite clock offsets
relative to GPS system time are known to the user, or the accuracy to which satel-
lite-to-user propagation errors are compensated, are important. Relevant errors are
induced by the control, space, and user segments.
To analyze the effect of errors on accuracy, a fundamental assumption is usually
made that the error sources can be allocated to individual satellite pseudoranges
and can be viewed as effectively resulting in an equivalent error in the pseudorange
values. The effective accuracy of the pseudorange value is termed the user-equiva-
lent range error (UERE). The UERE for a given satellite is considered to be the (sta-
tistical) sum of the contributions from each of the error sources associated with the
satellite. Usually, the error components are considered independent, and the com-
posite UERE for a satellite is approximated as a zero mean Gaussian random vari-
able where its variance is determined as the sum of the variance of each of its
components. UERE is usually assumed to be independent and identically distributed
from satellite to satellite. However, for certain cases of interest, it is sometimes
appropriate for these assumptions to be modified. For example, if one is considering

301