Differential GPS - Understanding GPS: Principles and Applications - page 379

CHAPTER 8
Differential GPS

Ronald J. Cosentino
Consultant
David W. Diggle and Maarten Uijt de Haag
Ohio University
Christopher J. Hegarty
The MITRE Corporation
Dennis Milbert
NOAA (retired)
Jim Nagle
ICAO

8.1 Introduction

As discussed in Chapter 7, a single-frequency SPS GPS user can often attain better
than 10m, 95% positioning and 20-ns, 95% timing accuracy worldwide. There are
many applications, however, that demand levels of accuracy, integrity, availability,
and continuity beyond even what a GPS PPS receiver can deliver. For such applica-
tions, augmentation is required. There are two general classes of augmentation: dif-
ferential GPS (DGPS) and external sensors/systems. This chapter introduces DGPS.
Chapter 9 will discuss various external sensors/systems and their integration with
GPS.
DGPS is a method to improve the positioning or timing performance of GPS
using one or more reference stations at known locations, each equipped with at least
one GPS receiver. The reference station(s) provides information to the end user via a
data link that may include:

Corrections to the raw end user's pseudorange measurements, corrections to
GPS satellite-provided clock and ephemeris data, or data to replace the broad-
cast clock and ephemeris information;

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Raw reference station measurements (e.g., pseudorange and carrier phase);

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Integrity data (e.g., “use” or “don't use” indications for each visible satellite,
or statistical indicators of the accuracy of provided corrections);

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Auxiliary data including the location, health, and meteorological data of the
reference station(s).

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