Differential GPS - Understanding GPS: Principles and Applications - page 380

Many types of data links may be used, such as radio links at frequencies ranging
from low frequencies below 300 kHz to L-band (1,000-2,000 MHz) and beyond to
the Internet, and importantly, the link may not be real time. For instance, it is possi-
ble to implement DGPS methods using two GPS receivers that each simply log data
to a hard drive or other storage device.
DGPS techniques may be categorized in different ways: as absolute or relative
differential positioning; as local area , regional area ,or wide area ; and as code based
or carrier based .
Absolute differential positioning is the determination of the user's position with
respect to an ECEF coordinate system (see Section 2.2.2). This is the most common
goal of DGPS systems. For absolute differential positioning, each reference station's
position must be accurately known with respect to the same ECEF coordinate sys-
tem in which the user position is desired. Aircraft use this type of positioning as an
aid for remaining within certain bounds of the desired flight path; ships use it as an
aid for remaining within a harbor channel.
Relative differential positioning is the determination of the user's position with
respect to a coordinate system attached to the reference station(s), whose absolute
ECEF position(s) may not be perfectly known. For instance, if DGPS is implemented
to land aircraft on an aircraft carrier, the ECEF positions of the reference stations
may be imperfectly known and time varying. In this case, only the position of the
plane with respect to the aircraft carrier is required.
DGPS systems may also be categorized in terms of the geographic area that is to
be served. The simplest DGPS systems are designed to function only over a very
small geographic area (i.e., with the user separated by less than 10-100 km from a
single reference station). To effectively cover larger geographic regions, multiple ref-
erence stations and different algorithms are typically employed. The terms regional
area and wide area are frequently used in the literature to describe DGPS systems
covering larger geographic regions with regional-area systems generally covering
areas up to around 1,000 km and wide-area systems covering yet larger regions.
There are not, however, universally agreed-upon demarcations in terms of distance
for the applicability of each term.
One final categorization of DGPS systems is between so-called code-based or
carrier-based techniques. Code-based DGPS systems rely primarily on GPS code
(i.e., pseudorange) measurements, whereas carrier-based DGPS systems ultimately
rely primarily on carrier-phase measurements. 1 As discussed in Chapter 5, carrier-
phase measurements are much more precise than pseudorange measurements, but
they contain unknown integer wavelength components that must be resolved.
Code-based differential systems can provide decimeter-level position accuracies,
whereas state-of-the-art carrier-based systems can provide millimeter-level

performance.
This chapter describes the underlying concepts of DGPS and details a number of
operational and planned DGPS systems. Section 8.2 provides a discussion of the spa-
tial and time correlation characteristics of GPS errors (i.e., how GPS errors vary
from location to location and how they change over time). These characteristics are

1. It should be noted that virtually all DGPS systems employ both pseudorange and carrier-phase measure-

ments, so the distinction between code-based and carrier-based techniques is a matter of degree of reliance
on the respective measurement type.