Differential GPS - Understanding GPS: Principles and Applications - page 381

extremely important, since they directly influence the performance achievable for
any type of DGPS system. The underlying algorithms and performance of code- and
carrier-based DGPS systems are presented in Sections 8.3 and 8.4, respectively.
Some important DGPS message standards are introduced in Section 8.5. The final
section, Section 8.6, details a number of operational and planned DGPS systems.

8.2 Spatial and Time Correlation Characteristics of GPS Errors

Many of the GPS error sources discussed in Chapter 7 are highly correlated over
space and time. All DGPS systems exploit these correlations to improve overall sys-
tem performance. For instance, in a simple local-area DGPS system with a single ref-
erence station (see Figure 8.1), the errors in the reference station's pseudorange and
carrier-phase measurements for visible satellites are expected to be very similar to
those experienced by a nearby user. If the reference station estimates the errors by
leveraging its known surveyed position and provides this information in the form of
corrections to the user, it is expected that the user's position accuracy will be
improved as a result. This section quantifies the correlation of GPS errors between
receivers separated over some distance (often referred to as the baseline , which may
be interpreted as a vector) and over time. Time correlations (i.e., how rapidly the
errors change with time), are also of interest, because in general DGPS systems can-
not instantaneously provide data to the end user—even with a high-speed radio link
there is some finite delay associated with the generation, transmission, reception,
and application of the data.

8.2.1 Satellite Clock Errors
Satellite clock errors are one of the simplest GPS errors to correct. This is because a
satellite clock error causes the same effect on pseudorange and carrier-phase mea-
surements, regardless of the location of the user. For instance, if the satellite clock

Satellite

User

Reference
station

Figure 8.1 Local-area DGPS concept.