Introduction - Understanding GPS: Principles and Applications - page 16

multipath and shadowing. Multipath and shadowing can be significant and some-
times dominant contributors to PVT error. These sources of error, their effects, and
mitigation techniques are discussed. The chapter concludes with a discussion on ion-
ospheric scintillation. Irregularities in the ionospheric layer of the Earth's atmo-
sphere can at times lead to rapid fading in received GPS signal power levels. This
phenomenon, referred to as ionospheric scintillation, can lead to a GPS receiver
being unable to track one or more visible satellites for short periods of time.
GPS performance in terms of accuracy, availability, integrity, and continuity is
examined in Chapter 7. It is shown how the computed user position error results
from range measurement errors and user/satellite relative geometry. The chapter
provides a detailed explanation of each measurement error source and its contribu-
tion to overall error budgets. Error budgets for both the PPS and SPS are developed
and presented.
Section 7.3 discusses a variety of important concepts regarding PVT estimation,
beginning with an expanded description of the role of geometry in GPS PVT accu-
racy determination and a number of accuracy metrics that are commonly used. This
section also describes a number of advanced PVT estimation techniques, including
the use of the weighted-least-squares (WLS) algorithm, the inclusion of additional
estimated parameters (beyond the user x , y , z position coordinates and clock offset),
and Kalman filtering.
Sections 7.4 through 7.6 discuss, respectively, the three other important perfor-
mance metrics of availability, integrity, and continuity. Detailed examination of
GPS availability is conducted using the nominal GPS constellation. This includes
assessing availability as a function of mask angle and number of failed satellites. In
addition to providing position, velocity, and timing information, GPS needs to pro-
vide timely warnings to users when the system should not be used. This capability is
known as integrity. Sources of integrity anomalies are presented, followed by a dis-
cussion of integrity enhancement techniques including receiver consistency checks,
such as receiver autonomous integrity monitoring (RAIM) and fault detection and
exclusion (FDE), as well as SBAS and GBAS.
Section 7.7 discusses measured performance. The purpose of this section is to
discuss assessments of GPS accuracy, which include but are not limited to direct
measurements of PVT errors. This is a particularly complex topic due to the global
nature of GPS, the wide variety of receivers, and how they are employed, as well as
the complex environment in which the receivers must operate. The section con-
cludes with a description of the range of typical performance users can expect from a
cross-section of today's receivers, given current GPS constellation performance.
DGPS is discussed in Chapter 8. This chapter describes the underlying concepts
of DGPS and details a number of operational and planned DGPS systems. A discus-
sion of the spatial and time correlation characteristics of GPS errors (i.e., how GPS
errors vary from location to location and how they change over time) is presented
first. These characteristics are extremely important to understanding DGPS, since
they directly influence the performance achievable from any type of DGPS system.
Next, the underlying algorithms and performance of code- and carrier-based DGPS
systems are described in detail. The Radio Technical Commission for Maritime Ser-
vices (RTCM) Study Committee 104's message formats have been adopted through-
out the world as a standard for many maritime and commercial DGPS applications.