Performance of Stand-Alone GPS - Understanding GPS: Principles and Applications - page 322

Table 7.3 GPS Precise Positioning Service Typical UERE Budget
Segment Source

Error Source 1 Error (m)

Space/control Broadcast clock 1.1

Broadcast ephemeris 0.8

User Residual ionospheric delay 0.1

Residual tropospheric delay 0.2

Receiver noise and resolution 0.1

Multipath 0.2

System UERE Total (RSS) 1.4

Table 7.4 GPS Standard Positioning Service Typical UERE Budget
Segment Source

Error Source 1 Error (m)

Space/control Broadcast clock 1.1

L1 P(Y)-L1 C/A group delay 0.3

Broadcast ephemeris 0.8

User Ionospheric delay 7.0*

Tropospheric delay 0.2

Receiver noise and resolution 0.1

Multipath 0.2

System UERE Total (RSS) 7.1*

*Note that residual ionospheric errors tend to be highly correlated among satellites
resulting in position errors being far less than predicted using DOP ⋅ UERE (see discussion

in Section 7.3.2).

7.3 PVT Estimation Concepts

Chapter 2 described some basic techniques for estimating the PVT of a possibly
mobile GPS receiver. This section discusses a variety of additional concepts regard-
ing PVT estimation, beginning with an expanded description of the role of geometry
in GPS PVT accuracy and a number of accuracy metrics that are commonly used.
This section also describes a number of advanced PVT estimation techniques includ-
ing the use of WLS, additional estimated parameters (beyond the user x , y , z position
coordinates and clock offset), and Kalman filtering.

7.3.1 Satellite Geometry and Dilution of Precision in GPS
As motivation for the concept of dilution of precision as it applies to GPS, consider
once again the foghorn example introduced in Section 2.1.1. In this example, a user
locates his or her position from ranging measurements from two foghorns. The
assumptions are that the user has a synchronized time base relative to the foghorns
and has knowledge of the location of the foghorns and their transmission times. The
user measures the TOA of each foghorn signal and computes a propagation
time that determines the user's range from each foghorn. The user locates his or
her position from the intersection of the range rings determined from the TOA
measurements.