Geoscience Reference
In-Depth Information
C/A is the civilian code available to all users of the Standard Positioning Service (SPS). Under
the
Anti-Spooing
*
(AS) policy imposed by the DOD, the P-code is encrypted by an additional
“W-code,” resulting in the “Y-code,” available exclusively to military users, and designated as Pre-
cise Positioning Service (PPS). PPS guarantees positioning accuracy of at least 22 m (95 percent of
the time) horizontally and 27.7 m vertically, while guaranteed positioning accuracy of SPS is 100 m
(95 percent of the time) horizontally and 156 m (95 percent of the time) vertically. However, the
practical accuracy of SPS is usually higher (see Section 9.5). Under AS policy, the civilian receivers
must use special signal tracking techniques to recover observables on L2, because no C/A code is
available on L2 (Hofman-Wellenhof et al., 2001).
9.5 the pRIMARy eRRoR SoURCeS
In general, biases and errors affect GPS measurements. Biases are defined as measurement errors
that cause the observation to differ from the “true” distance by a systematic amount, and errors
are considered synonymous to instrumental noise. Biases may occur due to imperfect knowledge
of constants, such as “fixed” parameters, for example satellite or station coordinates, or may have
a physical basis such as atmospheric effects on signal propagation, and can be normally accounted
for by using special mathematical models or data-processing techniques. The primary error sources
affecting GPS signals can be grouped into four basic categories based on the source of error: (1)
atmospheric and environmental (signal propagation), (2) satellite related, (3) receiver related, and
(4) reference station related. In addition, geometric factors and number of satellites tracked have a
significant impact on the final positioning accuracy. Radio interference and jamming, as well as
intentional signal degradation, have a considerable (yet random in magnitude), impact on GPS sig-
nals, which significantly affects the positioning accuracy.
9.5.1
a
it M o is P h e R i c
a n d
e
n v i R o n M e n t a l
e
f f e c t is
9.5.1.1
Ionosphere and troposphere delays
The satellite signal is subject to atmospheric delays (Figure 9.4) as it passes through the atmospheric
layers of the ionosphere and troposphere. The amount of delay depends on the state of the ionosphere
and troposphere and can be estimated from models in the case of the troposphere or removed by
using dual-frequency GPS measurements and relative (differential) processing (Hofman-Wellenhof
et al., 2001). An approximated ionospheric model is also included in the satellite navigation message
and can be used to partially remove ionospheric delay from GPS range measurements. The accuracy
of this model is about 50 percent of the total ionospheric delay (Klobuchar, 1987).
The presence of free electrons in the ionosphere causes a nonlinear dispersion of electromag-
netic waves traveling through the ionized medium, affecting their speed, direction, and frequency.
The largest effect is on the signal speed, and as a result, GPS pseudorange is delayed and is thus
measured too long, while the carrier phase advances and is thus measured too short. The total effect
can reach up to 150 m and is a function of the Total Electron Content (TEC) along the signal's path,
the frequency of the signal, the geographic location, the time of observation, the time of the year,
and the period within the 11-year sun spot cycle (the last peak in ionospheric activity was 2001; see
Figure 9.5). Because the ionospheric delay is different for L1 and L2, its effect can be removed from
the GPS observables by combining dual-frequency data into iono-free linear combination and by
relative processing, as will be explained in Section 9.7 (Hofman-Wellenhof et al., 2001).
Because the troposphere is a nondispersive medium for all frequencies below 15 GHz, the L1 and
L2 carrier phase and pseudorange measurements are delayed by the same amount. Consequently,
the elimination of the tropospheric effect by dual-frequency measurements is not possible; instead,
* Anti-spoofing policy guards against fake transmissions of satellite data by encrypting the P-code to form the Y-code.
