Differential GPS - Understanding GPS: Principles and Applications - page 397

systems. An excellent introduction to the methods of satellite orbit determination
may be found in [14].

8.3.3.2 Determining Ionospheric Propagation Delays
Ionospheric delays can be addressed in various ways within a WADGPS system.
The simplest approach is for the user to directly measure ionospheric delays using a
dual-frequency receiver. This option is currently available to authorized PPS users
or to civilian users using semicodeless methods (see Section 5.14) to track the L2
P(Y) code. Because of the fragility of semicodeless L2 P(Y) code tracking, some
important operational WADGPS systems discussed in Section 8.6 are designed to
support users with single-frequency L1 C/A code receivers. These systems estimate
ionospheric delays throughout their service volumes using dual-frequency
semicodeless receivers in their reference stations. The slant ionospheric delays mea-
sured by the reference stations are used by the central processing site, along with
models of the ionosphere, to develop estimates of vertical ionospheric delays for dis-
crete latitude/longitude points across the coverage volume. These vertical delay esti-
mates are broadcast to the user. The user equipment then interpolates among these
points to develop a vertical ionospheric delay correction for each visible GPS signal.
The vertical delay correction is mapped into an appropriate slant delay correction
based on the elevation angle for each visible satellite. The vertical delay corrections
for the visible satellites are generally not the same, since the points of intersection
between the signal paths and the ionosphere are not collocated.

8.4 Carrier-Based Techniques

The constant motion of the GPS satellite constellation requires that the GPS
receiver, in general, be capable of accounting for the changing Doppler frequency
shift on L1. Where dual-frequency receivers are used, both L1 and L2 are tracked.
The shift in frequency arises due to the relative motion between the satellites and the
receiver(s). Typical satellite motion with respect to an Earth-fixed observer can
result in a maximum range of Doppler frequencies of

4,000 Hz with respect to the
L1 and L2 carriers. Integration of the Doppler frequency offset results in an
extremely accurate measurement of the advance in signal carrier phase between
time epochs (see Section 5.7.3). Interferometric techniques can take advantage of
these precise phase measurements and, assuming sources of error can be mitigated,
real-time positional accuracies in the centimeter range are achievable. While
changes in signal phase from epoch to epoch can be measured with extreme accu-
racy, the number of whole carrier cycles along the propagation path from satellite to
receiver remains ambiguous. Determining the number of whole carrier cycles in the
propagation path is known as carrier-cycle integer ambiguity resolution and
remains an active area of investigation in the field of kinematic DGPS research.
Remondi [15] has made extensive use of the ambiguity function for resolving these
unknown integer wavelength multiples, but the pioneering work in this area arose
from the efforts of Counselman and Gourevitch [16] and Greenspan et al. [17]. As a
rule, the ambiguity function approach is successful for postprocessing applications

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