Differential GPS - Understanding GPS: Principles and Applications - page 416

if the user is airborne, there is a substantial averaging effect on the multipath seen by
the airborne antenna. In fact, with kinematic GPS implementations, multipath from
the ground site is the single biggest contributor to error in the overall airborne
system.
As a further aid to resolving the ambiguities, SVs in track by the receiver,
beyond the minimum five required, can be used for cross-checking, thereby acceler-
ating the ambiguity-resolution process. With six SVs, for example, two sets of four
DDs can be generated. This provides a second floating baseline solution and a corre-
sponding least-squares residual vector that can be searched. DD measurements that
are common between the two floating baseline solutions will produce associated
integer ambiguities, which can be compared for consistency. Such redundancy usu-
ally leads to faster isolation of the proper ambiguity set.

8.4.1.8 Wide-Lane Considerations
With some receivers, it is possible to track SVs in the GPS constellation on both L1
and L2 simultaneously. With dual-frequency tracking, the P(Y) code must be used,
because the C/A code is not modulated on both L1 and L2 carrier frequencies. Use of
dual-frequency techniques permits the ionospheric path delay to be precisely deter-
mined and, in some cases, eliminated. Additionally, there are advantages to using
the P(Y) code due to its higher chipping rate—10 times that of the C/A code. The
advantages include increased pseudorange accuracies (since receiver correlation of
the signal with added precision is possible) and reduced multipath errors. These pos-
itive aspects aside, great utility in isolating the carrier-cycle integer ambiguities can
be obtained by combining the two frequencies to produce a wide-lane metric, the
wavelength of which is roughly 86 cm. This is almost five times greater than the L1
wavelength. The wide-lane wavelength results from the beat frequency of the L1 and
L2 carriers:

f wl = 1575 42 , . − 1227 6 , . = 347 82 . MHz

λ = 8625 . cm

wl

When applied to searching the uncertainties of smoothed-code DD measure-
ments, the bound of

± 1-2m on the search volume can be spanned in theory with

± at L1. This results in a hundredfold decrease in the number of

integer-ambiguity set residuals that must be computed and examined during a given
epoch. The penalty for using the wide-lane wavelength is an increased noise level
noise level ( S wl ) as shown next:

3 λ wl instead of ± 11 λ

2 2

+

S S

L 1 L 2

S = λ (8.40)

wl wl

λ λ

1 2

Current receiver technology, however, can readily cope with this increase in
noise and, assuming the magnitude of the noise level on each carrier is approxi-
mately equal, the equation reduces to 5.7 times either S L 1 or S L 2 , the L1 or L2 noise
levels, respectively. Considering the increase in noise that will tend to expand the
search volume, in practice it may be become necessary to search beyond ± 3 λ wl .