USING THE L3 IONOSPHERE-FREE OBSERVABLE |
To process ionosphere-corrected double- or triple-differenced data requires
very few algorithm changes as the L1 and L2 data are linearly combined into
one "pseudo-measurement". Once combined, the data reduction continues
as before, as for the case of the ambiguity-free solution using single-frequency
data. However, the estimated results from the L3 combination are real-valued
ambiguity parameters 
n3 that are a combination
of n1 and n2 (section 6.4.2).
The wavelength of the n3 ambiguity is of
the order of 6mm, hence this makes the cycle
ambiguity resolution
of the L3 signal more complicated than the separate
resolution of the L1
or L2 ambiguities because there are several
combinations of integer n1
and n2
ambiguities
that produce almost the same n3
ambiguity. An
inspection of the Table
in
section 8.4.4 shows how several combinations of even small n1
and n2
cycles
(c1 and c2) will produce values of n3
cycles
(c3) that are near unity.
Hence it is often difficult to reliably decorrelate the integer ambiguities on L1 and L2 using the L3 observable alone. The use of L3 is usually justified so as to provide a better quality ambiguity-free solution than that which would have been obtained from single-frequency observations, or by processing L1 and L2 data separately. Commercial software capable of dual-frequency data processing usually allows, in addition to separate L1 and L2 solutions, for an L3 option to be used for medium and long baselines (> 20-30km).
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© Chris Rizos, SNAP-UNSW, 1999