GEOMETRY-FREE IONOSPHERE COMBINATION |
This is also sometimes referred to as the "L4" combination.
It is possible to isolate the ionospheric component using eqns
(6.4-3). Both measurements in these equations are converted to metric
units (scaling eqn (6.4-3a) by 
1 = c/f1, and eqn
(6.4-3b) by 2 = c/f2), and then differenced
to yield:
 |
(6.4-14a) |
or, in terms of only the L1 ionospheric delay (using eqn (6.4-2)):
 |
(6.4-14b) |
Note that the first two terms are constants, hence any variation
in the L4 combination represents entirely the variation in L1 ionosphere
effect (but it is only 0.646 the effect on the L1 observation) unless
there is a cycle slip on L1 (that is, n1 is not a constant) or
L2 (n2 is not a constant). The L4 signature is therefore a smoothly
varying L4 signature derived from L1 and L2 phase data. The ionospheric
delay changes slowly and any sudden "jumps" in this signature
could be interpreted as cycle slips on L1 and/or L2. Eqn (6.4-14) contains
no receiver-satellite range term, hence the reason for it being referred
to as a "geometry-free" quantity. The noise on the L4 combination
is 
times that of the L1 noise, in metric units -- if the L1 and L2
measurement noises when expressed in metric units are equal (Table
in section 6.4.6).
In the case of pseudo-range observations on L1 and L2, it is possible to write the L4 combination directly, and using eqns (6.4-2) and (6.4-10):
 |
(6.4-15) |
Eqn (6.4-15) implies that the ionospheric delay can be measured directly with two P code pseudo-range observations. This is not entirely correct because what is missing from the observation model is the data "noise" and multipath, both of which can be serious disturbances at the decimetre level or greater. So large in fact that they may "swamp" the ionospheric signal (which is already muted by the 0.646 factor). If a large enough data series is available, then the noise and multipath can be averaged out by "smoothing" the P(L4) series, or by curve-fitting a polynomial to the data series.
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© Chris Rizos, SNAP-UNSW, 1999