Section 6.2 · Further exercises · 8 of 13
Exercise 6.2.8: The ionospheric range error of GPS
About the further exercises of Section 6.2
Thirteen further exercises in the spirit of the problems, the worked examples and the home exercises, graded from easy to harder within each group. Exercises 6.2.1–6.2.3 are about lossy media and the loss tangent, Exercises 6.2.4–6.2.5 about good conductors and the skin effect, Exercises 6.2.6–6.2.8 about plasmas and the group velocity, and Exercises 6.2.9–6.2.13 come from the exercise lecture and mix all three. They use only the equations listed at the head of this subsection, together with and , .
The ionospheric range error of GPS
#A GPS signal crosses the whole ionosphere, whose electron density varies along the path, with everywhere. (a) Show that the envelope arrives later than in vacuum by the equivalent extra distance , where is the total electron content, and evaluate . (b) For a daytime find and the delay at (L1) and (L2). (c) A dual-frequency receiver finds the L2 pseudorange longer than the L1 one by . Find the TEC, and the combination of the two measured ranges that is free of the ionosphere. (d) By how much does the carrier phase path change?
Expand and to first order in : both errors are proportional to the line integral of and to , which is what makes the two-frequency correction possible.
, so for , and
gives . The true range obeys , and eliminating TEC,
: the phase path is shorter by the same , at L1.
Even a strong gives at L1, so the first-order expansion is excellent, and . The ionosphere is the largest error source of single-frequency GPS (metres to tens of metres). Because it is dispersive, two frequencies remove it almost completely, at the price of amplifying the receiver noise by about three.
cf. Problem 6.2.3