Prevent divergence of SaastamoinenModel.pathDelay() at low elevation
For near-zero elevation angles, the SaastamoinenModel class can compute non-physical path delays. See discussion here.
Suggested fix is:
- introduce an arbitrary low elevation threshold in
SaastamoinenModel
- for elevations lower than this threshold, return the path delay for the threshold, not the actual value => ensures stability and continuity during the estimation process
- do not forget to change the "Field" path delay method too
- default value for the threshold: 0.05rad (seems to return an acceptable path delay)
- have a setter method for this threshold in case the user wants to override the default value
- introduce a unit test for this behavior (current tests all have elevation values above 10deg)