1. With regard to satellites entering or leaving the constellation, please cite a source or some kind of Software Design Document from Garmin. Satellites lower on the horizon cannot produce the same accuracy as satellites at higher azimuths...therefore the position will suddenly have more error. The OP reported ~0.5mi jump. You seriously dont regard this as a step change in position?
AFAIK, Garmin does not publish their internal design documents.
The general method for determining the location is to find the location and time that minimizes the sum of the
weighted squares of the differences between the physical distances to the satellites and the pseudoranges* determined from the signals. (There is no closed form solution, but there are a number of iterative methods for solution.) The weights are chosen to reflect the expected error in the pseudorange to the corresponding satellite. A simpler, but less accurate method would be to declare the weights to be 0 or 1 (ie use each satellite equally or totally ignore it).
* The pseuodrange is determined from the time the signal took to travel from the satellite and the GPS.
Sudden loss or acquisition of a satellite is only one possible cause of a step in the estimated position,
but it is not the only possible cause. A change in multipath, for instance.... (Using the weighted solution can reduce the impact of some causes of degradation.) Note that modern GPS will generally use more than the minimum of 4 satellites if possible--the additional satellites generally increase the accuracy and improve the ability of the device to deal with a an inaccurate or erroneous pseudorange.
Satellites that are low on the horizon tend to give less accurate pseudoranges than those that are higher up, but if all visible satellites are in a tight group high up you will get large errors. In general, you need a constellation that covers a large portion of the sky for the best accuracy. (See Dilution of Precision (DOP).)
Some references covering much of the above:
http://en.wikipedia.org/wiki/Global_Positioning_System
http://www.edu-observatory.org/gps/gps_accuracy.html
http://en.wikipedia.org/wiki/Receiver_autonomous_integrity_monitoring
http://www.nbmg.unr.edu/staff/pdfs/Blewitt Basics of gps.pdf
FWIW, I am not observing (with a Garmin 60CSx) the sizeable jumps reported by others. In addition, my recorded tracks generally coincide very well with the published FS tracks for the WMNF.
EDIT: I thought about the maximum size of a jump caused by adding or deleting a satellite from the active constellation (assuming 4 or more satellites before and after)--it should be 15 meters or less. A .5 mile jump indicates that something else is going on.
Doug
