Hi Attila, Bob et al >>> Generally speaking, you do want satellites as low on the horizon as possible, because they give you the most accurate x/y position solution. >>> The idea is to get a location where you can see at least 2/3 of those tracks when they are above about 20 degrees to the horizon. >>> Hint: the holdover estimator does not handle warmup as well as it should Thanks for the interesting responses. Especially interesting re importance of accurate location / using low elevation sats / sky view / warm-up. Getting the optimum accuracy partly sounds like a multi-dimensional optimisation problem to me - I might just write the code for that :) I am enjoying the discussion atm on the list immensely :) Alan