Im glad if I can help. My knowledge is mostly gained from having an interest in what I do and wanting to find out about it, rather than just pushing buttons.
My understanding is that gps units with lots of channels is just help to get a quicker lock, not the position. I would preferably leave a gps unit on for 30min before relying on the position especially if it has not bean used for some time.
“If satellites on the horizon give better vertical resolution, will overhead satellites give better horizontal resolution? (I'm assuming my receivers will see more overhead satellites.)”
Yes I will give you a better position.
Quote from the gps page on wikipedia,
”Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth's atmosphere, especially the ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the path through the atmosphere is longer”
With the kind of units I normally use, you can enter a Elevation mask, around 8-10deg, is from memory what they are normally set to.
“Is there any way to tell receivers to ignore satellites? My application will be outdoor and so the receivers should be able to register many satellites, if I can tell them to only use the ones they can both see then my accuracy should increase. Shouldn't it?”
From memory without any manuals, im not sure if there is a way to ignore satellites on gps units i use. The only time I can think of when I have been able to disable sv, is when the raw gps is outputted from the gps card on a serial port to a computer and the position calculation is done on the pc(this is with Proprietary software, and the one I know of is very expensive)
As for the errors in accuracy from comparing the to gps positions if they see a different number of sv, I don’t know how much this will affect it, but am very interested to find out.
I take it your application needs to be fairly low cost? Because one of the easiest improvements would be to use a L1/L2 frequency gps receiver(and future proofing with L5) but these are lot more than just a standard L1 gps receiver, thousands of USD$ instead of hundreds.