hans wrote:

Why do you think that GPS has a bad coverage at the poles? The
inclination and orbital altitude of Galileo is a little bit higher than
for GPS, so there is potential for a small performance increases.

For some reason I thought the constellation was in low orbits and so had
limited polar visibility. I was wrong there.

It turns out GPS uses a 25,000 km orbit, inclined at 55 degrees to the
equator. The orbits reach 55 degrees north and south, which gives plenty
of polar visibility. There will still be at least 4 satellites visible
at any time. The satellites are closer to the horizon in polar regions
so positional accuracy isn't affected, though I suppose shielding by
surface features could be more of a problem than it is at lower latitudes.

However, height accuracy must deteriorate close to the pole because the
satellites are never overhead: you get max vertical accuracy with 3 or
more satellites near the horizon and one overhead.

I would expect that current GPS receivers have not implemented the
Galileo ICD.

I'm certain you're right, especially about the units with the original
Garmin 12 channel receivers (12XL, II+, III+, GPS35 etc), because apart
from anything else its hard to imagine that these could cope with the
different time codes used by GPS and Galileo. The time conversion
algorithms must have been published long after these receivers were
designed.

Thanks for prodding me into doing a quick search for the stuff I didn't
know.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |