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Originally Posted by Doc007
Thanks for the interesting read on GPS fixing. Maybe you can help me answer my questions based on your tutorial...
Why not have a program calculate the location of the satellites based on user input of a cross street (if you are on a road) or a time + city/town (if you are hiking)?
I figure since these satellites orbit the earth at calculated speed along with the earths rotation. Why not have a program which can make a temporary almanac/offline almanac until a true/real almanac can be downloaded from the primary and/or secondary satellites? Wouldn't something like this reduce the amount of trial and error? Which would result in a reduced acquisition time?
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You're right. Actually, some routing software and GPSr allow you to input a "known" latitude and longitude (or sometimes a city name) from which it starts the process of searching for satellites based on what's called an ephemeris (a fancy name for a table that lists the projected locations of all of the GPS). My Garmin Nuvi 660 does this and some of the standalone PPC software does as well.
Back in the "early" days of consumer GPSr use this was fairly common as the "time to acquisition" of these devices was pretty slow -- 10-20 minutes wasn't uncommon. Current devices can usually obtain a fix in far less time and, probably as a result, don't offer the option to input a "known" location to serve as a starting point.