PPRuNe Forums - View Single Post - what constitutes the nav msg of GPS?
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Old 21st December 2015 | 16:01
  #10 (permalink)  
msbbarratt
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Of course the vast majority of GPS receivers (i.e. mobile phones) these days don't get the ephemeris or almanac from the satellites themselves. They ask their friendly local cell tower for it, a set up called Assisted GPS (AGPS).

In a mobile this is very useful - it can take up to 20 minutes to receive the whole data set via the GPS signal itself. It's a very slow data rate. Getting it immediately from the cell stations means getting a fix very quickly, as the receiver can be primed on exactly the right channels at pretty much the right point in the PRN code for the satellites that are known to be in view. Saves a lot of power at the same too.

Presumably a proper GPS receiver in an avionics suite doesn't rely on AGPS.

AGPS is quite interesting. Having got a GPS fix before embarking on a long haul flight I was impressed when the mobile (in flight mode) was perfectly capable of getting both a GPS and GLONASS fix within a few seconds somewhere well north of Russia. Clearly the cached AGPS data is fairly global (I was a long, long way from the take off point by then) and it was still valid. Modern electronics!
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