[gps-talkusers] Situation 5, GPS signal parameters

>Situation 5:
>I can stand in the same place and the GPS signal strenth changes. Some 
>days I turn on the receiver and it takes 10 minutes to acquire and the 
>next day it takes 2 minutes.

Question 5:
Can you explain why there are so many variations in GPS reception?

>Answer 5: (from Charles)
>The GPS receiver keeps an internal almanac of where the satellites 
>"should" be.  This almanac is updated when the GPS receiver is linked to 
>the satellites.
>When the unit is first turned on it starts looking where it "thinks" the 
>satellites should be based on its current almanac.  If after a while it 
>can't find these satellites in the positions it thinks they should be then 
>it switches to a mode called Cold start where it forgets its current 
>almanac and starts searching for all satellites.  This happens because as 
>far as the receiver is concerned the satellites should be in a particular 
>position at a particular time for this particular part of the country but 
>they aren't. So the receiver starts over from scratch as if you just moved 
>the receiver half way around the planet, and it rebuilds it almanac of 
>where the satellites are.
>
>Cold start or cold boot as they call it can take a while to obtain a GPS 
>fix 5-10 minutes if you are lucky when out in an open area and the 
>satellites are in a favorable geometry.
>
>Warm start or warm boot usually takes less than a minute when the gps 
>receiver has an updated almanac of where the satellites are for the last 
>lat/lon the receiver was switched off at, and you are not too far from 
>that last position when you turn on your GPS receiver.
>
>Hot start usually takes less than 20 seconds, and this occurs with some 
>receivers if you just turn off your GPS receiver and turn it back on 
>within 30 minutes or so at the same location.
>
>Therefore if you are inside and turn on your GPS receiver it looks for the 
>satellites in a particular position in the sky but can't find them because 
>of a wall or roof blocking its view, then it starts searching the entire 
>horizon for all satellites, and even if you then go outside it can take a 
>while to find the satellites.
>
>Here is a real example of what I have tried.  I turned on my gps unit 
>inside my office, then walked outside and it was taking forever to find 
>any satellites.  I turned off the gps receiver turned it back on and 
>within 30 seconds had a gps fix.



Michael G. May


CEO Sendero Group

Developers and distributors of BrailleNote GPS
Now distributing BrailleNote, VoiceNote, Miniguide, The Tissot Silen-T 
tactile watch and the ID Mate, bar code reader

MikeMay@xxxxxxxxxxxxxxxx
http://www.SenderoGroup.com

(530) 757-6800, Fax (530) 757-6830, Mobile (530) 304-0007
Sendero Group, LLC
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