[gps-talkusers] Re: Situation 5, GPS signal parameters

I will add that the EarthMate claims an average cold start time of 45
seconds as they do not maintain an almanac in memory as Charles points
out that most receivers do.

Another factor to be considered on a day-to-day wide variation is that
A)  the satellites are not in the same place each day at the same time,
so you may see more or fewer at the time you try for a fix; B)
satellites do go offline for a variety of reasons, so at a given moment
you may have access to fewer than normal number of satellites.

The government agency responsible for GPS operation does publish a
schedule of planned outages and unplanned past outages, although making
use of it (trying to see how/when it effects you) is pretty difficult.

Lastly, some source of nearby RFI may cause you to have more difficulty
than normal in finding satellites; this could be a badly made TV set or
other electronic device that happens to be operating nearby, or it could
be that ham radio or CB radio operator, or someone's WIFI access point.

Hth,

Chip
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> -----Original Message-----
> From: gps-talkusers-bounce@xxxxxxxxxxxxx=20
> [mailto:gps-talkusers-bounce@xxxxxxxxxxxxx] On Behalf Of Michael May
> Sent: Thursday, October 14, 2004 10:27 PM
> To: GPS-talkusers-freelists.org
> Subject: [gps-talkusers] Situation 5, GPS signal parameters
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> >Situation 5:
> >I can stand in the same place and the GPS signal strenth=20
> changes. Some=20
> >days I turn on the receiver and it takes 10 minutes to=20
> acquire and the=20
> >next day it takes 2 minutes.
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> Question 5:
> Can you explain why there are so many variations in GPS reception?
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> >Answer 5: (from Charles)
> >The GPS receiver keeps an internal almanac of where the satellites=20
> >"should" be.  This almanac is updated when the GPS receiver=20
> is linked=20
> >to the satellites.
> >When the unit is first turned on it starts looking where it "thinks"=20
> >the satellites should be based on its current almanac.  If after a=20
> >while it can't find these satellites in the positions it thinks they=20
> >should be then it switches to a mode called Cold start where=20
> it forgets=20
> >its current almanac and starts searching for all satellites.  This=20
> >happens because as far as the receiver is concerned the satellites=20
> >should be in a particular position at a particular time for this=20
> >particular part of the country but they aren't. So the=20
> receiver starts=20
> >over from scratch as if you just moved the receiver half way=20
> around the=20
> >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=20
> >GPS fix 5-10 minutes if you are lucky when out in an open=20
> area and the=20
> >satellites are in a favorable geometry.
> >
> >Warm start or warm boot usually takes less than a minute=20
> when the gps=20
> >receiver has an updated almanac of where the satellites are for the=20
> >last lat/lon the receiver was switched off at, and you are=20
> not too far=20
> >from that last position when you turn on your GPS receiver.
> >
> >Hot start usually takes less than 20 seconds, and this=20
> occurs with some=20
> >receivers if you just turn off your GPS receiver and turn it back on=20
> >within 30 minutes or so at the same location.
> >
> >Therefore if you are inside and turn on your GPS receiver it=20
> looks for=20
> >the satellites in a particular position in the sky but can't=20
> find them=20
> >because of a wall or roof blocking its view, then it starts=20
> searching=20
> >the entire horizon for all satellites, and even if you then=20
> go outside=20
> >it can take a while to find the satellites.
> >
> >Here is a real example of what I have tried.  I turned on my=20
> gps unit=20
> >inside my office, then walked outside and it was taking=20
> forever to find=20
> >any satellites.  I turned off the gps receiver turned it back on and=20
> >within 30 seconds had a gps fix.
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> Michael G. May
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> CEO Sendero Group
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> Developers and distributors of BrailleNote GPS Now=20
> distributing BrailleNote, VoiceNote, Miniguide, The Tissot=20
> Silen-T tactile watch and the ID Mate, bar code reader
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> MikeMay@xxxxxxxxxxxxxxxx
> http://www.SenderoGroup.com
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> (530) 757-6800, Fax (530) 757-6830, Mobile (530) 304-0007=20
> Sendero Group, LLC
> 1118 Maple Lane, Davis, CA 95616-1723, USA
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> Latitude, 38 33 9.239 North
> Longitude, 121 45 40.145 West
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