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Satellite Acquisition


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I have noticed that at times I can power up my Garmin Legend C and get quick acquisition and other times from the the same location, I can power up and it takes several minutes. Why is this? I can understand being 100 miles from the last point I powered up but why the differences from my front yard?

 

(yes, a newbie question)

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I'll take a stab at this - I'm not a GPSr tech guru, but I did stay at a Holiday Inn Express not too long ago. <_<

 

The GPS satellites are constantly moving, so the number of satellites and their relative positions at one time won't be the same as some other time. Perhaps on one day you power up your GPSr and maybe you've got an unobstructed view of half a dozen satellites evenly spread across the sky, and your GPSr slurps up the signals and gets a fix right away. Another day you fire up the GPSr, and you've only got a clear view of a couple of sats, weak/intermittent signals from a few more that are closer to the horizon, and still others where the signal is either too weak or too jumbled by obstructions for the GPSr to make heads or tails of it - and it takes longer for the GPSr to come up with a fix. You can get a feel for the number and relative position of the GPS sats by looking at the Satellite display on your GPSr - the closer a satellite is to the center, the higher it is overhead (a satellite in the center is directly overhead), and the further out a satellite is the closer it is to the horizon. (On the Garmins I've owned, the outer ring represents the horizon, the middle ring is 45 degrees above the horizon, and the middle is overhead.) By looking at the locations of the satellites on the display and comparing them to nearby obstructions (buildings, tall hills, etc) you can usually get a decent feel for why certain satellites are coming in weak or not coming in at all. (Maybe you see that #8 is off to the west and low on the horizon, and there's a big apartment complex just west of you - probably going to have some problems getting a good signal from #8 until it climbs high enough to clear the apartments. And maybe #16 is nearly overhead and there's no obstructions above you, so you'd expect to see a strong signal from #16.)

 

Another possible cause might be storing the GPSr for (relatively) long amounts of time without batteries. I've noticed this with my 76CS - since it's no longer my primary GPSr, it may sit on the shelf for a week or three without batteries, and when I slap in some fresh batteries its clock will initially show a date from days or weeks ago. When it start searching for sats, it's scanning for the ones that it thinks should be visible based on the incorrect date that it thinks is the current time. It might get lucky and find that a few of the satellites actually do happen to be visible, update the clock from the information it receives from the sats, and update its predictions on which satellites really should be visible based on the new time, and proceed to get a lock. Or it might not get lucky, not see any of the expected satellites (from days/weeks ago), give up the search, and ask me if maybe I'm indoors right now since it can't seem to find what it's looking for. In that case telling the GPSr to do a New Location/Automatic can help - with an automatic location update, the GPSr will just start scanning for any satellites it can find, rather than only looking for the ones it expected to see - after it starts finding the real satellites it'll get the clock straightened out and get a position fix. Doesn't happen over short periods without batteries though - I've tried taking the batteries out for ten minutes or so and when I put them back in and fire the GPSr up it's still got the correct time.

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