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why does it take so long for gps to find sats?


sacred6
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My eXplorist gets a lock in about 1/2 minute. GPSr's can't get a lock in some cars that have a metallic windshield coating

 

Does your GPSr take long when you stand outdoors?

 

Some things you might try:

1. Update the firmware to the latest.

2. Do a clear all memory.

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Is it a long time (weeks) between uses? If so the almanac goes stale and it needs to download the almanacs before it can download the individual satellite information. Only then can it start navigating. If your moving during this time it can take longer.

 

Jim

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There's some misinformation here.

 

Two parts of the nav data will tell the receiver where satellites are.

 

The almanac takes 12.5 minutes to load completely. Your receiver get it from one satellite, but it tells it about all the satellites too so it knows where to look for them. This can be months old and still be good enough to get you started. If you don't have the almanac, the receiver needs to start looking for everything and it takes a little longer. It can also start working from a partial almanac even if it gets holes in the data from breaking lock. But you can't really calculate a position using just the almanac.

 

The ephemeris takes 18-30 seconds to load. You need to get this from each satellite that you start tracking. This is what the receiver uses to navigate and it's only good for a couple of hours. If breaking lock puts holes in the data before it's done, you'll need to wait until the next 30 second repeat to use that satellite.

Edited by GPSlug
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Actually if you want to get technical, after receiving the time signal from one satellite, the GPS receiver can set its internal clock. It then listens for any satellite it can "hear" to send out almanac or ephemeris data. Once the almanac is received the receiver can then listen specifically for the satellites that are near the one satellite it can presently hear at the time. If it hears them, it knows the almanac is relatively current. Finally, the GPS receiver gets the ephemeris data about where the satellites are located in space and with this information and the time signals from the satellites it can calculate its location and present the first "fix". The process of this cold fix can take as much as 20 minutes, but is usually done in as little as 3-6 minutes. Like I mentioned earlier, if you move the receiver during this first fix, the time may be extended significantly. The problem with movement is that if the GPS loses contact with a satellite in the middle of receiving an ephemeris or almanac string of data it has to wait until the next full cycle of the signals before it gets a new chance.

 

In any case, the point is if the reciever has been stationary for a significant period of time (I would leave it sitting for 30 minutes) and the receiver is still having issues identifying locations, it would be safe to say that the issue is with the unit. Also, the OP didn't state the terrain he is searching in. Heavy tree cover and tall city buildings, as well as overhead electircal stansions, will all cause receiving isseues known as multipath distortion.

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