Garmin GPS - Satellite lock times

Author

Satellite lock times

Bruce.

2007-06-13, 12:33 pm

My 650 seems to take a random amount of time to acquire (empty bar) and lock
(sold blue bar) satellites when powered on. The 650 has a 18 second boot
time and many times it has as many as 7 locks by the end of that boot cycle.
Other times it has no locks and can take up to 5 minutes to obtain the
minimum 4 locks. I eventually get 7-10 locks but it takes much longer.

In each test, I am located in the exact same location, inside my house in
the same room, the same chair, top floor. Reception wouldn't be as ideal as
being outside, but it has been the same in each test.

So why would the lock times vary so much given the same identical reception
conditions? From the satelite display, I have the same approximate number
of satellites overhead for each test.

Are there any rules, like how long the gps is powered off?

From the garmin manual, they describe different times: Wam < 1 second, cold
< 38 second, factory reset < 45 seconds. What's the difference between a
wam start and a cold start?

Bruce.

peter

2007-06-14, 4:33 am

Bruce. wrote:
> My 650 seems to take a random amount of time to acquire (empty bar) and lock
> (sold blue bar) satellites when powered on. The 650 has a 18 second boot
> time and many times it has as many as 7 locks by the end of that boot cycle.
> Other times it has no locks and can take up to 5 minutes to obtain the
> minimum 4 locks. I eventually get 7-10 locks but it takes much longer.
>
> In each test, I am located in the exact same location, inside my house in
> the same room, the same chair, top floor. Reception wouldn't be as ideal as
> being outside, but it has been the same in each test.
>
> So why would the lock times vary so much given the same identical reception
> conditions? From the satelite display, I have the same approximate number
> of satellites overhead for each test.

The solid bar means the receiver has valid ephemeris data giving the
exact location of the satellite. If the unit has been on recently and
still has valid data from before then the bar can come up solid as
soon as the signal is acquired. But if it doesn't have valid data it
needs to listen for the full data stream which takes 18 seconds and
repeats every 30 seconds. If there's any signal interruption during
that time it may need to listen for another cycle of data before it
gives you a solid bar (that's the main reason why lock-on takes so
much longer when moving - there tend to be many brief signal
interruptions from trees/poles/underpasses/buildings). Even when
stationary in your house there could be some temporary interruptions
from tree branches moving in the wind.
>
> Are there any rules, like how long the gps is powered off?

Usually the ephemeris data is considered to be good for about 2 hours,
but I'm not sure of the exact time used by the Nuvi.
>
> From the garmin manual, they describe different times: Wam < 1 second, cold
> < 38 second, factory reset < 45 seconds. What's the difference between a
> wam start and a cold start?
Warm: valid ephemeris data, not too far from last previous determined
position.
Cold: invalid ephemeris but does have less accurate almanac data and
proper time of day, not too far from last position.
Reset: invalid almanac, lost clock data, or moved hundreds of miles
from previous position - basically has to start from scratch listening
for all satellites and listening to the data messages.

Grumpy

2007-06-14, 4:33 am

On Wed, 13 Jun 2007 22:17:26 -0700, peter wrote:

> Bruce. wrote:
[color=darkred]
[color=darkred]
> The solid bar means the receiver has valid ephemeris data giving the
> exact location of the satellite. If the unit has been on recently and
> still has valid data from before then the bar can come up solid as
> soon as the signal is acquired. But if it doesn't have valid data it
> needs to listen for the full data stream which takes 18 seconds and
> repeats every 30 seconds. If there's any signal interruption during
> that time it may need to listen for another cycle of data before it
> gives you a solid bar (that's the main reason why lock-on takes so
> much longer when moving - there tend to be many brief signal
> interruptions from trees/poles/underpasses/buildings). Even when
> stationary in your house there could be some temporary interruptions
> from tree branches moving in the wind.
>
<<deleted>>

Thanks for such a detailed and interesting answer.. This was a point of
interest to me as well ... I guess it was to many other users also..

--
Cheers & Beers
Grumpy

Bruce.

2007-06-14, 10:33 pm

"peter" <prathman@comcast.net> wrote in message
news:1181798246.261634.157770@n15g2000prd.googlegroups.com...
> The solid bar means the receiver has valid ephemeris data giving the
> exact location of the satellite. If the unit has been on recently and
> still has valid data from before then the bar can come up solid as
> soon as the signal is acquired. But if it doesn't have valid data it
> needs to listen for the full data stream which takes 18 seconds and
> repeats every 30 seconds. If there's any signal interruption during
> that time it may need to listen for another cycle of data before it
> gives you a solid bar (that's the main reason why lock-on takes so
> much longer when moving - there tend to be many brief signal
> interruptions from trees/poles/underpasses/buildings). Even when
> stationary in your house there could be some temporary interruptions
> from tree branches moving in the wind.

> Usually the ephemeris data is considered to be good for about 2 hours,
> but I'm not sure of the exact time used by the Nuvi.

> Warm: valid ephemeris data, not too far from last previous determined
> position.
> Cold: invalid ephemeris but does have less accurate almanac data and
> proper time of day, not too far from last position.
> Reset: invalid almanac, lost clock data, or moved hundreds of miles
> from previous position - basically has to start from scratch listening
> for all satellites and listening to the data messages.

What an excellent reply! Exactly the details I was looking for. Thank you
so much for sharing that!

Bruce.

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