[time-nuts] GPS Modules Indoors

[Chris Albertson]

I don't think GPS is ever so in accurate as to have a 1 second error.  If
it has a solution at all it is going to be at least within a millisecond.

The way "everyone" handles "holdover" (Holdover is timekeeping with GPS is
not available) is to keep a local clock driven off a stable oscillator.
Then when you do have GPS you adjust the clock.  Then when GPS goes away
you "free wheel".

With your very loose one second requirements you can use a rather
inexpensive clock, perhaps just a TCXO.   The question is how long of a
hold over period must you be able to handle.   If GPS will is unavailable
for a month then you will need a VERY good clock but if you will only loose
GPS for a few hours at most it can be cheaper.

Active antenna are not a perfect solution.  What the GPS needs is good
signal to noise and the amplifier amplifies noise as well as signal.  It
can help in some cases and is worth doing if you have the space and power
budget.

So to design this step one is to select an oscillator that has the required
stability for "one second error durring the worst case holdover period" thn
build a clock based on that.   Next design a way to re-set the clock based
on GPS.


On Tue, Nov 20, 2012 at 3:33 AM, Tom Harris <celephicus at gmail.com> wrote:

> Greetings
>
> I know that the accuracy of GPS degrades indoors. However, suppose
> that I just want to turn a GPS module on, acquire the current time
> accurate to a second, and then turn it off. I can get a good deal on
> the U-Blox LEA-5H modules (same as used on Arduino shields I think),
> which have a high sensitivity, and I can use an active antenna if
> needed.
>
> Am I wasting my time. Sorry for not requiring the time more accurate
> than a second, but that's all the clients require.
>
> --
>
> Tom Harris <celephicus at gmail.com>
>
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-- 

Chris Albertson
Redondo Beach, California