[time-nuts] GPS Modules Indoors

Bob Camp lists at rtty.us
Tue Nov 20 18:55:35 UTC 2012


Hi

Ten seconds a month is "wrist watch" accuracy at constant temperature. For $3 Maxim will sell you a +/- 2 ppm real time clock.  That will hold your 1 second for 500,000 sec (5.787 days).  A cheap cell phone style TCXO at $1 or so should hold < 1 ppm over a modest range. That will get you out to > 10 days. 

A lot depends on how stable and repeatable your environment is. Most indoor areas cycle up and down, but they repeat the cycle. What happens this week is a lot like what happened a week ago. If you can take a time reading every few days, you likely can do some smarts to extend the holdover quite a bit. With a modern low power CPU, the smarts will pull a lot less current than your real time clock or your TCXO. 

Yup, this is all basement engineering. If somebody's life depends on it, maybe not such a good idea.

Bob

On Nov 20, 2012, at 10:46 AM, Chris Albertson <albertson.chris at gmail.com> wrote:

> 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
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