[clug] GPS and temperature

[Robert Edwards]

David Tulloh wrote:
> The GPS chips use a crystal of a known frequency, this is then scaled to 
> get the frequency that they scan for the GPS signals.  The more accurate 
> the frequency the less the GPS chip has to scan to find the satellites.  
> Once it's found the satellites it adjusts it's knowledge of the crystal 
> frequency based on the error it found and uses this calibrated value to 
> track the satellites while it's running.
> 
> The issue with all of this is that you assume that the crystal provides 
> a perfect set frequency.  In reality they don't, the frequency they 
> provide changes with the age of the crystal and the temperature.  So 
> they try to compensate for the temperature drift and allow for a degree 
> of frequency movement.  There are limits to how much they can do, if the 
> temperature changes too rapidly or the crystal is flawed the receiver 
> will lose it's tracking.
> 
> You can try this using one of those freeze spray cans.  Monitor the 
> number of satellites that the GPS device is tracking and then spray a 
> little of the freeze stuff onto the crystal, or just spray a whole lot 
> into the GPS box.  You should see it lose all the satellites, gradually 
> find the first one or two and then rapidly reacquire all the rest.
> 
I _definitely_ want to see you (or someone else) give a demo of this at
the next CLUG meeting!!! I'll even bring the freeze spray!

Bob Edwards.

> 
> David
> 
> Mike Carden wrote:
>> During GPS discussions this evening I made a comment about temperature
>> and oscillator stability in GPS receivers.
>>
>> Now, I know my etrex cares about temperature. If I put it into service
>> mode via UP + Page + power on, I get a look at all sorts of internal
>> bits including the output of its internal temp sensor.
>>
>> So I had a bit of a look online and pretty well every GPS chipset out
>> there is keen to boast about how good its temperature compensation is.
>> Seems it's used to make the internal clock more accurate and speed up
>> the process of acquiring an initial fix.
>>
>> So I wasn't entirely dreaming. :-)
>>
>