another GPS idea - speed-sensitive voicemail (Power usage)

Ian Stirling OpenMoko at
Mon Jul 30 17:59:51 CEST 2007

Visti Andresen wrote:
> While I do find most of the GPS based ideas intriguing I do have one comment.
> Power usage!
> ============
> I wonder if you have realised that the GPS consumes large quantities of current.
> (45 mW according to
> The 1200mAh battery will be drained in 1200mAh/(45mW/3.7V) = 98h ~= 4 days
> And this is for the GPS receiver alone and no power conversion losses,
> but GPS reception also requires CPU power..

This is not quite correct.

The GPS can be turned off and on.
45mW is for continuous positioning, 1s/45s will give completely fine 
positions, though not as accurate as if it was on for longer.

If the 'Resume in under 1s' from sleep entry on the table at the front 
of the above page is done, then the numbers change again.
If it takes 350mW for 1s to wake the device, and then 80mW for another 
second (CPU in slow mode and GPS on) that's 430mWs.

One position per 45s, that's some 9mW average, or around 5% of the 
battery per day.
It seems likely from the basic figures that this can be stretched to 1s 
wake every 3 minutes or so, for around 1.5%/day.
After this, you need to wake up for more than a second to keep the 
position current, so after 3 minutes, the next halving in battery use 
might end up at 4 hours or so.

This can also do bluetooth and WiFi polling of course (at a slight power 

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