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

Visti Andresen talpa at
Mon Jul 30 18:54:20 CEST 2007

On Mon, 30 Jul 2007 16:59:51 +0100
Ian Stirling <OpenMoko at> wrote:

> 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 
> cost)

But does a GPS not require large amounts of time to get a "new" fix?

Hot startup 12s, average
Warm startup 38s, average
Cold startup 60s, average 
(random data from

I must admit I don't know when one needs to reacquire the fix...

Of course even a 38s/60s power consumption reduction would be
quite noticeable, even if one has to switch to continues operation 
when moving (in order to notify me that I have to pickup groceries)

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