another GPS idea - speed-sensitive voicemail (Power usage)
talpa at galnet.dk
Mon Jul 30 18:54:20 CEST 2007
On Mon, 30 Jul 2007 16:59:51 +0100
Ian Stirling <OpenMoko at mauve.plus.com> 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 http://wiki.openmoko.org/wiki/Neo1973_Power_Management#Approximate_power_draw_of_various_subsystems)
> > 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
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 http://www.alibaba.com/catalog/11694061/Bluetooth_GPS_Receiver.html)
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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