thoughts on A-GPS offline
joerg at openmoko.org
Wed Mar 18 19:05:51 CET 2009
Am Mi 18. März 2009 schrieb Helge Hafting:
> Daniel Willmann wrote:
> > On Tue, 10 Mar 2009 15:18:23 +0100
> > Helge Hafting <helge.hafting at hist.no> wrote:
> >> It was just to be safe. The documentation states that you might not
> >> get a fix _at all_ if either position or time is outside the claimed
> >> accuracy. Now, maybe it works with 3km anyway after the fixes that
> >> prevents the chip-crashing exception. I happen to live about 6km away
> >> from where I work, so 9km was a nice safe value. The default is
> >> 300km, and "100km allows a more optimistic startup." Perhaps such
> >> rough estimates is all that is needed, if it is only used to figure
> >> which satellites that can be seen.
> > If you don't mind testing please try changing pacc to 100km and see if
> > it affects TTFF adversely in your case. If not we could just use that
> > as a default.
> Shouldn't be too hard to test.
> I think I know one side of having accurate pacc:
> The first fix can happen with only two satellites. I have seen this
> happen several times. It surprised me at first, but it makes sense. With
> two satellites (and a reasonable clock), you get a big circle of
> possible positions. But then there is the data from the "approximate
> position". It puts you at some height above sea level. The big circle
> intersects the earth surface at some angle, so with height, we now have
> two possible spots instead of a big circle. Usually, only one spot will
> be close to the approximate position, so that is where you are.
> That is an "optimistic startup scenario". A too spread out pacc means
> both possible spots are within pacc range, and the FR will have to wait
> for a third satellite to break the tie.
> If you travel a long way and still report the old position with a fake
> precision pacc, then you might be close to the other of the two possible
> satellite-based positions. You could then get a fake fix on the wrong
> spot. As you and/or the satellites move, the wrong spot will move around
> in strange ways at strange speeds. When more satellites show up, the
> device might get really confused if it keeps trusting the approximate
> position. Perhaps even rejecting them as "reflected signals" for a
> while. Of course, only the manufacturer will know the exact details of
> what might happen.
> Helge Hafting
Wow, I had to read your posting twice to get how brilliant this analysis is.
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