Am Mi 18. März 2009 schrieb Helge Hafting: > Daniel Willmann wrote: > > On Tue, 10 Mar 2009 15:18:23 +0100 > > Helge Hafting <helge.haft...@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. cheers jOERG
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