thoughts on A-GPS offline

Helge Hafting helge.hafting at
Wed Mar 18 09:15:41 CET 2009

Daniel Willmann wrote:
> On Tue, 10 Mar 2009 15:18:23 +0100
> Helge Hafting <helge.hafting at> 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

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