External GPS antenna question
Ruben Leote Mendes
ruben at nocturno.org
Mon Jul 6 23:12:48 CEST 2009
BTW, anyone have any info on the radiation pattern of the internal antenna?
Em Seg, Julho 6, 2009 13:31, Helge Hafting escreveu:
> Tomasz Suchan wrote:
>> Hi Armin,
>>> I use the atenna for GPS Tracking during biking and car tours.
>>> As the FR is always running (no sleep) and with activated GPS
>>> module - the battery last about 4h - then it's empty.
>> Is accuracy much better with external antenna?
> If the external antenna can be placed in a better location than the
> internal one - or if it gets better signal strength.
> An external antenna on the car roof may see more satellites than the
> FR sitting on the dashboard, because that metal roof block radio
> Now, the FR will see many satellites even inside a car, but those
> signals cannot penetrate directly through the roof. Some satellites will
> be seen directly through windows. Signals from the rest may bounce off
> the hood and come in through windows, or diffract (bend) around the roof
> edge. Such redirected signals are weaker, and they give greater position
> error. The latter because they really give the position where your FR
> would have been, _if_ the signal had not been diffracted or reflected.
> The GPS receiver is smart, and will discard data from satellites that
> seem to disagree with the rest. But there are two problems with this:
> * An extra satellite that gets discarded no longer helps improving
> * If two groups of satellites seems to disagree on position, then the
> receiver could lock onto the wrong group for a while.
> An external antenna see all the satellites directly, and
> don't suffer such problems. Unless you are near tall buildings or
> mountainsides, which cause the same kind of problems.
> So an external antenna is great if you have a roof above you.
> Such as in a car or boat. The problems above don't happen on a bike.
> Still, an external antenna might help, it may be bigger/better than the
> internal antenna and get more from a weak signal. For example, if you
> move around in heavy rain in a forest.
> Helge Hafting
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> community at lists.openmoko.org
Rúben Leote Mendes -- ruben at netureza.pt
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