External GPS antenna question

Helge Hafting helge.hafting at hist.no
Mon Jul 6 14:31:06 CEST 2009

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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