[OSM-talk] Choice of GPS for bicycle use in London

[Nick Hill]

There has been a lot of talk about the sirf star III chipset being so 
much more useful. They can after all pick up GPS signals in situations 
where other chipsets will not receive a signal.

This is not always a good thing. If you are in a concrete canyon with a 
sirfstar III chipset, the fact that it can pick up a GPS signal shows 
the signals it is using are bouncing off buildings. It is receiving 
multipath signals. Such multipath signals are bad for accuracy.

I say it is better to receive no signal rather than receive multipath 
(unless you are stuck down a mountain cravass and need a rough fix).

IMO, a receiver which is only sensitive enough to receive a line of 
sight signal has advantages in urban areas. ie for OSM.

I have seen tracks go way off course in my area, which I beleieve to 
have been captured with a SirfStar III. My Geko, OTOH, rarely lays 
off-course tracks.

The on-screen stated accuracy of my Geko is normally between 15ft-23ft. 
Actual lat/lon accuracy is better than that. It is nearly always clear 
which side of a small urban road I am on, even on different days, so 
long as I keep it still when I turn it on, until it gets a good fix. 
Don't bother turning the Geko on whilst moving.

My tests show Etrex Yellow=Geko 201.

Difference is that Etrex is bigger, has bigger batteries so lasts longer 
on a charge.

I highly recommend the Geko. It is small, light, tends to lay accurate 
GPX traces, and is not sensitive enough to receive most multipath. The 
down side is battery life. In normal mode, with 900mAh Uniross 
rechargeables, I get 9.5Hr. In battery save mode (some loss of accuracy) 
I expect 14 hr.





Erik Johansson wrote:
> On 5/22/06, Christian van den Bosch <cjb at cjb.ie> wrote:
> 
>> Emil Vaughan wrote:
>> > Would a "usb mouse" type gps receiver
>> > connected to my zaurus be a more accurate than a geko? And would it
>> > work ok under linux?
>>
>> I haven't tested it yet, but I believe that a Sirf Star III should be
>> /much/ more accurate than a Garmin log (because it won't lie,
> 
> 
> Actually I'm not so sure if  my GPS puck^W mouse is lying. It has just
> happend once:
> 
> http://www.openstreetmap.org/edit.html?lat=59.33833563680978&lon=18.08679437637329&zoom=16 
> 
> 
> There you have a diagonal line that starts between "Kommendörsgatan"
> and "Karlavägen", and then drifts up north east. The only explanations
> I have is that it's extrapolating.
> 
> This is done with a sirf III and a sirf II the sirf II just lost it's
> GPS lock. And the SIRF III PDOP is at 50 during that line.
> 
> /emj
> 
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