[OSM-talk] All radio begins with the antenna

[SteveC]

* @ 28/09/06 09:51:13 PM lars at aronsson.se wrote:
> 
> My new GPS receiver is a PDA, Mio P550, based on the new chip 
> SiRFstar III.  It's really a lot sharper than my old one, at least 
> as long as I measure latitude and longitude.  But the improvement 
> is not so evident when it comes to altitude.  In my car where the 
> GPS receiver is close to the windshield, I know reception is 
> always worse when I'm heading north, since the roof shades signals 
> from the south where many satellites are(*) if you're driving at 
> northern latitudes.  But this became very clear when I was driving 
> home from the Göteborg Book Fair this Saturday and turned north on 
> E4 after Huskvarna (57.93 N, 14.33 E).  The steep hills go up and 
> down, but my altitude reading varied almost independent of this. 
> Later I've been able to verify that lat-long are really sharp.
> 
> Today I went to an open place and measured the altitude with both 
> the old and new GPS receivers, while sitting still for almost two 
> hours.  My results are shown in a diagram in my OSM diary, 
> http://wiki.openstreetmap.org/index.php/User:LA2
> 
> My unexpected finding was how great a difference was made just by 
> positioning the device (and its built-in antenna).  Apparently, 
> the PDA must be held upright and should not lay flat on a table.

I sent similar graphs to the list years ago

http://www.fractalus.com/steve/stuff/gpstracks/index.html

It comes, I think, from the geometry. You can imagine the spehere of the
eath and the satellites flattened out to two planes. The range of values
you get x & y (lat, lon) is much greater than the range of values you
get in z (because they all fly at the same altitude).

Thats the analogy I use, anyway.

> 
> (*) The web that GPS satellites spin around the globe has large 
> holes over both poles.  GPS satellites never go higher than 55 N.  
> I covered this in my diary of February 27, 2006, 
> http://wiki.openstreetmap.org/index.php/User:LA2/Diary_for_Q1_2006 
> If you are in Belfast, Copenhagen or Moscow (55 N), you 
> occasionally see satellites at zenith and close to the northern 
> horizon (over the Pacific), but there is a dark hole between 
> there.  I live at 58.4 N, so I never see GPS satellites at zenith.
> 
> 
> -- 
>   Lars Aronsson (lars at aronsson.se)
>   Aronsson Datateknik - http://aronsson.se
> 
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have fun,

SteveC steve at asklater.com http://www.asklater.com/steve/