[OSM-talk] All radio begins with the antenna

[Lars Aronsson]

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.

(*) 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