GPS field trips
Werner Almesberger
werner at openmoko.org
Wed Apr 16 08:45:04 CEST 2008
FYI, reposting from an internal discussion:
In response to some truly devastating GPS results Will reported from
a field trip he did recently (basically: not a single fix in two
days, under sometimes ideal conditions), I gave my GTA02 a try as
well.
The main question was how long it would take for GPS to find the
position after a cold-start and how this compared to "typical" other
devices. Here's what I found:
>From my office, downtown Buenos Aires, indoors, 21th floor, facing
north, clear field about 120 deg horizontally, 45 deg vertically,
horizon at +5 deg, foggy afternoon, 15C, 50% humidity.
All devices cold-started.
Garmin eTrex (2001):
~45s sees the first satellite
~75s gets a firm lock on it
5+ min still no position
It does get the correct time.
TomTom GO (2004-ish ?)
<20s gets time
<20s sees satellites
7-8min later, gets a fix, but comes and goes
GTA02v5 (2008):
~10s sees 1st sat (in GSV sentence *)
~20s we have the time
~180s gets position
(*) GPS sentences are explained here:
http://www.gpsinformation.org/dale/nmea.htm
>From my rooftop terrace, outdoors, 22nd floor, horizon at -5...0 deg
for 240 deg hor, +10 deg very for 70 deg, +45 deg for 50 deg (yours
truly standing in the way), i.e., almost as good as standing in the
middle of the pampa in terms of clearance. Same weather conditions.
Garmin:
15s sees satellites
50s gets first firm satellite fix
110s gets position
TomTom:
50s gets position
GTA02:
45s gets first firm satellite fix (in GSA sentence)
60s gets position
Full disclosure: the GTA02 had a slight advantage because I left the
top off, so it could also see satellites that would normally be
covered by the earpiece.
Excerpts from a sample session:
# echo 1 >/sys/devices/platform/s3c2440-i2c/i2c-adapter/i2c-0/0-0073/neo1973-pm-gps.0/pwron
# tr -s '\012' </dev/ttySAC1 | grep GPGSA
$GPGSA,A,1,,,,,,,,,,,,,99.99,99.99,99.99*30
^ no fix
$GPGSA,A,2,22,30,31,,,,,,,,,,5.26,5.17,1.00*01
^ 2D fix (the best I got indoors, outdoors got 3D right away)
I also have a few GTA01 and HXD8 laying around, but they're not in a
state fit for quick GPS testing.
- Werner
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