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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