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Why do GPSs go crazy sitting still? — Parallax Forums

Why do GPSs go crazy sitting still?

RagtopRagtop Posts: 406
edited 2010-09-29 19:46 in General Discussion
I was wondering if anybody knew the reason why GPS readings seem to get more erratic sitting still. I have had quite a few and they all seem to have this behavior accept for one that had SA?, it didn't display new readings if you were going below a certain speed which doesn't work well with geocaching.

I was testing a gps datalogger that I put together to see how long the batteries would last.

Sitting on my desk for six and half hours, writing a coordinate to a kml file every four seconds; its icon danced around on the map the whole time and it reported that it traveled 1.7 miles.

Comments

  • kwinnkwinn Posts: 8,697
    edited 2010-09-24 07:48
    This is normal for electronics. Noise can cause small fluctuations in ADC's (last digit bobble) and counters may vary by a count or 2 due to signal jitter. Even the satellite signal propogation times can vary very slightly due to atmospheric conditions. Add up the difference of all those readings (15 per minute x 60 min/hr x6 hrs = 5400 readings) and it is not too hard to see where 1.7mi (2992 yds.) comes from.
  • FranklinFranklin Posts: 4,747
    edited 2010-09-24 08:08
    Try taking you GPS outside (in a watertight plastic box) and leave it there while it logs. How far from the base coords did the log report? Many little 15 ft corrections could add up over time.
  • ke4pjwke4pjw Posts: 1,173
    edited 2010-09-24 08:33
    Since you were indoors, I suspect you may have had a high HDOP value.
  • KeithEKeithE Posts: 957
    edited 2010-09-24 10:25
    You might check out these articles from GPS World:

    http://www.gpsworld.com/gps/gps-accuracy-lies-damn-lies-and-statistics-1127
    http://www.gpsworld.com/gps/gnss-accuracy-lies-damn-lies-and-statistics-1134

    If you're using a GPS chip/module that's intended for automotive applications, they may put some heuristics in to prevent the GPS from reporting motion/rotation when a car is stopped at a stoplight.

    Many PNDs/Smartphones are incorporating accelerometers,gyros,... into their Kalman filters too.
  • UnknownUnknown
    edited 2010-09-24 15:44
    I have a Tom Tom which I am pleased with but once in a while I have to reset it because it isn't reset when it is turned on. I really don't have the problems you describe. The problems I have experienced is when you don't know which lane you want to be in for the turning lane after you get out of a tunnel because it takes a while for it to update itself and the other problem is that it seems that the routes it takes are sometimes random if you direct it at shortly different times.
  • RagtopRagtop Posts: 406
    edited 2010-09-25 06:31
    KeithE wrote: »
    Many PNDs/Smartphones are incorporating accelerometers,gyros,... into their Kalman filters too.

    I wanted to add stop signs and time stopped to my kml file. I am planning on using this hiking,biking, and driving, so I can't really filter out the jumping, so I too am thinking about adding an accelerometer. I am just out of cogs now and need to figure out now to slip it in.
  • localrogerlocalroger Posts: 3,452
    edited 2010-09-25 13:10
    An individual GPS read just isn't very accurate, and they can be affected by all sorts of odd things; they work REALLY badly in cities full of tall buildings, because line of sight to the satellite might be blocked but you might get a strong reflection off a building, and that totally hoses the position calculation because it thinks the sat-to-you distance is longer than it really is. Commercial GPS take a moving average of a window of 20 or more readings to let the errors average out.
  • Buck RogersBuck Rogers Posts: 2,190
    edited 2010-09-29 19:46
    Ragtop wrote: »
    I was wondering if anybody knew the reason why GPS readings seem to get more erratic sitting still. I have had quite a few and they all seem to have this behavior accept for one that had SA?, it didn't display new readings if you were going below a certain speed which doesn't work well with geocaching.

    I was testing a gps datalogger that I put together to see how long the batteries would last.

    Sitting on my desk for six and half hours, writing a coordinate to a kml file every four seconds; its icon danced around on the map the whole time and it reported that it traveled 1.7 miles.

    Hello!
    Perhaps what you were observing was a case of jitter? The orbits that the constellation of GPS satellites are in, isn't a perfect one. The only bird in a perfect one, went silent many years ago, and is supposed to return to earth in a good many years.:smhair:
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