Are GPS fixes independent of each other?

[raptoroul]

Hello all.
I am using a proprietary system that uses a Gtop Ladybird 1 GPS module. It has worked fine for years and I have no complaints, but rather a curiousity. I was logging the GPS fixes coming every 3 seconds from the stationary unit at my location and decided to graph the results. That was when I saw that the fixes being obtained from the satellites formed a winding path that meandered around, staying close in proximity to my actual location, rather than a spatter or "shotgun" pattern as I had expected. This made me wonder if each set of GPS coordinates being logged is not independent, but is actually a calculation that is somehow based on the previous set(s) of coordinates.

As I said, this is just a curiosity. The unit accomplishes it's function perfectly, regardless of what the answer to my question is.

 

[Mark Hunter]

This is the normal "random walk" that is seen in lower grade receivers which have a failry large error (>1m) and is due to the kalman filters and errors in the system.

 

[raptoroul]

This makes perfect sense and confirms my suspicions. Thanks!

 

[Nuvi-Nebie]

As Mark says due to the application of Kalman filters there will be summing / averaging of measurements that are taken as a group over time.

A further explanation of "a winding path that meandered around" is the position of the Satellites, for example let us say you have 5/6 satellites in view that are giving you your current position, 300mS after the first measurement, the satellites are in much the same position, so the new guess (correct or incorrect) is also likely to be much the same, however over longer periods of time the satellite positions will change which can setup a more (or less) accurate overall guess of where you actually are because of their better (or worse) positions in relation to each other

Also GPS manufactures build-in other 'features' that can influence how measurements are stored, they include under what circumstances a new waypoint is made e.g. not allowing a new Waypoint unless it reports a shift from the last measurement by so many metres, Also some older Satnavs had a 'Snap to Road' feature so that your vehicle never drifts off the road, any in-built rules such as these will affect how the GPS is storing waypoints