TOM TOM - losing GPS position

I have a PDA with tomTom V6 Navigator. Just lately its become much more
unreliable at holding its correct GPS position. Frequently picking up
parallel roads and cul de sacs (upto 100/200 yds away) - leading to
confused/inapropriate verbal direction changes.

I have experimented with different baud rates (normally use 4800) on the
basis that strength or quality of signal might be an issue. This has not
produced any significant changes in performance.

Anyone else experienced similar and found a solution or improvement.

"A little knowledge and a screwdriver" springs to mind.

Don't mess with things you don't understand. (clearly demonstrated by
the statement, "I have experimented with different baud rates (normally use
4800) on the basis that strength or quality of signal might be an issue."

I'd say the problem is with the GPS, however, this could be down to
a multitude of faults. The GPS itself, an external antenna, the coax of
the antenna (if used), the positioning of the GPS etc.

Assuming that you are using a battery powered GPS mouse, try removing the
battery for about 30 seconds.

Your basic data in the GPS mouse has become corrupt (possibly due to a
voltage spike when connecting the charger).

Next time it is switched on it will reload all the GPS data and will
probably work accurately again.

HTH

--
Fred

The problem is not likely to be connected to your connection to the GPS but
rather to the calculation performed by the GPS unit to fix your position.
The data sent from the GPS to your PDA is simply a text stream showing
speed, position and other data, so if that degrades you are likely to get
complete garbage rather than a position that is slightly out. Your TomTom
software is simply picking up the data provided by the GPS and resolving
that to the nearest road. It may be doing some sort of filtering to reject
completely erroneous data, but that doesn't sound like its is an issue here.

It sounds like your GPS is giving out a "slightly" incorrect position. It
will do this if it is not able to receive a consistent signal from enough
satellites. Basically the GPS unit is determining your position, speed and
time by calculating the distance between the unit and several satellites.
It does this by working out the time taken for the signal to reach the GPS
unit from the satellite using clocking information in the signal. Since the
GPS does not have an atomic clock on board like the satellites, the GPS also
solves for its own time. To do all that it needs 4 signals (maths tells us
that we need 4 equations to solve a problem with 4 unknowns), which means
that your GPS needs to be receiving consistent signals from at least 4
satellites, but preferably more. If the signal from any satellite is
intermittent or weak the GPS might be calculating your position with
slightly out of date information for one or more of the satellites which
would give an incrrect position. Alternatively, one of the satellites might
be transmitting slightly incorrect data about its own position,although this
is likely to be corrected very quickly.

If it is simply a question of poor reception limiting the number of
satellites, try repositioning your GPS unit or adding an antenna. It may be
that you have been driving in built up areas where signals are reflected off
tall buildings, or under trees which cut out some of the signal.

For more information:
http://www8.garmin.com/manuals/GPSGuideforBeginners_Manual.pdf

Mark Williams wrote:


Now of course, my unit (HP6515, Tomtom5) may suffer from the same problems
as those of the OP... but

This happens to me from time to time (it used to happen a lot until I
upgraded the firmware a few months ago).

My solution to this problem is to turn the unit off and turn it back on
having covered 1 or 2 kilometers. When I turn it back on, the unit realizes
something strange has happened, and recomputes the itinerary from scratch
and I go on my merry way. It is my unsubstantiated belief that Tomtom is
performing some incremental correction (possibly based on a transient
error) and simply isn't able to correct itself (because it keeps trying to
increment off an error?). As an aside, you might think a navigation program
would snap-to-road when it's data suggests you're traveling at 130 km/h in
a field right next to a motorway, but no, it doesn't notice.

What really perplexes me is that it happened in an aggravated manner (i.e.
solution described above was useless for more than a minute or two) twice
in the same area, an area I don't visit every day (area is around Giverny
France).

Bernard Higonnet