Sat Nav position query

[McMasters]

Hi.
I have just replaced a very temperamental disk spitting Mk2 with a Mk3 ( BMW with a nice Rover75 splash) .
All works well and seemed OK but ... I noticed an odd thing when I turn a corner or change direction.
The voice asks to return to planned route and the direction arrow starts turning in all directions,flashing up various short meter distances, eg 40, 60, 30, etc
When I look at the map ( split screen) the position of the car seems to be approx. 50 meters beyond the route intersection.???? Some times it asks for a U turn after I have turned into the right street??? Then after a 100 meters or so the correct distance to the location appears again. On long stretches it runs OK but as soon as I change direction in small street areas... or near a corner , it starts getting loopy. I have tested it with known destinations and tried different route settings.
It always get you there but on freeway turnoffs I can see a problem as is stops tracking just when one needs the right info.
Can the positioning be made more accurate, this over run seems like a distance problem when the unit takes new readings.
Any tips from the Sat Nav specialist most welcome.

[damydyo1]

Hello Neil,

Try this: When your car was disconnected from the battery for a while, your navigation could fail in finding the right car position. Don't ask me why, because normally your navsat is guided by 6 to 8 satelites. For this I put a pdf in the attachment in which you can find how to set the correct vehicle position. Look for the last pages 32 and 33.
Let me know if this worked out for you.

Regards Ralph

[ChineseRover]

Suspect the compass is not working as the unit is not recognising your change of direction and is assuming you are travelling in the old direction until it works out the position from recalculating position via GPS.
The system does a lot of "dead reckoning", particularly in town when building might be blocking the sat signals.

Had this problem when the car was new (first week) and the sat nav computer was replaced.

[McMasters]

Thanks guys will have a look at that and report back. I have a Mk3 BMW unit and the menu looks different from the one in the attachment. That looks like my old Mk2 menu.
One other question.
I upgraded from a Mk2 to a Mk3. The "how to" posts said to just swap the antenna cable from the little silver box mounted over the old Mk2 Sat nav unit and plug it into the new Mk3 unit, Which I did. Should I also disconnect the black plug from the little silver unit that formally housed the cable or just leave it connected????

[Reebs]

Originally Posted by McMasters

Thanks guys will have a look at that and report back. I have a Mk3 BMW unit and the menu looks different from the one in the attachment. That looks like my old Mk2 menu.
One other question.
I upgraded from a Mk2 to a Mk3. The "how to" posts said to just swap the antenna cable from the little silver box mounted over the old Mk2 Sat nav unit and plug it into the new Mk3 unit, Which I did. Should I also disconnect the black plug from the little silver unit that formally housed the cable or just leave it connected????

Yes disconnect the plug from the Mark 2's GPS receiver, and tuck it away so it doesn't rattle, the GPS receiver should be completely out of circuit, as the Mark 3 has one of it's own already built in. This may be the cause of your issues.

[Departed]

Originally Posted by ChineseRover

Suspect the compass is not working as the unit is not recognising your change of direction and is assuming you are travelling in the old direction until it works out the position from recalculating position via GPS.
The system does a lot of "dead reckoning", particularly in town when building might be blocking the sat signals.

Had this problem when the car was new (first week) and the sat nav computer was replaced.

There isn't actually a compass in the system.

You are dead right it does dead reckoning, but it uses wheelspeed pulses and an accelerometer to do this, no compass.

[McMasters]

Well I disconnected black plug from the "Trimble" Mk2 receiver , ran the service mode menu , no errors found, tracking all satellites , direction forward position correct etc etc etc . All OK it seemed mmmm
So, another test drive, known route. Set the map scale to 500m instead of 200m just in case that was an issue.
Anyway it seemed a lot better snappy, less confused at the corners. Meter countdown seemed pretty good, the first screen refresh after the corner also good. But at one spot near the last turn it got confused again and said 'please return to the planned route' and spun the direction arrow around in several positions with various distances 100 m 50 m 30 m etc. I proceeded on the correct road then finally it kicked in with the right info and direction again till we arrived at the destination. Perhaps it is a satellite blind spot on that corner????
So thats the update report folks. Not perfect but better. I just hope it doesn't spit the dummy on the autobarn turn offs

[Kearton]

It may just be that the connection from the GPS antenna into the sat. nav unit is not fitted in tightly enough. That happened to me once. A simple enough thing to test.

Kearton

[ChineseRover]

I would not worry too much about the display occasionally showing you off the road. I think this is a problem with the accuracy of the maps. I have driven on lots of minor roads in Germany where the satnav shows me driving in the fields and forests alongside the road. It seems to snap back when I got to junctions.

The only time I have experienced this in the UK is driving down roads with tall building in London, when it can do the same thing, or worse, assume you are on a road running parallel with the road you are actually travelling on.

Originally Posted by lowedb

There isn't actually a compass in the system

Originally Posted by lowedb

You are dead right it does dead reckoning, but it uses wheelspeed pulses and an accelerometer to do this, no compass.

Er very difficult to navigate (er impossible?) by DR without an understanding the direction of travel!