Check carefully your vehicle speed sensor and gyro sensor. I believe these are the primary position inputs, with GPS as secondary.

My theory is that bad/variable speed sensor and gyro inputs can cause all sorts of positional havoc.

So record your normal speed sensor readings for some common speeds (ie 30mph, 50mph, 80mph) and gyro for common directions (n,s,e,w etc) and keep a note in the car.

Next time the nav plays up, quickly switch to service mode and compare sensor readings with your known, good readings that you noted earlier.

PS: helps if you have a navigator with you, please don't crash your car doing this!

Hello All,

I was able to obtain some gyro and wheel sensor data during a condtion of normal navigation function and a condtion of "map shift" inaccuract nav system function.

Wheel sensor readings were taken at 0 mph, 20 mph, and 35 mph. In all cases (i.e. both normal and abnormal Nav position calculation) the wheel sensor values remained the same (i.e. 0 mph - 0 value, 20 mph - 172 value, and 35 mph 320 value).

The Gyro sensor reads 2500 while at rest, and increases in value during a turn to the right and decreases in value during a turn to the left. For what it's worth, I also noticed the the accelerometer display appeared to be functioning normally.

My conclusion from this test was that the wheel sensor and the Gyro sensor are probably functioning correctly since there was no change in the sensor values between readings taking while the Nav system was providing a correct position, as well as an inaccurate ("map shifted") position.

However, there are still a number of unanswered questions about the GPS calculated position, and its influence on my unexplained "Map shift" problem.

- My green GPS symbol & Satellite indications (on the map page) refuse to appear until I move the gear selector into either forward or reverse.... is this normal? Could this delay in the presentation of the green GPS symbol be indicative of a GPS sensor related fault? I would have thought that the GPS would display its NAV ready green GPS symbol as soon as it had acquired satellites and calculated its postion irregardless of vehicle shiftor selection. Could another forum member look at their NAV display to help determine if the delay in the appearance of the green GPS symbology until a drive gear is selected is a normal function?

- I took my Garmin GPS with me during a test and compared both lattitude and longitude positions with the BMW calculated position. I have absolute faith in the Garmin system. The values of the BMW system were almost always different from the Garmin position by approximately 0.30 degrees lattitude and 0.20 degrees longitutde. For example Garmin: N 49 11.33 W 123 09.00
BMW : N 49 11.14 W 123 09.22
Is this position discrpency within an allowable tollerance? Could this position discrepancy be the root cause of my unexplained "map shift" of about 300 feet?

Any further insights into this issue would be greatly appreciated.

Cheers,

Marcus

Hello Again,

After thinking about this issue for a few moments I'm slowly forming the opinion that this might be a GPS sensor/cable connection issue.

In my opinion, one indication that might support the theory that there is a fault with the gps sensor/cable is a poor signal to noise ratio (S/N value) in the GPS Tracking Info sub menu. Next time I drive the vehicle I will record the satellite S/N ratio and post it in this forum.

In the mean time would someone please provide some satellite S/N ratios from a fully functioning BMW NAV system for reference?

Thanks,

Marcus

P.S. You can see the S/N ratio by entering the Navigation Service mode, selecting the GPS/Tracking Info from one of the sub-meuns.

I typically get between 2 (poor) and 10 or 11 (good) with most satellites around the 7 or 8 mark. I'm at 37 degrees south latitidue and 174 degrees east longitude. Not sure if latitude affects the S/N readings...

Also, note your HDOP and PDOP. PDOP = Positional Dilution of Precision, HDOP = horizontal... etc. They both reflect GPS acccuracy. A PDOP of 1.0 is very good, anything above 3 tends to be inaccurate. My PDOP tends to be about 1.2 to 1.5

I would suggest that, for the relatively low cost, you procure another GPS antenna and try it in a temporary location on your rear parcel tray.

Jochen

Thanks Kiwijochen and Terry Kennedy.

I'm at work right now (away from home base) and won't be back home for a couple of days.

When I'm back home I'll report further on this issue paying particular attention to my actual signal to noise ratio's and the HDOP, PDOP and VDOP values.

Thanks again for all your help with this issue so far.

Cheers,

Marcus