Anyone else having "wierd" effects on their nav system? Occasionally, (not always) the location shown on the map is several hundred feet "off" from actual. [This is NOT the lag due to the GPS system not keeping up with vehicle speed]. Example, we were traveling to a restaurant in the Portland, OR area, in traffic, going slow, no overpasses to block the satellite signal. About 2 blocks before we got there, the NAV computer told us to make a U-turn and go back. It thought we had already past the place! Fortunately, I could see where we were headed so ignored the instruction.
Talked to my "service advisor" about it and so far have no feedback.
Anyone else?

I also have a nav unit in my 320 CDI. The GPS reciever and corresponding locations plotted on the screen will have different levels of accuracy depending on several factors (from the Garmin Website):
Sources of GPS signal errors

Factors that can degrade the GPS signal and thus affect accuracy include the following:

Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

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sophie2
1987 300TD, 168K
1981 300D 220K
1968 250S W115 Diesel Converted,
2006 E320 CDI
2006 Yanmar 3YM30 / Vindo Sloop
29 Vintage Bicycles

You'll note our two cars in my signature and I can only state that the GL unit (Alpine) doesn't measure altitude as well as the Becker unit in the E-class. Have yet to use NAV in the GL.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles