I've done one long road trip with a large cargo box on top. Had no GPS navigation problems. Someone with more technical knowledge may chime in though.

So it sounds like you had no problems despite the box covering over the GPS attenna on the roof. Is this correct?

I don't know where the GPS antenna is positioned or how large it is. But yes, I had no problems .

Thanks. My understanding, and perhaps someone else can chime in, is the plastic spoiler sticking out of the center rear part of the roof is the GPS antenna. That's why I was curious to learn about GPS reception still working because cargo boxes clearly obstruct the antenna's view of the sky.

Hey, my GPS works while my car's in my closed garage so I don't think cargo boxes are going to create a problem BMW does say that "reception is best when you have unobstructed view to the sky".

If the cargo box is covering the spoiler over the rear window, it will interfer or completely block the reception. When the car is parked indoor, look at the right upper corner of the GPS screen, you can see a globe picture but no green dots to the left of the globe, the car will retain its last position before losing reception when indoor and not moving. Imagine if you go through a tunnel, say in Boston area... you will loos your guidance, and that is when the GPS goes to trip route (if you have a destination set), it will show a text version of the guidance list (like L on abc street in 2 miles, R on xyz Blvd in 1.5 miles and so on) until you are out of the tunnel, the GPS will regain access to the sky and the map reapears. DO not obstruct the GPS antenna view to open sky or you will get reduced accuracy from 20 to over 600 feet or you might completley loos the map display until the antenna gets a better reception. The spoiler itself is NOT the antenna but the antenna is either on the R or L side of the spoiler based on the L or R driver side (like in UK). It looks round omni-directional antenna. By the way the info above apply to MK4 screen and High version of map dvd.

The antenna, as mentioned, is in the rear spoiler on the tailgate.

Any obstruction of the antenna will reduce the signal level. Whether this has an impact depends mainly on the atmospheric conditions, as some types of rain/mist can also reduce the GPS signal level

Reduce the signal level toooo far, and you will not get a fix, or you will get poor fixes.

The golden rule is simply not to cover the antenna, so avoid having any roof box go over the top of the rear spoiler.

But if you do cover the antenna, you probably won't notice any impact anyway, as the antenna receives signals from all elevations, not just straight overhead. So some signal will likely still get through.