I have a Garmin Nuvi 2555 that I've used successfully both in my car and in the side, drivers window of a 45' tour bus. No problems - works great. Recently, with the correct Ram mount, I used it on my GL1500. Seems to work OK but occasionally a "pop up" message appears telling me I should be using an external antenna for best accuracy. I don't "get it" since the device sits out in the open with no "overhead" obstructions. The "pop up" stays on for 10 or 15 seconds, goes away, then we're "back to normal" until the message re-appears - maybe 10 minutes later (+ or -). Until using it on the bike, I never even saw this "pop-up" before.
This is not really a big deal except it's annoying and for the "mystery" of why it thinks an external antenna is required. Do any of you electronic "gurus" have an explanation? Thanks.