I have a Garmin Edge 200 GPS unit. I commute from the center of Melbourne CBD, where there are many tall buildings and not a good line of sight to satellites. I park in an underground garage, so there isn't an option to turn on the GPS for a few minutes before I set off.
I've found through experimentation that if I turn on the GPS as soon as I start riding, it takes longer to get a satellite lock than if I wait until I have clearer views of the sky and then turn it on.
So if you consider the first 5 kilometers of the ride, if I turn it on at kilometer 0, it can be 5 kilometers (or even longer) before it gets a lock. But if I turn it on at kilometer 2 (when I'm clear of the tallest buildings), it will get a lock almost immediately.
Is there any basis to my experience? Am I better off waiting until I'm a bit clearer, or is there another strategy I can use to try to capture my entire ride?
I've experimented with a few things, and have found that the most reliable method seems to be to turn it on and then not move until I get a fix. This works quickly regardless of the buildings around me. It seems that temporarily blocking a satellite by moving makes it take a lot longer.