how often does it loose the signal??? can you still geta good signal with pretty dense tree cover?

i work with GIS for a living and need to get one of these for "testing" purposes! it would be awesome to map out all my routes but i know this is sometimes easier said than done, especially when you GPS looses it's signal easily... in college i mapped some mtb trails with a real highend backpack unit. it was a big pain in the @ss because everytime we went under any tree cover you had to wait an eternity for it to get a signal. this thing would have been uselss at bike speed. luckily we were hiking and could just stop and wait for it.