I don't know for sure but I've read that newer units that can use GPS, Glonass and Galileo can get a good fix under a heavy canopy or even indoors.
That’s less a function of being able to work with multiple GNSS systems and more a function, I believe, of better antennas and receivers.
Yes, my understanding is that the biggest improvement in newer GPS units is a more sensitive antenna.
My older Garmin GPSMAP 60csx can get a lock in the center of the lower level the lower level of my house with only one window. To my knowledge the big improvement came with the SiRF chipset.
Again, that is mostly due to a more sensitive antenna.
Still, it couldn't get the lock on a narrow German street with two story buildings on either side.
Most places the GPS has been able to get a fairly quick fix, but last summer in the narrow streets of Regensburg, Germany it would not lock onto satellites and kept loosing the fix.
This is usually due to "multipath error". The direct GPS signal is blocked, but the a signal is bouncing off one of the buildings. This means that the path is longer, hence the time is longer, and the calculated distance from the satellite is longer (and erroneous). I've run into the same problem in urban areas with narrow streets, when the GPS will suddenly tell me I'm one block over from my actual location. I've also had it happen in narrow canyons. Sometimes the unit will ping back and forth between two locations.
QUESTION: Does anyone know how well a modern GPS does getting a lock along the Appalachian Trail? Would a mapping GPS with a topo map of the local area have likely been able to help Gerry?
Should work fine if there is a clear view of the sky. My understanding is that people us GPS all the time on the Appalachian Trail.
Sometimes GPS will have a problem in heavy tree cover, but newer units (with better antennas) will usually do OK. Unless it is really dense forest. If you can look up and see any significant amount of sky at all, you should be able to get a fix in most cases. And you can usually move a short distance to some spot with a slightly better view of the sky and get a fix. Sometimes moving only a few yards will make all the difference. The same thing is true in narrow streets in urban areas.
Remember that the satellites are always moving. So even if a given satellite is totally blocked, or you are getting a multipath error, eventually the unit will get a clear view of 4 satellites and be able to get a decent fix. It sometimes takes awhile though.
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