You would get more results on "The GPS Exchange" forum on TON.

I use a Magellan GPS 320. I've used it to GPS all the PMT in the Sam Houston National Forest. They are posted on the Trail Riders of Houston forum. I almost always have a minimum of 3 satellites, usually more. You must keep a medium steady speed. There is one dead spot in the NF north of 208 & west of 208B.