Will they work in canyon areas, or do they have dead zones like cell phones?
No easy answer to that. GPS is satellite dependent. They require line-of-sight to two or more (usually 3) satellites. Therefore, mass clearance becomes an issue. Since you usually won't know the direction and angle to the satellites your set is using, it is difficult to predict. In a narrow, deep canyon, it would be unlikely to get reception. Dense overhead cover will degrade or preclude reception, as will being inside some types of buildings.
For the most part, though, they are amazingly reliable and accurate. That is, until the Commander in Chief orders the degradation or encryption of the signals. GPS is probably what allowed terrorists who were inexperienced pilots (and inexperienced at navigating at 350 MPH and several thousand feet) to accurately find the WTC.
Bob VanAmburg
Kearney, NE
