Does anyone either know or have a link?

[zuben el genub]

We were Stargazing in Arches NP. I let the Vulcan play with my Nexus and Skeye. He was fascinated and would like some information about how the positioning work. GPS will give latitude and longitude (RA and DEC) but altitude has him puzzled. He is an EE so something slightly techy would be OK

Thanks