I'm looking into doing something similair myself. I contacted the guys at tessco. They have a in house engernieering team that can help with the design, you just need to get 4-5 gps settings for your area. Most of the radios you can buy with a license in them so you don't have to mess with the FCC ie: motorola canopy, alvarion and such.
This is CABL.com posting #246440. Tiny Link: cabl.co/mbcg0