can you check the cable between the uGPS and the radio. the link below will guide you through how to make this cable: http://community.cambiumnetworks.com/t5/PMP-FAQ/Cable-DiaCagram-for-450i-to-UGPS/m-p/52198#U52198
also, enable the uGPS power on the AP, if you are using the AP to power up the uGPS.
AP--->Configuration---->Sync Setting----->UGPS Power---->enable.