Even after the FAA brings in ADSB in the year 2020 it is going to keep all terminal radar units in class B and C airspace and also all enroute SSR units to cover FL180 and above. I wonder why? Yes- it's because they know that the GPS signals can be turned off at any time.
Also, it's the US Defence Department who have claimed concerns about spoofing of GPS based ADSB-not the FAA.