High accuracy AM Broadcast stations (probably GPS referenced)
Because of the possibility of multiple stations on the same frequency, I made sure I had a voice ID on all of these stations. You should do the same before you use them as a reference.
KARN 920 Little Rock
WWLS 640 Oklahoma City
KLIF 570 Dallas
KNX 1070 Los Angeles 50KW OMNI (info from KJ7E)
WWV-2.5 for reference (sky wave)