I was studying the Aeronautical Chart User's Guide and I came across the term 'radar minima' several times under the graphics depicting charted airports.
Blue – Airports with an Instrument Approach Procedure and/or RADAR MINIMA published in the high altitude DoD Flight Information Publications (FLIPs)
Green – Airports which have an approved Instrument Approach Procedure and/or RADAR MINIMA published in either the U.S. Terminal Procedures Publications (TPPs) or the DoD FLIPs
Brown – Airports without a published Instrument Approach Procedure or RADAR MINIMA