Early RDF receivers were costly, bulky and difficult to operate, but the simpler and less expensive ground installation allowed the easy addition of NDB based waypoints and approaches, to supplement the low-frequency radio system.[1] Modern RDF receivers, called "automatic direction finders" (or "ADF") are small, low cost and easy to operate. The NDB-ADF system remains today as a supplement and backup to the newer VOR and GPS navigation systems, although it is gradually being phased out.[24][3] All questions regarding NDB/ADF operation were removed from FAA pilot certification testing materials prior to October 2017.[26]