The 4000 is a portable NAV/COMM Test Set used to verify the operation and installation of ILS, VOR, and Marker Beacon receivers and VHF AM/FM and UHF AM Transceivers. The 4000 allows accurate measurement of VHF/UHF transmitter frequency, output power, modulation (AM and FM) and receiver sensitivity, VHF/UHF antenna and/or feeder Standing Wave Ratio (SWR). It can simulate Localizer and Glideslope (CAT I, II and III) signals with variable Difference in Depth of Modulation (DDM) settings. The instrument has a Swept Localizer DDM for coupled AutoPilot testing including simultaneous Localizer, Glideslope and Marker Beacon signals, as well as simulation of VOR beacon with variable bearing. It can also generate ARINC (Aeronautical Radio, Inc.) 596 Mark 2 airborne Selective Calling tones (SELCAL), allowing a ground radio operator to alert an air crew that the operator wishes to communicate with that aircraft.