TM 1-1510-223-10 GPU Ground power unit GS, G/S Glideslope GSPD Ground speed GUK Group unique key GUV Group unique variable HDG Heading HF High frequency HOM Homing HR Hours HSET Hopset HSI Horizontal situation indicator HSSP High speed signal processor I Inner marker ICS Intercom system ID Identification IDENT Identification IDL Interoperable data link IFF Identification, friend or foe IFR Instrument flight rules ILS Instrument landing system INIT Initialize INOP Inoperative INS Inertial navigation system INST Installed INSUFF Insufficient NTPH Interphone I/O Input/output IP Instructor pilot IPF Integrated processing facility IRU Inertial reference unit IRUE Inertial reference unit electronics kHz Kilohertz KU Keyboard unit L Left LAT Latitude, lateral LB Pounds LED Light emitting diode LH Left hand LOC Localizer LONG Longitude LRN Long range navigation LRU Line replaceable unit LSB Lower sideband LSET Lockout set LSS Lightning sensor system LVC Line voltage compensation LX Lightning sensor system MAG Magnetic MAN Manual MAX Maximum MDA Minimum descent altitude MEM Memory MFD Multifunction display MHz Megahertz MIC Microphone MIN Minimum MIS Mission MM Middle marker MON Monitor MSL Missile MSU Mode selector unit MUX Multiplex mV Millivolt N North N/A Not applicable B-7