8/15/19 1/30/20  Pilot/Controller Glossary Pilot/Controller Glossary  ALERTING SERVICE- A service provided to notify  appropriate organizations regarding aircraft in need  of search and rescue aid and assist such organizations  as required.  ALNOT- (See ALERT NOTICE.)  ALONG-TRACK DISTANCE (ATD)- The hori- zontal distance between the aircraft's current position  and a fix measured by an area navigation system that  is not subject to slant range errors.  ALPHANUMERIC DISPLAY- Letters and numer- als used to show identification, altitude, beacon code,  and other information concerning a target on a radar  display.  (See AUTOMATED RADAR TERMINAL  SYSTEMS.)  ALTERNATE AERODROME [ICAO]- An aero- drome to which an aircraft may proceed when it  becomes either impossible or inadvisable to proceed  to or to land at the aerodrome of intended landing.  Note: The aerodrome from which a flight departs  may also be an en-route or a destination alternate  aerodrome for the flight.  ALTERNATE AIRPORT- An airport at which an  aircraft may land if a landing at the intended airport  becomes inadvisable.  (See ICAO term ALTERNATE AERODROME.)  ALTIMETER SETTING- The barometric pressure  reading used to adjust a pressure altimeter for  variations in existing atmospheric pressure or to the  standard altimeter setting (29.92).  (Refer to 14 CFR Part 91.)  (Refer to AIM.)  ALTITUDE- The height of a level, point, or object  measured in feet Above Ground Level (AGL) or from  Mean Sea Level (MSL).  (See FLIGHT LEVEL.)  a.  MSL Altitude- Altitude expressed in feet  measured from mean sea level.  b.  AGL Altitude- Altitude expressed in feet  measured above ground level.  c.  Indicated Altitude- The altitude as shown by an  altimeter. On a pressure or barometric altimeter it is  altitude as shown uncorrected for instrument error  and uncompensated for variation from standard  atmospheric conditions.  (See ICAO term ALTITUDE.)  ALTITUDE [ICAO]- The vertical distance of a level,  a point or an object considered as a point, measured  from mean sea level (MSL).  ALTITUDE READOUT- An aircraft's altitude,  transmitted via the Mode C transponder feature, that  is visually displayed in 100-foot increments on a  radar scope having readout capability.  (See ALPHANUMERIC DISPLAY.)  (See AUTOMATED RADAR TERMINAL  SYSTEMS.)  (Refer to AIM.)  ALTITUDE RESERVATION (ALTRV)- Airspace  utilization under prescribed conditions normally  employed for the mass movement of aircraft or other  special user requirements which cannot otherwise be  accomplished. ALTRVs are approved by the  appropriate FAA facility.  (See AIR TRAFFIC CONTROL SYSTEM  COMMAND CENTER.)  ALTITUDE RESTRICTION- An altitude or alti- tudes, stated in the order flown, which are to be  maintained until reaching a specific point or time.  Altitude restrictions may be issued by ATC due to  traffic, terrain, or other airspace considerations.  ALTITUDE RESTRICTIONS ARE CANCELED- Adherence to previously imposed altitude restric- tions is no longer required during a climb or descent.  ALTRV- AMVER- APB- APD- APDIA- (See ALTITUDE RESERVATION.)  (See AUTOMATED MUTUAL-ASSISTANCE  VESSEL RESCUE SYSTEM.)  (See AUTOMATED PROBLEM DETECTION  BOUNDARY.)  (See AUTOMATED PROBLEM DETECTION.)  (See AUTOMATED PROBLEM DETECTION  INHIBITED AREA.)  APPROACH CLEARANCE- Authorization by  ATC for a pilot to conduct an instrument approach.  The type of instrument approach for which a  PCG A-10 