Minimum Safe Altitude Warning (MSAW) is a system designed to aid air traffic controllers to advise pilots when their aircraft appears to be at risk of colliding into terrain or obstacles. It is a computer system that projects the recent track history of a flight and compares that projection to a terrain model built into the computer database. When the projection indicates a hazard, the system generates alarms (visual and/or audible) to the controller, who then uses standard phraseology to alert the flightcrew. For example, a landing aircraft who appears to have descended too early might hear: “Low altitude alert. Check your altitude immediately. The (airport name) Decision Height is 1,200 feet.”
MSAW has two flight monitoring functions. General Monitoring protects flights from descending too low while flying en route, while Approach Path Monitoring protects aircraft flying charted approach procedures. To be functional, accurate digital terrain maps must be built into the computer database. The system is designed to trigger alerts with a buffer of 500′ above terrain (or obstacles such as antennas), and 100′ below the designed glidepath for arrivals.