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Discrete Code A four-digit SSR Code with the last two digits not being "00". Discriminant Analysis The procedure of classifying the individuals into two different groups on the basis of a test. Display Lead Time The time difference between the display of an alert and the predicted event. Distance Learning Any form of learning in which the teacher and student are not together in the same place. Distance Measuring Equipment 1- Usually coupled with a VOR beacon to enable aircraft to measure their position relative to that beacon. Aircraft send out a signal which is sent back after a fixed delay by the DME ground equipment. An aircraft can compute its distance to the beacon from the delay of the signal perceived by the aircraft's DME equipment using the speed of light.
2- A combination of ground and airborne equipment which gives a continuous slant range distance-from-station readout by measuring time-lapse of a signal transmitted by the aircraft to the station and responded back. DMEs can also provide groundspeed and time-to-station readouts by differentiation.
Distance Root Mean Square
The root-mean-square value of the distances from the true location point of the position fixes in a collection of measurements. 2 DRMS is the radius of a circle that contains at least 95 percent of all possible fixes that can be obtained with a system at any one place. Actually, the percentage of fixes contained within 2 DRMS varies between approximately 95.5 percent and 98.2 percent, depending on the degree of ellipsicity of the error distribution.
Distinctive competencies
Unique qualities or skills in an organisation that are difficult to copy or imitate.
Distinguishing Path Attribute
in ISO/IEC 10747 a DPA is used to discriminate among multiple routes to a destination, based on differences in the quality of service between the routes (for example, expense, transit delay or residual error probability).
Distress Phase
A situation wherein there is a reasonable certainty that an aircraft and its occupants are
threatened by grave and imminent danger or require immediate assistance.

Dither-Process
Manipulation of the satellite clock frequency to limit the accuracy available to users of Navstar's Standard Positioning Service.
See also Selective Availability
Diversity
The logical relation that holds (or exists) between two entities when and only when they are not identical.
Domain
Functional subdivision of the ATM system which:  a) With all other domains completely satisfies the ATM mission; b) Develops a strategy which is a component part of the global strategy (currently ATM 2000+); c) Undertakes the preparatory work for programmes and services; d) Provides a structure to communicate to the outside world; e) Is supported by one or more skill centres.

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