User Guide

Series III Avionics
Pilot's Guide
Page 37
DMS-44A Distance Measuring System
General Description
The DMS-44A is a digital, solid-state distance measuring system that can provide data from three DME
stations simultaneously. A system consists of a DM-441B transceiver and one or two SD-442B
Selector-Displays. The transceiver unit may be front or rear antenna connector mounted. An efficient,
advanced-design heat sink and cooling fin dissipate internal heat and keep the unit cool even at its full
325 Watt output.
Two DME stations are selectable during routine navigation frequency management - when the pilot
selects VOR/DME's, VORTAC's, MLS/DME's, ILS/DME's, or LOC/DME's. The third DME station is
selectable by an RNAV system for calculating distance to a waypoint, time to a waypoint, and
groundspeed. This third DME channel is selected automatically, independent of any pilot action, and so
is said to be "transparent" to the pilot.
Simultaneous three-channel operation is achieved by high-speed scanning of three different DME
stations. Lock-on to valid data takes less than 200 milliseconds with least-significant-bit accuracy of less
than 0.01 nautical mile. Range is up to 300 nautical miles and ground speed capability is from 0 to 999
knots.
The DMS-44A incorporates the latest microprocessor technology to achieve superior accuracy and
reliability. As soon as power is applied, the DMS-44A begins a continuous self-test of its circuitry. When
circuits do not function within specific parameters, no data is presented, and the word FAIL is
annunciated on the display. The DMS-44A also continuously monitors and analyzes the channel
selection commands being supplied by NAV Receivers and the RNAV System. Invalid command
structure or loss of command continuity results in a FAIL annunciation.
If an active DME signal is temporarily lost or becomes unintelligible, the DMS-44A will continue to
compute and display data based on the last valid signal for up to 12 seconds. This feature provides an
uninterrupted display during manual or automatic channel changes and during aircraft maneuvers that
block the aircraft antenna from line-of-sight to the DME station. If power to the DMS-44A is interrupted,
the last channel and mode used are retained in system memory. When power is reapplied, channel and
mode are restored without external adjustment.