The distance measuring equipment (DME) system is a remote-controlled transmitter/receiver system. The DME system operates in the N-mode with the VHF omni-range/localizer (VOR/LOC) channels. It has a range of 300 nmi and operates on 352 channels between 960 and 1,215 MHz. The DME system gives the aircraft's ground speed, slant range, and time-to-station data. It also gives the Morse code identification (IDENT) data.

The DME transceiver is one of the four modules installed in the very-high-frequency navigation (VHF NAV) units 1 and 2. The VHF NAV unit is also referred to as the Integrated Navigation Unit. The VHF NAV unit is the line-replaceable unit.

The VHF NAV units are installed in the main avionics compartment. A clamp holds each unit in a mounting assembly. Each DME transceiver connects to one DME antenna. The DME antennas installed on the bottom fuselage.

The DME transceiver connects to the cluster module in the VHF NAV unit through the radio communication bus (RCB). The cluster module supplies the interface between the RCB and the radio system bus (RSB). The DME transceiver receives the remote radio commands from the radio management system through the RSB. The cluster module also transmits the DME audio signals to the digital audio bus.

The DME system has a power-on self-test (POST) function that does a test of all of the system components. The POST starts after the system receives the pilot-activated self-test (PAST) command from the radio management system.

The DME transceiver weighs 3.4 lbs (1.54 kg) and operates with a +28 VDC power supply. Its transmitter supplies a minimum output power of 350 W.

The DME transceiver operates as a six-channel scanner. Each VHF NAV unit uses one channel. The flight management system (FMS) uses two channels. The DME transceiver monitors these four channels at the same time. The other two channels are for the IDENT decoder function. This configuration gives a fast IDENT when the pilot changes the DME channels from the standby mode to the active frequency mode.

The DME transceiver also operates as an interrogator. It transmits the interrogation pulses at 1,025 to 1,150 MHz and receives the replies at 960 to 1,213 MHz. It uses the pulses' elapsed time to calculate the aircraft's ground speed, slant range, and time-to-station data.

The DME transceiver supplies the digital DME data through one ARINC 568, four RSB, and two RS 422 outputs. It also supplies an analog DME data output and an audio IDENT output at 1,350 Hz. The distance data is accurate to ±0.05 nmi for the digital outputs and ±0.3 nmi for the analog output. The ground speed data is accurate to a maximum of 999 kts ±1% or ±1 kt. The time-to-station data is accurate to a maximum of 999 min ±1 min.

The DME antenna is a blade-type, vertically-polarized antenna. Each antenna has an N-type connector that connects to one DME transceiver through an in-line filter. The antenna has a frequency range of 960 to 1,220 MHz and a maximum weight of 0.25 lbs (0.114 kg). It is installed on the bottom of the forward fuselage.

The DME omnidirectional L-band antennas are located on the bottom of the aircraft for transmissions to and from ground facilities. An external in-line low pass filter is installed in the feedline of each DME transponder required to reduce the transmitter harmonics to a level required for registration in Germany. This filter has a cutoff frequency of 1,500 MHz.