NAV/DME Design

feature author: Paul Halliday


There are two NAV/DME worksheets. NAV_DME_1 contains models for the first set of VHF (commonly referred to as NAV) and DME navigation radio receivers. NAV_DME_2 contains the same models for the second set. The worksheets only differ by the inputs they accept from the host (NAV 1 versus NAV 2, etc). For purposes of this discussion, weíll only talk about the NAV_DME_1 worksheet. Iíve split the worksheet into two figures: the Plotís section (Figure 40) and the CoPilotís section (Figure 41). Weíll discuss the Pilotís section first.

There are 14 host inputs and 5 players for this design. The inputs labeled Pilot Audio NAV 1 and Pilot Audio DME 1 are switches on the cockpitís audio panel that allow the pilot to selectively listen to the NAV 1 or DME 1 radio signals. These eventually tie into the volume for the players. The NAV 1 and DME 1 Receiver Master Volume inputs are volume control knobs on the radio heads. The NAV 1 and DME 1 Signal-to-Noise Ratio inputs are signal strength ratios calculated by the host computerís radio models. These inputs are fed into an Automatic Gain Control (AGC) mini-model. This mini-model employs a High Pass Filter object and a Sum Float object to only produce a value when the Signal-to-Noise ratio is changing. The output of the AGC is then limited to 1 and is split into two paths. The first path goes to the Radio Noise player volume pin. Before it gets there, it is subtracted from a constant 1.0 which will give us the noise portion of the Signal-to-Noise ratio (the sum of the signal and the noise will always equal 1.0). The second path is combined with the output of a VOR/DME Keyer object, which is explained in the following paragraph.

The group of eight inputs labeled NAV1/DME1 Ident Letters represents character inputs of four identification letters each for the NAV 1 transmitter and the DME 1 transmitter. These are fed into the VOR/DME Keyer object. This keyer translates the letters into Morse Code pulses, which are eventually fed into the NAV and DME tone players [1].

 The 30Hz Modulation player represents a pair 30Hz sine waves on the VOR carrier that are out of phase with each other. These waves are used by the receiver to determine the azimuth to the station, which is then provided to an indicator of some sort (usually a Horizontal Situation Indicator).  Since 30Hz is effectively inaudible, what we hear are the harmonics of the modulation, which is kind of a clicking noise.

Note that all of the players of this design have fixed constants that feed the frequency, balance, and enable pins. There is no need to dynamically change these attributes of the players.

You may have noticed that there are five LFIs in this model, which are tied into the volume pins of the players. These are used to linearize the volume of a signal, which is a logarithmic DB volume. The curve for all of these LFIs is the same and is shown in Figure 39 .

Figure 39.  LFI for Volume Control of NAV/DME Tones

The CoPilot section of this design is largely dependent on the modeling performed in the Pilot portion. Worksheet connectors are used to take advantage of modeling something only once.

This is a confusing model. However, itís probably the most accurate audio radio model in the business because it incorporates subtleties not normally heard in most simulators. The beauty of this model is that itís already done, and can be reused on any simulator that has a VOR/DME radio system.


Figure 40.  NAV_DME_1 Design (Pilot)

Figure 41.  NAV_DME_1 Design (CoPilot)

A NAV (VOR) transmitterís tone pulses are 1020Hz. A DME transmitterís tone pulses are 1350Hz.

Beech 1900D Aircraft


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