![]() ![]() For digital units, the desired position of the needle is transmitted via a serial ARINC 429 signal from the radio or GPS unit, allowing the CDI design to be independent of the receiver and by multiple receiver types.The dizzy patient presents a formidable diagnostic challenge. The resolver position is sent to the converter which outputs the control signal to drive the CDI. More modern units are driven by a converter that is standalone or integrated with the radio. CDI units with an internal converter are not compatible with GPS units. Most older units and some newer ones integrate a converter with the CDI. When used with a VOR, a converter decodes this signal, and, by determining the desired heading or radial from a resolver connected to the OBS knob, provides a 150mV control signal to drive the CDI needle left or right. These receivers output a signal composed of two AC voltages. The CDI was designed to interpret a signal from a VOR, LDA, or ILS receiver. A CDI might incorporate a horizontal needle to provide vertical guidance when used with a precision ILS approach where the glideslope is broadcast by another transmitter located on the ground.Ī CDI is not used with an automatic direction finder (ADF), which receives information from a normal AM radio station or an NDB. When used for instrument approaches using a LDA or ILS the OBS knob has no function because the course line is usually the runway heading, and is determined by the ground transmitter.In this mode, the OBS knob may or may not have an effect, depending on configuration. ![]() Approach and terminal operations have a higher sensitivity up to frequently 0.3 nautical miles (0.56 km) at full scale. Sensitivity is usually programmable or automatically switched, but 5 nautical miles (9.3 km) deviation at full scale is typical for en route operations. When used with a GPS, or other RNAV equipment, it shows actual distance left or right of the programmed course line.(See Using a VOR for usage during flight.) Deflection is 10° deviation at full scale (each side), with each dot on the CDI representing 2°. This can be used to find and follow the desired radial. ![]() It then shows the number of degrees deviation between the aircraft's current position and the "radial" line emanating from the signal source at the given bearing. The course line is selected by turning an "omni bearing selector" or "OBS" knob usually located in the lower left of the indicator.
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