Figure 4 3.  Example of a Data Loader System

4.2.3.3  Flight Instrument Interface Unit

Some GPS receiver designs will pass analog signals direct ly to the flight instruments,

but many designs may have a digital only output via an ARINC 429 interface.  The

reason for a digital only design is the anticipation of all digital flight instruments in the

future.  Aircraft with analog flight instru ments may require a separate digital to analog

converter to convert the digital data to the synchro, analog and discrete signals needed

to drive these instruments.

As an example, the Signal Data Converter (SDC) unit, developed for the U.S. DoD,

performs this function.  In concept, the SDC process is simple; the SDC takes the

digital ARINC 429 data stream and converts those parameters to analog signals that

can be handled by analog flight instruments.  Not all of the parameters can be used

(e.g. waypoint, latitude, and longitude) since the analog flight instruments have no way

of processing or displaying such data.  Data which can be used by analog flight

instruments include:

Distance to Waypoint

Waypoint Bearing

Desired Track (or radial)

Vertical/Horizontal Deviation From Selected Track (2 D or 3 D)

Data Validity Discretes

To/From Indication

The use of GPS for navigation in a mili tary aircraft is often seen as a substitute for

the Tactical Air Navigation (TACAN) system.  Therefore, it may be desirable to use

TACAN procedures with GPS, and it may also be desirable for the GPS displays to

emulate the TACAN displays.  The SDC includes the capability to function as a

TACAN digital to analog  converter by means of a simple discrete switch.  This

provides a simplified method

4 6