for GPS access to the analog flight instruments, using the existing TACAN wiring

path (i.e., replace the existing TACAN D to A with the SDC).

Since GPS is still a relatively new system, some of the TACAN system characteristics

need to be considered.  Identified below are GPS flight instrument display and

procedures comparisons to TACAN and other radio navigation aids.

4.2.3.3.1  Deviation Scale Factor

With TACAN, a 2 dot horizontal deviation displacement represents 10 degrees off the

required radial.  An Instrument Landing System (ILS) Localizer has a 2 dot

displacement of approximately 3 degrees (runway dependent).  In the case of the U.S.

DoD equipment, the GPS 2 dot displacement represents either 4 nmi, 1 nmi, 0.3 nmi

linear displacement, or 3 degrees depending on the scale factor selected (Enroute,

Terminal, Nonprecision Approach, or Approach respectively).

These GPS horizontal scale factors were generally derived from airway track keeping

requirements for the various phases of flight.  The Enroute scale factor was derived

from the typical  4 nmi U.S. National Air Space (NAS) Airway width.  The Terminal

scale factor was selected based on U.S. Air Force Instrument Flight Center flight

testing.  The Non Precision Approach scale factor corresponds with U.S. FAA non 

precision approach tolerance.  The Approach scale factor simulates an ILS localizer

display.

If 3 dimensional waypoints are used, then the U.S. DoD GPS receiver can present

vertical deviation information.  The vertical 2 dot deflections are 1000 ft, 500 ft and 200

ft linear displacement, and 0.7 degrees corresponding to the En Route, Terminal, Non 

Precision Approach, and Approach scale factors respectively.  The linear scale factors

provide the opportunity to someday utilize GPS for vertical navigation in level flight. 

The Approach vertical scale factor simulates an ILS glideslope display.

4.2.3.3.2  TACAN and GPS Flight Procedural Differences

In the TO/FROM TACAN Navigation mode, the Omni Bearing Select (OBS) knob on

the Horizontal Situation Indicator (HSI) allows the pilot to select the radial (to or

from the current waypoint) along which he wishes to fly.  As the knob is turned and

the radial changes, the horizontal deviation bar swings to show the pilot whether he

is left or right of that radial.  In the case of TACAN, the OBS knob feeds back to the

TACAN Digital to Analog Converter (DAC), where the left/right computation is

carried out (see Figure 4 4).  The deviation bar is driven by angular differences. 

The U.S. DoD SDC can mimic the TACAN DAC as shown in Figure 4 4.

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