Figure 4 4.  Flight Instruments and TACAN

In the case of U.S. DoD GPS receivers using GPS TO/FROM Navigation mode, the

receiver is programmed with waypoint information which includes desired track.  This

can be analogous to selected TACAN station (waypoint) and OBS radial setting

(desired track).  The deviation bar deflection will be a function of linear distance (when

not in approach mode) of the aircraft perpendicular to the desired track which was

programmed in the receiver (see Figure 4 5).  The SDC provides a desired track output

synchro signal that can drive the HSI OBS to the appropriate radial setting.  The pilot,

however, can not turn the OBS knob to select a new GPS desired track (other similar

products may choose to incorporate the OBS knob setting).  The pilot wishing to

change the desired track value must enter it into the CDU.  The pilot alternatively can

select the Direct To navigation function to get a direct course to the waypoint.

Pilots generally steer magnetic headings.  GPS is an inher ently "true" system.  One

must therefore be careful that the SDC always has a designated magnetic or true

heading source and the GPS receiver has knowledge of local magnetic variation, or

assigned magnetic variation (in the case of Navaids used as waypoints).

4.2.3.4  Inertial Navigation Systems

A GPS receiver integrated with an Inertial Navigation Systems (INS) forms a

particularly effective navigation system.  The GPS receiver can compensate for the

long term drift of an INS and an INS can compensate for the short term noise and

relatively low data rate of a GPS receiver.  (Additional discussion of GPS integration

architectures is provided in Chapter 8).

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