S-TEC ST-901, Pilot Operating Handbook

1-4

5th Ed.  Feb 15, 08

S–TEC

It has the following modes of operation:

HDG Mode - Used to turn onto a Selected Heading and Hold it

GPSS Mode - Used to Laterally Steer along a Course defined by Waypoints,

that have been programmed into the GPS Navigator

Pressing the HDG / GPSS Selector Switch will alternately engage the HDG
mode and GPSS mode.  This switch is shown in Fig. 1-1.  As indicated, when the
HDG mode is engaged, the HDG lamp is illuminated and the GPSS lamp is
extinguished, but when the GPSS mode is engaged, the GPSS lamp is illuminated
and the HDG lamp is extinguished.

The GPSS Converter is internally configured to be compatible with the particular
AC or DC Heading System installed in the aircraft, be it a Directional Gyro (DG)
or Horizontal Situation Indicator (HSI).  The output of the Heading System will be
either an AC or DC Heading Error Signal.  When the GPSS Converter's HDG
mode is engaged, the GPSS Converter processes the AC or DC Heading Error
Signal at its input, to produce a proportional DC Heading Error Signal at its
output.  This signal is sent to the input of the autopilot's heading error channel.
When the autopilot's HDG mode is also engaged, the autopilot will turn the
aircraft onto the selected heading and hold it.

The GPS Navigator is programmed with a sequence of waypoints, as a means
to define a course.  This course is realized at the output of the GPS Navigator in
the form of two digital signals, the Bank Angle Signal and Ground Speed Signal.
When the GPSS Converter's GPSS mode is engaged, the GPSS Converter
processes the Bank Angle Signal and Ground Speed Signal at its input, to
produce a DC Turn Rate Signal at its output.  This signal is sent to the input of
the autopilot's heading error channel.  When the autopilot's HDG mode is also
engaged, the autopilot will laterally steer the aircraft along the course defined
by the waypoints.

1.4 Block Diagram

The GPSS Converter Block Diagram is shown in Fig. 1-2.