Dead Reckoning - Garmin Cirrus Perspective SR20 Cockpit Reference Manual

dead reCKoning

While in Enroute or Oceanic phase of flight, if the Perspective
invalid GPS solution or is unable to calculate a GPS position, the system automatically
reverts to Dead Reckoning (DR) Mode. In DR Mode, the system uses its last-known
position combined with continuously updated airspeed and heading data (when
available) to calculate and display the aircraft' s current estimated position.
NOTE: Dead Reckoning Mode only functions in Enroute (ENR) or Oceanic
(OCN) phase of flight. In all other phases, an invalid GPS solution produces
a "NO GPS POSITION" annunciation on the map and the system stops
navigating in GPS Mode.
DR Mode is indicated by the appearance of the letters 'DR' superimposed in yellow
over the 'own aircraft' symbol as shown in the following figure. In addition, 'DR'
is prominently displayed, also in yellow, on the HSI slightly above and to the right
of the aircraft symbol on the CDI as shown in the following figure. Also, the CDI
deviation bar is displayed in yellow. Lastly, but at the same time, a 'GPS NAV LOST'
alert message appears on the PFD.
Normal navigation using GPS/WAAS source data resumes automatically once a
valid GPS solution is restored.
It is important to note that estimated navigation data supplied by the system in
DR Mode may become increasingly unreliable and must not be used as a sole means
of navigation. If, while in DR Mode, airspeed and/or heading data is also lost or
not available, the DR function is not be capable of estimating your position and,
consequently, the system may display a path that is different than the actual movement
of the aircraft. Estimated position information displayed by the system through
DR while there is no heading and/or airspeed data available should not be used for
navigation.
DR Mode is inherently less accurate than the standard GPS/WAAS Mode due to the
lack of satellite measurements needed to determine a position. Changes in wind speed
and/or wind direction compounds the relative inaccuracy of DR Mode. Because of this
degraded accuracy, the crew must maintain position awareness using other navigation
equipment until GPS-derived position data is restored.
190-00821-03 Rev. A Cirrus Perspective ® by Garmin – Cockpit Reference Guide for the SR20/SR22
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