INS Modes of Operation. The INS can be
operated in four modes: Standby (STBY), Align,
navigation (NAV), attitude (ATT). When the INS is
turned on it will (under normal conditions) proceed
through the STBY and ALIGN modes and enter the NAV
mode, where it will remain until it is downmoded at the
end of the flight. The only exception will be in the event
of a malfunction, in which case the INS will be shut
down or placed in the ATT mode. In the ATT mode, the
computer is turned off and the INS will only provide
Standby mode. The characteristics of
the standby mode vary, depending upon whether the
INS is being turned on or is being downmoded from
NAV, ALIGN, or ATT. During standby mode the
alignment state number is 9.
Standby mode (INS being turned
on). If the INS is being turned on by setting the mode
selector to STBY, the navigation unit is brought up to
operating temperature by fast warm-up heaters and the
gyro wheels are brought up to speed. The INS platform
is caged to the aircraft axes and all instrument warning
lamps controlled by the INS indicate warning.
downmoded). If the INS is being downmoded from a
higher mode by setting the mode selector switch to
STBY, the INS platform retains its alignment with local
horizontal, and all instrument flags controlled by the INS
except ATTITUDE and PLATFORM HEADING flags,
will indicate warning.
Present position, waypoint coordinates, and
TACAN station data can be inserted into the
INS while in the standby mode.
Alignment mode. In the alignment
(ALIGN) mode the INS computer progresses through a
series of submodes called alignment states.
alignment state 8, the INS platform is aligned to local
horizontal and the INS battery is tested. Attitude
warnings are removed at the beginning of state 8, but all
other instrument flags controlled by the INS continue to
indicate warnings. The battery unit is tested during the
first 12 seconds of this state. The INS spends a
minimum of 51 seconds in state 8.
During the transition of state 8 to state 7, the
heading is set to the stored value. If this value
magnetic variation, the status will remain unchanged. If
the comparison is unfavorable,
the status will be set invalid, and the system must be
upmoded to NAV before a heading may be stored.
Present position must be loaded before the
system will progress from state 8 to state 7.
Alignment states 7 and 6. The
primary function of states 7 and 6 is to refine the
relationship between the INS platform and true north.
This function continues unchanged throughout both
states 6 and 7. In state 6 the INS compares the latitude
of the loaded present position with the latitude it has
computed as part of the alignment process, and it uses
the results of this comparison (with other information) to
update certain self-calibration data. During these states
it is possible to enter the navigation mode if the stored
heading is valid. Entry to NAV will immediately follow
the setting of the mode selector to NAV. The alignment
will be entirely dependent upon the stored values
regardless of the length of time in states 7 or 6.
Alignment states 5 through 0.
Entry into alignment state 5 indicates that normal or
standard accuracy alignment is complete. The NAV
mode may be selected at any time after entry into this
alignment should be allowed to progress until the
READY NAV lamp is illuminated, and preferably to
alignment state 2 or 1. A standard accuracy alignment
takes about 20 minutes, while a high accuracy
alignment requires about 45 minutes.
Alignment states 4 through 0 indicate continuing
operating of the self calibration process begun in
alignment states 6 and 5. In addition, automatic
calibration data for the azimuth gyro is now available. If
the INS is left in ALIGN, the self-calibration data will
continue to be refined and the alignment state will
decrease toward 0.
Navigation mode. In the navigation
(NAV) mode, all navigation data is computed and
available for display. All warning flags controlled by the
INS will disappear from view. The INS depends on its
own inertial instruments for all position and velocity
data. The present position computed by the INS can be
updated by TACAN, GPS, data link, or manually when
the aircraft passes over a known position reference.
Attitude mode. In the attitude (ATT)
mode, the computer is shut down and no navigation
information is available. The ATT mode is entered at
the discretion of the pilot if navigation and/or steering
data signals become unreliable. Attitude signals and
platform heading outputs continue to be available. The
NAV mode can only be reentered from the ATT mode
after the system has been realigned. Alignment can
take place only on the ground.