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mode selection. See figure 24-1.
24.1.6.1 INS Mode Select Knob. The INS mode select knob has switch positions of OFF, CV, GND,
NAV, IFA, GYRO, GB, and TEST.
OFF Removes power from the INS.
CV Commands the INS carrier align mode with the MC providing the carrier align display.
GND Commands the INS ground align mode with the MC providing the ground align
display.
NAV Commands the INS navigation mode which enables the MC to use INS information
to provide navigation steering.
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VII-24-9 ORIGINAL
IFA Commands the INS IFA (in-flight alignment) mode with an IFA display.
GYRO Commands the AHRS (attitude heading reference set) mode.
GB Commands the gyro bias mode enabling the INS to do a gyro bias calibration.
TEST Enables the INS to perform an initiated BIT upon command from the MC.
24.1.7 CRS Select Switch. The CRS select switch is used to set a course to the selected waypoint,
OAP, or TACAN station. When the switch is actuated with waypoint/OAP or TACAN direct great
circle steering already selected, a course line appears through the waypoint/OAP or TACAN symbol on
the HSI display and steering information appears on the HUD. The course line rotates clockwise when
the switch is held to the right and counterclockwise when the switch is held to the left. When a course
is selected, a digital readout appears on the lower right corner of the HSI display.
24.1.8 IFF Control Panel. This panel contains two ILS controls: ILS UFC/MAN switch and the ILS
channel thumbwheels.
24.1.8.1 ILS UFC/MAN Switch. When the switch is in the UFC position, ILS power and channelization
is controlled by the UFCD. With the switch in the MAN position, ILS power is enabled and ILS
channel changes are controlled by the ILS channel thumbwheels.
24.1.8.2 ILS Channel Thumbwheels. These thumbwheels are used to select ILS channels when the
ILS UFC/MAN switch is set to MAN.
24.2 INS/GPS INERTIAL NAVIGATION SYSTEM /GLOBAL POSITIONING SYSTEM
The AN/ASN-139 INS is a self-contained, fully automatic dead reckoning navigation system. The
INS detects aircraft motion and provides acceleration, velocity, present position, pitch, roll, and true
heading to related systems. Correction signals from accelerometers provide constant leveling. The INS
is coupled to the GPS (in AINS mode) to provide a more accurate aided source of position and velocity.
The INS uses both periodic and initiated built in test (BIT). Periodic BIT monitors essential
parameters within the system and provides in flight, shipboard, and ground failure detection and
isolation. Initiated BIT is performed on the ground and accomplishes that portion of the failure
detection and isolation capability which periodic BIT is unable to do.
NOTE
While it is acceptable to taxi with the INS OFF, the INS should be
ON, whenever possible, to reduce accelerometer wear. It is preferable
to switch to the GYRO mode and wait for course level complete (NO
ATT disappears) before taxiing.
24.2.1 GPS - Global Positioning System. The Global Positioning System provides position, velocity,
and time (PVT) data that can be used to aid the INS or as an independent navigation sensor. The GPS
consists of an aircraft mounted receiver/processor which receives modulated signals through the GPS
antenna from 24 high orbit satellites. Satellite data is used to determine aircraft position and velocity.
GPS can be initialized with crypto keys, enabling encrypted P-code (precise) navigation signals to be
received.
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VII-24-10 ORIGINAL
GPS has four modes of operation. In NOT READY mode the system is off. In Initialize mode (INIT)
the power supply is turned on, almanac data and waypoint data are loaded into the MC and Crypto key
loading may be performed with a KYK-13. In NAV mode the GPS tracks the best four satellite
constellations possible to provide the most accurate PVT solution. TEST mode is provided for
maintenance and inflight testing.
It is important to load Zulu time to aid in satellite acquisition. Local time, if desired, should be set
after takeoff. GPS is loaded with GPS precise time only once per cold start. Changing the aircraft time
or date with WonW re-initializes the GPS.
24.2.2 Inertial Navigation (NAV) Mode. Without GPS selected or operating, NAV mode is the
primary mode of operation for the INS. In NAV the INS provides smoothed attitude and attitude rates
to the MC for use in sensor stabilization. The INS provides position, velocity and acceleration
information for navigation and weapon delivery.
· If the INS shuts down abnormally (power loss) the INS mode selector
knob must be set to OFF for a minimum of 5 seconds.
· If the INS mode selector knob is turned OFF in less than 40 seconds
　
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