(c)  
The control/display unit (Fig. 8) is used to insert initial position, waypoint, and destination data into the system and to display various navigation data generated by the system. These data are also provided to the horizontal situation indicator (HSI) and the autopilot; however, such data are controlled by mode selectors associated with those systems.

 (d)  
Terminology used in navigation with an inertial system is similar to that used in other forms of navigation with the possible exception that the former is referenced to true north rather than magnetic north. Fig. 3 illustrates INS terminology. The abbreviations correspond to those used on the selector switch of the control/display unit.

 (2)  Operating Modes
 (a)  The INS has five major modes of operation that are controlled by the mode selector switch on the MSU. These modes include OFF, STBY (standby), ALIGN, NAV (navigate) and ATT REF (attitude reference).
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 (b)  
STBY and ALIGN modes are used in leveling the INS platform in preparation for the NAV mode of operation. Platform leveling from STBY through ALIGN to NAV (status 90 through 02) requires a minimum of 905 seconds (max is 1265 seconds). NAV mode may be selected at status 02 without a READY NAV light. Status 02 without a READY NAV light indicates an MSU failure. Moving the mode selector switch to STBY or ATT REF, while the INS is in NAV mode, destroys the INS alignment. Selecting ATT REF shuts off the INU digital computer.

 (c)  
ALIGN may be selected at turn on but the INS will not complete the align mode until after present position has been entered into the system. The NAV mode may be selected when the MSU READY NAV light illuminates to indicate correct IRU platform alignment.

 (d)  
When only attitude outputs are desired from the INS, ATT REF may be selected directly at turn-on. Attitude signals may be used three minutes after setting the mode selector switch from OFF to ATT REF.

 (3)  
System Startup

 (a)  The MSU mode selector switch is moved from OFF to STBY (standby). This applies power to the INS system so that present position and destination data can be inserted and allows for system warmup. As an alternative, the mode selector switch can be moved directly to ALIGN, and the system will automatically cycle through STBY but will remain in status 80 until present position is inserted.

 (4)  
Present-Position Data Entry

 (a)  
Set display selector switch to POS (present position). Present-position latitude is entered from the data entry keyboard by first pressing N or S for north or south latitude, then the most significant digit to the least significant digit. Depressing N (or S) determines that data will be read on the left display and also illuminates the INSERT pushbutton. When the numbers are entered, the most significant number will start in the 0.1-minute position and advance to the left as the remaining numbers are inserted. When the correct latitude is displayed, the INSERT pushbutton is pressed to load the latitude number into the computer. The INSERT pushbutton will remain illuminated.

 (b)  
After the number has been inserted, the displayed number should be rechecked. When the INSERT pushbutton is pressed, the number is transferred to the computer, and the display then reads directly from the computer. Rechecking confirms that an error did not occur during the data transfer.

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 (c)  Present-position longitude is entered in the same manner as latitude except that E or W for east or west longitude is depressed first. Depressing E (or W) determines that the data will be read on the right display. The number is then entered on the keyboard and inserted in the computer by pushing the INSERT pushbutton. The INSERT pushbutton now goes off.
 (5)  Waypoint Data Entry
 (a)  Waypoint latitude and longitude are entered in a similar manner as present position except that the display selector switch is moved to the WPT (waypoint) position, and the waypoint thumbwheel should be set to the proper waypoint number (1 thru 9). Waypoint data can be entered when the mode switch is in STBY, ALIGN, or NAV.
 NOTE: When entering waypoints, the INSERT pushbutton goes out
____ after entering the first coordinate.
 Waypoint 0 is reserved for the computer to establish a track from the airplane present position, and cannot be used by the operator to enter waypoint coordinates.
 (6)  
Clearing a Data-Entry Error

 (a)  
　
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