AN/ASN-92 INS Part II: NAV MODES and COMP Panel

AN/ASN-92 INS Study: Table of Contents

Part I: Components
Part II: NAV MODES and COMP Panel
Part III: Carrier Operations
Part IV: Fix Update and Troubleshooting
MUST know: Fix an F-14 Departing from a Carrier (Simplified discussion post AHRS bugfix)

Navigation Modes

The navigation mode can be selected via the NAV MODE knob, located in the top-left corner of the Tactical Information Display.

Listed below is a general summary of navigation mode data sources used for general navigation:

INS Mode

After the completion of the alignment, the primary navigational mode selected by the Radar Intercept Officer is INS.
The prime sensor is the IMU, supplying velocity pulses to the CSDC which in turn computes all inertial outputs. The IMU also provides data about roll and pitch.
The IMU and the CSDC provide, when the INS Nav mode is selected:

Aircraft latitude and longitude;
Aircraft magnetic or true heading (depending on CAP prefix pushbutton selected);
System altitude (barometric damped inertial altitude);
Platform wander angle;
Velocity components (x, y, z);
Vertical acceleration.

The magnetic heading of the aircraft is derived from the AHRS. This input is used by the WCS to update the computed data, by weighting them against its calculations made in true north coordinates for steering and other purposes.
The wind parameters are calculated from the difference between the inertial velocities and the air mass velocities.

IMU/AM Mode

The first backup mode available is the IMU/Air Mass mode. The RIO can select this mode, or it can be entered automatically if the CSDC determines that inertial velocity data from the IMU is unreliable (quantizer fail).
To provide general navigation functions in this mode, the true airspeed provided by the CADC and the stored or entered wind parameters are combined to provide ground speed and true heading. Pitch and roll data is provided by the IMU.
Note that if an aligned platform exists with no NAV COMP failure and the RIO switches to IMU/AM, the READY light will flash, indicating that if the switch is not returned to INS within 5”; the INS mode cannot be re-entered without performing a new alignment.
If an IMU reset is required, the procedure is the following:

switch NAV MODE to OFF for a few seconds;
switch NAV MODE to IMU;
fly straight and level for 5 minutes;
verify the IM acronym.

AHRS/AM Mode

The most degraded navigational mode available is AHRS/Air Mass. Similarly to IMU/AM, it is either selected by the Radar Intercept Officer or can be entered automatically when the CSDC detects a total INS failure.
To provide basic navigation information, the heading is derived by the magnetic heading plus the entered or stored magnetic variation. True Air Speed, barometric altitude and altitude rate are provided by the CSDC, and the entered or stored wind parameters are used. Pitch and roll data is provided by the AHRS.
Note thatm if the IMU fails, the navigation mode switches automatically to AHRS, but the STBY and READY lights remain on until the “AHRS/AM” Nav mode is manually selected.

The AHRS/AM supports three additional submodes, controlled by the Pilot by means of the Compass Controller Panel displayed above (this panel is discussed more in-depth later).

SLAVED: Default mode used when reliable magnetic heading reference is available. The directional gyro is slaved to the magnetic north reference (flux value).
DG: Used when the magnetic reference is unreliable. The gyro is not slaved, but it is compensated for drift.
COMP: In this mode, HUD, VDI, HSD and ECMD use the manual magnetic variation value (vM) automatically. Direct magnetic north reference with no gyro damping.

FUNDAMENTAL!
If you have not set vM (perhaps because you have spawned the F-14 hot), switching to COMP will not work. Since the player has not initialised vM, the F-14 will use vC, thus showing unexpected results. Thus, remember to re-insert vM before switching to COMP!

Nav modes: Nota Bene

It is important to note that the RIO may switch from either INS mode to AHRS/AM, or from IMU/AM to AHRS/AM modes in flight comparison purposes without introducing a degradation, since the AHRS is a separated system. However, this operation cannot be done between the INS mode and the IMU/AM mode because the IMU is used in both cases, and a permanent degradation of the IMU alignment will be introduced.

Other Positions

On top of the mentioned INS, IMU/AM and AHRS/AM; the other positions of the NAV MODE knob allow to:

OFF: disconnect power to the IMU;
ALIGN: starts the INS alignment program, using one of the following methods (note that each of them requires different inputs and parameters):
1. GND: “feet-dry” alignment method. The aircraft latitude and longitude are required for initialization;
2. CVA: mode used when operating from an aircraft carrier. It may or may not take advantage of the data provided by the ship’s inertial navigation system (SINS). If these parameters are not provided, then the RIO has to input the ship’s latitude and longitude, true heading and speed;
3. CAT: allows the continuation of the alignment with the aircraft on the catapult and with the handbrake released;

Important!
If the ship is turning, do not switch the NAV MODE knob to INS, even if the fine alignment has been completed. Otherwise, the resulting alignment quality will be greatly affected.
The handset alignment is not affected by this scenario.

Compass Controller Panel

The following is a more in-depth look at the Compass Controller Panel mentioned before.

NOMENCLATURE	FUNCTION
SYNC IND	Indicator is deactivated in DG and COMP modes. In SLAVED mode, it indicates the synchronization between AHRS directional gyro and magnetic azimuth detector.
N−S Hemisphere select switch	Used in DG and SLAVED modes, it allows the selection of the hemisphere (N or S) in which the aircraft is operating. The correct value must be preselected to provide accurate Earth−rate corrections.
LAT correction control	Knob used in DG and SLAVED modes: the latitude between 0° and 90° must be pre-selected to allow for earth rate corrections.
COMP−SLAVED−DG mode switch	COMP – This position is used if the directional gyro malfunctions, and for emergencies only. The compass heading is obtained directly from the magnetic detector, without stabilization by the platform directional gyro. The digital displays (HUD, VDI, HSD, and ECMD) use manual magnetic variation (vM) automatically in this mode. SLAVED – Standard operational position. The magnetic heading is provided from the Magnetic Azimuth Detector (MAD). DG – This mode does not use the Magnetic Azimuth Detector (MAD). The directional gyro mode uses the selected gyro heading.
HDG pushbutton	This knob has a pushbutton function. The action depends on the selected mode: In SLAVED mode, the pushbutton synchronizes the directional gyro with the magnetic azimuth detector and sets magnetic heading on the BDHI. The pushbutton must be depressed and held until the synchronization indicator needle is bracketing the null mark. Note that the synchronization cannot be accomplished if the aircraft is accelerating or decelerating by more than 75 kts per minute. In DG mode, the pushbutton set the selected heading on the BDHI. Depressed the knob and rotate it CW or CCW to set the desired value. In COMP mode, the pushbutton has no function. To obtain a fast pitch and roll erection (for up to 12°/minute minimum), depress and hold the pushbutton for up to three minutes. Allows at least 1 minute before repeating the 3-minute cycle.
AHRS advisory light	The green light located in the Caution-Advisory Indicators panel turns on when the attitude or the heading information provided by the AHRS is unreliable.

Bottom part of the Caution-Advisory Indicators panel, with the AHRS light illuminated.

Magnetic Variation Sources

The MagVar used in the computation can be either the vC or the vM depending on what the crew selects:

CONDITION	MAG VAR SOURCE
COMP mode selected by the pilot.	Manual MAG VAR (vM)
RIO enters manual MAG VAR after selecting AHRS navigation mode.	Manual MAG VAR (vM)
RIO updates MAG VAR after IMU or AHRS failure.	Manual MAG VAR (vM)
All other conditions.	Current or last value computed MAG VAR (vC)
The computed MagVar value (vC) will continue to be the reference MagVar source if the RIO switches to AHRS/AM and does not enter (or re-enter in case no changes are required) a manual MagVar value.

The AWG-9 continuously calculates the value of the computed magnetic variation by comparing the true heading provided by the IMU with the magnetic heading from the AHRS. The difference between the two values is stored as the computed MagVar (vC).

Part III will focus on Carrier Operations, showing a ubiquitous issue caused by flying off a huge chuck of metal.