Forum: F-16 Procedures

No data yet, shoot!!!! Come on, please? Crying or Very sad

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How did I miss this question the first time around...!

Aside from the Stored Heading alignment (already covered), there is the Best Available True Heading alignment (BATH) that is/was never used because of it's massive inherent error. The BATH was intended for cases where you wish you'd done a Stored Heading, but for some reason moved the aircraft. Operationally, this would never happen because if you're putting an aircraft on alert, and some knucklehead moves it after the hot-cock, you would simply do another hot-cock.

If anyone wants the boring explanation of what acutally happens during the alignment, and why it still takes 8 minutes to align a solid-state RLG, PM me and I'll give you the scoop.

Some information surrounding the Ring Laser Gyro (RLG) (Un-classified version, training but yet technical).

Keep in mind, while the end results of a RLG are the same as that of a "gimballed" INS, the theory of operation is quite different.

I'll try and explain.....Sagnac Effect (gets technical):

Optical gyroscopes, such as the RLG, operate on an optical principle called the Sagnac effect that deals with the properties of two light beams traveling in opposite directions around a closed loop. For example...lets say two atheletes running in opposite directions but at the same speed around a race track is a good analogy. If they start together at the same point, then they will cross the finish line at the same time.

In the Sagnac effect: the runners can be likened to light beams and the track to the closest light path formed by a set of careful aligned mirrors. Two beams of light are directed to shine into this ring of mirrors, one beam in the clockwise direction and the other in the counterclockwise direction. If the ring rotates clockwise while the light beam are in transit, then the clockwise beam will seem to travel a shorter distance then the counterclockwise beam and so exit the ring at the finish line.

This can be used as a measure of rotation by observing the interference (or fringe) pattern created at the finish line by the two beams of light when they mix together. The faster the rotation, the larger the fringe pattern displacement. A rigorous derivation of the Sagnac effect from a mathematical point of view involves the use of Einstein's theory of relativity. The simple pictoral analogy described above is only meant as a guide to aid in understanding the operation of optical gyros. Believe it or not, this effect was first discovered back in 1913 about the same time the first crude gyro compasses (the forerunners of today's mechanical gyros) were being manufactured.

Lasing Medium:

This medium of the RLG is helium and neon gas. When a high voltage is applied to this gas mixture, the necessary light is produced just as in the red neon signs seen in todays advertising displays. The laser beams travel in opposite directions around the light cavity as described in the Sagnac effect but now they make close to a million round trips before exiting at one of the mirrors. This means the mirrors used in RLG's must have very low losses, more than 1000 times less than a typical shaving mirror. As the beams exit the cavity, they mix together and form a fringe pattern to move across the detector: thus, by counting the number of fringes crossing the detector, a measure of rotation can be obtained.

Whats all this mean...I'll tell you

The RLG uses two physical phenomena.
1. The Sagnac effect that produces a path - length difference between two counter-propagating beams that depend on the rate of rotation.
2. The resonance properties of a laser cavity that convent the path - length difference into a frequency difference that is much easier to measure and makes the device much more sensitive to rotation.

The F16C/D aircraft incorporates a strapdown ring laser gyro inertial navigational system. The inertial sensors (gyros and accelerometers) are fixed with respect to the vehicle body and as such, measure vehicle angular rate and acceleration vector components. As a result, in a strapdown mechanization, the accelerometer output (in body axes) is transformed into local level (computational axes) and then fed to the navigation processor for navigational solution....thats the best detail I can come up with.

Long story shortened......in NORM....gyrocompass alignment mode in which external heading or stored heading information is not required for alignment, course alignment is achieved after 6 to 10 minutes depending on initial platform temperatures. At this time, navigation MODE may be selected for navigation at reduced accuracy. If navigation mode is not selected, fine alignment is achieved after an additional 2 minutes. Selecting navigation mode at this time will result in navigation at specified accuracy.

I hope this helps you understand the RLG effect better and times involved

Peros-
Here's a couple of pics for ya, hopefully what you wanted. These are really small files sizes and I don't know how to resize them so they don't look so pixilated, sorry. If anybody knows how, i'm all ears.

Good photo...works for me

From my personal collection...

NOT from any Lockheed Martin proprietary documentation
NOT from any Honeywell proprietary documentation
NOT from any USAF web site
NOT Classified or restricted in any manner

Best Regards,

Very clear and concise photos....

Thanks for the pics, not everybody can climb up in an E&E bay to see what it looks like sitting in there. Reminds me of how tight and cramped it is in there from the few times I have been in one.

Oh dear lord.....nice pics but.....Block 40/50 do take 8 minutes for a full alignment. If you are "hot cocking" a jet, you run the INS for 8 minutes then go straight to OFF, this preserves the alignment for the next pilot (stored heading!) Only active duty Vipers that take 4 minutes are the (guard owned) block 30's. Once you start a hot cocked (never knew why they called it that) it takes about 1 minute for a full alignment.

Enthusiast Joined: Apr 20, 2005 - 03:38 PM Posts: 74

I remember on the RNoAF F-16A OCU's the INS needed about 9 minutes to get ready.

What the MLU's of today require I have no idea.

Hi,

Could someone please tell me, does the F-16 use the same INS inertial sensors (gyros & accelerometers) for its FLCS, as for navigation/position keeping, or is it a separate system?.

Also is the system an AHRS or an INS?

Thanks
Obi

Usually FCS uses it's own rate sensing gyros. As I know F-16C have not an additional AHRS, only standby ADI as the backup attitude source.

Olgerd wrote:

Usually FCS uses it's own rate sensing gyros. As I know F-16C have not an additional AHRS, only standby ADI as the backup attitude source.

Thanks Olgerd.

I'm interested in the older systems, not the modern strapdown systems such as RLG. A Google search suggests that the F-16 used the Litton LN-39 INS. I can't find any info on FLCS sensors unfortunately.

Cheers
Obi

Wow.. I was perusing the net looking for some F16 INS info and stumbled on this thread.

Brings back some memories! I used to work on the old ASN-92 (RF-4B and S3) and subsequently the AN/ASM-130 (F/A-18A & Harrier) made by Litton G&C. The older 92 system weighed in at about 300 lbs if you add up all the boxes and the main computer (ANCU?) still had the old ferrite core memory.

I left the service just before the RLG made it's way onto the scene. Would have really liked to work on those platforms.

Does anyone know why sustained climbs or dives (3 minutes plus) can cause INS vertical velocity errors, particularly at low speed and or low altitude?

Cheers
Obi

Forum: F-16 Procedures

INS alignment time and time to takeoff from cold start