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Altitude in the DED INS page


Musis
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The altitude displyas in the INS page of the DED is linked with the baro altimeter. That means that the DED altitude is corrected during INS alignment when we set the baro altimeter in the same time (In my case the altitude in the INS has changed from -502 ft to 50 ft Senaki airport during a settting at 30.52 inch Hg.

I don't understand this behaviours for an INS.

Is it the reality ?

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It helps to understand the various altitude values in the F-16. The "SALT" on the INS page stands for system altitude which is distinct from a pure internal INS altitude reference.

 

From CADC pressure altitude, temperature, and Kollsman knob input a baseline nonstandard barometric altitude is formed. A mixture of this altitude and the integration of the INS vertical motion combines to form the system altitude. On top of system altitude are deltas from the A-CAL page either manual or automatic.

 

The only theoretical potential for the pilot to make system altitude bad using the barometric altimeter cockpit controls is to mess with the Kollsman knob. That would change one aspect of the mix. What I don't know is how badly changing the baro input would screw up sys alt especially of other updates are thrown in.

 

Usually when available GPS auto-calibration is used and every few seconds sys alt is getting updated back to GPS, Kalman filtered out to close to truth. If the baro alt input was way wrong it would just generate a huge delta number on the A-CAL page.

 

The SALT value on the INS page isn't an INS value but a starting value to which INS vertical movement adjusts it and is the first change for the airplane to generate vertical delta corrections. Ranging-sensor AG attacks (FCR AGR, lasing TGP) skip the "baro" calculation and your system altitude doesn't have an effect on aiming.


Edited by Frederf
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As I'm not sure about the impact of the baro setting on the INS, I have made it a personal habit of mine to ALWAYS set the correct baro pressure and switch it to elec BEFORE I start the INS alignment in the F-16.


Edited by QuiGon

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Thanks for you answer, as the SALT is a System ALTitude (mix of Baro and INS computed altudes), we don't need to confirm the altitude in the first aligment minute. What does-it append if we push ENTER and we set the baro altimeter during the aligment process ?

I do that recently with a non nomial behavoiurs of CCIP mode the "chute line" of my M82 has pointed the sky.

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From what I gather system alt is INS alt but it's subject to filtered updating from CADC. CADC alt might (because I don't know the algorithm only that there is one) be the long-term target value. Changing the CADC altitude wildly almost certainly doesn't change system altitude wildly at least not immediately.

 

INS is pretty good about vertical rate, e.g. if it says 1000 ft/min that's going to be quite close to the truth. However vertical rate accelerometer over long periods of time small errors in rate can add up to large displacements. So CADC makes a great long term displacement reference but a poor short term rate instrument. INS is the exact opposite. So a good design is going to appeal to the strengths of each.

 

What you'd want is to distrust any short term discrepancies. CADC can do weird things supersonic, changes of altimeter setting, INS updates, etc. CADC alt being wildly different than INS alt would tend to converge over long periods of time because INS can be trusted not to be hugely off in vertical velocity but slight errors are expected. If system altitude is 10,000' but CADC is 20,000' I would expect that the algorithm would drag the system altitude upward slowly over time, i.e. hours.

 

CCIP normally uses FCR AGR or TGP laser which does a completely different calculation and doesn't care about system altitude at all. CCIP can do baro if the radar is off. This is all assuming a more complete systems modeling that I think is justified at this stage of development. The idea that CCRP and CCIP are actually using different references for computing and displaying aiming is some detailed modeling stuff.

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