Some INS and Mirage real facts and figures

[diditopgun]

Hi all ! :)

 

Here are some informations about INS given by a real Mirage crew member (thanks to him and sorry for my English, I try to do my best) on Check-six french forum:

 

I'll answer some questions about the INS.

 

I will talk mainly about Mirage 2000N and D and IRL. I suppose it is the same for others M2000 versions.

 

Alignment can take between 1'30 and 8mn for standard use, but can be up to 30 minutes!

 

When we want to align the inertial unit, we have a doc with Lat-Lon-Alt coordinates for shelter and other points of parking, as well as heading.

 

An inertial unit is a platform kept horizontal by gyros on which there are three accelerometers.

It measures the speed of rotation of the Earth thanks to the accelerometers and has the latitude information you enter to determine true heading.

 

If you do a mistake introducing this latitude, the heading that the inertial unit will find will be wrong.

 

Once you have entered into DCI 00 those two coordinates (plus the heading that allows the inertial unit to not use a high speed of gyros precession and to know roughly at near 18° the actual course - I'm not going to go into details) and check its temperature in DCI 516 (important for the expansion of the elements are optimum and avoid premature wear of the inertial unit), we launch alignment.

 

If the inertial unit has been aligned prior to (and the aircraft has not moved), we can switch to NAV after 1mn30. It's ALCM.

We have a inertial unit drift at about 3 NM / h.

 

At 4'20, we can make an ALI (ALignement Interrompu - Interrupted ALignment). The heading is strongly rough out, there is also a drift of about 3 NM / h

 

8 mins, heading research is over. We can switch to NAV. Accuracy of about 0.8NM / h

We can also let the inertial unit go into a second heading research. This is the fine cap phase. It takes 5 minutes more. Total 13mn. The accuracy will be better and the drift slightest.

 

You should know that when the platform has an accelerometer aligned to the north and the other set to almost 90 ° from the one to the north (plus the vertical one). These two horizontal accelerometers are not welded together to be at 90 °, but they are articulated.

 

At each alignment, the platform rotates by 90 °, and thus aligns to north an accelerometer, the other rotate by 90 °.

 

When aligning, the one facing east (or west depending) does not move. And accelerometer that is in alignment phase moves north through the articulation between the two accelerometers.

 

So an alignment phase aligns only one accelerometer!

 

Then we can make a double alignment:

 

We switch to NAV, there is at this point accelerometers tilt by 90 °, then we switch off the inertial unit, we wait two minutes for the gyros stop, then we relaunch an alignment. Here we go again for the time we want.

 

So we can go to: 8+5+2+8+5min = 28min!

 

As for the altitude information inserted manually, it is important to determine the plateform horizontality.

 

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When the recorded heading during shut down of the inertial unit is greater than 18 ° of the current airplane heading (if we moved the plane meantime so), the inertial unit makes precess the heading gyro at high speed and when 18 ° are reached, the precession speed decreases.

The problem is that there is residual inertia in this high-speed movement, so the heading won't be very stable.

 

So we insert a coarse heading in the INS before aligning (a heading given by the compass decreased of declination is enough, we have 1-2 ° precision then) and heading gyro will be in small precession speed.

 

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About inertial unit alignment they always do that on GPU or there is enough battery power time ?

 

It's done on Park group because it needs 400Hz to feed the inertial unit.

We avoids to do it while engine running due to vibrations.

 

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Have to always check the heading in the INS in DCI 100so

 

The ALCM is a particular alignment mode for takeoff on alert, mainly used by the PO (Permanence Opérationnelle - Quick Reaction Alerte). You must have done previously an ALN, so, did not have move the plane after his return to parking.

 

An ALCM requires having the coordinates lat / lon / alt and exact heading.

 

An ALN, normal ALIgnment, always takes 8mn, whatever the heading inserted.

 

To determine the heading, the INS uses the the latitude's cosinus and measures the Earth's rotation speed.

 

Maybe it can help you Zeus ?

Edited June 28, 2016 by diditopgun

[Zeus67]

Hi all ! :)

 

Here are some informations about INS given by a real Mirage pilot (thanks to him and sorry for my English, I try to do my best) on Check-six french forum:

 

Maybe it can help you Zeus ?

 

Yes, it does. Thanks.

 

But I feel this thread should be moved form the bug reports into the general forum section.

[diditopgun]

Yes I did a location mistake.... I switch between too much internet forums and tabs, sorry ^^