curious about why 105s behave like this

[maxmax]

Not complaining here, just very curious.

 

Normally 105s are green in dsms when flying. Then all of the sudden they go white and can't be released. To get them green again you roll the hog a couple of times, i.e. quick left and right on the stick, and then level flight again.

 

Why is this?

[Frostiken]

Just the unique way the alignment system programmed into the WCMD works. I imagine it's like a joystick calibration - you flex it around to all the limits and once you're done it uses that to find the 'center'.

[159th_Viper]

I' m pretty sure the INS of the JDAM degrades over time and requires re-alignment, hence the need to maneuver. That said, wait for one of the avionics specialists to confirm :)

 

Edit: Norrised.....

[maxmax]

Cool, some kind of calibration then. It can be quite annoying when you forget to check if they are actually active. And you press to release.. and then nothing. Nowadays i always keep dsms open when using 105s :)

[Frostiken]

Wait for one of the avionics specialists to confirm :)

 

Ammo would be a safer bet. Avionics learns the weapon integration, but the weapons themselves aren't us at all. Anything I know about them I've gleaned from other sources, and to be fair it really doesn't impact our job enough to be worth knowing.

[ralfidude]

That is correct, Aligment is degraded over time.

 

Look under the 105s in the DSMS, when they turn white it also says ALN DEG

 

Im not eaxctly sure why it degrades when flying in a straight line, and it fixes itself upon changing course and direction, but it does. So i have the habit to fly off angle on a target until i get close, then turn into it so that i have no aligment issues before drop.

 

This has been confirmed on another thread.

[Tucano_uy]

I wish they had a laser seeker instead of INS/GPS guidance, especially to use them against moving targets. I guess there's a good reason for them to be the way they are.

[Jona33]

I wish they had a laser seeker instead of INS/GPS guidance, especially to use them against moving targets. I guess there's a good reason for them to be the way they are.

 

Yep clouds, all to easy for the laser to go wrong an the bomb to hit the wrong place.

[Frostiken]

Yep clouds, all to easy for the laser to go wrong an the bomb to hit the wrong place.

 

If you're using a cluster bomb, odds are you don't give a shit about what's in the area anyway.

 

 

Kind of why I find it funny whenever JTACs ask for CBUs all the time...

[Depth]

If you're using a cluster bomb, odds are you don't give a shit about what's in the area anyway.

 

 

Kind of why I find it funny whenever JTACs ask for CBUs all the time...

Crazy bastard once wanted me to use a CBU to take out a single stationary tank when I was loaded with 4 mavs and a couple of GBU-12s

[RodBorza]

That is correct, Aligment is degraded over time.

 

Look under the 105s in the DSMS, when they turn white it also says ALN DEG

 

Im not eaxctly sure why it degrades when flying in a straight line, and it fixes itself upon changing course and direction, but it does. So i have the habit to fly off angle on a target until i get close, then turn into it so that i have no aligment issues before drop.

 

This has been confirmed on another thread.

From Wikipedia:

"All inertial navigation systems suffer from integration drift: small errors in the measurement of acceleration and angular velocity are integrated into progressively larger errors in velocity, which are compounded into still greater errors in position. Since the new position is calculated from the previous calculated position and the measured acceleration and angular velocity, these errors are cumulative and increase at a rate roughly proportional to the time since the initial position was input. Therefore the position must be periodically corrected by input from some other type of navigation system. The inaccuracy of a good-quality navigational system is normally less than 0.6 nautical miles per hour in position and on the order of tenths of a degree per hour in orientation. If Navigation Systems malfunction, they can send planes off course.

 

Accordingly, inertial navigation is usually used to supplement other navigation systems, providing a higher degree of accuracy than is possible with the use of any single system...

 

... By properly combining the information from an INS and the GPS system (GPS/INS), the errors in position and velocity are stable. Furthermore, INS can be used as a short-term fallback while GPS signals are unavailable, for example when a vehicle passes through a tunnel."

 

Link: http://en.wikipedia.org/wiki/Inertial_navigation_system

 

What maneuvering does is "mimic" the correction input from other type of navigation system. Actually, maneuvering gives the system a new set of coordinates to start from, and thus, for this new set of coordinates the error is zero. Maneuvering "forces" the system to find the new position, not relying on old information. That's why the error is diminished.

 

Our not so beloved SAMs use this to correct their inputs when launching. When sitting down on the ground, their info position is already badly degraded. When lauching, the maneuver that the missile does not only puts it on the optimal position to intercept, but also corrects the position error close to zero.

What if means is, if you don't see it, don't maneuver, and don't desperately release chaffs and flares, you will end up like the wingie: a big fireball on the ground.:pilotfly:

Edited March 5, 2012 by RodBorza

[RobC]

Yep clouds, all to easy for the laser to go wrong an the bomb to hit the wrong place.

 

I was under the impression there was a cost consideration behind this as well (same reason they use an INS rather then GPS), but I might be wrong.

[sobek]

I was under the impression there was a cost consideration behind this as well (same reason they use an INS rather then GPS), but I might be wrong.

 

You can't use a GPS without an INS, because GPS is too slow in providing updates for a flight control computer and it is also prone to jamming. You have to remember that a JDAM flight duration is on the order of minutes, not hours. This is short enough for a conventional INS to be pretty accurate.

[Tucano_uy]

What if means is, if you don't see it, don't maneuver, and don't desperately release chaffs and flares, you will end up like the wingie: a big fireball on the ground.:pilotfly:

 

Would you mind to elaborate?

 

Obrigado

[RodBorza]

Would you mind to elaborate?

 

Obrigado

 

It was a sad attempt on a joke of my part. What I meant is that when a missile is launched, it is better to take evasive action, and fast!

 

But elaborating, as you requested:

 

1) A ground radar station detects an aircraft approaching. The radar station knows its own position (position A), and knows the position of the incoming aircraft (position B). See picture below.

 

2) A few meters to a hundred meters away sits a missile battery. The missiles have their own inertial system, so they, theoretically, know their position very well (position C).

 

3) It then, should be fairly easy: the missiles are launched from the battery, they receive information from the ground station where the aircraft is (position A) and, a by a little triangulation and course correction, bam!, the missile hits and that's it!

 

4) The problem is that inertial systems are subjected to drift (as explained before). So even if position A and position B are accurate, position C is not. It happens because the missiles are siting there for so long, that their position is off by, at least, hundreds of meters. In other words, the missiles don't know exactly where they are anymore. They think they have moved and now are in a new position (position C'), but, in reality, they are exactly where they were initially (position C). Position C' is a "virtual" position, a position given to the missile guidance system by its own errors.

 

5) As the missile thinks it is in a new position, it also thinks that the position B, and consequently position A, have moved as well.

 

6) So, if the missile is launched in this condition, when the ground station gives the coordinates for the missile to intercept, as the initial position is off, the missile will either over steer or under steer to a "virtual " Position A', and will miss.

 

7) Enter then, a strange, yet very useful phenomenon. It is called accuracy improvement (error reduction) of inertial systems by rotational motion. The actual mathematics behind this is very complex, but basically, what it means is this: every time a inertial system is rotated on one of its axis (pitch, roll or yaw), the system error is greatly reduced.

 

8) A missile when is launched from the battery, first go straight out, to get off of the platform. Then it makes an upward curve, to get to the incoming aircraft altitude. This maneuver counts as one rotational motion, in this case, in pitch.

Then, the missile makes another maneuver, in order to get level to intercept. This second maneuver counts as a second rotational motion in pitch, which causes the error to be reduced even more.

 

9) By doing this, which is natural to missiles operation, the error is greatly reduced, up to four times the initial value. This causes the position of the missile to be accurate again. Now, with information received from the ground station, the missile has a great chance of hitting its intended target. That's why a pilot has to start evasive maneuvers, because he has a great chance to being hit.

 

10) What is described here is one type of error reduction, in this case, a self-contained method. This is what applies to the CBU-105, which has only its own inertial navigation system to look at.

For other systems is used what is called sensor integration. Sensor integration on inertial navigation systems causes the error to be greatly reduced. It is always the gyros plus some other sensor.

 

11) In the case of missiles, the fused sensor is radar (its own or ground/platform based). That's why the radar paints its target all the time when tracking, to update target position to the missile's guidance system.

 

12) In the case of the A-10, the inertial system is called the EGI, which stands for Embedded GPS/INS Navigation System. What it means is that the system already comes from factory ready to be used integrated with the aircraft's GPS.

Also, if I can extrapolate a little more, I would risk saying that the INS from the A-10 also takes signals from the altimeter and radar altimeter. All to help reduce system drift error.