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Subject: How does guided arty work?
scholar    11/9/2004 4:34:57 PM
I just read that the US is firing guided 155mm shells out of howitzers at positions in Falluja. GPS. How does that work? Do little fins pop out of the shells? Or does the GPS simply guide the targeting computer on the gun itself?
 
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scholar    RE:How accurate is the present system?   11/10/2004 3:17:50 PM
That's still pretty far away! So this marks serious progress over other technologies, but remains a far cry from the precision of "smart" strikes. Thanks for the insight.
 
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ArtyEngineer    RE:How does guided arty work?   11/10/2004 6:52:12 PM
Yes it is, a lot of people fail to understand that artillery is an area weapon. Minor variations in the surface finnish of the projectiles will cause round to round variations even when firing on exactly the same QE and Deflection. A 1 m/second change in muzzle velocity can cause a change in range of up to 20 meters!!, the variation in muzzle velocity can be due to variations in the charge, or a poorly constructed ammo pit which means the charges are not kept in the shade and there are considerable temperature variations. Having an accurate propellant emperature is critical to having the correct firing solution to engage tyhe target. The meteorological data used in the firing solution may be up to 4 hours old!!!! At certain periods of the day mainly dawn and dusk the wind direction in the upper atmosphere where the bullet may spend 35 or 40 seconds can change by 180 degres in a few seconds!!!! After each round due to weapon displacement the Gunner and A Gunner have to get the tube back on Deflection and QE, these guys are usually 18 or 19 year old kids being hollered at by the section chief to move it. They have approximately 6 seconds to use levelling knobs and elevation and traverse handwheels to centre 2 or 3 bubbles in level vials. Im sure someone will reply in this thread saying this system or that system has an accuracy of so and so which is vastly better than 20 - 30 meters. True, under very controlled test conditions where you stack the deck in your favour with perfect conditions in all the areas I have mentioned this may be the case, but that is not the real world. Hope this calrifies some things for you.
 
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Worcester    RE:How does guided arty work?   11/10/2004 7:36:03 PM
ArtyEngineer: since the kill radius of a 155mm is at least 50m, accuracy of 25m is very good - it means the target will always be in the kill zone. Put another way, the collateral damage area to worry about will be anything up to 75m from the target.
 
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neutralizer    RE:How does guided arty work?   11/11/2004 3:54:45 AM
It's not quite that simple. First is that accuracy and consistency are different things. Accuracy is a measure of the distance between aimpoint and mean point of impact of the shells. Consistency is the measure of the shells' distribution around a gun's mpi. Both these distances are probability distributions, they are not absolute numbers. Arty measures these distributions in terms of 'probable errors', basically the maximum spread is 4 PEs from the mpi. Accuracy can be measured in the same terms. Accuracy is determined by the conditions of the moment, such as met conditions. Of course talking in terms of the relationship to the aimpoint highlights that there are errors in positioning the aimpoint on the actual target. PEs for consistnecy are given in firing tables. For at example at 10km one PE for a M109 or M198 is about 52 metres, this means that 50% of the shells will fall with 52 metres of their mpi along the line of fire (it's a lot less laterally). If you are fortunate enough to have a 155mm gun firing a modernish ammo system the the PE is down to about 28 metres. I'd have guessed that the PE for a GPS shell is a bit less, although the purpose of the shells is to improve accuracy at longer ranges, not short ranges of 10 km or so. Of course the GPS shell is also going to be closer to the aimpoint because the GPS is compensating for that as well. I'd guess that in Fallujah something like 1:15,000 or better scale maps ('town plans') are being used, these indicate individual houses so I'd expect target location errors to be very small. I'm basing this on the UK experience in Basra last year, they found they could destroy small bunker type positions with indirect 155mm fire from a battery converged (ie all guns aiming at the same point)opening with fire for effect (ie no adjustment). They put this down to good maps, good met, good survey and every gun having an MV radar. I assume the US Army and USMC have similar capabilities, although their PEs are larger than the Brit AS90s. Finally, HE shell lethal areas need caution, they do not destroy/kill everything with about 50 metres. First there is the fragment spread pattern, basically butterfly shape not circular, next there are probabilities and the spread of fragments, this means that you can get unlucky well beyond 50 metres, but also that there will be plenty of survivors within 50 metres, albeit with a decreasing number the closer to the burst. Of course this is complicated by the angles of descent and the height of burst if VT is being used (some VT fuzes have HoB options)
 
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neutralizer    RE:How does guided arty work?   11/11/2004 6:26:51 AM
Correction to my last post, the PEs quoted are 2 x PE, so 1 PE is half those numbers.
 
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neutralizer    RE:How does guided arty work?   11/11/2004 6:26:51 AM
Correction to my last post, the PEs quoted are 2 x PE, so 1 PE is half those numbers.
 
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ArtyEngineer    RE:How does guided arty work?   11/11/2004 12:55:33 PM
I did actually say that I considered 20- 30 meter accuracy to be very good. Was trying to give an appreciation to Scholar as to why this is the case.
 
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neutralizer    RE:How does guided arty work?   11/11/2004 10:38:31 PM
Accuracy is a tricky subject, but also seems so obvious. Interestingly in 1915 one of the biggest challenges the Brit arty faced was trying to get the infantry to understand what accuracy and consistency meant, and it was the latter that was more important to them! I think we'd find that accuracy, lets define it as average distance between aimpoint and mpi to keep it simple has been in the 20 - 30 m or less magnitude since computers were first used for tech fire control. The trouble is it's very seldom that actual mpi is accurately measured for a battery, so it's a bit subjective. The average probably wasn't much bigger pre-computers although I suspect the variance was substantially more because measuring map data was far less accurate and the methods of calculating met fairly crude compared to today where the computer 'flies' the trajectory applying met every second or so of time of flight, and keeps repeating the flight until the trajectory hits the aimpoint (or some small tolerance around it). (Well tht's one way of doing it, MLRS is different and I'm still trying to find out what the NABK expects).
 
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Mike From Brielle    RE:How does guided arty work?   12/9/2004 10:32:32 AM
I don't think their refering to the actual shells being guided their instead refering to the precise location of the tubes, target, and Foward observers which would give a much more accurate shot.
 
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Texastillidie    RE:How does guided arty work?   4/4/2005 8:13:10 PM
A guided artillery shell such as a "copperhead" 155mm is fired in the general vicinity of a target by the cannon. Once the target is designated by a laser, the shell is terminally guided by controllable fins on the rear of the shell. The terminal guidance is quite accurate. Years ago I watched a Paladin hit a 4X8 sheet of plywood from 7 miles on Blackhorse range at Ft. Hood.
 
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