Radar fire control

Guns, torpedoes, mines, bombs, missiles, ammunition, fire control, radars, and electronic warfare.
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marcelo_malara
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Radar fire control

Post by marcelo_malara » Wed Sep 03, 2008 1:53 am

Hi all:

Not to disturb the Yamato vs. Iowa thread, I created this new one to discuss some aspects of the radar fire control system.

I am in the process of reading Dreadnought Gunnery and the Battle of Jutland, a must for naval enthusiasts, and finally got a grasp on the intricacies of optical fire control.

First a brief description of an optical system.To begin with, a fire control system needs to determine:

1-enemy course and speed
2-range to the enemy ship
3-bearing to the enemy ship
3-own ship course and speed

The data is obtained from:

1-enemy course and speed -> estimated at first
2-range to the enemy ship ->pointing the rangefinder to the enemy ship
3-bearing to the enemy ship -> reading the bearing of the rangefinder
4-own ship course and speed -> from gyrocompass and log

Then some derived data is needed:

5-range rate: a velocity. How the range varies with time ALONG the line of sight to the enemy.
6-speed across: a velocity too. How quickly the enemy moves PERPENDICULAR to the line of sight.
7-bearing rate: an angular velocity. How the bearing varies with time. It is equal to speed across/range

To do this (1), (3) and (4) are entered in some type of vector calculating device (a Dumaresq for example) and obtained.

Then the range, range rate, bearing and bearing rate are entered in a rangekeeper, a device that starting with the entered range and bearing and at the range rate and bearing rate speed, display the values as time runs. The initial values of range and bearing are also plotted in a paper. The rangefinder goes on taking ranges and bearings, these are plotted along with the initial values and compared with the rangekeeper values. If the enemy course and speed were correct, the values coincides. If they do not, enemy course and speed are corrected, range rate and speed across recalculated, and the new values entered in the rangekeeper. This is called a "cross-cut" (British terminology) and so process goes on.

The range and bearing are corrected for time of flight and other ballistic parameters. The guns are elevated and trained and fire starts. The spotting corrections are added as separate values to the bearing and range, and eventually used to correct them.

Now, it would be interesting to see how all this tasks are acomplished in a radar equipped ship. If any member can describe it would be wonderfull. Anyway I have a couple of questions:

-I know that radar is a poor bearing indicator. So how are the bearings obtained and with what accuracy? Remember that this is necesary for range rate and speed across (and from the last derive bearing rate).
-With lack of accurate bearing, how are enemy course and speed obtained?

Sure more questions would appear.

Regards to all

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Re: Radar fire control

Post by Bgile » Wed Sep 03, 2008 3:44 am

This is my understanding. I'm fairly sure I'm correct in general ... in the details I may be off a bit.

Here is the way radar range determination works with an "A" scope:

The operator has a crt display showing a horizontal line drawn by the radar equipment. At some point along the line there is a spike representing the contact. The operator turns a crank, lining up his cursor with a point on the spike. His crank is connected to the rangekeeper by an electrical circuit, so as long as he keeps the cursor on the target, the mechanical computer in FC is getting continuous range input. It's more accurate than optical systems at long range, and operator fatigue isn't nearly as much of a factor.

Bearing Input:

Optical bearings were preferred for much of the war. The director operator simply lines up his reticle with a point on the ship and keeps it there. This provides bearing input to the computer in FC.

Late war radars (Mk 8 and Mk13 in US service) provided acceptable bearing input. The operator lined up his cursor with the center of a "blip" and kept it there. That provided continuous bearing info to the FC computer.

Target course and speed:

As long as there is bearing and range data coming from the above sources (along with own ship course and speed from gyrocompass and pit log), the computer in FC provides an instant and continuously corrected target course and speed to the computer. There is a mechanical counter which one can read it off of. It also computes gun bearing and elevation continously, and the guns on late war US battleships were normally under full remote power control, so any time the guns are ready to fire, they can be fired. First salvo usually straddles the target, and you can fire as often as you want, since if the system is aligned properly each salvo will straddle the target unless it makes a radical change of course. If it DOES do that, you know it almost immediately since the inputs are continuous, and the FC system keeps pointing the guns at the new predicted location. I know the FC officer can input different types of patterns, such as deliberately alternating slightly short and long salvoes onto the target. I believe that particular thing is what Washington was doing at 2nd Guadalcanal.

I'm not aware of any paper coming out of any of the machines. There is a table with a glass surface and I think there is a lighted "bug" under the glass showing own ship's location relative to the beginning of the engagement. At fixed intervals you put a dot where the bug is and draw a bearing line to the target and another dot at the proper range. This isn't part of the FC solution; it's just to give the personnel a paper record of the engagement and a visual representation.

This computer is entirely electromechanical, with gears and cams. The US Navy considered replacing it with a digital computer when reactivating New Jersey for service in Viet Nam, but decided it wouldn't be any more accurate so they didn't bother with the additional expense.

I'm sure there are some errors here, but I think I have the general function of this system down fairly well.

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Re: Radar fire control

Post by Dave Saxton » Wed Sep 03, 2008 8:58 am

marcelo_malara wrote:.......


Now, it would be interesting to see how all this tasks are acomplished in a radar equipped ship. If any member can describe it would be wonderfull. Anyway I have a couple of questions:

-I know that radar is a poor bearing indicator. So how are the bearings obtained and with what accuracy? Remember that this is necesary for range rate and speed across (and from the last derive bearing rate).
-With lack of accurate bearing, how are enemy course and speed obtained?

Sure more questions would appear.

Regards to all
In 1935 GEMA first demonstrated to the German Navy a radar prototype that was accurate enough for blind fire. The bearing accuracy of 0.1* was actually more accurate than typical optical derived bearing accuracy. The bearing accuracy was fixed via lobe switching.

Lobe switching or "Peil Verfaren A/N" used two alternately switched beams, one aimed slightly to the right and the other aimed slightly to the left of centerline. When the signal returns from the two beams were equal on the scope, then the antenna was aimed directly at the target. The USN called this procedure "pip matching".

The German Navy rejected the use of this type of lobe switching as being too complex and too demanding on the operators. The early model Seetakt radars only fixed the bearing using maximum signal or S-Max. In this method the antenna was aimed were the strongest signal appeared on the scope. This only allowed accuracies of one or two degrees. The early model British Type 284 50cm radars also used max signal. In 1942 the British began to phase in Type 284M that used a pip matching Type lobe switching. The USN Mk3 and MK4 also used pip matching. The problem was making sense of, and properly matching, multiple pips of closely grouped targets on the bearing scope. It was all very abstract.

When GEMA introduced it's second generation Seetakt radars it also introduced a new method of lobing. The new method called "Radattel-Peiling" or S-Rad, manipulated the phase of each recieving dipole (phased array concepts). A pickup was circulated among a full wave delay line (hence a rattling sound) and when a minimum signal was recieved at the center of the delay line, then the antenna was aimed directly at the target. The typical accuracy was also 1/10 of a degree. The display for for S-Rad had a bearing scope with the null mark at the center of the screen. The target pip was saddle shaped and when the apex of the saddle was positioned right on the null mark the antenna was "zeroed in". If the target wandered off to the left or the right the pip moved accordingly. There were two bearing indicating guages along with two crank wheels to tranverse the assembly left and right. After awhile, in addition to the two indicator gauges there were automatic transfer of the bearing position to the fire control computors.

There was also automatic transfer of range data to the firecontrol computor rooms via selsyns with the second generation model Seetakt radars. With Seetakt the range was measured electronically by the operator moving the pip to the zero mark at the center of the scope. The amount of the time base presented on the scope could be zoomed in or out. In practice the operator help the pip on the zero mark and range data was automatically and continously recorded on the guages, and automatically transfered to the firecontrol computors.

With the British 50cm radars the operators moved an indicator in the form of a wire overlaying the screen from the zero mark at the left end of the A -scope to the position the target pip. This measured the distance on the time base mechanically. When the wire reached the pip; the operator stomped on a foot pedal and the range was recorded.
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Re: Radar fire control

Post by dunmunro » Wed Sep 03, 2008 5:56 pm

This is the best site on the web for details and 3-D animations of the fire control process:

http://dreadnoughtproject.org/tech/

the video on the Dreyer table is excellent, and although this is WW1 technology, the concepts are fully applicable to WW2.

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Re: Radar fire control

Post by marcelo_malara » Thu Sep 04, 2008 12:21 am

I watched them Dunmunro. I have also read one article in Warships called The Admiralty Fire Control Tables and a three part one in WI called The Dreyer and the Dumaresq. But what I like of Dreadnought Gunnery.... is the fact that in describing the evolution of the first fire control system, the author details all the arrangements tried and why some of them failed, for example why was plotting deeemed necesary and what would happened if it was not used.

Thanks Bgile and Dave for your answers. Dave, if the German system was so advanced, why didn´t Scharnhorst return fire in North Cape?

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Re: Radar fire control

Post by Dave Saxton » Thu Sep 04, 2008 4:41 am

Regarding North Cape; the pivotal event was the destruction of Scharnhorst's foretop radar by a hit from HMS Norfolk right at the begining of the first skirmish. The Scharnhorst was handicapped thereafter, not only by the loss of it's primary firecontrol position and primary radar, but also because it meant that the Scharnhorst could not see the forward sectors. The ship's command must have been having a difficult time in going about accessing, or being truly aware of the evolving tactical situations, in real time. This set in motion a train of events; among which, to borrow a phrase, was Duke of York being able to "get the drop" on the Scharnhorst; surprizing the Scharnhorst with star shell and devastating first salvo hits at decisive ranges, that damaged Scharnhorst's heavy battery, further hindering it's ability to fight back. Such a train of events in combat often have a snowballing effect. British accounts indicate that the Scharnhorst's shooting was rather accurate throughout, including many, many, straddles, even beyond the effective range of it's night optics, but it never got near the number of shells into the air from as favorable shooting situations. As I recall, DoY fired more than 50 salvoes, many of them broadsides.

I don't think North Cape really tells us that much about German radar directed fire capabilities.
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Re: Radar fire control

Post by Dave Saxton » Mon Sep 08, 2008 4:30 pm

To continue a disscussion on radar directed fire, we musn't forget the very important arena of flak direction by radar.Flak direction radars need to be able to accurately determine the elevation of a target as well as the bearing. To provide this capability many of the decimetric radars had additional antenna elements to provide lobe switching facilities on the vertical axis. These can be seen as orange peels or owls ears in addition to the main radar mattress.

The USN Mk4 used two different antenna elements to provide pip matching for the vertical as well as the horizontal axis. Later developments of the Mk4, included a conversion to 33cm (Mk12) and the addition of owls ears for better height finding (the 3cm Mk22). The bearing and elevation data had to be feed into mechanical predictors and they could barely keep up. The Mk4 had problems tracking low flying aircraft, something that did not escape the notice of the Japanese, but it was far superior to purely optical means. The Mk4 lost in a competition to 150cm wave length Army SCR268.

The SCR268 had a large mattress antenna with large owls ears on the sides. It garnered respect from the Luftwaffe in the Med, and at Rendova in the Pacific a Marine Corps 268 shot down 12 of 16 Japanese bombers using only 88 rounds of 90 mm ammo. The antenna was so large that it would not have been practical to install it on ships with the Mk37 type directors. The Japanese had captured a few examples intact and they eventually built many exact copies of this blind fire radar. The developing Japanese 150cm technology certainly was helped by the examination of the captured American radars.

The Seetakt with owls ears, installed on PG for awhile, also provided lobe switching on both vertical and horizontal axis. Tests shown that it was virtually as accurate as the Wuerzburg, but the Wuerzburg clearly utilized a better concept for flak direction. The Wuerzburg and it’s get were the best flak direction radars of WWII. The KM asked Telefunken to develop a navalized Wuerzburg using a much smaller dish and with more advanced stablized directors and computors.

In a stroke of genius Telefunken engineers placed a rotating dipole at the focus of a 3 meter diameter dish antenna. They called the rotating dipole a quirl. The quirl caused the center of the radar beam to rotate around the center axis, tracing the perimeter of an imaginary cone extending outward. This was called conical scanning. The conical scan rotated around the target at 25 times per second. As long as the signal returns from each quadrant was equal then the target was held firmly within the center of the cone. If the target began drift out of the cone then the signal strength would be greater in that quadrant and adjustment could be made. Once an aircraft was seized by a conical scan, it was as good as finished. Wuerzburg’s accuracy was such that the bearing/elevation accuracy was within 0.1* 60% of the time and within 0.2* 80% of the time.

Conical scan could also be easily converted to automatic tracking. The Mannheim version of Wuerzburg utilized auto tracking. The US Army SCR584 was essentially a Wuerzburg copy with auto tracking, but operating on 10cm wave length. Brown believes that the 584 was the single best radar of WWII.

Another important advantage of the Wuerzburg concept was the electronic measure of radial velocity. Radial velocity is essentially a measure of the target closure rate in all three dimensions in real time. This unique capability became even more pronounced once the Wuerzburg could be operated in Pulse Doppler mode after Aug 1943. The 584 used a similar capability to back track the trajectories of incoming artillery shells to their point of orgin.
Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

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Re: Radar fire control

Post by marcelo_malara » Mon Sep 08, 2008 10:13 pm

Dave, for low angle control, how long had the radar to feed the conputer with bearing and range to obtain an accurate target´s speed and course?

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Re: Radar fire control

Post by Dave Saxton » Tue Sep 09, 2008 4:56 am

By low angle do you mean ship to ship? In any case I don't know the elapsed time.

With Seetakt the range was continously transmitted direct by the operator holding the target pip on the zero mark at the center of the screen. There was no need for a talker to read the radar measured range changes over the phone. After lobing was instituted, the bearing was likewise automatically transmitted direct from the relative position of the radar mounting assembly. I doubt that it took very long to attain an initial solution and updates could be more or less continous.

In most cases I have found were radar was the primary method; the result was usually a first salvo straddle. The whole exercise of shooting test salvos and "finding the range" could be by-passed.
Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

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Re: Radar fire control

Post by Dave Saxton » Tue Sep 09, 2008 5:36 am

Regarding the speed of the computors in tracking low flying aircraft, Brown wrote concerning the Mk4 and it's associated director and computor:

"While the Army was beginning to demonstrate excellant AA fire, the Navy's performance with FD radar with the mechanical-anolog predictor, was failing all too often....this still left the Mk12 with manual tracking, not suited to the agile attackers and still followed by the slow, mechanical predictor, and most ships retained the Mk4. No Mk57 directors appeared until early 1945."
Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

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Re: Radar fire control

Post by marcelo_malara » Thu Sep 11, 2008 1:00 am

Dave, I understand that the gyrocompass has an accuracy of 1°, and you need it as a reference to isolate the change of bearing of the target due to its motion from the change due to own ship yaw. How does the fire control deal with this?

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Re: Radar fire control

Post by tommy303 » Thu Sep 11, 2008 5:07 pm

My understanding is the 1* figure may be an average theoretical error, although in a properly functioning gyrocompass the actual error is 0.0 to 0.5*. Power failure to the gyro compass can completely throw off the accuracy, even after being restored, thus requiring recalibration of the master gyro compass. In the Kriegsmarine this was the job of the Navigation officer who would use an azimuth circle or parallel motion protractor fitted to an above deck gyro compass repeater. These would measure the azimuth of the sun indexed against true north and the results checked against the compass card. The card would then be adjusted to true north, and all other repeaters and the master gyro compass would be adjusted to suit. The regulations required the gyro compass be checked for errors daily, but usually, unless there had been a major failure in the power system, little or no recalibration was necessary, which was just as well as bad weather would make it difficult or impossible to measure solar azimuth.

As the fire control system was dependent on the master gyro compass for directional reference, it's accuracy was only as good as the calibration of the gyro itself. If for some reason, extensive cloud cover or sailing in long arctic nights with little or no opportunity to check the gyro against the solar azimuth, the gyro compass could be checked against a magnetic compass properly adjusted for declination. This would be less accurate, as the amount of deviation in the magnetic compass varies with the course the ship is on--i.e., a course of 90* may well have a different deviation than when on a course of 180*. One would normally utilize a compass engineer to either eliminate the deviation after a ship is built, or if unable to completely eliminate it (usually the case), compile a deviation card for each bearing so the navigator can utilize the card to adjust for whatever course is being planned.

In radar directed gunnery, as well as optically directed gunnery, minor errors caused by deviation would be compensated for my marking fall of shot, either by radar or by optical spotting and applying the appropriate correction to the bearing dial of the computer.

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Re: Radar fire control

Post by Brad Fischer » Thu Sep 11, 2008 10:30 pm

I can give a brief synopsis of the American surface fire control system and an illustration on how radar was integrated into it and how radar “changed the game”. First the primary components:

Director
Stable Vertical (gyro)
Switch board
Rangefinder(s)
Rangekeeper (computer)
Range Receiver (later plotting rooms deleted this and incorporated it into the RK itself)

The basic operation is similar but more refined than what you’d find in WWI systems. Tracking is accomplished in a similar manner as you suggest; director tracks the target in bearing and ranges are obtained from each rangefinder station. The ranges are plotted on a moving graphic plot automatically (the range receiver or a type 3 graphic plotter on the rangekeeper) and the plotter operator manually derives a mean range line. The mean range line cuts down the spread from range to range and that allows him to calculate the range rate. Both of these values are sent to the rangekeeper operator (literally standing across the instrument from him) periodically, usually about every 15 to 30 seconds or so, depending on the range density.

Tracking on the rangekeeper is accomplished by observing the motion of the “observed bearing pointers” on the target dial cluster and comparing generated range and range rate to those being computed on the graphic plotter. The target and own ship dial clusters are stacked vertically, one above the other with a fixed index between which represents generated bearing (bearing being computed by the rangekeeper based on the current target course and speed). On the “own ship” dial cluster, which is the ones on the bottom, an “observed bearing pointer” is located. This pointer represents where the target actually is relative to where the computer is generating. Thus is the solution is correct, the pointer will be matched to fixed index at the top of the dial. If the solution is predicted a faster bearing rate to the left, then the pointer will start to move slowly to the right.

Solving for range is different but straightforward. The rangekeeper operator compares the generated “present range” and the generated range rate as displayed on the rangekeeper to the values that are being passed to him by the graphic plotter operator. He combines these indications with what the observed bearing pointers motion is to build a picture one what the target is doing. Each time new values were received; he’d “drive in” in the new present range then adjust target course and speed. Example, say that the observed bearing pointer starts to slowly move to the right and the present range and range rates being received is more than the predicted values on the rangekeeper, the target has turned towards the ship and he’d need to adjust the course and speed accordingly. This is the basic procedure and there are of course tricks and techniques not discussed here to facilitate tracking but at least the fundamentals are described here.

Now radar didn’t change things overnight. Initially it was in effect a more accurate all weather rangefinder. Initial installations such as the American Mark 3 and British Type 284 installations had the operator tracking the target either in the director or radar office and passing down ranges to the graphic plot over the IC circuits. The plot operator then manually plotted them and derived the range rate (no need for a mean range line as there was very little ranging spread for radar). The speed of operation was improved a bit but there was still a latency compared to later operations.

Later installations such as the Mark 8 series transmitted the range directly to the rangekeeper. The range was displayed by range indicators installed to the immediate left of the rangekeeper operator. This is where the latency was drastically reduced. Instead of waiting for the ranges to be transmitted and plotted, he merely adjusted the present range on the rangekeeper when ever the generated and observed present ranges became unsynched (he still had the graphic plotter to back him up and provide a range rate).

When the generated present range differed from observed range and the bearing pointers started to drift, he’d have to adjust course and speed and could estimate how much by observing the difference in observed and generated ranges along with the bearing pointer deflection. This process takes but a couple seconds depending on how long the target is in a turn. I happen to have a 2D real time simulator of this particular set up and I can attest that the process is quick and impact on accuracy of fire minimal. To be sure the simulator is not and never was intended to be a high fidelity model but is accurate in the function of the rangekeeper, the tracking process and the motion of ships.

There is another point to consider when talking about tracking with optics vs. radar and that is ranging accuracy. To be succinct, rangefinders are subject to more systematic errors (than radar) and include:

Average RF unit of error (average instrument error)
RF ranging accuracy (fixed, variable, and unit of error),
Transmitted accuracy (ranging accuracy and transmission accuracy)
Transmitted error (transmitted accuracy from each RF station)
Total system error (transmitted accuracy and plotting error)

Thus even under ideal ranging conditions and with highly experienced operators optical ranging is at a disadvantage to radar in terms of accuracy (as well as the afore mentioned latency) as exemplified below. These figures represent ideal conditions based on fleet experience for an Iowa class battleship:

40Kyds +/- 357yds
35Kyds +/- 275
30Kyds +/- 205
25Kyds +/- 139
20Kyds +/- 102
15Kyds +/- 72

This is the radar error budget:

40Kyds +/- 63yds
35Kyds +/- 58
30Kyds +/- 54
25Kyds +/- 50
20Kyds +/- 46
15Kyds +/- 42

Lastly is of course spotting. When the later radars developed small resolution cells, they were able to spot better (at least in range) than most optical spotters with very little training required. For direct spotting – that is spotting directly to the target without ladders or brackets – generally a well trained optical spotter would be able to spot to within about 1.5 units of error. So for the 46’ Mk 52 in Turrets 2 and 3, the average direct spotting error is approximately 205yds at 30,000yds. The smaller Mk 48 in the director is approximately 355 yards. The Mk 8 radar by contrast averaged approximately 80yds regardless of range with 3-gun salvos and approximately 60 yards with larger 6 to 9-gun salvos.

In the end, optics were hard pressed to keep up with the range aspect of fire control problem. They did have advantages particularly in survivability and durability. They also were superior in bearing tracking and deflection spotting. Thus a combination of optical bearing track and spot with radar ranging and range spotting was considered the ideal setup.

Brad Fischer

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Re: Radar fire control

Post by marcelo_malara » Thu Sep 11, 2008 11:05 pm

Thanks Brad for your long explanation.

So I understand that the radar only changed the instrument used to measure the range and bearing, the plot still being necesary to determine range rate. Am I right?

And a couple of questions:

-Bearing was plotted too?
-Teoretically you can derive target course and speed (and from them range rate and deflection, and from deflection and range bearing rate) from a series of target´s bearings and ranges. Was in WWII any computer (or rangekeeper) capable of solving this?

Regards

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Re: Radar fire control

Post by Karl Heidenreich » Thu Sep 11, 2008 11:37 pm

Marcelo,

I have the "NAVAL ORDNANCE AND GUNNERY VOLUME 2, FIRE CONTROL" which weights about 5 MEGA. I can send it to you if you want.

Best regards.
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