title graphic


1. Intelligent operation of any rangekeeper requires a thorough understanding of how it functions. Contrary to popular belief, rangekeepers are not too complicated for the average per- son to master. Given a good instruction book, any officer can, in a comparatively short time, thoroughly grasp the construction and functioning of any rangekeeper. When the Mark I rangekeeper is mastered, other and more complicated rangekeeper s can be easily understood.

2. Before studying this chapter O.D. 1661 should be studied, and FTP 134, Articles 500 to 517 should be read. It is assumed that Chapter 1 of this book (PGS 5) has been thoroughly studied.


3. The primary function of the Mark I Mod. 3 rangekeeper is:


From these quantities, other instruments obtain and transmit to the turrets:

4. If the Mark I Mod. 3 rangekeeper were perfect, and if all of the data introduced were correct, the rangekeeper would continuously generate the hitting advance range, deflection and target bearing. Since neither of these conditions can be counted upon the rangekeeper is designed to facilitate the correction of errors in the data, and the application of ballistic corrections. The general scheme by which the rangekeeper is made to accomplish its mission is indicated in the following steps:


5. The initial data introduced is:

6. Plate 1 is a graphical representation of the above data. Own ship's course and speed are represented by the direction and length of d e. Relative Target Bearing (which incidentally defines own ship's course with respect to the line of fire) is e d a. Range is d a. Target's course and speed are represented by the direction and length of a b. Target Angle (which really defines target's course with respect to the line of fire) is bad.

Computation of Rates.

7. Referring again to Plate 1, it will be seen that own ship's motion d e may be resolved into two components: d f in the line of fire and f e across the line of fire. Similarly, the target's vector may be resolved into a c in the line of fire and c b across the line of fire, a c plus d f is the speed at which the two ships are approaching each other, and is called the RANGE RATE. c b plus f e equals the relative speed at which the ships are moving across the line of fire and may be called the LINEAR BEARING RATE. Note that these two speeds or

rates are independent of the range. They may be positive or negative, and in obtaining them the correct signs must be used. In this case illustrated in Plate 1 the range rate is decreasing or negative, but the linear bearing rate, since the line of fire is being swung to the right, is positive.

8. The rangekeeper mechanically computes these two rates as indicated schematically in Plate 2. The Own Ship and Target component solvers resolve their vectors into components in and


across the line of fire. One differential adds algebraically the two components in the line of fire to obtain RANGE RATE. Another differential adds algebraically the two components across the line of fire to obtain the LINEAR BEARING RATE (or "Deflection Rate"). These rates may be expressed in knots, yards per minute, or similar units. The Range Rate is indicated on a dial in knots, but there is no bearing rate dial.

9. The Range Rate (dR) may be used directly to change the range. Since bearing is measured in angular units (degrees) it is necessary to have the angular rate at which the bearing is changing. This depends not only upon the relative speed across the line of fire but also upon the range. In fact the Angular Bearing Rate (usually called simply the "bearing rate") is equal (in radians per minute) to the Linear Bearing Rate (in yards per minute) divided by the Range.(in yards). It may be converted into degrees per minute by a conversion factor.

10. The above relationship may be expressed as follows, omitting the conversion factor:

Linear Bearing Rate = Angular Bearing Rate x Range or using convenient symbols:

RdB = dB × R
dB = RdB/R

11. The rangekeeper performs the operation indicated in the last equation. The Linear Bearing Rate (RdB) goes to a multiplier (which can of course divide) where it is divided by the Range (R) giving as its output the (angular) Bearing Rate . (dB)

Integrating the Changes in Range and Bearing. (See Plate 5)

12. The Range Rate (dR) obtained from the component solvers is used as an input to the range integrator. The other input is "time" from the constant speed driving motor. The out-put of the integrator is a change of range which is added continuously to the existing range to give the generated present range.

13. In a similar manner the (angular) Bearing Rate (dB) is put into the Bearing integrator and the change of bearing obtained as the output. This change of bearing is added to the initial true bearing and thus produces generated true bearing. Generated relative bearing is easily obtained by subtracting own true course.

Comparing Generated Values with Observed Values of Range and Bearing.

14. The graphic plotter furnishes the best method of comparing the value of present range thus generated with the observed


Plate 1


Plate 2


Plate 3


Plate 4


(range finder) range. It is mounted at the top of the range-keeper. A sheet of paper is moved steadily by the rangekeeper, as time passes. A pencil automatically traces the generated present range. The operator of the graphic plotter, who wears the telephone to the rangefinders, plots each range as it is received, and fairs the mean rangefinder line. If the set up is correct, and if the rangefinder readings are excellent, the mean rangefinder line will coincide with the (generated) present range line. The slopes of the lines depend upon the rates at which the respective ranges are changing. If the lines are parallel, the rates are equal. If not, then the rangekeeper range rate (dR) is in error - assuming that the rangefinder range rate is correct. If dR is incorrect, then the settings of target's course and speed (either or both) are in error.

15. There are other ways of detecting errors in dR. The Mark II plotting board, though less convenient, can be used in the same manner. The HORIZONTAL WIRE is also, available, although seldom used. It is so controlled by the rangekeeper that when a new range or range ballistic correction is cranked into the rangekeeper it will move up if the range is increased or down if it is decreased. The wire is normally over the speed button which shows the target's course and speed settings. The direction in which the wire moves indicates the corresponding direction in which the settings should be corrected. The details of the horizontal wire mechanism, being unimportant, will not be described.

16. Comparison of the observed relative target bearing with the generated relative target bearing is shown by the VERTICAL WIRE. The Vertical Wire is driven from a differential whose inputs are observed target Bearing (Relative) and generated Target Bearing (Relative). If they differ, the vertical wire will move off the target Speed button in the direction in which the button should be moved to correct the target's course and or speed settings.

17. If the target's course and speed settings are correct, both the range rate (dR) and the bearing rate (dB) must be correct. The rangekeeper operator, by trial and error, adjusts the target's course and speed settings until these two conditions are met. That is, until the graphic plotter lines show him that the range rate is correct and the vertical wire shows him that the bearing rate is correct. However, in practice, the rangekeeper operator depends largely on the spotter's estimate of the target's course, or target angle. Later rangekeepers employ a vernier in place of the vertical wire, but this does not change the method of operation.


Computing Certain Ballistic Corrections.

18. Rangekeepers vary greatly in the corrections which they automatically compute and apply. In general, some of the ballistic corrections are automatically applied by the rangekeeper, while others must be computed separately and inserted like spots. The corrections computed by the Mark I Mod. 3 rangekeeper will be described.

Corrections in Range.

19. Own Ship's and Target's Motions. There is a small time delay in getting Advance Range from Rangekeeper finally applied (as gun elevation) to the guns. This is called the transmission interval. There is also a much longer time from the firing of the gun until the projectile arrives at the target (Time of Flight) . During the total of these two intervals (called the "prediction interval") the target may move several hundred yards, and motion of the gun may have a similar effect. The Range correction for the motion of the gun and target is computed by the Rangekeeper and applied continuously to Present Range, thus giving (after the input from the shot crank has been added) Advance Range. This ballistic is computed in the Rangekeeper-as follows: The time of flight is assumed to be directly proportional to the Present Range. Although this introduces an error, the error is small and the assumption makes it possible to avoid using a cam, thus simplifying the machine. The shafting from Present Range to the corrector simply has the proper gear ratio, thereby introducing time of flight. The transmission interval (set by hand) is added to the time of flight by a differential before it reaches the corrector. The corrector itself is merely a multiplier which receives the "prediction interval" as just described, and the Range Rate (dR), multiplies them, and puts out the predicted change of range. The generation of Advance Range is as indicated by Plate 4.

20. Wind. There is no corrector for the true wind nor for the wind due to gun's motion. Consequently the wind ballistic must be computed separately, using apparent wind.


21. Own and Target Motions and Wind. The Rangekeeper also computes the deflection correction due to motion of gun and target, adds in drift, and indicates the total on the deflection dial. This is done by the Bearing Multiplier as shown in Plate 5. The deflection correction due to wind must be computed separately using apparent wind," and applied like a spot. The bearing multiplier has two functions. One is to compute the deflection due to own ship's motion, target's motion, and drift. This is done with the aid of the drift cam, as indicated. The


Plate 5


Plate 6


Plate 7


Plate 8


RdB input represents the relative movement across the line of fire of own ship and target. This does not correct for either true wind or wind due to gun's motion, so that the correction for apparent wind must be computed and applied like a spot.

Application of Ballistic Corrections.

22. Other corrections must be computed and introduced like spots. Due to the approximations, especially in range, it is best, if time permits, to compute the entire ballistic corrections (range and deflection) and "force" them on the rangekeeper using the spot cranks. Whenever the ship changes course the apparent wind changes, which will cause errors unless a corrected ballistic is applied. The later rangekeepers are much improved; most of them need not be corrected for changes in course. Also more corrections are handled by the rangekeeper.

Generation of Relative Target Bearing.

23. The Bearing Integrator was discussed in Par. 13. The output of the integrator is generated true bearing. For use in in direct fire and in making comparisons, the generated relative bearing is required. Target Course Relative to Own Course and Target Angle are also required for the face dials and component solver. The relationships between the various angle is shown in plate 6. The methods of obtaining them, and the connections between the various angle dials is shown in Plate 7.


24. The various units in the Rangekeeper have been taken up in detail. It remains to place them in the Rangekeeper and show their relation to each other. This is shown schematically in Plate 8. No attempt has been made to show details such as the. connections for the horizontal and vertical wires and the graphic plotter. Later modifications also have servo motors and selsyns for transmitting some of the outputs electrically. The method of operation and essential mechanisms are shown so that all inputs may be traced thru the system to obtain the final outputs.


Table of Contents
Previous Chapter (6) Next Chapter (8)

Transcribed and formatted by Thomas Wildenberg