Repeater Plan Position Indicators
A. Introduction--need for repeaters.
B. Standard repeater PPI: Function; characteristics; model VC; model VD; model VH; delay box.
C. Precision repeater PPI: Characteristics; relation between B scope and PPI; operation of precision repeater PPI.
D. Projection PPI: Characteristics; operation of VG repeater PPI.
E. Off-center PPI:
Advantages; relation to normal PPI.
As the great usefulness of radar on board ship became apparent, it was realized that full benefit could not be taken of the information provided by this equipment if only the radar operator could see the indicator. Therefore, repeater indicators were produced so that radar information could be made readily available at many positions in the ship remote from the radar console. The use of repeater indicators permits great flexibility in radar operation. For example, the range scale on a repeater indicator can be selected for whatever application the repeater is to be used, independently of the range scale in use on the master indicator. Large repeater plan position indicators have been developed with which it is possible to keep directly on the face of the PPI a summary plot of all detectable targets. Methods of improving the display of radar information are under study in order to make the repeater PPI more useful. Smaller repeater plan position indicators are available for use in positions where it is necessary to keep track of every target.
B. STANDARD REPEATER PPI
The function of a standard repeater plan position indicator is to duplicate the information shown on the master PPI to which the repeater is connected. At present, the repeater presentation may differ from the master only in the size of the cathode-ray tube and the sweep length used, although modifications will probably soon be available to increase the usefulness of the repeaters.
Standard repeater PPIs do not have facilities for the generation of a range ring incorporated in their circuits; the ranges of targets can only be estimated by reference to range markers. Therefore, the accuracy of range determination is not of high order, and it is dependent on the range scale used. If the range of a target can be estimated within 2% of the length of the range scale in use, then the accuracy expressed in yards will be poorer the longer the range scale is. For example, on a 10-mile scale (20,000 yards), range can be estimated within ±400 yards, but on an 80-mile scale (160,000 yards) ranges can be estimated within only ±1,600 yards.
The resolution of a repeater PPI is controlled primarily by the characteristics of the radar to which the repeater is connected, but the range scale used may also affect range resolution to some extent. Thus, a short range scale on the repeater PPI will provide better resolution of near targets than is possible on the master PPI if a long range scale is used for some reason on the master. However, if both indicators are on the same range scale, the resolution should be nearly the same for both. Bearing resolution cannot be improved by the use of a repeater PPI, since this characteristic is controlled primarily by the beam width of the radar antenna.
2. Model VC and VC-1 RPPI.
Since the VC type of repeater PPI is not of splash-proof construction, it can be used only in protected locations. The early models were provided with 7-inch cathode-ray tubes, but some of these equipments subsequently have been modified to accommodate a 12-inch tube. The range rings on the VC and VC-1 are scribed on a plastic overlay rather than their being produced electronically. These range marker rings can be calibrated by comparison with the range marks produced by the master PPI. The range scales used on the VC and VC-1 are 20, 75, and 200 nautical miles. In general, the VC and VC-1 are very similar to the master PPI of the SC-2 and SK radars.
3. Model VD, VD-1, and VD-2 RPPI.
The VD type of repeater PPI is of splashproof construction, so that it can be installed in locations exposed to the weather. The cathode-ray tube is
seven inches in diameter, mounted with the screen horizontal. Range rings are produced electronically to serve as an aid in estimating the ranges of targets. Most of the VD, VD-1 and VD-2 repeater PPIs employ the standardized range scales of 4, 10, 20, 80, and 200 nautical miles. The 10-mile scale was not included in the first 50 VDs that were produced.
4. Model VH RPPI.
The VH type of repeater PPI is built in two units so that the cathode-ray tube can be put in restricted spaces or exposed locations where other types of repeaters would not fit. A five-inch cathode-ray tube is contained in a small waterproof case approximately 7 inches wide, 9 inches deep and 18 inches high. The driver unit, in which most of the electronic circuits are located, is contained in a separate case that may be located as far as 50 feet away from the cathode-ray tube unit. The range scales to be provided with the VH are 2, 4, 10, 20, 80, and 200 nautical miles. Another repeater PPI unit, which is very similar to the VH in form except that it uses a 12-inch cathode-ray tube, is currently under procurement. This unit is to be called the VJ.
5. Delay Box.
The range resolution of which a PPI is capable on long range scales is relatively poor. This failing is caused by the limitation on the minimum spot size that can be produced on the screen of the cathode-ray tube. For example, on a 12-inch tube one microsecond occupies only 0.0025 inch of the 200-mile range scale, while the minimum possible spot of light that can be produced on the screen may be
Figure 143--Standard repeater PPIs.
approximately 0.03 inch in diameter. Under these conditions, even a 1-microsecond pulse would appear to be a mile wide, so that the PPI is far from able to display the resolution of which the radar is capable. On a shorter range scale, such as the 10-mile range, for example, the spot size is small enough in terms of yards that it does not limit the resolving power of the PPI, and the range resolution is greatly improved. Therefore, if it were possible to delay the start of the sweep for a time equivalent to 50 miles, for example, targets at 56 miles could be viewed on the 10-mile scale with good resolution, instead of on the 80- or 200-mile scale with relatively poor resolution.
Normally the sweep on any indicator is made to start as nearly as possible at exactly the instant that the radar transmits a pulse. If the trigger pulse that is used to start the sweep is delayed, the sweep will start at some time later than the instant at which the transmitted pulse is sent out. Thus, if a sweep normally starts at zero and extends to 10 miles, it will start at 50 miles and sweep to 60 miles if the sweep trigger is delayed a time equivalent to a range of 50 miles. This process, then, expands the presentation of a part of the area covered by the radar. Figure 144 shows an illustration of the increased resolution provided by a delayed PPI. In A the 100-mile sweep shows a convoy in Massachusetts Bay, indicated by the arrow. Very little information can be gained from this presentation except the knowledge that many ships have been detected. The expansion of the ring on the main PPI between 20 and 40 miles is shown in B. Notice that the single ships of the convoy are shown clearly because of the greater
Figure 144--Normal and delayed PPI.
resolution possible on the 20-mile range scale. These pictures were taken on a developmental radar.
The device which is being produced to permit the use of repeater indicators in this manner is called a delay box. The amount of delay is continuously variable from zero to 180 miles, and the amount of delay to be used is indicated by a range ring that appears on the normal PPI. When the repeater indicator is switched to produce the expanded presentation, this indicating range ring collapses to a spot at the center of the screen, while at the same time the sweep length is changed to the range scale selected to provide the desired degree of expansion in the area under observation. Since the amount of delay introduced by the Delay Box is indicated directly by the calibration on the controls, the range of any target can be measured to an accuracy of one per cent by adjusting the delay so that the echo is brought to the center of the delayed PPI presentation. The Delay Box is so designed that it can be applied to any remote repeater plan position indicator.
The use of a delayed PPI will create a new problem for fighter directors and intercept officers because the expansion of the presentation is accompanied by rather great distortion in the relative positions of the several targets. On a normal PPI, as in figure 145A, the presentation is undistorted, so that the bearing of one target from another can be readily estimated from the indicator screen. Thus the operator could find that the bearings of targets W, X, Y, and Z from the friendly target at F are as indicated on the drawing. However, if the start of the sweep were delayed 40 miles and the 20-mile range scale used, target X would be at the center of the scope, F and Z at the
Figure 145--Distortion in delayed PPI.
outer edge, and W and Y midway out from the center. Because of the expansion, the bearings of the several targets are now different. However, both F and Z are expanded the same amount, since they are at the same range, and their apparent relative position is unchanged. The bearings to targets W, X, and Y on the other hand are considerably changed. Notice that any target at a 40-mile range must bear 210° from F because the 40-mile range circle collapses to a point at the center of the scope. Since the amount of distortion is dependent on several factors, it is very difficult to predict. For example, both W and Y are at the same range but at different bearings. The errors in the bearings to these two targets from F are by no means equal.
However, the range and true bearing of each target from the radar antenna can still be read correctly from the indicator. Therefore, if it is desired to control aircraft from a delayed PPI, the correct bearing of one target from another can be found only by taking the time to plot the data obtained from the indicator. Vectors cannot be taken directly from the delayed PPI.
C. PRECISION REPEATER PPI
The precision PPI is designated VF. This unit displays radar information on a 5-inch PPI and a 5-inch B scope. The PPI is quite conventional, and the range scales provided are 4, 20, 80 and 200 nautical miles, with four range marker rings produced electronically on each range scale. Thus, range can be estimated directly from the PPI presentation if desired. However, a range unit is also provided for
Figure 146--Type VF precision repeater PPI.
precise range measurement on the accompanying B scope. The B scope displays on an expanded scale the echoes selected from a small area on the PPI. The expansion afforded on the B scope makes possible the measurement of range to an accuracy of ±20 yards, and provides improved range resolution within the area that the B scope can display.
The B-scope sweep is approximately 4,000 yards long, and arranged to show targets 2,000 yards on each side of a selected range. The range unit is so designed that only targets within 20 miles can be displayed on the B scope. The total angular width of the B scope is 30°, so that the area 15° to either side of the cursor on the PPI is shown expanded.
The VF precision PPI incorporates provisions for transmitting both range and relative bearing to remote locations by 1-speed and 36-speed synchros.
2. Relation Between B Scope and PPI.
The area on the PPI which is to be shown expanded on the B scope of the VF is selected by adjustment of a bearing cursor and a ribbon marker on the PPI screen. The bearing cursor is a black line scribed on a plastic overlay. The operator sets the cursor over the desired target, and an area 15° on either side of the cursor is automatically displayed on the B scope each time the PPI trace passes through this sector. The black line in figure 147A, at approximately 085° true, is the cursor, and the center vertical line on the B scope in figure 147B corresponds to this bearing. Since the bearing of the cursor is always represented on the B scope by the center dot, it is apparent that only bearings relative to the cursor can be read directly from the screen of the B scope. The B scope is not stabilized in azimuth, so that true north does not always appear at twelve o'clock as on the PPI. The ribbon marker is the brightened annular segment shown in figure 147A. It is approximately 4,000 yards wide so that it can show on the PPI the range limits of the area that will appear on the B scope. Thus, the actual segment of the PPI selected for enlargement is shown by the intensified area on the PPI screen.
The B-scope pattern is traced once every revolution, during the time that the PPI sweep is traversing the intensified segment. As a result, the B-scope echoes are retraced intermittently only, causing a. fast-moving target to seem to move in a series of jumps. Because of the intermittent nature of the B-scope echoes, it is difficult to align the echo with the center range line. Therefore a correction circuit is provided in the VF as an aid to keeping the desired echo centered on the B scope. While the PPI sweep is continuing beyond the intensified segment, the range and bearing knobs can be moved to place a spot directly under the desired echo. The turning of these knobs corrects the position of the selected segment so that the echo will be nearly centered on the B scope when the pattern is retraced.
It was originally intended that the VF would provide more accurate bearing information than could be obtained from the radar itself. To make this improved accuracy possible, a synchro system using both 1-speed and 36-speed elements was provided. However, none of the radars available are capable of supplying antenna train information precisely enough to allow advantage to be taken of the precision built into the VF. As a result, the bearing accuracy obtainable from the VF is little better than that obtained from the radar. Such improvement in accuracy that is obtained is largely due to the expansion of the echoes on the B scope. Since only a 30° sector is shown on this screen, while the full 360° are shown on the PPI, the line produced by the echo is longer on the B scope than the arc on the PPI. When the VF is connected to a radar having a narrow antenna beam, it may be possible to center the longer line on the B scope somewhat more accurately than to bisect the echo arc on a PPI with a cursor. Since a reference line is provided on the B scope for measuring bearing, this process is capable of greater accuracy than simple estimation on a PPI which has no cursor. However, cursors are being supplied to those equipments that do not already have them, since bisecting the echo arc is faster and perhaps more accurate than determining bearing on an A scope by maximizing the echo pip.
Figure 147--Model VF PPI and B scopes.
3. Operation of Precision Repeater PPI.
It is difficult to keep the desired echo centered on the VF B scope on every revolution of the antenna. Hence, it is difficult for the operator to get accurate data on every revolution of the antenna, and considerable practice is required to enable the operator to realize the full accuracy of which the equipment is capable. Probably it will be more satisfactory to have the VF operator call his data when he obtains it accurately, rather than to attempt to have him read at fixed time intervals.
Aircraft are hard to track on the VF because they move so fast that the echo changes position radically between the successive retracings of the pattern. The fact that the maximum range on the B scope on the VF is 20 miles also makes tracking aircraft difficult. The echoes received by many air search radars may extend all the way across the VF B scope screen because of the wide antenna beam. In these circumstances it is not possible to measure bearings on the B scope, so that it is necessary for the operators to estimate bearing from the PPI but the B scope can still be used for ranging.
Since echoes from shell splashes last for only 3 to 6 seconds, it is possible that they may not show on the 30° sector displayed on the VF. However, if the antenna is rotated at high speed, the likelihood of missing the splash echo will be decreased. High receiver gain usually is required to make the splashes visible on both the PPI and B scopes. Unfortunately, when the gain is increased, echoes may be received from the antenna side lobes, resulting in confusion on the indicator. However, preliminary tests indicate that fairly reliable spotting is possible with the VF connected to an SG radar to an accuracy of ±100 yards in range and ±2° in deflection.
The B scope on the VF is a distorted presentation which may be misleading unless the distortion is understood. For example, any target on a collision course will travel straight down a vertical line on the indicator, but this may not be apparent without an understanding of the type B presentation. On a B-scope screen, own ship's position is represented by a horizontal line, not by a point as on a PPI. The B scope will be very useful in station keeping if it is desired to keep at a specified range and bearing from the guide. However, it is not possible to determine the bearing of one target from another directly from this indicator alone. As an illustration, suppose that four vessels are arranged in a square like that shown in the PPI presentation of figure 148A. It is apparent that true bearings cannot be determined without first finding the bearing of the center vertical line on the B scope. Even if this heading is known and taken into account, the bearing of targets 2, 3, and 4 from target 1 will all be incorrect on the companion B scope shown in figure 148B. Notice that the square that the targets formed on the PPI has been stretched at the bottom and compressed at the top to form a trapezoid on the B scope. The distortion introduced in this way is especially severe at the short ranges where the VF may be used for station keeping. Therefore, it is necessary that the operator realize that the B scope distorts the relative positions of targets when the VF is used for this purpose.
Figure 148--Distortion in B scope.
D. PROJECTION REPEATER PPI
The projection repeater PPI is designated VG, VG-1, and VG-2. The VG and VG-1 are identical except that one is a "mirror view" of the other, so that the two equipments may be mounted end to end as a single operating unit. The electrical and optical systems of the VG-2 are identical with those of the VG, but the VG-2 has a Dead Reckoning Tracer built in. The complete unit is approximately the size of a desk, 35 inches high.
In these repeaters the scope pattern is projected on to a large horizontal surface instead of being observed directly on the face of a cathode-ray tube. The face of a 4-inch cathode-ray tube is strongly
Figure 149--Type VG projection repeater PPI.
illuminated so that its image may be reflected and projected as a pattern 24 inches in diameter. The image on the screen consists of magenta-colored echoes on a white background. The image of a gyro compass repeater is projected on the same screen concentric with the radar pattern as a means of indicating the ship's course. Although the first projection PPIs produced had plain screens on which the pattern was viewed, later models will be supplied with a standard plotting surface engraved on the screen. Thus, if the equipment should fail, the surface can still be used for plotting.
The range scales provided are 4, 10, 20, 80, and 200 nautical miles. Electronically generated range markers divide the screen into four concentric rings to facilitate estimation of range.
2. Operation of VG Repeater PPI.
The cathode-ray tube used in the VG is of special construction. Before the electron beam strikes the screen, it is white. The places where the electron beam strikes do not glow as in the conventional cathode-ray tube, but instead they turn a dark magenta color. The nature of the coating on the screen is such that the dark spots will stay dark almost indefinitely if neither heat nor light is directed at the screen from outside. Thus, the VG pattern is of very long persistence. To prevent the pattern from lingering too long, the VG should be operated in the "Fast Erase" position, in which hot air is blown across the cathode-ray tube screen in addition to the strong light that is normally shone on it. Such operation will not seriously shorten the life of the tube, as was originally feared, and it will result in
more satisfactory operation of the VG. The nature of the coating on the screen of this dark-trace cathode-ray tube is such that the fading of the image can be hastened if the screen is uniformly bombarded by the electron beam while heat and light are applied externally. The electron bombardment can be made reasonably uniform by turning the GAIN control on the repeater to its lowest setting and advancing the INTENSITY control, while the unit is connected to a radar that uses a high repetition rate, since the higher the repetition rate of the radar, the more often the electron beam will sweep across the tube. Thus, to erase the pattern on the projection PPI most quickly, the unit should be connected to a radar, such as the SG, that uses a fairly high PRR, the gain reduced, the intensity increased, and the repeater operated in the "Fast Erase" position. However, even in the "Fast Erase" condition, the persistence of the screen is longer than that of a conventional PPI tube, so that range scales cannot be changed without confusing the presentation for at least a few seconds.
The transmitted pulse and sea return may cause the center of the VG scope to become permanently darkened. If the burning of the screen takes place on a short range scale, detection of targets within the first few miles will be impossible on the long range scales. Burning of the screen can be prevented by the installation of a very simple modification which prevents any targets from appearing for approximately the first 500 yards of any sweep.
When the pulse repetition rate of the radar is low, relatively few echoes are returned from a given target as the antenna beam sweeps across it. Therefore, the echo on the VG cathode-ray tube screen is paler at low repetition rates than at high rates because the spot on the screen is excited less often. It is for this reason that the echoes from an SK arc less intense on a VG than those from radars that operate at higher repetition rates. Consequently, the VG is not able to show the weak echoes that are returned from targets at the maximum range of the radar, which causes the maximum range of detecting targets to be somewhat less on the VG than on the more conventional PPI.
The resolution possible on long range scales with a four-inch cathode-ray tube is relatively poor because of the limitation on the minimum size of the spot. The minimum spot size on the dark trace tube is somewhat larger than on the conventional cathode-ray tube because of the nature of the coating on the screen. Enlargement of the image of the face of the four-inch tube by projecting it as a 24-inch circle on the plotting surface provides no increase in resolution because all parts of the picture are enlarged proportionally. In fact, if the optical system is not properly adjusted, the projected image may exhibit less resolution because of defocusing. No loss will result if the optical system is carefully aligned. Comparative tests indicate that somewhat poorer resolution in both range and bearing is obtained on a VG connected to an SG radar than is obtained on the SG PPI scope.
The combination of reduced resolution and long persistence of the image impose limitations on the VG that may handicap the control of aircraft to some extent. Fast moving targets produce a blur on the screen that makes accurate plotting difficult, and the inferior resolution impedes both the control of interception and the interpretation of the echoes returned from crossing raids. Experience in the use of the equipment probably will lead to ways of minimizing these handicaps. The long persistence of the image is very detrimental when the range scales are changed, and even more troublesome if any kind of jamming is encountered. Both electronic and mechanical jamming will produce large darkened areas that will remain on the screen for a long time. It will be very difficult to see targets through even a small amount of jamming because of the reduced contrast on the screen.
The procedure for adjusting the optical system of the VG must be thoroughly learned by the maintenance personnel because the whole system must be disturbed whenever the cathode-ray tube is replaced. If the adjustment procedure is not well known, the equipment may not be kept in good working order, and considerable time may be lost in changing the tube.
E. OFF-CENTER PPI
Recently it has been found possible to expand a section of a PPI screen without introducing appreciable distortion. The indicator which produces this expansion presents a virtual pattern on a large area of which only the small circular area of the cathode-ray tube screen is actually visible. The expansion is accomplished by moving the center of the virtual pattern off the screen to allow the outer, expanded part of the virtual presentation to fill the scope. The decentering can be accomplished in several different ways, but all result in a pattern called an off-center PPI. The method used in the SX radar produces some distortion, but an improved circuit which is to be incorporated in the VK repeater PPI presents a pattern that is as true as the one shown in a normal PPI.
The relationship of the off-center pattern to a normal PPI pattern can be clarified by reference to figure 150. The normal PPI pattern is shown as the dark circle in A. If the cathode-ray tube screen is
Figure 150--Relationship between off-center and normal PPI patterns.
12 inches in diameter, the sweep applied to the tube must be sufficient to deflect the spot to any point within a 48-inch circle. Since fluorescent material is available in this area only on the 12-inch cathode-ray tube screen, the only area that can show a trace is the 12-inch circle of the tube face. This arrangement is much like the effect of shining a spotlight on a large darkened screen. All that can be seen by the audience is the area within the brightened circle, but other parts of the screen can be brought into view by moving the spot. In figure 150, parts other than the center of the virtual 48-inch PPI can be made visible on the cathode-ray tube screen by moving the center of the pattern relative to the screen, as in B, C, and D.
If the range scale in use on the normal PPI is 10 miles long, it is possible to show targets out to 40 miles on the off-center scope by decentering the pattern. This arrangement makes it possible then to display distant targets undistorted on a sweep that permits maximum use of the resolution inherent in the radar. For example, if the normal PPI in figure 150A is set to the 80-mile range scale, targets at 70 miles are difficult to resolve. A great improvement can be made in the resolution if the 20-mile range scale is put in use, and the center of the PPI moved off to bring the targets onto the screen as in D. The direction in which the pattern is decentered is controlled by the cursor over the PPI. It would be possible to improve the resolution further if the 10-mile or 4-mile range scales could be used. However, since technical limitations prevent decentering the pattern by a distance greater than three times the range scale in use, neither of these range scales
could show the targets mentioned above, even with maximum decentering. The amount of decentering is continuously variable from zero to a maximum of three times the radius of the cathode-ray tube screen.
Limitations of pulse repetition rate prevent using a range scale longer than 200 miles on a repeater PPI. After the spot has swept 200 miles, the election beam in the cathode-ray tube is cut off and nothing shows on the screen beyond 200 miles. Thus, if an 80-mile range scale were used and the center of the pattern moved three radii off the tube face, which is a distance of 240 miles for this range scale, nothing at all would show on the screen. If the center were shifted only two radii, or 160 miles, approximately half the screen would be blank and the remainder would show the area between 160 and 200 miles, centered about the bearing of the cursor. The 80-mile range scale does not provide much expansion, so that an additional 40-mile scale is being supplied in the VK to allow a reasonable degree of expansion out to a maximum range of 160 miles.
The decentering method used in the SX off-center PPI distorts the bearing from one target to another by an amount that varies with the positions of the targets. The error can be as large as 36° when both targets are far off to the same side of the cursor, near the edge of the screen. This large error can readily be reduced to a few degrees simply by moving the cursor so that the two targets are centered on the screen. The error is small for targets symmetrically placed about the cursor, for targets separated by a long range, and for targets near the center of the pattern. Close inspection of the photograph of the SX off-center PPI shown in figure 151 will show the cause of the distortion that occurs in this indicator. The sweep starts from a straight line just off the tube face at 7 o'clock in figure 151. In order for the pattern to be true, the several traces should start from a circular arc rather than a straight line. It is evident in the photograph that the nearest arc of a range circle is too straight, although at greater ranges the arcs of the range circles become more nearly circular.
The principal advantage of the off-center PPI of the VK over both the B-scope and the delayed PPI is that the pattern on the off-center scope is not distorted. Therefore, an intercept officer can determine vectors to give aircraft under his control by reference to the indicator alone. The high degree of resolution provided in the off-center PPI allows the friendly plane to be followed until it is very close to the enemy plane--possibly until a "tally-ho" is reported--depending on the range at which the interception is being conducted.
Another possibility that arises with the advent of the off-center PPI is that of connecting the Dead Reckoning Tracer to the decentering circuits. If this is done, a particular section of shoreline can be made to remain stationary on the off-center screen as the ship moves. This property may be useful in conducting shore bombardments. The connection of the DRT to the indicator does not prevent manual control of either the selection of the extent of de-centering or the direction along which the decentering takes place.
Figure 151--SX off-center PPI.
Table of Contents
Previous Chapter (7) ** Next Chapter (9)