3 March 1944 NRL Report No. R-2241
GUIDED MISSILE COUNTERMEASURE SYSTEMS
NAVAL RESEARCH LABORATORY
WASHINGTON 20, D. C.
Number of Pages: Text--16 Plates--17 Authorization BuShips ltr S-S67-5(923-2) S-920-06126 of 5 Oct. 1943 Prepared by S.G. Lutz, Radio Engineer Submitted by E.H. Krause, Section Head, Communication Security Section Reviewed by A.H. Taylor, Superintendent, Radio Station Approved by A.H. Van Keuren, Rear Admiral, USN, Director Distribution BuShips (40)
TABLE OP CONTENTS
Subject PART I. GENERAL INFORMATION 1 1. Purpose 1 2. Information Concerning German Guided Missiles 1 a. PC-1400-FX 1 b. HS-293 1 3. Information Concerning the Radio Control System 2 a. Missile Control Signals 2 b. German Radio Control Equipment 2 c. Results of Jamming 3 d. Information Needed 3 4. Methods of Jamming 3 PART II. GUIDED MISSILE COUNTERMEASURE (GMCM) EQUIPMENT 4 1. High Frequency Receivers 4 a. RDC - 28 to 140 Mc Broad Band Panoramic 4 b. RBK-1 - 28 to 143 Mc Manually Tuned Receiver 4 c. RBW Panoramic Adaper 5 2. Low Frequency Receivers 5 a. RDG - .5 to 30 Mc Broad Band Panoramic 5 b. RAO-2 - .5 to 30 Mc Manually Tuned Receiver 5 c. RCX Panoramic Adapter 5 3. Jamming Transmitters 5 4. Auxiliary Equipment 6 a. XCA Modulation Analyzer 6 b. DuMont Oscilloscope 6 c. Soundscriber 7 5. Broad Band Antennas 7 a. Sleeve Transmitting Antennas 7 b. Fan Receiving Antennas 8 c. Antenna Feed Installations 8 PART III. LOOK-THROUGH OPERATION AND INTERCONNECTIONS 9 1. Theory of Look-Through Operation 9 2. Look-Through Modifications 9 a. Panoramic Adapter Modifications 10 b. Receiver Modifications 10 3. Look-Through Interconnections 10 4. Precautions 11 5. InstallationRecommendations 11 PART IV. OPERATING PROCEDURE 12 1. Watch Procedure 12 2. Jamming Practice 12 3. Procedure for Tuning the Jammer 12 4. Identification of Guided Missile Signals 13 5. Choice of Jamming Method 14 a. S-not Jamming 14 b. Noise Modulated Barrage 14 c. Band Sweeping 14 d. IF Heterodyning 14 6. Operational Data 15 APPENDIX 16
PART I. GENERAL INFORMATION
These instructions supply general information concerning the purpose, composition, installation and operation of guided missile countermeasure systems. Other attached instruction books include more specific information concerning the theory, operation, and servicing of component equipment.
2. Information Concerning German Guided Missiles
This missile, most commonly called the "FX", is a 1400 kilogram high angle armor-piercing radio guided "bomb. It has no rocket propulsion but generally is equipped with a colored flare in its tail to aid the "bombardier in guiding it visually. The "bomb is a modified German 1400 kilogram "Fritz" "bomb, equipped with fins projecting from the "body and with a different tail structure. The "bomb is aimed and dropped from a normal "bombing run at altitudes always in excess of 4000 meters and possibly as high as 8000 meters. The "bomb is guided "by means of small spoiler vanes in the tail structure which are controlled "by a radio circuit operated "by the bombardier. Only slight corrections to the trajectory, of the order of 500 meters fore and aft and 300 meters laterally, can "be made. Since this is a large expensive armor "piercing missile it generally is used only against larger combat shins such as cruisers or "bat tie shins. The accuracy attained with these missiles has "been exceptionally good but fortunately they have been used infrequently. Plate 1 shows a drawing of the FX bomb.
The HS-293 (shown in Plate 2) is a rocket propelled, radio guided glide bomb, having a wing span of about 11 feet and a gross weight of about 800 kilograms. The bomb, itself, weights about 500 kilograms. The rocket propulsion equipment is housed in a metal cylinder suspended below the body of the bomb. Observers have described this missile as resembling a small and clumsy pursuit ship. This bomb is launched from its parent plane at altitudes of about 1000 meters at distances of 6 to 8 kilometers from the target. The rocket propulsion is effective only during the first 12 to 15 seconds after release, accelerates the missiles to perhaps 650 k.p.h., and thereafter, the missile continues its flight in a free glide. The attacking aircraft usually flys a course paralleling that of its target, launches its missile when nearly at a minimum distance, and guides it toward the target in a curved path, keeping it aligned with the target visually. This missile generally is used against merchant shins and lightly armored naval vessels. It has been used much more extensively than the FX but, fortunately, its accuracy has been relatively poor, Many of
these missiles appear to have "been defective and have fallen out of control. Attacks with either of the above missiles usually have "been accompanied "by torpedo or high angle "bombing attacks, apparently to help spread confusion.
3. Information Concerning the Radio Control System
Little information concerning the radio control equipment in these missiles has been obtained from the remains of the missiles and information obtained from PW's and from captured equipment should be considered as uncertain.
a. Missile Control Signals
Previous countermeasure missions have heard many signals in the 47.5 to 50 Mc frequency range during attacks by HS-293 glide bombs which appear to be the missile control signals. These signals had a distinctive chirping sound, somewhat like an automatic transmitter sending dots, with the space between dots being filled with a different tone. Recordings of these signals have been analyzed and have been found to contain only two distinct audio tones, 1000 and 1500 cycles. These two tones are transmitted alternately, never simultaneously, at a relatively constant recurrence rate of approximately 10 per second. That is, a 1000 cycle tone lasting 80 milliseconds may be followed by the 1500 tone for 20 milliseconds, followed in turn by the 1000 cycles, then the 1500 cycle tone, recurrently. The relative duration of these two tones may change. It is believed that the missile is guided by varying the relative duration of these two tones but the exact mechanism of this is not yet clear as there does not appear to be sufficient information contained in this type of signal to control both the lateral and vertical directions of the missile. While most of these signals have been heard in the 47.5 to 50 Mc range, the same signals have been heard during recent attacks on frequencies as low as 43 Mc. It is entirely possible that the enemy may shift their frequencies anywhere in the 15 to 100 Mc range but they are most likely to remain in the general vicinity of 50 Mc because extensive modifications to the missile receiver would be required for a large frequency change. It is also t>ossible that different types of control modulation may be used so all operators should be alert for any unusual signals which cannot be identified. It is still possible, though now improbable, that the signals which have been heard thus far are merely "spoof" signals, intended to draw our jamming to other frequencies. In recent attacks many more signals have been heard than the number of missiles which were seen. This appears to indicate that the enemy knows that his frequencies have been identified and has adopted the tactics of turning on many transmitters as each missile is dropped so that we cannot tell which of these signals are the correct ones being used to guide missiles.
b. German Radio Control Equipment
A tire-selector stage, presumably from an FX bomb, was found at Foggia, undamaged and only partially dismantled. Tests showed
that this unit had a "band pass inputt circuit tuned to accent frequencies "between 47.5 and 50 Mc. The converter plae was connected to a 3 Mc IF transformer. The local oscillator and IF amplifier were missing. The antenna coupling unit recovered from a control Diane was also tuned to 50 Mc. Portions of an IF amplifier recovered from an excluded HS-293 glide bomb also indicated that the IF frequency -probably was 3 Mc. Part of this circuit resembled a discriminator, suggesting that FM may "be used. The control Diane antenna coupling circuit, previously referred to, contained a dummy antenna for transmitter tuning consisting of two 15-watt resistors. This indicates that the control transmitter Dower is less than 30 watts. Both transmitter and receiver presumably are crystal control and PW's report that a choice of 19 frequencies is available.
c. Results of Jamming
Previous countermeasure groups equipped with inadequate low power jammers have reported highly successful jamming of HS-293 glide "bombs using tone modulation, noise modulation, and even an unmodulated carrier, though subsequent analysis of these reports casts some doubt on the number of missiles jammed. Such a large percentage of HS missiles have proved defective in earlier attacks that it is very easy to see a missile fall out of control and assume that it has been successfully jammed. Thus far, there has been no adequate correlation between the start of jamming and supposed deviations in the missile's flight.
d. Information Needed
More recordings of missile signals, made with any available recording equipment, are urgently needed. In -particular there is urgent need for correlation between changes in the path of the missile and changes in the nature of the control signal which will clarify the method of control and enable us to use a jammer modulation which will be certain to take control of the missile. Accurate correlation is needed between jamming activities and observations of missiles being jammed to correlate positively the effects of jamming. It should be remembered that frequency modulation may be used.
4. Methods of Jamming
The two fundamental methods of jamming consist of (a) block-ing the missile receiver to prevent its control by the enemy and (b) causing false control to deflect the missile by modulating the jammer with a signal sufficiently similar to the control signal to deflect the missile. Blocking may be produced by a strong carrier, modulated or unmodulated, which saturates or overloads the missile receiver, or by noise modulation which blankets the modulation of the control signal. Either of these methods of jamming is only effective as long as the jammer is held continuously on the missile frequency. If the missile
is "blocked for only a "brief period, the enemy may regain control. False control jamming is the most effective method when it can "be used "but demands an accurate knowledge of the control modulation being used. If the missile can "be deflected sufficiently, "by a "brief "burst of jamming, a considerable time may "be required for the enemy to bring the missile "back on its course.
PART II. GUIDED MISSILE COUNTERMEASURE (GMCM) EQUIPMENT
The following listing of GMCM equipment includes all equipment which may "be supplied in the near future and includes items which probably will not "be available. The jamming transmitters have "been designed to cover frequencies from 15 to 55 Mc, "but no one receiver capable of covering- this entire frequency range. is available thus far. Consequently, communication receivers are used "below 30 Mc and special high frequency receivers are used above 28 Mc. Broad band panoramic receivers covering approximately a 2:1 band width on each frequency range are provided whenever available for use in preliminary search and to warn of presence of new signals. These receivers give the operator a view of the strong signals in any one band but do not provide sufficient frequency resolution for setting the jammer frequency. Manually tuned receivers with panoramic adapters are used to provide a magnified view of a small band of frequencies and to -permit setting of the jammer frequency with the necessary accuracy. The jammer and receivers are interconnected for "look-through" operation as will be explained later.
1. High Frequency Receivers
a. RDC-28to140 Mc Broad Band Panoramic
This receiver is a conversion of the RBK or Hallicrafter S27 receiver. The conversion involves replacing the tuning condenser with a motor driven high speed scanning condenser, together with suitable modifications of the output circuits. This condenser rotates at 900 r.p.m. and scans through any one frequency band in three-fourths of a revolution, retracing itself in the remaining one-fourth revolution. A synchronizing switch on the condenser shaft is used to synchronize the sweep circuit of an oscilloscope display (either the XCA, Dumont 241, or other available oscilloscopes). The ETC output is applied to the vertical deflection elates and any signal present produces a vertical Dip on the horizontal trace each time the receiver scans through its frequency. As a result, all signals present appear as pips on the trace with their relative amplitudes and positions in the frequency spectrum.
b. R3K-1 - 28 to 143 Mc Manually Tuned Receiver
This receiver is a Navy model of the Hallicrafter S27 receiver with certain modifications to be discussed later. In some
installations, standard S27 receivers may "be used. These receivers have been modified for look-through operation as explained later.
c. RBW Panoramic Adapter
This is an attachment to the RBK receiver to provide panoramic display of frequencies within 500 Kc either side of the frequency to which the RBK is tuned. Signal Corps BC-1032A panoramic adapters may be provided if insufficient EBW's are available but there is no essential difference between them. The panoramic adapter is essentially a reactance scanned panoramic receiver tuned about the IF frequency of its companion receiver, and it is connected to the converter output ahead of the first IF transformer of the receiver. The local oscillator frequency of the panoramic adapter is swept through a narrow range of frequencies "by a reactance tube which is driven from the same saw-tooth ware used to provide the sweep voltage for its oscilloscope display. Therefore, there is a definite frequency corresponding to each value of sweet) voltage and all signals within the frequency range of the adapter will appear on the display scone in their relative frequency relation. These adapters have teen modified for look-through operation as explained later. Instruction "books are provided with these panoramic adapters.
2. Low Frequency Receivers
a. RUG- - .5 30 Mc Broad Band Panoramic
The RDG is a panoramic conversion of the National HRO receiver, similar in principle to the RDC receiver. The RDG probably will not "be available in the immediate future.
b. RAO-2 - .5 to. 30Mc Manually Tuned Receiver
This receiver is a Navy model of the National NC-100. Other receivers in this frequency range may "be provided if RAO's are not available. Instruction "books for these receivers are provided,
c. RCX Panoramic Adapter
This adapter is intended for use with the RAO or other communication receivers having 456 Kc IF amplifiers. Signal Corps BC-1031A adapters may be provided when RCX's are not available. These adapters are very similar to the RBW and also have "been converted for look-through operation. Instruction books for these adapters are provided.
3. Jamming Transmitters
Most jamming transmitters to be provided at present will be of type CXGE. This jammer is similar electrically to the XCJ. The CXGE is a conversion of a 1 Kw final amplifier built for General Electric FM broadcast transmitters. It covers 15 to 55 Me with three
plug-in coils, uses 833A's in a push-pull self-excited single control oscillator and delivers output powers between 500 and 1000 watts. The transmitter is modulated by a grid keyer producing on-off square wave modulation. Tone modulation is available over a wide range of audio and supersonic frequencies. In addition, noise modulation also is provided for barrage jamming of restricted band widths. The transmitter is equipped for look-through operation with its associated receivers as will be discussed later. Instructions for the operation of this transmitter are provided with the transmitter.
4. Auxiliary Equipment
a. XCA Modulation Analyzer
The XCA analyzer incorporates instruments for studying those types of modulation which cannot be recorded or studied aurally, and also includes two 5" scopes intended to be used as displays for the broad band panoramic receivers and panoramic adapters. The XCA incorporates a video frequency analyzer which is essentially a very low frequency reactance scanned panoramic receiver. Three frequency ranges, 0-15, 0-50 and 0-200 Mc, are provided. The XCA also includes a servo sweep oscilloscope for studying pulse modulation. Separate instructions for the XCA are provided.
b. PuMont Oscilloscope
This is a standard DuMont, Type 241, 5-inch oscilloscope, to be used as a display for the RDC and RDG broad band panoramic receivers, in lieu of an XCA. It may be used also as a general test instrument. When used as a panoramic display the synchronizing output of the RDC or RDG should be connected to the synchronizing terminals of the 241 and the signal output of the RDC or RDG should be connected to the vertical deflection terminals of the 241. The sweet) rate of the 241 should be set at a value lower than 15 per second so that the synchronizing pulses from the panoramic receiver may lock it at this rate. The horizontal trace may be so positioned that most of that portion of the trace corresponding to the 90 degree retrace portion of the panoramic condenser's rotation is not visible. In addition, the synchronizing signal may be applied to the "Z axis" terminals on the oscilloscope to produce an intensity modulation of the sweep. With proper adjustment of the "Z gain" a break will be produced in the baseline at the point where the retrace starts, so any signal seen beyond this point may be ignored. It is impossible to obtain complete suppression of signals during the retrace because of the short time constant in the Z axis amplifier. Use of excessive Z axis gain in an attempt to secure complete suppression of the retrace will defocus other parts of the sweep trace. Further instructions on the operation and servicing of this instrument are supplied in its instruction book. The same comments apply to any other oscilloscope which may be used.
A Soundscriber or other recorded may be provided. This should be used as a dictation instrument and as a means for recording signals. More extensive use should be made of this equipment for recording observations during combat when it is impractical to write notes. Dictate the frequencies on which you hear missile signals as you hear them, not sometime later. Dictate the time the signal is heard, how many signals are heard simultaneously and at what frequencies, when jamming is started, what type modulation is used, how long the signal is heard, how many missiles are observed, whether they fly a smooth course or go out of control, whether they appear to be deflected by jamming or merely appear unable to maneuver when blocked by the jammer. It is suggested that the recorder be bridged across the intercommunication circuit between the aerial recognition officer and the countermeasure officer to record the observations of both with positive time correlation, Whenever possible, record missile signals, particularly if these signals are heard on a new frequency or appear to have any new or unusual characteristics. Turing an attack the recording equipment should be used almost continuously. However, be careful to avoid over-modulation and to protect the instrument from shock. Identify all records with the date, time, ship, and nature of attack. Write this information in the center of the record in ink, or scratch it into the center with a pin. DO NOT WRITE OVER THE GROOVES. Soundscriber recording is an embossing process and writing over the grooves with a pencil breaks down the walls of the grooves, as well as fouling them with graphite.
5. Broad Band Antennas
The impedance of ordinary antennas changes rapidly with frequency over wide limits and it has been customary to use antenna impedance matching networks with communication transmitters. This is impractical with GMCM jammers because their antennas usually must be at a remote location, connected by a transmission line, and too much time is required to adjust an impedance matching network or to adjust the length of the antenna to resonance. Furthermore, sharply resonant antennas are objectionable for use with broad band panoramic receivers because they cause bad inequality in sensitivity. Consequently, broad-band transmitting and receiving antennas have been developed for use with GMCm installations.
a. Sleeve Transmitting Antennas
These antennas resemble quarter wave stub antennas having their lower eighth wavelength surrounded by a grounded sleeve, like the outer conductor of a coaxial line. Inside this sleeve is located an impedance transforming line section. These antennas are installed at an appropriate point on the ship's superstructure, with the superstructure forming the ground plane for the antenna. The antenna should be inclined 45 degrees from the vertical and deviate
45 degrees from the ship's axis to provide both horizontal and vertical polarization. When properly installed, these antennas will match the impedance of a 50 ohm cable with a standing wave ratio not exceeding 2 to 1 over almost a 2 to 1 band width. Two antennas are provided, one covering 15 to 30 Mc and the other covering 30 to 55 Mc. The ability of these antennas to meet a 2 to 1 standing wave ratio over the above band widths defends largely upon the location of the antenna on the ship. Satisfactory locations for these antennas have been found on DE's and DD's. They should not be installed on other type ships without making careful impedance measurements and changing the location or the antenna, dimensions as required. Such impedance measurements must be made by skilled personnel with equipment not generally available in the field. Plate 3 is an assembly drawing of the 30 to 55 Mc sleeve antenna showing its impedance matching transformer and the method of coupling to the RG-18/U cable. Plate 4 shows the correct location of sleeves on a BE. Plate 5 is a. photograph of one of the two 30 to 55 Mc sleeve antennas installed on a DD. Two such antennas, port and starboard, are required to avoid dead spots. Plate 6 shows the method of attaching the cable to this antenna, inside of the skin of the ship. Plate 7 shows one of the two 15 to 30 Mc sleeve antennas mounted on a DD. Since this antenna extends out over the water, beyond the ship's side, it was necessary to hinge the antenna so it could be secured inboard when the ship docked. Plate 8 shows this hinged mounting in the operating position, while Plate 9 shows it with the antenna secured. Particular attention is called to the care which was taken to provide continuous electrical contact at the base of this antenna. The contact surfaces were faced with a bronze bead from a welding arc and then filed for complete around the ring contact when the antenna, was held in place by the four dogs. Oversized holes were made for the hinge-pin so that the pin did not restrict the contact between the surfaces.
b. Fan Receiving Antennas
These antennas consist of a network of wires having the general shape of a fan. The dimensions of this antenna are shown in Plate 10. When properly installed, this antenna, will match a 70 ohm coaxial line with less than a 5 to 1 standing wave ratio over the entire 15 to 55 Mc band. The impedance match to a 50 ohm line is nearly as good. Such large standing wave ratios are far less objectionable for receiving than for transmitting. Plate 4 also shows the fan installed on a DE, while Plate 11 is a photograh of a fan installed on a DD.
c. Antenna Feed Installations
RG--18/U (CASSF-50-2A) 50 ohm 0.9 inch diameter armored cable should be used to connect the jammer to the sleeve antennas.
The more common RG-10/U (armored PT-5) is too light except for very short lengths, and becomes warm when carrying a kilowatt. ASE "long range-home" (Washington Institute of Technology N. 3557-1) switches are provided to use for connecting the jammer to the appropriate antenna. These switches take British angle fittings (type 701) for PT-5 cable and adapters to fit the RG-18/U cable are provided with the antennas. Plate 12 is an assembly drawing of these adapters. Plate 13 shows the switch and cable arrangement used to feed two sets of sleeve antennas for DD installations. Short lengths of RG-10/U may be used for the connections between switches. HG-12/U (small 75 ohm armored cable) should be used to connect the fans to the broad band panoramics, though RG-10/U maybe used if necessary. Do not switch antennas under load. Always interrupt the jammer before switching antennas. The fan antennas may also be used with the manually tuned receivers, though ordinary wires or whips are satisfactory.
PART III. LOOK-THROUGH OPERATION AND INTERCONNECTIONS
1. Theory of Look-Through Operation
The term "look-through" refers to a -panoramic method of monitoring both the jammer and victim signal simultaneously for the purpose of adjusting the jammer frequency. The time division method of look-through is used in the CXGE and XCJ jammers and their associated receivers. With this method the jammer is interrupted periodically for the time required for one sweep of the panoramic adapter, then is energized for the time required for three sweeps of the panoramic adapter. This cycle is repeated continuously. During each three sweeps that the jammer is energized the sensitivity of the panoramic adapter is reduced sufficiently by 'a blocking pulse to avoid saturation. During every fourth sweep, while the jammer is keyed off, the sensitivity of the panoramic adapter is restored to normal, permitting observation of the victim signal. In this manner the operator can observe the pips on the panoramic adapter from both the jammer and the victim signal, and can adjust the jammer frequency to cause these pips to coincide. All circuits for keying the jammer in this manner and for generating the blocking voltages are contained in the transmitter. Further discussion of look-through operation is contained in the instructions for the CXGE and XCJ jammers.
2. Look-Through Modifications
Certain simple modifications have been made to the manually tuned receivers (RAO and RBK) and to the panoramic adapters (RBW and RCX) to permit look-through operation.
a. Panoramic Adapter Modifications
Synchronization of the look-through circuit with the panoramic adapters is accomplished by differentiating the saw-tooth sweep voltage of the panoramic adapter through a small condenser and -using the resultant pulse occurring at the start of each sweet} to trip the electronic switches in the look-through circuit. The blocking pulse derived from the look-through circuit is supplied to the screen grids of the first and third (amplifier) tubes in the panoramic adapter, through a volume control located on the panel of the panoramic adapter. This volume control may be adjusted for the desired degree of blocking to equalize the two pips. With normal panoramic adapters, having a sweep ratio of 30 per second, the repetition rate of the look-through is 7-1/2 per second and the resultant flicker has been found tiring to the eyes. Consequently, the more recent adapters which have been converted at the factory have had their sweep rate increased to 60 per second. This reduces the flicker at the expense of a slight loss in panoramic resolution. This modification can be made to other adapters by readjusting their synchronization and increasing a resistor to restore the length of the trace.
b. Receiver Modifications
By-passed resistors have been placed in the grid leads of the RF amplifier and converter stages of both the RAO and RBK receivers to protect these tubes from excessive RF voltages from the jammer. Any RF voltages sufficient to draw grid current bias these tubes back and limit the grid currents to safe values. The time constants of these by-passed resistors are negligible in comparison with the look-through period. In addition, it has been found necessary to apply a slight negative blocking pulse of fixed amplitude to the screen of the RF stage in the RBK to minimize "frequency pull" of the local oscillator. With no such blocking pulse, the jammer may pull the frequency of the RBJC local oscillator as much as 50 Kc, producing this same error when the jammer frequency is adjusted by look-through. The value of this negative blocking voltage is not critical. No blocking is applied to the RAO receiver because its local oscillator has no appreciable frequency-pull.
3. Look-Through Interconnections
Plate 14 shows the interconnections required between the CXGE or XCJ and its associated receivers for look-through operation. These connections should be made with RG-8/U or R&-10/U with Selector fittings. A switch is provided in the look-through circuit of the jammer to select either the high or low frequency receiver. On the CXGS and later XCJ transmitters the operate-standby switch also operates the look-through circuits. Thus, the receivers are blanked only when the transmitter is operating. Furthermore, only one of the two receivers, the one chosen by the above selector switch, is blanked at a time and the remaining receiver may be used without look-through to cover frequencies other than the one being jammed.
Look-through operation should be used only for adjusting the jammer frequency. When set on frequency the look-through should be turned off, letting the jammer run steadily, except possibly for occasional brief "peeks" to determine whether the victim signal is still present. This method of look-through produces time holes in the jamming signal, through which it may be possible for the enemy to retain control of the missile. This is not likely, but it is better to play safe.
No provision for look-through has been made in the RDC and RDG broad band panoramics because the mechanically scanned condensers in these receivers cannot be rotated safely at sufficient speed. It is best to disconnect these receivers when actually operating the jammer.
5. Installation Recommendations
Each new type of ship-presents new -problems in installing GMCM equipment but certain general recommendations can be made for all installations to insure maximum effectiveness and convenience of operation. A floor space of approximately 50 square feet is re- quired for the equipment and operating personnel The equipment should be so placed that one operator can monitor the low frequency receiver, the second operator can monitor the high frequency receivers and both of these operators can reach the main tuning dial of the jammer conveniently, while watching their Panoramic adapters. A third operator, if available, should adjust the jammer plate voltage and antenna coupling, change soundscriber records and perform other functions as required. The countermeasure officer should be able to observe both receivers and supervise their operators. It is suggested that the high and low frequency receivers be mounted with shock mounts on benches on either side of the jammer with the panoramic adapter secured to the to-Ds of their respective receivers. The broad band panoramic receivers, RDC and RDG, may be suspended on a bracket or mounted on a shelf in less accessible positions because the operator need only have occasional access to the volume control. The DuMont oscilloscope, if used for the broad band panoramic display, should be mounted as close as possible to the associated panoramic adapter scone so that the operator may observe both scopes together. If an XCA analyzer is available, it should be used as the broad band panoramic display and its right hand scope may be used as the panoramic adapter display by removing the scope from the panoramic adapter and bringing the connections from the scope circuit out through the scope o-opening to the panoramic adapter terminals of the XCA. The transmitting antenna selector switches and the Soundscriber, or other recording equipment, may be mounted in any readily accessible location. Plates 15, 16 and 17 are photographs of an XCJ installation on a DD, illustrating the suggested placement of the equipment. If an XCA is provided, it
should be located beside the TBK-RBW, instead of the DuMont scone, since it is most likely to be used only for high frequency signals.
PART IV. OPERATING PROCEDURE
The following suggestions concern routine watch, practice jamming, and combat jamming procedure.
1. Watch Procedure
Prompt location of missile control signals depends upon being able to identify and eliminate all ordinary communication signals promptly. This requires extensive practice to develop a high degree of familiarity with all signals which normally are [resent in the frequency spectrum. Each new signal appearing on the broad band panoramic display may be identified with the manually tuned receiver and located according to frequency. Known signals which are generally present may be masked from the broad band display with Scotch tape or an ink mark, if desired. Considerable practice is required to tune the manual receiver to any given signal on the broad band display. It will be observed that radiation from the local oscillators of the manually tuned receiver produces a marker pip on the broad band display. This marker pin corresponds to the local oscillator frequency which differs from the signal frequency by an amount equal to the intermediate frequency of the manually tuned receiver. The suggested tuning procedure is to align this marker with the desired signal pip then detune the receiver by an amount equal to its intermediate frequency. This should place the desired signal near the center of the panoramic adapter display. After some practice the operator will be able to detune the correct amount automatically without using the receiver frequency calibration.
2. Jamming Practice
Jamming practice should be held, whenever conditions permit, to develop familiarity and teamwork. A TBY or even a signal generator may be used as a victim signal. Jamming practice may be conducted most effectively between two shins, the shin carrying the TBY monitoring the accuracy of jamming from the other shin. The TBY may be keyed slowly or modulated in any agreed manner for identification. An unmodulated carrier should be used for practice jamming to avoid disclosing the nature of the equipment and to avoid possible interference with other services. Practice adjusting the jammer as rapidly as possible after each shift in frequency of the TBY.
3. Procedure for Tuning the Jammer
Keep the filaments of the jammer hot when expecting operation. On identifying a signal seen on the broad band panoramic tune...
4. Identification of Guided Missile Signals
5. Choice of Jamming Method
Choice of the proper jamming method defends largely upon tactical considerations.
a. Spot Jamming
If the missiles are launched singly so that only one or two are in the air at any instant, and if only one or two control signals are present, best results may be obtained with "spot jamming"; that is, "spotting" the jammer exactly on the control frequency. This type of operation concentrates the jammer's energy. The jammer may be modulated with 1000 or, preferably, 1500 cycles, the two tones identified in recordings of the missile control signals, with the possibility that this may cause false control. Unfortunately, this method is impracticable if several frequencies are present simultaneously.
b. Noise Modulated Barrage
If several signals appear together on closely spaced frequencies, noise modulation may be applied to barrage jam all frequencies within a band of approximately 1/2 Mc. This type jamming is less certain in its results because less energy is concentrated at the frequencies of individual missiles.
c. Band Sweeping
Previous GMCM groups have reported successful jamming of several simultaneous signals by sweeping an unmodulated carrier slowly back and forth through their frequencies. Their report indicated that this jamming deflected the missiles, presumably by beats between the jammer and the control carriers causing false control. This observation was surprising, and it seems questionable that false control will be produced in this manner. A more effective method would be to modulate the jammer with 1000 cycles, or, more preferably, 1500 cycles, adjusting these frequencies after carefully listening to the signal, and sweeping this modulated signal slowly back and forth through the several missile frequencies.
d. IF Heterodyning
The fourth method of jamming, advocated by the British, consists of using the beat between two jammers to block the IP amplifier of the missile receiver, and makes use of the fact that the missile receiver has a band pass preselector. Two powerful jammers are tuned to frequencies exactly 3 Mc apart, preferably to 47.25 and 50.25 Mc. These two signals, if sufficiently powerful, will produce a 3 Mc beat in the converter of the missile receiver, regardless of the local oscillator frequency of this receiver, and the beat will, therefore, enter the receiver's IF amplifier. One jammer should be left unmodulated, while the other may be modulated with 1000 or 1500
cycle tone or with noise. This method of jamming requires an accurate knowledge of the intermediate frequency of the missile receiver and very careful setting of the frequencies of the two jammers. It is believed that our -present knowledge of the missile receiver's intermediate frequency is not sufficiently accurate and that the degree of coordination required between the two jammers is too great to justify this method of jamming at t>resent.
6. Operational Data
During guided missile attacks keen complete data on all signals heard, their frequencies, the tone jamming used, and the results observed. Use the Soundscriber for this purpose and avoid longhand notes. Make recordings of missile signals whenever possible. Submit your report of the attack as promptly as possible. Remember that FX signals have not yet been recorded, that the frequencies and control modulation of the HS-293 may be changed at any time, and that new missiles with totally new control signals may be introduced. Our ability to cone with such new systems defends upon the accuracy and completeness of the information which you supply.
APPENDIX I -- Recent Information on Missile Receiver
a. Examination of receiver parts from recently recovered missiles confirms the use of 1000 and 1500 cycle modulation frequencies but also indicates the use of additional modulation frequencies somewhere in the range 7 to 10 Kc. The frequencies of the 1000 and 1500 cycle tones have teen established with an accuracy of 2% but the higher modulation frequencies have not been accurately determined. It is assumed that the 1000 and 1500 cycles operate one set of controls, either elevation or direction, and that the higher frequency modulations operate the re maining control. It may now be considered certain that the signals which have been recorded have not been "spoof". Despite the lack of adequate knowledge concerning the higher modulation frequencies, the missiles should be vulnerable to jamming modulated with 1000 or 1500 cycle tones providing that these frequencies are set with ± 2% accuracy. Use of 1500 cycle modulation is recommended because examination of recordings indicates that an excess of 1500 cycles occurs much less frequently than an excess of 1000 cycles.
b. The best present information on the receiver indicates that the IF frequency is 2.93 Kc and the IF bandwidth is 50 Kc at the 6 db points. There is also considerable evidence that the receiver is not crystal controlled but employs a self-excited local oscillator with automatic frequency control from a discriminator in the IF amplifier. If so, the missile should be even more vulnerable to jamming since it may lock to the jammer frequency even though this frequency has not been set with greater than 25 Kc accuracy.
c. It is recommended that all ships equipped with XCA analyzers watch the modulation frequencies on the 0 to 15 Kc range noting the number of frequencies observed, their exact values and relative amplitudes, and whether they occur simultaneously or alternately. Confirmation of and information on these higher modulation frequencies is needed urgently.
Plate 5: Sleeve Type (High Frequency) Transmitting Antenna
Plate 5-1: Sleeve Type (High Frequency) Transmitting Antenna
Plate 10: Dimensions for Fan Antennas
Plate 13: Schematic Diagram of Sleeve Antenna Switch Arrangement
Plate 14: Inter Connections for Lock-Through Operation
Plate 15: RBK Receiver, RBW Panoramic Adapter, SCJ-1 Transmitter, RCX Panoramic Adapter,
RAO Receiver, Transmitting Antenna Switches
Plate 16: RDC Panoramic Receiver, DuMont Scope, RBW Panoramic Adapter, RBK Receiver
Plate 17: P.J. Sound Scriber, XCJ-1 Power Unit