RADAR BEACONS--PRINCIPLES OF OPERATION Page I. Pulse-and-Echo vs. Direct Transmission 100 II. Characteristics 100 III. Range/Altitude 101 IV. Coding 101 YJ RADAR BEACON I. Function 105 II. Description 105 III. Tactical Employment 107 AN/CPN-3 (YK) AND AN/CPN-6 (YM) I. Function 109 II. Description 109 III. Tactical Employment 110
RADAR BEACONS (RACONS)--PRINCIPLES OF OPERATION
I. PULSE AND ECHO VERSUS DIRECT TRANSMISSION
A radar transmitter sends out pulses of radio energy in a beamed path. When an object is in the path of the beam, it reflects back a small portion of the transmitted energy to the source, where it is displayed visually on a cathode-ray tube screen to indicate the presence, bearing, and range of the object. This main function of a radar employs the pulse-and-echo cycle of operation.
However, if the beamed pulses from the radar strike the antenna of a radar beacon installation, the radar beacon, normally dormant, is triggered into action and in turn transmits powerful omnidirectional radio waves. (See fig. 3-1.)
Beacon transmissions have three predominant characteristics which contribute to the function of the beacon as an aid to navigation:
1. The transmission occurs at a frequency slightly higher than that of the initiating radar pulse. Therefore, these transmissions are not displayed visually on the radar scope screen in the aircraft until the radar receiver is retuned away from the radar frequency to accept the higher beacon frequency. Retuning the radar receiver not only allows the beacon signal to be observed visually but also rejects radar echoes which might otherwise cause a confused screen pattern.
2. Beacon signals as transmitted are coded so as to furnish the identity of the beacon station to the interrogating aircraft.
3. The transmissions from the beacon station are not beamed in any one given direction, but, rather, are radiated in all directions. In the interrogating aircraft, the relative bearing of the beacon is indicated by the azimuth position
Figure 3-1.--The outward-going pulses from the aircraft's radar strike the beacon transmitter, which is triggered into operation and sends out a powerful coded signal in all directions.
of the antenna and the corresponding location in azimuth of the signal indication on the scope screen (fig. 3-2).
Figure 3-2.--Since the trace on the PPI rotates in azimuth in step with the antenna,
the azimuth position of the coded signal on the PPI denotes bearing of the beacon station.
The range at which radar beacon signals can be received is greater than that obtained in normal radar operation. This is because in radar operation the strength of the indication, all other factors being equal, is dependent upon the strength of the reflected echo, whereas in beacon operation the received signal displayed on the scope is produced by a strong, directly transmitted signal from the beacon station.
Range is also dependent upon the altitude of the interrogating aircraft. If the aircraft is flying low, the path of the radar beam to the horizon is short. A beacon transmitter beyond the horizon at that altitude may not be triggered into operation. Increasing the altitude increases the distance to the horizon and brings the radar beacon station, previously hidden by the horizon, into the path of the radar beam.
Transmissions from radar beacons operating in the P-L frequency bands (YJ) are gap coded. See L-scan, figure 3-3. A motor-driven coding disc having raised cams operates a micro-switch to key the transmitter circuit for long or short periods of time, producing two-letter Morse Code signals.
The speed of rotation of the code disk is such that the complete series of signals comprising a two-letter code is transmitted within a 30-second period.
At the receiving end (radar interrogator), these signals, displayed on the scope, appear as recurring blips on the trace, the duration of the blips signifying the "dots and dashes" of the two-letter code.
Figure 3-3.--Depending upon the type of air-borne radar equipment used and the radar beacon with which it is used, beacon signals are eiiher displayed as gap-coded indications or range-coded indications. For instance, on airborne radar equipment employing the L-scan the beacon signal blinks on and off in accordance with 2-letter signals of the International Morse Code. On equipments employing the PPI or B-scan radar signals appear range coded; that is, a series of vertically spaced light spots appear on the scope screen, their separation and number denoting the specific coding employed.
Range to the radar beacon is indicated by the position of the blip along the trace, up from the bottom of the scale. Bearing of the radar beacon with respect to the aircraft is indicated by the lateral displacement of the blip on the trace. When the beacon is dead ahead the lengths of the blip on either side of the trace would be equal.
Transmissions from radar beacons operating in the S and X frequency bands (AN/CPN-3 and AN/CPN-6, respectively) are range coded; i.e., the coded response consists of a number of time-spaced signals (six or less) occurring so rapidly (15 to 35 microseconds) that, at the radar receiving location, they appear on the PPI or B-scan screen of the indicator as a series of spots of light arranged as to number and spacing to indicate the radar beafeon's identity. See PPI and B-scan, figure 3-3.
Range to the radar beacon is indicated by the position on the trace of the lowest signal in the coded series. Bearing is indicated by the azimuth position of the entire coded series of spots.
YJ Radar Beacon
YJ RADAR BEACON
The YJ radar beacon, furnishing coded signals to radars operating in the L and P frequency bands, is designed to act as an aid to homing and as a means of locating an aircraft's position with respect to the radar beacon.
A. MAIN COMPONENTS. YJ radar beacon equipments consist of:
- Antenna assembly.
- Beacon transmitter-receiver and power supply unit.
1. The antenna assembly comprises two separate radiator units (one for operation on the L band of frequencies, the other for operation in the P band of frequencies), a supporting mast, and a coaxial feeder cable for connecting the antenna to the transmitter-receiver unit.
2. The transmitter-receiver unit consists of two separate transpondors, designated "A" and "B," for operation in the P and L frequency bands, respectively.
The power supply unit operates from a 110-volt, 800-cycle a. c. source and furnishes the
required operating voltages to the "A" and "B" transpondor units.
B. OPERATION. 1. YJ is a transpondor (transmitter-receiver) type of electronic equipment, ship-based or land-based, which, when triggered into operation by the reception of beamed radar pulses from ship or aircraft, transmits gap-coded replies omnidirectionally.
At the radar-interrogating ship or aircraft, these coded replies are received by the radar and displayed on the radar scope to denote identity, range, and bearing of the radar beacon.
The receiver circuits of the YJ transpondor are tuned to accept pulses from radars operating at 176 megacycles and 515 megacycles. The transpondor responds by alternately transmitting signals of 177.5 and 520 megacycles on a time-sharing basis.
NOTE.--Specific mention of operating frequencies is made here to facilitate complete understanding of the operation of the equipment in question. With reference
Figure 3-4.--The YJ radar beacon contains two separate transpondors-an "A" unit and a "B" unit-which furnishes replies to radars interrogating on 176 and 515 megacycles, respectively. The coded reply enables the beacon to be identified.
Figure 3-5.--As long as the equipment continues to receive an interrogating signal on either channel it transmits a coded reply on the same channel. The code, which can consist of two letters, or only one, as shown here, repeats every 30 seconds.
to correspondence or conversation regarding specific operating frequencies, readers of this manual are cautioned to observe security regulations as outlined in Part I, pages 5 and 6.
On board the radar-interrogating ship or aircraft, the radar receiver must purposely be tuned away from the regular pulse-and-echo frequency to the radar beacon frequency in order to receive the beacon replies. As a result, the radar scope does not present radar echo displays but shows only the beacon signal in the form of a series of blips of dot and dash duration whose coded sequence denotes the beacon's identity.
2. P and L band triggering.--P-band radar pulses which are beamed directionally to strike the P-band antenna are fed to the "A" transpondor to trigger it into operation as a transmitter. Its reply, interrupted by a coding disk, is transmitted omnidirectionally by the same antenna receiving the radar pulses.
Figure 3-6.--To prevent one transmitter from firing the other, one is blocked for about one-fortieth of a second during which the other transmitter can reply as interrogated. Then the second one is blocked while the first one is allowed to answer on interrogation. The blocking period is, however, so short that the reply appears continuous to an observer at the cooperating radar.
In a like manner, radar pulses from radars operating in the L-band of frequencies are picked up by the L-band antenna and fed to the "'IV transpondor to trigger it into transmitting operation. See figure 3-4.
As long as the equipment continues to receive an interrogating signal on either channel, it transmits a coded reply on the same channel. See figure 3-5.
3. Time sharing.--Reception of both P- and L-band radar pulses can be simultaneous. Transmission of coded replies to simultaneous reception is performed by the equipment on a time-sharing basis. See figure 3-6.
4. Selective response.--Adjustments to the operating controls of YJ permit either the "A" and "B" transpondor to be selected for full-time response. During the period the equipmerit is so adjusted, only the selected transpondor operates.
5. Coding.--Coded replies to radar interrogations are furnished through the operation of a motor-driven code disk whose arrangement of interruptor cams key the transmitted replies in one- or two-letter Morse Code.
NOTE.--Shipboard YJ's will not normally have an A-band unit operating in the P-band of frequencies. It will utilize only the B-band unit operating in the L-band of frequencies for use with air-borne radar equipments installed in carrier-based aircraft.
III. TACTICAL EMPLOYMENT
For the tactical employment of YJ as a beacon homing system for radar-equipped (ASB) aircraft, see p. 35.
The YJ radar beacon, when triggered into operation by radars operating on either the P-or L-band of frequencies will transmit radio signals which can be received by the enemy to provide him with directional information as the bearing of the YJ location. Tactical commanders, therefore, are faced with the problem of determining whether aircraft employed in a strike should be permitted to return to base by YJ homing or whether it is more important from the standpoint of tactical surprise to maintain all radar silence.
AN/CPN-3 (YK) AND AN/CPN-6 (YM)
A. AN/CPN-3 and AN/CPN-6 radar beacons, furnishing signals to radars operating in the S and X bands, respectively, are designed to act as aids to homing and as a means of locating an aircraft's position with respect to the radar beacon location.
A. MAIN COMPONENTS. Radar beacons AN/CPN-3 and AN/CPN-6 consist of:
- Antenna assembly.
- Beacon transmitter-receiver and power supply unit.
Fundamentally, these radar beacons are similar to each other. Their difference are chiefly in circuit design and in the frequencies on which they receive and transmit.
These beacons may be ship-borne or land-based.
B. MODE OF OPERATION. 1. Beacon triggering.--One characteristic which makes these beacons different from the YJ type of radar beacon (L- and P-band operation) is that they do not respond to all radar pulses received within their respective operating frequency bands. While the beacon station can receive pulses from all radars operating within the band of frequencies accepted by the beacon (provided, of course, that the radar pulses are directed toward the beacon station), the use of a discriminator circuit in the beacon receiver allows the beacon to respond only to those radar pulses which are of 2 microseconds duration.
The normal pulse length of air-borne radars employed for search varies from 0.5 to 1 microsecond. These pulse lengths will not trigger the beacon into operation. When an air-borne radar is switched to beacon operation, its pulse length is automatically increased to 2 microseconds duration. These pulses, when received by the beacon will be accepted by the discriminator and will trigger the beacon into operation.
It follows, then, that even though a beacon station is within the field of the beam from an air-borne radar, the beacon will not make known its presence by replying to radar pulses until the air-borne radar has been set to transmit the 2-microsecond beacon triggering pulses.
2. S-Band.--AN/CPN-3 will receive interrogations from S-band air-borne radars whose individual operating frequencies may be anywhere within a 66-megacycle band, from 3267 megacycles to 3333 megacycles. The beacon t ransmitter, however, will respond only on 3256 megacycles, and the receiver portion of the airborne S-band radar equipments must be tuned to this frequency in order to receive the beacon response.
3. X-Band.--AN/CPN-6 will receive interrogations from air-borne X-band radars whose individual operating frequencies may be anywhere within a 110-megacycle band, from 9320 megacycles to 9430 megacycles. The beacon (ransmitter, however, will respond only on 9310 megacycles and the receiver portion of air-borne X-band radar equipments must be tuned to this frequency in order to receive the beacon response.
NOTE.--Specific mention of operating frequencies is made here to facilitate complete understanding of the operation of the equipment in question. With reference to correspondence or conversation regarding spe-ciflc operating frequencies, readers of this manual are cautioned to observe security regulations as outlined on pages 5 and 6.
4. Coding.--The coded responses of both the AN/CPN-3 and AN/CPN-6 appear on the radar screen of the interrogating aircraft as a vertical series of horizontal dashes, correct in azimuth and range. The number and spacing of the dashes furnish the radar beacon's identity.
5. Range and bearing.--Range is indicated by the placement of the coded response along the time trace as viewed on the radar indicator. In the PPI display, range is measured from the bottom of the lowest dash to the center of the screen. In the B-scan display, range is measured from the bottom of the lowest dash to the bottom of the rectangular screen.
Bearing of the radar beacon station is indicated by the azimuth position of an imaginary line drawn through the center of the coded signals.
III. TACTICAL EMPLOYMENT
The air-borne radar equipments which are served by the AN/CPN-3 (S-band) and AN/CPN-6 (X-band) are as follows:
S-band X-band ASG-AN/APS-2 Series, page 52
AN/APS-2F, page 52.
AN/APS-15, page 41.
AN/APS-15A and -15B, page 49.
AN/APS-3, page 55.
AN/APS-4, page 61.
AN/APS-6A, page 67.
For detailed descriptions of the uses of these equipments for beacon bombing refer to the pages listed above.
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