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CHAPTER X
Radar Beacons--RACON

A. Introduction--Uses of beacons.

B. Method of operation: Similarity to IFF; types of coding; distinction between radar and radio beacons.

C. Types of radar beacons.

D. Capabilities and limitations.

A. INTRODUCTION

Radar beacons, or Racons as they have been named, are used to return a distinctive signal to a radar in much the same way that an IFF transpondor replies to an interrogator. Their principal use is to provide a strong signal so that airplanes can home by radar on a carrier or land base from a long distance. Consequently, beacons are classified as navigational aids. Other beacons may be used to provide a strong signal on a featureless shore as a means of facilitating radar navigation of ships, or to provide a reference point for shore bombardment. Radar beacons are installed in many shore establishments, and are on the allowance list for all CV, CVB, CVL, and CVE's. The radio call letters and the codes by which each racon identifies itself are published in a series of six books entitled "Racon List" published by the Hydrographic Office, U. S. Navy, with the approval of the Combined Communications Board. The short titles of these books are CCBP 02 to CCBP 07, inclusive.

B. METHOD OF OPERATION

A radar beacon is a fixed-frequency transpondor that is interrogated by the pulses transmitted by a radar. Therefore, a separate racon transpondor must be produced for every frequency band on which beacon responses are desired. Since many radar frequency bands are in use, there are many different types of beacons in use.

The radar beacon transpondor normally is quiescent, in a receiving condition. When a pulse of the proper frequency and duration is received, the equipment automatically becomes a transmitter to return a coded response that identifies the beacon as well as indicating the range from the challenging radar.

The receiver tuning is made rather broad in order that a single racon can respond to challenges from any radar within a given band, because a beacon transpondor does not sweep in frequency as the Mark III IFF transpondor does on the A band. Since the beacon replies to each challenging pulse, the responses appear as continuous pips on the radar indicator.

The radar beacon transmitter frequency is slightly different from the frequency to which the receiver is tuned. For example, if one type of racon is actuated by a pulse at 176 megacycles, it will respond on 177.5 megacycles. Thus, the receiver of the radar that challenges a radar beacon must be detuned from the radar frequency in order to accept the beacon response. As a result, a radar can be used either to challenge a racon or to search for targets, but never for both operations simultaneously. The reason for designing the beacons for this type of operation is to prevent interference between radar echoes and racon responses. Therefore, signals returned from racons are clearly visible on the indicator because the clutter of sea return and echoes from land is eliminated. Another beneficial feature of this type of operation is that racon responses may be viewed on a clear scope in one installation, while an adjacent radar in the same frequency band may search normally without interference from the radar beacons.

Most airborne radars have been designed for use with beacons, so that such operation is made easy and convenient. The microwave beacons that are designed to be challenged by airborne radars will not respond to the normal search pulses transmitted by the radar. Therefore, when it is desired to interrogate a radar beacon, the "Beacon" switch is thrown, causing the radar transmitted pulse duration to be increased, while at the same time the radar receiver is automatically tuned to the racon response frequency. Shipboard radars, on the other hand, have seldom been used with racons, and tuning the radar receiver to the racon response is a manual process not facilitated by any special provisions for beacon operation.

There are two different means of automatic coding of the racon responses in use at present. On the P and L band beacons the responses are coded by interrupting the responses at intervals to form the Morse code letters that identify the beacon. Normally the beacon signal appears steady on the indicator, but every 30 seconds the response pip winks off and on like a blinker light. This type of code is called "gap coding" because the identification is

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Figure 159--Range-coded beacon response
Figure 159--Range-coded beacon response.

accomplished by inserting gaps in the normally continuous response.

Microwave beacons, however, use range coding as a means of identification. In this system several pulses separated by spaces of variable width are returned for each interrogating signal, causing the response to extend out in range from the actual range of the racon. The number of pulses and the relative spacing between each pulse form the code. Between two and six pulses and either narrow or wide spacing are used. The code usually is specified by giving the number of pips in each closely-spaced group. For example, a particular beacon might transmit the following: pip-long space, pip-short space-pip, long space-pip. The code would then be specified as 1-2-1.

The range to a beacon is given by the range to the first code pip. The bearing is given by the center line of the pips. Figure 159 shows the appearance of a PPI and a B scope with a range-coded beacon response showing. This type of reply is advantageous because the beacon identifies itself on every response, which eliminates any delay in recognizing its geographic position.

Radio homing beacons, or sector-coded beacons, are designed for use on aircraft carriers or at airfields to provide an omnidirectional homing or navigational signal for the guidance of aircraft. The transmitter is keyed in twenty-four 15° sectors with 000° centered on true north. The directional antenna, rotating at a speed of two revolutions per minute, radiates a differently coded modulated CW signal in each sector for nine revolutions, and during the tenth revolution it radiates an identification signal. The

homing beacons, which are designated YE and YG, are very different from radar beacons in that instead of being actuated by an interrogating signal from the plane, these beacons transmit continuously without the plane's having to do more than receive the signals sent out. Since the YE and the YG are radio beacons which operate on principles entirely different from those on which radar beacons operate, these equipments are not racons.

C. TYPES OF RADAR BEACONS

Since racon transpondors operate on fixed frequencies, there are many racons in existence. Those that are likely to be found on board ship are given in the table on opposite page.

The YJ serves two frequency bands since it can respond alternately to a frequency within the A band of the Mark III system and to the frequency on which the ASB radars operate. The YJ is widely installed, and the identifying gap-coded response from each shore station is indicated in the Racon List. The AN/CPN-3, 6, 8 and 17 will not respond to interrogation from shipboard radars because the frequencies assigned to these radars within the S and X bands are different from those assigned to the airborne radars.

The Mark 2 is simply an improvement on the Mark 1 to reduce weight and to permit response either to the Mark 3 or 4 or to the Mark 12 fire control radars. The Mark 4 beacon is to be produced in three models having different power supplies. The Mark 5 is a modification of a racon designated AN/UPN-1 or AN/UPN-2 which may be available during the first part of 1945.

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Designation Frequency Band For Use With Installation
YJ-1 P and L ASE, SCR 521, SCR 729, AN/APX-2. All ASB type radar. Ship, shore.
YJ-2
AN/CPN-3, AN/CPN-8 S All S-band airborne radar. Shore.
AN/CPN-6 X All X-band airborne radar. Ship, shore.
AN/CPN-17 S All S-band airborne radar. Ship, shore.
Mark 1, Mod 1 L Mark 3/4 fire control radar. Portable,100 pounds.
Mark 2, Mod 0 L Mark 3/4 fire control radar. Portable,40 pounds.
Mark 2, Mod 1 L Mark 12 fire control radar. Portable.40 pounds,
Mark 4, Mods 0, 1, 2 S Mark 8, Mark 28 fire controlradar. Shore.
Mark 5, Mods 0, I S Mark 8, Mark 28 fire controlradar. Portable.

D. CAPABILITIES AND LIMITATIONS

The range at which a racon response can be received is dependent on the height of the challenging craft, the power radiated by the racon, and the height of the racon antenna. For low position angles the range is greatly affected by propagation conditions. Usually, if a plane cannot get a response from a racon it should climb to a higher altitude. However, when the weather is such as to cause excessive bending of the radiated rays, the plane may be able to get a response by flying at a low altitude. If the plane flies within the duct formed by non-standard propagation conditions, exceptionally long ranges may be observed on racon stations.

The curves shown in figure 160 are very approximate indications of the reliable ranges that can be obtained on the YJ, AN/CPN-3, and AN/CPN-6 racons under standard conditions as the height of the interrogating plane is varied. The curves show clearly that the S-band energy of the AN/CPN-3 stays much closer to the surface of the earth than does the L-band energy of the YJ.

The Mark 1 Mod 1 racon, being portable, is a low-power equipment. When it is set up on land, the racon response can be detected at a maximum range of approximately 25,000 yards. If the same set is installed in a spotting plane, the response can be detected out to approximately 50,000 yards. The ranges on the Mark 2 beacon will probably be of the same order.

In all racon transpondors there is a small constant error in the indicated range because of the delay that the signal suffers in actuating the transpondor. This delay is inherent in the set and it cannot be eliminated entirely because the response of the transpondor cannot be made absolutely instantaneous. In navigational racons this delay causes a negligible error. However, in fire control accurate data are

desired, so that the error caused by the transpondor delay must be considered in beacons used for providing a reference point for shore bombardment. The delay in the Mark 1 beacon is approximately one microsecond, which causes all the indicated ranges to be approximately 150 yards longer than the true range. Therefore, in using beacons in connection with shore bombardment, the inherent error must be determined, and the error subtracted from the ranges indicated on the radar.

Since a radar that is used to challenge a beacon cannot be used simultaneously as a detection device, it is not possible to spot the fall of shot with a radar while observing beacon responses on it. The Mark 4 and Mark 12 radars have rather poor bearing resolution, which makes them of limited use in ranging on land. With either the Mark 1 or Mark 2 racons available, the utility of these radars would be considerably increased in shore bombardment actions. Any radar spotting that is to be done can be performed by some radar other than the one observing the racon responses.

After sufficient practice, it is claimed that the technician can retune a Mark 4 fire control radar for normal radar operation in about 10 seconds. Thus, using this radar for interrogating a beacon does not limit to any great extent the use of the equipment as a radar. If retuning from beacon to radar is to be accomplished within this short time, however, the receiver sensitivity for the beacon response will be far less than for a radar echo. This difference is due to the use of radio-frequency amplifiers in the Mark 4 receiver. These amplifiers are either left tuned to the radar frequency or by-passed, reducing the radar receiver sensitivity for beacon responses in either case. If it were decided not to attempt to change back to normal radar operation

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Figure 160--Approximate maximum reliable range on radar beacons
Figure 160--Approximate maximum reliable range on radar beacons.

very quickly, the beacon responses could be detected well beyond 25,000 yards. With radars that are provided with an Automatic Frequency Control Circuit (AFC) the receiver local oscillator can be manually tuned to the beacon response, and the radar retuned for normal operation almost instantly by turning on the AFC. This type of operation could be very helpful in extending the use of the radar.

Since portable beacons must be supplied power from batteries, the useful life of the racon is limited by the life of the batteries. The Mark 1 Mod 1 racon weighs 100 pounds complete, of which 60

pounds are in batteries. With these batteries the racon can operate continuously for 6 hours. If half the weight of batteries is used, the set can operate continuously for only two and a half hours.

The shore fire control party is considerably handicapped by having to carry equipment as heavy as the Mark 1 Mod 1 racon. The Mark 2 Mod 0 racon, which weighs only 40 pounds, was therefore developed to reduce the load. With the lighter equipment the shore fire control party will be enabled to get in position more quickly and to operate successfully over rougher terrain than would be possible with the heavier equipment.

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