PART ONE
INTRODUCTION

			Page
I.	Nomenclature		3
II.	Frequency Bands and Frequency Designations		4
III.	Categories		4
	A.	Radar Systems	4
	B.	Radar Beacons (Racons)	4
	C.	Identification Systems	4
	D.	Navigation Systems	5
	E.	Altimeters	5
IV.	Security		5

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[BLANK PAGE]

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Part One
Introduction

I. Nomenclature

An "AN Nomenclature System," used by both Army and Navy, has been developed to establish one standard plan of nomenclature for communication, noncommunication, and associated air-borne electronic equipments.

Under the "AN Nomenclature System" a typical model number might be as follows:

AN/APS-8B

The letters "AN" before the slant bar show that this is the new Army-Navy system of nomenclature. The first letter after the slant bar shows the type of installation; i.e., "A" for air-borne. The second letter after the slant bar shows the type of equipment; i. e., "P" for radar. The third letter after the slant bar shows the purpose for which the equipment is used; i. e., "S" indicates search, detection or interception equipment designated under the "AN" system of nomenclature. The number "8" indicates the model number; and "B" indicates the modification.

The following table lists the various designating letters used in this system, and their meaning.

LETTER SYMBOLS USED IN MAKING BASIC INDICATORS*

Installation	Type equipment	Purpose
A--air-borne (installed and operated in aircraft).	B--pigeon.	A--assemblies.
C--air transportable (designed to be air transportable as stated in specification of military characteristics).	C--carrier (wire).	C--communications (receiving and transmitting).
F--ground, fixed.	F--photographic.	D--direction finder.
G--ground, general ground use (includes two or more ground installations).	G--telegraph or teletype (wire).	G--gun directing.
M--ground, mobile (installed as operating unit in a vehicle which has no function other than transporting the equipment).	I--interphone and public address.	L--searchlight control.
P--ground pack or portable (horse or man).	M--meteorological.	M--maintenance and test assemblies (including tools).
S--shipboard.	N--sound.	N--navigational aids (including altimeters, beacons, compass, and instrument landing).
V--ground, vehicular (installed in vehicle designed for functions other than carrying radio equipment, etc., such as tanks).	P--radar.	Q--special.
T--ground, transportable.	R--radio.	R--receiving.
U--general utility, includes two or more general installation classes, air-borne, shipboard and ground.	S--special types (heat, magnetic, etc.).	S--search and/or detecting.
	T--telephone (wire).	T--transmitting.
	V--visual and light.	W--remote control.
	X--facsimile or television.	X--identification and recognition.

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*An "X," "Y," or "Z," used after the main designation indicates a change in voltage, phase or frequency of the input power supply (AN/APX-8X). Suffix letters "A," "B," "C," etc., indicate a change in design (other than the voltage, phase, or frequency of the input) which will not affect the equipment's military characteristics or inter-changeability of the equipment as a whole.

II. FREQUENCY BANDS AND FREQUENCY DESIGNATIONS

All conversational or written reference pertaining to the operating frequencies of airborne electronic equipments should avoid mention of specific bands or frequencies unless it is absolutely necessary. Particularly, the practice of describing or classifying an equipment or system by naming its frequency or wavelength is dangerous. To avoid the necessity for this, the following code should be used wherever it will serve in lieu of naming specific frequencies or wavelengths. This list is classified RESTRICTED

Megacycles		Code
10,900 to 33,000		K band
5,200 to 10,900		X band
1,650 to 5,200		S band
390 to 1,650		L band
225 to 390		P band

For a list of equipments and the frequency bands in which they operate see the table in the appendix, page 155.

III. CATEGORIES

Air-borne electronic systems and their associated equipments may be classified, for the purposes of this manual, into five broad categories:

A. Radar systems.
B. Radar beacons (racons).
C. Recognition systems.
D. Navigation systems.
E. Altimeters.

A. RADAR SYSTEMS. Air-borne radar systems are employed mainly for the purpose of detecting, at long range, through overcast, or under conditions of poor visibility, the presence of targets, and measuring their range and bearing.

In addition to its main function, a radar system may have one or more of the following specific uses:

1. Search.
2. Navigation.
3. Homing.
4. Bombing.
5. Interception.
6. Gun aiming.

Air-borne radars employed in combination with certain other electronic systems can also be used for:

7. Target identification.
8. Beacon homing.
9. Ordnance control:
   1. Low- or high-altitude bombing.
   2. Torpedo dropping.
   3. Mine laying.
   4. Gun sighting.

PART TWO of this manual describes:

1. The theory and principles of operation of the radar system.

2. The physical and operating characteristics of air-borne radar equipments, and their tactical employment.

B. RADAR BEACONS (RACONS). Radar beacon stations serve either meterwave or microwave interrogating equipments. Radar beacons comprise a means for receiving the pulses from radars or special interrogating equipment and using the received signal to trigger a relatively powerful beacon transmitter into operation.

The transmissions of a beacon station are gap-or range-coded to furnish its identity to the interrogating craft. On board the interrogating craft, the bearing of the beacon relative to the craft is indicated by the position of the antenna or of the signal display on an indicator. The range in nautical miles is read directly from the calibrated indicator screen.

PART THREE of this manual describes:

1. The theory and principles of operation of radar beacons.

2. The physical and operating characteristics of radar beacons, and their tactical employment.

C. RECOGNITION SYSTEMS. Radar indicates the presence, range, and bearing of targets. Until such targets are positively identified as friendly, they are assumed to be "bogeys" (unidentified aircraft targets), or "skunks" (unidentified surface targets).

Air-borne electronic IFF (Identification Friend or Foe) equipment, when interrogated by shipboard, land-based, or other air-borne radars, operates to identify as friendly the aircraft in which it is carried. Some Mark III IFF air-borne equipment currently in use employs only a transpondor. Upon receipt of an interrogating signal, it responds with a coded reply to furnish identity.

Other Mark III IFF air-borne equipment, in addition to a transpondor, contains an interrogator-responsor unit. This unit is employed

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to interrogate targets which appear on the radar scope, and to receive their replies which are then displayed on the scope.

PART FOUR of this manual describes:

1. The theory and principles of operation of IFF equipments.

2. The physical and operating characteristics of IFF equipments and their tactical employment.

D. NAVIGATION SYSTEMS. Through the use of a system called "loran" (LO-ng RA-nge N-avi-gat ion), lines of position and "fixes" can be obtained with an estimated accuracy of approximately 1 percent of the distance separating the craft from the special loran transmitting stations.

A loran system comprises three basic elements:

1. Loran transmitters (arranged in pairs).
2. Loran receiver-indicator.
3. Loran charts (or tables).

The loran transmitters, situated on islands or along coastal promontories, transmit short pulses at a low repetition rate. At a point out at sea, the time-difference in the reception of pulses from a pair of transmitters is displayed and measured on the screen of a cathode-ray tube (scope). On specially prepared loran charts, a line of position may be found which has the same time-difference designation as the t ime-difference reading obtained from the scope display. Obtaining a second time-difference reading from a second pair of transmitters provides a second line of position. A fix is indicated where one of the selected lines of position intersects the other.

Effective ranges of the loran system are:

		Nautical miles
Daytime		700
Nightime		1,400

The advantageous features of loran are:

1. Its use is independent of and not affected by adverse weather conditions.

2. No transmission from the aircraft or ship is required. It does not reveal an aircraft's position by the necessity of breaking radio silence.

3. Its effective range is greater and it provides a higher degree of accuracy than RDF systems.

4. It is a rapid yet highly accurate method of obtaining a fix. Three lines of position may be obtained in approximately 5 minutes.

5. It is limited to the use of friendly craft by virtue of the fact that only aircraft appropriately equipped can receive the pulsed signals and measure the time difference.

PART FIVE of this manual describes:

1. The theory and principles of operation of loran.
2. The physical and operating characteristics of airborne loran and its tactical employment.

E. ALTIMETERS. Electronic altimeters do not operate on atmospheric pressure principles. They indicate the altitude of the aircraft above the terrain directly below, regardless of the height above sea level of that terrain.

Electronic altimeters work on the following principles:

1. A frequency-modulated signal is generated, varying in frequency over a fixed frequency band.

2. A small portion of the generated signal is fed to the receiver, the major portion going to the antenna where it is directed toward the terrain.

3. The receiver, which accepts the returned transmitted signal, compares that frequency to the new frequency being generated at the time of reception and converts this frequency difference into current output.

4. The resultant current output actuates a meter, calibrated in feet, to indicate the absolute altitude. Altitude limit lights also give a general indication of altitude within prescribed limits.

The efficient employment of electronic altimeters depends upon the accuracy with which they are initially calibrated and the effectiveness with which they are maintained.

PART SIX of this manual describes:

1. The theory and principles of electronic altimeters.
2. The physical and operating characteristics of electronic altimeters and their tactical employment.

IV. SECURITY

Present security instructions concerning radar and radio equipments are quoted herewith:

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(The letter which follows outlines current security doctrine. Omitted from RADTWOA are some thirty-five typewritten pages, constituting ENCLOSURE A, referred to in the following security classification letter, and containing specific security classification for each piece of naval electronic and communication gear now in use.)

NAVY DEPARTMENT
OFFICE OF THE CHIEF OF NAVAL OPERATIONS
WASHINGTON 25, D.C.

Op-413-B23/cvk
Serial 203P4I3	20 November 1945.
RESTRICTED
From:	Chief of Naval Operations.
To:	Chief of the Bureau of Ships. Chief of the Bureau of Ordnance. Chief of the Bureau of Aeronautics.
Subject:	Electronics Equipment--Security Classification of.
References:	(a) CNO restr ltr Op-25-A/ab, Serial 31825A dated 24 May 1945. (b) CNO restr ltr Op-25-3/jeh, Serial 34725 dated 18 July 1945. (c)Art. 76, U. S. Navy Regulations, 1920.
Enclosure:	(A) Security Classifications of Eleectronics Equipments.

1. Reference (a) and (b) are hereby canceled. Enclosure (A) hereto, with respect to the equipments listed therein, supersedes enclosure (A) to reference (b).

2. The classifications assigned to classified equipments are based upon the need for maintaining military security. Equipment is classified when disclosure of any one or combination of the following may benefit a potential enemy to such an extent as to prejudice national interest:

1. the use of certain newly developed or otherwise publicly undisclosed component;
2. the use of certain newly developed or otherwise publicly undisclosed circuit;
3. the use of certain newly developed or otherwise publicly undisclosed technique;
4. the assembly of various units which- individually are unclassified but which collectively satisfy the operational characteristics for a military application;
5. the use of an unique manufacturing process; or
6. the fact that the equipment is used by the Navy.

In promulgating these rules, Chief of Naval Operations appreciates that decisions involving technical details and their relations to military security must be determined by the Bureaus as prescribed in subparagraphs (7) (b) and (c) of reference (c). Accordingly, the addresses are requested to exercise care to avoid classifying individual components, circuits, or units of a complete equipment which is classified when those components, circuits or units do not satisfy at least one of the conditions stated above.

3. Attention is directed to items indicated-by hatch marks (#). These items may contain some British influence; therefore, no technical disclosures thereon shall be nlade to the public until notified by subsequent correspondence that this restriction has been removed.

4. Reference (c) contains the instructions for safeguarding classified material. It should be noted that RESTRICTED documents, information or material may not be given to the public or to the press.

5. Recommended changes in or additions to enclosure (A) should be submitted to CNO promptly.

6. It is requested that the Bureau of Ships include in SHIPS 242A and the Bureau of Aeronautics include in CO-NAVAER 08-5Q-227, the security classifications of the equipments listed in enclosure (A), in order that the information may reach all activities which require that information.

(s) W. S. Farber,
Deputy Chief of Naval Operations (Logistics).

Authenticated:

W. A. Hammond,
Captain, USN.

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