二战雷达研发总结：MIT辐射实验室《雷达系统工程》V4-LORAN导航技术详解

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《MIT辐射实验室丛书 V4-LORAN》是一套由美国麻省理工学院辐射实验室在二战期间雷达研发经验基础上编撰的系列书籍，共28本，于1947年由麦克劳希尔出版社出版。这套丛书的核心内容是《雷达系统工程》，专注于介绍LORAN系统，这是一种利用无线电导航的重要工具。LORAN（Long Range Navigation）系统旨在为移动车辆提供独立于其他导航手段（如罗盘和航速表）的位置定位以及引导服务。在第一部分"The LORAN System"的第一章，作者Byron W. Sitterly和David Davidson首先概述了LORAN系统的概念。它是一种无线电信号导航系统，它使船只或飞机能在任何时刻确定自己的位置，并能准确地引导到预设的地点或沿着预设路径行进。相较于视觉导航通过识别地标来确定航向的固定位置方法，LORAN采用了不同的原理和技术。在一般情况下，航海员通过观测某些特定量（如地标的方向、卫星信号、无线电波等）来确定自己的位置，这些观测值决定了他们在地球表面的两个位置线的交点。例如，视觉罗盘可以用来测量与已知陆地标之间的相对方位，从而形成一个与经线类似但更为精确的航向线。LORAN系统则在此基础上发展出更复杂的定位技术，它不依赖于单一的地标或观测方式，而是通过接收和解析多个同步发射的信号来实现全球范围内的定位。在讨论LORAN之前，有必要了解其他无线电导航原理，如无线电信标、测距仪、甚高频导航等，因为它们与LORAN一起构成了早期航空和航海导航的重要组成部分。通过对比分析，读者能够更好地理解LORAN的独特之处，包括其可靠性、精度和覆盖范围。《MIT辐射实验室丛书 V4-LORAN》中的《雷达系统工程》深入探讨了LORAN系统的设计原理、工作机制以及与其他导航手段的异同，对于理解二战后无线电导航技术的发展具有重要的历史价值。这套书不仅适合专业技术人员，也对了解和研究早期航空和航海技术变迁的爱好者具有极高的参考价值。

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INTRODUCTION

[SEC. 1.6

measure of his approach to the destination; when th~ distance coincides

with the known separation between mark and destination, he has arrived.

The marks are generally radar beacons. Aa described, the method is

simply a particular use of the H-system of position finding that has been

discussed in Sec. 1.3.

Hyperbolic Tracks.-A constant reading, maintained on the Gee,

Loran, or Decca indicator carried by a vehicle, guides the vehicle along

one member of the family of hyperbol= generated by the pair of trans-

mitters whose signals are compared.

Location along this track is shown

by the changing reading on another pair of transmitters as the aircraft cuts

across its associated hyperbolic family. A hyperbolic track will also be

followed if a radar navigator maintains a constant difference between his

distances from two marks or radar beacons, so that these increase or

decrease at the same rate. The constant difference maybe automatically

added to the nearer reading by introducing a corresponding time delay

either into the response of the radar beacon on the ground or into the

action of the receiver in the aircraft. This difference is then maintained

between the actual distances, and the apparent distances remain equal.

The radar indicator displays this equality clearly, and the navigator

can easily hold to it. The Micro-H bombing system used circular

or hyperbolic tracks at will; along the latter, the bomb was released

when both apparent distances together reached the precomputed

value.

1,6. Homing.-A track defined in one of the ways just described may

be chosen so as to pass through any desired destination within range,

but it will be a unique track, except in the special case that the destina-

tion is the center from which a family of radial tracks proceed (as in

direction finding). In that case only is a navigator in any arbitrary

location able to travel directly to the destination along one line of

position. In every other case he must follow the line of one family that

passes through his location until it intersects the line of another family

that passes through the destination, or else he must cut across both

families.

The obvious and most commonly used method of homing is to steer

so that a radio signal emitted at the destination is always received from

dead ahead, according to the navigator’s direction-finder, or so that the

image of the destination on the PPI screen of his radar set always appears

dead ahead. This is the special case just mentioned. It is to be noted

that if this procedure is followed when there is any cross wind or current,

the aircraft or vessel will not travel in the direction in which it is headed

but in a curve concave toward the direction from which the deflecting

force acts and increasing in curvature as the goal is approached. The

navigator may maintain a straight approach by heading to windward of

SEC.17]

GENERAL COMPARISON OF BASIC TECHNIQUES

the direction of the destination, but this requires accurate knowledge of

the drift.

Homing may beaccomplished inthe general case, cutting across the

two coordinate families, by steering so that the two instrument readings,

which indicate the present position of the craft, approach their destina-

tion values at rates approximately proportional to the amounts by which

they respectively differ from those values. (For example, if the present

Gee coordinates of an aircraft are 6.4 and 38.5 and the destination is at

16.6, 35.1, the craft should be steered so that the first coordinate increases

about three times as fast as the second diminishes. ) In a system like

Gee, where both the destination readings can be set into the indicator

beforehand, an almost direct homing track maybe held by eye estimation

of the motion of the pips on the screen.

Strict proportionality of change

of coordinates could be maintained by the use of rather simple auxiliary

equipment, which would direct the craft along a definite track to the

destination. Such equipment has not been developed as yet, but sug-

gestions regarding its form and use in the Loran system are given in

Chap. 4. The technique might be adapted to any system in which

position is referred to a coordinate grid based upon instrument readings.

1.7. General Comparison of Basic Techniques.—Some comparison

of the basic techniques can be made without entering into details of

instrumentation or operation.

1. The direction-finding and hyperbolic systems, which do not use the

echo or bounce-back principle of radar, have a great advantage in

range or coverage, both because they are not subject to the two-

fold dispersal of a signal traveling over a return path and because

they can operate with long waves, for which attenuation is much

less than it is for short waves and for which range maybe extended

by ionospheric reflection. Radar systems must use ultrahigh or

microwave frequencies in order to get definition in the optical

sense.

2. Systems that measure distance by timing’ of signals give greater

line-of-position accuracy than direct angle-measuring systems.

With radar systems, accuracy of the circular lines of position is

independent of distance or direction. With angle systems, accu-

racy is inversely proportional to distance. With hyperbolic sys-

tems (and the Sonne direction-finding beacon) accuracy depends

on both distance and direction.

At long distances the hyperbolic

systems effectively determine angle by distance-measuring tech-

nique, with a considerable advantage in precision.

Both angle

and hyperbolic systems are affected at long ranges by errors due

to variations in the ionosphere, but these affect the direction of

CHAPTER 2

HISTORY OF LORAN

BY J. H. HALFORD, D. DAVIDSON,AND J.

A. WALDSCHMITT

2.1. Origin of Pulsed Hyperbolic Navigation in the United States.

The UHF Proposal.—The pulsed, hyperbolic, radio grid-laying system for

long-range navigation which finally evolved into the Loran system was

first proposed to the Microwave Committee in October 1940, by its

chairman, Alfred L. Loomis. This proposal involved the use of synchro-

nized pairs of high-power high-frequency pulse-transmitting stations

separated by distances of the order of several hundred miles. The

families of confocal hyperbolic lines of constant time difference generated

by these pairs of transmitting stations could then be interpreted as lines

of position by observers equipped with electronic receiver-indicators

capable of measuring the elapsed time between the arrival of correspond-

ing pulses from the two members of each pair of stations. Ranges from

300 to 500 miles for high-flying aircraft were anticipated.

The Army Signal Corps Technical Committee had set up the following

requirements for a “ Precision ATavigational Equipment for Guiding

Airplanes” at its meeting on Oct. 1, 1940:

a. General: it is desired to have precision navigational equipment for

guiding airplanes to a predetermined point in space over or in an

overcast by radio beams, detection apparatus, or direction-finders.

b. Distance: maximum possible. Five hundred miles desired.

c. Altitude: the ceiling of present heavy bombers; about 35,000 ft.

d. Accuracy: the greatest accuracy obtainable. One thousand feet at

200 miles is desired,

The Loomis navigation system proposal was quickly accepted by the

Microwave Committee and established as Project 3. A coordination

subcommittee whose first recorded minutes are dated Dec. 20, 1940, was

formed to arrange for the procurement, installation, and field testing of

one pair of transmitting stations and suitable navigation equipment as

outlined in the original proposal. This committee was made up of

representatives from several of the large electronic manufacturing

companies.

Although the precise method of synchronizing the transmitters had

not been determined, and no agreement had been reached concerning the

HISTORY OF LORAN

[SEC.?.1

most practical method of constructing a suitable navigator’s receiver-

indicator, the committee initiated the following orders for about $400,000

worth of equipment during December 1940:

a. 2 receiver-indicators—RCA.

b. 2 receiver-indicators (independent design) —Sperry.

c. 2 crystal-controlled timers—Bell Laboratories.

d. 1 1.5-megawatt transmitter—Westinghouse.

e. 1 1.5-megawatt transmitter—General Electric.

f. 6 high-frequency pulse triode transmitting tubes—RCA.

When the orders for equipment were placed, it was estimated that most

of the items would be available for test during the summer of 1941.

In addition to much discussion concerning the most practical methods

of synchronizing the stations and providing the navigator with time-dif-

ference readings, there was considerable indecision concerning the best

places to locate the experimental stations. Several mountain peaks were

considered before it was finally decided to request permission to use two

abandoned Coast Guard lifeboat stations located at Montauk Point,

Long Island, and Fenwick Island, Del. This provided a 209-nautical

mile baseline entirely over water and at the same time kept the stations

within a reasonable distance of the Bell Telephone Laboratories in New

York, from which Project 3 was coordinated.

Meetings of the Project 3 Committee and several subcommittees set

up to coordinate transmitter, transmitting tube, and receiver-indicator

development were held regularly throughout the winter and spring of

1941. The minutes of these various meetings contain numerous discus-

sions of possible methods of synchronization of the stations and of measur-

ing the time difference between the received pulses in the field.

Several distinguished British visitors attended many of these meetings.

E. G. Bowen of the British Mission was present at the Project 3 Commit-

tee meeting on Dec. 20, 1940. I.ater, Squadron Leader G. Hignett of the

British Embassy also participated. Although these British scientists

were aware that a somewhat similar navigational system of hyperbolic

grid-laying was under development in England, they were not completely

familiar with it and were, therefore, able to give only general advice to

the Project 3 Committee.

That the project, new in concept and untried experimentally, was

foredoomed to failure under control of a loose administrative committee

became apparent. This weakness was recognized particularly by Mel-

ville Eastham, who advised turning the central authority over to a full-

time group at Massachusetts Institute of Technology that could make

trials, suggest changes, and guide the whole development as experiment

indicated.

Experiments at Lower Frequencies.—As soon as a few personnel had

been hired, Eastham suggested to the Project 3 Committee that some of

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