upon which the radio beacons are shown, with their characteristics and operating schedules. These charts may be obtained free from the offices of the District Lighthouse Superintendents.

Radiobeacons are the most valuable fog signals and are also available for navigation in clear weather. All radiobeacons operate during fog or low visibility; also in clear weather during scheduled intervals. The clear weather schedules are given in the Light Lists, Hydrographic Office Publication No. 205, and other Radio References.

The radiobeacons in or near the area covered by this volume are given in the following table:

For details as to frequencies, geographic positions, signals, and thick and clear weather transmission, see Light List-Atlantic Coast, United States, of the Lighthouse Service or Hydrographic Office Publication No 205. Changes are announced in the Notices to Mariners.

These radiobeacons are intended for the use of vessels equipped with radio compasses; radiobeacons are now recognized to be allweather aids to navigation as well as fog signals. While the use of radiobeacons should not lead a mariner to neglect other precautions such as the use of the lead, etc., during thick weather, the bearings will greatly reduce the dangers incident to navigation in the fog.

The bearing of the radiobeacon may be determined with an accuracy of approximately 2° and at distances considerably in excess of the range of visibility of the most powerful coast lights. The apparatus is simple and may be operated by the navigator without the assistance of a radio operator or with the knowledge of the telegraph code. The radio direction-finding apparatus consists of a radio receiving set, similar in operation to those used for radiotelegraph or radiotelephone reception, and a rotatable coil of wire in place of the usual antenna. By rotating the coil the intensity of the signal received from the transmitting station is made to vary, and by noting the position of the coil when the signal is heard at its minimum intensity, the bearing of the transmitting station is readily obtained.

It is important to note that the bearing of an incoming radio wave is subject to errors not unlike the deviation of the magnetic compass. Those using radio direction-finding apparatus aboard ship are cautioned to bear these errors in mind and to keep the radio

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All

compass calibrated at all times. This may be done during clear weather by comparing the bearing obtained with the radio compass with the bearing as obtained by visual methods in general use. radio bearings are subject to what is called "night effect", a variable error sometimes experienced near nightfall and sunrise. The uncertainty due to this cause may be lessened by taking repeated radio bearings.

The signals from the radio beacons have definite characteristics for identifying the station, and bearings may be obtained with even greater facility than sight bearings on visible objects. At some of the stations the radio and sound signals are synchronized for distancefinding purposes as explained under the following heading.

Distance Finding.-Radiobeacon stations that are equipped for distance finding transmit a special radio signal and sound a corresponding air or submarine signal at the same period of time. Thus, whenever the sound signal of a radiobeacon distance-finding station is operating, there is transmitted at the end of the radiobeacon minute of operation (1 minute out of every 3) a group of two radio dashes (a short dash and a long dash), 1 second and 5 seconds, respectively; and a group of two sound signals of corresponding length is sounded at the same time. These signals are uniformly the same for all the distance-finding stations, except that the long fog signal blast and corresponding long radio dash may be made as short as 3 seconds where desired. They are entirely distinctive and should not be confused with the characteristic signals which identify the stations.

The reception of the sound signal by a vessel will be delayed, depending upon the distance the sound has to travel; whereas the radio signal will be received practically instantaneously. This delay is measured by observing the time that elapses between hearing a part of the group of radio dashes and the corresponding part of the group of sound blasts, say the end of the long radio dash and the end of the long sound blast. This elapsed time in seconds (time interval of sound travel in air), divided by 5 (or more exactly 5.5), gives the approximate distance in nautical miles the observer is off the radiobeacon. The error of such observations should not exceed 10 percent.

The 1-second dash preceding the long dash is a stand-by signal, as is also the 1-second blast. The latter serves as an identification signal to assure the observer that he is taking time on the correct sound signal blast.

Where the submarine oscillator is synchronized with the radiobeacon, the method of distance finding is the same excepting that the interval of time in seconds (sound travel in water) is divided by 1.25 to obtain the distance in nautical miles.

Along the Gulf Coast the radiobeacons have been synchronized with sound signals for distance finding at the following stations:

South Pass, West Jetty.

Southwest Pass, East Jetty.

Heald Bank Lightship.

For further details, see Light Lists, published by the United States Lighthouse Service and other radio references.

Radio bearings from other vessels.-Any vessel equipped with a radio direction finder can give a bearing to a vessel equipped with a radio

transmitter. Such service will generally be furnished when requested, particularly by Government vessels. These bearings, however, should be used only as a check, as comparatively large errors may be introduced by local conditions surrounding the radio direction finder unless known and accounted for. Any radio station, the position of the transmitter of which is definitely known, may serve as a radio beacon for vessels equipped with a radio direction finder. However, mariners are cautioned that stations established especially for maritime service are more reliable and safer for use by the mariner, for numerous reasons.

Many navigators are using the ship's radio direction finder as a help in avoiding collision in fog, detecting with it the presence and observing the direction of approaching vessels.

Caution in approaching lightships.-The attention of mariners is directed to the serious dangers which may arise from the misuse of radio signals, and particularly to the danger of collision with lightships operating such signals. The mariner, who in thick weather approaches a radiobeacon directly ahead on a radio bearing and relies on hearing the fog signal in sufficient time to alter course to avoid danger, is taking an unjustifiable risk. The vagaries of sound fog signals are well known, and warnings regarding them are widely printed; such signals may not be heard with certainty in time to avoid collision. Safety demands that all precautions be taken in approaching lightships. The risk of collision can be avoided by getting the radiobeacon on the bow, taking successive bearings, and ensuring that the radio bearing does not remain constant; the angle between the course and the radiobeacon should increase for a vessel passing clear.

Nearly half of the radiobeacons on the coasts of the United States are placed on lightships, and these radio signals provide valuable marks for approaching the coasts and ports. In using radio bearings for approach, it is important that the courses at all times be set to pass safely clear. Radiobeacons offer to navigators a convenient means of checking this by methods well known in ordinary navigation, but sometimes overlooked in navigation by radio bearings. The navigator may check his position with respect to the lightship radiobeacon when steering to pass to one side, by taking successive radio bearings and using these with the intervening distances run, to ascertain the distances off from time to time. Radio cross bearings should also be taken, and the effect of currents should not be overlooked. Where warning radiobeacons and direct distance finding facilities are provided, they should be utilized. Soundings should not be neglected, as a few soundings in connection with a radio bearing or a chain of soundings alone usually give an excellent position. See Soundings and Echo Soundings, page 349.

The caution as to passing lightships at a safe distance, and approaching them using all care, of course applies to all lightships, whether or not they are provided with radiobeacons, and it applies also to radiobeacons at lighthouses which may be approached close-to. In preparing these notes, advantage has been taken of many reports from practical mariners, who systematically make use of radio direction finders or radio-compasses in navigation, and it is appreciated that great interest is already taken in the problems of radiobeacon navigation. Further suggestions and reports on these subjects are

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invited, and may be sent to the Commissioner of Lighthouses, Washington, D. C.

The Nantucket Lightship is equipped with a warning radiobeacon. (See Light List, Atlantic Coast.)

RADIO DIRECTION-FINDER STATIONS

United States naval direction-finder stations will furnish radio bearings to mariners of all vessels equipped with radiotelegraphic transmitters. While the use of these bearings should not lead a mariner to neglect other precautions, such as use of the lead, etc., during thick weather, these bearings will greatly reduce the dangers incident to navigation in the fog.

Accuracy of bearings.-The accuracy with which bearings can be taken depends on various conditions and, while bearings taken by a station can generally be considered accurate to within 2°, the Government cannot accept any responsibility for the consequences of a bearing being inaccurate.

In the case of bearings which cut the coast line at an oblique angle, errors of from 4° to 5° have been reported. Bearings obtained between about one-half hour before sunset and one-half hour after sunrise are occasionally unrelibale. It is probable that the accuracy of a bearing is also affected if the ship's transmitting instrument is not adjusted to the correct wave length. Bearings signaled as "approximate" or "second class" should be regarded with suspicion as being subject to considerable error. The maximum distance for which bearings from these stations are accurate is 150 miles.

Direction-finder stations used as radio beacons.-A naval directionfinder station when used as a radio beacon transmits upon request, its call sign for 45 seconds on 375 kilocycles (800 m).

For further information regarding radio bearings and radiodirection-finding stations see the Light Lists of the United States Lighthouse Service or Hydrographic Office Publication No. 205, Radio Aids to Navigation.

The following list of naval radio direction-finder stations will be of use to vessels navigating the approximate limits of this volume; these stations are noted on the charts by the symbol "N. R. C."

CONVERSION OF RADIO BEARINGS TO MERCATOR BEARINGS

The increasing use of radio directional bearings for locations of ships' positions at sea, especially during foggy weather, has made it particularly desirable to be able to apply these radio bearings taken on shipboard or sent out by the shore stations directly to the nautical chart. These radio bearings are the bearings of the great circles passing through the radio stations and the ship, and unless in the plane of the Equator or of a meridian would be represented on a

Mercator chart as curved lines. Obviously it is impracticable for a navigator to plot such lines on his chart, so it is necessary to apply a correction to a radio bearing to convert it into a Mercator bearing; that is, the bearing of a straight line on a Mercator chart laid off from the sending station and passing through the receiving station. On page 33 is given a table of corrections for the conversion of a radio bearing into a Mercator bearing. It is sufficiently accurate for practical purposes for distances up to 1,000 miles.

The only data required are the latitudes and longitudes of the radiobeacons or direction-finder stations and of the ship by dead reckoning. The latter is scaled from the chart, and the former either scaled from the chart or taken from the list of radiobeacon and radio direction-finder stations found in the Light Lists published by the United States Lighthouse Service or Hydrographic Office Publication No. 205.

The table is entered with the difference of longitude in degrees between the ship and station (the nearest tabulated value being used), and opposite the middle latitude between the ship and station, the correction to be applied is read.

When bearings are taken from the ship, the sign of the correction (bearings read clockwise from the north) will be as follows: In north latitude, the minus sign is used when the ship is east of the radiobeacon and the plus sign is used when the ship is west of the radiobeacon. In south latitude, the plus sign is used when the ship is east of the radiobeacon, and the minus sign is used when the ship is west of the radiobeacon.

To facilitate plotting, 180° should be added to the corrected bearing, and the result plotted from the radiobeacon.

Should the position by dead reckoning differ greatly from the true position of the ship as determined by plotting the corrected radio bearings, a retrial should be made, using the new value as the position of the ship.

If the bearing is from a radio direction-finder station ashore, the sign of the correction will be reversed to that given when the bearing is taken from the ship.

Example. A ship in latitude 39°50′ N., longitude 67°35′ W., by dead reckoning, obtains a radio bearing of 299° true on the radiobeacon of the Nantucket Shoals Lightship, located in latitude 40°37′ N., and longitude 69°37′ W.

Entering the table with difference of longitude equals 2°, which is the nearest tabulated value, and opposite 40° middle latitude, the correction of 39′ is read.

The ship being east of the radiobeacon, the correction is minus. The Mercator bearing will then be 299°-0°39′-298°21'. To facilitate plotting, add 180° and plot from Nantucket Shoals Lightship,