Seaward G3278-1000 User Manual download pdf (Page 28)

XXII

This whistle signal is to be repeated until it is acknowl-

edged by the station through the starting of the transmitter.

The same group of signals should be sounded at the termi-

nation of calibration.

The work of the station personnel is not confined to

standing watch and there may be times when the whistle

request for calibration is not immediately heard, due to the

noise from operating station machinery, etc. Usually, a

repeated signal not too far from the station will attract

attention.

“COMPENSATED” RADIO DIRECTION-FINDERS:

Many radio direction-finders are “compensated” and no

calibration chart or curve is used. Attention is invited to

the fact that such compensation is just as vulnerable as the

calibration data due to changes made in the set or its

surroundings.

CHECK THE CALIBRATION: The calibration of

compensation should be checked frequently by taking

bearings when the ship’s position is accurately known and

the results should be recorded for future reference.

CALIBRATION RADIOBEACONS: In the United

States and certain other areas special radiobeacons,

primarily for calibrating shipboard direction-finders are in

operation. These radiobeacons transmit either

continuously during scheduled hours or upon request, as

indicated in station details.

COASTAL REFRACTION (OR LAND EFFECT):

Errors may occur in bearings taken by ships so located that

the line of observation to the radiobeacon passes over land

or along the shore line. However, many observations seem

to indicate that such errors are negligible when the

observing vessel is well out from the shore. Bearings

secured entirely over water areas are to be preferred since

“land effect” is thus eliminated. Bearings taken at sunset

and sunrise are likely to be erratic, and observations taken

at these hours should therefore be repeated and checked as

may be feasible.

PROGRAM BROADCASTING STATIONS: Before

taking bearings on a station broadcasting entertainment

programs a mariner should consider that frequency may

differ widely from the frequency for which the set is

calibrated, that the published location of the station may

be that of its studio and not that of its transmitting antenna,

that if the station is synchronized with other stations it

may be impossible to tell on which station the bearing was

taken, and that as the majority of these stations are inland,

the coastal refraction may be excessive.

Station Details

FREQUENCY: The frequency listed is that used by the

station in transmitting its “Characteristic Signal.” Calling

frequencies, if any, will be given under “remarks.”

RANGE: In this book the range of radiobeacons is only

approximate and is given merely to assist mariners in

planning their voyages and to inform them of several

radiobeacons they will probably hear first. Frequently,

when conditions for radio reception are good,

radiobeacons may be heard at greater distances than

indicated. The mariner who is at a greater distance than the

range indicated should attempt to obtain bearings when

necessary, and not assume that the radiobeacon will be

unheard beyond its indicated range.

GROUP SEQUENCE: Selected radiobeacons are

grouped together on the same operating frequency and are

assigned a specific sequence of transmission within this

group. This reduces station interference and unnecessary

returning.

ANTENNA LEAD-IN: Included in the details of many

radiobeacons located at or near light stations is a statement

of the distance and bearing of the radiobeacon transmitting

antenna from the light tower. Use should be made of this

information when calibrating the ship’s direction-finding

equipment by means of simultaneous visual and radio

bearings.

Plotting Radio Bearings

The procedure for converting radio (great circle) bear-

ings as received by direction-finder equipment aboard ship

is identical with that used in converting radio bearings

supplied by direction-finder stations on shore and is

described in section 100E “Plotting Radio Bearings” of

Pub. 117, Radio Navigational Aids.

Synchronization for Distance Finding

At some radiobeacon stations, sound signals, either sub-

marine or air or both, are synchronized with the radiobea-

con signals for distance finding. Ordinarily, the sound

signals do not operate during the transmission period of

the radio signal in clear weather. The methods in use

employ, as a rule, distinctive signals to indicate the point

of synchronization, and make use, for determining dis-

tance, of the lag of signals traveling through air or water as

compared to the practically instantaneous travel of the

radio signals.

In the case of some sound signals, a series of short radio

dashes is transmitted at intervals following the synchroniz-

ing point, so that by counting the number of such short

dashes heard after the distinctive radio signal and before

hearing the corresponding distinctive sound signal, the

observer obtains the distance, in miles equal to the number

of dashes counted, from the sound signal apparatus unless

stated otherwise.

In the case of other signals, the observer notes the num-

ber of seconds intervening between the reception of the

distinctive radio signal and the corresponding sound signal

and uses a factor to determine distances in miles as fol-

lows:

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Seaward G3278-1000 User Manual Page 28

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