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10 STANDARDS FOR RADIO DIRECTION-FINDING SYSTEMS

10.1 INTRODUCTION 

10.1.1 The direction-finding equipment required by the Navigating Appliances and Equipment Regulations is to indicate both bearing and sense of radio transmissions in the frequency bands specified in paragraph 10.2.1 of these standards.

10.1.2 In addition to the provisions of the Ship Station Technical Regulations the equipment shall comply with the following minimum performance requirements.

10.2 FREQUENCY RANGES AND CLASSES OF EMISSION ^

10.2.1 The equipment shall be capable of receiving signals of classes of emission A1, A2, and A2H in the frequency range 255 to 525 kHz and A1, A2, A2H, A3 and A3H in the frequency range 2167 to 2197 kHz.

10.3 SELECTIVITY ^

10.3.1 The selectivity shall be such as to allow a bearing to be taken readily without interference from other radio transmissions on frequencies more than 2 kHz from the desired signal.

10.4 SIGNAL IDENTIFICATION ^

10.4.1 Means of audio-monitoring shall be provided regardless of the method used for direction-finding.

10.4.2 The equipment shall be suitable for use with headphones. A loudspeaker, if provided, shall be capable of being rendered inoperative by simple means.

10.5 BEARING INDICATION ^

10.5.1 Means shall be provided to indicate the bearing of the desired transmission. Such indication shall be capable of being easily, rapidly and precisely resolved within 0.25 degrees.

10.6 BEARING ACCURACY ^

10.6.1 The instrumental accuracy in taking bearings shall be within ± 1° . This requirement shall be met at all frequencies in the frequency bands specified in paragraph 10.2.1 of these standards and throughout the whole 360 degrees of azimuth at field strength values between 50 m V/m and 50 mV /m.

Note: The instrumental accuracy referred to above does not include the operational accuracy attainable in service, which shall be determined for each installation taking into account the provisions of the Ship Station Technical Regulations. In particular the operational accuracies in the 2 MHz band shall be sufficient for homing purposes.

10.6.2 Preset facilities to correct the quandrantal error shall normally be provided for the frequency band 255-525 kHz.

10.7 MANUAL CONTROLS AND THEIR OPERATION ^

10.7.1 A tuning scale or indicator shall be provided, calibrated to indicate directly the carrier frequency of the signal to which the equipment is intended to be tuned.

10.7.2 .1 If a tuning scale is provided, at all points in its range,

1. mm shall correspond to not more than 2.5 kHz in the frequency range 255-525 kHz.
2. The maritime distress frequencies shall be prominently marked.
3. Where other means of frequency indication are provided, the resolution shall be at least 1 kHz.

10.7.3 All controls shall be of such size and location as to permit normal adjustments to be easily performed, and shall be easy to identify and use.

10.7.4 The sense switch, if fitted, shall be of a non-locking type.

10.8 OPERATIONAL AVAILABILITY ^

10.8.1 The equipment shall be ready for operation within 60 seconds of switching on.

10.9 POWER SUPPLY ^

10.9.1 If provision is made for operating the equipment from more than one source of electrical energy arrangements for rapid change-over shall be provided.

10.9.2 Means shall be incorporated for the protection of the equipment from excessive voltages, transients and accidental reversal of power supply polarity.

10.9.3 The equipment shall be capable of operating in accordance with the requirements of these standards in the presence of such variations of the power supply as are normally expected in a vessel.

10.10 DURABILITY AND RESISTANCE TO EFFECTS OF CLIMATE ^

10.10.1 The equipment shall be capable of continuous operation under the conditions of vibration, humidity and change of temperature likely to be experienced in the vessel in which it is installed.

10.11 SPECIAL REQUIREMENTS FOR DIFFERENT METHODS OF DIRECTION-FINDING ^

10.11.1 Aural minimum method

1. With a field strength sufficient to ensure a signal/noise ratio of at least 50 dB, a change in the setting of the bearing indicator of 5° in either direction from the position of minimum output shall cause the audio-frequency output to increase by not less than 18 dB. Similarly, a change of 90° in either direction shall cause an increase of not less than 35 dB.
2. The equipment shall be provided with a minimum-clearing control giving a noticeable minimum of the output at all settings.
3. The sense shall be determined with reference to the lower output.
4. The sense ratio in the frequency ranges 255-525 kHz and 2167-2197 kHz shall be 15 dB and 10 dB, respectively.
5. The automatic gain control, if provided, shall be rendered inoperative automatically when the equipment is used for bearing determination.

10.11.2 Other methods

1. There shall be means of indicating that the receiver gain and signal strength are sufficient to enable a correct bearing to be taken.
2. With a field strength of 1 mV/m the indicated bearing shall not change by more than 1° when the receiver is detuned to a point where the indication referred to in sub-paragraph 10.11.2.1 above shows that the signal strength is just sufficient to take a bearing.
3. For any signal of strength sufficient to give a bearing indication, there shall be no observable change of indicated bearing when the beat frequency oscillator is switched on.
4. Fluctuations of the indicated bearing caused by any servo mechanism shall not exceed + 0.5° from the mean value.
5. If, after identifying a station the bearing of which is required, it is necessary to check or alter the adjustment of any control as part of the process of direction-finding, this check and adjustment shall be capable of being made within 10 seconds.

10.12 MISCELLANEOUS ^

10.12.1 The equipment shall be protected from excessive voltages induced in the aerials.

10.12.2 The equipment shall be clearly marked with the minimum safe distance at which it may be installed from a standard or a steering magnetic compass.

10.12.3 The equipment shall be provided with an indication of manufacturer, type and/or number.

10.12.4

1. The equipment shall be so constructed that it is readily accessible for maintenance purposes.
2. Information shall be provided to enable competent members of a ship's staff to operate and maintain the equipment efficiently.

10.13 INTERNATIONAL STANDARD ^

10.13.1 The International Maritime Organization Resolution "A.223(VII) Performance Standards for Radio Direction-Finding Systems" is the adopted standard.

10.14 EFFECTIVE DATE ^

10.14.1 This Standard came into force on 25 May 1980.

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