gmdss

Below are some Frequently asked Questions in GMDSS and Competency Exam. This page keep updating as more question keep coming. Keep visiting for more updates.

In the VHF Band there is always inherent receiver noise when no signal is being received. Squelch control is used to suppress this noise. it cuts out the AF output from the speaker when there is no incoming signal.

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For Bridge to forward or Aft communication VHF Channel 15 or 17 using low power mode i.e., 1 Watt.

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The input is 12 V DC. This is usually obtained from the Mains supply using a power supply unit which steps down and rectifies the mains voltage.

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Antenna used for the VHF Transceiver is Omni-directional half wavelength dipole 1.5 meter whip antenna.

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In USA a number of ITU International Duplex Channels are converted and used as Simplex channels i.e., both ship and shore transmit on the ship’s frequency. (e.g., Ch 01, 05, 07, 18, 19, 21, 22, 23, 63, 65, 66, 78, 79, 80, 81, 82, 88)

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F3E or G3E (F =Frequency Modulation, G =Phase Modulation, 3 =Analogue Signal, E =Telephony). So either Frequency or Phase modulation can be used. Both are compatible with each other.

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SCAN: is an advanced form of dual watch. In Scan mode the VHF set can monitor more than one channel besides Channel 16.

SCAN TIME: is the number of seconds the VHF dwells on a channel in the SCAN program on which a signal has been picked up. SCAN TIME is set by the operator.

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In Dual Watch mode, the VHF set monitors one channel plus channel 16. The set monitors Ch. 16 for 0.1 Sec and the other Channel for 1.2 Sec. Channel 16 has priority over the other channel being monitored.

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Not by a ship station as it has to transmit / receive in all directions. Coast Radio Stations may use a directional Antenna to transmit / receive signals out to sea. This Directional Antenna is called a Yagi-Uda Antenna.

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In Sea Area 1 (this is a ship to shore as well as a ship to ship alert). In Sea Area A3 and Sea Area A4 (this is ONLY a ship to ship Alert).

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It continuously monitors VHF DSC Channel 70 for reception of Alerts. It processes all DSC Alerts which are then transmitted through the VHF Transceiver RM-2048.

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There are 3 ways of sending Distress Alert

  • Undesignated
  • Designated using Distress Button
  • Designated using Menu or Call and Edit Button

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Dummy load is an artificial aerial consisting of a relative load. It does not radiate the R.F. power supplied to it, but rather dissipates the power I the form of heat. It is used for testing / running / repairs without radiation.

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It is a fine tuning control. It helps to reduce adjacent station interference and also serves to clarify SSB Speech.

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Fading is a continual variation of signal amplitude experienced at the input of a receiver. AGC is a negative feedback given from the detector stage of the receiver to the input R.F. Amplifier by automatically adjusting the Gain of the input R.F. Amplifier to a fixed level, AGC keeps the output of the receiver constant no matter how the incoming signal varies.

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Maximum power is transferred from the transmitter Power Amplifier to the Antenna for radiation into space when the Antenna is perfectly tuned circuit i.e., it is in resonance with the frequency being transmitted.

This usually achieved when the length of the Antenna is half the wavelength of the frequency being transmitted.

The Antenna is of fixed physical length (6-8m). It has to cover a large band of frequencies (from 300 kHz – 40 Mhz). Therefore the electrical length of the Antenna is adjusted by means of capacitors and inductors inside the Antenna Tuning Unit (ATU) which makes the Antenna a perfect tuned circuit so that maximum power is transferred from the power amplifier to the antenna and out into space.

By pressing Tune buttons a motorised control brings in different values of capacitors and inductors to electrically match the antenna length to the frequency being transmitted.

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AGC is used to combat the effects of fading on both MF and HF Bands. Fading occurs on HF bands throughout the 24 hours while on MF only at night.

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Adding inductors / coils in the circuit increases the electrical length while adding capacitors decreases it.

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  • Format Specifier (Distress)
  • MMSI No of Station in Distress
  • Nature of Distress (Undesignated)
  • Position and Time (UTC)
  • Frequency of Transmission.

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The kind of alert sent will depend upon the type of assistance required in a particular situation.

For Example: Send a Distress Alert if your vessel is disabled and drift and in danger of running aground and breaking up. Send and urgency alert if your vessel has lost propeller and requires a Tow. Send a safety alert if your vessel is disabled & adrift undergoing engine repairs and you wish to inform all ships in the area to give a wide berth.

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A single call on MF/HF DSC varies between 6.2 – 7.2 seconds while on VHF it varies between 0.45 – 0.63 seconds.

To repeat a DSC Distress alert we can do it after 3.5 to 4.5 minutes from the beginning of the initial call.

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A CRS should acknowledge a DSC Distress Alert between 1 – 2.75 Minutes

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