GMDSS basics, frequencies and sea areas

Sea areas by definition, not by distance

Equipment logic by area

The watches you must keep

Radio from zero: frequency and wavelength

Channel versus frequency, simplex versus duplex

Propagation: why the band decides the reach

Channel plans: how the numbers are built

GOC exam check

1. Which is the definition of sea area A2?

   1. Within 100 NM of an MF coast station with continuous DSC
   2. Outside A1, within coverage of an MF coast station with continuous DSC
   3. Outside A1, within coverage of an HF coast station with continuous DSC
   4. Within radiotelephone coverage of any MF coast station
   Show the answer

   B. Outside A1, within coverage of an MF coast station with continuous DSC

   Why: A2 is defined by declared MF coast-station coverage with continuous DSC alerting, outside A1. 100 NM is only a typical figure, not the definition.

2. A ship in A3 relies on MF/HF. Which DSC watch must it keep on MF/HF?

   1. 2182 and 8291 kHz, plus one other HF voice frequency
   2. 2187.5 kHz only, with HF used when needed
   3. 8414.5 kHz plus every other HF DSC frequency
   4. 2187.5 and 8414.5 kHz, plus one other HF DSC frequency
   Show the answer

   D. 2187.5 and 8414.5 kHz, plus one other HF DSC frequency

   Why: The watch is 2187.5 and 8414.5 kHz plus one of 4207.5, 6312, 12577 or 16804.5 kHz. 2182 and 8291 kHz are the voice partners, not DSC.

3. Roughly what is the wavelength of the 8291 kHz HF voice frequency?

   1. About 36 m
   2. About 9 m
   3. About 3.6 m
   4. About 137 m
   Show the answer

   A. About 36 m

   Why: 300 / 8.291 MHz gives about 36 m. About 9 m is the quarter-wave, a common slip; 137 m is the wavelength at 2182 kHz.

4. Which ionospheric layer absorbs the lower HF frequencies strongly by day and largely disappears at night?

   1. The F2 layer
   2. The E layer
   3. The D layer
   4. The F1 layer
   Show the answer

   C. The D layer

   Why: The D layer absorbs the low HF bands by day and fades at night. The F layers carry long-distance HF links rather than absorbing them.

5. On one HF frequency a station 1,500 NM away hears you well, but a station 300 NM away hears nothing. What is the most likely explanation?

   1. The frequency is above the MUF for the path
   2. The near station lies in the skip zone
   3. Your transmitter power is too low
   4. D-layer absorption is blocking the sky wave
   Show the answer

   B. The near station lies in the skip zone

   Why: The gap between the ground wave and the first sky-wave return is the skip zone. Above the MUF the distant station would not hear you either, and more power does not fix skip.

6. What does the HF radiotelephone duplex channel number 822 stand for?

   1. The 22nd channel in the 8 MHz band
   2. The 8th channel in the 22 MHz band
   3. The 82nd channel in the 2 MHz band
   4. A simplex channel on 8.22 MHz
   Show the answer

   A. The 22nd channel in the 8 MHz band

   Why: The number is the band in MHz followed by a two-digit serial, so 822 is the 22nd channel in 8 MHz. Each number stands for a ship and a coast frequency, not one simplex frequency.

7. Which group of VHF channels is intership only?

   1. 6, 13, 16 and 70
   2. 8, 10, 12 and 14
   3. 6, 72, 77 and 16
   4. 6, 8, 72 and 77
   Show the answer

   D. 6, 8, 72 and 77

   Why: Channels 6, 8, 72 and 77 are intership only. Channel 16 is distress, safety and calling, so any set containing it is wrong.

8. Why does a 2182 kHz signal usually reach further over the sea by day than a VHF signal?

   1. Lower frequencies travel faster than higher ones
   2. MF sky waves are refracted by the F layer by day
   3. MF travels as a ground wave over conducting sea water
   4. MF sets always transmit with more power than VHF
   Show the answer

   C. MF travels as a ground wave over conducting sea water

   Why: MF ground wave follows the sea, while VHF is essentially line of sight. Every radio wave travels at roughly the speed of light, so speed is never the reason.

9. Your ship is 400 NM offshore at 1400 local time, outside VHF and MF coast coverage, and needs a voice link with a coast station about 1,200 NM away. The MF/HF installation is working. Which band is the sensible first try?

   1. 2182 kHz, because MF ground wave is reliable over sea
   2. 4 MHz, because the lower HF bands carry far by day
   3. 12 MHz, because higher bands suit a long daytime path
   4. Channel 16 on high power with the tallest antenna
   Show the answer

   C. 12 MHz, because higher bands suit a long daytime path

   Why: By day the D layer absorbs the lower HF bands, so a higher band such as 12 MHz is the usual first guess for a long path. 4 MHz is the tempting near miss, but it is the band the daytime D layer hurts most; MF ground wave and VHF line of sight cannot reach 1,200 NM.

10. Same scenario: the 12 MHz link works well until evening, then the signal fades away as night falls along the path. What do you try next?

    1. 16 MHz, because the D layer has gone at night
    2. More power on 12 MHz to push through the fade
    3. Stay on 12 MHz, as night improves every HF path
    4. A lower band such as 8 MHz, as the MUF drops
    Show the answer

    D. A lower band such as 8 MHz, as the MUF drops

    Why: At night the MUF for the path falls, so 12 MHz may no longer return to earth and a lower band such as 8 MHz becomes the better choice. Going higher is the tempting error: the D layer has gone, but a higher band is even further above the falling MUF, and more power does not cure that.