Login or Register for FREE!
Subelement H
Maritime Safety Information (M.S.I.):
Section 51
Navtex-1: Operations:
How is mutual interference on 518 kHz among NAVTEX stations avoided?
  • All stations transmit at the same time but stations are limited to daytime operation only to reduce the radius of propagation.
  • Transmitter power is limited, station assignment codes are not shared by other NAVAREAS and stations alternate between daytime and nighttime operations.
  • Correct Answer
    Transmissions scheduled on a time-sharing basis, power limited and station assignment codes are geographically separated.
  • Station codes are not shared by other NAVAREAS, transmissions scheduled on a time-sharing basis and power is limited.

How is mutual interference on 518 kHz among NAVTEX stations avoided?

(C). Transmissions scheduled on a time-sharing basis, power limited and station assignment codes are geographically separated.

Although, NAVTEX stations are constantly sending information, they do not broadcast at night, since atmosphere layers change and the radio wave propagation increases in scope. So, they transmit at daytime only.

The 518 kHZ frequency is for English only broadcasts. Regional stations are also broadcasting on 490 kHz or even 424 kHz in local languages, other than English.

For great overview of NAVTEX, please see MySea Time site article on 20 Navtex Question (and answers) to Make Naxtex Your Friend For Life

Last edited by markadlerdallas. Register to edit

Tags: none

When do NAVTEX broadcasts typically achieve maximum transmitting range?
  • Local noontime
  • Afternoon
  • Sunset
  • Correct Answer
    Middle of the night

When do NAVTEX broadcasts typically achieve maximum transmitting range?

(D). Middle of the night

Medium Frequency (MF) of 518 kHz or 490 kHz can reach up to 200 nautical miles during the day. But, at night it can reach up to 400 nautical miles.

The MF uses ground bounce of radio waves which attenuates energy with every bounce, BUT on salt water, the propagation scope increases dramatically.

For overview of MF, please see Wikipedia's article on Medium frequency

Some of the energy absorbing layers, which limit radio wave propagation, disappear during the night, allowing MF to have a significantly larger broadcast scope.

For more information about the radio wave focused atmospheric layers composition, please see Wikipedia's article on Ionosphere

Last edited by markadlerdallas. Register to edit

Tags: none

What should a GMDSS Radio Operator do if a NAVTEX warning message is received but it contains too many errors to be usable?
  • Correct Answer
    Do nothing. Vital NAVTEX messages will be repeated on the next scheduled broadcast.
  • Contact the NAVAREA coordinator and request a repeat broadcast.
  • Initiate a request for Category A, B, L and D messages.
  • Listen to appropriate VHF weather channel for repeat warnings.

What should a GMDSS Radio Operator do if a NAVTEX warning message is received but it contains too many errors to be usable?

(A). Do nothing. Vital NAVTEX messages will be repeated on the next scheduled broadcast.

NAVTEX messages are categorized as vital, important, or routine. When vital message is transmitted, other stations cease transmissions. If other stations do not stop in time, messages can be distorted.

The important messages are send at the subsequent open time slot. And, routine messages are sent during scheduled broadcasts.

For more information, please see gmdsstesters.com article on NAVTEX System

Last edited by markadlerdallas. Register to edit

Tags: none

Which of these cannot happen when a paper model NAVTEX receiver runs out of paper?
  • The unit is unable to print messages and all subsequent MSI broadcasts may be missed until the paper is replaced.
  • Correct Answer
    The system will automatically change from receiving MSI by NAVTEX to receiving it by SafetyNET™ so that no messages will be lost.
  • It may give off either an audible and/or visual alarm.
  • MSI messages may be missed because the unit cannot print them out.

Which of these cannot happen when a paper model NAVTEX receiver runs out of paper?

(B). The system will automatically change from receiving MSI by NAVTEX to receiving it by SafetyNETTM so that no messages will be lost.

I believe in the answer is SafetyNET(tm) for trade mark.

As example NAVTEX operations manual with illustrations, please see NAV-7 NAVTEX Receiver User Manual 35-821N Iss1 NAV-7 Orolia

For comprehensive info, please see pdf INTERNATIONAL SafetyNET Services

For Inmarsat info on SafetyNET(tm) pdf booklet, please see International SafetyNET Handbook

Last edited by markadlerdallas. Register to edit

Tags: none

Which of the following is the primary frequency that is used exclusively for NAVTEX broadcasts internationally?
  • 2187.5 kHz
  • Correct Answer
    518 kHz
  • 4209.5 kHz
  • VHF channel 16 when the vessel is sailing in Sea Area A1, and 2187.5 kHz when in Sea Area A2.

Which of the following is the primary frequency that is used exclusively for NAVTEX broadcasts internationally?

(B). 518 kHz

The 518 kHZ frequency is for English only broadcasts. Regional stations are also broadcasting on 490 kHz or even 424 kHz in local languages, other than English.

For great overview of NAVTEX, please see MySea Time site article on 20 Navtex Question (and answers) to Make Naxtex Your Friend For Life

Last edited by markadlerdallas. Register to edit

Tags: none

What is the transmitting range of most NAVTEX stations?
  • Typically 50-100 nautical miles (90-180 km) from shore.
  • Typically upwards of 1000 nautical miles (1800 km) during the daytime.
  • Correct Answer
    Typically 200-400 nautical miles (360-720 km).
  • It is limited to line-of-sight or about 30 nautical miles (54 km).

What is the transmitting range of most NAVTEX stations?

(C). Typically 200-400 nautical miles (360-720 km).

Medium Frequency (MF) of 518 kHz or 490 kHz can reach up to 200 nautical miles during the day. But, at night it can reach up to 400 nautical miles.

The MF uses ground bounce of radio waves which attenuates energy with every bounce, BUT on salt water, the propagation scope increases dramatically.

For overview of MF, please see Wikipedia's article on Medium frequency

Some of the energy absorbing layers, which limit radio wave propagation, disappear during the night, allowing MF to have a significantly larger broadcast scope.

For more information about the radio wave focused atmospheric layers composition, please see Wikipedia's article on Ionosphere

Last edited by markadlerdallas. Register to edit

Tags: none

Go to 50 Go to 59