Subelement A: VHF-DSC Equipment & Operation— Topic :
Question 9A5
Element 9 (GMDSS Maintainer)What is the time diversity reception interval for a VHF-DSC call?
Explanation
VHF-DSC (Digital Selective Calling) utilizes time diversity to enhance reliability, particularly in the presence of fading or impulse noise. To achieve this, each 10-bit character within a DSC message is transmitted twice. The interval between the start of the first transmission of a character and the start of its retransmission (the second identical character) is precisely specified by the ITU-R M.493 standard as 33-1/3 milliseconds.
This specific interval is chosen to be long enough for short-term channel impairments to decorrelate, improving the likelihood that at least one of the two transmissions is received correctly. This duration is also exactly four times the duration of a single 10-bit DSC character (4 * 8-1/3 ms = 33-1/3 ms). The other options do not correspond to this standardized time diversity interval for VHF-DSC.
Related Questions
9A3 How might a DSC radio check be accomplished with a coast station?9A4 You attempt to program a new MMSI into a VHF radio with an existing MMSI. The radio will not accept the entry. What is the best course of action to correct this problem?9A6 What might a voltmeter indicate when testing a DSC circuit for an outgoing message data stream?