Subelement A: VHF-DSC Equipment & Operation— Topic :
Question 9A2
Element 9 (GMDSS Maintainer)What is the correct modulation rate and index of modulation for VHF-DSC?
Explanation
VHF-DSC (Digital Selective Calling) employs Frequency Shift Keying (FSK) to transmit digital messages reliably over marine radio frequencies. International standards, particularly ITU-R Recommendation M.493, precisely define the modulation characteristics for this system.
The standard signaling rate, or **baud rate**, for VHF-DSC is **1200 baud**. This ensures universal compatibility among all DSC-equipped marine radios. The system uses a frequency shift of ±400 Hz from a 1700 Hz center frequency (i.e., mark at 1300 Hz, space at 2100 Hz). The associated **index of modulation** is specified as **2.0 +/- 10%**.
Options A, C, and D are incorrect because their listed baud rates (800, 1500, and 1800 baud) do not conform to the established international standard for VHF-DSC communication. Therefore, 1200 baud and a modulation index of 2.0 are the correct parameters.
Related Questions
8A6 What terms are used to describe a device used to test the RF power output of a marine VHF transmitter without emitting a signal on the air?9A1 What is the correct frequency shift and sub-carrier for VHF-DSC?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?9A5 What is the time diversity reception interval for a VHF-DSC call?