General Description
The GMDSS/DSC is a maritime safety system. It is defined by the ITU specification R-REC-M.493. There are two main operational modes: 100 baud FSK for HF/MF (offset 170 Hz) and 1200 baud FSK (offset 800 Hz) for VHF. SkySweeper supports both of these modes.
The decoder searches for the transmission within the given frequency boundaries. Also you can set the lower FSK frequency by double clicking the mouse on the required frequency position, and then SkySweeper allows you to set a second (=higher) frequency, that can be set by a mouse click. The values of frequencies and the frequency offset are also shown on the screen.
When a transmission has been detected within the given range, the decoder will then lock itself to that frequency. When locked, the decoder draws a bold blue line on the spectrum display at the currently locked frequency. If the decoder is not locked, then the blue line is not shown as bold.
In the non-locked state, the carrier frequencies are automatically monitored within the given frequency boundaries. If the carrier frequency changes, then the decoder moves to a new frequency. If 'Lock' is pressed in the user interface, then the decoder will not automatically change the frequency. ‘Reset’ resets the decoder and the received text can be saved by using ‘Save’. The parameters can be modified by pressing the ‘Config’ button.
SkySweeper supports both FFT and waterfall display modes for the decoder tuning. The Decoder Spectrum Dialog chapter gives more information about the spectrum display options.
Parameters can be modified by pressing the ‘Config’ button. Note: you can install several decoders in one configuration, if required.
Configuration
Pressing the ‘Config’ button opens the following Configuration dialog box.
Parameters
Mode
This selects the operational mode (HF/MF or VHF).
Tone scan area
The FSK transmission has two tones named as Mark and Space. The scanned frequency range for Mark and Space will be set with two values, namely ‘low’ and ‘high’. The Mark and Space tones are searched for only inside this frequency window.
Fixed Offset
If the Fixed Offset check box is selected, the user can enter the fixed frequency offset between the Mark and Space carrier frequencies. The fixed frequency offset is used in automatic scanning mode.
Negative Polarity
This negates the bits (1->0 and 0->1).
Print Timestamps
If this is selected, the timestamp is printed before the message
Print Symbols
If this is selected, the GMDSS symbol numbers are printed before the message
Print CRC Failed
If this is selected, also messages not passing the error check (CRC) will be printed.
FFT
The ‘FFT’ button opens and closes the power spectrum window. You can, for example, close the power spectrum window when the decoder has been locked onto the correct frequency in order to get more space for the text window.
Save
The ‘Save’ button opens the log file control Dialog, which is used to open or close a log file. The detector text window can also be saved into a file or sent into a socket.
Lock
The ‘Lock’ button locks or unlocks the decoder frequency. Decoders have the ‘automatic frequency tracking’ feature. Push the ‘Lock’ button when you want to lock the decoder onto the selected frequency. Push the ‘Unlock’ button when you want to release frequency locking.
Reset
The ‘Reset’ button resets the decoder and clears the text window.