SITOR-ARQ is a simplex system where both stations transmit alternately on the same frequency.
The CCIR recommendation 476-3 defines a cycle of 450 ms and the two stations are designated as follows:
Ø ISS - information sending station
Ø IRS - information receiving station
The WAVECOM software always decodes the information of the ISS station.
|
Parameter |
Value |
|
Frequency range |
HF |
|
Operation modes |
Simplex ARQ |
|
Modulation |
FSK |
|
Symbol rate |
100.0 Bd |
|
Shift/Bandwidth |
170 Hz |
|
Receiver settings |
DATA, CW, LSB or USB |
|
Input format(s) |
AF, IF |
SITOR systems almost exclusively operate at a symbol rate of 100 Baud and with a shift of 170 Hz.
After the receiver has been correctly tuned, the SITOR mode may be started by selecting a baud rate.
Fully automatic tuning to the signal center and shift may be achieved by selecting the Auto button.
The SITOR 7 bit alphabet employs a 3:4 mark-space ratio. The ISS transmits blocks of 210 ms duration each containing 21 bits. In turn the IRS transmits acknowledgement bursts with a duration of 70 ms.
If the ISS does not receive an acknowledgement, a block containing the RQ character is sent. In the case of blocks being received in error by the IRS, the last data block sent is repeated.