AFSK and GMSK in ICOM IC910 H

Question

I am a newbie in HAM.The transceiver I have in my lab is ICOM IC910 H. My goal is to use it to communicate with our cubesat which uses GMSK. According to the manual, on page 50, there are notes for operating the radio at 9600 bps. In the note, the manual instruct to set AFSK/GMSK output level to some value. My question is does this mean AFSK mode can also be used to receive GMSK signals as there is nowhere else in that manual where the term GMSK is used? Could you elaborate more on the implication of attempting to demodulate GMSK by using AFSK communication chain? Regards.

Answer 1

Background

GMSK is a type of FSK (frequency-shift keying) with specific parameters and pulse shaping.

AFSK is "audio FSK" — that is, taking some choice of analog modulation and using FSK with some audio-frequency carrier frequency (e.g. around 100 Hz to 3 KHz) as the modulating signal.

• If the analog modulation chosen is SSB, then this is identical on the air to FSK transmitted without a secondary modulation step (because SSB modulation is essentially just a frequency translation and bandpass filter), assuming the bandwidth of the signal fits within the ≤3 KHz bandwidth of a SSB signal.

• If you use FM then the signal on the air is different from a non-AFSK system. Do this only if your satellite is actually using AFSK on FM.

Usage

If your cubesat is transmitting GMSK, then you will want to set your radio to SSB mode. Then you will need to connect its output to a separate GMSK demodulator — which could be a computer running suitable software and with a sound card (or dedicated radio interface unit) connected to the radio. To learn about the appropriate connections/devices, read about amateur radio “digital modes”.

Also: If the radio gives different advice for "1200 baud" vs "9600 baud", then choose the 9600 option. This minimizes unwanted filtering not suitable for digital signals.