[amsat-bb] Thoughts on ISS packet switch back to 145.825 MHz (long)

Kevin M n4ufo at yahoo.com
Sun Apr 16 23:00:15 UTC 2017

> No, you are not creating QRM if the ISS does not repeat your packet. Listen to the path of the ISS and
> you will hear that 50% of the time the radio is silent. It does not TX since there are no valid packets.
> Where is the QRM?

I don't have a dog in this fight since I don't work packet any more... but at one time I was a statewide packet coordinator and personally took care of numerous dual band packet nodes. I've listened to the audio while at those sites. There may be one thing to consider than a regular 'ground op' doesn't think of...

At high altitude, the receiver can hear a LOT and there are a lot of packets that do not get decoded because two, three, even four or more stations will all transmit at once and cover some part of another stations packet, thereby negating each other. (FM capture effect does not really apply for packet unless it's near 100% with clear audio, which is rare.) Because those stations are not hearing each other, they will key up and transmit at will, whether another station is transmitting at the time or not. From the 'digipeaters' point of view, it hears only partial packets, with lots of overlapping signals, so it decodes none.

Whether or not the activity is appropriate, I'm not commenting on that... but I'm addressing your question so that you can better decide for yourself that question. My answer to 'where is the QRM?' is... 'The radio is silent 50% of the time, because it cannot decode enough valid packets... true. But the reason it can't decode enough valid packets may well be (and most likely IS) that all the transmitting stations continually overlap each other. (QRM each other)' The error in the logic here is assuming that it doesn't retransmit because it doesn't 'hear'... On the contrary, it HEARS TOO MUCH and can't make sense of the mess to decode it. In other words, it manages to hear a packet in the clear, about 50% of the time.

In simple terms, it's most likely silent BECAUSE of the QRM... as opposed to the silence being an indicator of a LACK of QRM as you have suggested. Which would be no different than SO-50 staying silent because 3-4 stations are all keying up at once and SO-50 can't make sense enough to decode someone's PL tone. In both cases... too many stations, bird can't hear... bird does not retransmit.

73 and good luck to all in finding a workable solution.  Kevin N4UFO

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