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I was able to implement a half-duplex system using an SPDT switch and a relay board and tested it in a lab environment. I'm about to test it against orbiting CubeSats. I understand that the return time for a packet to LEO could be less than 8 ms, depending on altitude and elevation angle. This might be too quick for the mechanical SPDT switch. Do all CubeSats add some delay to enable half-duplexing? If that's the case, what is the typical delay time added to enable half-duplexing?

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I have no intimate knowledge of any amateur cubesats, but:

It's very unlikely; these satellites are about all primarily transverters. This a) means they can themselves operate in TX and RX simultaneously, by isolating RX from TX by means of frequency, and b) that they don't need any memory to operate, but just receive and transmit amplification, filtering and a mixer.

So, the bad news is, it's very unlikely you get more than a roundtrip at the speed of light to switch. The good news is that you don't need that much isolation, anyways, as you can use a frequency duplexer.

Of course, there's also digipeaters, which actually use the same downlink frequency as uplink frequency.

Let's assume the digipeater would artificially wait much more than necessary:
If you delay relaying, then you "risk" another transmission happening while you wait. Two options there:

  1. You go to "deaf" mode while you wait: This reduces the time transmissions could be captured to a duty cycle of $\frac{T_{\text{packet}}}{T_{\text{wait}}}$; this could very well lead to repeat transmissions, and thus congestion of the medium.
  2. You accumulate messages for a while, then relay them "en bloc" after some delay. This increases your duty cycle, but can lead to much bigger latency (which might not be desirable in a network!)

So, if I remember correctly (and that's a big if) at least PSAT2 had something like a 10s "hold time", so, mode 2, for APRS packets. However, that thing was never in APRS digipeater mode (again, IIRC), because that was just a fallback mode.

I have a faint memory that things like PSK31 are simply transverted.

So, in the end, this depends on the operating mode of your satellite; can't give a general statement.

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