[amsat-bb] Digital Satellites Question

Robert Bruninga bruninga at usna.edu
Mon Jul 28 14:11:23 UTC 2014

> The biggest load in a communications satellite....
> is almost always the downlink RF power amplifier(s)...

True for some, not true for others.  The locations of ham operators are
only about 10% of the earths surface.  90% of transmitter power can be
saved if the transmitter is not left on 100% of the time.

An APRS satellite takes advantage of this by knowing not only are there
users only 10% of the time, but even over a saturated ham area, the TX/RX
duty cycle cannot exceed 40% or so.  The result is that the transmitter is
only on for 4% of the orbit.  This means we can run a 5W transmitter to
make the packets hearable on an HT with a stock antenna, but that the
transmitter AVERAGE power is only 200 mW (less power than the receiver).

And by putting the downlink on 2m instead of UHF, there is another 9 dB
advantage to OMNI anennas.  Add that to the 10 dB advantage of running a 5W
transmitter instead of 0.5W and you can see that an APRS cubesat can have
almost 19 dB downlink advantage over the typical UHF 0.5W downlink.

Many other factors of course are involved, but we need a mix of satellites
and modes, but the above is why I like AX.25 for short bursty texting and
data for a large number of people to share in a short period of time.


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