[amsat-bb] Re: ARRISSat Reception 14.45 UTC
karn at philkarn.net
Mon Apr 11 20:58:54 PDT 2011
On Mon, Apr 11, 2011 at 5:47 PM, Joe Fitzgerald <jfitzgerald at alum.wpi.edu>wrote:
> I am curious to see how your BPSK1000 fares on a rapidly tumbling
> platform. Let's hope ISS doesn't start tumbling more than once per orbit!
It's a pretty sensitive mode, but it still won't work with a zero-watt
> If you do convince them to leave the ISS powered up on board ISS, we
> could evaluate rapit deep fades in the channel by putting middle school
> students in charge of holding an arrow antenna.
My concern is that the on/off cycling won't play well with my convolutional
interleaver. It takes 16.384 seconds to fill the interleaver at AOS. You
might get decoded data up to 8 seconds earlier than that if what you do get
is very clean, but there's little margin for additional error correction.
And when the transmitter switches off, the interleaver will drain over
16.384 seconds as it fills with noise. If the signal in the last 16.384
seconds before switch-off is unusually strong, you may be able to decode
data up to 8 seconds before LOS. But anywhere from 8 to 16 seconds will be
chopped off *each end* of each already very short 40-60 second
I designed this signal to deal well with occasional deep fades lasting up to
1-1.5 seconds -- not for total "fades" lasting 2 minutes at a time. Had I
known that this "emergency low power mode" was actually going to be used, I
would have designed the whole mode completely differently, with block
interleaving aligned to the transmit on/off times.
The golden rule of the modem designer: "know your channel". Optimizing for
one impairment usually pessimizes it for something else.
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