[amsat-bb] Re: ARRISSat Reception 14.45 UTC
aa2tx at comcast.net
Tue Apr 12 01:59:14 PDT 2011
At 02:50 AM 4/12/2011, Phil Karn wrote:
>As I recall, ARISSat-1 data frames can be up to 512 bytes long. Ignoring
>HDLC flags, bit stuffing, CRC, etc, that's 4K bits. At a data rate of 500
>bps (the FEC is rate 1/2), 512 bytes will take 4096/500 = 8.192 seconds to
>8.192 seconds is longer than 7.232 seconds.
>But wait, there's more. If the satellite sends a series of back-to-back 512
>byte frames, and the transmitter comes on too late after one has already
>started, you'll have to wait for it to end before you can begin decoding the
>next one. Meanwhile, the clock is quickly ticking down until the transmitter
>goes OFF again...
That's why it doesn't work that way.
In low power mode, the transmission is started clean
every time. A single telemetry data frame is only 256
bytes so about 4 seconds of data. After the 1 frame, the
transmitter is left on until the interleaver is emptied.
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