[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: CRC



> Could someone give me a reference /discussion on the meaning of CRC'c
> referenced for the data content of AO-40 Telemetry?  thanks, Farrell
Winder,
> W8ZCF.
>

Farrell,

Good question.  "CRC" stands for Cyclical Redundancy Check, an error
checking error detection scheme widely used in floppy disk transfers, LANs
etc.  In our telemetry application, the spacecraft generates a block of
telemetry, then makes a calculation on the bits comprising that block and
comes up with a CRC number which is sent along with the block.  When the
receiver gets the telemetry block, it makes the same calculation on the
received bits and compares its CRC result with the one the spacecraft sent.
If it matches, there is an extremely high probablity that all data in the
block is correct.  Older telemetry schemes we have used in OSCAR's are less
reliable ... with just straight ASCII as sent by other spacecraft, it is
difficult to be _sure_ you got all the data correct.  Simple schemes like
the parity bit can be easily fooled by a two bit error.

Some work has been done to combine the efforts of several receive stations
to generate good telemetry from telemetry from data with "bad" CRC's.
Imagine taking all data from three stations, and comparing frames bit by
bit.  If we take a "vote" on each bit and calculate a CRC on a newly
constructed frame of bits that won the vote, we have a better chance of
calculating a good CRC, as different stations are likely to receive
different "bad" bits.  That's why you often hear Paul Willmott say "EVERY
BLOCK COUNTS GOOD OR BAD!".

If you want more details on CRC, check
http://www.iol.unh.edu/training/vganylan/teach/vgconcepts/frame/crcsum.html



----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home