Corrections

[Fausto A. A. Barbuto]

Good day,

This is perhaps more of a technical question than one on SkyRoof, but here it is: what are the "Corrections" that often appear in the telemetry reports? E.g.:

META:
  Bytes: 223
  CFO: 1927.6 Hz
  SNR: 3.7 dB
  CRC: OK
  Corrections: 15
  Erasures: 0

Does this mean that the telemetry data just received are invalid? 

I rarely get zero corrections, so they seem to be the rule and not

the exception. Are those corrections somehow related to the low

SNR? According to what you said previously, transmissions with 

low SNRs should not be decoded, and 3.7 dB seems too low to me.

Also, what does CFO mean?

Thanks for any help. 73s.

Fausto

[Alex VE3NEA]

Some telemetry transmissions include Forward Error Correction (FEC), when extra bits are added to the payload in such a way that on the receive side they allow us to detect and correct the errors - in some cases, up to 16 corrections. When FEC is not included, but the message contains a checksum (CRC), then we can tell if there are errors in the received frame but not which bits are wrong. In such cases it is possible to correct 1 or 2 errors, by flipping some bit and re-computing the checksum. These error correction methods greatly improve decoding, but they are statistical and thus sometimes produce false positive detection, as in your example. Red flags to look for: low SNR and high number of corrections.