Re: AIS message 1, Rate of turn

[Gary E. Miller]

Yo Владимир!

On Thu, 18 May 2023 15:53:29 +0300
Владимир Калачихин <v.kalachikhin@gmail.com> wrote:

> I am, of course, stupid, but:
> 
> https://www.navcen.uscg.gov/ais-class-a-reports :
> 'Rate of turn' field has 8 bits.
> It may contain:
> "-127 = turning left at more than 5 deg per 30 s (No TI available)"
> 
> How to cram in 8 bits the '-127' number?
> 
> 
> Next:
> "-128 (80 hex) indicates no turn information available (default)."
> 
> But '80 hex' === 128, not the -128.
> 
> Can anyone clarify?

That's just the way twos complement binary works.

I can't improve upon Wikipedia:

    https://en.wikipedia.org/wiki/Two%27s_complement

Maybe think of it as 7 bits and the sign bit, but 128 is a fudge.

RGDS
GARY
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