[CA] [CA11] Understanding GPS COG data vs p70RS Heading data

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I have an EV1 and p70RS modules on the same network as an A127.

The A127 shows a COG value with a 'T' setting. The p70RS shows a degrees value as well and also shows a 'T' indication. I figure the T stands for True vs Magnetic compass heading. Why does the A127 show a different degree value vs the p70RS? And the p70RS degrees seem to change alot faster than the A127. Is it because the p70RS gets it degress from the EV1, while the A127 has to calculate the COG from satellite signals? Should they ever be exactly the same (ie. in sync). There are times when the delta in degrees for the devices are 3 degrees, and at other times a larger number of degrees.

tks

Dave