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

[Chuck - Raymarine - Moderator]

Welcome back to the Raymarine Forum Dave,

Q1. 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.
A1. Correct. When configured for True bearings, bearings and heading are referenced to True North (i.e. the geographic north pole). When configured for Magnetic Bearings, bearings and heading are referenced to the Magnetic North (i.e. the magnetic north pole). When configured for Magnetic Bearings and operating ahead on a constant heading in the absence of leeway, the value of COG, Heading, and that indicated on a properly calibrated ship's compass will all be very close in value.

Q2. Why does the A127 show a different degree value vs the p70RS? 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.
A1. GPS latency, and leeway are the most common causes for differences between the values reported for COG and SOG. Also COG may also differ greatly when maneuvering. These values will be closest when operating on a constant heading in the absence of leeway and seas which would cause yawing. A few degrees is not uncommon and the amount of this difference will typically vary with heading. The internal GPS receivers of the a-Series MFDs calculate position updates at 1Hz, whereas the EV-1 CCU updates at 10Hz. Onboard my own boat which features MFDs running LightHouse II v19.03 software, an Evolution autopilot, and Raystar 130 GPS sensor, I have heading and COG vectors set to infinite and generally don't see more than +/- 2-3 degrees of difference when operating on a steady heading in calm seas.