"Gordon Wedman" wrote in message
news:tZRfd.10435$df2.5362@edtnps89...
In " The Radar Book - Effective Navigation and Collision Avoidance " by
Kevin Monahan, he points out that the compass bearing given by a GPS will
be
unstable at low speeds. If you want your radar to show proper compass
bearings and work in the North-up/Course-up modes he says you must connect
to some type of fluxgate or gyro compass. So if you just connect your GPS
NMEA output to your radar it seems you may not get correct heading
information while creeping along at low speed, for example, in fog.

Not to mention the huge difference between COG and heading, when on a slow
speed and a tidal current....

Meindert