compass bearing to the MARPA target, all of the collision calculations become unstable. The rate gyro on our (oldish Raymarine unit) results in satisfactory predictions. Of course when the target signal begins to get weak, it will sometimes accelearate to warp speed like some UFO. That's as the automatic target tracking feature "loses" it. The rate gyro makes a bigger difference when seas are rough and there's a lotta yawing going on.