This conversation comes up frequently and it turns into a short discussion on exactly what a GPS receiver can and cannot do when it comes to figuring out which direction you are going...or in this case, heading.
SCX20 Satellite Compass
A GPS receiver alone has to do some calculations to figure out what it calls the COG, or coarse over ground. If you are moving along then the GPS has a place that it knew it was just a second ago and where it is right now. Some processing then produces a vector, or a velocity in a direction. That is your COG. As soon as you go very slow the GPS cannot determine your COG with any accuracy.
If you point your bow 5 degrees starboard to compensate for a drift and to maintain your COG, then your heading is 5 degrees more than your course. A GPS receiver cannot compensate for this difference. The one exception being a satellite compass, which is really a heading sensor using multiple GPS receivers spaced a couple of wavelengths away from each other.
This issue of heading versus course is a real issue when using an autopilot or radar. The autopilot is an obvious application needing heading input. Most major brand kits come packaged with what is called a flux gate compass. These solid-state sensors can detect the change in the earth's magnetic field as their orientation is changed. The newest models are NMEA 2000 compliant and are powered by the network at the same time.
Using a heading sensor with a marine radar system opens up more features, such as MARPA target tracking, real motion target trails, and radar chart overlay that matches on the chart what your bow is really pointing to. It is also reassuring to be able to acquire a target on the radar and within a few sweeps, then know it's heading and speed. That information can be displayed as a motion vector on the chart showing where that guy is headed and if he is going fast or slow. Even though AIS can do that and give more detailed information, AIS installations are on few boats and it is more practical to use radar for target tracking at this point.
I have mentioned the two major types found today, a fluxgate type rate compensated solid state heading sensor, or a satellite compass.
In both types, NMEA 2000 should be used to assure fast update times required by most manufacturer's equipment. Older NMEA 0183 type sensors may not refresh fast enough. Look for 100ms update times or better.
Because of the features gained in the radar capability alone, the purchase of a heading sensor may be one of the most value packed electronics investments you can make. Now when you choose "heading up" orientation on a chart plotter, the bow is really pointing at what the chart shows even if you are stopped or ahead slow, circling for that perfect fishing mark.
Capt Brett