Article by Capt. Steve Katz
Heads-Up
Whether low cost or high cost - today's navigation electronics and even a smartphone can supply the modern Captain with an abundance of navigation information and perform numerous calculations that would have been a chore years ago. Whether you are headed to your favorite back bay fishing spot or traveling across an ocean, learning about and understanding your marine navigation data can help make the journey safer and more accurate.
North
The magnetic compass is familiar to many boaters but is often overlooked in favor of today’s marine electronics. In basic terms, this compass will point to the earth’s magnetic north, no matter which way we turn the boat and no electricity needed! The compass will constantly re-align itself to the north and allow us to interpret the heading printed on the card in degrees, 0-360, indicating where our vessel’s bow is pointed. For those purists out there: the compass actually points in the direction of the horizontal component of the earth’s magnetic field where the compass is located, and not to any single point.
Magnetic North or True North?
Magnetic north is the direction the compass points, which is the earth’s magnetic North Pole. This is not the same as the earth geographic North Pole. The magnetic north pole is a continuously moving point, dependent on the changes in the earth’s core and therefore magnet field. True north then can be calculated by applying the magnetic declination for your geographic area.
So why is this important? Today’s modern electronics calculate magnetic north through the use of your position obtained by GPS signals so that your ships compass, and the marine electronics are all on the same reference format. There are some electronic navigation systems that allow you to change the amount of magnetic north variation and even some that allow you to set up your system on true north (the geographic North Pole). Most pleasure boats should use magnetic north for all of their navigational needs and it’s not recommended to mix true north and magnetic north – this will cause much confusion at the helm.
12,000 Miles in the Sky
How can modern GPS chart plotters calculate your direction of travel without a compass? A GPS chartplotter uses the data received from multiple GPS satellites orbiting the earth in a medium Earth orbit (MEO) 12,550 miles from the earth surface to calculate your vessel's speed, location AND course over ground. A vessel's course is an imaginary line based on the vessel’s past locations – calculated very fast on a chartplotter, and often provided as a graphical track line and numerical compass degrees abbreviated as COG (course over ground) or GPS heading.
A vessel's heading is the direction the bow is pointed and is not always the same as the vessel's course. A boat on the water can be subject to wind and current, causing the vessel to be pushed or pulled in direction other than the intended path of travel. While this variation could be very slight, the captain may have to steer the boat in a direction not exactly dead center of the bow to get the vessel to stay on track to the intended waypoint, buoy or desired destination.
My chartplotter or autopilot screen shows a heading, where does this come from?
If you have an autopilot, you usually have a heading sensor. The autopilot needs to know which way the bow is pointed so it can calculate how best to keep you on course while engaged. Most autopilots have a screen that reads a compass heading from its internal heading sensor. While this number should be close to the ship's compass, it may not be exact due to numerous variations including mounting location, local magnet fields etc. This autopilot heading sensor is similar to the standard ships compass you may have on the helm, but instead of a clear window to read the card, electronic sensors read the card. Today’s modern heading sensors consist of solid-state components without a moving card.
These heading sensors often contain a 3-axis rate gyro with a 3-axis accelerometer or even a 9-axis solid-state gyro. Heading sensors output electronic data using industry standard protocol such as NMEA0183 or NMEA2000 and are read by the modern autopilots and chart plotters. You can interconnect the autopilot heading sensor to your chartplotter, which will then display heading information along with GPS course information provided by the chartplotter. Remember - the course the vessel is traveling and the direction the bow is pointed are two different pieces of information, while they are the same number at times, they are calculated differently.
Some GPS chartplotters report a GPS heading without a heading sensor, this a calculated value based on the vessel's past position and is only useful when the vessel is underway and is actually course over ground (COG). This is most helpful to keep the boat’s icon on the GPS screen pointed in the direction of travel and this GPS heading cannot be used as a source of heading for an autopilot system or other shipboard system requiring actual heading.
If you do not have an autopilot, you can install a standalone heading sensor and connect it to your chart plotter to provide heading, these start at a few hundred dollars. A heading sensor needs to be calibrated after installation, either automatically or manually depending on the exact model. The newest heading sensors use multiple GPS antennas to calculate your vessels heading, not requiring magnetic sensors, these were at one time very expensive and only practical for larger boats. Today, a few manufactures offer satellite heading sensors for the pleasure boat market, one of those was recently introduced by Furuno - the SCX 20, selling for less than $1000. Another benefit of a satellite compass is that it can be connected to some autopilot systems, putting an end to autopilot compass errors and issues.
Modern electronics often need to know the ships heading to properly display information on the screen such as a radar overlay. A radar overlay is when the GPS chart plotter overlays the radar screen image over an electronic chart in a fashion that allows you to see both the radar and chart at the same time. This can be extremely useful as a navigation and collision avoidance tool. In order for this to work properly and accurately, the chart plotter needs to know the actual heading (not course) so it can precisely adjust the radar image in regard to the front of your boat.
Another use of a heading sensor is to enable features like Mini-Automatic Radar Plotting Aid (MARPA) or ARPA target tracking and real motion target trails. Using ARPA/MARPA is a great feature allowing you to be able to acquire a target on the radar screen and within a few sweeps know the other vessels heading and speed while tracking the other vessel. Because of the features gained in the radar capability alone, the addition of a heading sensor may be one of the most valuable add-on navigation electronics investments you can make.
Incorporating a heading sensor in your system allows you to choose "heading up" orientation on a chart plotter, the result is that the bow is really pointing where the electronic chart screen shows - even if you are stopped or moving slowly, especially circling for that perfect fishing spot.
That’s the head-up on heading! §
