New On The Roof: Satellite Compasses
A new piece of equipment you might see on a larger boat is the satellite compass. While already familiar to commercial mariners and competitive sailboat racers, improvements in electronics and affordability means that for the first time these devices are now seen among recreational boaters. (The first units marketed for recreation debuted in 2021.)
The very familiar GPS receivers on boats determine a boat’s latitude and longitude by triangulating radio signals from four GPS satellites. A satellite compass is a similar idea, but it is an array of multiple GPS units (up to four) inside one enclosure. Each GPS unit calculates position, then minute differences in signal timing between the units are compared. This provides the same information as a traditional GPS, but adds accurate 3-D positioning in space. A new satellite compass can thus replace both a magnetic/electronic compass and a traditional GPS.
A satellite compass can report your latitude/longitude position as a traditional GPS would, but it can also accurately report heading, pitch, roll, yaw, heave, rate of turn, and more. It updates much more quickly than a GPS position, is much more accurate, and works while the boat is stationary.
Why is this important? There are a number of reasons and some are not obvious. A satellite compass can improve navigation, safety, boat handling, and even your success at fishing.
A boat’s autopilot requires a heading in order to operate. Today on recreational boats heading is often determined by a fluxgate (electronic) compass. However, an error of 3 degrees is considered normal for a fluxgate, it is affected by the magnetic anomalies noted on nautical charts, and it is disturbed by nearby metal objects and other electronics. This is a major problem for many boat owners (especially metal boat owners), who just get used to living with a fluctuating heading number. Studies by the marine industry have shown that eliminating this heading error can significantly improve fuel efficiency when navigating by autopilot as it makes paths shorter and reduces the boat's tendency to "hunt" for a heading.
Boaters that rely on electronics are familiar with the positioning and “speed over ground” number from a phone GPS or chartplotter that relies on a GPS and/or “paddlewheel” speed sensor. With these tools there is a noticeable delay between updates of the lat/lon position from the GPS and a delay for the measurement of knots to change when the boat accelerates. The boat's position icon sometimes moves around on the screen when the boat is not moving. All of these delays and inaccuracies are eliminated by the satellite compass.
Improved heading information can also sharpen your RADAR situational awareness by eliminating the “wavy lines” on radar tracks of other boats on the screen (these are caused by heading error).
Sea state awareness is also improved because the satellite compass directly reports “heave,” a name for the vertical up-and-down motion of the boat due to wave action. Wave height no longer needs to be estimated by eye, it can be directly read off the chartplotter’s screen.
Both imaging sonar (such as fish-finders) and computer-controlled active fin stabilization devices also benefit from a more accurate 3D position, producing clearer imagery and less boat rolling.
Tools/Resources:
- Installing and testing a satellite compass (DIY) https://youtu.be/PfGzQZfRfx8?t=341
- The principles of GPS are discussed in AUXNAV Chapter 9, and the satellite compass array is the same in principle as an array used for radio direction finding as discussed in AUXCOM Chapter 9.
Linking to videos that discuss particular radios as examples is not intended as a recommendation or endorsement.
Updated 2023-Feb-11 by Christian Sandvig, SO-CM