Fluxgate Compass vs MEMS Gyro: Autopilot

What a heading sensor does

An autopilot needs two pieces of information to steer a boat: the direction the boat is pointing and how fast that direction is changing. The heading sensor is the part of the system that supplies that information. It is separate from the drive unit, which moves the steering, and from the control head, which you use to set a course.

On a listing, the heading sensor appears under different names: a compass, a heading source, a 3D heading sensor, or a sensor. Some packs put one in the box. Others expect you to have one already on the boat, usually connected through the NMEA 2000 network.

Fluxgate compass basics

A fluxgate compass measures the direction of the earth's magnetic field and reports it as a heading relative to magnetic north. Because it reads the earth's field directly, it gives an absolute direction. That makes it a reference sensor: it can tell the autopilot which way the boat is pointed even when the boat is sitting still.

A fluxgate compass is sensitive to its surroundings. It needs to be mounted away from magnetic interference from engines, wiring, and metal objects, and it should be calibrated for the boat it is installed on. Mounting and calibration are installation questions; the maker's manual and a qualified installer are the ones to answer them.

MEMS gyro basics

A MEMS gyro is a small solid-state sensor that measures rotation rate. It provides a rate signal the autopilot can use to detect a turn as it begins. The word MEMS stands for micro-electro-mechanical system; it is the same kind of technology used in many consumer devices.

A gyro measures change, not absolute direction. Its sense of heading drifts over time unless something corrects it. That is why a gyro is usually paired with a reference, such as a compass or a GPS-derived heading, rather than used alone.

The term MEMS describes the manufacturing technology of the sensor. You will see it on listings as a name, not as a performance claim. What matters for a buyer is whether the pack includes a gyro-based sensor and whether it needs a compass reference. That is stated in the pack contents, not in the name alone.

How modern autopilots use both

In a combined sensor, the gyro provides the fast rate information and the compass supplies the absolute heading reference. The autopilot uses the compass to correct the slow drift in the gyro. This combination is often described with the term sensor fusion. The exact behavior of a given pack is set by the maker, so the maker's page is the place to confirm what a sensor does.

The stated details on the source pages show how differently packs handle the heading sensor. Raymarine describes the Evolution sensor as 'the autopilot central processor and 3D heading sensor at the heart of every Evolution autopilot system.' The 3D wording indicates a sensor that uses more than a single magnetic compass measurement.

Furuno takes a different approach with the NavPilot 300. The maker notes that 'a Furuno PG700, SC33, SCX20, or other compatible NMEA2000 vessel heading source is required.' So a NavPilot 300 listing depends on an external heading source being present on the boat or added separately.

What this means on a listing

When you read an autopilot listing, the heading sensor is one of the first parts to identify. A complete pack may include a control head, a drive, and a sensor. A core pack often leaves the sensor out. The complete packs guide lists packs that include these parts, and the core packs guide lists packs for boats that already have gear.

If a listing does not state whether a sensor is included, that missing detail is a signal to check the maker's page for the model line. The same applies when a title says 'w/o' or 'requires' next to a sensor name. The seller may not know; the maker's stated details do.

The marine autopilot guide compares these systems on stated details, and the Raymarine autopilot guide and Furuno autopilot guide explain how each line labels its packs.

The difference matters when you are replacing an old autopilot. An older fluxgate compass may still be aboard and able to provide a heading reference to a new pack. A pack that requires a heading source can reuse that compass if it is compatible. A pack that includes its own sensor does not need the old one. The maker's page is the source for both questions.

Questions to ask before choosing

Before you settle on a pack, ask the questions below. The answers come from the maker's page and from your own boat's existing equipment. A qualified installer can also help you confirm what your boat has and what the pack needs.

• Does the pack include a heading sensor in the box, or does it require one from another instrument?
• If it requires one, does your boat already have a compatible heading source, such as a fluxgate compass, a GPS compass, or a chartplotter with a built-in sensor?
• Is the sensor a combined 3D unit that uses both a gyro and a compass, or is it a simple fluxgate compass?
• Where will the sensor be mounted, and is that location free of magnetic interference? The maker's manual and a qualified installer decide this.
• If you are keeping an existing drive unit, does the new sensor and processor work with that drive? The maker's page is the source.
• Does the pack connect to your existing NMEA 2000 network, or does it use a proprietary network? The NMEA 2000 article explains the standard in general terms.

Safety and the watch

Whatever sensor an autopilot uses, the autopilot does not replace a lookout. The U.S. Coast Guard's Boating Safety Division describes its mission as reducing loss of life and injuries on U.S. waterways by improving the knowledge, skill, and abilities of recreational boaters. Its page points to boating safety courses, vessel safety checks, and float plans.

This site makes no safety claims for any autopilot. If you want to know what alarms or watch features a control head offers, read the maker's page for that control head and ask a dealer.

What to pick for your situation

Questions

Is a fluxgate compass the same as a heading sensor?

A fluxgate compass is one type of heading sensor. A heading sensor is any device that tells the autopilot the boat's direction. A MEMS gyro is another type, and many modern sensors combine a compass and a gyro in one unit.

Do all autopilots include a heading sensor?

No. Some packs include a sensor in the box; others require an existing heading source, such as a Furuno NavPilot 300 needing a PG700, SC33, SCX20, or other compatible NMEA 2000 heading source. Read the sensor requirements article and the maker's page to confirm what a pack includes.

Does a MEMS gyro replace a compass?

A MEMS gyro measures rotation rate but not absolute direction, so it drifts unless corrected. In an autopilot it is usually paired with a compass or another reference rather than used alone.

Why does the Furuno NavPilot 300 need an external heading source?

The maker states that a Furuno PG700, SC33, SCX20, or other compatible NMEA 2000 vessel heading source is required. That means the pack is designed to use a heading source already aboard or added separately, rather than including one.

What should I look for in a listing title?

Look for words like 'sensor', 'compass', '3D heading sensor', 'w/o sensor', or 'requires heading source'. If the title does not say, check the maker's page for the model line. For a closer look at how the two sensor types interact, see the integrated compass vs external compass article.

Is a GPS compass the same as a MEMS gyro?

No. A GPS compass derives heading from satellite positions, a fluxgate compass reads the earth's magnetic field, and a MEMS gyro measures rotation rate. Some autopilots can use a GPS compass as an external heading source over NMEA 2000.