Autopilot Heading Sensors: Compare Compass and Gyro

What a heading sensor does

Every autopilot needs to know the boat's heading before it can steer a course. The heading sensor is the part that supplies that information to the autopilot's processor. In the Raymarine Evolution system, the autopilot sensor is described as the central processor and 3D heading sensor at the heart of every Evolution autopilot system, which tells you that the sensing and the processing are handled together.

The heading sensor is not the drive unit. The drive unit is the part that interfaces with the steering system to keep the boat on the right course, as Raymarine's drive unit page says. The sensor decides what the boat is doing; the drive does the actual steering. When you compare two listings, keep these two jobs separate.

Magnetic compass: fluxgate and solid-state

The most common heading reference in a marine autopilot is a magnetic compass. The two names you will see on listings are fluxgate and solid-state. A fluxgate compass measures the direction of the Earth's magnetic field with a magnetometer. A solid-state compass does the same job with chip-based sensors. Both output a magnetic heading in degrees, and the autopilot uses that heading as its reference for the course you command.

A magnetic compass can be disturbed by nearby metal and by electric current, which is a general fact about magnetism, not a claim about any particular unit. If the boat's compass will be a separate sensor, read the maker's manual about where it can be mounted. The separate article on magnetic interference describes this in more depth.

What a rate gyro adds

A rate gyro measures how fast the boat is rotating, not the absolute direction it points. Because a gyro does not give a heading by itself, it is normally combined with a magnetic compass. The autopilot reads the compass for the absolute heading and the gyro for the rate of change, which helps the processor respond smoothly during a turn.

The word gyro may not appear in every listing title. Some makers describe the sensing package as a single sensor, like Raymarine's 3D heading sensor. When a listing is quiet on this point, look at the bullet list or the maker's page.

GPS compass and separate heading sources

A GPS compass derives heading from satellite signals rather than from the Earth's magnetic field. A boat that already has a compatible GPS heading source can feed that heading to the autopilot. The NMEA 2000 standard is the network that lets a heading sensor share data with the autopilot and other instruments; the standard describes a network for interconnecting marine electronics so they can share data, including commands and status, over a single channel.

Some listings make the heading source a separate item. The Furuno NavPilot 300 page states that a Furuno PG700, SC33, SCX20 or other compatible NMEA 2000 vessel heading source is required. The same page lists the pack's contents as cables, autopilot pumps and miscellaneous accessories, so the heading source is not in the box. When you see a note like that, the question is not which sensor to choose; it is whether the boat already has a compatible heading source.

What the listing tells you about the sensor

Read the first words of the title. Self-contained cockpit tiller autopilots, like Raymarine's ST1000+ and ST2000+, carry the sensing inside the cockpit unit. A below-deck pack, by contrast, may name the drive and the control head in the title but leave the sensor out, which means you have to check the bullet list.

The bullet list is where the listing states what is in the box. Look for phrases such as sensor included, requires a heading source, or NMEA 2000 heading source. If the detail is missing, ask the seller. A complete autopilot pack generally includes the parts for a full system, while a core pack leaves parts out on purpose. The comparison between core packs and complete packs explains the difference, and the guides on core packs and complete packs show how listings name these parts.

Which sensor setup suits your boat

For a small sailboat with a tiller, the self-contained tiller pilot is the simplest choice: the compass and the processor are in the same unit, and the list of parts is short. The tiller pilot guide compares self-contained units on stated details.

For a larger sailboat or a powerboat with hydraulic steering, the autopilot is usually a multi-part system, and the heading sensor may be separate. If the boat already has a compatible heading source on its NMEA 2000 network, a core pack can make sense because you are not buying a sensor you do not need. If you are building a system from scratch, a complete pack that names the sensor is easier to plan. On a boat with an electronic helm or steer-by-wire, the pack title will usually name the helm system, and the steer-by-wire guide explains how those listings read.

Questions to ask before you buy

Ask the seller or the dealer: Is the heading sensor in this pack, or do I need to buy it separately? Does the autopilot use a magnetic compass, a rate gyro, or both? Is the heading source expected on NMEA 2000? If the listing does not say, the maker's page is the place to check.

Once the autopilot is installed and steering, it does not replace the watch. The U.S. Coast Guard's Boating Safety division works to improve the knowledge, skill and abilities of recreational boaters, and the operator remains responsible for keeping watch. This site makes no safety claims for any unit.

What to pick for your situation

Questions

What is a heading sensor in an autopilot?

It is the part that tells the autopilot which way the boat points. It can be a magnetic compass, a rate gyro, a GPS compass, or a combined sensor, and it feeds the heading to the autopilot's processor.

Is a rate gyro enough on its own?

A rate gyro measures rotation speed, not absolute heading, so it is normally combined with a magnetic compass. Some makers describe the combined sensing in one package, such as Raymarine's 3D heading sensor.

Do tiller pilots have the heading sensor built in?

Self-contained cockpit tiller autopilots, like Raymarine's ST1000+ and ST2000+, carry the sensing inside the cockpit unit, so you do not buy a separate heading sensor.

Does the Furuno NavPilot 300 include a heading sensor?

The NavPilot 300 page states that a Furuno PG700, SC33, SCX20 or other compatible NMEA 2000 vessel heading source is required, and the listed contents are cables, autopilot pumps and miscellaneous accessories. Plan on a separate heading source.

How does NMEA 2000 relate to the heading sensor?

NMEA 2000 is the network standard for interconnecting marine electronics so they can share data. A heading source on the network can feed the autopilot and other instruments over a single channel, per the NMEA standard's description.

Can a GPS compass replace the magnetic compass?

Some systems accept a GPS heading source as the heading reference. The GPS compass gets its heading from satellite signals rather than the magnetic field, and the listing or the maker's page will say whether the autopilot accepts one.