Rate Gyro vs Compass: Autopilot Sensors

Fluxgate compass and rate gyro: two different jobs

An autopilot needs to know two things about the boat: where it points and whether that direction is changing. Those two jobs go to different sensors inside the system. The compass, often a fluxgate compass, tells the autopilot the boat's magnetic heading. The rate gyro tells it how quickly that heading is rotating. Together they give the control unit the information it needs to command the drive.

Listings do not always name each sensor separately. A pack may simply include a heading sensor, or it may state that an external heading source is required. Understanding the difference between the two sensors helps you read those listings and know what a pack includes before you order. The heading sensor types article explains the full range of sensors you may see on a listing.

What a fluxgate compass measures

A fluxgate compass senses the direction of the earth's magnetic field relative to the boat. It provides a stable heading reference in steady conditions, which is exactly what an autopilot needs to hold a course. The compass reading changes slowly as the boat turns, because the boat must actually change direction before the reading moves.

Raymarine describes the sensor at the heart of its Evolution autopilots as the 'autopilot central processor and 3D heading sensor.' That wording shows how the heading sensor is more than a compass: it also carries the processing role for the system. When a listing mentions a 3D heading sensor, the compass part is inside that unit.

What a rate gyro measures

A rate gyro measures angular velocity, meaning how fast the boat rotates around its vertical axis. It does not tell you where north is. Instead, it tells you whether the boat has started to swing, even before the change appears in the compass reading.

In dynamic conditions, that early information matters. Wind and waves push the boat's bow, and the rate gyro lets the autopilot sense the beginning of that motion. The control unit can respond sooner than it could if it waited for the compass to report a new heading, which is why rate gyros are common in marine autopilot heading sensors.

Why an autopilot uses both

A compass alone gives a steady reference but responds slowly to a swing. A rate gyro alone gives a fast response but cannot tell you the actual heading. An autopilot that uses both gets the stability of a magnetic reference and the fast response of a rate gyro.

That combination is why many modern autopilots describe a single sensor unit instead of listing a compass and a gyro separately. The sensor unit contains both and feeds the control unit. The Furuno NavPilot 300, for example, requires a Furuno PG700, SC33, SCX20, or other compatible NMEA 2000 heading source, which means the heading sensing is part of the network rather than inside the display. See the sensor requirements article for more on what a system needs.

Reading a listing for sensors

When you compare autopilot listings, the sensor words appear in different places. Raymarine's autopilot pack page lists the EV-100, EV-150 and EV-200 packs, and the boat autopilot page states that the Evolution sensor is the 'autopilot central processor and 3D heading sensor at the heart of every Evolution autopilot system.' A Furuno NavPilot listing may state that a heading source is required, which tells you the pack does not include the compass.

Words like 'heading sensor,' '3D heading sensor,' or 'heading source required' are the phrases to look for. If a listing says a heading source is required, plan to supply one from a compatible instrument or a separate sensor, as the maker's page describes. The complete autopilot packs guide on this site compares packs on stated details such as what is in the box and what the maker lists as required companion parts. For a broader overview, see best marine autopilot.

Questions to ask before you choose

Before you choose between a complete pack and a core pack, ask a few questions about the sensor. The answers tell you whether the pack will work with the equipment you already have.

• Does the pack include a heading sensor, or does the maker list an external heading source as required?
• Does the sensor unit also act as the processor, as Raymarine describes for its Evolution sensor?
• Does the listing name a specific sensor, like a Furuno PG700 or SC33, or say 'compatible NMEA 2000 heading source'?
• Is the sensor unit in the box, or is it listed as an accessory?
• Does the maker's page describe the heading source before you compare two packs on contents?

Sensors on the NMEA 2000 network

Many modern autopilots read heading data over a NMEA 2000 network. The NMEA 2000 standard uses a CAN-based communications system to interconnect marine electronics, so a compatible heading source can share data with the autopilot. The Furuno NavPilot 300 page names a PG700, SC33, SCX20, or other compatible NMEA 2000 heading source as a requirement.

When a listing says 'compatible NMEA 2000 heading source,' the sensor is part of the network, and you should confirm that your boat has a compatible source before ordering. The NMEA 2000 page explains how certified products share data over a single channel. You can also read the NMEA 2000 connectivity article for more on how autopilots join the network, and the NMEA 2000 backbone article for details on the wiring.

What to pick for your situation

Questions

Is a rate gyro the same as a compass?

No. A compass measures magnetic heading, while a rate gyro measures how fast the boat is turning. An autopilot uses both for different jobs, and many modern sensor units combine them in one housing. The gyro vs GPS compass article compares other heading sources.

Do all autopilots include a rate gyro?

Modern sensor units often combine a compass and a rate gyro in a single unit. A listing may use the term '3D heading sensor' instead of naming the gyro separately, as Raymarine does for its Evolution sensor.

What does 'heading source required' mean on a Furuno NavPilot listing?

It means the pack does not include the heading sensor. The maker's page says the NavPilot 300 requires a Furuno PG700, SC33, SCX20, or other compatible NMEA 2000 heading source.

Can I compare two packs by the sensor name alone?

No. The sensor is one part of the system. Compare the full stated details: drive type, control head in the box, and whether the maker lists any required companion parts.

Does an autopilot replace a lookout?

No. An autopilot steers the boat, but the operator remains responsible for keeping watch. The U.S. Coast Guard Boating Safety page provides guidance on boating safety courses and safe operation.