Built-in GPS, external GPS/compass or GNSS compass – what should you choose for your boat?

A modern chartplotter often has a built-in GPS receiver, but it does not always show which direction the boat is pointing when the speed is low or the boat is stationary. For many boats, the built-in GPS receiver is more than sufficient, while installations with radar, AIS and other advanced features may benefit from a separate heading sensor.

In this article, we compare three different solutions:

• Built-in GNSS receiver in, for example, Raymarine Axiom
• External GNSS and heading sensor such as Raymarine AR200
• Professional GNSS compass like Hemisphere Vector V200

We look at the differences in position, compass heading, accuracy, startup time and installation, and explain what the specifications mean in practice.

GPS and GNSS – what is the difference?

GPS is the American satellite-based positioning system. GNSS is a broader collective name for satellite systems such as GPS, GLONASS, Galileo and BeiDou.

A modern marine GNSS receiver can receive signals from multiple satellite systems. This gives the receiver access to more satellites, but actual performance is still affected by antenna placement, visibility to the sky, interference, and receiver specifications.

In everyday speech, GPS is still often used as a collective term, even when the product is actually a GNSS receiver.

Level 1: Built-in GPS in the chartplotter

Modern chartplotters like the Raymarine Axiom have a built-in GNSS receiver. This means you can get your boat's position on the chart without installing a separate GPS antenna.

For many recreational boats, this is a simple and cost-effective solution.

Benefits of built-in GPS

• No separate GPS antenna needed
• Fewer components and cables
• Easy installation
• Lower total cost
• Sufficient accuracy for normal recreational navigation

The verified specification for a previous Raymarine Axiom generation states support for GPS, GLONASS and BeiDou, tracking up to 28 satellites and position updates at 10 Hz. The stated position accuracy is 15 meters maximum without SBAS and 5 meters maximum with SBAS, in both cases 95 percent of the time. Exact specifications vary between models and generations.

A clear view of the sky is crucial

A built-in GPS receiver only works as well as the installation location allows. Since the antenna is inside the chartplotter, reception is directly affected by where the screen is mounted.

Reception conditions may deteriorate if the plotter is located:

• Inside a wheelhouse
• Under a hardtop
• Under a flybridge
• Behind metallized or heavily tinted glass
• Near larger metal structures
• In a place where large parts of the sky are obscured

In an open cockpit with a good view of the sky, the built-in antenna usually works under significantly better conditions than in a closed cabin.

So it's not just the receiver you use that matters. The location of the antenna and the clear view of the satellites are also very important.

How long does it take to get position after starting?

Just because the chartplotter has started does not automatically mean that the GNSS receiver has already established a stable position.

The stated cold start time for the verified Raymarine Axiom receiver is less than two minutes. A cold start means that the receiver needs to search for satellites without being able to use all previously saved data.

How quickly the position is displayed in practice is affected by, among other things:

• The view towards the sky
• Antenna placement
• If satellite information is saved
• How long the system has been off
• If the boat has been moved since the receiver was last used

After a short shutdown, the position can therefore be reached quickly, while a cold start or an obscure installation can mean a longer wait.

A separate, externally mounted sensor may have better reception conditions if it is placed on, for example, a hardtop or radar arch with a clear view of the sky. However, this does not mean that an external sensor always guarantees faster position lock. The result depends on the entire installation.

GPS does not automatically show which direction the boat is pointing

It is important to distinguish between course over ground , COG, and compass course , heading.

The GPS receiver can calculate COG by comparing how the boat's position changes. COG therefore shows in which direction the boat is actually moving over the Earth's surface.

Heading instead shows which way the bow is pointing.

These values may differ due to:

• Crosswind
• Current
• Drift
• Reversing
• Maneuvering
• Very low speed

When the boat is stationary, there is no movement that the GPS receiver can use to calculate a stable COG. Therefore, the chartplotter's built-in GPS is not a substitute for a true heading sensor.

It can be noticed by:

• The boat symbol is twisting or moving erratically
• The head-up orientation becomes less stable
• Radar overlay does not align correctly when heading is missing
• The presentation of surrounding targets becomes less intuitive

Level 2: External GPS/compass – Raymarine AR200

The Raymarine AR200 combines a GNSS receiver with an AHRS, Attitude and Heading Reference System. The sensor provides position, magnetic compass heading, pitch and roll to compatible Raymarine Axiom systems.

This means that the AR200 provides information that a standard GPS receiver cannot provide when the boat is stationary.

External placement provides better installation options

The AR200 has an internal GNSS antenna, but the entire sensor is mounted externally, allowing you to choose a location with a better view of the sky than the chartplotter's location offers.

Common mounting locations may be:

• Hard-top
• Radar arch
• Targa arch
• Other open space with good satellite visibility

The location must also be chosen with consideration for the magnetic compass in the device. The sensor should not be mounted near equipment or wiring that creates magnetic interference and then calibrated.

Stable heading when the boat is stationary

The most important difference between the AR200 and a regular GPS is the access to heading.

The AR200 uses an AHRS solution with an accelerometer, digital magnetic compass and MEMS gyro. The unit provides heading, pitch, roll and rate of turn at a 10 Hz update rate. Raymarine states a static magnetic compass accuracy of 1° RMS and a dynamic accuracy of 3° RMS.

This gives the system access to the boat's direction even when the boat is stationary or moving slowly.

Heading information is valuable for:

• Stable chart orientation
• Radar overlay
• ClearCruise Augmented Reality
• Video stabilization
• Presentation of AIS and chart objects
• Functions that require pitch and roll information

The AR200 is specifically designed as a sensor and video stabilization module for ClearCruise AR on compatible Axiom chartplotters. Together with a compatible camera, the system can overlay navigation objects, AIS targets and seamarks on the camera image.

Raymarine AR200 Technical Data

The Raymarine AR200 tracks up to 28 satellites and updates the GNSS position at 10 Hz. The stated position accuracy with SBAS is less than 5 meters, 95 percent of the time. Cold start is stated at less than two minutes, typically less than 60 seconds, while hot start is stated at less than 45 seconds.

This means that the AR200 should not primarily be seen as a dramatic upgrade in the accuracy of the stated GPS position compared to Axiom's internal receiver.

The big upgrade is instead:

• An externally placed GNSS receiver
• Magnetic heading when the boat is stationary
• Pitch and roll
• Stabilized price information
• ClearCruise AR support

The AR200 must be installed and calibrated correctly to provide reliable heading data. The system's selected data sources and compatibility should also be checked before heading information is used by other networked functions.

Level 3: Professional GNSS Compass – Hemisphere Vector V200

The Hemisphere Vector V200 is a professional GNSS compass that uses two GNSS antennas in the same housing. The antennas are 20 centimeters apart and are complemented by gyro and tilt sensors.

Unlike the AR200, the V200 does not need to use a magnetometer to determine heading. Instead, the direction is calculated using the satellite signals received by the two antennas.

Heading without magnetic compass

Since heading is determined using a dual GNSS antenna, the course calculation is not dependent on the Earth's magnetic field.

This means that the heading function is not affected by magnetic deviation from, for example:

• Speaker
• Electric motors
• Generators
• Heavy-duty power cables
• Steel components
• Magnetic objects

The device also does not need to be calibrated like a traditional magnetic compass. However, installation still needs to be done correctly. The orientation of the sensor, a clear view of the sky, and distance from interfering radio transmitters or obscuring structures are still important.

Position, heading and movement data

V200 can leave:

• GNSS position
• Heading
• Pitch
• Role
• Heave
• Turn rate

The manufacturer's datasheet states 424 channels, 10 Hz as standard and 20 Hz as an option. Cold start is stated to be 60 seconds, warm start to typically 30 seconds, hot start to typically 10 seconds and heading fix to typically 10 seconds.

The position accuracy is specified as:

• 1.2 meters RMS when autonomously positioning
• 0.3 meters RMS with SBAS

Heading accuracy is 1.5° RMS as standard and 0.75° RMS as an option. Pitch and roll are specified at 1.5° RMS.

It is important to distinguish between the standard and optional versions. The 0.75° accuracy should not be stated as general performance for all V200 configurations.

When is the V200 the right choice?

The V200 is particularly interesting when the installation places higher demands on position and heading, or when magnetic interference makes a standard electronic compass difficult to place.

Examples of uses are:

• Professional navigation installations
• Workboats
• Measuring boats
• Installations requiring GNSS-based heading
• Advanced radar and AIS systems
• Systems using NMEA 2000 or NMEA 0183

The V200 supports NMEA 2000 or NMEA 0183 depending on the product version and connection selected. Therefore, the correct variant needs to be selected based on the boat's existing network.

Technical comparison

Axiom GNSS specifications vary between models and generations. The table is therefore based on the Raymarine generation whose documentation has been verified here. Always check the manual for the specific Axiom model.

The values for the Axiom and AR200 are given by Raymarine as 95% confidence limits, while the V200 values are given as RMS, so they cannot be compared as if they were calculated using exactly the same statistical method.

What do the differences mean in practice?

Built-in GPS in Axiom

The built-in receiver is a good choice when:

• The plotter has a good view of the sky
• You mainly need a stable position on the chart
• You want to keep the installation simple
• You don't need heading when the boat is stationary
• You do not have features that require a separate heading sensor

For normal recreational navigation, the built-in receiver may be sufficient.

Raymarine AR200

The AR200 is a natural step up when:

• The plotter is mounted in a hidden location.
• You want to be able to position the GNSS receiver with a better view of the sky
• You need heading when the boat is stationary
• You are using ClearCruise AR
• You need pitch and roll
• You want to improve features that depend on stable heading data

The most important gain is not necessarily better position accuracy. It is the combination of external positioning and correct heading data that makes the difference.

Hemisphere Vector V200

The V200 is best suited when:

• Professional GNSS heading required
• Magnetic interference is difficult to avoid
• Higher position accuracy is required
• Heading should be calculated without a magnetometer
• Installation requires NMEA 2000 or NMEA 0183
• Standardized and documented sensor performance is prioritized

Here you take the step from a magnetic AHRS to a dual-antenna-based GNSS compass.

Which solution should you choose?

Choose built-in GPS if:

• You have a simple navigation setup
• The plotter has a good view of the sky
• You mainly need position and COG
• You want to avoid additional components

Choose Raymarine AR200 if:

• You have a compatible Axiom system
• You need heading when stationary
• You want to use ClearCruise AR
• You need pitch and role information
• The plotter's location provides less suitable reception conditions

Choose Hemisphere Vector V200 if:

• You need GNSS-based heading
• You want to avoid magnetic compass problems
• You require documented, professional sensor performance
• You need higher position accuracy
• The installation uses NMEA 2000 or NMEA 0183

Conclusion

The built-in GPS receiver in a chartplotter like the Raymarine Axiom is more than adequate for normal navigation, especially when the chartplotter is mounted with a good view of the sky. However, it does not provide a true compass heading when the boat is stationary. It primarily displays position and can calculate COG when the boat is moving.

The Raymarine AR200 completes the system with heading, pitch and roll. The external mounting also allows you to choose a location with better satellite visibility than the plotter location. For a compatible Axiom system with ClearCruise AR or other features that require stable heading, it is a clear step up.

The Hemisphere Vector V200 goes further by calculating heading using two GNSS antennas instead of a magnetometer. This reduces the problems of magnetic deviation and provides higher documented position accuracy, along with heading, pitch, roll and heave for more demanding installations.

The most important question is therefore not only how accurate a GPS position you need. You also need to determine whether the system needs to know where the boat is , which way it is moving , or which way the bow is actually pointing .

Short summary

Built-in GPS: Position and COG for normal navigation.

Raymarine AR200: Position plus magnetic heading, pitch and roll.

Hemisphere Vector V200: Position and GNSS-based heading with dual antennas for more demanding and professional installations.