%20(1).png)
The Ultimate Guide to Magnetic Motion Capture
- Rebecca Flynn
- 8 hours ago
- 7 min read
One of the lesser known types of motion capture technology, magnetic motion capture is to some a mystery, and to others a lifesaver! Read on to become a magnetic mocap master.
What is magnetic motion capture?
What is magnetic motion capture and how does it actually work?
A magnetic mocap system uses a source (transmitter) that emits an electromagnetic dipole field into the surrounding space. Small, lightweight sensors attached to a person or object detect that field. By analysing how the field reaches each sensor, the system computes the sensor's precise position (X, Y, Z) and orientation (yaw, pitch, roll) in real time. Polhemus calls this "true 6DOF" because both position and orientation motion data are measured natively by the technology, with no hybrid calculations needed.
How is it different from optical motion capture systems like OptiTrack?
The most fundamental difference is that it does not need cameras. Because no cameras are used, line-of-sight is not necessary for continuous tracking, and the technology can even be used in complete darkness. Optical systems lose data whenever a marker is occluded (a body part blocks another, for instance) whereas magnetic systems track continuously through obstructions.
Do the sensors need line-of-sight to work?
No, and this is one of its biggest selling points. Magnetic motion capture technology can track through people and most walls, which is simply impossible with any camera-based approach.
What is magnetic motion capture used for?
Where is magnetic mocap actually used today? (e.g. medical rehab, military, sports biomechanics, VR/AR?)
Magnetic motion capture is used in a number of industries. For example, medical training and simulation, flight training and simulation, pilot head tracking on live aircraft (including the Air National Guard F-16, A-10, and the USAF C-130 Gunship), sports motion analysis, biomechanics research, neuroscience (including Parkinson's tremor tracking and autism research), and virtual physical rehabilitation. The US Olympic Committee has also used it for weight lifting and gymnastics analysis.
Why would someone choose it over other mocap?
Drift is never an issue with Polhemus technology because it is solid-state. There are no MEMS, gyros, accelerometers, or magnetometers involved, so there is never a need to correct data due to drift. This is a meaningful advantage over wearable inertial systems, which accumulate error over time and require periodic recalibration.
Additionally, the main advantage to electromagnetic motion tracking over optical mocap is its ability to track in dynamic environments. Often, optical motion capture requires a direct line-of-sight and a well lit area to track optimally. For Polhemus, this is never an issue. Motion capture can occur without any line-of-sight or specific lightning needed.
Polhemus specifically uses AC (alternating current) electromagnetics, which they describe as their proprietary, award-winning approach. Their AC electromagnetics deliver a longer tracking range than similar competing technologies and operate at lower power levels. They also offer the best signal-to-noise ratios, and multiple tracking systems can be used in the same environment without cross-talk interference.
Can it be used outdoors or in large spaces?
It works outdoors in principle. The electromagnetic field doesn't care about daylight, but the practical constraint is that the working volume is defined by the transmitter's field size, not an open sky.
That said, range can be meaningfully extended. By simply adding a second source you get twice the tracking range, and using up to four sources achieves the maximum coverage expansion with VIPER. LIBERTY LATUS is specifically designed for large-area work: tracking coverage of up to several hundred square feet can be achieved, with the system scalable up to 16 receptors. G4 also lists outdoor use as a specific use case for its digitising stylus option.
Can you stream data to third party applications and platforms?
Yes! Polhemus technology is compatible with many different software platforms. For more information on third party plug-ins we provide, see their page here.
What are some limitations of magnetic motion capture?
What happens near metal objects or ferromagnetic interference? Does it just break completely?
The main limitation is metallic distortion. Metallic distortion is a non-linear signal corruptor which induces accuracy degradation in the tracker's output. The majority of the time, Polhemus electromagnetic tracking solutions do not encounter any issues with it, but it is highly dependent on the application. Beyond that, the working volume is bounded by the transmitter's field rather than an infinite size like an outdoor optical setup, and the technology isn't well-suited to tracking very large numbers of people across a stadium-sized space.
How big can the transmitters be and how far do they track the sensors?
Sources can be as large as a 4x4 in cube (~10.7cm), or as small as approximately 1x1 in (~2.5cm).
Tracking range is determined by the size of the source, with larger sources offering up to 72 in
(182cm) useful operation range, and beyond.
How accurate is it compared to optical systems?
Polhemus positions their accuracy as extremely high. Their systems have measured the slightest tremor of a Parkinson's patient's hand, the subtle movement on the barrel of a gun, and the swiftest golf swings by the world's top professionals. The key distinction vs. optical is consistency: optical systems drop data whenever a marker is occluded. With Polhemus electromagnetics, there is no lost data, no data interruption due to line-of-sight occlusions associated with camera technologies. For many real-world applications, uninterrupted data is more valuable than theoretical peak precision.
Is there meaningful latency?
On some older systems, yes. However - VIPER achieves update rates up to 960Hz with latency as low as 1 millisecond, which is essentially imperceptible and rivals or beats what many optical systems offer in practice.
How does magnetic motion capture work?
What's the difference between DC and AC magnetic systems?
Polhemus uses AC (alternating current) electromagnetics specifically. Their proprietary AC technology delivers a longer tracking range than similar competing technologies, operates at lower power levels, offers the best signal-to-noise ratios, and allows multiple tracking systems to be used in the same environment without cross-talk interference. DC systems (used by some competitors) work differently at the hardware level and are more susceptible to certain types of interference; AC systems like Polhemus's can better distinguish the intentional signal from environmental noise.
How do the receivers calculate position and orientation from the field?
The source emits an electromagnetic dipole field. The sensors read how that field arrives at their location, and the system uses those measurements to calculate where the sensor is and how it is oriented.
How many sensors can a single system track simultaneously?
It varies. VIPER supports up to 16 sensors per system, while LIBERTY LATUS can track up to 12 independent, self-contained markers. PATRIOT handles up to two sensors, making it an option for simpler applications.
What are the operating frequencies?
Up to 960Hz (frames of data per second) depending on the tracking system.
Why would someone use magnetic motion capture?
Why isn't it more widely used if it has real advantages?
A few honest reasons, drawing from what Polhemus acknowledges and the broader context.
First, environmental sensitivity. Metal-heavy environments have historically been problematic, though Fly True Technology, a filter offered with VIPER, is now addressing this.
Second, bounded working volume. A single transmitter covers a limited area, which makes it less suited to large free-roam applications.
Third, sensor counts. Optical systems can track many more markers simultaneously across a full body, which matters for high-fidelity character animation. Magnetic mocap's strengths - the ability to embed sensors, no occlusion, darkness operation, zero drift - tend to shine most in specialised fields like medical simulation, military and research rather than in mainstream film and game production.
What are the major commercial systems?
From Polhemus specifically, the current lineup includes:
VIPER (their highest fidelity, lowest latency flagship), G4 wireless tracker, PATRIOT (a cost-effective two-sensor system), SCOUT (designed for military head tracking on live aircraft), LIBERTY LATUS (large-area untethered tracking), and the long-running FASTRAK, which they describe as having "set the gold standard in motion tracking" and remains a top seller. Other companies in the space include NDI (Northern Digital Inc.) and Ascension Technology (now part of NDI).
How much does a system cost compared to optical alternatives?
Generally in the industry, magnetic systems tend to be more accessible than high-end multi-camera optical rigs, though premium magnetic systems with many sensors are still professional-grade investments.
How is magnetic motion capture improving?
How does magnetic interference affect data?
When highly conductive metal is present very close to the sensor, or source, it is possible
to encounter magnetic distortion which affects the fidelity of tracking data. In almost all
cases, mechanical changes to your setup can resolve this behaviour.
For the most challenging tracking conditions, VIPER is equipped with Fly True Technology
(FTT), a proprietary data integrity tool.
How does it fit into the current wave of IMU-based systems?
Magnetic and IMU (inertial) systems are often compared since both are worn on the body and need no cameras. The key difference is drift: drift is never an issue with Polhemus technology because it is solid-state.
There are no MEMS, gyros, accelerometers, or magnetometers involved, so there is never a need to correct data due to drift. IMU suits can accumulate orientation error over time and require periodic correction. Magnetic systems give you absolute position natively, which IMUs can't do at all without additional infrastructure. The two technologies are complementary rather than competing in the same niche.
Could it have a role in inside-out VR tracking?
Polhemus themselves list VR and AR as application areas for LIBERTY LATUS. The system is listed for use in virtual reality, augmented reality, training and simulation, animation, and motion capture. The core appeal for VR is the same as elsewhere: no cameras, no occlusion, no drift. The limitation is that the transmitter defines a fixed working volume, so it's best suited to room-scale or lab-scale VR rather than building-scale or outdoor experiences.
For inside-out tracking (where the headset tracks itself relative to the world), it would need to be the headset tracking relative to a fixed transmitter - which is essentially what Polhemus already does for military helmet tracking on aircraft.
Stay tuned as we dive further into magnetic motion capture!
Comments