Key points in 30 seconds
- SBG Systems’ Ellipse-D pairs an inertial measurement unit with a GNSS receiver fed by two antennas. Its heading is true heading, measured between the antennas: no magnetic deviation, no compass calibration.
- Accuracy depends on how far apart the antennas are: SBG quotes 0.2° RMS with a baseline over 2 m, 0.5° over 0.5 m. Roll and pitch: 0.1° without RTK corrections. Heave (5 cm or 5%) is only available on the Marine version.
- It does not plug in like an NMEA 2000 compass: NMEA 0183 and binary over serial, NMEA 2000 only on the CAN variant. We found no documented integration with H5000, nke or MADBrain: it connects through standard protocols and must be bench-tested.
- Verdict: worth it if your programme can spread the antennas 2 m apart and make use of high-rate data. For fast cruising, a satellite compass or a 9-axis NMEA 2000 compass is enough and much simpler to install.
Table of contents
What a dual-antenna INS measures
An inertial measurement unit measures the boat’s rotations and accelerations with three MEMS gyroscopes and three MEMS accelerometers. On its own, it drifts. The Ellipse-D couples it to a dual-frequency GNSS receiver connected to two antennas. The receiver computes the direction of the line joining the antennas and, knowing where they sit on board, the boat’s orientation relative to geographic north: true heading, available even alongside the dock, about 25 seconds after the first fix according to SBG.
A fusion filter combines the two: GNSS keeps the inertial unit in check (the receiver computes at 5 Hz), the inertial unit fills the gaps and provides attitude, with outputs of up to 1 kHz for inertial data. SBG’s published figures for marine use, RMS:
| Measurement | Condition | SBG figure |
|---|---|---|
| True heading | antenna baseline > 0.5 m | 0.5° |
| True heading | antenna baseline > 2 m | 0.2° |
| Roll and pitch | no corrections (single point) | 0.1° |
| Roll and pitch | RTK corrections | 0.05° |
| Real-time heave | Marine version, wave period 0 to 20 s | 5 cm or 5%, whichever is greater |
Offshore, with no RTK correction stream, work with the single-point figures.
Why the D rather than the N or the A
The single-antenna Ellipse-N initialises its heading from motion (at least 3 m/s) or from its magnetometer, and SBG considers a boat’s dynamics insufficient for the first option. The Ellipse-A (AHRS) has a magnetic heading: 0.8° static, after calibration, in a clean magnetic environment. Only the Marine version of each model (±8 g, ±450°/s) outputs heave.
Against a magnetic compass and a satellite compass
A magnetic compass, fluxgate or 9-axis, measures the local field. Ferrous masses and current-carrying cables deflect it; calibration removes the fixed part of that deviation, not a circuit that starts drawing current. Its heading is magnetic and needs variation applied to become true. nke points out another limit of the classic fluxgate: its heading is not corrected for the boat’s accelerations, so it is filtered, at the cost of responsiveness.
A one-piece satellite compass removes the magnetic problem, but its antennas share a single housing: its baseline is short by design. The Ellipse-D lets you choose the antenna separation, and the quoted accuracy depends directly on it.
| Technology | Example | Heading | Quoted heading accuracy | Quoted roll and pitch | Output |
|---|---|---|---|---|---|
| 9-axis compass | B&G Precision-9 | magnetic | ±2° after calibration | output, accuracy not published | NMEA 2000 |
| 9-axis compass | nke Compas 9X | magnetic | 2° dynamic, after calibration | 1° dynamic | nke Topline bus |
| One-piece satellite compass, 35 cm long | Simrad HS75 | true | 0.75° RMS | output, accuracy not published | NMEA 2000 |
| Dual-antenna INS | SBG Ellipse-D | true | 0.2° RMS (baseline > 2 m) | 0.1° RMS | NMEA 0183, binary; NMEA 2000 on the CAN variant |
Manufacturer figures under non-uniform conditions (RMS for SBG and Simrad, “after calibration” or “dynamic” for B&G and nke): orders of magnitude, not a ranking. The Precision-9 and the HS75 also output heave, with no published accuracy; SBG publishes its own.
Why offshore racing programmes are interested
True wind direction is heading plus true wind angle: one degree of heading error becomes one degree of error in wind direction, and so in the shifts you think you are reading. Current is computed by comparing GNSS course and speed over ground with heading and boat speed through the water: heading error ends up there too.
Attitude matters just as much. nke gives a figure for the wind sensor: without heel correction, 2.5° of true wind angle error at 25° of heel. An autopilot in compass mode receives a heading that depends neither on the magnetic field nor on fluxgate filtering. Angular rates, accelerations and standard deviations also feed performance analysis.
We found no public reference to an offshore racing team using an Ellipse-D (September 2026); the closest SBG case study covers a solar-powered foiling boat built by EPFL students, fitted with an Ellipse-N. These arguments are technical, not references.
Installation and autopilot integration
Installation
The unit (65 g, 5 to 36 V DC supply) mounts rigidly anywhere in the boat, under cover: SBG states that IP68 does not make it proof against salt-water corrosion. The two antennas: same model, cables of identical length, same view of the sky, and a baseline of at least 2 m, as SBG recommends. Lever arms (from the unit to each antenna) are measured to the centimetre, or to within 10 cm before SBG’s calibration tool refines them. Configuration is done in the sbgCenter software over the USB cable, which also powers the unit. After the fix and the heading solution, SBG allows 2 to 15 minutes of alignment, sped up by figure-of-eight manoeuvres a few tens of metres across: power the electronics up before leaving the dock.
The Ellipse-D is sold on quotation, not as a direct online purchase. Our Ellipse-D kit pairs the Marine version with a serial interface and two TW3972 antennas recommended by SBG; the unit on its own is also available. Variant and antenna cables are specified with our workshop, boat by boat. The range is grouped on our SBG Systems page.
Three routes to the processor
| Output | What goes through | Condition |
|---|---|---|
| NMEA 0183 (version 4.1), RS-232 or RS-422 | HDT (true heading), ROT, GGA, RMC, VTG…; roll, pitch and heave only in proprietary sentences (PASHR, PRDID, SBG sentences) | the processor must read these sentences, at the right baud rate |
| NMEA 2000 on the CAN port | heading, rate of turn, attitude, heave, rapid position and COG/SOG, time | CAN variant only; bus disabled by default |
| sbgECom binary, or CAN messages described in DBC files | the most complete data set, with status flags and standard deviations | software able to decode it; C library published under the MIT licence |
The stand-alone unit in our catalogue is SBG P/N 100-2422 (ELLIPSE-D-G4A2-B1): Marine version without CAN, main port in RS-232 or RS-422 depending on the cable. The CAN variant (100-2423) adds NMEA 2000 but limits that port to RS-232. The choice is made before ordering.
H5000, nke, MADBrain: what is documented
None of the B&G, nke or Madintec documents we consulted mentions SBG. B&G describes its H5000 Serial Expansion module as intended, among other things, for “3rd party heading sensors” over NMEA 0183, with configurable sentences and baud rate. nke’s Box N2K links the Topline bus, NMEA 0183 (4,800 to 38,400 baud) and NMEA 2000; its page does not list the sentences it passes on. Madintec’s brochure describes an in-house IMU, MADMotion (MAD Sensor Motion in our catalogue), on its own CAN bus, and lists off-the-shelf NMEA 2000 compasses including the Precision-9 (see our MADBrain architecture article). In all three cases the connection relies on a standard protocol: what decides is the list of sentences or PGNs the processor accepts, checked on the bench before launch.
Limits and pitfalls
- The baseline is decided on the deck plan. With 0.5 m between antennas, quoted heading accuracy drops to 0.5°: better than a magnetic compass, close to a one-piece satellite compass. Both antennas must see the same sky; SBG advises against placing them either side of a structure that masks part of it. On a sailing boat, mast, sails and heel all come into play.
- No good GNSS signal, no guaranteed heading. When the standard deviation exceeds the configured threshold, SBG leaves the relevant NMEA fields empty: the autopilot must switch to another source. Keep a calibrated magnetic compass on board.
- Heave: Marine version only. SBG reserves heave messages (PASHR, TSS1, Ship Motion) for Marine hardware, and its accuracy assumes velocity aiding or, failing that, no turns and no speed changes.
- NMEA 0183 saturates quickly. Too many high-rate sentences for the chosen baud rate saturate the port, a cause SBG cites when the unit stops responding. Its GGA sentence is also longer than the standard allows (extra decimals): check that the receiving device accepts it.
- Set-up is a technician’s job. Axes, lever arms, motion profile, outputs and validity thresholds are set in sbgCenter. For residual misalignment, SBG suggests averaging roll and pitch over a long period in harbour and entering them as misalignment angles: the boat’s trim at the dock becomes zero.
Workshop verdict
Fast cruising, off-the-shelf autopilot already fitted. No Ellipse-D. A well-calibrated 9-axis compass from your autopilot’s brand (Precision-9 for B&G, Compas 9X for nke) is enough; if the magnetic environment is poor, an NMEA 2000 satellite compass such as the HS75 solves it more simply.
Offshore racing on B&G or nke instruments, reliable true heading needed. The satellite compass remains the simple answer. The Ellipse-D is justified if you can spread the antennas 2 m apart and if your processor’s reading of its sentences is validated before the season.
Open processor, data acquisition, performance analysis. This is its home ground: high output rates, standard deviations, a documented binary protocol, raw GNSS data for post-processing. Pick the serial or CAN variant to suit the boat’s network.
Motion only. If heading already comes from a satellite compass, the Ellipse-A Marine (AHRS, on quotation) provides roll, pitch and heave.
Who it is not for. Anyone who wants a sensor that plugs straight into the backbone, has no room to separate two antennas, or has nobody to keep the configuration up to date. For a quotation or an installation study, go through our workshop services: survey, installation and commissioning.
Sources
Manufacturer documents consulted on 25 September 2026.
- SBG Systems: Ellipse-D and Ellipse-A product pages, Ellipse-N/D datasheet (MK068EN v1.1, April 2026), Swiss Solar Boat case study
- SBG Support Center: performance specifications (firmware 3.x), interfaces, hardware codes, marine applications, FAQ, antennas, cables
- SBG sbgECom 5.8: introduction, NMEA 0183 messages, NMEA 2000 messages, third-party formats
- B&G: H5000 Serial Expansion, Precision-9
- nke: Compas 9X, Box N2K
- Simrad: HS75 GNSS Compass
- Madintec: MADBrain AutoPilot, MADBrain brochure (French)
