Output Specifications

• Overview of data outputs
  • MTData2 output in XBus protocol
  • ASCII output (NMEA)
• Coordinate systems
  • Calibrated inertial data and magnetic field data
  • Delta_angle and delta_velocity
  • Orientation data
  • Velocity data
  • Position data
• Orientation performance specification
• Position and velocity performance specification (MTi-G-710)
• Sensor data performance specifications
  • Gyroscopes
  • Accelerometers and magnetometer
  • Barometer
  • GNSS receiver
• Built-in self-test
• Test and Calibration parameters
• Sensor data outputs
  • Physical sensor model
  • Calibrated delta_q and delta_v outputs
  • Calibrated inertial and magnetic data outputs
  • High-rate (HR) inertial data outputs
  • Free acceleration
  • Uncalibrated raw output mode
• Reset of reference co-ordinate systems
• Timestamp and packet counter output
• Status byte

In this chapter, the various output modes of the MTi are described. The MTi’s have tens of different output options; it is possible to select a different output frequency and/or output format (e.g. float or double) per output or group of outputs. A full overview of outputs can be found in the MT Low Level Communication Protocol Document.

Overview of data outputs

The MTi supports two different data protocols: the binary (hexadecimal) XBus protocol and NMEA. Refer to the MT Low Level Communication Protocol Document to learn how to switch between data protocols.

MTData2 output in XBus protocol

The MT Low Level Communication Protocol Document contains a full list of all data outputs. Refer to the table in the MTData2 message description. 

ASCII output (NMEA)

The MTi also supports a variety of strings in ASCII, amongst others, messages in the NMEA protocol. The list of available ASCII strings can be found in the the MT Low Level Communication Protocol Document, in the table in the description of the SetStringOutputType message.

Coordinate systems

Data from the MTi is represented in various coordinate systems, which are explained below.

Calibrated inertial data and magnetic field data

The sensor coordinate system (S) is a right-handed coordinate Cartesian system that is body-fixed to the device. It is possible to rotate the sensor coordinate system to an object coordinate system (O) when the MTi is not exactly aligned with the axes of the object or vehicle the MTi is attached to. See #Reset of reference co-ordinate systems for more information on alignment matrices.

(S) and (O, when applied) are used in the rate-of-turn (DataID 0x8020), acceleration (DataID 0x4020) and magnetic field (DataID 0xC020) outputs. The encased version of the MTi shows the coordinate system on the sticker. Depicted below is the sensor coordinate system on the encased MTi and the OEM version. Small x, y and z are used for (S) and the object coordinate system (O). Capital X, Y and Z are generally, but not always, used for velocity. They stand for the local-earth fixed coordinate system (L), see section #Orientation data.

Coordinate system of the encased and OEM MTi (Note: origin is located at the accelerometers)

The aluminum base plate of the MTi is carefully aligned with the output coordinate system during the individual factory calibration. The alignment of the bottom plane and sides of the aluminum base-plate with respect to the sensor-fixed output coordinate system (S) is within 0.1 deg. Convenient alignment points are added to the base plate of the MTi. 

The non-orthogonality between the axes of the body-fixed co-ordinate system, (S), is <0.05°. This also means that the output of 3D linear acceleration, 3D rate of turn (gyro) and 3D magnetic field data will all have orthogonal xyz readings within <0.05° as defined in the figure above.

Delta_angle and delta_velocity

The SDI output of the MTi includes delta_angle (dq, DataID 0x8030) and delta_velocity (dv, DataID 0x4010). These values represent the orientation change and velocity change during a certain interval. In the MTi, this interval is 2.5 ms (400 Hz) by default. The values dq and dv are always represented in the same coordinate system as calibrated inertial data and magnetic field data (see previous section), which can be (S) or (O).

Orientation data

By default, the local earth-fixed reference coordinate system (L) used is defined as a right-handed Cartesian co-ordinate system with[1]:

• X positive to the East (E).
• Y positive to the North (N). 
• Z positive when pointing up (U).

This coordinate system is known as ENU and is the standard in inertial navigation for aviation and geodetic applications. Note that it is possible to change the local coordinate system (L) using a different convention (NWU or NED), by changing the alignment matrix or applying an orientation reset.

The 3D orientation output (DataID 0x2010, 0x2020, 0x2030) is defined as the orientation between the body-fixed co-ordinate system, (S) or (O), and the earth-fixed co-ordinate system, (L).

Orientation output modes

The output orientation can be presented in different parameterizations:

• Unit Quaternions
• Euler angles[2]: roll, pitch, yaw (XYZ Earth fixed type) are output following the aerospace sequence (Z-Y’-X”)
• Rotation Matrix (directional cosine matrix)

A positive rotation is always “right-handed”, i.e. defined according to the right-hand rule (corkscrew rule). This means a positive rotation is defined as clockwise in the direction of the axis of rotation.

Refer to BASE to learn more about how quaternions, Euler angles and the rotation matrix relate to each other.

Interpretation of yaw as heading

Heading is defined as the angle between north and horizontal projection of the vehicle roll axis. Heading is positive about the local vertical axis following the right-hand rule. For a local level navigation frame, yaw is the angle from a horizontal navigation axis to the projection of the longitudinal axis in the horizontal plane; positive is about the positive vertical axis following the right-handrule[3].

With the default ENU (L) coordinate system, the Xsens yaw output is defined as the angle between East (X) and the horizontal projection of the sensor roll axis (x), positive about the local vertical axis (Z) following the right-hand rule. The table below shows the different yaw values corresponding to the different local coordinate systems that are available for the MTi.

Yaw in different coordinate systems (applies only to VRU/AHT, AHRS and GNSS/INS product types). The MTi is assumed to be mounted with its roll-axis (X) aligned with the roll-axis of the vehicle (front of the vehicle).

When using the ENU convention (default), the yaw output is 0º when the vehicle (x-axis of the MTi) is pointing East. When it is required that the yaw output is 0º when the vehicle (x-axis of the MTi) is pointing north, it is recommended to select NWU or NED as the local coordinate system. In section #Reset of reference co-ordinate systems the various alignment resets are described.

When using the MTi-G-710 in the Automotive filter profile, as best practice, pay proper attention to mounting of the MTi on the automotive platform/vehicle. It is recommended to always mount the MTi with the x-axis of the MTi-G-710 pointing to the front of the vehicle, irrespective of the local coordinate frame used for the output data.

True North vs. Magnetic North

As defined above, the output coordinate system of the MTi is with respect to the local Magnetic North. The deviation between Magnetic North and True North (known as the magnetic declination) varies depending on the location on earth and can be roughly obtained from the World Magnetic Model of the earth’s magnetic field as a function of latitude and longitude. The MTi accepts a setting of the declination value. This is done by setting the position in the MT Manager, SDK or by direct communication with the sensor. The output will then be offset by the declination calculated internally and thus referenced to “local” True North. The MTi-G-710 GNSS/INS calculates and applies the local magnetic declination automatically when the GNSS position is available.

Velocity data

Velocity data, calculated by the sensor fusion algorithm (DataID 0xD010) is provided in the same coordinate system as the orientation data, and thus adopts orientation resets as well. It is available only on the MTi-G-710.

Velocity data from the navigation solution from the GPS receiver in the MTi-G-700 (DataID 0x8840) is represented in Earth Centered – Earth Fixed (ECEF). 

Velocity data part from the PVT estimates retrieved from the GNSS receiver of the MTi-G-710 in the GNSS format (DataID 0x7010) is represented in the NED reference frame.

Position data

Position data, calculated by the sensor fusion algorithm (DataID 0x5040) is represented in Latitude and Longitude in the WGS84 datum. It is available only on the MTi-G-710.

It is possible to retrieve position data, calculated by sensor fusion algorithm, in Earth Centered – Earth Fixed (ECEF) format. Use DataID 0x5030 to retrieve this output. Note that a position in ECEF cannot be represented in Fixed Point values because of the limited range of fixed point representations. Use double or float representation instead.

Altitude is output in WGS84 datum (DataID 0x5020).

Position data (latitude and longitude), part of the PVT output from the GNSS receiver in the MTi-G-710 (DataID 0x7010) is in the WGS84 datum.

Orientation performance specification

In the MTi product portfolio, several products provide roll, pitch and (un)stabilized yaw. The table below provides an overview of the specific products and orientation performances. The MTi-10 IMU and the MTi-100 IMU are not listed, as they do not provide orientation.

Orientation performance specification

All specifications listed in the above table are based on typical application scenarios.

The performance specifications in this chapter are subject to the following assumptions (see also footnotes);

• Correct filter profile: If an unsuitable filter profile is chosen, the sensor fusion algorithm will use erroneous assumptions. Depending on the specific situation, this can lead to large errors or even instability of the filter. Take care to select the correct filter profile. If you are uncertain, do not hesitate to contact Xsens or your local distributor.
• Magnetic distortions: In several filter profiles, the on-board 3D magnetometer is used to observe the heading. It is assumed that a magnetic field mapping is performed and that the magnetometers are not distorted by nearby ferromagnetic materials in the environment.
• Long-lasting accelerations (all, except for MTi-G-710): Each filter profile copes differently with long-lasting accelerations. Although the MTi’s are able to discard these transient accelerations, they will have an effect after a certain period of time. In the MTi 10-series, this period is shorter than in the MTi-100 series. The MTi-G-710 with a good GNSS fix can compensate for these long-lasting accelerations.
• Settling time: Parameters in the MTi’s such as the rate gyro bias or the pressure at sea level are continuously estimated. The sensor fusion algorithms need some time to estimate such parameters. Depending on the time since the last calibration of the MTi’s, the chosen filter profile and the quality of the GNSS fix, this can take up to 5 minutes. There is no need to have the MTi static during the settling time. On the contrary, movement (accelerations, turns) of the MTi’s will help to estimate for example the gyro bias quicker as they become observable.
• Good GNSS fix (MTi-G-710 only):
  • If there is no GNSS fix, the position estimates based on the IMU will degrade very rapidly (in the order of a few seconds).
  • If there is no GNSS fix, the MTi-G cannot calculate the correct attitude and heading under all dynamic conditions due to “apparent gravity” (e.g. centripetal and linear accelerations). Based on the rate gyros the attitude and heading estimates will degrade slowly. Attitude accuracy will be the same as the accuracy of an MTi-300. The heading will typically drift up to a degree every minute.
• Valid holonomic constraints (MTi-G-710 only): In the Automotive filter profiles of the MTi-G-710 it is assumed that the x-axis of the MTi-G is mounted in the driving direction of the vehicle. Accuracy of the heading estimate improves as the alignment error decreases and vice-versa.
• Barometric conditions (MTi 100-series only): In some of the filter profiles, the altitude of the MTi-G-710 is partly estimated by the on-board barometer. It is assumed that the pressure near the MTi-G reflects the atmospheric pressure outside of the vehicle. So, if you select a filter profile that uses barometer data, the MTi-G cannot be used in e.g. a pressurized cabin. Furthermore, the height accuracy may reduce with very rapid changes in atmospheric pressure that could occur in e.g. severe thunderstorms.

Position and velocity performance specification (MTi-G-710)

The MTi-G-710 has the ability to give position and velocity output. The table below states the position and velocity accuracy according to Xsens’ reference trajectories.

Position and velocity performance specifications (MTi-G-710)

All specifications listed in the above table are based on typical application scenarios.

Sensor data performance specifications

This section describes the specifications of the physical sensors of the MTi’s. Not all MTi’s feature all sensors. Per sensor, the applicable MTi’s are mentioned.

Gyroscopes

The main difference between the MTi 10-series and the MTi 100-series is the type of gyroscopes used. The two different specifications are listed below. A full range of 1000 º/s is available upon request.

Gyroscopes in MTi 10-series: MTi-10, MTi-20, MTi-30

Gyroscopes in MTi 100-series: MTi-100, MTi-200, MTi-300, MTi-G-710

Gyroscope specifications

Accelerometers and magnetometer

The MTi 10-series and MTi 100-series use the same accelerometers and magnetometer. The output of the magnetometer is in arbitrary units (a.u.), one a.u. is the magnetic field strength during calibration at Xsens’ calibration lab. This is approximately 49 µT.

Accelerometers/magnetometer: all products: MTi-10, MTi-20, MTi-30, MTi-100, MTi-200, MTi-300, MTi-G-710.

Accelerometer specifications

Magnetometer specifications

Barometer

The barometer measures barometric (atmospheric) pressure. The MTi-100 series features this barometer. The MTi 100-series has three holes with a protective vent in its casing in order to ensure fast adaptation inside the MTi to atmospheric pressure. Typical latency because of the vent is <10 ms.

Barometer: MTi 100-series only: MTi-100, MTi-200, MTi-300, MTi-G-710

Barometer specification

GNSS receiver

The MTi-G-710 is the only MTi that features a GNSS receiver. It requires an active antenna, which is delivered with the Development Kit and can be ordered separately from Xsens as well. It is possible to use a different antenna that better suits your application.

GNSS receiver specification

Note that when you are not using the default GNSS antenna, it is important to use an antenna that is suitable for the MTi-G-710. Refer to BASE for more information.

Built-in self-test

All MTi’s feature a built-in self-test (BIT). The self-test actuates the mechanical structures in the MEMS accelerometer and gyroscope by inducing an electric signal. This allows checking the proper functioning of the mechanical structures in the MEMS inertial sensors as well as the signal processing circuitry. For the magnetometer, the self-test checks the integrity of the sensor component.

A passed self-test will result in a valid self-test flag in the status byte. Because the self-test influences the sensor data, the self-test is only available in Config mode. For more information, refer to the MT Low Level Communication Protocol Documentation, function RunSelftest.

Test and Calibration parameters

Every MTi is calibrated and tested for calibration residuals that correspond to the specified performance of the MTi. This way, a device that leaves our facility has the same high quality that can be expected from Xsens.

The MTi applies calibration parameters internally and each MTi therefore contains individual test and calibration data in its eMTS (electronic Motion Tracker settings). It is digitally signed by a Test Person and states the calibration values determined during the calibration of the MTi at Xsens’ calibration facilities. For reference, the values can be read by connecting the MTi to MT Manager and navigating to Device Settings – Modeling Parameters. It is also supplied as a hardcopy with the MTi. The values are explained here in short:

“Modeling Parameters” are the values that describe the conversion from the physical phenomenon to a digital output in an orthogonal coordinate system:

Offsets (bits): Digital reading in bits of the sensor when no physical signal is measured. The barometer is a digital sensor and does not require calibration, hence the value is always 0. 

Gain (bits): Gains (or scale factor) describe the relation between the digital reading in bits and the measured physical signal. The barometer is a digital sensor and does not require calibration, hence the value is always 0. 

Alignment matrix:  Non-orthogonality of the sensor triad. This includes non-orthogonality in the orientation of the sensitive system inside the MEMS sensor, the mounting of the sensors on the PCB of the MTi, the mounting of the PCB’s and the misalignment of the OEM board in the MTi housing.

Next to the calibration values shown in MT Manager, each device is calibrated according to more complicated models to ensure accuracy (e.g. non-linear temperature effects and cross-coupling between acceleration and angular rate[5]).

Sensor data outputs

Physical sensor model

This section explains the basics of the individual calibration parameters of each MTi. This explains the Calibration parameters in Device Settings (see also #Test and Calibration parameters).

The physical sensors inside the MTi (accelerometers, gyroscopes and magnetometers) are all calibrated according to a physical model of the response of the sensors to various physical quantities, e.g. temperature. The barometer and GNSS receiver do not require calibration. The basic model is linear and according to the following relationship:

(s=K_T^{-1}(u-b_T)) 

During factory calibration, each MTi has been assigned a unique gain matrix, (K_T) and the bias vector, (b_T) This calibration data is used to relate the sampled digital voltages, (u), (unsigned integers from the 16 bit ADC’s) from the sensors to the respective physical quantity, (s).

The gain matrix is split into a misalignment matrix, (A), and a gain matrix, (G). The misalignment specifies the direction of the sensitive axes with respect to the ribs of the sensor-fixed coordinate system (S) housing. E.g. the first accelerometer misalignment matrix element (a_{1,x}) describes the sensitive direction of the accelerometer on channel one. The three sensitive directions are used to form the misalignment matrix:

(A= begin{bmatrix} a_{1,x} & a_{1,y} & a_{1,z} \ a_{2,x} & a_{2,y} & a_{2,z} \ a_{3,x} & a_{3,y} & a_{3,z} end{bmatrix})

(G= begin{bmatrix} G_{1} & 0 & 0 \ 0 & G_{2} & 0 \ 0 & 0 & G_{3} end{bmatrix})

(K_T= begin{bmatrix} G_{1} & 0 & 0 \ 0 & G_{2} & 0 \ 0 & 0 & G_{3} end{bmatrix} begin{bmatrix} a_{1,x} & a_{1,y} & a_{1,z} \ a_{2,x} & a_{2,y} & a_{2,z} \ a_{3,x} & a_{3,y} & a_{3,z} end{bmatrix} +O)

With (O) representing higher order models and temperature modelling, g-sensitivity corrections, etc.

Each individual MTi is modeled for temperature dependence of both gain and bias for all sensors and other effects. This modeling is not represented in the simple model in the above equations, but is implemented in the firmware with the temperature coefficient being determined individually for each MTi device during the calibration process. The basic indicative parameters in the above model of your individual MTi can be found on the “MT Test and Calibration Certificate” and in the MT Manager (Device Settings dialog).

Calibrated delta_q and delta_v outputs

This output is coning and sculling compensated strap-down integrated data in the sensor-fixed coordinate system (S) or (O). Note that the value of the output depends on the output frequency, as the values are integrated over a specific time. Delta_q can also be noted as dq, delta_angle, del_q or OriInc. Delta_v can also be noted as dv, delta_velocity, del_v or VelInc.

Output specifications ∆q and ∆v outputs

It is possible to multiply consecutive delta_q values to find the total orientation change over a specific period. Note that this data is not drift free, as it has not been processed by the sensor fusion filters. Use the orientation output for drift free orientation.

Calibrated inertial and magnetic data outputs

Output of calibrated 3D linear acceleration, 3D rate of turn (gyro) and 3D magnetic field data is in the sensor-fixed coordinate system (S) or (O). The units of the calibrated data output are as shown in Table 15.

Output specifications inertial and magnetometer data outputs

Calibrated data has been going through Strapdown Integration and Inverse Strapdown Integration.

High-rate (HR) inertial data outputs

High-rate calibrated 3D acceleration (accelerometer) and 3D rate of turn (gyroscope) are outputted in the sensor-fixed coordinate system (S) or (O). The units of the calibrated data output are as shown in Table 16. HR calibrated data is available at a higher rate than regular calibrated inertial data outputs. It is outputted as a separate data packet next to the other data outputs. The maximum output rate, degree of signal processing, and calibration applied depends on the device type.

Refer to MT Low Level Communication Protocol Documentation for more details.

Output specifications high-rate calibrated inertial data outputs

Free acceleration

Free acceleration (Data ID 0x4030) is the acceleration in the local frame from which the local gravity is deducted. Output is in m/s².

Uncalibrated raw output mode

The MTi can give a sensor component readout output (SCR, DataID 0xA010 for the sensor data and 0xA020 for the gyroscope temperatures), i.e. digitized voltages of all sensors, before they are filtered or calibrated using Xsens’ proprietary firmware and calibration parameters. These sensors are the gyroscopes (rate of turn), accelerometers (acceleration), magnetometer (magnetic field), barometer (static pressure) and temperatures (gyroscope temperatures and a general temperature sensor). When selecting the sensor component readout, the following outputs are available:

Output specifications Sensor Component Readout (SCR)

Note that the SCR-values of the gyroscopes and accelerometers are not calibrated for offset, gain, misalignment and temperature. Also, coning and sculling compensation is not applied. It is possible to post-process SCR data with MT Manager, and this output is very suitable if you need to perform your own calibration. Note that this output cannot be combined with any other output.

Reset of reference co-ordinate systems

In some situations, it may be that the MT sensor axes are not exactly aligned with the axes of the object of which the orientation has to be recorded. It may be desired to retrieve the orientation and/or calibrated inertial data in a different sensor-fixed frame (S’ instead of S) or a different earth-fixed local frame (L’ instead of L). Refer to BASE for more information on the available orientation resets.

Timestamp and packet counter output

Each data message can be accompanied by a packet counter and/or timestamp. Refer to the MT Low Level Communication Protocol Documentation for detailed information on the various time outputs.

Status byte

The status byte includes information about the status of the MTi, its sensors, the filter and user inputs. Refer to the MT Low Level Communication Protocol Documentation for detailed information on the Status Word output.