Document MT0605P.E
© Xsens Technologies B.V.
MTi User Manual
27
True North vs. Magnetic North
As defined above the output coordinate system of the MTi is with respect to 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
(Xsens uses WMM 2010-
2015) 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 an
d thus referenced to “local” True North. The MTi-G-700 GPS/INS
calculates True North automatically when GPS-position is available.
4.2.4
Velocity data
Velocity data, calculated by sensor fusion algorithm (DataID 0xD010) is outputted in the same
coordinate system as the orientation data, and thus adopts orientation resets as well. It is available
only in the MTi-G-700.
Velocity data from the navigation solution from the GPS receiver (DataID 0x8840) is represented in
Earth Centered
– Earth Fixed (ECEF).
4.2.5
Position data
Position data, calculated by the sensor fusion algorithm (DataID 0x5040) is represented in Latitude,
Longitude in the WGS84 datum. It is available only in the MTi-G-700.
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.
Altitude is outputted in WGS84 datum (DataID 0x5020).
Position data from the navigation solution from the GPS receiver (DataID 0x8840) is represented in
Earth Centered
– Earth Fixed (ECEF).
4.2.5.1
Linearization errors and relation WGS84 and LTP
The following section describes in more detail the (linearization) errors made and other artefacts
introduced when using global coordinate systems. It also describes the relation between WGS84
(LLA) and ENU (LTP). Most users will not experience these artefacts; neither should they need to take
these into account. This section is primarily in the User Manual for completeness and accuracy
reasons.
World coordinates WGS84 and LTP
Navigating around the world (f)using GPS and inertial sensors requires an understanding of the
Cartesian and Spherical coordinate systems commonly used for describing a position on the Earth.
For purposes of measuring and determining the orbits of the GPS satellites, it is convenient to use an
Earth Centered Inertial (ECI) coordinate system in which the origin is at the centre of the mass of the
Earth and which axes are pointing in fixed direction with respect to the stars. For the purpose of
computing the position of a GPS receiver, it is more convenient to use a coordinate system that
rotates with the Earth, known as an Earth Centered Earth Fixed (ECEF) system.
In the ECEF system, the xy-pla
ne coincides with the Earth’s equatorial plane, the x-axis points in the
direction of 0° longitude (Greenwich meridian) and the y-axis points in the direction of 90°E. It is typical
to transform these Cartesian coordinates to latitude, longitude and height (or altitude), which are often
projected on maps. In order to carry out this transformation, it is necessary to have a physical model
