Document MT0605P.E
© Xsens Technologies B.V.
MTi User Manual
28
describing the Earth. The standard physical model of the Earth used for GPS applications is the World
Geodetic System 1984 (WGS84). WGS84 is also what is used in the MTi-G.
WGS84 provides an ellipsoidal model of the Earth’s shape, as well as Earth’s gravitational
irregularities. Major parameters are the semi-major axis
a
(=6,378,137 m) and the semi-minor axis
b
(=6,356,752 m), see figures below. There are several local models (datums) which will increase local
accuracy using modified
a
and
b
and shift parameters (x, y, z) of the origin. However, if a selected
datum is used beyond its ‘borders’, accuracy will deteriorate fast. The MTi-G uses the default WGS84
model and not a specific datum.
Earth Centered Earth Fixed
– ECEF
WGS-84 parameters:
a = 6,378,317 meter
b = 6,356,752 meter
Figure 5: Earth-Centered Earth-Fixed Coordinate System
Spherical coordinates - LLA:
λ = longitude
φ = latitude
h = altitude
Figure 6: Definition of Ellipsoidal Coordinates (Latitude, Longitude, Altitude) in WGS-84 Ellipsoid
In order to combine the output of the GPS receiver with the IMU, both systems have to be converted to
an appropriate coordinate system. Inertial sensors measure properties in the ECI system. However,
the MEMS inertial sensors used in the MTi-
G are not accurate enough to measure the Earth’s rotation
rate or the transport rate of the MTi-G over the curved Earth surface if the MTi-G has a velocity.
Therefore, we can work with a local linearized tangent plane without making any significant errors.
