Geodesy module¶
Geodesy is the science of accurately measuring and understanding the Earth's geometric shape, orientation in space, and gravity field (NOAA).
Shape models are required to compute position in geodetic coordinates (ellipsoidal latitude and longitude). Gravity must be compensated in accelerometer sensor readings in order to calculate total acceleration. Earth's rotation must be compensated in gyroscope measurements and induces apparent forces (centripetal and Coriolis forces) that must be compensated in accelerometer measurements also.
The inertialsim.geodesy module provides classes to support these operations.
Geodetic datums¶
A datum describes a coordinate system (origin, direction of axes, etc). A geodetic datum describes a coordinate system with respect to Earth. By default InertialSim uses the World Geodetic System 1984 as detailed in Standard [07]. It is implemented in WGS84 which is of type GeodeticDatum. Advanced users can specify alternative datums but note that InertialSim does not support converting coordinates between different geodetic datums.
Gravity models¶
InertialSim supplies three gravity models.
The first, standard_magnitude
is a constant scalar value of 9.80665 m/s/s. This is approximately the value
of gravity at sea level and 45 degrees latitude and is internationally
standardized. It is appropriate for use when converting units of g or if a
single approximation is adequate for the application. See Standard
[01].
The second, standard simply returns the
standard magnitude in local coordinates (zero horizontal components, 9.80665
m/s/s in the vertical component).
The third is an exact ellipsoidal model. This model is exact in the sense that it is error free under the assumption of an ellipsoidal Earth for all altitudes on and above the ellipsoid surface (sea level). It is the state of the art closed form, analytical model (Standard [07]). An approximation is supplied (Reference [01]) extending it to altitudes below the ellipsoid surface. This exact ellipsoidal model includes the effect of Earth's rotation which induces a centripetal force. It can be returned in any common coordinate system (local, ECEF, spherical, etc.).
Other datums¶
InertialSim also supplies TopocentricDatum, VehicleDatum, and LocalDatum classes.
TopocentricDatum describes a coordinate system fixed to the Earth at a given
set of geodetic coordinates (latitude, longitude, altitude). It supports the
common axis orderings NED (North, East, Down, default) and ENU (East, North,
Up).
VehicleDatum describes a coordinate system fixed to a vehicle. It supports
the standard FRD (forward, right, down), and FLU (forward, left, up) axis
orderings. See Standard [02].
LocalDatum can be extended to describe any axis ordering that might be
relevant to a user's application.