﻿ GroundVehicle Attitude Reference Frames

# GroundVehicle Attitude Reference Frames

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The Attitude Reference Frame defines the coordinate system that the Ground Vehicle's attitude is referenced to. The reference frames available in FreeFlyer are described below. The AttitudeConvert function can be used to convert attitudes between the different systems. The GroundVehicle Attitude Reference Frames available in FreeFlyer are:

## MJ2000 Earth Equator

Description:

 •Z-axis: Vector normal to the mean equatorial plane at Julian year 2000.0, pointing towards the Northern Hemisphere •X-axis: Vector pointing from the center of the Earth to the mean vernal equinox at Julian year 2000.0 •Y-axis: Vector perpendicular to the x-axis and z-axis, forming a right-handed coordinate system •Origin: Center of the Earth

Notes:

 •This is the principal coordinate system used for computations in FreeFlyer •Commonly referred to as Mean of J2000, J2000, ECI, or GCI

Prior to setting the Ground Vehicle's attitude state, its attitude reference frame must be set:

 GroundVehicle1.AttitudeRefFrame = "MJ2000";

## VLT

Description:

 •X-axis: Vector pointing in the direction of the body-fixed velocity of the GroundVehicle •Y-axis: Vector perpendicular to the GroundVehicle's z-axis and x-axis, forming a right-handed coordinate system •Z-axis: Vector pointing in the direction normal to the reference ellipsoid at the central body fixed position of the GroundVehicle •Origin: Center of the GroundVehicle

Notes:

 •In the case that the VLT frame is not defined, it will fall back to the ENU frame. •When using terrain the Z-axis is defined as the normal to the terrain at the location of the GroundVehicle on the terrain.

Prior to setting the Ground Vehicle's attitude state, its attitude reference frame must be set:

 GroundVehicle1.AttitudeRefFrame = "VLT";

## ENU

Description:

 •X-axis (East): Vector pointing in the east direction of the Ground Vehicle's position vector •Y-axis (North): Vector perpendicular to the GroundVehicle's z-axis and x-axis, forming a right-handed coordinate system •Z-axis (Up): Vector pointing in the direction normal to the reference ellipsoid at the central body fixed position of the GroundVehicle •Origin: Center of the GroundVehicle

Notes:

 •In the case that the ENU frame is not defined (at the Poles), the frame will fall back to x pointing along the prime-meridian, z pointing along the body-fixed z-axis, and y completing the triad.

Prior to setting the Ground Vehicle's attitude state, its attitude reference frame must be set:

 GroundVehicle1.AttitudeRefFrame = "ENU";