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Coordinate Frames

In this manual, coordinate frame systems are simply referred to as frames. This page is to assist the developer in choosing and implementing the appropriate coordinate frames for their respective application. It should be noted that the following frames are in relation to the uINS itself.

Sensor Frame

IMU, magnetometer, and INS velocity data are in the Sensor Coordinate Frame, or Sensor Frame, and are identified by the X, Y, and Z axes labeled on the hardware. The z-axis is positive down into the image.

coordinate_frames

INS Output Frame

The INS output data (DID_INS_1, DID_INS_2, DID_INS_3) is in the INS Output Frame which is the same as the Sensor Frame after default rotations and offsets are entered to the Flash Configuration. Translation from Sensor Frame to INS Output Frame is defined as:

  1. Sensor Frame → Intermediate Output Frame by rotation of DID_FLASH_CONFIG.insRotation euler angles (in order of heading, pitch, roll angle) In radians.
  2. Intermediate Output Frame → INS Output Frame: Offset by DID_FLASH_CONFIG.insOffset in meters.

If DID_FLASH_CONFIG.insRotation and DID_FLASH_CONFIG.insOffset are zero, the Sensor Frame and the INS Output Frame are the same.

North-East-Down (NED) Frame

Position estimates can be output in the North-East-Down (NED) coordinate frame defined as follows:

* Right-handed, Cartesian, non-inertial, geodetic frame with origin located at the surface of Earth (WGS84 ellipsoid). * Positive X-axis points towards North, tangent to WGS84 ellipsoid. * Positive Y-axis points towards East, tangent to WGS84 ellipsoid. * Positive Z-axis points down into the ground completing the right-handed system.

Earth-Centered Earth-Fixed (ECEF) Frame

The Earth-Centered Earth-Fixed (ECEF) frame is defined as follows:

* Right-handed, Cartesian, non-inertial frame with origin located at the center of Earth. * Fixed to and rotates with Earth. * Positive X-axis aligns with the WGS84 X-axis, which aligns with the International Earth Rotation and Reference Systems Service (IERS) Prime Meridian. * Positive Z-axis aligns with the WGS84 Z-axis, which aligns with the IERS Reference Pole (IRP) that points towards the North Pole. * Positive Y-axis aligns with the WGS84 Y-axis, completing the right-handed system.

ECEF_coordinate_frames

Coordinate Frames Transformation Functions

This section is intended to be an example of how to rotate between frames using utility functions defined in the InertialSenseSDK.

Body frame to NED frame

The following example converts body velocity DID_INS_2.uvw to NED velocity vel_ned.

#include "SDK/src/ISPose.h"
quatRot( vel_ned, DID_INS_2.qn2b, DID_INS_2.uvw );

This following example removes gravity from the IMU measured acceleration.

#include "SDK/src/ISPose.h"
Vector gravityNED = { 0, 0, -9.80665 }; // m/s^2
Vector gravityBody;
Vector accMinusGravity;
// Rotate gravity into body frame
quatConjRot( gravityBody, DID_INS_2.qn2b, gravityNED );
// Subtract gravity from IMU acceleration output
sub_Vec3_Vec3( accMinusGravity, DID_DUAL_IMU.I[0].acc, gravityBody );

ECEF frame to NED frame

This example converts ECEF velocity vel_ecef to NED velocity vel_ned.

#include "SDC/src/ISPose.h"
quat_ecef2ned( lla[0], lla[1], qe2n );
quatConj( qn2e, qe2n );
quatRot( vel_ned, qn2e, vel_ecef );