Reference frames must be decided when calculating the attitude and position of the satellite for calculations. Below are a few standards of reference frames based on common fixed positions.
Earth Centered Inertial (ECI)
For an Earth Centred Inertial (ECI) reference frame, the Earth's centre is defined as (0,0,0) but does not rotate with Earth. For objects in orbit, it is much easier to represent positions and velocities in this non-moving frame, even if the earth continues to rotate.
This type of coordinate system is typically used by bodies in space as it is easier to represent them on a large 3D coordinate system.
Earth Centered Earth Fixed (ECEF)
Earth Centred, Earth Fixed (ECEF), also known as Earth Centred Rotational (ECR) reference frames represent positions at X,Y,Z with earth's centre defined as (0,0,0) This axis is aligned with the international reference pole:
- The Z-axis extends through true north
- This X-axis intersects the sphere of the earth at 0º latitude and longitude
This axis alignment has its advantages since the earth does not rotate about the Z-axis and wobbles, which is known as polar motion. ECEF rotates with the earth and thus coordinates of a fixed point on the surface of the earth do not change.
An example of fixed coordinates would be the north pole has coordinates of (0,0,6371km) where the layout is (x,y,z) and the earth's radius is 6371km. Earth Centred, Earth Fixed reference frames are typically used for ground stations or other earth-based locations since the coordinates referencing the station do no change as the earth rotates.