Coordinate Frames & Epoch – Lesson 3

All orbital elements need to be defined in a coordinate system and at an epoch (specified time).

Coordinate Systems:

Earth Centered Inertial (ECI) -

• Reference plane is Mean Equatorial Plane.
• X direction is fixed relative to the celestial sphere. The vernal equinox is used as the reference direction.
• Y direction is 90 degrees from the X axis within the equatorial plane.
• Z direction is pointed upward from the north pole.

Earth Centered, Earth Fixed (ECEF) -

• Reference plane is Mean Equatorial Plane.
• X direction is fixed at the zero meridian line (0° longitude).
• Y direction is 90 degrees from the X axis within the equatorial plane.
• Z direction is pointed upward from the north pole.

Epoch:

A reference time, or timestamp, in which time-varying celestial data is defined.

A commonly used epoch is J2000 - January 1, 2000 at 11:58:56 GMT

ECI vs ECEF – Orbital Tracks:

The ECI and ECEF coordinate frames both have their advantages and disadvantages.  When defining the orbital parameters of a satellite, the ECI coordinate frame is largely superior. The video to the right demonstrates why.