Coordinates reference frames for Orbital motion
The relative position of moving objects in space is determined by usage of coordinate reference frames that compare the position of the object with respect to earth. This concept helps in defining the equations of motion for orbital movement of celestial objects in space. The extension of the same concept is used to determine the relative position of a spacecraft during the course of its ascension into the space using the earth centered system. The position of a spacecraft in its ascending node can be determined by using an equatorial vector that ascertains its relative position with respect to the centre of the earth (Erickson 2010, 193). The Geocentric-equatorial system is a similar reference system which provides the relative position of a spacecraft when it passes from the earth’s equatorial plane from south to north.
Geocentric-equatorial system and its inertial nature
The Geocentric-equatorial system is an earth centered inertial system which uses a reference system that is not assumed to rotate along with earth but its origin is the earth’s center. So, as the earth’s center revolves around the sun during its revolutionary motion, the Geocentric-equatorial system is not truly inertial in nature. Still, there are some specific practical uses which make it useful for a spacecraft to use the coordinate reference system.
Why the Geocentric-equatorial system is used for the spacecraft?
The spacecraft move in the space orbits and the use of earth-centered orbits makes it difficult to measure the position of any celestial object as the earth’s centre itself is not steady. Hence, the concept of Geocentric-equatorial position vector comes into picture which uses a different coordinate system based on earth’s axis and helps in estimating the coordinates and velocity of the spacecraft for a given point of time, known as the epoch (Erickson 2010, 193). This estimate of velocity and position is accurate enough to be used for all the position and velocity estimation for a spacecraft.
Erickson, Lance, 2010. “Space Flight: History, Technology, and Operations.”Government Institutes .1st ed. Plymouth: The Rowman and Littlefield Publishing group.