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# What Is The Inertial Coordinate System?

In inertial navigation, the inertial coordinate system is a very important concept. According to the definition of Newtonian mechanics, it is an “absolute space” where the acceleration vector is always zero, or it is absolutely stationary, or it moves in a straight line at a uniform speed in the strict sense.

In the field of inertial technology, there are two commonly used inertial systems: one is the “heliocentric inertial system” with the sun center as the coordinate origin, and the three coordinate axes point to three stars; the other is the Geocentric Inertial System “, one coordinate axis is along the Earth’s polar axis (rotation axis), the other two axes are in the Earth’s equatorial plane, the three axes are orthogonal and point to three stars in space, respectively. . Please note that the geocentric inertial system is not fixed to the earth and does not rotate with the earth.

The heliocentric inertial system is mostly used in astronomical observations and space vehicles; the geocentric inertial system is mostly used in navigating bodies that move relative to the surface of the earth (for example, aircraft, missiles, ships, and tanks, etc.). 