Before changing or correcting the trajectory of a spacecraft it is necessary to have determined the position and velocity of the vehicle relative to the Sun, the Earth or a planetary body, ie. navigation is a prerequisite for guidance. Since spacecraft are usually moving in a gravitational field due predominantly to one body such as the Earth or the Sun, the estimation of position and velocity is equivalent to the determination of the orbital parameters. Even when other gravitational sources are significant, we have seen that the use of osculating (varying) orbital elements is merely an alternative characterization to the use of the six components of position and velocity.
KeywordsOrbital Element Orbit Determination Orbital Parameter Bias Term Sequential Estimation
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