In astrodynamics, orbit phasing is the adjustment of the time-position of spacecraft along its orbit, usually described as adjusting the orbiting spacecraft's true anomaly. Orbital phasing is primarily used in scenarios where a spacecraft in a given orbit must be moved to a different location within the same orbit. The change in position within the orbit is usually defined as the phase angle, ϕ, and is the change in true anomaly required between the spacecraft's current position to the final position.
The phase angle can be converted in terms of time using Kepler's Equation: where This time derived from the phase angle is the required time the spacecraft must gain or lose to be located at the final position within the orbit. To gain or lose this time, the spacecraft must be subjected to a simple two-impulse Hohmann transfer which takes the spacecraft away from, and then back to, its original orbit. The first impulse to change the spacecraft's orbit is performed at a specific point in the original orbit, usually performed in the original orbit's periapsis or apoapsis. The impulse creates a new orbit called the “phasing orbit” and is larger or smaller than the original orbit resulting in a different period time than the original orbit. The difference in period time between the original and phasing orbits will be equal to the time converted from the phase angle. Once one period of the phasing orbit is complete, the spacecraft will return to the POI and the spacecraft will once again be subjected to a second impulse, equal and opposite to the first impulse, to return it to the original orbit. When complete, the spacecraft will be in the targeted final position within the original obit. To find some of the phasing orbital parameters, first one must find the required period time of the phasing orbit using the following equation. where Once phasing orbit period is determined, the phasing orbit semimajor axis can be derived from the period formula: where From the semimajor axis, the phase orbit apogee and perigee can be calculated: where Finally, the phasing orbit's angular momentum can be found from the equation: where To find the impulse required to change the spacecraft from its original orbit to the phasing orbit, the change of spacecraft velocity,∆V, at POI must be calculated from the angular momentum formula: where Remember that this change in velocity, ∆V,is only the amount required to change the spacecraft from its original orbit to the phasing orbit. A second change in velocity equal to the magnitude but opposite in direction of the first must be done after the spacecraft travels one phase orbit period to return the spacecraft from the phasing orbit to the original orbit. Total change of velocity required for the phasing maneuver is equal to two times ∆V. Orbit phasing can also be referenced as co-orbital rendezvous like a successful approach to a space station in a docking maneuver. Here, two spacecraft on the same orbit but at different true anomalies rendezvous by either one or both of the spacecrafts entering phasing orbits which cause them to return to their original orbit at the same true anomaly at the same time. Phasing maneuvers are also commonly employed by geosynchronous satellites, either to conduct station-keeping maneuvers to maintain their orbit above a specific longitude, or to change longitude altogether.
