Orbit phasing

Phase Angle

In astrodynamics,orbit phasingis 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:

$$t={\frac {T_{1}}{2\pi }}(E-e_{1}\sin E)$$ $$E=2\arctan \left({\sqrt {\frac {1-e_{1}}{1+e_{1}}}}\tan {\frac {\phi }{2}}\right)$$ where

• tis defined as time elapsed to cover phase angle in original orbit
• T₁is defined as period of original orbit
• Eis defined as change ofeccentric anomalybetween spacecraft and final position
• e₁is defined asorbital eccentricityof original orbit
• φis defined as change intrue anomalybetween spacecraft and final position

Phase Orbit
If spacecraft is behind the final position on the same orbit, the spacecraft must slow down to enter a smaller, faster phasing orbit to catch up to final position.

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 (point of impulse, POI), usually performed in the original orbit'speriapsisorapoapsis.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 orbit.

To find some of the phasing orbital parameters, first one must find the required period time of the phasing orbit using the following equation.

$$T_{2}=T_{1}-t$$ where

• T₁is defined as period of original orbit
• T₂is defined as period of phasing orbit
• tis defined as time elapsed to cover phase angle in original orbit

Once phasing orbit period is determined, the phasing orbitsemimajor axiscan be derived from the period formula:

$$a_{2}=\left({\frac {{\sqrt {\mu }}T_{2}}{2\pi }}\right)^{2/3}$$ where

• a₂is defined as semimajor axis of phasing orbit
• T₂is defined as period of phasing orbit
• μis defined asStandard gravitational parameter

From the semimajor axis, the phase orbit apogee and perigee can be calculated: $$2a_{2}=r_{a}+r_{p}$$ where

• a₂is defined as semimajor axis of phasing orbit
• r_ais defined as apogee of phasing orbit
• r_pis defined as perigee of phasing orbit

Finally, the phasing orbit's angular momentum can be found from the equation: $$h_{2}={\sqrt {2\mu }}{\sqrt {\frac {r_{a}r_{p}}{r_{a}+r_{p}}}}$$ where

• h₂is defined as angular momentum of phasing orbit
• r_ais defined as apogee of phasing orbit
• r_pis defined as perigee of phasing orbit
• μis defined as Standard gravitational parameter

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: $$\Delta V=v_{2}-v_{1}={\frac {h_{2}}{r}}-{\frac {h_{1}}{r}}$$ where

• ∆Vis change in velocity between phasing and original orbits at POI
• v₁is defined as the spacecraft velocity at POI in original orbit
• v₂is defined as the spacecraft velocity at POI in phasing orbit
• ris defined as radius of spacecraft from the orbit’s focal point to POI
• h₁is defined as specific angular momentum of the original orbit
• h₂is defined as specific angular momentum of the phasing orbit

Co-orbital Rendezvous
If the target (satellite) is behind the spacecraft (shuttle) in the same orbit, the spacecraft must speed up to enter a larger, slower phasing orbit to allow the target to catch up.

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 rendezvouslike 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 spacecraft 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.

See also

• Orbital maneuver
• Hohmann transfer orbit
• Clohessy-Wiltshire equationsfor co-orbit analysis
• Space rendezvous

References

General

• Curtis, Howard D (2014).Orbital Mechanics for Engineering Students(Third ed.). Butterworth-Heinemann.ISBN978-0-08-097747-8.
• Francis, Hale J (1994).Introduction To Space Flight.Prentice-Hall, Inc.ISBN0-13-481912-8.
• Sellers, Jerry Jon; Marion, Jerry B. (2005).Understanding Space An Introduction to Astronautics(Third ed.). McGraw-Hill.ISBN978-0-07-340775-3.
• Hall, Christopher D.; Collazo-Perez, Victor (November 2003)."Minimum-Time Orbital Phasing Maneuvers".Journal of Guidance, Control, and Dynamics.26(6).RetrievedJanuary 2,2023.
• Phasing Maneuver