Orbit Overview
In order to achieve whole-Earth high-resolution imaging once per day, while maintaining lower operation cost over a 10 year orbit lifetime, the satellite orbits were carefully constructed to maximize ground resolution while maintaining constellation stability. Through imaging trade studies, it was determined that 86 satellites are required to satisfy global coverage. The 86 satellites are arranged in a Walker-Delta constellation, common for Earth-observation applications. The particular geometry chosen is a 4-plane constellation, with 22 satellites each in 2 planes and 21 satellites each in the remaining 2 planes. This spacing provides ample distance between satellites to reduce the number of spacecraft simultaneously in range of a ground station.
Orbit ElementsThe constellation orbit parameters are listed to the right. The constellation was chosen to have a low altitude (500 km) to improve image resolution. At this altitude, each 3U cubesat is guaranteed to have an orbit lifetime greater than the 10-year mission life but less than the IADC 25-year orbit lifetime limit [1].
The chosen orbits are circular to minimize the altitude and avoid perigee-drift perturbation concerns. Since the orbit is circular, argument of perigee is undefined. The inclination was selected to reject node precession perturbations and maintain a consistent constellation orientation. Orbit planes are separated by 30 degrees in longitude to separate communication passes from each plane of satellites. For true anomaly, the satellites in each plane are arranged such that there are no gaps in ground coverage. |
Classical orbit elements for the MARCO POLO constellation (true anomaly omitted since it varies for each satellite).
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An illustration of the concept of sun-synchronous orbits, where the green orbit maintains consistent orientation with respect to the sun over the course of the year [2].
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Perturbation RejectionSatellites in LEO are subject to many orbit perturbations that could cause drift of the constellation over time, which would require station-keeping maneuvers to avoid gaps in coverage. However, some perturbations can be cancelled with careful orbit design, saving station-keeping fuel. For instance, since the orbit is circular, the drift of perigee can be neglected. But more importantly, the constellation arrangement must be maintained over the mission lifetime to avoid coverage gaps.
Sun-synchronous orbits are commonly used for Earth observation missions because they maintain a consistent orientation with respect to the sun over the course of the year. This is accomplished by taking advantage of the precession of the line of nodes, an orbit perturbation caused by mass concentrations on Earth [3]: 
By matching this precession rate to the rate of Earth's revolution around the sun (360-degrees in 365.25 days), a sun-synchronous inclination is determined based on the satellite semi-major axis (directly related to altitude). This ensures consistently good lighting conditions throughout the mission while maintaining consistent orientation of the planes to avoid coverage degradation.
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References:
[1] - Inter-Agency Space Debris Coordination Committee (IADC) and ISO 24113 "Debris mitigation requirements and compliance"
[2] - "Heliosynchronous orbit" by BrandirXZise - Licensed under CC BY-SA 3.0 via Wikimedia Commons.
[3] - R. Bate, D. Mueller, J. White (1971) Fundamentals of Astrodynamics, Dover Publications, New York. ISBN 0486600610.
[1] - Inter-Agency Space Debris Coordination Committee (IADC) and ISO 24113 "Debris mitigation requirements and compliance"
[2] - "Heliosynchronous orbit" by BrandirXZise - Licensed under CC BY-SA 3.0 via Wikimedia Commons.
[3] - R. Bate, D. Mueller, J. White (1971) Fundamentals of Astrodynamics, Dover Publications, New York. ISBN 0486600610.