Chuck Quintero, Johns Hopkins University Applied Physics Laboratory; Robert Duggan, Johns Hopkins University Applied Physics Laboratory
Keywords: Waves Constellation, Constellation Design, Polar-Star, Latitude Coverage, Satellite Access, Coverage Persistence, Satellite Communications, Satellite Custody
Abstract:
This presentation will describe and explain the underlying mathematical basis for a new constellation design, the Waves constellation, that phase shifts conventional Walker-Delta orbits to optimal high latitude cover. We propose a novel method to specify constellation parameters optimized for maximum coverage at relevant latitudes with additional design characteristics that minimize the number of satellites in the constellation and provide for both in-plane and cross-plane datalinks.
Large numbers of satellites (colloquially called mega-constellations) in low earth orbits (LEOs) are planned, operated by either private companies or nations. From the largest to the smallest systems, the list includes: SpaceXs Starlink (41,927), OneWeb (6,372), Samsung (4,700), Amazons Project Kuiper (3,236 satellites planned), Boeing (3,116), Sat Revolution (1,024), Chinas Hongwan (864), RosCosmoss Sfera (640), Indias Astro Tech (600), Canadas Telesat (512), and more than 2,000 others. If we include other recent proposals the total number increases to over 100,000 new satellites in orbit. More than 18,000 of this total have been proposed for launch in the next 5 years.
Current communications constellation designs require Polar-Star orbits (exemplified by Iridium) which divide the ascending nodes of their orbital planes over 180 degrees of right ascension (RAAN). This approach, cuts the number of satellites in half, but requires that all of the satellites are in polar orbits. Walker-Delta constellations provide good Earth coverage, but near-neighbors in the constellation are constantly changing geometries and widely varying Doppler shifts that make it difficult to maintain a datalink. Unfortunately, there are two difficulties with expanding the use of these constellations to mega-constellations: 1) they each converge at the North and South poles with substantially increased collision risk, and 2) there is a limited capacity for growth in the number of satellites, since altitude is the only available variable.
The reason polar orbits are required is that other ways of diversifying orbital parameters cause difficulties in maintaining earth coverage. In particular, any large change in inclination causes the Polar-Star constellation to lose earth coverage at both seams in significant black-out periods each day.
This presentation presents design principles for optimizing phase in a Walker-Delta constellation designs. The new constellation design, the new Waves constellation. This new constellation offers reduced satellite counts and greatly increased orbit availability. This presentation will show the results of a study on the optimal satellite configurations and latitude coverage performance numbers. The optimization of latitude coverage is extremely important to improving the efficiency of the constellation design.
The same design principles for communications also apply to mega constellations used for Earth monitoring, whether it is a satellite constellation system for imagery, like Planet’s PlanetScope, or a constellation for RF systems monitoring, like HawkEye 360. Relatively constant steering and Doppler are also desirable for imagery and RF monitoring, for several reasons, including: 1) direct hand-off between adjacent satellites, and 2) coordinated behavior between the neighboring satellites.
With optimization, whether for communications, imagery or RF monitoring, the new Waves constellation provides:
The ability to reduce the altitude of the constellation so that less effective isotropic radiated power (EIRP) is required to achieve the required Energy per bit-to-Noise (Eb/No) is reduced.
The ground station utilization vs latitude optimization for specific latitudes.
The reduced black-out gaps of Walker-Delta constellations
Reduced steering (pointing) at cross-link areas at the latitudes where they are most useful.
Cross-links between satellites for data transfer between planes and coordination achieved with a single antenna system with fixed pointing forward and aft.
Date of Conference: September 14-17, 2021
Track: Astrodynamics