Satellite constellation

Other satellite groups

Satellite constellations should not be confused with:

• satellite clusters, which are groups of satellites moving very close together in almost identical orbits (see satellite formation flying);
• satellite series or satellite programs (such as Landsat), which are generations of satellites launched in succession;
• satellite fleets, which are groups of satellites from the same manufacturer or operator that function independently from each other (not as a system).

Overview

Satellites in medium Earth orbit (MEO) and low Earth orbit (LEO) are often deployed in satellite constellations, because the coverage area provided by a single satellite only covers a small area that moves as the satellite travels at the high angular velocity needed to maintain its orbit. Many MEO or LEO satellites are needed to maintain continuous coverage over an area. This contrasts with geostationary satellites, where a single satellite, at a much higher altitude and moving at the same angular velocity as the rotation of the Earth's surface, provides permanent coverage over a large area.

For some applications, in particular digital connectivity, the lower altitude of MEO and LEO satellite constellations provide advantages over a geostationary satellite, with lower path losses (reducing power requirements and costs) and latency. The propagation delay for a round-trip internet protocol transmission via a geostationary satellite can be over 600ms, but as low as 125ms for a MEO satellite or 30ms for a LEO system.

Examples of satellite constellations include the Global Positioning System (GPS), Galileo and GLONASS constellations for navigation and geodesy in MEO, the Iridium and Globalstar satellite telephony services and Orbcomm messaging service in LEO, the Disaster Monitoring Constellation and RapidEye for remote sensing in Sun-synchronous LEO, Russian Molniya and Tundra communications constellations in highly elliptic orbit, and satellite broadband constellations, under construction from Starlink and OneWeb in LEO, and operational from O3b in MEO.

Design

Walker Constellation

There are a large number of constellations that may satisfy a particular mission. Usually constellations are designed so that the satellites have similar orbits, eccentricity and inclination so that any perturbations affect each satellite in approximately the same way. In this way, the geometry can be preserved without excessive station-keeping thereby reducing the fuel usage and hence increasing the life of the satellites. Another consideration is that the phasing of each satellite in an orbital plane maintains sufficient separation to avoid collisions or interference at orbit plane intersections.

A class of circular orbit geometries that has become popular is the Walker Delta Pattern constellation. This has an associated notation to describe it which was proposed by John Walker. His notation is:

: i: t/p/f

where:

• i is the inclination;
• t is the total number of satellites;
• p is the number of equally spaced planes; and
• f is the relative spacing between satellites in adjacent planes. The change in true anomaly (in degrees) for equivalent satellites in neighbouring planes is equal to f × 360 / t.

For example, the Galileo navigation system is a Walker Delta 56°:24/3/1 constellation. This means there are 24 satellites in 3 planes inclined at 56 degrees, spanning the 360 degrees around the equator. The "1" defines the phasing between the planes, and how they are spaced. The Walker Delta is also known as the Ballard rosette, after A. H. Ballard's similar earlier work. Ballard's notation is (t,p,m) where m is a multiple of the fractional offset between planes.

Another popular constellation type is the near-polar Walker Star, which is used by Iridium. Here, the satellites are in near-polar circular orbits across approximately 180 degrees, travelling north on one side of the Earth, and south on the other. The active satellites in the full Iridium constellation form a Walker Star of 86.4°:66/6/2, i.e. the phasing repeats every two planes. Walker uses similar notation for stars and deltas, which can be confusing.

These sets of circular orbits at constant altitude are sometimes referred to as orbital shells.

Orbital shell

In spaceflight, an orbital shell is a set of artificial satellites in circular orbits at a certain fixed altitude. In the design of satellite constellations, an orbital shell usually refers to a collection of circular orbits with the same altitude and, oftentimes, orbital inclination, distributed evenly in celestial longitude (and mean anomaly). For a sufficiently high inclination and altitude the orbital shell covers the entire orbited body. In other cases the coverage extends up to a certain maximum latitude.

Several existing satellite constellations typically use a single orbital shell. New large megaconstellations have been proposed that consist of multiple orbital shells.

List of satellite constellations

Navigational satellite constellations

Main article: Satellite navigation

Communications satellite constellations

Broadcasting

• Sirius Satellite Radio until 2013
• XM Satellite Radio until 2011
• SES
• Othernet
• Molniya (discontinued)

Monitoring

• Spire (AIS, ADS-B)
• Iridium (AIS, ADS-B, IoT)
• Myriota (IoT)
• Swarm Technologies (IoT)
• Astrocast (IoT)
• TDRSS

Internet access

Other Internet access systems are proposed or currently being developed:

Some systems were proposed but never realized:

; Progress

• Boeing Satellite is transferring the application to OneWeb
• LeoSat shut down completely in 2019
• The OneWeb constellation had 6 pilot satellites in February 2019, 74 satellites launched as of 21 March 2020 but filed for bankruptcy on 27 March 2020
• Starlink: first mission (Starlink 0) launched on 24 May 2019; 955 satellites launched, 51 deorbited, 904 in orbit ; public beta test in limited latitude range started in November 2020
• O3b mPOWER: first 6 satellites launched December 2022-November 2023 with service start April 2024. 7 more in 2024–2026.
• Telesat LEO: two prototypes: 2018 launch
• CASIC Hongyun: prototype launched in December 2018
• CASC Hongyan prototype launched in December 2018, might be merged with Hongyun
• Project Kuiper: FCC filing in July 2019. Prototypes launched in October 2023.

Earth observation satellite constellations

• RADARSAT Constellation
• Planet Labs
• Pléiades 1A and 1B
• Satellogic
• RapidEye
• Disaster Monitoring Constellation
• A-train
• SPOT 6 and SPOT 7
• Spire
• Synspective

Notes

References

References

1. "On the increasing number of satellite constellations".
2. [https://www.satelliteevolutiongroup.com/articles/LEO-Constellations&Tracking.pdf ''LEO constellations and tracking challenges''] Satellite Evolution Group, September 2017, Accessed 26 March 2021
3. [https://www.telesat.com/wp-content/uploads/2020/07/Real-Time-Latency-Rethinking-Remote-Networks.pdf ''Real-Time Latency: Rethinking Remote Networks''] {{Webarchive. link. (2021-07-21 Telesat, February 2020, Accessed 26 March 2021)
4. J. G. Walker, Satellite constellations, Journal of the British Interplanetary Society, vol. 37, pp. 559-571, 1984
5. A. H. Ballard, Rosette Constellations of Earth Satellites, IEEE Transactions on Aerospace and Electronic Systems, Vol 16 No. 5, Sep. 1980.
6. J. G. Walker, Comments on "Rosette constellations of earth satellites", IEEE Transactions on Aerospace and Electronic Systems, vol. 18 no. 4, pp. 723-724, November 1982.
7. [https://fcc.report/IBFS/SAT-MOD-20181108-00083/1569860 SPACEX NON-GEOSTATIONARY SATELLITE SYSTEM, Attachment A, TECHNICAL INFORMATION TO SUPPLEMENT SCHEDULE S], US Federal Communications Commission, 8 November 2018, accessed 19 November 2019.
8. (2019-07-08). "Amazon lays out constellation service goals, deployment and deorbit plans to FCC".
9. "Land Xpress".
10. "Globalstar satellites".
11. (21 February 2018). "This is how Elon Musk plans to use SpaceX to give internet to everyone".
12. (14 February 2018). "SpaceX Set to Launch 2 Starlink Satellites to Test Gigabit Broadband". [[ISPreview]].
13. (2020-09-09). "SpaceX's Satellite Internet Service Latency Comes in Under 20 Milliseconds".
14. (2018-03-20). "OneWeb asks FCC to authorize 1,200 more satellites".
15. Thierry Dubois. (Dec 19, 2017). "Eight Satellite Constellations Promising Internet Service From Space". Aviation Week & Space Technology.
16. (11 September 2018). "Telesat says ideal LEO constellation is 292 satellites, but could be 512".
17. Telesat Canada. (August 24, 2017). "Telesat Technical Narrative".
18. Telesat Canada. (August 24, 2017). "SAT-PDR-20170301-00023".
19. Zhao, Lei. (5 March 2018). "Satellite will test plan for communications network". [[China Daily]].
20. Jones, Andrew. (13 November 2018). "China to launch first Hongyan LEO communications constellation satellite soon". [[GBTimes]].
21. ((EL2squirrel (cedar))). (12 December 2019). "Chinese version of OneWeb: The Hongyan system consists of 864 satellites, with 8 Tbps of bandwidth, Orbital altitude 1175km".
22. Jewett, Rachel. (31 March 2022). "Hanwha Systems Plans 2,000-Satellite LEO Constellation for Mobility Applications". Via Satellite.
23. Porter, Jon. (2019-04-04). "Amazon will launch thousands of satellites to provide internet around the world".
24. (2017-12-17). "Boeing wants to help OneWeb satellite plans". Advanced Television.
25. "LeoSat, absent investors, shuts down".
26. (2020-03-21). "OneWeb increases mega-constellation to 74 satellites".
27. (2020-03-30). "Coronavirus: OneWeb blames pandemic for collapse".
28. (2020-03-27). "Voluntary Petition for Non-Individuals Filing for Bankruptcy".
29. Samantha Mathewson. (6 November 2020). "SpaceX opens Starlink satellite internet to public beta testers: report".
30. [https://www.satellitetoday.com/connectivity/2024/04/24/ses-o3b-mpower-meo-system-is-now-operational-service-rollout-to-follow/ ''SES’ O3b mPOWER MEO System is Now Operational, Service Rollout to Follow''] Via Satellite. 24 April 2024. Accessed 29 April 2025
31. Barbosa, Rui C.. (21 December 2018). "Chinese Long March 11 launches with the first Hongyun satellite". [[NASASpaceFlight.com]].
32. Barbosa, Rui. (29 December 2018). "Long March 2D concludes 2018 campaign with Hongyan-1 launch". [[NASASpaceFlight.com]].
33. (14 December 2019). "Notice that these satellites from CASC are mentioned as part of a "national satellite Internet system". There are rumors that several of the planned Chinese private LEO comsat constellations have been recently absorbed into one big nationalized one.".