Satellite constellation

The GPS constellation calls for 24 satellites to be distributed equally among six orbital planes. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 40°N, changes with time.

A satellite constellation is a group of artificial satellites working together as a system. Unlike a single satellite, a constellation can provide permanent global or near-global coverage, such that at any time everywhere on Earth at least one satellite is visible. Satellites are typically placed in sets of complementary orbital planes and connect to globally distributed ground stations. They may also use inter-satellite communication.

Other satellite groupsEdit

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).

OverviewEdit

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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.<ref>LEO constellations and tracking challenges Satellite Evolution Group, September 2017, Accessed 26 March 2021</ref> The propagation delay for a round-trip internet protocol transmission via a geostationary satellite can be over 600Template:Nbspms, but as low as 125Template:Nbspms for a MEO satellite or 30Template:Nbspms for a LEO system.<ref>Real-Time Latency: Rethinking Remote Networks Template:Webarchive Telesat, February 2020, Accessed 26 March 2021</ref>

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.

DesignEdit

Walker ConstellationEdit

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.

File:Walker-Delta Constellation.webp

Walker-Delta Constellation

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.<ref>J. G. Walker, Satellite constellations, Journal of the British Interplanetary Society, vol. 37, pp. 559-571, 1984</ref> 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°:Template:Nbsp24/3/1 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.<ref>A. H. Ballard, Rosette Constellations of Earth Satellites, IEEE Transactions on Aerospace and Electronic Systems, Vol 16 No. 5, Sep. 1980.</ref><ref>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.</ref> Ballard's notation is (t,p,m) where m is a multiple of the fractional offset between planes.

File:Walker-Star Constellation.webp

Walker-Star Constellation

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°:Template:Nbsp66/6/2, 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 shellEdit

In spaceflight, an orbital shell is a set of artificial satellites in circular orbits at a certain fixed altitude.<ref name="fcc20181108b">SPACEX NON-GEOSTATIONARY SATELLITE SYSTEM, Attachment A, TECHNICAL INFORMATION TO SUPPLEMENT SCHEDULE S, US Federal Communications Commission, 8 November 2018, accessed 19 November 2019.</ref> 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).Template:Citation needed 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.Template:Citation needed

Several existing satellite constellations typically use a single orbital shell. New large megaconstellations have been proposed that consist of multiple orbital shells.<ref name=fcc20181108b/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

List of satellite constellationsEdit

Navigational satellite constellationsEdit

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Satellite constellations used for navigation
Name	Operator	Satellites and orbits (latest design, excluding spares)	Coverage	Services	Status	Years in service
Global Positioning System (GPS)	USSF	24 in 6 planes at 20,180 km (55° MEO)	Global	Navigation	Operational	1993–present
GLONASS	Roscosmos	24 in 3 planes at 19,130 km (64°8' MEO)	Global	Navigation	Operational	1995–present
Galileo	EUSPA, ESA	24 in 3 planes at 23,222 km (56° MEO)	Global	Navigation	Operational	2019–present
BeiDou	CNSA	Template:Ubl	Global	Navigation	Operational	Template:Ubl
NAVIC	ISRO	Template:Ubl	Regional	Navigation	Operational	2018–present
QZSS	JAXA	Template:Ubl	Regional	Navigation	Operational	2018–present

Communications satellite constellationsEdit

Template:See also

BroadcastingEdit

MonitoringEdit

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

Internet accessEdit

Operational communications satellite constellations
Name	Operator	Constellation design	Coverage	Freq.	Services
Broadband Global Area Network (BGAN)	Inmarsat	3 geostationary satellites	82°S to 82°N		Internet access
Global Xpress (GX)	Inmarsat	citation	CitationClass=web }}</ref>		K_a band	Internet access
Globalstar	Globalstar	citation	CitationClass=web }}</ref>	70°S to 70°N<ref name=":0" />		Internet access, satellite telephony
Iridium	Iridium Communications	66 at 780 km, 86.4° (6 planes)	Global	Template:Ubl	Internet access, satellite telephony
O3b	SES	20 at 8,062 km, 0° (circular equatorial orbit)	45°S to 45°N	K_a band	Internet access
O3b mPOWER	SES	8,062 km, 0° (circular equatorial orbit) 6 operational, 2 being commissioned (Template:Asof), 5 more to be launched by end 2026	45°S to 45°N	K_a (26.5–40 GHz)	Internet access
Orbcomm	ORBCOMM	17 at 750 km, 52° (OG2)	65°S to 65°N		IoT and M2M, AIS
Defense Satellite Communications System (DSCS)	4th Space Operations Squadron				Military communications
Wideband Global SATCOM (WGS)	4th Space Operations Squadron	10 geostationary satellites			Military communications
ViaSat	Viasat, Inc.	4 geostationary satellites	Varying		Internet access
Eutelsat	Eutelsat	20 geostationary satellites			Commercial
Thuraya	Thuraya	2 geostationary satellites	EMEA and Asia	L band	Internet access, satellite telephony
Starlink	SpaceX	LEO in several orbital shellsTemplate:Ubl	Template:Ubl	Template:Ubl	citation	CitationClass=web }}</ref><ref>Template:Cite news</ref><ref>{{#invoke:citation/CS1\|citation	CitationClass=web }}</ref>
OneWeb constellation	Eutelsat (completed merger in Sep 2023)	882–1980<ref>Template:Cite magazine</ref>(planned) Total number of operational satellites: 634 as of 20 May 2023	Global	Template:Ubl	Internet access

Other Internet access systems are proposed or currently being developed:

Proposed internet satellite constellations<ref name="AvWeek19dec2017">Template:Cite news</ref>
Constellation	Manufacturer	Number	Weight	Template:Abbr	Template:Abbr	Altitude	Offer	Band	Inter-sat. links
IRIS²	European Space Agency	TBD	TBD
Telesat LEO	Template:Ubl	117–512<ref>Template:Cite magazine</ref>	Template:N/A	2016	2027	Template:Cvt	Fiber-optic cable-like	K_a (26.5–40 GHz)	Optical<ref>{{#invoke:citation/CS1\|citation	CitationClass=web }}</ref><ref>{{#invoke:citation/CS1\|citation	CitationClass=web }}</ref>
Hongyun<ref>Template:Cite news</ref>	CASIC	156		2017	2022	Template:Cvt
Hongyan<ref name=hongyan>Template:Cite news</ref>	CASC	citation	CitationClass=web }}</ref>		2017	2023	Template:Cvt
Hanwha Systems<ref name=hanwha>Template:Cite news</ref>		2000		2022	2025
Project Kuiper	Amazon	3236		2019	2024	Template:Cvt	citation	CitationClass=web }}</ref>

Some systems were proposed but never realized:

Abandoned communication satellite constellation designs
Name	Operator	Constellation design	Freq.	Services	Abandoned date
Celestri	Motorola	63 satellites at 1400 km, 48° (7 planes)	K_a band (20/30 GHz)	Global, low-latency broadband Internet services	1998 May
Teledesic	Teledesic	Template:Ubl	K_a band (20/30 GHz)	100 Mbit/s up, 720 Mbit/s down global internet access	2002 October
LeoSat	Thales Alenia	78–108 satellites at 1400 km	K_a (26.5–40 GHz)	High-speed broadband internet	2019

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Progress

Boeing Satellite is transferring the application to OneWeb<ref name="boeing-to-oneweb">{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

LeoSat shut down completely in 2019<ref>Template:Cite magazine</ref>
The OneWeb constellation had 6 pilot satellites in February 2019, 74 satellites launched as of 21 March 2020<ref>Template:Cite news</ref> but filed for bankruptcy on 27 March 2020<ref>Template:Cite news</ref><ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

Starlink: first mission (Starlink 0) launched on 24 May 2019; 955 satellites launched, 51 deorbited, 904 in orbit Template:As of; public beta test in limited latitude range started in November 2020<ref>Template:Cite news</ref>
O3b mPOWER: first 6 satellites launched December 2022-November 2023 with service start April 2024. 7 more in 2024–2026.<ref>SES’ O3b mPOWER MEO System is Now Operational, Service Rollout to Follow Via Satellite. 24 April 2024. Accessed 29 April 2025</ref>
Telesat LEO: two prototypes: 2018 launch
CASIC Hongyun: prototype launched in December 2018<ref>Template:Cite news</ref>
CASC Hongyan prototype launched in December 2018,<ref>Template:Cite news</ref> might be merged with Hongyun<ref>Template:Cite tweet</ref>
Project Kuiper: FCC filing in July 2019. Prototypes launched in October 2023.