Satellite constellation

Asatellite constellationis a group ofartificial satellitesworking together as a system. Unlike a single satellite, a constellation can provide permanent global or near-globalcoverage,such that at any time everywhere on Earth at least one satellite is visible. Satellites are typically placed in sets of complementaryorbital planesand connect to globally distributedground stations.They may also useinter-satellite communication.

TheGPSconstellation calls for 24 satellites to be distributed equally among sixorbital 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.

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 (seesatellite formation flying);
• satellite seriesorsatellite programs(such asLandsat), 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

A bright artificial satellite flare is visible above theVery Large Telescope.Satellite constellations could have an impact on ground-based astronomy.

Satellites inmedium Earth orbit (MEO)andlow 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 highangular velocityneeded to maintain itsorbit.Many MEO or LEO satellites are needed to maintain continuous coverage over an area. This contrasts withgeostationarysatellites, 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 lowerpath losses(reducing power requirements and costs) and latency.Thepropagation delayfor a round-tripinternet protocoltransmission 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 theGlobal Positioning System(GPS),GalileoandGLONASSconstellations fornavigationandgeodesyin MEO, theIridiumandGlobalstarsatellite telephony services andOrbcommmessaging service in LEO, theDisaster Monitoring ConstellationandRapidEyeforremote sensinginSun-synchronousLEO, RussianMolniyaand Tundra communications constellations in highlyelliptic orbit,and satellite broadband constellations, under construction fromStarlinkandOneWebin LEO, and operational fromO3bin 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. Circular orbits are popular, because then the satellite is at a constant altitude requiring a constant strength signal to communicate.

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:

• iis the inclination;
• tis the total number of satellites;
• pis the number of equally spaced planes; and
• fis the relative spacing between satellites in adjacent planes. The change in true anomaly (in degrees) for equivalent satellites in neighbouring planes is equal tof× 360 /t.

For example, theGalileo navigation systemis 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 theequator.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 byIridium.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

Inspaceflight,anorbital shellis a set ofartificial satellitesincircularorbitsat a certain fixedaltitude.In the design of satellite constellations, an orbital shell usually refers to a collection of circularorbitswith the samealtitudeand, oftentimes,orbital inclination, distributed evenly incelestial longitude(andmean anomaly). For a sufficiently high inclination and altitude the orbital shellcoversthe entire orbited body. In other cases the coverage extends up to a certain maximumlatitude.

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

List of satellite constellations

Navigational satellite constellations

Main article:Satellite navigation

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	3 geostationary at 35,786 km (GEO) 3 in 3 planes at 35,786 km (55°GSO) 24 in 3 planes at 21,150 km (55° MEO)	Global	Navigation	Operational	2012–present, Asia 2018–present, globally
NAVIC	ISRO	3 geostationary at 35,786 km (GEO) 4 in 2 planes at 250–24,000 km (29° GSO)	Regional	Navigation	Operational	2018–present
QZSS	JAXA	1 geostationary at 35,786 km (GEO) 3 in 3 planes at 32,600–39,000 (43° GSO)	Regional	Navigation	Operational	2018–present

Communications satellite constellations

See also:Satellite communicationandCategory:Communications satellite constellations

Broadcasting

• Sirius Satellite Radio
• XM Satellite Radio
• SES
• Othernet
• Molniya(discontinued)

Monitoring

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

Internet access

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	5 Geostationary satellites		Kaband	Internet access
Globalstar	Globalstar	48 at 1400 km, 52° (8 planes)	70°S to 70°N		Internet access, satellite telephony
Iridium	Iridium Communications	66 at 780 km, 86.4° (6 planes)	Global	L band Kaband	Internet access, satellite telephony
O3b	SES	20 at 8,062 km, 0° (circular equatorial orbit)	45°S to 45°N	Kaband	Internet access
O3b mPOWER	SES	6 at 8,062 km, 0° (circular equatorial orbit) 7 more to be launched by end 2026	45°S to 45°N	Ka(26.5–40 GHz)	Internet access
Orbcomm	ORBCOMM	17 at 750 km, 52° (OG2)	65°S to 65°N		IoTandM2M,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	EMEAand Asia	L band	Internet access, satellite telephony
Starlink	SpaceX	LEO in several orbital shells ~5000 satellites at 550 km (Oct 2023) 12000 satellites at ~350–550 km (planned)	44°S to 52°N (Feb 2021) Global	Ku(12–18 GHz) Ka(26.5–40 GHz)	Internet access
OneWebconstellation	Eutelsat(completed merger in Sep 2023)	882–1980(planned) Total number of operational satellites: 634 as of 20 May 2023	Global	Ku(12–18 GHz) Ka(26.5–40 GHz)	Internet access

Other Internet access systems are proposed or currently being developed:

Proposedinternet satellite constellations

Constellation	Manufacturer	Number	Weight	Unveil.	Avail.	Altitude	Offer	Band	Inter-sat. links
IRIS²	European Space Agency	TBD	TBD
TelesatLEO	AirbusSSTL SS/Loral	117–512	—	2016	2027	1,000–1,248 km 621–775 mi	Fiber-opticcable-like	Ka(26.5–40 GHz)	Optical
Hongyun	CASIC	156		2017	2022	160–2,000 km 99–1,243 mi
Hongyan	CASC	320-864		2017	2023	1,100–1,175 km 684–730 mi
Hanwha Systems		2000		2022	2025
Project Kuiper	Amazon	3236		2019	2024	590–630 km 370–390 mi	56°S to 56°N

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)	Kaband (20/30 GHz)	Global, low-latency broadband Internet services	1998 May
Teledesic	Teledesic	840 satellites at 700 km, 98.2° (21 planes) [1994 design] 288 satellites at 1400 km, 98.2° (12 planes) [1997 design]	Kaband (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	Ka(26.5–40 GHz)	High-speed broadband internet	2019

Progress

• Boeing Satelliteis transferring the application to OneWeb
• LeoSatshut down completely in 2019
• TheOneWeb constellationhad 6 pilot satellites inFebruary 2019,74 satellites launched as of 21 March 2020but 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 as of 25 November 2020;public beta test in limited latitude range started in November 2020
• O3b mPOWER:first 2 satellites launched December 2022; 9 more in 2023–2024, with the initial service start expected in Q3 2023.
• TelesatLEO: two prototypes: 2018 launch
• CASICHongyun:prototype launched in December 2018
• CASCHongyanprototype launched in December 2018,might be merged withHongyun
• Project Kuiper:FCC filing in July 2019. Prototypes launched in October 2023.

Earth observation satellite constellations

See also:List of Earth observation satellites

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

See also

• Satellite internet constellation
• Light pollution
• Space debris
• Types of geocentric orbit
• Orbital mechanics

External links

Satellite constellation simulation tools:

• AVM Dynamics Satellite Constellation Modeler
• SaVi Satellite Constellation Visualization
• Transfinite Visualyse Professional

More information:

• Internetworking with satellite constellations - a PhD thesis (2001)
• Lloyd's satellite constellations- last updated 20 July 2011
• Examination and analysis of polar low Earth orbit constellation-IEEE