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 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.LEO constellations and tracking challenges Satellite Evolution Group, September 2017, Accessed 26 March 2021 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.Real-Time Latency: Rethinking Remote Networks Telesat, February 2020, Accessed 26 March 2021

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

Optimization-based Design

Designing a satellite constellation (i.e., determining the orbital distribution of its constituents) is a complex problem that typically uses satellite-to-target coverage as the primary figure of merit. Naturally, the resulting constellation depends on the mission requirements and objectives and can be characterized by its geometry. That is, either symmetric or asymmetric constellations.

Symmetric constellations are traditionally proposed for applications that require global coverage (e.g., telecommunications). Notable symmetric constellations proposed in the literature include Walker Delta and Star,J. G. Walker, Circular Orbit Patterns Providing Continuous Whole Earth Coverage, TR 70211, Royal Aircraft Establishment, Farnborough, England, U.K., 1970.J. G. Walker, Continuous Whole Earth-Coverage by Circular-Orbit Satellite Patterns, TR 77044, Royal Aircraft Establishment, Farnborough, England, U.K., 1977. and Rosette.A. H. Ballard, Rosette Constellations of Earth Satellites, IEEE Transactions on Aerospace and Electronic Systems, Vol 16 No. 5, 1980.

Asymmetric constellations are typically adopted when regional coverage (i.e., the distribution of targets is concentrated around specific locations rather than being globally uniform) is required. Asymmetric constellations are proposed to address disaster monitoring,P. Chadalavada, and A. Dutta, Regional CubeSat Constellation Design to Monitor Hurricanes, IEEE Transactions on Geoscience and Remote Sensing, Vol. 60, pp. 1–8, 2021. orbital debris remediation,D. O. Williams Rogers, M. C. Fox, P. R. Stysley, and H. Lee, Optimal Placement and Coordinated Scheduling of Distributed Space-Based Lasers for Orbital Debris Remediation, Advances in Space Research, Vol. 76, No. 9, pp. 5265–5293, 2025. and cislunar space domain awareness.T. H. Clareson, M. C. Fox, D. K. Amato, and H. Lee, Embedded State Estimation for Optimization of Cislunar Space Domain Awareness Constellation Design, Journal of Spacecraft and Rockets, Vol. 62, No. 3, pp. 898–914, 2024.

Large satellite constellations are evaluated against equivalent power flux density (EPFD) limits to ensure their combined transmissions remain compatible with geostationary satellite services.

The need for optimization in constellation design arises from the large number of degrees of freedom in the design space. For example, payload characteristics, required temporal coverage resolution (e.g., continuous or discontinuous), targets' distribution, and candidate orbits. Conventional optimization methodologies used for constellation design include:

Full enumerationD. C. Beste, Design of Satellite Constellations for Optimal Continuous Coverage, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-14, No. 3, pp. 466–473, 1978.Y. Ulybyshev, Near-Polar Satellite Constellations for Continuous Global Coverage, Journal of Spacecraft and Rockets, Vol. 36, No. 1, pp. 92–99, 1999.C.-H. Lin, and Z.-C. Hong, Mission and Constellation Design for Low-Cost Weather Observation Satellites, Journal of Spacecraft and Rockets, Vol. 42, No. 1, pp. 118–123, 2005. Mixed integer linear programmingH. Lee, S. Shimizu, S. Yoshikawa, and K. Ho, Satellite Constellation Pattern Optimization for Complex Regional Coverage, Journal of Spacecraft and Rockets, Vol. 57, No. 6, pp. 1309–1327, 2020.H. Lee, and K. Ho, Regional Constellation Reconfiguration Problem: Integer Linear Programming Formulation and Lagrangian Heuristic Method, Journal of Spacecraft and Rockets, Vol. 60, No. 6, pp. 1828–1845, 2023.D. O. Williams Rogers, D. Won, D. Koh, K. Hong, and H. Lee, Optimal Satellite Constellation Configuration Design: A Collection of Mixed Integer Linear Programs, Journal of Spacecraft and Rockets, Vol. 63, No. 4, pp. 1144–1162, 2026. Metaheuristics E. A. Williams, W. A. Crossley, and T. J. Lang, Average and Maximum Revisit Time Trade Studies for Satellite Constellations Using a Multiobjective Genetic Algorithm, The Journal of the Astronautical Sciences, Vol. 49, pp. 385–400, 2020.M. P. Ferringer, R. S. Clifton, and T. G. Thompson, Efficient and Accurate Evolutionary Multi-Objective Optimization Paradigms for Satellite Constellation Design, Journal of Spacecraft and Rockets, Vol. 44, No. 3, pp. 682–691, 2007.I. Meziane-Tani, G. Métris, G. Lion, A. Deschamps, F. T. Bendimerad, and M. Bekhti, Optimization of Small Satellite Constellation Design for Continuous Mutual Regional Coverage with Multi-Objective Genetic Algorithm, International Journal of Computational Intelligence Systems, Vol. 9, No. 4, pp. 627–637, 2016.

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.J. G. Walker, Satellite constellations, Journal of the British Interplanetary Society, vol. 37, pp. 559-571, 1984 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.J. G. Walker, Comments on "Rosette constellations of Earth satellites", IEEE Transactions on Aerospace and Electronic Systems, vol. 18 no. 4, pp. 723-724, 1982. 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.SpaceX Non-Geostationary Satellite System, Attachment A, Technical Information to Supplement Schedule S, US Federal Communications Commission, 8 November 2018, accessed 19 November 2019. 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

+ Satellite constellations used for navigation Name Operator Satellites and orbits (latest design, excluding spares) Coverage Services Status Years in serviceGlobal Positioning System (GPS)USSF (US)24 in 6 planes at 20,180 km (55° MEO) No expansion planned as of June 2026. GlobalNavigationOperational1993–presentGLONASSRoscosmos (Russia)24 in 3 planes at 19,130 km (64°8' MEO) No expansion planned as of June 2026.GlobalNavigationOperational1995–presentGalileoEUSPA, ESA (EU)26 in 3 planes at 23,222 km (56° MEO) No expansion planned as of June 2026.GlobalNavigationOperational2019–present BeiDou CNSA (CN) Global Navigation Operational NAVIC ISRO (India) Regional Navigation Operational 2018–present QZSS JAXA (JPN) Regional Navigation Operational 2018–presentPulsarXona Space systems (US)1 in LEO . A total of 258 are planned globalPNT navigationoperational2025-presentTrsutpointTrustpoint (US)0 in LEO as of June 2026. A total up to 300 is planned.globalPNT navigationPLANNED2027PNT-LEOArkEdge Space(JPN)/ JAXA (JPN)0 in LEO as of June 2026. Unknown total planned.TBDPNTPLANNEDTBD

Communications satellite constellations

Broadcasting

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