Satellite Types for Communications

2.1 Overview of Orbital Regimes

The choice of orbital regime fundamentally shapes every aspect of a satellite communication system—from the number of satellites required and system latency to terminal complexity and overall service characteristics. Each orbital regime represents a distinct engineering trade-off between coverage, capacity, latency, system complexity, and cost.

The three primary orbital regimes used for satellite communications are Geostationary Earth Orbit (GEO), Medium Earth Orbit (MEO), and Low Earth Orbit (LEO). Additionally, Highly Elliptical Orbits (HEO) serve specialized applications requiring coverage at extreme latitudes. Understanding the characteristics of each regime is essential for selecting the optimal architecture for specific communication requirements.

2.2 Geostationary Earth Orbit (GEO) Systems

Geostationary satellites occupy the unique orbital position at 35,786 km altitude where their orbital period exactly matches Earth's rotation. This causes them to appear motionless relative to ground observers, enabling fixed ground antennas without tracking requirements and providing continuous coverage over approximately one-third of Earth's surface.

GEO Slot Allocation

The geostationary arc is a limited resource, with orbital positions (slots) allocated by the International Telecommunication Union (ITU) through a complex international coordination process. Typical spacing between satellites is 2-3 degrees of longitude, allowing approximately 120-180 satellites across the visible arc from any ground location. Satellites using the same frequency bands must be adequately separated to prevent interference.

Station-Keeping Requirements

Despite appearing stationary, GEO satellites require continuous station-keeping maneuvers to maintain their orbital position. Various perturbations—including solar radiation pressure, gravitational effects from the Sun and Moon, and Earth's non-spherical gravitational field—cause satellites to drift from their assigned positions. Typical station-keeping requirements:

• North-South Station-Keeping: Counters inclination growth caused by solar and lunar gravitational perturbations; requires approximately 45-50 m/s of delta-V per year
• East-West Station-Keeping: Maintains longitude position against gravitational anomalies; requires approximately 1-2 m/s per year
• Eccentricity Control: Maintains circular orbit against solar radiation pressure; typically combined with other maneuvers

Major GEO Communication Operators

2.3 Medium Earth Orbit (MEO) Systems

Medium Earth Orbit spans the vast region between LEO and GEO, typically defined as 2,000 to 35,786 km altitude. MEO offers a compelling middle ground—significantly lower latency than GEO while requiring fewer satellites than LEO for global coverage. Navigation systems (GPS, Galileo, GLONASS) operate in MEO, and communication systems like SES's O3b/mPOWER constellation exploit this regime for high-throughput, medium-latency services.

MEO Orbital Characteristics

Unlike the unique GEO altitude, MEO encompasses a wide range of possible orbits with different characteristics:

• Lower MEO (~2,000-8,000 km): Approaches LEO characteristics with lower latency but faster ground track motion, requiring more frequent handovers
• Mid-MEO (~8,000-15,000 km): Optimal balance for communication systems; O3b operates at 8,062 km
• Upper MEO (~15,000-35,000 km): GPS/navigation regime at ~20,200 km; slower orbital motion provides longer visibility windows

Van Allen Radiation Belts

A critical consideration for MEO systems is the Van Allen radiation belts—zones of high-energy charged particles trapped by Earth's magnetic field. The inner belt extends from approximately 1,000 to 5,000 km altitude, while the outer belt spans roughly 13,000 to 60,000 km. MEO satellites must either avoid these regions or employ extensive radiation hardening, significantly impacting design and cost.

2.4 Low Earth Orbit (LEO) Constellations

Low Earth Orbit, spanning 160 to 2,000 km altitude, has emerged as the dominant architecture for next-generation satellite broadband. LEO's proximity to Earth dramatically reduces latency and path loss but requires large constellations—often hundreds to thousands of satellites—for continuous global coverage. The rapid orbital motion (completing an orbit in roughly 90-100 minutes) necessitates sophisticated satellite tracking and seamless handovers between satellites.

Starlink (SpaceX)

OneWeb

Amazon Kuiper

Other LEO Communication Systems

2.5 Highly Elliptical Orbits (HEO)

Highly Elliptical Orbits provide an alternative architecture for serving high-latitude regions poorly covered by GEO satellites. By using orbits with high apogee over the target hemisphere, satellites spend the majority of their orbital period at high altitude, appearing nearly stationary for extended periods.

Molniya Orbit

The Molniya orbit, developed by the Soviet Union in the 1960s, features a 12-hour period with high apogee (~40,000 km) over the Northern Hemisphere and low perigee (~500 km) over the Southern Hemisphere. The 63.4° inclination is chosen to eliminate apsidal rotation (the slow rotation of the orbit's orientation), keeping apogee fixed over the northern regions.

A satellite in Molniya orbit spends approximately 8 hours of each 12-hour period above 20,000 km altitude, visible from northern regions with high elevation angles. Three satellites phased 8 hours apart can provide continuous coverage of the entire Northern Hemisphere, including polar regions unreachable by GEO.

Tundra Orbit

The Tundra orbit is a 24-hour highly elliptical orbit, also at 63.4° inclination, providing extended dwell time over a single region. With apogee at approximately 46,000 km, a Tundra satellite appears nearly stationary for roughly 12 hours over its assigned coverage area. Two satellites provide continuous coverage of the target region.

HEO Applications

• Russian Communications: Molniya satellites have provided communications to Russia's northern territories since the 1960s
• Arctic Coverage: Growing interest in Arctic communications for shipping, resource extraction, and military applications
• Sirius XM Radio: Uses HEO satellites for North American mobile radio service
• Military Applications: Specialized reconnaissance and communication systems

2.6 Multi-Orbit Integration Strategies

The future of satellite communications increasingly involves integration across multiple orbital regimes. Rather than competing, GEO, MEO, and LEO systems can complement each other, with intelligent network orchestration directing traffic to the optimal satellite based on application requirements.

SES Multi-Orbit Strategy

SES operates both GEO and MEO (O3b/mPOWER) satellites, offering integrated services that leverage the strengths of each regime. Broadcast and throughput-intensive applications use GEO capacity, while latency-sensitive enterprise applications route through MEO. The common ground infrastructure and unified management platform enable seamless service delivery across orbits.

Eutelsat OneWeb Convergence

The merger of Eutelsat (GEO) and OneWeb (LEO) creates a multi-orbit operator capable of addressing the full spectrum of connectivity needs. GEO assets serve broadcast, maritime, and enterprise customers, while the LEO constellation expands reach to underserved areas and enables low-latency services.