Medium Earth Orbit

Medium Earth Orbit (MEO) refers to the region of space around the Earth that lies between Low Earth Orbit (LEO) and Geostationary Earth Orbit (GEO). It is typically defined as the range of altitudes between 2,000 kilometres and 35,786 kilometres above the Earth’s surface. Satellites operating within this orbital zone serve diverse functions including navigation, communication, and scientific observation. MEO plays a crucial role in modern satellite systems, particularly in global positioning and timing networks.

Background and Orbital Characteristics

The concept of orbital zones is used to categorise satellites based on their altitude, speed, and period of revolution around the Earth. MEO represents the intermediate zone:

• Low Earth Orbit (LEO): Extends up to about 2,000 km; used for imaging, observation, and some communication satellites.
• Medium Earth Orbit (MEO): Extends from about 2,000 km to 35,786 km; used for navigation and communication systems.
• Geostationary Orbit (GEO): At approximately 35,786 km; used for weather monitoring, broadcasting, and high-coverage communications.

Satellites in MEO have orbital periods ranging from about 2 to 12 hours, depending on their altitude. Unlike GEO satellites, which appear stationary relative to the Earth, MEO satellites move relative to the planet’s surface, meaning that multiple satellites are typically needed to ensure continuous coverage of any region.
The most common inclination for MEO satellites ranges between 55° and 65°, enabling wide coverage of both hemispheres. The gravitational effects of the Earth’s shape (oblateness) and lunar–solar perturbations are important factors in determining the stability of MEO satellite orbits.

Applications of Medium Earth Orbit

1. Navigation Systems: The most prominent use of MEO is in global navigation satellite systems (GNSS). These include:

• Global Positioning System (GPS) of the United States.
• GLONASS of Russia.
• Galileo of the European Union.
• BeiDou of China.

These systems use constellations of satellites placed at altitudes of around 20,000 km. Each satellite continuously transmits time and position data that enables receivers on Earth to determine their location with high accuracy.
2. Communication Satellites: Although MEO is less commonly used for communication than LEO or GEO, it offers a balance between latency and coverage. Systems such as O3b (“Other 3 Billion”) Networks use MEO satellites at approximately 8,000 km altitude to provide broadband Internet to remote regions, achieving lower latency than GEO satellites and broader coverage than LEO systems.
3. Scientific and Environmental Missions: Certain research satellites operate in MEO to study radiation belts, space weather, and magnetospheric dynamics. The region intersects the Van Allen radiation belts, making it suitable for studying the interaction between solar wind and Earth’s magnetic field.

Orbital Dynamics and Technical Considerations

The orbital mechanics of MEO require precise management due to its exposure to the Van Allen belts, regions of intense charged-particle radiation that can degrade satellite components. Consequently, spacecraft operating in MEO must incorporate radiation shielding and hardened electronics to ensure operational reliability.
Typical parameters of MEO orbits include:

• Altitude: 2,000 km – 35,786 km.
• Orbital Period: 2 – 12 hours.
• Orbital Velocity: 3 – 7 km/s.

MEO satellites generally require station-keeping manoeuvres to counter gravitational perturbations caused by the Moon, the Sun, and Earth’s equatorial bulge. Advanced propulsion systems or electric thrusters are often used to maintain orbital stability.

Advantages of Medium Earth Orbit

• Wider Coverage: A single MEO satellite can cover a larger portion of the Earth’s surface than an LEO satellite.
• Moderate Latency: Communication latency (about 100–150 milliseconds) is lower than GEO but higher than LEO, offering a practical compromise for data services.
• Reduced Satellite Count: Fewer satellites are needed to provide global coverage compared to LEO constellations.
• Stable Observation Zone: Suitable for consistent signal transmission, especially in navigation systems.

Disadvantages and Challenges

• Radiation Exposure: Proximity to the Van Allen belts subjects satellites to high radiation levels, necessitating protective design measures.
• Higher Launch Costs: Reaching MEO requires more energy than placing satellites in LEO.
• Complex Tracking: Since MEO satellites move relative to the Earth’s surface, ground stations require advanced tracking and switching capabilities.
• Signal Delay: Though improved compared to GEO, latency can still affect time-sensitive communication applications.

Examples of Prominent MEO Systems

1. GPS (United States):
   • Altitude: ~20,200 km
   • Constellation: 24 operational satellites (with spares)
   • Orbital Period: ~12 hours
   • Purpose: Navigation, timing, and military use.
2. Galileo (European Union):
   • Altitude: ~23,222 km
   • Constellation: 24 satellites + spares
   • Purpose: Civilian global positioning and navigation accuracy enhancement.
3. GLONASS (Russia):
   • Altitude: ~19,100 km
   • Constellation: 24 satellites
   • Purpose: Navigation and geolocation.
4. BeiDou (China):
   • Altitude: 21,500 km (MEO component)
   • Hybrid configuration including MEO, GEO, and inclined geosynchronous orbits for global coverage.
5. O3b Networks:
   • Altitude: ~8,000 km
   • Purpose: High-speed, low-latency Internet connectivity for remote regions.

Emerging Trends in MEO Usage

With the expansion of global communications and navigation demands, MEO is witnessing renewed interest. Hybrid satellite constellations are being developed to combine the strengths of LEO, MEO, and GEO systems. These configurations enhance data throughput, redundancy, and service resilience.
Developments in electric propulsion, miniaturised satellite technology, and radiation-resistant materials are further increasing the feasibility and cost-effectiveness of MEO missions.