The Middle Ground: Finding the "Goldilocks" Orbit

The landscape of satellite communication is often presented as a choice between two extremes. On one end, you have the Geostationary Earth Orbit (GEO), where massive satellites orbit at a distant 35,786 kilometers, appearing stationary in the sky. They provide vast coverage but suffer from a significant, unavoidable signal delay. On the other end, there is the bustling new frontier of Low Earth Orbit (LEO), where megaconstellations of thousands of small satellites zip around the planet at low altitudes, offering fiber-like speeds and low latency but requiring immense complexity and investment.

Between these two lies a third, vital region of space known as Medium Earth Orbit, or MEO. MEO represents the "Goldilocks" zone of satellite orbits, it is not too high, not too low, but often just right for a specific set of critical applications. MEO satellites operate at altitudes above the 2,000-kilometer upper boundary of LEO and below the 35,786-kilometer altitude of GEO. This intermediate positioning allows them to skillfully balance the competing demands of coverage area, signal latency, and constellation size, creating a unique sweet spot in orbital mechanics that has proven indispensable for global navigation and is increasingly important for high-performance data communication.

The Dynamics of the Intermediate Orbit

As dictated by Kepler's Third Law of Planetary Motion, a satellite's altitude directly determines how long it takes to orbit the Earth. MEO satellites, being further away than their LEO counterparts, travel at a more leisurely pace and have significantly longer orbital periods.

• Orbital Period: Typically ranges from 2 to 12 hours.
• Visibility from Ground: For an observer on the ground, a MEO satellite will be visible in the sky for several hours at a time as it passes overhead, much longer than the fleeting minutes of a LEO satellite pass.

The Need for Constellations and Tracking

Despite the longer visibility window compared to LEO, MEO satellites are not stationary from our perspective. They steadily move across the sky. This has two critical implications for system design.

First, like LEO systems, continuous global coverage requires a constellation of multiple satellites. However, the advantage of MEO's higher altitude is its much larger coverage footprint per satellite. A single MEO satellite can "see" a much wider area of the Earth's surface than a LEO satellite. Consequently, a full MEO constellation requires far fewer satellites to achieve global coverage, typically numbering in the dozens (e.g., 20 to 30 satellites) rather than the thousands required for LEO.

Second, ground-based user terminals must actively track the satellites. A fixed dish, like that used for GEO satellite TV, will not work. MEO ground stations require antennas mounted on motorized gimbals that can smoothly follow a satellite as it moves from horizon to horizon. For communication systems that require an uninterrupted link, this often means employing dual-antenna setups. One antenna tracks the currently active satellite, while the second antenna positions itself to acquire the next satellite that is about to rise. A seamless "make-before-break" handoff is then performed to switch the connection between the two antennas, ensuring continuous service.

MEO's Killer Application: Global Navigation Satellite Systems (GNSS)

While LEO is making headlines for broadband internet and GEO dominates television broadcasting, Medium Earth Orbit is unequivocally the home of global navigation. Nearly every Global Navigation Satellite System (GNSS) in operation today utilizes MEO because this orbit offers the perfect combination of geometric stability and global coverage needed for precise positioning.

The Principle of Trilateration: Finding Your Place in the World

The magic behind GNSS is a geometric principle called trilateration. Each satellite in a GNSS constellation continuously broadcasts a highly precise timing signal. A receiver on the ground (in your phone or car) picks up these signals from multiple satellites. By measuring the tiny difference in the arrival time of the signals, the receiver can calculate its distance to each of those satellites. If you know your distance from one satellite, you know you are somewhere on the surface of a giant sphere with that satellite at its center. If you know your distance from two satellites, your location is narrowed down to the circle where those two spheres intersect. With a third satellite, your location is narrowed down to just two points. A fourth satellite is required to resolve this ambiguity and, crucially, to solve for any errors in the receiver's own clock, allowing for an extremely precise calculation of your latitude, longitude, altitude, and the current time.

MEO: The Perfect Orbit for Navigation

MEO is the ideal orbit for this task for several reasons:

• Global Visibility: The MEO altitude is high enough that a constellation of just 24 to 30 satellites can ensure that at least four satellites are visible in the sky from almost any point on Earth at all times.
• Stable Geometry and Predictability: The satellites move in highly predictable paths and at a slower angular velocity than LEO satellites. This provides a stable and well-distributed geometry of visible satellites, which is essential for accurate trilateration.
• Less Orbital Decay: MEO is high enough to be above almost all of the Earth's residual atmospheric drag. This means the orbits are very stable and require minimal station-keeping maneuvers, making their long-term positions highly predictable, a key requirement for a timing-based system.

The World's Premier GNSS Constellations

• GPS (Global Positioning System): The original and most well-known GNSS, operated by the United States Space Force. It consists of a constellation of about 31 satellites orbiting at an altitude of approximately $20,200 \text{ km}$ with an orbital period of about 12 hours.
• GLONASS (Global Navigation Satellite System): Russia's global navigation system, also operating in MEO at a slightly lower altitude of around $19,100 \text{ km}$.
• Galileo: The European Union's global navigation system, designed for civilian control and use. It orbits at an altitude of about $23,222 \text{ km}$ and is designed to offer higher accuracy and reliability than earlier systems.
• BeiDou: China's navigation system, which uses a combination of GEO, MEO, and inclined geosynchronous orbit (IGSO) satellites to provide both regional and global services.

MEO for Data: The Case of the "Other 3 Billion"

While GNSS is the primary user, MEO has also been successfully exploited for high-performance data communication, notably by the O3b constellation, now part of SES. O3b, which stands for the "Other 3 Billion," was designed to address a market that LEO and GEO were not optimally serving.

Operating in an equatorial MEO at an altitude of approximately $8,000 \text{ km}$, O3b provides a fiber-like, low-latency broadband service. With a round-trip latency of around $150 \text{ ms}$, it is dramatically faster than GEO and is suitable for demanding enterprise applications, tele-medicine, cellular backhaul for mobile operators, and providing high-speed internet to cruise ships and governments located in a wide band around the equator. O3b's success demonstrates the viability of MEO as a powerful data communication solution for high-value markets where latency matters and terrestrial infrastructure is lacking.

Summary of Trade-offs: MEO in a Multi-Orbit World

Medium Earth Orbit represents a masterclass in engineering trade-offs, carving out a vital role in a world where a single orbital type cannot meet all needs.

• Advantages: It offers a much lower latency than GEO, requires a significantly smaller constellation for global coverage than LEO, and resides in a stable orbital environment with minimal atmospheric drag.
• Disadvantages: MEO systems require complex and expensive tracking antennas on the ground. Their latency, while good, is still higher than what LEO systems can achieve. Furthermore, their orbits must often traverse the Van Allen radiation belts, which contain high-energy charged particles that can damage satellite electronics. This necessitates robust radiation hardening, which adds to the satellite's mass, complexity, and cost.

In the evolving architecture of global communications, the future is multi-orbit. MEO's unique balance of properties ensures it will remain the indispensable foundation for global navigation systems. Simultaneously, it will continue to serve critical data markets, acting as a crucial intermediate layer that bridges the gap between the terrestrial-like performance of LEO and the vast, stationary broadcasting power of GEO.