Acronym of the day – MEO

Last time I explained what the acronym LEO stands for. Today, we’re going to look at a similar one called MEO. Can you guess what this stands for? Or do you know something that you’re probably using regularly that uses MEO itself? Let’s find out.

We’ll start with the acronym itself. MEO stands for Medium Earth Orbit.

What is a Medium Earth Orbit?

A Medium Earth Orbit is an orbit around Earth with an altitude between 2,000 km and 35,780 km above the Earth’s surface. The orbital period is between 2 hours and less than 24 hours. This is the time it takes the satellite to complete one orbit.

Compared to the Low Earth Orbit, the Medium Earth Orbit has a higher altitude and takes more time to complete one orbit. So, before we take a closer look at Medium Earth Orbits, let’s explore how altitude and speed are connected.

Step on the break – to speed up!

Here’s a question for you. What do you have to do to speed up how fast your satellite is going around Earth? I probably should rephrase that a bit. The goal is to reduce the time it takes your satellite to complete one orbit.

1. Fire the thruster in the direction of travel (speed up)
2. Fire the thruster against the direction of travel (slow down)

If you said 1, sorry, you’re wrong. You have to fire against the direction of travel. This reduces the altitude of your orbit, which in turn increases how fast you are going around Earth. As seen from Earth. If you want to go slower, fire your thruster in the direction of travel and increase your altitude. Because the higher the altitude, the longer it takes to complete an orbit.

This relation also influences which altitudes and shapes of orbits are chosen for satellites.

Shapes of orbits

So, what parameters aside from altitude and speed influence your orbit?

Eccentricity

Eccentricity describes the shape of the orbit. A near circular orbit has a very low eccentricity, while a highly elliptical orbit has a high eccentricity. Or to put numbers to it, a near circular orbit has an eccentricity near 0. A highly elliptical orbit has an eccentricity near 1. It, however, is always less than 1.

Because it is accelerated by our planet’s gravity, the satellite moves quickly when it is close to the Earth. As it moves away, its speed slows, so it spends more time at the top of its orbit, farthest from the Earth.

But what is it good for? We’ll talk about that a bit later when we take a look at two types of Medium Earth Orbits. For now, let’s take a look at the inclination of an orbit.

Inclination

The inclination of an orbit describes its angle in relation to Earth’s equator. An orbit directly above Earth’s equator has an inclination of 0 degrees. An orbit, which is going over the North Pole and the South Pole, has an inclination of 90 degrees. Furthermore, an inclination of 180 degrees means, that the satellite is going in the opposite direction of Earth’s rotation.

Why is the inclination important? Because it basically defines how much of the Earth surface a satellite can cover. The higher the inclination, the more of the surface the satellite can see. A satellite with inclination 0 degrees can only observe the equator. Unless it is far enough out to see more of the Earth surface. But it will never directly fly overhead. In contrast, a satellite with an inclination of 90 degrees can observe the complete surface.

What are MEO orbits used for?

Alright, let’s get back to Medium Earth Orbits. What kinds of satellites are using them? They are mostly used by satellites for navigation and positioning systems, communications, and weather forecasting. That’s because of their high altitude, they are visible from a large portion of the Earth’s surface. While at the same time, they can observe a particular region on Earth surface for a longer time.

There are two notable Medium Earth Orbits. Let’s take a look at the semi-synchronous orbit and the Molniya orbit.

The semi-synchronous orbit

A semi-synchronous orbit is an orbit where the satellite needs half as long for one orbit as the planet takes for one rotation on its axis. For Earth, this means the satellite requires 12 hours for one orbit. It has an altitude of about 20,200 km above the Earth surface and a very low eccentricity. This orbit is consistent and highly predictable.

A satellite in this orbit passes over the same spot on the Earth surface twice every day. That might be a reason why the GPS constellation is using the semi-synchronous orbit. Its 24 satellites orbit the Earth with an inclination of 55 degrees.

The GLONASS, Galileo and BeiDou constellations use quite similar orbits. Which leads to them needing a couple of days to pass over the same location on the Earth surface. Here’s an overview of their MEO orbits:

The Molniya orbit

Soviet engineers proposed the Molniya orbit first in the 1960s. They were looking for a way to cover high latitudes with satellites. As a satellite placed over the equator doesn’t work well for far northern or southern locations. Seen from the ground, the satellite is only visible near the horizon.

The Molniya orbit was designed to have a much better coverage of these areas. It combines a high inclination (63.4°) with a high eccentricity (0.722). Which leads to a very elliptical orbit. The satellite flies fast, when it’s near the Earth and slows down, when it’s going farther away. This also means, the satellite spends more time (two-thirds of its orbital period) farther away.

As one orbit lasts about 12 hours, the satellite flies over the same location every day and night. This, combined with the inclination of the orbit, made it possible to have good coverage of the far northern locations. For example, for communication satellites like the ones from Russia or the Sirius radio satellites.

Differences between LEO and MEO

Let’s compare LEO and MEO. Looking at the altitude, MEO is above LEO. Satellites in LEO take less time for one orbit of Earth than satellites in MEO. Because of the higher altitude, satellites in MEO can be seen from a large region on the surface, than satellites in LEO. They are also longer in range than satellites in LEO.

However, MEO orbits are also more distant from the Earth’s surface than LEO orbits, which means that the satellites in this orbit have a longer round-trip time for communications, which can impact the speed and latency of the signal.

To sum it up, both types of orbits have their pros and cons. Which orbit you choose mainly depends on the purpose of your satellite and which drawbacks you’re willing to take.

Do you want to know more about acronyms in space? Check out other acronyms.