Orbits : Wonder where the satellites go?

What are orbits?

The planets in our solar system orbit the Sun along well-defined paths, and the Moon follows a fixed trajectory around the Earth. This consistent trajectory is known as an “orbit.” Similarly, when we hear about satellites being launched into space, it’s not a random toss into the void. Each satellite’s path is meticulously calculated and strategically determined for a specific purpose.

There are 3 orbits defined with respect to the purpose it may serve,

1. Low Earth orbit

As the name implies, Low Earth Orbits (LEO) are the closest orbits to our planet. These orbits range from just above the Earth’s surface to around 1,000 kilometres in altitude. In fact, LEO can extend as low as 160 kilometres above the Earth's surface.

To put this in perspective, consider that commercial airplanes generally fly at altitudes up to 14 kilometres. LEO satellites, however, orbit at distances more than ten times this height. This significant difference underscores the vast scale of space compared to our daily experiences.

One of the remarkable features of LEO is its flexibility in terms of orbital inclination. Unlike some other orbits that require alignment with the equator or poles, LEO orbits can be inclined at various angles according to mission needs. This versatility allows satellites to pass over different regions of the Earth, providing comprehensive coverage.

Polar Orbits: A Subset of LEO

Polar orbits are a specific type of LEO, where satellites travel from north to south or vice versa. These orbits are particularly useful for observations of the Earth’s poles and surrounding areas. Polar orbits don’t need to pass directly over the North and South Poles; a deviation of 20 to 30 degrees is still classified as a polar orbit.

Polar orbits typically range from 200 kilometres to 1,000 kilometres above the Earth. This range makes it relatively easier to deploy satellites into these orbits compared to the more complex higher orbits, contributing to the crowded nature of LEO.

Sun-synchronous Polar orbits (SSO) is a sub-type. Satellites in an SSO are placed in in such a way that they are always in a fixed position relative to the Sun, which also means, the satellite will be at a location exactly over a point on Earth, at a given time of the day, for the rest of it’s time in the orbit. In layman terms, imagine a satellite is spotted over Paris at sharp 1300 hrs, the next day it can again be seen at that very spot, at that exact time.

Time is relative to the Sun, and the satellite is fixed relative to the Sun, hence the recurrence of the event.

2. Medium Earth orbit

Medium Earth Orbits are like the middle child of the orbital family. They're sandwiched between Low Earth Orbit (LEO) and Geostationary Orbit (GEO). Picture it like this: LEO is where you’d find your friendly neighbourhood spy satellites and the International Space Station, cruising at around 200 to 2,000 kilometres above Earth. GEO, on the other hand, is where the big shots like weather satellites and communication satellites hang out, sitting at a distance of 35,786 kilometres away, just chilling out there for eternity. MEO is right in between—generally ranging from 2,000 to 20,000 kilometres above the Earth’s surface.

The Sweet Spot of MEO

Why is MEO so special? Well, it's the Goldilocks zone for a variety of missions. For instance, MEO is where you’ll find Global Navigation Satellite Systems (GNSS) like GPS. These satellites need a good balance between being high enough to cover a large portion of the Earth’s surface and low enough to maintain a decent signal strength. Think of them as the omnipresent but not overbearing guides who always seem to know exactly where you are—creepy, but in a helpful way.

Another notable resident of MEO is the communications satellite constellation. Unlike their GEO counterparts that are stationed in a fixed position relative to the Earth, MEO satellites are in constant motion relative to the Earth's surface, which makes them perfect for systems requiring frequent updates or data transmissions over larger swaths of the globe. It’s like having a set of super-efficient, constantly rotating space satellites making sure your satellite TV signal doesn’t drop during that crucial moment of your favourite reality show.

Why Bother with MEO?

You might ask, “Why not just use LEO or GEO for everything?” Well, that’s a fair question. MEO orbits offer an ideal middle ground for balancing coverage, signal strength, and orbital lifespan. They avoid the congestion of LEO and the high costs and maintenance of GEO orbits. Plus, they allow for a network of satellites that can provide consistent global coverage without the need for a massive fleet of high-altitude GEO satellites.

So next time you find yourself marvelling at how your GPS can pinpoint your location within a few meters, give a nod to the trusty MEO satellites. They’re the unsung heroes of the orbital world—keeping your adventures on track and your reality shows uninterrupted. After all, in the grand theatre of space, every orbit has its role.

3. High Earth orbit (HEO)

If Low Earth Orbit (LEO) is the bustling downtown and Medium Earth Orbit (MEO) is the cozy suburb, then HEO is the luxury penthouse suite, far above the fray.

High Earth Orbits are like the high rollers of the orbital world, positioned well beyond the range of MEO, typically starting from around 20,000 kilometres and extending up to the vast reaches of space. They include orbits like the Geosynchronous Orbit (GSO) and its more specific subset, the Geostationary Orbit (GEO).

Geosynchronous vs. Geostationary: A Tale of Two Orbits

First, let’s clear up a common mix-up: Geosynchronous and Geostationary Orbits are often confused, but they have distinct characteristics. Geosynchronous Orbits are those where a satellite’s orbital period matches the Earth’s rotation period of approximately 24 hours. This means the satellite follows the same path over the Earth each day, albeit not necessarily fixed above the same point.

Geostationary Orbits (a special type of Geosynchronous Orbit) are the crème de la crème of HEO. These orbits are positioned exactly 35,786 kilometres above the equator, and they rotate in sync with the Earth’s rotation. This cosmic choreography means that a satellite in GEO appears to be stationed over the same spot on Earth at all times. Imagine a satellite with a front-row seat to the Earth’s show, always watching the same view without missing a beat.