SATELLITE ORBITS
We've mentioned in our previous article that Diwata-1 took an ISS orbit while Diwata-2 will be launched to a Sun-synchronous orbit. We've also mentioned that Agila-1 and Agila-2 were previously positioned to a certain degree of orbital slot.
But what are these things called orbit? Do I need to know these things? Why should I care?
Orbit is the path followed by a satellite. You are probably aware that the moon orbits the Earth -- and moon is a natural satellite! There are several types of orbits, and each has their pros and cons. Let us take a look at some of the most common satellite orbits.
Low Earth Orbit (LEO)
Satellites in this orbit resides from 180 km to 3000 km from the Earth's surface. Due to its closeness to the surface, this orbit is easily accessible and launch opportunities are common, keeping the launch cost low. This orbit is popular for remote sensing applications since small and relatively less powerful cameras can work well from this distance. Communications can also be a good mission choice since transmit power requirements can be small.
However, satellites in this orbit can cover only a small area and they pass any specific spot on Earth only for a short time. The orbital period of a satellite at 600 km to 700 km plays at around 97 minutes or higher, and overhead communications time only runs at approximately 10 minutes or higher. Thus, for remote sensing, you will need to have camera pointing capabilities -- requiring attitude maneuvers. For communications, a constellation might need to come in play for better coverage.
The ISS orbit of Diwata-1 is a special LEO. It is around 400 km from the Earth's surface and is inclined by 51.64°. As you already probably guessed, this is the International Space Station's orbit. This orbit is very popular to small satellites particularly cube satellites due to regularity of launch opportunities.
The Sun-synchronous orbit (SSO) is another special orbit such that the satellite will pass any point on the Earth at the same local time everyday. This makes it more ideal for monitoring than the ISS orbit. Diwata-2 will take a 10:30 LTDN (Local Time of the Descending Node) SSO at about 600 km.
Medium Earth Orbit (MEO)
This orbit occupies the range 3000 km to <35,786 km from the Earth's surface. This orbit is usually occupied by navigation satellites. It has longer orbital period and also provides longer communication time than LEO. This makes the number of satellites needed for constellation smaller.
Geosynchronous Orbit (GSO) and Geostationary Earth Orbit (GEO)
The Geosynchronous Orbit (GSO) is located 35,786 km from the Earth's surface. It has an orbital period that is approximately 24 hours, matching the Earth's rotation on its axis. The Geostationary Earth Orbit (GEO) is a special type of GSO wherein the satellite is placed on the equator (0° inclination), thus, it is also known as Geosynchronous Equatorial Orbit (GEO).
In this orbit, satellites can provide a constant coverage and can see almost 50% of the Earth's surface, making it popular to communications satellite. Satellites in this orbit are designed for longer lifetime and would require propulsion mechanisms, larger power capacity, higher-powered transmitters. This makes the satellite size and cost large. Further, satellites in this orbit acts as main payload in rockets, making the launch cost also high.
There is a special orbit called the Geostationary Transfer Orbit (GTO). It is an inclined elliptical orbit connecting LEO and GEO and is used to transfer satellites to their operational orbits in GEO. You can actually think of this as the temporary orbit of the satellite as it moves to its target orbital slot. Orbital slot, which we have mentioned for Agila-1 and Agila-2, is the designated location of the satellite in space. Basically, the satellites line-up along the 35,786 km equatorial space and the slot assignments are spaced across the longitudes.
High Elliptical Orbit (HEO)
This is an inclined orbit with an altitude of a few hundred up to 100,000 km. The Space Observatory ISO is located at 1000 km (perigee) x 70,500 km (apogee).
There is a special type of HEO named the Molniya Orbit. This orbit is at 500 km x ~40,000 km with 63.4° inclination. It is popularly used by Russia since it can provide better coverage at high latitudes than GEO. It is also easier to launch to this orbit from high latitude launch sites.
Going back, why should you care about orbits?
If you haven't noticed it yet, your chosen orbit helps you achieve a successful mission. Say, you want to do optical imaging and monitoring, you need to make your satellite pass to your location at a regular time everyday, and at a time when there is less cloud cover. Thus, you choose a Sun-synchronous orbit passing your location in the morning. Your orbit can also drive the cost. At higher orbit, the development, test and launch costs tend to be higher.
Most importantly, your choice of launcher depends on your desired orbit. This is because not all rockets can bring your satellite to your targeted orbit. Further, when there is rare launch opportunities, launch cost can also be higher.
Low Earth Orbit (LEO)
Satellites in this orbit resides from 180 km to 3000 km from the Earth's surface. Due to its closeness to the surface, this orbit is easily accessible and launch opportunities are common, keeping the launch cost low. This orbit is popular for remote sensing applications since small and relatively less powerful cameras can work well from this distance. Communications can also be a good mission choice since transmit power requirements can be small.
However, satellites in this orbit can cover only a small area and they pass any specific spot on Earth only for a short time. The orbital period of a satellite at 600 km to 700 km plays at around 97 minutes or higher, and overhead communications time only runs at approximately 10 minutes or higher. Thus, for remote sensing, you will need to have camera pointing capabilities -- requiring attitude maneuvers. For communications, a constellation might need to come in play for better coverage.
The ISS orbit of Diwata-1 is a special LEO. It is around 400 km from the Earth's surface and is inclined by 51.64°. As you already probably guessed, this is the International Space Station's orbit. This orbit is very popular to small satellites particularly cube satellites due to regularity of launch opportunities.
The Sun-synchronous orbit (SSO) is another special orbit such that the satellite will pass any point on the Earth at the same local time everyday. This makes it more ideal for monitoring than the ISS orbit. Diwata-2 will take a 10:30 LTDN (Local Time of the Descending Node) SSO at about 600 km.
Medium Earth Orbit (MEO)
This orbit occupies the range 3000 km to <35,786 km from the Earth's surface. This orbit is usually occupied by navigation satellites. It has longer orbital period and also provides longer communication time than LEO. This makes the number of satellites needed for constellation smaller.
Geosynchronous Orbit (GSO) and Geostationary Earth Orbit (GEO)
The Geosynchronous Orbit (GSO) is located 35,786 km from the Earth's surface. It has an orbital period that is approximately 24 hours, matching the Earth's rotation on its axis. The Geostationary Earth Orbit (GEO) is a special type of GSO wherein the satellite is placed on the equator (0° inclination), thus, it is also known as Geosynchronous Equatorial Orbit (GEO).
In this orbit, satellites can provide a constant coverage and can see almost 50% of the Earth's surface, making it popular to communications satellite. Satellites in this orbit are designed for longer lifetime and would require propulsion mechanisms, larger power capacity, higher-powered transmitters. This makes the satellite size and cost large. Further, satellites in this orbit acts as main payload in rockets, making the launch cost also high.
There is a special orbit called the Geostationary Transfer Orbit (GTO). It is an inclined elliptical orbit connecting LEO and GEO and is used to transfer satellites to their operational orbits in GEO. You can actually think of this as the temporary orbit of the satellite as it moves to its target orbital slot. Orbital slot, which we have mentioned for Agila-1 and Agila-2, is the designated location of the satellite in space. Basically, the satellites line-up along the 35,786 km equatorial space and the slot assignments are spaced across the longitudes.
High Elliptical Orbit (HEO)
This is an inclined orbit with an altitude of a few hundred up to 100,000 km. The Space Observatory ISO is located at 1000 km (perigee) x 70,500 km (apogee).
There is a special type of HEO named the Molniya Orbit. This orbit is at 500 km x ~40,000 km with 63.4° inclination. It is popularly used by Russia since it can provide better coverage at high latitudes than GEO. It is also easier to launch to this orbit from high latitude launch sites.
Going back, why should you care about orbits?
If you haven't noticed it yet, your chosen orbit helps you achieve a successful mission. Say, you want to do optical imaging and monitoring, you need to make your satellite pass to your location at a regular time everyday, and at a time when there is less cloud cover. Thus, you choose a Sun-synchronous orbit passing your location in the morning. Your orbit can also drive the cost. At higher orbit, the development, test and launch costs tend to be higher.
Most importantly, your choice of launcher depends on your desired orbit. This is because not all rockets can bring your satellite to your targeted orbit. Further, when there is rare launch opportunities, launch cost can also be higher.
References:
Spacecraft Systems Engineering, 3rd edition
Satellite Orbits: Models, Methods and Applications, Montenbruck, Gill
Handbook of Space Technology by Wilfried Ley, Klaus Wittmann, Willi Hallmann
Comments
Post a Comment