Which orbit is best for imaging satellites?
Suppose you have a satellite. Now you need to launch it into orbit so that you can photograph the Earth’s surface and conduct science or provide satellite images for your clients or yourself. To which orbit should you launch it? In today’s article we will briefly tell you, which orbit is best for imaging satellite and why!
Brief summary of orbit types
Firstly – what is an orbit? It is the path an object takes in space around another body (or, more precisely, a barycenter) as a result of the force of gravity. Let us narrow our considerations to the Earth’s orbit.
For the purposes of our considerations, we can distinguish between several main types of orbit:
- LEO – Low Earth Orbit
- MEO – Medium Earth Orbit
- GEO – Geostationary
- SSO – Sun-synchronous orbit
LEO is the lowest orbit, which extends from the lowest stable orbit that an object can maintain to about 1,000 km above the Earth’s surface. It is a popular orbit for many communication satellites (e.g. Starlink, Iridium) and perhaps the world’s most famous satellite, the International Space Station. It is the orbit that is easiest to access due to the low energy required to launch a satellite. In addition, the low latency and high bandwidth makes this orbit very useful for many applications.
MEO orbit spans above Low Earth orbit and it’s commonly used by navigation satellite systems, such as American GPS or European Galileo. Its boundary is the GEO orbit.
GEO orbit is 35,786 km above the equator. Each object placed in this orbit takes 23 hours, 56 minutes and 4 seconds to complete a full orbit – matching the Earth’s period of rotation. This results in the apparent lack of movement of the satellite – it remains above a specific point on the Earth. This makes satellite TV possible, as well as other use cases such as ESA’s European Data Relay System.
And then there is the SSO orbit. This is a type of LEO orbit, but is usually distinguished as a separate type of orbit. Unlike 'standard’ orbits, objects in an SSO move from north to south, rather than from west to east. What is special about this orbit? Satellites launched in this orbit are synchronous with the Sun, which means that the satellite passes over the same place at the same time, e.g. over Wrocław every day at 15:00.
Of course, there are many different types of orbits like GSO, GTO, Molniya or Tundra orbits, but we will not discuss them today.
Low Earth Orbit downsides
Why do we want to pass over the same place at the same time? Because if we want to compare changes on the surface, it is easier to do so under the same lighting conditions, and not once at midnight and once at sunset.
In the table below there are shown some of the next scheduled ISS passes nearly Wrocław:
As you can see, not only does the ISS pass near Wrocław at different times (and additionally at night, which makes it very difficult to observe any changes; however, it should be mentioned that there are periods when the ISS passes through our area at different times), but also the time of visibility from Wrocław changes (which means that the ISS passes at different distances). Wouldn’t it be easier if the satellite passed over our hometown at the same time each time?
What information can we get from photo comparison?
As we know, imaging from space can help us in many ways. As explained in a previous article, Scanway’s satellites are capable of observation of urban development, agriculture, object recognition (cars, buildings), defense purposes, natural disasters and climate change.
Some of those use cases are long-term, and some are short-term. For example, climate change is visible over months, if not years. Natural disasters, on the other hand, happen quickly, within days. Therefore, there is a need to be able to take a picture of an area periodically. Only the SSO orbit offers this possibility. A satellite in LEO, such as the ISS mentioned earlier, may not fly over a given area for many days, hence there would be no opportunity.
The images below show photographs of the Hunga Tonga-Hunga Ha’apai volcano eruption, which occurred in early 2022. As the island nation was cut off from global communications because the tsunami destroyed the undersea fibre-optic cable, scientists were only able to assess the damage in almost real time due to the presence of an imaging satellite in SSO orbit.
Other use cases for comparative photography can be intelligence gathering (such as troop movements), analyzing crop yields and determining the correct harvest time, analyzing urban development and much more. With a variety of sensors, the limitation of the use of satellite imaging is often only the right software and the user’s imagination. The important thing is that data analysis can be done using artificial intelligence.
How to launch a satellite into Sun Synchronous Orbit?
This is not very different from launching satellites into standard LEO orbits. The main difference is the north-south trajectory of the rocket; also, some launch pads cannot use certain launch azimuths for safety reasons, so choosing a special launch pad is another challenge (e.g. you can’t launch directly to SSO from Florida as well as to a standard one from west to east from Vandenberg).
Since you are not launching in line with the Earth’s rotation, you will have to spend more fuel and oxidizer to get the rocket to orbital velocity, as you will not get the 'free boost’ resulting from the said rotation. For example, SpaceX launches 54 Starlink satellites into standard Low Earth Orbit, but „only” 46 when launching into Sun-Synchronous orbit.
Our STAR VIBE mission was launched to orbit SSO on the January 3 this year on a SpaceX Transporter-6 rideshare mission. It was launched from Florida on a Falcon 9 rocket. As previously mentioned, a direct launch to SSO from Florida is not possible, as rockets are not allowed to fly over populated areas and nearby islands for safety reasons. For this reason, the rocket performed a so-called 'dogleg’ maneuver, changing its trajectory in flight.