How to Keep Your Satellite Pointing at Earth

The guest on this show is Jack Reed, a PhD student at the MIT Media Lab. He started out in mechanical engineering and then later on moved into aerospace engineering. At the MIT Media Lab, he is part of an interdisciplinary research group alongside lab mates with backgrounds in data science, ethics, and art. Together they work on making space sustainable, and using space-based assets and imagery to help promote sustainability on Earth.

Why do We Need to Orient Satellites?

We tend to assume that since space is a vacuum, then there is nothing that would make the satellite drift from its original course. If this is the case, then there should really be no need to worry about orientation since the satellite should keep pointing to the same direction.

In reality, satellites are affected by forces like atmospheric drag, especially in low Earth orbits. Atmospheric drag slows down satellites and pulls them out of orbit.

Another reason to orient satellites is that as they revolve around the Earth, at 180 degrees they would be pointing directly away from the Earth. Unless they are rotated it will have to go another 180 degrees before they point directly at earth again.

In order to keep the satellite pointing at Earth at all times, they need to be rotated constantly, otherwise, they lose half of their utility.

How Satellites Orientate Themselves in Space?

The ability to control a satellite’s position in three dimensions, and where it is pointing is critical to increasing its lifespan. Controlling it in three-dimensional space helps to keep the satellite in the correct orbit, while controlling where it is pointing to ensure that the satellite is capturing the right data, and sends communications back in the right direction.

If it goes off its orbit, or points in the wrong direction and is unable to get back, its lifespan would be cut short, and a huge amount of resources would be wasted.

Propulsion is used to move a satellite through three dimensions. It enables satellites to get back to orbit if they get off. Propulsion can be achieved by having rockets or thrusters at different corners of a spacecraft for turning. Large spacecraft also use propulsion to control where they are pointing. Ideally, engineers want to achieve propulsion with the least possible amount of fuel, as the weight of the fuel at launch can make the endeavour far more expensive and complicated.

When talking about flight, attitude is the information about an object’s orientation and position on its axis in relation to the plane below it. These properties are pitch, roll, and yaw. For attitude control, most satellites use reaction wheels. Speeding up or slowing down a reaction wheel on a spacecraft, will cause the spacecraft to rotate in the opposite direction. By having at least three wheels, a satellite can precisely orient itself towards wherever it needs to be. You could hypothetically call this an attitude adjustment. 

How Do Satellites Determine Their Attitude?

In order to keep pointing at Earth, satellites need to know how they are oriented relative to the Earth’s location at any particular moment. A spacecraft subsystem, the ADCS (Attitude Determination and Control System) serves the purpose of maintaining a satellite’s optimal orientation. It consists of IMUs (Inertial Measurement Units) and a variety of other sensors working together to tell where a satellite is pointing in relation to the Earth’s location. 

Some of these sensors include:

Horizon Trackers

Since the Earth is warmer than space, infrared cameras can see the cut off of the earth’s horizon in space. In this way, a satellite is able to figure out where the Earth is. The downside though is that due to the horizon’s huge size, it does not give a precise point as to where all satellites will be pointing, producing a more generalized target direction.

Sun Trackers

The sun can be tracked through sensors that look for the hottest thing in the sky. Unlike the Earth’s horizon, which may appear as a huge circle of horizon, the sun tends to be in a very particular spot in the sky that the satellite can point at. By tracking the sun, a satellite can determine how they are oriented in space. During the times in its orbit when the Earth is between the sun and the satellite, sun tracking cannot be used.

Star Trackers

Star tracking is the most accurate method that satellites use to determine their attitude and figure out the location of the Earth. In fact, even the early astronauts using sextants, an instrument used for measuring the positions of stars since the 18^th century. Star trackers compare the position of very particular stars against a star catalogue in order to tell the direction in which a satellite is pointing.

Magnetometers

Satellites on Earth orbits can use the Earth’s magnetic field to orient themselves. A magnetometer which detects the Earth’s magnetic field can tell a satellite which direction is north, and with this, a satellite can determine its orientation.

Earth based Ground Stations

Broadcasted signals from ground stations on Earth can be used by satellites to figure out where the Earth is relative to their current position. A major limitation to this method is that the signals’ strength fades out the further the satellite travels, and many ground stations are needed to keep continuous coverage and not lose data.

Can GNSS Satellites Be Used to Orientate Low Orbit Satellites?

GNSS constellations live about 20,000 km above Earth, and hypothetically they can potentially help orient low orbit satellites, which are usually below 2000 Km. While it is possible, there are certain caveats that limit using GNSS satellites from orienting lower satellites. Firstly, GNSS satellites are broadcasting specifically to Earth, so it is possible to receive a signal from at least four of them at any point on Earth, and triangulate an accurate position. This is how we use GNSS for earth navigation systems, and GPS. As you move higher, the field of view gets narrower and satellites which are very high up may not receive signals from enough GNSS satellites to triangulate an accurate position. Additionally, GNSS predominantly gets a satellite’s position, but not its attitude or orientation.

Want to learn more about using GNSS on earth? Listen here to learn about Where Does the Blue Dot Come From?

Edge Computing

Tones of high-resolution imagery is being captured by earth observation satellites every day, and edge computing has an important role to play here. It is not necessarily feasible to transmit everything that a satellite captures, since some of the data may not be usable. For instance, running algorithms on captured images out in space will help to sort out the ones that have too much cloud in them, and only downlink the images that would be usable. Algorithms can also complete some light processing and correction of the images, so the data is already in a more usable state before being sent down to a ground station on earth.

Space and Geospatial

Compared to previous generations, we are certainly living in a golden age of both geospatial data, and the space industry. There are a lot of commercial players that are designing, building, and launching new Earth observation satellites; and many others who are figuring out new and innovative ways of processing that data and turning it into useful products for various applications. Earth observation satellites are surely becoming an important pillar for many geospatial applications.

Want to learn more about the growth of earth observation, remote sensing, and imagery? Listen to our podcast with Dr. Aliastair Graham

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