The advent of Earth observation has revolutionised our understanding of the planet and its dynamic processes. With steady advancements in technology, we now have a vast array of tools, from satellites to balloons, to near-surface drones and high-altitude platforms. These eyes in the sky provide massive amounts of data every minute of every day. This has put us in the most informed position in our history and makes the challenges we collectively face even clearer.
77 Years Out Of 300,000 Years
The first image: Earth from space taken from onboard a V2 rocket test out of New Mexico, USA, 24 October 1946. (Photo credit: US Army)
Our species has wandered the planet for over 300,000 years. We moved across continents and built civilisations that grew, thrived, and fell; the ages of stone, iron, bronze, and the revolution of agriculture and industry have passed by.
Fifteen thousand generations passed, and in 1946 for the first time, we developed the technological capability that allowed us to see where we actually are.
It’s easy to miss the enormity of the moment; a grainy black and white image, taken from a film camera that caught a ride to space on an old weapon of war. But the photograph was truly momentous. It was the dawn of when our species could observe the Earth’s surface, its thin protective atmosphere, and the cosmic abyss beyond. We had observed Earth from space. The era of Earth observation had begun.
A Cosmic Perspective
The Blue Marble: The first time a human captured the full sphere of the Earth in one image. Apollo 17, the last mission to the Moon, December 7th 1972. This photo was taken 29,000 km from Earth, somewhere between the orbital height of current GPS satellites and geostationary orbit. (Photo Credit: NASA)
When you place it in the context of life on our planet, existing for nearly 4 billion years, the momentousness is elevated far further. The first time a species on a planet reaches a technological inflection point to be able to view itself is a big deal. Some people may point out that it is critical to a species’ ability to persevere in the cosmos at all, a sub-step within The Great Filter. It provides the critical template for true reflection on our actual existence in the vastness of space. We then truly realise our uniqueness and vulnerability.
When that point is reached, a narrative of stewardship is hard to ignore. This was, of course, enhanced by the images that followed, the most powerful of which was perhaps the Blue Marble, captured by Apollo 17 on the last mission to the Moon in December 1972. The image captured “spaceship earth”, visually reinforcing the reality that our home is a small stage in a vast cosmic arena.
It’s no surprise that revealing these new types of images inspired the birth of the environmental movement in the 1970s, because how can we even begin to understand something before we can observe it? This holds true today, and the realisation of the value of earth observation has driven the continued expansion of a burgeoning earth observation satellite sector.
A Satellite’s View
In 2023 there are over one thousand earth observation (EO) satellites in orbit tasked with monitoring our planet, providing invaluable insights into natural and human processes. We can now image the entire planet every day and have a constant eye on our world using many different observation technologies. National space programs and government agencies have traditionally designed, launched and operated EO satellites. Today, because of the value they serve, commercial enterprises have scaled to step up to the plate. Dozens of companies now provide end-users with high-value data from orbit, which informs critical policy, governance and industry decisions, which have helped advance knowledge across many scientific disciplines.
EO satellites now play a fundamental role in observing the state of our planet. They are critical in monitoring our changing climate, providing routine information about sea surface temperatures, winds, sea level, glacier and ice sheet mass change, air quality, wildlife, etc. They are now a crucial part of accurate weather forecasting services critical to industries from aviation to agriculture, plus everyday life decisions by the public. Using many different instruments, they provide continuous data streams for terrain mapping, land use change, ecosystem monitoring, maritime domain awareness, and disaster monitoring.
Across The Spectrum: Different sensors can be used to create useful and stunning images. Infrared signal from the cloud tops is used here, combined with visual imagery at night showcasing Hurricane Laura before landfall in 2022. (Photo credit: NASA)
Our eyes have evolved to see visible light, but information is available across the electromagnetic spectrum. EO satellites use remote sensing to take advantage of this, gaining insights about our world from the cloud tops, to the shallow seas. Observing systems make use of either passive or active sensors. Passive sensors collect information that is naturally emitted to them, such as visible light for photographs, just like a camera. The highest-resolution commercial sensors say they can provide visual imagery with a ground sampling distance of 30 cm. In practical terms, you should be able to differentiate between objects larger than 30 cm. This provides the most basic kind of visual image, but to gather more data, the resolution typically gets worse as you involve more bands, referred to as multispectral or hyperspectral visual imagery.
Light is emitted across many more wavelengths than what’s visible, so additional information can be gathered if the sensor is tweaked to receive those; common wavelengths used in EO are infrared and microwave energy.
Infrared and thermal bands allow us to identify different classes of objects not only based on their colour but also based on their heat signature. This opens the door to many applications, including heat spot detection for wildfire identification and mapping, and determining the temperature of many parts of the climate system.
Passive microwave sensors take advantage of the fact that all surfaces emit at different frequencies based on their temperature and moisture content; this allows scientists to differentiate between different surfaces from space. Using this technique, they can identify the areas of ocean covered by ice (colder) instead of water (warmer) in the polar oceans, map soil moisture, sea surface temperature, atmospheric water vapour and rainfall rates.
Synthetic Aperture Radar: Day and night, blue sky or cloud, synthetic aperture radar can provide information about the surface. This image shows ships anchored in Vancouver Harbour, showcasing the technologies used for maritime domain awareness. (Photo credit: ESA)
Active sensors emit a signal which is fired at the Earth’s surface, bounces off and is interpreted upon its return to the sensor, a classic example being RADAR – RAdio Detection And Ranging. At shorter wavelengths, lasers do exactly the same thing using LiDAR – LIght Detection and Ranging, but often with more precision. A current NASA mission measures the height of the Earth’s surface every 70 cm with incredible detail. Historically, the cornerstone objective of RADAR missions has been measuring the changing heights of the global oceans. Since the early 1990s, multiple satellite RADAR altimetry missions have been keeping a near-constant check on our rising sea levels. RADAR can also be used to create images of the Earth’s surface by compiling all of the reflected energy into pixels scaled for the power of the intensity of the backscatter. Synthetic Aperture RADAR (SAR) uses the motion of the satellite or aircraft to create a synthetically large aperture that improves the resolution of the resultant image.
Most EO satellites are in either Low Earth Orbit (LEO) or Geostationary Orbit (GEO). LEO is defined as below 2,000 km from the Earth’s surface; most satellites in this configuration are around 500 km above us and will complete an orbit every 90-120 minutes. In GEO, the orbital velocity matches that of the Earth’s rotation. At 35,786 km altitude, far beyond LEO, the satellite is fixed above a specific point on the Earth’s surface. To an observer on the surface, if they could see it, the satellite would appear stationary in the sky. This fixed viewing frame is useful for continuously monitoring large regions of the planet. These orbits are commonly used for weather observation. As these sensors are farther away, they sacrifice spatial resolution, which is much higher than LEO. Therefore, all high-resolution satellite imagery comes from satellites in LEO.
Satellite orbit comparison: Satellite orbits extend from several hundred kilometres above us to tens of thousands of kilometres. (Photo credit: cmglee)
A constellation is achieved by placing multiple satellites in LEO in a sort of web encircling the Earth. With a bit of planning, a constellation can provide impressive coverage. In the design phases of EO instruments, a compromise must be struck between the amount of the Earth’s surface imaged, referred to as ‘swath’, and the detail available in that image, known as its ‘spatial resolution’. The higher the spatial resolution, the smaller the swath and vice versa. So, if you want to achieve great detail in your image, you can only capture smaller regions at a time.
At low resolution, broad coverage can be achieved at a low cost and is commonly provided to the public for free. Higher-resolution products are more expensive to obtain and require more satellites to achieve good daily coverage. These must typically be ordered through a provider like Maxar or Planet. Large amounts of data are archived every day for future users, while clients can also provide acquisition plans to task the satellite to image something at a specific time and location in the future.
Closer To Home
Aircraft also play an important role in EO and do the heavy lifting regarding very high-resolution tasking for things like urban surveying. Typically, resolutions reach 5 cm, at least six times better than satellites. The limitation of aircraft is their range and restricted ability to cover large regions. They also burn large quantities of carbon-intensive aviation kerosene, making it difficult to justify their use if carbon-neutral alternatives are available. They also require a pilot, crew, and generally fair weather conditions to conduct surveys. Small battery-powered drones have begun to play a significant role but are again very limited in range and require an on-site pilot.
A New Era: High Altitude Platforms
Satellites and low-altitude aircraft cannot provide all of the answers we need in our complex, changing world, particularly for questions involving fast processes or the need to be continuously monitored. High-altitude platforms will offer the latest advancements in EO. Operating in the stratosphere, they fill the gap between satellites and crewed aircraft with a vast array of applications; their niche mostly concerns events that happen quickly and that also need consistent monitoring at high spatial resolution. This is very difficult to achieve with satellites. With the deployment of fleets of HAPS around the planet with highly flexible tasking, it will become near-impossible to miss a critical event. And once that event is identified, it can be monitored consistently, taking data availability for decision-making to the next level.
EO is entering a new era with an exciting new capability nearly within our technological reach. Satellites and low-altitude aircraft will continue to be of great value, with HAPS providing an additional tool to access information that hasn’t been available to date. Earth observation was born in an era where the goal was looking to the stars; now, with the challenges we face, it is clear that looking back on ourselves is equally important.