Earth Observation: Our Eyes in the Sky

Since the launch of Sputnik 1 in 1957, the number of artificial satellites in orbit around the Earth has increased enormously. Since 1962, the United Nations Office for Outer Space Affairs has maintained a Space Object Register including objects known to be in orbit around the Earth. The Register currently lists over 13,000 objects in Earth orbit. Of course, not all of these are operational satellites, but the number nonetheless highlights the enormous growth in humanity’s utilisation of Earth orbit.

What are all these satellites used for?  Some are used for navigation purposes, such as the US Global Positioning System or the EU’s Galileo constellation. Many others are used for telecommunications. Nearly half of the 13,000 objects mentioned above are satellites in SpaceX’s Starlink constellation, which provides satellite internet service around the globe. However, probably the earliest application for satellites, which is still extremely important today, is Earth observation.

Earth observation satellites are those used for taking measurements of the Earth’s surface and atmosphere. Even the relatively simple Sputnik 1 enabled measurements of the Earth’s ionosphere, based on the effects on radio transmissions from the satellite, and of the density of the upper atmosphere, based on how quickly the satellite’s orbit decayed.

In the early days of spaceflight, the launching of satellites was almost exclusively the preserve of governments. However, spaceflight today is a very different landscape. The emergence of commercial launch providers has drastically reduced the cost of launching objects into Earth orbit. This makes the deployment of Earth observation technologies a realistic prospect for a much wider range of commercial and scientific operations.

Imaging

As for early government surveillance satellites, imaging the Earth’s surface is still an important application of satellite technology. Various private companies operate imaging satellites and sell high-resolution images for many familiar commercial and civilian purposes such as Google Maps. However, as well as traditional visible-light imaging, modern applications use many other types of imaging.

One example is LiDAR (Light Detection And Ranging), which bounces pulsed lasers off objects to measure distances. This can be used for 3D mapping of the Earth’s surface, providing much more detailed information than is available from normal optical cameras alone. This can be used for terrain mapping, surveying, and many other applications.

Other imaging techniques make use of frequencies of electromagnetic radiation outside of the visible spectrum. Synthetic aperture radar is a technique in which a small radar antenna takes many measurements as it moves over a target region. This allows the measurements to be compiled to produce results that are equivalent to measurements made using a much larger antenna, thereby providing greatly increased resolution.

Satellites equipped with radar are ideal for synthetic aperture measurements. Their orbit moves them along an extremely consistent and predictable path over large distances relative to a target area of the Earth’s surface, thereby allowing composite measurements to simulate a very large antenna. Since radio waves can easily penetrate obstacles such as smoke or cloud cover, satellites equipped with radar can take extremely high-resolution measurements of the Earth’s surface even when the surface would be obscured for visible light imaging.

Weather forecasting

Earth observation from satellites has helped to greatly improve our ability to forecast weather. In contrast to radio waves, microwaves are much more strongly affected by moisture in the atmosphere. Microwave imaging from satellite-based instruments is therefore often used for meteorological applications such as tracking cloud movement, detecting precipitation, or even making measurements of the level of moisture in soil.

Satellite-based instrumentation can also be used to track other variables that are important for understanding and predicting the weather. The European Space Agency’s Aeolus mission, which ended with the re-entry of the satellite in 2023, used the Doppler shift of pulses of UV light fired into the atmosphere to measure wind speeds around the globe. Other satellite-based instruments can measure sea or land surface temperatures, water salinity, and many other variables such as the movement of ocean currents. All of these data can be fed into weather forecasting models to provide accurate forecasting around the world.

Climate

As well as short- and medium-term weather forecasting, meteorological measurements are critical in understanding how our climate is changing and in predicting the longer-term effects of those changes. Satellite measurements contribute to this area as well.

Satellite imagery such as that discussed above can be used to monitor deforestation and other changes to the distribution of vegetation that may be caused by changes in climate. Imagery can also monitor changes in the extent of glaciers, while radar and LiDAR measurements provide estimates of the thicknesses of ice sheets even where their extent has not changed significantly.

One very recent new capability is provided by the EarthCARE mission launched by the European Space Agency in May 2024. This satellite uses a combination of lidar, radar, and thermal radiation sensors to track the balance of incoming radiation from the sun and outgoing radiation emitted by land and reflected from the sea and clouds. This balance is critical for understanding how the overall temperature of the Earth is changing as more or less thermal energy is retained by the Earth and its atmosphere.

Other applications

More esoteric applications of Earth observation satellites are also exploited for scientific and research purposes. Satellite-based instruments have been used for measuring the Earth’s magnetic field and mapping the strength of gravity at different points on the Earth’s surface. The latter can provide insights into the Earth’s structure and the behaviour of currents in the deep ocean.

Why use space?

Many of these measurements can be made, sometimes much more accurately, using ground- or air-based systems. So why is satellite monitoring so important?

The obvious answer is visibility of any part of the Earth. From orbit, even inaccessible regions such as Antarctica or parts of the ocean far from any landmass can be monitored relatively easily. Regularity of monitoring is another advantage. Satellites in low-Earth orbit will typically make 10 or more orbits of the Earth per day, allowing them to make frequent measurements to monitor changing conditions in a specific region.

However, the use of space for these applications also has significant disadvantages. One of the main disadvantages is the difficulty of reaching space. While this has reduced in recent years as launch costs drop, launching a satellite is still expensive. Launch vehicles are also limited in their payload, putting severe restrictions on the mass of a satellite and the instrumentation that it can carry. The difficulty of reaching space also means that servicing satellites once they have been launched is typically not practical or cost-effective.

Nonetheless, despite these potential drawbacks, the advantages of space-based platforms mean that Earth-observation satellites will continue to play an extremely important role in monitoring the Earth.

Innovation in Space

As falling launch costs makes space more accessible, companies and governments are collecting and using increasing amounts of data from Earth observation satellites, and are developing techniques and instrumentation to obtain new types of data. This is leading to increased levels of investment and innovation in the space sector.

In the UK alone, a report published in May 2024 by the UK Space Agency estimated that turnover in the space sector increased by over 50% to £17.5 billion between 2010 and 2020 when adjusted for inflation. In the same month, the UK Space Agency announced a £9 million funding round to support investment in projects related to satellite-based Earth observation technology.

It seems clear that innovation and investment in Earth observation is only likely to increase in the coming years. For the increasing number of private companies involved in this sector, proper management and protection of their innovations will be key to their continued commercial success.

J A Kemp has specialists in all of the diverse technical fields that are involved in space technology, from electronics and electrical engineering and optics and photonics to materials and metallurgy.