Earth observation sensors and technology: the current state of play

By offering the ability to track wildfires, flooding, land use, infrastructure and urban development, satellite imagery fuels smarter geospatial analysis for planning, risk assessment and environmental monitoring. This article outlines the evolution of Earth observation satellites over time and where we stand today in the USA, Europe and Asia. Against the backdrop of declining launch costs, miniaturization, reduced barriers to entry, artificial intelligence (AI) and multi-sensor fusion, it also looks ahead to the exciting new era of Earth observation for geospatial data acquisition.

Earth observation (EO) satellites come in many forms. They can differ in their orbital paths, the instruments they carry, and how those instruments capture data – from viewing angles and spatial resolution, to spectral sensitivity and swath width. These technical parameters are carefully chosen during mission planning, depending on the specific purpose. To observe weather patterns on a large scale and with high frequency, a geostationary orbit is ideal. From this high vantage point, approximately 36,000km above Earth, a satellite can continuously monitor almost an entire hemisphere. While the high altitude limits spatial resolution, this is generally sufficient for applications like tracking cloud movements over continents.

In contrast, applications that require detailed imagery of specific areas – e.g. monitoring the size of glacial lakes, mapping damage after a natural disaster, tracking deforestation or inspecting urban infrastructure – rely on high-resolution sensors. These are typically mounted on satellites in low Earth orbit (LEO), between 500 and 1,200km above the planet. Since satellites in LEO move relative to the Earth’s surface and cannot continuously observe the same area, images can only be captured during the brief period when the satellite passes overhead. The frequency of observations over the same location can be significantly increased by using satellite constellations – groups of satellites working together in coordinated orbits.

The origins of Earth observation

Sputnik 1, launched by the Soviet Union in 1957, was technically the first artificial satellite, albeit not an Earth observation satellite in the modern sense. Nor was Sputnik 2, which was launched just one month after Sputnik 1 and was famous for carrying Laika the dog, the first living creature to orbit Earth. While it did not perform Earth observation activities, Sputnik 2 was the first platform capable of making scientific measurements in orbit because it was equipped with instruments to measure cosmic rays, solar radiation and other space-related parameters.

The Americans responded to the launch of the Sputniks by launching their first satellite – Explorer 1 – in 1958, marking the start of the ‘Space Race’ during the Cold War between the Soviet Union and the USA. Later, after Explorer 3, which included a tape data recorder in the payload, it was concluded that the original Geiger counter had been ‘saturated’ by strong radiation coming from a belt of charged particles trapped in space by the Earth’s magnetic field. This belt of charged particles is now known as the Van Allen radiation belt.

Further developments followed quickly. TIROS-1, launched by NASA in April 1960, is widely considered the first true Earth observation satellite, specifically designed for meteorological applications. This was the first satellite to capture television images of the Earth’s weather systems from space, and the visual cloud cover data significantly improved weather forecasting capabilities at the time. More importantly, although it didn’t yet offer the multispectral detail or sharp resolution that Landsat would later bring, TIROS-1 demonstrated that satellites could be used operationally for Earth observation, thus laying the foundation for today’s environmental monitoring systems.

Early days of Landsat

The early 1970s saw a rising wave of environmental awareness. The first Earth Day, held in April 1970, symbolized growing public concern for the planet and helped catalyse political action on environmental issues. Against this backdrop, the Earth Resources Technology Satellite (ERTS) was launched as a proof of concept to demonstrate that satellite-based remote sensing could support better management of the environment and natural resources.

Based on the data and experience gathered from this pioneering mission, the USA decided to establish a long-term, operational Earth  observation programme. ERTS was renamed Landsat 1, and the Landsat space programme has since become the world’s longest-running EO initiative. Landsat 1 orbited Earth in a near-polar path at an altitude of approximately 900km. This type of orbit had a distinct advantage: as the satellite circled the globe, the Earth slowly rotated beneath it, allowing Landsat to scan long, continuous swaths of the planet’s surface. The orbit was also synchronized with the Sun, meaning the satellite passed over each region at roughly the same local solar-time with the Sun always behind the satellite on its daylight pass, ensuring consistent lighting conditions.

Landsat 1 completed an orbit every 103 minutes and returned to the same location every 18 days, building up a systematic, repeatable record of Earth’s surface. On board, the satellite carried a multispectral scanner (MSS) and three return beam vidicon (RBV) video cameras, capable of capturing imagery in both visible and infrared wavelengths. This ushered in a new era of Earth observation from space.

Landsat’s technological leaps

While the instruments aboard Landsat 2 were identical in specification to those on Landsat 1, from Landsat 3 onwards satellite sensor technology underwent a remarkable evolution. Launched in March 1978 operated solely by NASA, marking the last time the agency managed a Landsat mission without a civilian partner, Landsat 3 had a multispectral scanner system that included a thermal infrared band, intended to capture temperature variations on Earth’s surface. Unfortunately this did not go entirely to plan, as the thermal IR band failed shortly after launch, limiting its utility. Nevertheless, the RBV sensor offered enhanced spatial resolution of 40m (compared to 80m in the earlier versions), providing better detail in the captured images.

The Landsat 4 and 5 satellites signalled a significant technological leap by introducing the thematic mapper (TM) sensor alongside the existing MSS. The TM represented a major advancement in remote sensing technology, setting a new benchmark for both spatial and spectral performance in terms of sharper imagery, improved spectral separation, greater geometric precision and finer radiometric resolution.

The TM collected data across seven spectral bands simultaneously, resulting in a much richer and more detailed view of Earth’s surface. Bands 1-5 and Band 7 recorded data at a spatial resolution of 30 by 30m, while Band 6 – which measured thermal infrared radiation, effectively capturing surface temperature – had a coarser resolution of 120 by 120m. Interestingly, Landsat satellites could only acquire scenes in this thermal band during nighttime. These capabilities made the TM sensor a powerful asset for applications ranging from environmental monitoring to land use and resource management.

Setbacks on the path to progress

Not every step of the Landsat programme was a success. Landsat 6 failed to reach orbit after its launch in 1993. The satellite had been upgraded by equipping it with the familiar MSS plus an enhanced thematic mapper (ETM). Despite this initial failure, Landsat 7 successfully introduced the enhanced thematic mapper plus (ETM+) six years later, in 1999. The ETM+ added a 15m-resolution panchromatic band to the existing TM bands, allowing for significantly sharper black-and-white images and bringing the total of spectral bands to eight. In addition, the thermal band resolution was improved to 60m. With a 16-day revisit interval, these improvements enabled more precise monitoring of land cover changes, urban development and a wide range of environmental phenomena.

The journey continued in 2013 with the launch of Landsat 8, which brought significant advancements with two new instruments: the operational land imager (OLI) and the thermal infrared sensor (TIRS). OLI provided nine spectral bands, including a coastal aerosol band and a cirrus cloud detection band, increasing atmospheric correction and water studies. TIRS introduced two thermal bands with 100m resolution, improving the accuracy of surface temperature measurements. The instruments collectively offered 12-bit radiometric resolution, allowing for finer distinctions in detected energy levels.

The programme’s most recent addition is Landsat 9, which was launched into orbit in 2021. It continues the mission with instruments similar to Landsat 8 but with improved performance. The OLI-2 and TIRS-2 sensors maintain the same spectral and spatial resolutions but benefit from better calibration and data quality. Now with 14-bit radiometric resolution, Landsat 9’s sensors can capture 16,384 levels of brightness per spectral band, which is far more than previous sensors. This higher sensitivity enables detection of subtle changes in vegetation, water quality, soil conditions and urban surfaces. It also improves performance in low-contrast environments such as shaded terrain, coastal zones and snow-covered areas. With reduced quantization error and smoother tonal transitions, the data is better suited for scientific analysis, time-series monitoring and visual interpretation.

Landsat 8 and 9 both record imagery in 11 spectral bands with spatial resolutions of 15m (panchromatic), 30m (most reflective bands) and 100m (thermal bands). Like Landsat 7, they each revisit the same location every 16 days, but when operated together, they offer eight-day global coverage. The images are typically divided into scenes enabling smooth downloading, each covering an area of approximately 185 by 185km.

Global archive of millions of images

Each generation of Landsat satellites has contributed to more detailed, accurate and comprehensive Earth observations, reinforcing the programme’s pivotal role in environmental monitoring and scientific research. Over the decades, it has produced millions of satellite images. These images, which are archived in the USA and at Landsat receiving stations around the world. They are a unique resource providing invaluable data for scientific research into the changing conditions on our planet as well as for a wide range of practical applications.

Europe’s Copernicus programme

The European counterpart to the Landsat programme is the firmly established Copernicus programme, led by the European Union (EU) in collaboration with the European Space Agency (ESA). The programme is named after Nicolaus Copernicus (1473-1543), the scientist whose theory of a heliocentric universe – revolving around the Sun rather than the Earth – marked a big shift in our understanding of the cosmos and laid the groundwork for modern science. Like Landsat, the Copernicus programme is built on the principle of free and open access to satellite data and has rapidly become a cornerstone of global Earth observation.

At the heart of the programme is the Sentinel-2 mission, with Sentinel-2A (2015) and Sentinel-2B (2017) forming a twin-satellite constellation dedicated to high-resolution optical imaging of land and coastal surfaces. Each satellite is equipped with a multispectral imager (MSI), allowing imagery to be captured in 13 spectral bands: four at 10m resolution (visible and near-infrared), six at 20m (red edge and shortwave infrared) and three at 60m (for atmospheric correction). With a 290km swath width and a five-day revisit cycle, Sentinel-2 delivers frequent wide-area coverage with consistent image quality.

Game-changing complementary datasets

Thanks to its accuracy, accessibility, and compatibility with established tools, Sentinel-2 has become an essential resource in both operational workflows and scientific research. In fact, it is often regarded as a game changer for the geospatial community, particularly when used in conjunction with the Landsat programme. While Landsat offers an unmatched historical archive, Sentinel-2 adds more frequent revisits, additional spectral detail (especially in the red edge) and also sharper resolution in certain bands. Together, the two programmes provide complementary datasets for time-series analysis, land cover monitoring, precision agriculture and disaster response.

ESA is currently developing six new Sentinel Expansion missions to further broaden the Copernicus Space Component, including CO2M, CHIME and LSTM. These missions are set to deliver high-resolution, application-ready data on carbon emissions, vegetation health and land temperature. For geospatial professionals, this means even richer datasets, more frequent updates and expanded analytical possibilities across domains like climate modelling, precision agriculture and land-use planning.

New era of Earth observation

Due to the enormous financial resources and advanced technical expertise required to build, launch and operate Earth observation satellites, space-based EO systems were the exclusive territory of national space agencies and a few state-backed programmes for many decades. However, over the past decade, declining launch costs, miniaturization of components and reduced barriers to entry have opened the market to private companies, investors and venture capitalists. This shift is especially evident in the rise of small satellite constellations focused on geospatial data acquisition.

Companies like Planet have deployed hundreds of compact Dove satellites, providing daily, medium-resolution imagery of nearly every point on Earth. Planet’s continuously refreshed dataset has become a valuable source for tracking land use changes, vegetation cycles and urban development. Its combination of global coverage and high temporal frequency enables users to detect subtle changes over time that well could be missed by less frequently updated systems. Meanwhile, BlackSky brings rapid, high-resolution optical imagery with a strong emphasis on real-time situational awareness. Another player, ICEYE, is pioneering small radar satellites capable of imaging through clouds and darkness, which is crucial for consistent monitoring in challenging conditions.

Looking further ahead, we can expect the coming years to bring even greater innovation in the EO sector. Advancements in onboard processing, AI-enhanced analytics and multi-sensor data fusion are set to accelerate the delivery of timely, precise geospatial intelligence. As public and private efforts increasingly converge, we are entering a new era of Earth observation – faster, more accessible and more responsive to the world’s changing needs.

Earth observation in Asia

Besides North America and Europe, Asia has also grown into a powerhouse in Earth observation, with countries like Japan and China making significant advances in satellite-based geospatial intelligence:

Japan’s Advanced Land Observing Satellite (ALOS) programme, operated by JAXA, plays a key role in generating high-quality geospatial data. ALOS-1 and ALOS-2 are twinning optical sensors with L-band synthetic aperture radar (SAR), enabling the monitoring of land deformation, infrastructure stability, forest cover and natural disasters – even under cloud cover or at night. ALOS-4 was launched in July 2024 and further strengthens Japan’s ability to support disaster response, climate analysis and large-scale mapping through advanced radar imaging. ALOS-4 observes areas up to 35km² and significantly improves imaging capabilities compared to its predecessor. In high-resolution mode (3m), it expands the swath from 50km (ALOS-2) to 200km. In wide-area mode, it increases coverage from 350km at 100m resolution to 700km at 25m resolution. This high-end combination of resolution and swath allows for imaging all of Japan up to 20 times a year, compared to just four times with ALOS-2.

China’s Gaofen satellites, part of the China High-resolution Earth Observation System (CHEOS), together form a rapidly growing constellation that delivers diverse geospatial datasets. With satellites offering both high-resolution optical and SAR capabilities, the Gaofen programme supports applications such as precision agriculture, urban expansion tracking, environmental monitoring and land-use planning. By 2030, China is expected to operate at least 40 EO satellites, marking a significant expansion of its space-based monitoring capabilities.

Both ALOS and Gaofen exemplify how Asian EO programmes are not only expanding regional capabilities but also enriching the global pool of Earth observation data. Their outputs are already widely and increasingly used for numerous applications, such as to map earthquake-affected regions in near real time and to track seasonal shifts in crop production across large agricultural zones.

Further reading

UCS satellite database: https://www.ucs.org/resources/satellite-database

Landsat science: https://landsat.gsfc.nasa.gov

Ginger Butcher, Linda Owen, and Christopher Barnes. Landsat: the cornerstone of global land imaging, GIM International Vol 33, January/February 2019, https://www.gim-international.com/content/article/landsat-the-cornerstone-of-global-land-imaging

Copernicus: https://www.esa.int/Applications/Observing_the_Earth/Copernicus

ALOS Research and Application Project: https://www.eorc.jaxa.jp/ALOS/en/index_e.htm

Shusong Huang, Wenping Qi, Shuai Zhang, Tian Xia, Jingqiao Wang, and Yong Zeng. Recent progress of Earth observation satellites in China, Chinese Journal of Space Science, Volume 44, Issue 4: 731 - 740 (2024), https://www.sciengine.com/CJSS/doi/10.11728/cjss2024.04.2024-yg23

Spacepage: https://www.spacepage.be