# What is Earth observation? Optical, radar and free data Earth observation (EO) is the gathering of information about the Earth's surface, waters and atmosphere by sensors on satellites, aircraft or the ground, and its processing into maps and measurements. Optical sensors record reflected light; radar sensors send microwave pulses and work through cloud and at night. Copernicus data and services are free of charge. Earth observation is behind many satellite-based services for cities and companies, from heat maps to the monitoring of construction sites. This page explains how it works, which types of data exist, what is free and what its limits are. **Definition: Earth observation (EO).** Earth observation is the gathering of information about the Earth's surface, waters and atmosphere through satellite, airborne or ground-based remote sensing, and the processing and analysis of that information to monitor the state of the natural and built environment and how it changes. ## How Earth observation works A satellite carries one or more sensors. Passive sensors record natural energy that is reflected or emitted by the Earth's surface; active sensors send their own signal and record what comes back [3]. Optical instruments are passive: they record reflected energy in visible light and in infrared bands. Radar instruments are active: they send microwave pulses to the Earth and record how these pulses are reflected [2]. **Figure:** Optical sensors are passive: they record sunlight reflected by the surface, so clouds hide the ground. Radar sensors are active: they send microwave pulses and record the echo, through cloud and at night. Satellite images are usually shown as pictures, but they are digital data, and the same raw data can be processed in many different ways [2]. Providers deliver them at defined processing levels. For Sentinel-2, for example, Level-1C products are top-of-atmosphere reflectance, resampled to a ground sampling distance of 10, 20 or 60 m depending on the band, and Level-2A products are surface reflectance, corrected for the effects of the atmosphere [4]. **Figure:** Satellite image of Graz in late summer: the dense red-roofed centre, the river Mur as a dark line from north to south, forested hills to the west and north, and suburbs and fields to the south and east. Graz in a Copernicus Sentinel-2 image of 1 September 2025, a Level-2A product shown in true colour at 10 m resolution. The image covers 16 by 10 km. The river Mur runs from north to south through the city; the green hill east of the river in the centre is the Schlossberg. Contains modified Copernicus Sentinel data 2025, processed by LETO SPACE. Data accessed through the Sentinel-2 collection maintained by Element 84 on AWS Open Data. [20] The Committee on Earth Observation Satellites (CEOS) defines Analysis Ready Data as satellite data that have been processed to a minimum set of requirements and organised so that they can be analysed immediately, with little additional effort, and combined over time and with other datasets [5]. Analysis ready does not mean ready for your decision: whether a product fits a task still has to be checked against the requirements of that task. ## Optical, radar, thermal and hyperspectral data Each type of sensor shows a different aspect of the surface. The choice follows the question you need to answer. **Main sensor types, with one example mission each** | How it works | Example mission | Typical uses | | ----------------------- | ----------------------------------------------------------------------- | ------------------------------------------------------- | | Optical (multispectral) | Records reflected sunlight in several wavelength bands | Sentinel-2: 13 bands, 10 m at best, 290 km swath | | Radar (SAR) | Sends microwave pulses and records the echo, through cloud and at night | Sentinel-1 | | Thermal infrared | Records heat emitted by the surface | Landsat 9: two thermal bands, 100 m | | Hyperspectral | Records the spectrum in many narrow, adjacent bands | EnMAP: 420 to 2450 nm, sampled every 6.5 to 10 nm, 30 m | Mission figures and uses: Sentinel-2 [6], Sentinel-1 [7] [8], Landsat 9 [9], EnMAP [10] [11]. Radar mainly measures surface roughness rather than light or heat [2]. Repeated radar images also show how buildings, bridges and the ground move over time, with millimetre precision at continental scale in the European Ground Motion Service [8]. This technique, InSAR, has its own page: [InSAR ground motion monitoring](https://leto.space/use-cases/ground-motion-insar/). ## Resolution and revisit "High resolution" is not one number. Four kinds of resolution describe what a sensor can deliver: - **Spatial resolution:** the minimum size of detail visible in an image [2]. - **Spectral resolution:** the number and width of the wavelength bands an optical instrument records. More bands allow materials to be told apart more accurately [2]. - **Temporal resolution:** how often the satellite images the same place, also called revisit. - **Radiometric resolution:** how finely the sensor distinguishes differences in the recorded energy. The imager on Landsat 9, for example, distinguishes 16,384 levels per band [9]. These properties pull against each other. The higher the spatial resolution of an instrument, the narrower its swath, the strip of ground covered in one pass, and this generally means a longer revisit time [2]. Sentinel-2 images a 290 km swath at up to 10 m and revisits a location at the equator every five days with two satellites [6]. Pléiades Neo, a commercial constellation, delivers 30 cm panchromatic images with a swath of 14 km and can revisit a site twice a day, because its satellites can be pointed at a target [12]. Steerable sensors like these shorten the revisit time [2]. A requirement should therefore name the resolution it means and the value the task needs. ## Earth observation and remote sensing Remote sensing is the science of obtaining information about objects or areas from a distance, typically from aircraft or satellites [3]. The term describes the method. Earth observation is the wider activity: it gathers information about the Earth by remote sensing from satellites, aircraft or the ground, and analyses it to monitor the environment and its changes [1]. In practice, aircraft and drones complement satellites. In selected activations, for example, the Copernicus emergency mapping service complements satellite images with imagery from aircraft and drones; aircraft can often deliver information even in cloudy weather [19]. In German, Fernerkundung is the usual word for remote sensing and Erdbeobachtung for Earth observation. ## Copernicus, free data and commercial imagery Copernicus is the Earth observation component of the European Union's Space Programme. It delivers its information through six services, for the atmosphere, the marine environment, land, climate change, security and emergencies [13]. The legal basis is the EU Space Programme Regulation, which provides Copernicus data and information under a free, full and open data policy, with limitations such as predefined formats and security restrictions [14]. Sentinel data are available through the Copernicus Data Space Ecosystem, which provides free access; users can register for further functions [16]. Landsat products from the US Geological Survey have been available at no cost since 2008 [17]. Very high-resolution images are available from commercial missions such as Pléiades Neo, which Airbus Defence and Space owns and operates [12]. Such data are offered either from an archive of past acquisitions or as new images that a satellite is tasked to acquire. SPOT 6 and 7 data, for example, are available both ways; since SPOT-7 ended in March 2023, new acquisitions come from SPOT-6 only [18]. Prices and conditions of use depend on the provider, so we do not quote them here. Free data is not a free service: processing, delivery and integration still need effort and budget. The [procurement guide](https://leto.space/knowledge/procurement-guide/) explains what to buy around the data. ## What Earth observation is used for Most of the use cases in our library rely on Earth observation. A selection, from cities to agriculture: [Browse all use cases](https://leto.space/use-cases/) ## Limits **Limits** - **Optical sensors need clear skies:** Optical instruments record reflected light, so clouds hide the ground. Radar sees through cloud and darkness, but it shows surface roughness, not colour \[2]. - **Detail costs coverage:** The finer the detail, the smaller the area per pass and, as a rule, the longer the revisit \[2]. Objects smaller than a pixel cannot be seen reliably. - **The right image may not exist:** In emergency mapping, the first available image is often of limited quality: partly cloudy, taken at an oblique angle far from the vertical or covering only part of the area. An image from before the event is not always available \[19]. - **A map is not ground truth:** A classified map is the result of a model. Before it informs a decision, it needs validation against reference data from the area, with a stated method and accuracy. ## Key terms - **Swath:** the width of the area a sensor covers in one pass. - **Revisit time:** the time between two images of the same place. - **Ground sampling distance:** the distance on the ground between the centres of two neighbouring pixels. - **SAR:** synthetic aperture radar, the imaging radar used on Earth observation satellites. - **InSAR:** interferometric SAR, which compares the phase of radar images taken at different times to measure ground motion. - **Processing level:** the stage of processing a product has reached, for example Level-1C or Level-2A for Sentinel-2. - **Analysis Ready Data:** data processed to a minimum standard so that they can be analysed directly, as defined by CEOS. - **In-situ data:** observations from ground-based, airborne or seaborne sensors rather than satellites. Copernicus uses them alongside satellite data, among other things to calibrate and validate its products [15]. ## How we help We help organisations decide whether Earth observation fits a task, which data and level of processing it needs, and how to procure a service around it. If you are new to the field, start with the definition of [space-enabled solutions](https://leto.space/knowledge/space-enabled-solutions/), which places Earth observation next to satellite navigation, satellite communication and AI. **Next steps** - [Strategy and use cases](https://leto.space/consulting/strategy/): Find the use case that pays off in your organisation. - [Procurement guide](https://leto.space/knowledge/procurement-guide/): Six steps from the decision to the evaluation of offers. - [Use cases](https://leto.space/use-cases/): Applications of space technology by industry and technology. ## Frequently asked questions ### Is satellite data free? Much of it is. Copernicus data and information are provided under a free, full and open data policy, with defined limitations [14], and the Copernicus Data Space Ecosystem gives free access to data from the Sentinel missions [16]. Landsat products have been available from the US Geological Survey at no cost since 2008 [17]. Data from commercial very high-resolution missions are not openly available. ESA, for example, offers the full Pléiades Neo archive and new acquisitions for scientific research and application development only on an accepted project proposal and within a quota [12]. ### Can satellites see through clouds? Radar satellites can. They send microwave pulses and record the echo, so they see through cloud and darkness. Optical satellites record reflected light and cannot see the ground under cloud [2]. ### How often is an area imaged? It depends on the mission. With two satellites, Sentinel-2 revisits a location at the equator every five days [6]. Landsat 9 has a 16-day repeat cycle, offset by eight days from Landsat 8 [9]. Some commercial constellations, such as Pléiades Neo, can revisit a site twice a day [12]. For optical data, clouds reduce the number of usable images. ### What is the difference between Earth observation and remote sensing? Remote sensing is the method: obtaining information about objects or areas from a distance, typically from aircraft or satellites [3]. Earth observation is the wider activity of gathering information about the Earth's surface, waters and atmosphere by remote sensing from satellites, aircraft or the ground, and analysing it to monitor the environment and its changes [1]. ## Sources 1. [What is Earth Observation?, EU Agency for the Space Programme (EUSPA), 2026-07-13](https://www.euspa.europa.eu/eu-space-programme/copernicus/what-earth-observation) 2. [What is Earth observation?, European Space Agency (ESA), accessed 2026-10-06](https://www.esa.int/Applications/Observing_the_Earth/What_is_Earth_observation) 3. [What is remote sensing?, U.S. National Oceanic and Atmospheric Administration (NOAA), National Ocean Service, 2026-09-23](https://oceanservice.noaa.gov/facts/remotesensing.html) 4. [Sentinel-2 (product documentation), Copernicus Data Space Ecosystem, accessed 2026-10-06](https://documentation.dataspace.copernicus.eu/Data/SentinelMissions/Sentinel2.html) 5. [CEOS Analysis Ready Data, Committee on Earth Observation Satellites (CEOS), accessed 2026-10-06](https://ceos.org/ard/) 6. [Sentinel-2: Facts and figures, European Space Agency (ESA), accessed 2026-10-06](https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-2/Facts_and_figures) 7. [Sentinel-1, European Space Agency (ESA), accessed 2026-10-06](https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-1) 8. [Sentinel-1 mission… did you know?, European Space Agency (ESA), 2025-11-03](https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-1/Sentinel-1_mission_did_you_know) 9. [Landsat 9, U.S. Geological Survey (USGS), accessed 2026-10-06](https://www.usgs.gov/landsat-missions/landsat-9) 10. [EnMAP mission, German Aerospace Center (DLR), EnMAP, accessed 2026-10-06](https://www.enmap.org/mission/) 11. [EnMAP HSI instrument specification, German Aerospace Center (DLR), EnMAP, accessed 2026-10-06](https://www.enmap.org/data/doc/EnMAP_Specs.pdf) 12. [Pléiades Neo, European Space Agency (ESA), Earth Online, accessed 2026-10-06](https://earth.esa.int/eogateway/missions/pleiades-neo) 13. [Copernicus, European Commission, DG Defence Industry and Space (EU Space portal), accessed 2026-10-07](https://eu-space.europa.eu/earth-observation/copernicus) 14. [Regulation (EU) 2021/696 of 28 April 2021 establishing the Union Space Programme and the European Union Agency for the Space Programme, European Union, EUR-Lex, 2021-05-12](https://eur-lex.europa.eu/eli/reg/2021/696/oj/eng) 15. [What is in situ data?, European Union, Copernicus In Situ Component (European Environment Agency), accessed 2026-10-07](https://insitu.copernicus.eu/about/what-is-in-situ-data) 16. [Copernicus Data Space Ecosystem, Copernicus Data Space Ecosystem, accessed 2026-10-06](https://dataspace.copernicus.eu/) 17. [Landsat Data Access, U.S. Geological Survey (USGS), accessed 2026-10-06](https://www.usgs.gov/landsat-missions/landsat-data-access) 18. [SPOT-6 to 7 full archive and tasking, European Space Agency (ESA), Earth Online, accessed 2026-10-06](https://earth.esa.int/eogateway/catalog/spot-6-to-7-full-archive-and-tasking) 19. [Using Earth Observation Data, Copernicus Emergency Management Service, accessed 2026-10-06](https://mapping.emergency.copernicus.eu/about/using-of-earth-observation-data/) 20. [Copernicus Sentinel-2 Level-2A scene S2B_33TWN_20250901_0_L2A, true colour image, 1 September 2025, Copernicus Sentinel data (European Union, ESA), Sentinel-2 Cloud-Optimized GeoTIFFs maintained by Element 84 on AWS Open Data, accessed 2026-10-08](https://earth-search.aws.element84.com/v1/collections/sentinel-2-l2a/items/S2B_33TWN_20250901_0_L2A) Page: