Earth Under Surveillance: Observation Satellites

Scientific Summary

This article provides an overview of Earth observation satellites, classified into three main categories (optical, radar, altimetric). These instruments provide essential data for climate, disaster management, agriculture, and urban planning. Iconic missions like Copernicus Sentinel, Landsat, and Terra are detailed. In 2025, the total number of active satellites in orbit is estimated at around 11,700, of which about 1,000 are dedicated to Earth observation, a number constantly increasing thanks to commercial constellations and civil and military space programs.

How do satellites monitor our planet and how many are in orbit?

Observation satellites scan Earth from all angles using three technologies: optical (visible images), radar (vision through clouds), and altimetry (height measurement). Their data are crucial for tracking climate, disasters, or deforestation. In 2025, approximately 11,700 active satellites are in orbit, of which 1,000 are dedicated to observing our planet. This number is constantly growing, driven by commercial constellations like Pléiades Neo and international programs (Copernicus, NASA, CNSA).

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The Era of Space Surveillance

Since the advent of the space age, satellites (artificial Satellites) for Earth observation have allowed monitoring of our planet with unprecedented precision. These instruments provide data on climate, oceans, forests, as well as urban and agricultural areas. Their observations are essential for disaster management, deforestation monitoring, and the study of climate change.

Main Types of Observation Satellites

Observation satellites are mainly divided into three categories:

Optical satellites: capture images in the visible and near-infrared spectrum.
Radar satellites: use microwave signals to obtain images even through clouds or at night.
Altimetric satellites: measure ocean and land surface topography using instruments like Synthetic Aperture Radar (SAR).

Scientific and Environmental Applications

These satellites provide crucial data for:

Monitoring natural disasters such as floods, fires, or earthquakes.
Studying the evolution of glaciers and polar ice caps.
Monitoring urbanization and land-use changes.
Climate modeling and weather forecasting.
Monitoring air quality and greenhouse gas emissions.
Observing oceans, currents, and sea surface temperatures.
Tracking deforestation and forest regeneration.
Managing water resources, including groundwater.
Monitoring crops and food security.
Assessing the environmental impact of mining and industrial activities.
Mapping coastal areas and preventing coastal erosion.
Detecting and monitoring volcanic and geological phenomena.
Urban planning and analyzing population growth.
Protecting biodiversity through monitoring natural habitats.
Validating and improving global climate models.

Examples of Iconic Satellites

Some satellites have become benchmarks for Earth observation:

Copernicus Sentinel-2: for vegetation and soil monitoring.
Terra and Aqua: NASA missions for studying the atmosphere and oceans.
Envisat: ESA mission for global environmental observation.

Comparison Table of Satellites

Characteristics of Selected Observation Satellites
Satellite	Mission	Main Instrument	Agency
Sentinel-2	Soil and vegetation monitoring	Multispectral	ESA
Terra	Climate and atmosphere study	MODIS, ASTER	NASA
Envisat	Global environmental observation	MERIS, ASAR	ESA
Aqua	Oceans and atmosphere observation	MODIS, AMSR-E	NASA
Sentinel-1	Radar for land and maritime surveillance	SAR	ESA
GOES-16	Real-time meteorological observation	ABI, GLM	NOAA
Landsat 9	Soil and environmental monitoring	OLI-2, TIRS-2	NASA / USGS
ICEsat-2	Glacial ice and land topography measurement	ATLAS	NASA
SMAP	Soil moisture monitoring	Radar and radiometer	NASA
Copernicus Sentinel-5P	Air quality and pollutant monitoring	TROPOMI	ESA
Jason-3	Ocean altimetry observation	Radar altimeter	NASA / CNES
Suomi NPP	Global climate monitoring	VIIRS, CERES	NASA / NOAA
GRACE-FO	Measurement of Earth's gravity variations	Gravimeter	NASA / GFZ
ADEOS-II	Atmospheric and oceanic observation	Multispectral and radars	JAXA
TerraSAR-X	Radar imaging for mapping and surveillance	SAR	DLR
Gaofen-1	Multispectral soil and agriculture observation	Multispectral camera	CNSA / CAST
Gaofen-2	High-resolution optical imaging	High-resolution optical camera	CNSA / CAST
Gaofen-3	Synthetic Aperture Radar (SAR) for maritime and land surveillance	SAR	CNSA / CAST
Gaofen-6	Optical imaging for soil monitoring and mapping	Multispectral optical camera	CNSA / CAST
Gaofen-7	Stereoscopic observation for topography and mapping	Stereoscopic camera	CNSA / CAST

Source: ESA - Observing the Earth (new window), NASA Earthdata (new window), and Wikipedia – Gaofen (new window).

How Many Observation Satellites Are Orbiting in 2025?

In 2025, approximately 11,700 active satellites orbit the Earth, most in LEO (low Earth Orbit) below 2,000 km altitude. These satellites perform a variety of missions, including telecommunications, navigation, Earth observation, scientific research, and defense.

Among this total, around 1,000 satellites are specifically dedicated to Earth observation. This category includes civil, military, and commercial satellites such as the Pléiades Neo, CO3D, NISAR, FLEX, TRISHNA, Sentinel-6B constellations, as well as satellites from Planet Labs, ICEYE, and other private operators.

The total number of observation satellites is constantly increasing. Future projects aim to deploy new constellations to improve coverage and observation frequency. In 2025, these satellites represent approximately 8% of the total of active satellites in low Earth orbit.

N.B.:
The exact number of Earth observation satellites is actually an estimate, not an absolute figure, due to satellites whose information is not publicly disclosed, notably those of China, Russia, or certain military programs.

FAQ: All about observation satellites

What are the three main types of observation satellites and their differences?

We distinguish: 1) Optical satellites, which photograph Earth in the visible and infrared spectrum (like Sentinel-2). 2) Radar satellites, which use microwaves to see through clouds and at night (like Sentinel-1). 3) Altimetric satellites, which precisely measure the height of oceans and land surfaces (like Jason-3).

What are the concrete uses of data from observation satellites?

They are used for many applications: disaster monitoring (floods, fires), climate studies (glaciers, greenhouse gases), resource management (agriculture, water, forests), urban planning, and ocean surveillance. They are also used for weather forecasting, mapping, and biodiversity protection.

Why is the number of observation satellites difficult to determine precisely?

The figure of 1,000 satellites is an estimate. It is difficult to be exhaustive because some programs, particularly military ones or those from countries like China and Russia, do not make all their launches public. The total number is constantly increasing with the arrival of new commercial constellations and scientific missions.