Satellites: More than Just Pictures

Satellite data is much more than just pictures – optical imaging is only a fraction of the capability

When thinking about satellites most people think of images, as on Google maps or the more traditional aerial photography. Indeed, satellites routinely produce these images – and can produce phenomenal pictures with amazing quality and resolutions.

However, satellites have a far wider range of sensors, continuously monitoring the earth’s surface, providing a rich and dense source of data that can provide detection of millimetre changes in height, spot subtle changes in pollutants around the world, and report moving vehicles from 600km way.

The European Space Agency (ESA) Copernicus programme currently has six missions, SENTINEL-1 to 6, with an additional six planned for the Sentinel Expansion. Only one of these (SENTINEL 2) provides traditional photographs, the other five are using a diverse set of sensors, from radar to spectrometer sensors working across a variety of different wavelengths. Much of the data can even be collected at night, or with cloud cover.

Data – More than Pictures

Broadly speaking there are three areas of satellite sensors:

  • Optical –Images/photographs providing high levels of resolution.
  • Radar – These use radio waves to reflect off surfaces providing incredible precision for height.
  • Atmospheric Sensors – This can detect trace gases, providing near real-time reporting on air quality.

Optical

Optical images from commercial satellites range in resolution from 0.3m to 30m – this means that a single pixel will represent 30cm to 30m on the ground. Given that these pictures are taken from a distance of 700km the quality and details are amazing.  Optical satellites frequently also collect additional spectral information with a range far greater than what can be perceived with an unaided human eye – for example, fine differences in levels of Chlorophyll in leaves of plants, or algae blooms can be detected.

Sample 10m resolution image
10m resolution image
Sample 1m resolution image
1m resolution image
Sample 0.3m resolution image
0.3m resolution image

Optical – Example Uses:

  • Aquaculture: identification of new sites, spotting damaged nets, detecting and predicting algae blooms, seeing variations in water clarity, detecting runoffs from farmland.
  • Agriculture: Understanding the quality of crops, spotting diseases.
  • Trading: Predicting the yields of crops, detecting where commodities are stored and measuring the volume of these stores.
  • Insurance: Quickly assess pre- and post-event conditions for onboarding and claims adjustment.

ESA’s SENTINEL-2 mission and NASA Landsat are examples of optical satellites.

Radar

Satellites equipped with radar provide multiple capabilities, including – the ability to see through clouds, operate at night and measure in incredible details. These satellites are equipped with active sensors which bounce radio signals of the surface of the earth and manmade objects. While the resolution of an image is typically 2m to 30m (low compared to some optical images) they can measure elevation with millimetre accuracy.

Satellite SAR Image

Satellite radar Image – credit: ICEYE

Radar – Example Uses:

  • Aquaculture: Detecting currents, wave height, wind speed.
  • Agriculture: Combined with optical images, enables the understanding of crop types and their quality with improved detail.
  • Trading: Detecting changes in the height of oil floating roof storage tanks, understanding when industrial facilities and plants are in operation, even detecting oil being extracted from the ground.
  • Insurance: Understanding changes in buildings, construction, subsidence, and other geological changes.

ESA’s SENTINEL-1 mission is an example of a radar satellite.

Oil spill observed off the Belgian coast near Zeebrugge in 2015, after a collision between two vessels

Oil spill observed off the Belgian coast near Zeebrugge in 2015, after a collision between two vessels

Atmospheric Sensors

By using a variety of sensors, such as infrared and microwave, and combining the results, highly accurate measurements can be obtained for air and water temperature, water vapor, Carbon Dioxide, and several other gasses resulting from human and natural activities, at various altitudes. During the COVID-19 lockdown, there were numerous reports about decreases in Nitrogen Dioxide as transport ground to halt – this data came from satellites monitoring the earth’s atmosphere.

The use of this information extends beyond that of climate research and has proven to be very valuable in detecting emissions from individual industrial plants, where this is typically a direct function of production levels.

Satellites can even measure the depth of shallow water. Depending on the water clarity this can be possible to a depth of 25m to 30m. The great advantage of this technique is that large areas can have subsurface features mapped without the need for expensive lidar and sonar measurements conducted from the surface, and this capability is available globally with short lead times.

Satellite mapping sub-surface plankton with LIDAR

Satellite mapping sub-surface plankton distribution with LIDAR

Atmospheric Sensors – Example Uses:

  • Aquaculture: Measuring sea temperature, detecting plankton and pollutants. 
  • Agriculture: Measuring temperature, humidity, and soil moisture levels.
  • Trading: Understanding supply (production) and demand (based on human activity).
  • Insurance: Gaining a better understanding of the environment and their risks of flood and fire.

ESA’s SENTINEL-4 and SENTINEL-5 are examples of missions for detecting atmospheric properties.