SSS10 - Case Study: Earth Observation

Space Science and Sector

UKSA Space Cluster Microcredentials

10

case study:Earth observation

• In this case study module you will study Earth Observation (EO) as a means of protecting the health of our planet.
• You will become familiar with some basic science which is used to obtain information from light and other forms of electromagnetic radiation. This will enable you to understand how EO satellites work.
• You will also make use of online videos to help you learn the concepts better.
• Let's begin !

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Welcome to module 10

1. Be able to describe the main environmental challenges to the earth and how earth observation (EO) can help to protect the planet,
2. Identify the main regions of the electromagnetic spectrum and describe their use in EO.
3. Recognise and understand new terms used to describe EO science and instruments.
4. Describe the Copernicus EO program and major satellites in use.

Learning Objectives

learning objectives

• The environment of Earth is facing an unprecedented challenges from many directions.
• Global warming due to greenhouse gas emissions.
• Deforestation
• Pollution
• Melting of the polar ice caps.
• Changing weather patterns.
• To monitor these changes Earth Observation (EO) satellites measure both the light reflected from the Sun and the natural infrared radiation generated by the earth’s surface, oceans and atmosphere.

The Earth – A Planet in Crisis

• This infographic shows that satellites analyse both light reflected by the earth including clouds and oceans, and the thermal infrared (IR) emitted by the earth’s environmental systems including clouds.
• Instruments that measure the reflected light or the emitted IR are called passive sensors. This means that they don’t actively illuminate the target themselves.
• To go further we need to get a very basic understanding of the electromagnetic spectrum – the big family to which visible light and infrared belong.

Reflected Light and Emitted Infrared

• Both visible light and infrared are examples of electromagnetic radiation. The full electromagnetic spectrum is shown to the right.
• The spectrum is divided into regions, where the waves in each region behave in similar ways.

• We will be interested mainly in three regions, namely the visible light (or optical) region, the infrared region which is often associated with heat emissions and TV remote controls, and the radio wave region, associated mainly with communication and RADAR.
• The wavelengths we will briefly study will range from kilometres for radio waves down to microns for visible light. A micron (μ) is a millionth of a metre.
• 1 μm = 1/1,000,000 mPut another way, a micron is a thousand times smaller than a millimetre ! That’s small !

The electromagnetic spectrum

• The key science concept we need here is the wavelength of the waves. Like water waves, electromagnetic waves have peaks (crests) and troughs, but this time instead of physical heights they are peaks of electric and magnetic fields.

• The distance between two crests is the wavelength, and the different regions of the spectrum have different wavelengths. For example, radio waves have wavelengths starting from as small as 1 mm going up to 100's of kilometres.
• The wavelengths of visible light waves are MUCH smaller than that of radio waves and in fact so small we need a unit much smaller than a metre or even a millimetre. We can use microns or nanometres (nm).

The electromagnetic spectrum

• Visible (often broken down into Red, Green, Blue RGB channels). 
• Red has the longest wavelength while blue has the shortest wavelength.
• Blue has a wavelength of between 0.45 µm and 0.50 µm. Green from 0.49 µm to 0.58 µm. Red from 0.62 µm to 0.78 µm. YOU DO NOT HAVE TO REMEMBER THESE VALUES.
• Put another way, about a hundred wavelengths of red light would stretch across the width of one of your hairs !
• Near infrared (NIR), just beyond the red region of the visible spectrum. Wavelength range 0.7 µm to 1.1 µm. 
• Long Wave IR (LWIR) also called Thermal Infra-Red (TIR) or Far Infra-Red. Wavelength range from 8 µm and 15 µm. Again, you don't need to remember these values.

Regions or Bands of the EM Spectrum

• Optical cameras can be carried on the satellite to monitor land use changes, deforestation, urban sprawl and many other pieces of data.
• Higher resolution images are often commercial products. The ESA Sentinel series offer free high-resolution images for public use. Opposite is an image of Milan from Sentinel-2.

• If the colours correspond to what you would see as you were flying over the area, the image is called a True Colour Image.

Visible band observation

• Blue sensors are optimized for studying oceans. Humanmade structures such as roads also show up well in blue light.
• Green sensors are best for studying plants on land and phytoplankton in the oceans.
• Red sensors have many uses in geology and also, perhaps surprisingly, in studying plants, as chlorophyll absorbs strongly in red light.

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RGB Bands

• This is a false color image of a fire at Hermit's Creek. in New Mexico from 2022.
• The scorched earth from the fire has been assigned the colour red.
• False color images don't represent the actual colors you would see if flying over the area.

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false color imaging

• When sunlight reflects of objects, not all colours reflect off equally. For example, when sunlight reflects of a green leaf, all the colours except green are absorbed, and the green is reflected. That’s why leaves appear green !
• In fact materials reflect parts of the spectrum in unique ways, to such an extent that measuring the reflected spectrum is like a fingerprint for the material. This is a spectral fingerprint.
• By observing in different bands, a satellite can remotely sense what a material is made of. This shows the importance of measuring in different spectral bands using multispectral instruments.
• An instrument designed to measure between 2 and 20 bands is called a multispectral instrument. Hyperspectral instruments measure hundreds of bands. For example, Landsat-8 carries a multispectral instrument with 11 bands, and Sentinel-2 has a 13 band instrument.

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spectral fingerprints, multispectral and hyperspectral instruments

• Some satellites use lasers to illuminate a target and measure the reflected laser beam.
• By doing this many things can be measured such as the height of the satellite above the ground or even the flow of air in the atmosphere. An instrument to measure height is called an altimeter.
• Lidar – Light Detection and Ranging
• The ESA Aeolus satellite uses Lidar to map atmospheric winds above the planet.
• A sensor which illuminates its target is called an active sensor.

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Active Lasers (lidars)

• Play the video opposite to see how satellite imagery can be used to analyse the San Francisco Bay area. The satellite was Sentinel-2 which is part of Europe's Copernicus program. The video is from the European Space Agency (ESA).
• Throughout the course, videos are only meant to reinforce your learning, and are not assessed - you certainly don't have to remember any details of what is shown in videos. But they are very useful to make a topic much clearer.

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activity

• The band just beyond red is called the Near Infra-Red (NIR) and is invisible to the human eye.
• Healthy plants reflect NIR much more than unhealthy vegetation and so satellites with NIR sensors can give vital information on vegetation health.
• Recall we cannot see NIR, and so to represent it we code the NIR with visible colours. This is a False Colour Image.
• The image on the right is a false colour image of Florida taken by Landsat-5 showing NIR coded as green, indicative of healthy vegetation.

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Near infrared (NIR) monitoring vegetation health

• All objects above absolute zero emit long wavelength infra-red (LWIR) and so by measuring this radiation the temperatures of objects on earth can be measured, even at night.
• The false colour image on the right shows the infra-red emitted by two people as a heat map.
• LWIR is also known as Thermal infra-red (TIR), because it maps heat emission.

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Thermal infrared band to monitor heat and temperatures

• This is a false colour infra red picture of clouds over the central USA from the GOES-17 satellite.
• Clouds at different heights are at different temperatures which can be detected in IR and coded as different colours.
• In the image red and black represent very cold clouds, which indicate these clouds are very high up.

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infrared satellite image example

• RADAR – RAdio Detection And Ranging
• Radars use radio waves which travel to a target and then bounce back of it to a detector. This allows a radar receiver to measure the precise position as well as velocity of the target. This can be used by satellites as well.
• By using radar a satellite can send radar pulses down and time how long it takes to return from reflecting off whatever surface they encounter. They can measure the heights of surfaces such as oceans and sea ice. This is then referred to as a radar altimeter.To the right we see Mount Teide illuminated by a radar from the Space Shuttle.

• Like lidar, radar is active imaging, meaning that it illuminates the image with EM radiation. It is not dependent on the sunlight illuminating the image. It can be used at night as well as through clouds.

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active Radar imaging through clouds and at night

• Play this video which neatly summarizes all the new learning we have saccomplished about spectral bands.
• Remember, you will NEVER be asked for the wavelength of any spectral band in an assessment.

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activity

• An EO satellite usually covers the Earth’s surface in swaths, imaging along the swath as it moves along in its orbit. The image shows a satellite swath. 
• The swath width of Landsat satellites is 185 km. 
• As the satellite orbits North-South eventually the whole surface of the earth is imaged.
• Click on the button below to view a Sentinel-2 document from ESA. Note the multispectral nature and the swath width for this saetllite. Is it wider or narrower than the Landsat swatch width?

Go to esa

swaths and swath widths

• Watch this short video of the Landsat orbit showing swaths and the ability to completely image the entire surface of the earth.
• How long does it take Landsat to view the entire surface of the earth ?

Activity - swath width video

• As a vital part of humanity’s response to climate change and to understand our planet better, Earth Observation satellites are tracking these changes globally and over long timescales, giving us an accurate picture of what is happening and delivering vital accurate data to decision makers.
• The European Union’s Copernicus Program is the biggest Earth Observation program in history and is administered by the European Space Agency (ESA).

Europe's copernicus program

• ESA operates a fleet of large Earth Observation satellites known as Sentinels to track changes on the Earth.
• These are large and highly sophisticated satellites built specifically to measure critical parameters of the earth’s ecosystem.

Europe's sentinel satellites

• Spatial resolution refers to the detail which satellites can observe on the ground. It is the area 1 pixel represernts on the ground.
• Here is an excerpt from Sentinel 2 satellite performance from ESA: “Sentinel-2 will carry an optical payload with visible, near infrared and shortwave infrared sensors comprising 13 spectral bands: 4 bands at 10 m, 6 bands at 20 m and 3 bands at 60 m spatial resolution.”
• Common commercial spatial resolutions are between 2 and 5 m although down to 30 cm is available.

spatial resolution

• There are some common instruments which it is good to know about. The most common are:
• Radiometers, which measure the intensity of EM radiation in some band.
• Spectrometers, which measure the spectral content of the radiation.
• Altimeters, active instruments which measure the height of the instrument above the Earth.
• Scatterometers are active radar instruments which provide information about near-surface winds over the oceans.

common eo instruments

• The video from Meet the Satellite walks you through the main instruments of Sentinel -3 satellite.
• You do NOT need to remember these.
• Note how many times your new knowledge is used as you view the various instruments.

activity - meet sentinel-3 and its instruments

• A single satellite may fly over a target but then not return to that target for days or even weeks. Or it may be that the target is only visibe for a very short time as the satellite flies overhead. This is a problem if there is a fast developing situation which needs monitored daily or hourly.
• If, however, a group of satellites is used, all cooperating, then the area can be viewed much more frequently.Conversely, the satellites will be in view much more often, perhaps with always at least one satellite in view.

• A fleet of satellites is called a constellation, and constellations are used for EO, for navigation and for communication applications.The image shows NASA's A Train constellation of EO satellites. The A stands for 'afternoon'.

satellite constellations

• Weather satellites monitor the world’s weather and climate. They are often placed in a special orbit 36,000 km up above the earth. At this height they take 24 hours to orbit the earth and are called geostationary, as they seem to be stationary in the sky. Weather satellites commonly use at least Visible and IR channels.
• EUMETSAT currently operate Meteosat satellites in geostationary orbit to provide this service as well as transitioning to the new Meteosat Third Generation (MTG) series., shown in the image to the right.

Our final topic - weather satellites

• Click the button below for a live weather satellite imagery from sat24.com
• Notice the menu down the right hand side of visible, IR and radar. Click these in turn to get an appreciation of what the different bands are showing.

activity

• As your final activity for this Module, watch this video here to get a good understanding of the way Eumetsat monitors weather and climate.

activity

You should now be able to:

• Describe why satellite EO is so important for the future of the planet.
• Describe some uses of different bands of the electromagnetic spectrum in earth observation.
• State the difference between passive and active sensors, and recognise an example of each.
• Know what the term spatial resolution means.
• Know the terms radiometers, spectrometers, altimeters, multipectral and hyperspectral.

• Briefly describe the Copernicus program and state that the main EO satellites are called Sentinels.
• Explain what a satellite constellation is and why they are needed.
• State that the major European weather satellites are the Meteosats operated by Eumetsat.

module summary

Test your Knowledge

Test your Knowledge

Test your Knowledge

Test your Knowledge

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