Blog of Many Colours

Image featuring the sister cities of Sault Sainte Marie, Ontario, and Sault Sainte Marie, Michigan. ESA’s Proba satellite acquired this image on 11 August 2006 with its Compact High Resolution Imaging Spectrometer (CHRIS), designed to acquire hyperspectral images with a spatial resolution of 18 metres across an area of 14 kilometres. Data courtesy of SSTL through ESA.

Image featuring the sister cities of Sault Sainte Marie, Ontario, and Sault Sainte Marie, Michigan. ESA’s Proba satellite acquired this image on 11 August 2006 with its Compact High Resolution Imaging Spectrometer (CHRIS), designed to acquire hyperspectral images with a spatial resolution of 18 metres across an area of 14 kilometres. Data courtesy of SSTL through ESA.

The aspect of art at school that really stuck with me was learning about the main colours of the rainbow and how they fit together – like with like, such as yellow, green, blue, and like with unlike such as shades of green with a fleck of red to put spark into a picture. Based on these ideas, when I was a teenager I used to construct geometric mandalas coloured in with gouache. As I began studying remote sensing, it seemed natural that hyperspectral imaging would hold a special fascination.

The term Hyperspectral Imaging was coined by Goetz in 1985 and is defined as ‘the acquisition of images in hundreds of contiguous, registered, spectral bands such that for each pixel a radiance spectrum can be derived.’ Put simply, whereas a picture is made using three colour components for television (red, green and blue), for hyperspectral imaging the spectrum is split into many, sometimes hundreds, of different grades of colour for each part of the image. The term made its way into scientific language by way of the intelligence communities – the military became interested in it as it offered them the ability to tell plastic decoys from real metal tanks, as well as an object’s precise colour.

When the first field spectral measurements were conducted in the early 1970s, technology was not advanced enough for it to be put into operation. However, developments in electronics, computing and software throughout the 1980s and into the 1990s, brought the hyperspectral imaging to the EO community.

A series of parallel hardware development began in the 1980’s, such as at NASA JPL with the Airborne Imaging Spectrometer (AIS) in 1983, followed by AVIRIS (Airborne Visible/IR Imaging Spectrometer). The AVIRIS sensor was first flown in 1987 on a NASA aircraft at 20km altitude and to this day, it is still a key provider of high-quality HS data for the scientific community.

The hardware advances were matched by improvements in software capabilities, with the development of the iconic image cube method of handling this type of data, by PhD students Joe Boardman and Kathryn Kierein-Young, from the University of Colorado. Spectral libraries have been amassed for over 2,400 natural and artificial materials, to enable them to be identified. The most famous is the ASTER spectral library which includes inputs from Johns Hopkins University (JHU) Spectral Library, the Jet Propulsion Laboratory (JPL) Spectral Library, and the United States Geological Survey (USGS – Reston) Spectral Library.

Hyperspectral imaging was primarily developed for the mapping of soils and rock types; and the spectra of these are rich in character. Taking regions from the contiguous spectrum makes it possible to identify surface materials by reflectance or emission and also allows precise atmospheric correction which can only be approximated if you are using discrete, wide colour bands. The shape of the reflectance or emittance spectrum yields information about grain size, abundance and composition as well as the biochemistry of vegetation, such as the concentration of chlorophyll and other pigments and life forms in water bodies.

Earth observation hyperspectral imaging really began with NASA’s Earth Observing-1 Mission (EO-1) launched in 2000, with the Hyperion imager on board that has 200 wavelengths. Since then, various other missions have been launched such as the Compact High Resolution Imaging Spectrometer (CHRIS) on the Proba-1 satellite also in 2001, with 63 spectral bands; or the Infrared Atmospheric Sounding Interferometer (IASI) on board the MetOp series of Meteorological satellites whose first version was launched in 2006.

The coming years for hyperspectral imaging looks exciting with a whole series of planned missions including the Italian PRISMA (PRecursore IperSpettrale della Missione Applicativa), German EnMAP (Environmental Mapping and Analysis Program), NASA’s HyspIRI (Hyperspectral Infrared Imager), and JAXA’s (Japan Aerospace Exploration Agency) Hyperspectral Imager Suite (HIUSI).

So for me, and anyone with the same fascination, the future really will be filled with many colours!


Blog written by Dr Louisa Reynolds

The Small and Mighty Proba Missions

This week the European Space Agency announced the latest mission in the Project for OnBoard Automony (PROBA) mini-satellite programme. Proba-3 is planned to launch in four years; and will be a pair of satellites flying in close formation, 150m apart, with the front satellite creating an artificial eclipse of the sun allowing its companion views of the solar corona; normally only visible momentarily during solar eclipses.

Tamar estuary captured in October 2005, data courtesy of ESA.

Tamar estuary captured in October 2005, data courtesy of ESA.

The Proba missions are part of ESA’s In-orbit Technology Demonstration Programme, which focuses on testing, and using, innovative technologies in space. Despite Proba-3’s nomenclature, it will be the fourth mission in the Proba programme. The first, Proba-1, was launched on the 22nd October 2001 on a planned two year Earth observation (EO) mission; however despite the planned lifecycle, thirteen years later it is still flying and sending back EO data. It’s in a sun synchronous orbit with a seven-day repeat cycle and carries eight instruments. The main one is the Compact High Resolution Imaging Spectrometer (CHRIS), developed in the UK by the Space Group of Sira Technology Ltd that was later acquired by Surrey Satellite Technology Limited. CHRIS is a hyperspectral sensor that acquires a set of up to five images of a target, with different modes allowing the collection of up to 62 spectral wavebands.

Plymouth, where Pixalytics is based, and our lead consultant, Dr Samantha Lavender, have a long history with Proba-1. Rame Head point, along the coast from Plymouth, is one of the test sites for the CHRIS instrument and she’s been doing research using the data it provides for over a decade. Over Plymouth Mode 2 is used, which focuses on mapping the water at a spatial resolution of 17m; this mode was proposed by Sam back in the early days of CHRIS-Proba. The image at the top of the page, captured in October 2005, shows the Tamar estuary in the UK that separates the counties of Devon and Cornwall; for this image CHRIS was pointed further North due to planned fieldwork activities. At the bottom of the image is the thick line of the Tamar Road Bridge and below it, the thinner Brunel railway bridge. Plymouth is to the right of the bridge, and to the left is the Cornish town of Saltash.

Proba-V image of the Nile Delta in Egypt, courtesy of the Belgian PROBA-V / ESA Earth Watch programmes

Proba-V image of the Nile Delta in Egypt, courtesy of the Belgian PROBA-V / ESA Earth Watch programmes

Proba-2 was launched in 2009, carrying two solar observation experiments, two space weather experiments and seventeen other technology demonstrations. ESA returned to EO for the third mission, Proba-V, launched on the 7 May 2013; the change in nomenclature is because the V stands for vegetation sensor. It is a redesign of the ‘Vegetation’ imaging instrument carried on the French Spot satellites; it has a 350m ground resolution with a 2250km swath, and collects data in the blue, red, near-infrared and mid-infrared wavebands. It provides worldwide coverage every two days, and through its four spectral bands it can distinguish between different types of land cover. The image on the right is from Proba-V, showing the Nile delta on 2nd May 2014.

Despite their small stature all the Proba satellites are showing their resilience by remaining operational, and they’re playing a vital role in allowing innovative new technologies to be tested in space.