INVESTIGA I+D+i 2013/2014

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1 INVESTIGA I+D+i 2013/2014 SPECIFIC GUIDELINES ON AEROSPACE OBSERVATION OF EARTH Text by D. Eduardo de Miguel October, 2013 Introducction Earth observation is the use of remote sensing techniques to better know the basic functioning and state of our planet. For many researchers, Earth observation and remote sensing are practically synonyms, although for others the former is a more global concept and may include other techniques and activities. In an ordinary definition, remote sensing is the technique that allows us to capture images of earth s surface with sensors installed on aircrafts or space platforms. One can see that, for several reasons, some other techniques are excluded, like conventional aerial photography (difficult to be used quantitatively), sonar, rawinsondes, fire watch towers Curiously enough, people s knowledge on remote sensing covers both extremes of the rage of images taken from Earth s observation. On the one hand, the Meteosat, with low resolution images, wide coverage and a scientific focus; and on the other, Google Earth, with highly detailed images and limited coverage and with a clear commercial objective. Additionally, each one is a good example of two large and quite different areas of the spectrum; the visible range and the thermal range.

2 Meteosat thermal infrared > 3000 meters per pixel. Low space resolution of the images, high repetition, high coverage, scientific focus, thermal data. Google Earth and similar 1 meter per pixel approximately High space resolution, low coverage, low repetition, commercial use, similar data to conventional photography. Between these two widely known applications, we have many other missions to generate images of Earth. But, besides the in between, we have side missions, with very different technologies. Basically those based on microwaves, like radar images, which belong to a completely different field that will not be detailed in this paper. There are currently 1000 active satellites in space. Most of them are communications ones, but around 100 are devoted to Earth observation. Some are SPOT, Pleiades, Landsat, ENVISAT/MERIS, AVHRR o MODIS (one of its images is shown the cover of this document). Lifetime of these satellites varies between 5 and 15 years. Most of them belong to public institutions both at a national or international level (NASA, ESA, CNES...), but there are also

3 commercial satellites (p.e., WorldView o DEIMOS). Some of them are built at universities with an educational objective and at a minimum price, therefore they are quite interesting. General image of the Pleiades satellite (left) and the inside of it (right). Photo: CNES Concerning Earth observation from a plane, there are several instruments, similar to those on space platforms but normally more powerful because they are not affected by weight limitations (and others). Its normal use is devoted to special research, pilot projects, instrument validation, etc. They are not efficient when they need continuity or a wide geographical coverage. They perform on order and very rarely can their data be available for researchers who don t apply for them. INTA s remote sensing area, with its instruments, AHS and CASI, and its platform C-212, is one of leading European centers for this technology.

4 C-212 of INTA and an image taken with its AHS instrument in Ostend, Belgum. The possible and the impossible It is interesting to approach Earth observation thinking that it is actually possible; this way we can imagine new ideas and applications but we will also be aware of the limitations of this technique, that is to say, those applications that are not possible.

5 Each one of these entries has its own problem. As we cannot get into detail, we will summarize it by saying that it is actually possible to capture images of the ground: In narrow wavelength intervals In areas of the spectrum where our eyes are not sensitive to High resolution (detail of the image) in both intensity and spatial (geographical location) With rigorous calibration And, on the other hand, it is possible to systematically cover Earth from sun-synchronous orbits at around 800 km high and from a geostationary orbit at km. It is impossible (or difficult): to simultaneously have high spatial, radiometric and spectral resolution. to have high spatial resolution in the thermal infrared interval. to have high temporal frequencies from low orbits recognize low-contrast objects see under surface. It is also impossible to have a geostationary orbit at a different height from km, or fly very slowly to improve spatial resolution of the image. Why Earth Observation? Because it is very useful for many things, although not for everything and not even for all that we would be interested in. Mainly we can observe Earth to study any attribute that involves the color of the Earth s surface in a wide concept, and also to measure Earth s temperature in the thermal range. We can also study some aspects of the atmosphere. In all cases, we can benefit from the global and repeatable vision that a satellite provides, making it more efficient and cheap than other alternative methods.

6 Remote sensing is being used in many fields; measurement of sea temperature, cartography of soil uses, delimitation of burnt areas in forest fires, estimation of biomass on our planet But there are still applications under development. For example, it is not possible to distinguish easily all the species of plants and bushes to make a forestry map, or estimate all the biomass of a forest. Research derived from Earth observation The basis of the scientific method is to infer laws in nature from observation and validate them through experiments whose results confirm those laws. That is to say, that research is based on data collection and analysis. However, not all scientific activity consists in running experiments to suggest or confirm laws. It is also necessary to develop technology to collect and analyse data or find the mathematic tools to create those laws. Therefore, Earth observation can also have two types of scientific activities: develop technology to capture, calibrate and analyse images use of images to improve our knowledge on Earth. On the latter, we normally need external data that will allow law inference and its later validation. Let s see a simple and rather realistic example. There is a need to measure the quality of waters in reservoirs. Currently, samples are taken in situ and it calls for expensive and hard working field visits. Some researchers have tried to find an alternative through remote sensing imagery. For this reason, they have used the following procedure. we chose a few representative reservoirs we collect some samples to identify the quantity of chlorophyll in water, which is related to the quantity of unicellular algae and, consequently with the quality of water. we chose satellite images that cover these reservoirs and have the characteristics (collecting date, certain spectrum bands, space resolution, etc) we are interested in we build through math/statistics techniques a model that relates the quantity of chlorophyll taken in situ with the signal strength registered by the satellite in different bands.

7 we apply this model to other satellite images and then obtain the estimate of the quantity of chlorophylls in other reservoirs. we validate this estimate in a few reservoirs again with the new in situ data In practice, these steps are full of challenges and problems and finding a relatively-efficient model to estimate the chlorophyll in reservoirs, is still an issue. The same happens with other applications in remote sensing imagery. Sometimes it is possible to investigate using images from remote sensing imagery without having the external data, using instead, information provided by the images. One example is the study of the changes on the ground of a certain area and compare all the images taken in different dates. How to obtain Remote Sensing Imagery To obtain the necessary images, a researcher has several options. There are some commercial missions whose images can be bought and some other scientific missions whose data their data are not on sale. Some missions are a combination of both and images are offered for free if the use has a scientific purpose but are sold if it is commercial. In scientific missions, there is a need to justify the use and purpose to be able to apply for them. There are several ways to obtain images to get started into remote sensing imagery. We highlight some of the easiest and most accessible ways ESA provides the student with some images. They are available at ESA Eduspacedatabase, NASA keeps a similar resource, specialised in satellite images LANDSAT, en CREPAD s data basis included images from different sensors. CREPAD is a remote sensing imagery program of INTA, whose main objective is to facilitate users access to Earth observation products. In order to obtain them, it is only necessary to

8 register and apply for the images on the available catalogue on Another broad data resource is the USGS catalogue (United States Geological Service): In this catalogue one can obtain MODIS, Landsat and AVHRR images, which are also free and just needs the user s registration. Last recommendation is data access to the VEGETATION instruments on: Screen capture of ESA / EduSpace s browser. Working on remote sensing imagery Remote image is the basic element in Earth observation. An image is a conjunct of computer data that covers a certain part of Earth. Each one of these data is called a pixel. (Word derived from picture element)

9 Although initially each datum is not different from any other taken with a different technique (for example the height of each student in a college), its ordered layout in space confers an additional value. Relationship of a datum with its neighboring data becomes important, for both the visual analysis and the math analysis, contrary to the example above. Usually the same scene has been observed at different wavelengths at the same moment, therefore giving different overlapped images, which we call bands. Hence, each pixel has from one to many values of intensity associated. These values can simply be the digital data registered, calibrated data indicating the exact energy received, data transformed into more advanced magnitudes like reflectance, emissivity or temperature of the surface observed or even those data derived from this, like the vegetation index. In order to investigate Earth observation images, it is necessary to use informatics tools to visualize and access the values of each pixel. There are several commercial and free products for this purpose. The most popular ones among the first ones are ERDAS, Geomatica y ENVI and ArcGIS geographic information system. Among the free ones, we recommend LEOworks, especially designed by ESA for remote sensing teaching in secondary/bachillerato: For educational activities, NASA recommends the use of MultiSpec; Other free products which have not been designed for educational purposes but professional ones are: quantumgis, initially a geographic information system better than an image processing tool: BEAM, toolbox launched by ESA for MERIS image processing: HDFview, for images in a HDF format, which is used in MODIS: Likely topics for discussion

10 Although most of the hot spots under Earth observation require broad and deep knowledge on the state of art in many fields (space instrumentation, radiometry, Earth Science, statistics ) some of them can benefit from some imagination, creativity and lack of prejudice of the newcomers. Here are a few ones. How to efficiently use the Petabytes which are continuously obtained from space and never analyzed? Is sensor calibration actually working or does each one measure a different value on observing the same plot of land? How to benefit from Earth observation technology to fight forest fires? What new fields of applications can be suggested? What mission can be suggested to cover those fields, or other ones that are not well defined? What can we suggest to improve the efficacy of aero transported sensors? Are there too many remote sensing data and few funds to analyze them? Why? How can it help the fact that citizens have easy access to data and processes through their ipads, PCs or mobiles phones? How can western countries Earth observation leadership and experience be transferred to other nations who may need it. Bibliography One can find easily two introductory books in Spanish. Both have been thought for college students, but their level is quite reasonable for secondary education, if the students focuses on the introductory part and leaves out the detailed explanations. 1. TELEDETECCIÓN AMBIENTAL. EMILIO CHUVIECO, ARIEL, ISBN ELEMENTOS DE TELEDETECCIÓN. CARLOS PINILLA RUIZ, RA- MA, 1995 ISBN Of course, there are a lot of resources on the Internet and many webs devoted to this topic. See the list below:

11 1. ESA s tutorial guide is the best one and gives a better idea on this topic ml 2. Similar material can also be found: Earth_observation 3. NASA s prepared version focuses on satellites Landsat: 4. Finally, 6. Two typical glossaries useful for doubts are: 5. Natural Resources Canada includes good: Finally, CEOSS EO handbook is an on-line list of satellites and instruments, including revision of fields of application: