Module 1 - Mission to Planet Earth

C) What is Remote Sensing?

Objectives

Students will understand how remote sensing is used to study the earth.
Students will learn that three different technologies (optical, infra red and radar) are commonly used in remote sensing and that the three give different information about the earth.
Students will learn that different platforms are used in remote sensing.

Image data for Module 1 contained in the /MODS1TO5/MODULE01/IMAGES directory should be copied over to your computer before beginning this module. The files you will need are in this directory are GalOptic.gif, GalInfra.gif and AndesVIR.gif

Looking At An Object From A Distance

Remote Sensing means "LOOKING AT AN OBJECT FROM A DISTANCE" and can be applied to any attempt to observe interesting phenomena from afar. Thus an astronomer, looking at the constellation of Orion through a telescope, or a bird-watcher observing a rare swallow or eagle through a pair of binoculars, could both be said to use "REMOTE SENSING". In the scientific community, remote sensing has come to mean observing the earth (or other planets in the solar system) using instruments which measure reflected electromagnetic radiation. In the 1990's, this means using sophisticated cameras and radars mounted on high-flying aircraft or orbiting spacecraft to form pictures or images of the earth as seen at different wavelengths of reflected electromagnetic radiation, (i.e. light). These images are then used to aid our understanding of the physical processes which can affect the earth's atmosphere or it's surface.

History - From Pigeons To Satellites

An early example of a remote sensing tool is the telescope, mentioned above, which was used by navigators to spot land while at sea, as well as for observing the stars and planets. Until the hot-air balloon was invented, our ancestors had no means of observing the earth from up above, other than by climbing a high mountain and looking down. During the nineteenth century, the need for long-range observations of enemy positions for artillery targeting produced some interesting experiments in remote sensing. An example was a French experiment to obtain long-range photographs by strapping a camera to a (presumably well-developed) pigeon. During the American Civil War, balloons were hoisted aloft during several battles to observe the disposition of enemy troops and direct artillery fire on them through signaling (using semaphore). Unfortunately for the balloonists, this improvement in military technology arrived on the scene at the same time as the rifle, which allowed sharpshooters on the ground to use them for target practice.

During the First World War, airplanes were introduced in support of artillery, in the role formerly played by pigeons and balloons. In the early stages of the war, unarmed observation planes would take off with a camera strapped to the undercarriage, to take pictures of the trenches and troop movements of the enemy. When someone figured out how to strap a machine gun onto an aircraft, the era of the fighter and the Red Baron was born. The initial objective of the fighter plane was thus to shoot down the observation planes which carried remote sensing equipment.

After the First World War had ended, and aircraft began to be produced for civilian use, it was realized that pictures taken from cameras mounted on aircraft could be used to produce highly detailed, accurate maps. Most of our modern maps are generated in this way. Prior to that, maps were produced by survey teams on the ground, in a similar fashion to the maps produced by the Lewis and Clark Expedition in North America or by Stanley and Livingstone in Central Africa.

As technology progressed, cameras improved, infra-red and ultra-violet films were developed, and other types of sensors, such as radar and X-ray instruments, added to the range of the electromagnetic spectrum which could be used in remote sensing. With the dawn of the space age, it became possible to place remote sensing instruments on orbiting satellites, and to look down on all the corners of the earth, even the most remote locations. The development of satellite remote sensing technology also coincided with the dawn of the missile era. Thus, the first remote sensing satellites were used by the USA and the Soviet Union to target or at least identify each others' long-range ballistic missile sites. The U-2 spy plane, whose revealing pictures gave the first indications to President Kennedy that the Soviet Union had installed missiles in Cuba, was a further example of the military use of remote sensing.

As often happens with new technological developments, initial exploitation by the military is followed by scientific utilization, then commercial use. A good example in remote sensing is the case of weather satellites, which were initially developed to determine weather conditions over sites of military significance, then the data from these satellites was used by scientists studying the Earth's atmosphere. Nowadays, no TV news broadcast would be complete without the sight of the weather 'expert' waving his or her arms about over a picture of clouds or storm systems provided by a weather satellite.

In the 1970's, the National Aeronautics and Space Administration (NASA) launched the first in a series of civilian remote sensing satellites

called LANDSAT. LANDSAT provided scientists and cartographers with unprecedented access to remote sensing images of the Earth from space. Maps could be reliably drawn for areas of the world which were previously inaccessible, geologic surveys could be conducted from an office desk, and vegetation cover could be mapped over large portions of the Earth's land surface.

In the 1990's, NASA plans to launch a series of Earth Observing Satellite (EOS) as part of the Mission to Planet Earth. The remote sensing instruments carried by these satellites will revolutionize scientists' understanding of the Earth as a global system, including the atmosphere, oceans and land. Other remote satellites have already been launched or planned by nations such as Russia, Japan, Canada and the countries of the European Economic Community. Radars which can image the Earth's surface, such as NASA/JPL's Spaceborne Imaging Radar-C (known as SIR-C for short) are a key part of these programs.

What Do Scientists Use Remote Sensing Data For?

Take a look at the picture of the Earth below. The image was collected by a satellite at a very high altitude, so that one side of the whole Earth is visible. On this scale, it is possible to see only large-scale features, such as the continents, the oceans and clouds in the atmosphere.

When scientists look at remotely sensed images of the Earth, they are interested in different aspects of the large-scale features visible in the image above: the land, the oceans and ice, and the atmosphere. Scientific use of remote sensing data can be divided up into these areas of interest, i.e.,

Land Studies (e.g. geology, agriculture, land management)
Ocean Studies (e.g. oceanography, ice studies)
Atmospheric Studies (e.g. meteorology)

Within each area of interest, there are many measurements which can be made using remote sensing. Examples are given on the next page. It is only recently that scientists from the different disciplines have got together to incorporate their results and insights into the first attempts at an understanding of the Earth as a global system. For example, a scientist studying the Antarctic ice sheet (a land study) might make use of meteorological data (from an atmospheric study) to estimate the annual snowfall rate over the continent. Combining these data with measurements of seasonal ice surface temperature, obtained from a remote sensing satellite, the scientist could then produce a model which would be used to estimate the rate of increase or decrease in the ice sheet thickness over the continent. This data could then be used by an oceanographer to estimate the rise or fall in sea level which would occur if the ice sheet thickness model were correct.

More recently, scientists have developed models of the Earth as a complete system, including land, ocean and atmosphere. Remote sensing measurements are used as inputs to these models. The models are used to predict the consequences and estimate the likely causes of global warming, or an increase in the size of the hole in the ozone layer which has been observed over the polar regions.

Scale

The scale of the features to be measured governs the scale of the remote sensing data required. For example, looking at vegetation cover over a continent can be done at very coarse scale (e.g. a grid with points every 1 kilometer). Mapping vegetation type in California's Central valley would require a finer scale (e.g. a grid with points every 30 meters).

Measurements Derived From Remote Sensing By Scientists:

Land Studies
- Vegetation cover (type, seasonal variation, effects of drought, deforestation)
- Surface roughness and moisture content
- Hydrology (watersheds, runoff rates, snow and ice cover)
- Geology (fault lines, earthquake damage, volcanoes, oil, gas and mineral exploration)
- Cartography (producing maps of topography, land cover type)
- Disaster monitoring
Ocean Studies
- Monitoring ocean currents (e.g. Gulf stream)
- Wind speed and direction at the ocean surface
- Wave height and direction of travel
- Ocean surface temperature
- Presence of plankton
- Extent and nature of polar ice caps
Atmospheric Studies
- Rainfall
- Extent and thickness of cloud cover
- Hurricane and tropical storm monitoring
- Wind speeds and directions
- Atmospheric temperature
- Atmospheric Chemistry (aerosols, ozone, volcanic dust, etc.)

Images

Students will do an exercise where they are introduced to the use of digital numbers (DNs) and X,Y coordinates as seen in the Values table. The DNs indicate gray scale values from 0 to 256, and range in shade from white to black. They will looks at various shades of gray and then turn their gray scale images into false color images, also ranging from 0 to 256.

Students will then return to the images reviewed in Section B and check the images for gray scale values. They will then display an image of the Andes taken using both visible and infrared wavelengths and see what a three color, "FALSE COLOR" image looks like. The image the students will display is a false color mosaic of the central part of the Andes mountains of South America (70^oW, 19^oS). It is made up of 42 images taken by the Galileo spacecraft from an altitude of about 25,000 kilometers. The combination of visible and near infrared filters (green, 0.76 micron, and 1.0 micron) was chosen for this image to separate regions with distinct vegetation and soil types.

The mosaic shows the area where Chile, Peru, and Bolivia meet. Lakes Titicaca and Poopo can be seen as nearly black patches from top to bottom; a large light blue area below and left of Lake Poopo is Salar de Uyuni, a dry salt lake some 120 kilometers across. These lakes lie in the Altiplano, a region between the western and eastern Andes, which is covered by clouds. The water/ice content of the clouds is indicated by their shade of pink The vegetated Gran Chaco plains east of the Andes are pale green (right of image). Light blue patches in the mountains to the north are glaciers.

Teacher's Guide - Table of Contents

Converted to the IBM-PC by Al Wong, sirced03@southport.jpl.nasa.gov

Jet Propulsion Laboratory
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