HOW WILL WE STUDY THE EARTH


Remote Sensing Overview

We will launch a series of spacecraft carrying instruments that will gather data for studying the Earth. This is known as the Earth Observing System (EOS). These instruments will scan the Earth measuring radiation reflected or emitted by the Earth system components, and relay those data back to ground stations to be processed and used by scientists worldwide for studying the Earth. This process of gathering data is called remote sensing.

You can begin the video by clicking on the EOS Movie link below.

EOS Movie



Satellite Orbits for Remote Sensing

Consistent, long-term measurements are needed of the key physical variables that define Earth system processes. A full set of observations requires different orbits. For global coverage by EOS, polar orbits will view the entire Earth over the course of many orbits over several days.

Low-inclination orbits will permit observation of a portion of the Earth over several days, with the observations on successive days being made at different times of day.

Geostationary orbits permit continuous observations in time, but only for a limited view of the Earth. Since a geostationary satellite progresses in its orbit at the same rate as the Earth's rotational rate, it can provide a fixed view of the Earth's disk that is determined by its selected position above the equator.


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Satellite Ground Tracks

As a satellite in polar or near-polar (high-inclination) orbit passes over the Earth, the Earth's rotation shifts the satellite ground track westward, so that after a certain number of days and orbits, the entire Earth is covered and the cycle begins again.

If orbits are sun-synchronous, the satellite passes over each latitude at the same local time, providing consistent lighting and allowing easier comparison between data and images taken of the same area on different days. Most of the EOS satellites will be in polar, sun-synchronous orbits.

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Remote Sensing Methods

Remote Sensing involves the detection and measurement of radiation of different wavelengths emanating from distant objects or materials, by which these may be identified and categorized by class/type, substance, and spatial distribution. In Earth science studies, the objects or material are located at the Earth's surface or in the atmosphere. There are two types of remote sensing instruments-passive and active. Passive instruments detect natural energy which is reflected or emitted from the observed scene, and active instruments provide their own energy to illuminate the scene they observe. Remote sensing instruments are further divided into scanning and pointing instruments, with scanning instruments being the most commonly used. Also, remote sensing instruments are typically multispectral-that is, they detect multiple wavelengths of radiation.

A remote sensing instrument that transmits its own electromagnetic radiation to detect an object or to scan an area for observation and receives the reflected or backscattered radiation is called an active instrument. Examples are radars, scatterometers, and lidars.

Radar (radio detection and ranging)

A radar uses a transmitter operating at either radio or microwave frequencies to emit electromagnetic radiation and a directional antenna or receiver to measure the reflection or backscattering of radiation from distant objects. Distance to the object can be determined since electromagnetic radiation propagates at the speed of light.

Examples (non-EOS): TRMM Rain Radar, SAR

Scatterometer

A scatterometer is a radar that measures the backscattering coefficient of the surface of the viewed object. The backscattering coefficient can be used to define surface characteristics such surface roughness, moisture content, and dielectric properties. Over ocean surfaces, measurements of backscattering coefficient in the microwave spectral region can be used to derive maps of surface wind speed and direction.

EOS examples: SeaWinds

Lidar (light detection and ranging)

A lidar uses a laser (light amplification by stimulated emission of radiation) to transmit a light pulse and a receiver with sensitive detectors to measure the backscattered or reflected light. Distance to the object is determined by recording the time between the transmitted and backscattered pulses and using the speed of light to calculate the distance traveled. Lidars can determine the profile of aerosols, clouds, and other constituents in the atmosphere.

EOS examples: GLAS

Laser Altimeter

A laser altimeter uses a lidar (see above) to measure the altitude of the instrument platform by measuring the distance to the surface below. By independently knowing the location of the platform, the topography of the underling surface can be determined.

EOS examples: GLAS

Passive instruments sense only radiation emitted by the object being viewed or reflected by the object from a source other than the instrument. Reflected sunlight is the most common external source of radiation sensed by passive instruments. Various types of passive instruments are used, including radiometers and spectrometers.

Radiometer

An instrument that quantitatively measures the intensity of electromagnetic radiation in some band of wavelengths in the spectrum. Usually a radiometer is further identified by the portion of the spectrum it covers, for example, visible, infrared, or microwave.

EOS Examples: ACRIM, CERES, AMSR

Imaging Radiometer

A radiometer including a scanning capability to provide a two-dimensionalarray of pixels from which an image may be produced. Scanning can be performed mechanically or electronically by using an array of detectors.

EOS examples: ASTER, Landsat

SpectroRadiometer

A radiometer having the capability for measuring radiation in manywavelength bands (i.e., multispectral), often with bands of relatively high spectral resolution designed for the remote sensing of specific parameters such as sea surface temperature, cloud characteristics, ocean color, vegetation, trace chemical species in the atmosphere, etc.

EOS examples: MISR, MODIS

Spectrometer

A device to detect, measure, and analyze the spectral content of theincident electromagnetic radiation. Conventional, imaging spectrometers use gratings or prisms to disperse the radiation for spectral discrimination.

EOS examples: AIRS, MOPITT, SAGE III, TES, SOLTICE


Atmospheric Absorption/Emission

Most materials possess unique radiation properties or signatures by which they can be identified. Properties of primary interest for Earth remote sensing are the absorption/emission spectra of atmospheric constituents and surface features. Examples of atmospheric constituents are clouds, aerosols, and gases like water vapor and ozone. Examples of surface features are vegetation, water, snow and ice, soils, and rocks.

The ability of remote sensing systems to selectively examine portions of the wavelength spectrum enables satellite sensors to map globally important Earth system features such as surface temperatures, cloud cover, atmospheric ozone, vegetation, ocean currents, and many others. Portions of the wavelength spectrum where radiation can penetrate the atmosphere are called atmospheric windows.

In the charts, the "Wavelength Spectrum" describes the radiation spectrum from very short wavelengths, high energy gamma rays to very long wavelengths, low energy radiowaves. The most important wavelength regions for Earth remote sensing are the "Ultraviolet and Visible" region, the "Infrared" region, and the "Microwave" region. The absorption/emission spectra of important atmospheric gases, the atmospheric windows, and the data measured in each wavelength region are shown on the charts.

The red color represents the degree that radiation can penetrate the atmosphere, whereas the white color represents the degree that radiation is absorbed or emitted by the atmospheric gases noted. For example, the large white area in the ultraviolet region indicates that oxygen and ozone absorb this radiation. By contrast, the large red area in the microwave region, shows that radiation with these wavelengths can very effectively penetrate the atmosphere.

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