Remote Sensing, Physics and Techniques

David L. Glackin: deceased.

Synonyms

Earth observation; Earth remote sensing; Teledetection

Definition

Remote Sensing. The study of objects at a distance, without direct physical contact.

Introduction to remote sensing platforms

Before delving into physics and techniques, it is appropriate to briefly discuss the types of remote sensing platforms that are used to host remote sensing instruments, so that the various remote sensing techniques can be put into context. Remote sensing of the Earth can be carried out from many types of platforms, including towers, balloons, unmanned airborne vehicles (UAVs), manned aircraft, satellites, and space stations, using techniques principally in the ultraviolet (UV), visible, infrared (IR), and microwave portions of the spectrum (see Observational Systems, Satellite ). Because it is impractical to instrument the entire globe with ocean buoys, land-based sensors, weather balloons, etc., the emphasis to date has been on remote sensing from aircraft and unmanned satellites.

Satellite orbits

Most Earth remote sensing satellites are placed into circular, near-polar, sun-synchronous orbits at altitudes of approximately 600–900 km above the Earth’s surface. A satellite in a sun-synchronous orbit passes over a given region on the ground at approximately the same local time every day. This means that the solar illumination angle changes relatively slowly for that area from day to day, which simplifies data interpretation. Some sun-synchronous satellites are in exact repeat orbits, meaning that they fly over the same region at exactly the same local time every day. Instruments on sun-synchronous LEO satellites can cover the globe in a synoptic manner, which means they can provide coverage of large areas within a short period of time.

If an instrument onboard a satellite can gather data over a geographically wide swath (approximately 3,000 km in the direction perpendicular to the direction of satellite motion), then that instrument can cover the entire globe in 12 h, if it is designed to collect data both day and night. The visible and infrared instruments currently flying on LEO satellites typically have spatial resolutions (a measure of the size of discernible features) of approximately 1 m–1 km on the ground. Higher spatial resolution generally means less geographical coverage, owing to practical considerations of focal-plane size, onboard data storage, and downlink data rates (see Mission Operations, Science Applications/Requirements ).

Other remote sensing satellites, all weather satellites, occupy geosynchronous orbits. Positioned at 36,000 km above the equator, their orbital periods of 24 h make them appear to hang over one spot on Earth. With broad-area imagers on five “GEO birds,” it is possible to cover the globe up to approximately 60° latitude, with spatial resolutions as good as approximately 1 km.

The L1 libration point, a gravitationally stable point between the Earth and the sun, has been discussed as a possible location for a satellite to study the Earth as a planet for climate research. Other, less used, orbits include Medium Earth Orbit (MEO) and Molniya orbits (see Observational Systems, Satellite ).

Aircraft

Despite the growth in satellite observations, aircraft are still used to gather much remote sensing data and dominate the field for some applications. Because they can fly specific paths at selected times, aircraft offer more flexibility than spacecraft, and the higher spatial resolution that is possible provides the finer detail necessary for applications such as urban planning, disaster assessment, and mapping (see Observational Platforms, Aircraft, and UAVs ). Bridging the gap between conventional aircraft and orbiting spacecraft are the UAVs. These aircraft, capable of long-duration, high-altitude flight, have undergone successful tests and have the potential to provide a valuable supplement to satellites (see Observational Platforms, Aircraft, and UAVs ).

The remote sensing spectrum

The portions of the electromagnetic spectrum (see Radiation, Electromagnetic ) that are most useful for remote sensing can be defined as follows: The ultraviolet (UV) extends from approximately 0.1 to 0.4 μm, the visible (VIS) from 0.4 to 0.7 μm, the near infrared (NIR) from 0.7 to 1.0 μm, the short-wave infrared (SWIR) from 1 to 3 μm, the mid-wave infrared (MWIR) from 3 to 5 μm, the long-wave infrared (LWIR) from 5 to 15 μm, and the far infrared (FIR) from 15 to 100 μm. These ranges are typically defined in terms of wavelength, but other ranges can be defined in terms of frequency as well. Thus, the submillimeter-wave (sub-mm or sub-mm-wave) range encompasses wavelengths from approximately 100 to 1,000 μm or frequencies from approximately 3 THz (THz) to 300 GHz (GHz). The millimeter-wave (mm or mm-wave) range extends from 300 to 30 GHz or 1 mm to 1 cm (cm) and the microwave region from 30 to 1 GHz or 1 to 30 cm (Figure 1).

Figure 1

Electromagnetic spectrum from the ultraviolet to the microwave.

The special case of the gamma-ray spectrum is not discussed here (see Gamma and X-Radiation ).

Within these spectral regimes, there are “window bands” of low atmospheric absorption, in which imaging instruments typically operate and “absorption bands” of relatively high atmospheric absorption in which atmospheric profiling or sounding instruments operate. From the UV through the SWIR, the spectrum is dominated by reflected sunlight. In the MWIR during the daytime, the spectrum is a mix of reflected solar and emitted thermal radiation (Figure 2); the fraction of each depends on the exact circumstances of observation. From the LWIR to the FIR, the spectrum is dominated by the natural thermal emission of the environment. The sub-mm or terahertz regime (between the electro-optical and mm/microwave regimes) is in its relative infancy with regard to remote sensing techniques, showing particular potential for measuring ice in the atmosphere. The mm-wave and microwave regimes are especially useful for measuring environmental phenomena that are water related. These include atmospheric water vapor, cloud liquid water, and rainfall (see Atmospheric General Circulation Models ); sea-surface temperature, wind vector, and salinity (see Ocean, Measurements and Applications ); sea ice extent and concentration, snow extent, and water equivalent (see Cryosphere, Measurements and Applications ); and soil moisture, land surface temperature, land freeze/thaw state, and vegetation (see Land-Atmosphere Interactions, Evapotranspiration ).

Figure 2

Example of solar/thermal crossover characteristics in the mid-wave infrared. During the day, the region from 3 to 5 μm is a mix of reflected solar and emitted thermal radiation.

The portion of the spectrum from the UV through the FIR is generally called the electro-optical (E-O) portion of the spectrum, while the portion of the spectrum from the mm-wave through microwave is generally loosely called the microwave (sometimes abbreviated as μW) portion of the spectrum.

Instruments operating in the E-O spectrum are adversely affected by clouds (see Optical/Infrared, Scattering by Aerosols and Hydrometeors ) unless they are designed to be meteorological imagers or other specialized instruments. E-O imagery in the VIS-SWIR spectrum has low utility at night, although moonlight is sufficient for high-sensitivity low-light-visible systems and hot fires are visible at night, particularly in the SWIR. E-O imagery in the LWIR-FIR is effective day or night, since it relies on natural thermal emission, not sunlight (see Thermal Radiation Sensors (Emitted) ). Microwave instruments, on the other hand, can operate day or night and can penetrate nearly all types of weather, including most clouds. They thus offer views of areas that are frequently cloud covered, such as tropical countries, or that are in darkness for extended periods, such as the wintertime polar caps.

With the above background, an overview of remote sensing techniques and the physics upon which they rely will now be presented. The list of instruments below is not exhaustive, but it includes the major ones in use today.

Types of instruments

Remote sensing instruments fall into the general classes of passive and active electro-optical and passive and active microwave instruments. Passive instruments collect and detect natural radiation from the environment. They receive upwelling radiation from the Earth-atmosphere system, collect it, focus it, detect it, and convert it into electrical signals. These signals are then processed, stored, compressed, formatted, and transmitted to ground stations. Active instruments emit radiation and measure the returning signals. They emit a pulse of radiation that interacts with the environment and then backscatters or reflects off of it. Some of the radiation travels back to the instrument for detection and processing.

Passive electro-optical instruments include multispectral imagers, hyperspectral imagers, atmospheric profilers or sounders, spectrometers, radiometers, and polarimeters (as well as digital CCD (charge-coupled device) cameras, analog vidicon (tube-type) cameras, and film cameras). Active electro-optical instruments include backscatter lidars, differential absorption lidars, Doppler wind lidars, fluorescence lidars, and Raman lidars (“lidar” is an acronym for “light detection and ranging”). Passive microwave instruments include imaging radiometers, atmospheric sounders, spectrometers, synthetic-aperture radiometers, and submillimeter-wave radiometers (assuming for now that the sub-mm spectrum is organized under the microwave spectrum). Active microwave instruments include real-aperture radars, synthetic-aperture radars (SARs), altimeters, and scatterometers. Instruments that observe signals from GPS satellites as they propagate through the Earth’s atmosphere or scatter off the Earth’s surface, namely, GPS occultation instruments and GPS reflectometers, are organized under active microwave instruments, because the GPS signals are actively transmitted. (“Aperture,” for microwave instruments, refers to the effective size of the antenna. “Synthetic aperture” refers to the process of making the instrument function as if it has a larger or more continuous antenna than it does. For SAR, this is done by exploiting the motion of the satellite to make the antenna effectively larger. For radiometers, this is done by exploiting the properties of an array of thin antennas that only partially populate the desired aperture (“sparse aperture”), to make the aperture effectively filled in.)

The special case of using accurate measurements of the Earth’s gravitational field to infer measurements of underground water resources is not treated here. For more information, see Water Resources .

Passive electro-optical

There are three basic classes of passive E-O imagery: panchromatic, multispectral, and hyperspectral. Panchromatic imagery in a single broad spectral band (“black-and-white”) is useful for maximum spatial resolution, because more light is available over the wide spectral range. Multispectral imagery in several to tens of moderately wide spectral bands (“color”) adds information for discrimination, classification, and analysis of objects based on their spectral properties. Hyperspectral imagery in hundreds of narrow contiguous spectral bands further improves discrimination, classification, and analysis.

Passive electro-optical multispectral imagers observe Earth’s natural thermal radiation in the MWIR-FIR and solar radiation that has been reflected and scattered back toward space in the UV-MWIR (see Calibration, Optical/Infrared Passive Sensors ). In the case of panchromatic or multispectral imagers looking down at the Earth from LEO, scanning optics moving in a cross-track “whiskbroom” motion, perpendicular to the direction of satellite travel, can be used to collect a swath of data. Alternatively, the motion of the spacecraft can simply carry the imager’s field of view along track in a “pushbroom” fashion, parallel to the direction of satellite motion (Figure 3). The radiation captured by the front-end telescope optics is transferred through a set of back-end optics and band-pass filters to one or more focal-plane arrays, where it is converted to electrical signals by a number of detectors in the focal plane. These signals are then digitized and may be compressed to reduce downlink bandwidth requirements. Whiskbroom imagers can scan a wide swath of the planet with relatively few detectors in the focal-plane array, while pushbroom imagers can be built with no moving parts; each approach involves trade-offs (Slater, 1980). Whiskbroom scanners result in a “bow tie” effect in the data, because the projection of the focal-plane array on the Earth grows progressively larger as the instrument scans progressively farther toward the edge of the swath. The resultant data swath grows larger in both directions perpendicular to the direction of satellite motion and in outline looks like a bow tie. This effect is often removed in onboard processing.

Figure 3

Examples of whiskbroom (left) and pushbroom (right) scan geometries. The whiskbroom scan may either be a sinusoidal scan (oscillating back and forth) or a barrel roll scan (constant direction of spin) perpendicular to the direction of flight.

If the multispectral imagers are calibrated to quantitatively measure the incoming radiation (as indeed most are), they are termed imaging radiometers. Such instruments detect radiation in a number of (typically less than 20) spectral bands. Multiple wavelengths are almost always required to retrieve the desired environmental phenomena. Multispectral imagers have applications in the study of clouds, aerosols, volcanic plumes, sea-surface temperature, ocean color, vegetation, precision agriculture, forestry and deforestation, land cover and land use, urban planning, snow, ice, pollution and fire monitoring, disaster assessment, emergency response, news reporting, etc. Multispectral imagers typically fly in LEO orbits, although they are also hosted on GEO weather satellites.

Some of these applications make use of the technological advances in spatial resolution offered by the high-resolution (1 m panchromatic or less) commercial remote sensing satellites that began flying in LEO orbits in 1999. The emphasis on higher-spatial-resolution systems for mapping and other applications is sparking an increasingly close tie between remote sensing and the geographic information system (GIS) and geoprocessing technologies. GIS allows the combination of remote sensing imagery with infrastructure information such as roads, bridges, waterways, power lines, and pipelines. The ongoing increases in spatial and spectral resolution, data rate, and data volume emphasize the need for improved image compression techniques, data fusion, image archiving and browsing techniques, and advanced computer hardware (Glackin and Peltzer, 1999).

In contrast to multispectral imagers, hyperspectral imagers (HSI) typically cover 100–200 spectral bands, producing simultaneous imagery in all of them (Figure 4). Moreover, these narrow bands are usually contiguous, typically extending from the visible through SWIR regions. This makes it easier to discriminate surface types by exploiting fine details in their spectral characteristics. Exploitation of imagery from HSI systems is done with spectroscopy, whereas exploitation of imagery from multispectral systems is often done with multivariate classification. Hyperspectral imagery is used for mineral and soil-type mapping, precision agriculture, forestry, and other applications. Because the desired spatial resolution is usually on the order of 30 m, HSI instruments typically fly in LEO orbits. A few airborne hyperspectral imagers operate in the thermal (MWIR-LWIR) infrared, but this technique has yet to advance to space.

Figure 4

Conceptual example of a hyperspectral imager, showing the implementation that uses an area array detector, in which one direction along the array is the spatial dimension and the other direction along the array is the spectral dimension. Spectra of each ground footprint along a line are collected simultaneously, and then the instrument pushbrooms along the direction of satellite motion to collect spectra of the next line on the ground. The result is a two-dimensional “image cube” of data, with two spatial axes and one spectral axis.

Profilers or sounders exploit the properties of a spectral band characteristic of a particular species of gas (e.g., the 15 μm band of carbon dioxide). Typically operating in the thermal infrared, they are most often used to measure the vertical profile of atmospheric temperature, moisture, ozone, and trace gases. Sounding while looking down at the Earth exploits absorption bands in the spectrum, in which the gaseous species of interest absorbs the radiation that is upwelling from the Earth. The vertical profile of the atmospheric property or species of interest can be inferred by observing in spectral regions that are narrow compared to the absorption bandwidth. When the spectral region observed is in the center of the absorption band, the instrument can see only so far down into the atmosphere, because of the high absorption. As the wavelength being observed is increasingly shifted from the absorption band center out into the “wings” of the absorption band, absorption steadily decreases, allowing the instrument to see increasingly deeper into the atmosphere. By observing in relatively narrow portions of the spectrum, one can infer the properties of different layers of the atmosphere, because different amounts of absorption allow us to examine different depths in the atmosphere. The raw observations in the different narrow regions of the absorption band, when combined with the physics of spectroscopy and radiative transfer (see Radiative Transfer, Theory ), and a model of the structure of the atmosphere, can yield the desired vertical profile. This process is known as “inversion”.

Sounding while looking above the edge of the Earth at the atmosphere is known as “limb sounding” (see Limb Sounding, Atmospheric ). Limb sounding can be done by observing an atmospheric absorption band against the bright background of the solar disk (“solar occultation”). Limb sounding can also be done by observing the spectral band of a gaseous species against the black background of space, in which case the spectral band is seen in emission (i.e., a bright spectral feature against a dark background, as opposed to a dark spectral feature against a bright background) (Figure 5). Sounding may also be done with active E-O or passive microwave techniques (see below).

Figure 5

Conceptual example of (a) vertical sounding and limb (both (b) solar occultation and (c) emission) sounding.

Spectrometers exploit the spectral “fingerprints” of environmental species and surfaces, providing much higher spectral resolution than multispectral imagers. They use a grating, prism, or more complicated method [such as a Fourier transform spectrometer (FTS) or Fabry1Perot spectrometer (FPS)] to spread the incoming radiation into a spectrum that can be detected and digitized. Spectrometers are typically used for measuring trace species in the atmosphere or the composition of the land surface. Various types of vegetation, soil, and minerals have unique spectral signatures.

A special class of spectrometer is the gas correlation spectroradiometer (GCS), which is similar to the pressure-modulated radiometer (PMR). The GCS is a simple, compact, and robust method for spectral detection. It uses an optical cell filled with a sample of the gas to be detected, and it compares the spectral signature of that gas with the spectral signature being observed in the Earth’s environment. Very high discrimination is possible in the presence of interference from other species. Another special class of spectrometer is the acousto-optical tunable filter (AOTF). The AOTF is an electrically tunable band-pass filter. The observed radiation from the Earth is passed through a crystal. An oscillating radio-frequency signal is applied to piezoelectric transducers (PZTs) attached to the crystal. The PZTs vibrate and generate sound waves in the crystal that turn it into an optical diffraction grating by locally perturbing the refractive index of the material. This is a compact and effective technique for spectral discrimination.

The distinction between the various classes of instruments is often blurred. For example, a sounder might use band-pass filters to observe discrete spectral bands or it might employ a spectrometer to observe a spectrum from which the appropriate sounding frequencies can be extracted. Similarly, a hyperspectral imager will typically use a spectrometer for spectral discrimination (in which case, it is known as an imaging spectrometer).

Nonimaging radiometers are typically used to study the Earth’s energy balance. They measure radiation levels across the spectrum from the ultraviolet to the far infrared, with low spatial resolution. They can measure such quantities as the incoming solar irradiance at the top of the atmosphere and the outgoing thermal radiation caused by the sun’s heating of the planet (see Thermal Radiation Sensors (Emitted) ). These are two of the principal quantities that determine the net heating and cooling of the Earth. Such instruments typically fly in LEO, although they have application to GEO and L1 orbits.

A special technique for the study of the Earth’s energy balance makes use of earthshine from the Moon. Earthshine is most easily visible to the eye when the Moon is a crescent, and it can be seen that the “dark” portion of the moon is actually faintly illuminated. This illumination is due to sunlight reflected from the Earth, off the dark portion of the Moon, and back to the Earth. Measurement of earthshine provides a measurement of the Earth’s global albedo, a measure of the fraction of incident sunlight that is directly reflected back into space. Changes in the global albedo infer changes in the Earth’s energy balance, which relate to climate change. This measurement is currently made from Big Bear Solar Observatory in California. This is an example in which ground-based observations looking up at the Moon have an advantage over satellite-based observations looking down at the Earth and attempting to measure the same quantity. The ground-based lunar observation measures global albedo, while the satellite-based Earth-observing instruments measure the albedo over a fraction of the globe at any given time (Goode, 1998).

Polarimeters, which can be imaging or nonimaging devices, exploit the polarization signature of the environment (see Reflected Solar Radiation Sensors, Polarimetric ). As anyone who has experimented with polarized sunglasses knows, the sky on a clear day is polarized, particularly at a 90° angle from the sun. Sunlight reflected from metal or glass structures can also be polarized, as drivers with polarized sunglasses know. Electromagnetic radiation can be characterized as a wave, having both electric and magnetic fields, each of which can be expressed as a vector that is perpendicular to the direction of the motion of the wave (Figure 6). If the direction of the electric vector varies randomly with time, the radiation is said to be unpolarized. If the vector has a preferred orientation as a function of time, the radiation is polarized. Radiation from Earth-atmosphere system can be unpolarized, linearly polarized, circularly polarized, or elliptically polarized, depending on the physics of reflection and scattering. Polarization is fully characterized by the Stokes parameters (see Reflected Solar Radiation Sensors, Polarimetric ). The resulting information can be used to study phenomena such as cloud-droplet size distribution and optical thickness, aerosol properties, vegetation, and other land surface properties.

Figure 6

Diagram showing the electric (E) and magnetic (H) vector in an electromagnetic wave (top). For linear polarization (left), the direction of the E-vector is fixed. For circular polarization (center), the direction of the E-vector, when viewed along the direction of propagation of the electromagnetic wave, continually moves in a circle (right- or left-hand). For the general case of elliptical polarization (right), the E-vector moves in an elliptical manner.

Stereo imagers are a special case of E-O instruments and fly in LEO orbits. Stereo imagers that afford both fore and aft looks as the satellite passes over the Earth can provide “3-D” data useful for topographic mapping. A nadir (straight down) look is often added for accuracy in mountainous terrain (see Land Surface Topography ). Multi-angle imagers are also a special case, often having many more view angles than three. When implemented as a polarimeter, multi-angle instruments are particularly useful for measuring the properties of clouds and aerosols (see Reflected Solar Radiation Sensors, Multiangle Imaging ).

UV instruments are also something of a special case (see Ultraviolet Sensors ). Their observations are limited to the Earth’s upper atmosphere and near-Earth space environment (Huffman, 1992). UV instruments are used for ozone monitoring (see Stratospheric Ozone ) and for imaging of the Earth’s aurora, airglow, polar mesospheric clouds (“noctilucent” clouds), and ionosphere.

Calibration of passive electro-optical instruments is a very important issue, if their raw observations are to be turned into scientifically and operationally useful data (National Research Council, 2007). The four types of calibration are radiometric, spectral, geometric, and polarimetric. Radiometric calibration is important both in absolute terms (conversion of raw counts to absolute radiance or irradiance units, e.g., for temperature measurement) and in relative terms (for combining observations in different spectral bands). Spectral calibration, the determination of the exact positions in the spectrum at which the data were taken, must be performed for spectrometers and imaging spectrometers. Geometric calibration is performed to determine the position on the ground at which data is taken and to remove instrumentally induced distortions. Polarimetric calibration, the determination of the type and percent polarization that is observed, must be performed for polarimeters (see Calibration, Optical/Infrared Passive Sensors ).

Active electro-optical

A lidar sends a laser beam into Earth’s environment and measures what is returned (Figure 7) via reflection and scattering. This typically requires a large receiving telescope to capture the returning photons. The returning signal can be measured either by direct detection or by heterodyne detection (NASA ESTO, 2006). With direct detection, the receiving telescope acts as a simple light bucket to collect photons, which means that the phase information in the electromagnetic wave is lost. With heterodyne (“coherent”) detection, the returning photons are combined with the signal from a laser onboard the satellite (a “local oscillator”), whose frequency is close to that of the lidar’s frequency. The act of combining these two signals generates an “intermediate frequency” that is much lower in frequency than either the lidar or the local oscillator (like a heterodyne circuit in a radio), making it easier to detect while maintaining the frequency and phase information.

Figure 7

Example of a lidar pulse that is emitted from the instrument and is backscattered from the atmosphere, clouds, and the Earth’s surface.

Only a handful of lidars have flown in space, owing to limitations involving high power, high cost, and the availability of robust laser sources. With a few notable exceptions, lidar remote sensing is typically carried out with an aircraft.

Lidars can potentially generate high-resolution vertical profiles of atmospheric temperature and moisture because the returns can be sliced up or “range gated” in time (and thus space) if they are strong enough. Lidar also has potential for profiling winds, determining cloud physics, measuring trace-species concentration, etc.

Backscatter lidar is the simplest in concept: A laser beam scatters off of aerosols, clouds, dust, and plumes in the atmosphere. The data can be used to generate vertical profiles of these phenomena, except where the beam is absorbed by clouds. A related device is the laser altimeter, which records the backscatter from the Earth’s surface to measure features such as ice topography and the vegetative canopy (e.g., the tops of trees for biomass studies).

Differential absorption lidar (DIAL) transmits at two wavelengths, one near the center of a spectral absorption band of interest, the other just outside of it. The difference in the returned signal can be used to derive species concentration, temperature, moisture, or other phenomena, depending on the spectral band selected (akin to passive E-O sounding). The differential technique requires no absolute calibration, so it is relatively easy to achieve high accuracy (e.g., parts-per-million to parts-per-billion for species concentration).

Doppler lidar measures the Doppler shift of aerosols or molecules that are carried along with the wind. Thus, wind speed and direction can be determined if two separate views of each atmospheric parcel are acquired to measure velocity in the horizontal plane. In concept, this can be done with a conically scanning lidar and a large receiving telescope. The available aerosol backscatter is too low to measure the complete wind profile as desired (from the surface to 20 km in altitude), but molecular scattering can be used to cover the aerosol-sparse regions. Strong competition has existed between two schools of thought that propose using direct or heterodyne detection. Although wind lidar has been studied since 1978, according to current plans, approximately 30 years would have passed before the first Doppler lidar is launched into space (Kramer, 2002). Of all of the environmental parameters that can only be crudely measured today, improved wind measurements would have the greatest impact on the accuracy of numerical weather prediction (see Tropospheric Winds ).

Fluorescence lidar transmits a spectral frequency that is absorbed by the species of interest and then reradiated at a different frequency, which is then detected on orbit by a radiometer. A related technology, Raman lidar, exploits the Raman scattering from molecules in the air, a process in which energy is typically lost and the scattered light is reduced in frequency.

Calibration of lidars is a very important issue, if their raw observations are to be turned into scientifically and operationally useful data.

Passive microwave

Passive microwave imaging radiometers (usually called microwave imagers; see Microwave Radiometers, Conventional ) collect the Earth’s natural radiation (see Microwave Radiometers ) with an antenna and typically focus it onto one or more feed horns that are sensitive to particular frequencies and polarizations. From there, it is detected as an electrical signal, amplified, digitized, and recorded for the various frequencies and polarizations (linear or circular; see Microwave Radiometers, Polarimeters ). The antenna usually rotates such that it scans the Earth in a conical fashion (Figure 8), and these instruments have all flown in LEO. The amount of radiation measured at different frequencies and polarizations can be analyzed to produce environmental parameters such as soil moisture content, precipitation, sea-surface wind speed, sea-surface temperature, ocean salinity, snow cover and water content, sea ice cover, atmospheric water content, and cloud water content (NASA ESTO, 2004). Unlike visible imagers, microwave imagers can operate day or night through most types of weather, including most clouds, affording views of portions of the globe that are usually cloud covered, including the polar ice caps and many tropical countries (Janssen, 1993). Microwave instruments are ideal for imaging sea ice during the polar winter when the polar region is not illuminated by the sun and they can see through the persistent cloud cover that often plagues these regions (see Ulaby et al., 1981).

Figure 8

A conventional conically scanning microwave imager, which maps out a data swath as a result of the motion of the satellite.

A special class of microwave radiometer is a passive polarimetric radiometer that measures not only sea-surface wind speed but also sea-surface wind direction. By measuring various combinations of linear and circular polarization from the sea surface at different frequencies (e.g., 10, 18, and 36 GHz), a signal can be derived that varies with the conically scanning look angle as a function of wind direction. This provides a potential alternative to scatterometers (see section “Active Microwave” below).

Microwave profilers or sounders, like electro-optical sounders, operate in several frequencies around a spectral band characteristic of a target gas and may be either vertical sounders or limb sounders. They are often used to measure the vertical profiles of temperature and moisture in the atmosphere. The oxygen band near the frequency 60 GHz, which becomes more or less opaque as a function of atmospheric temperature, is usually used for temperature sounding, while the water vapor band at 183 GHz is typically used for moisture sounding. The advantage of microwave over electro-optical sounding is that it can be done through most types of weather and cloud cover.

Passive microwave imagers and sounders generally operate at frequencies ranging from 6 to 183 GHz. Higher frequencies have recently been used in submillimeter-wave radiometers for measuring cloud ice content. Lower frequencies, around 1 GHz, can be used to measure soil moisture and ocean salinity; however, such low frequencies are not always practical. For a given antenna size, spatial resolution decreases as the frequency decreases. Most microwave imagers are limited to a lower frequency of about 6 GHz because a large antenna would be required at 1 GHz to achieve acceptable resolution.

This difficulty can be overcome through a technique known as aperture synthesis (see Microwave Radiometers, Interferometers ). In this concept, which has long been used in radio astronomy, the operation of a large solid dish antenna is simulated by using only a sparsely populated aperture or “thinned array” antenna. In such an antenna, only part of the aperture physically exists and the remainder is synthesized by correlating the individual antenna elements (see Microwave Radiometers, Correlation ). This technique has been proven in aircraft flight demonstrations, but has not yet been flown in space. This type of instrument is often referred to as a STAR (Synthetic Thinned Array Radiometer – Figure 9).

Figure 9

Conceptual example of a Synthetic Thinned Array Radiometer (STAR) passive microwave interferometer, in which the full aperture is synthesized by correlating the signals from a small number of stick antennas.

A significant problem in the field of microwave radiometry is the existence of man-made sources of radio-frequency interference (RFI) on the Earth, especially in advanced and highly populated regions. Research on various techniques for addressing this problem is ongoing.

Calibration of microwave radiometers is a very important issue, if their raw observations are to be turned into scientifically and operationally useful data (Skou, 1989). Radiometric calibration is important both in absolute terms for conversion of observed radiation to temperature (“brightness temperature”) of the Earth’s environment and in relative terms (one frequency relative to another). It is usually done in spaceborne instruments using a regulated “warm load” and a view to cold deep space, each of which is viewed periodically as the antenna and feedhorn assembly rotates past the warm load and the space view (see Calibration, Microwave Radiometers ).

Active microwave

Active microwave instruments can be broadly divided into real-aperture and synthetic-aperture radars and operate from LEO. They all transmit microwaves toward Earth and measure what is reflected and scattered back. “Real aperture” typically means that the instrument has a solid, dish-shaped antenna that defines its performance properties, such as spatial resolution on the ground. “Synthetic aperture” usually means that the performance of a hypothetically very large real aperture is achieved by letting the satellite’s motion along its orbit sweep the field of view of the radar along with it for a period of time while data is collected, such that the length of the effective aperture equals the amount of space swept out by the satellite during that period of time (Ulaby et al., 1982; Cantafio, 1989; NASA ESTO, 2004).

Real-aperture radars can be further categorized as atmospheric radars, altimeters, and scatterometers. Atmospheric radars are useful for studying precipitation and the three-dimensional structure of clouds. The use of more than one frequency is beneficial for separating the effects of cloud and rain attenuation from those of backscatter. The first spaceborne atmospheric radar to fly was the precipitation radar on the US/Japan Tropical Rainfall Measuring Mission. The second was on NASA’s CloudSat mission, which performed the first 3-D profiling of clouds. This mission is especially important because clouds and aerosols are the primary unknowns in the global climate-change equation.

Altimeters measure surface topography, and radar altimeters are typically used to measure the surface topography of the ocean (which is not as uniform as one might think). They operate using time-of-flight measurements and typically use two or more frequencies to compensate for ionospheric and atmospheric delays (see Radar, Altimeters ). Altimeters have been flying since the days of Skylab in 1973. Aperture synthesis and interferometric techniques can also be employed in altimeters, depending on the application.

Scatterometers are a form of instrument that uses radar backscattering from the Earth’s surface (see Radar, Scatterometers ). The most prevalent application is for the measurement of sea-surface wind speed and direction. This type of instrument first flew on Seasat in 1978. A special class of scatterometer called delta-k radar can measure ocean surface currents and the ocean wave spectrum using two or more closely spaced frequencies.

Synthetic-aperture radars (SARs) also flew for the first time on Seasat. These radars sometimes transmit in one polarization (horizontal or vertical) and receive in one or the other (Campbell, 2007). A fully polarimetric synthetic-aperture radar employs all four possible send/receive combinations (see Radar, Synthetic Aperture ). Synthetic-aperture radars are powerful and flexible instruments that have a wide range of applications, such as monitoring sea ice, oil spills, soil moisture, snow, vegetation, and forest cover.

Some active microwave instruments (predominantly SARs) are interferometric (InSAR), meaning that they exploit the signals that are seen from two somewhat different locations but are sufficiently close in time. This is a powerful means of elevation/topographic measurement. Interferometry can be done by using two antennas separated by a rigid boom, or by using a single antenna on a moving spacecraft that acquires data at two different times or by using similar antennas on two separate spacecraft. InSAR can be used to monitor surface motion due to earthquakes and volcanoes and to create Digital Elevation Models that represent the three-dimensional topography of the Earth’s surface, information that has a wealth of applications.

GPS occultation instruments exploit a technique that was first used for remote sensing of planetary atmospheres. They observe the GPS satellites as they rise and set over the Earth’s limb (NASA ESTO, 2004). They detect the signals emitted from the GPS satellites and track the phase of those signals as a function of altitude above the limb (see GPS, Occultation Systems ). Those phase changes can be converted to atmospheric density changes, which in turn can be used to compute the temperature profile in the lower atmosphere, as well as the electron density profile in the ionosphere (the moisture profile in the lower atmosphere can also be derived, but with more difficulty). Temperature profiles derived using this technique are more accurate and structured than those using weather balloons. The other type of GPS-based instrument, the GPS reflectometer, measures the difference between direct and ocean-reflected GPS signals, to measure ocean surface height (topography). This is a newer technique that has flown on three satellites through 2008.

Calibration of active microwave instruments is a very important issue, if their raw observations are to be turned into scientifically and operationally useful data (see Calibration, Scatterometers and Calibration, Synthetic Aperture Radars ).

Conclusion

Remote sensing is a field that involves a wide variety of instruments that exploit the physics of the Earth’s environment (atmosphere, oceans, land, solid Earth, ice cover, and near-Earth space environment) to measure a large number of environmental phenomena. The field has blossomed tremendously since the launches of Landsat-1 in 1972, Skylab in 1973, and Seasat and Nimbus-7 in 1978. New and more sophisticated techniques are being applied, resulting in the ability to measure a wider range of environmental phenomena and an increase in the accuracy with which those measurements are made. Technologies for access to and manipulation of the data have increased dramatically. The field has benefited from international proliferation, as the number of countries owning remote sensing satellites increased by a factor of 6 from 1980 to 2008. As the field evolves, there will be a continued need for trained scientists and engineers with a broad and interdisciplinary perspective to work in the remote sensing field.

Cross-references

Calibration, Microwave Radiometers

Calibration, Optical/Infrared Passive Sensors

Calibration, Scatterometers

Calibration, Synthetic Aperture Radars

Cryosphere, Measurements and Applications

Gamma and X-Radiation

Geophysical Retrieval, Inverse Problems in Remote Sensing

GPS, Occultation Systems

Land Surface Topography

Lidar Systems

Limb Sounding, Atmospheric

Microwave Horn Antennas

Microwave Radiometers, Conventional

Microwave Radiometers, Correlation

Microwave Radiometers

Observational Platforms, Aircraft, and UAVs

Observational Systems, Satellite

Ocean, Measurements and Applications

Optical/Infrared, Atmospheric Absorption/Transmission, and Media Spectral Properties

Optical/Infrared, Radiative Transfer

Optical/Infrared, Scattering by Aerosols and Hydrometeors

Radar, Altimeters

Radar, Scatterometers

Radar, Synthetic Aperture

Radiation, Electromagnetic

Radiative Transfer, Theory

Radio-Frequency Interference (RFI) in Passive Microwave Sensing

Reflected Solar Radiation Sensors, Multiangle Imaging

Reflected Solar Radiation Sensors, Polarimetric

Thermal Radiation Sensors (Emitted)

Tropospheric Winds

Ultraviolet Sensors

Water Resources