Earth Remote Sensing: An Overview

DOD, NOAA, and NASA have merged their separate polar-orbiting environmental satellite programs into a single program called NPOESS. Aerospace provides support in requirements development, system and payload specification and evaluation, systems engineering, mission operations planning, and acquisition and contract oversight for this interagency program. (NOAA)

Earth Remote Sensing: An Overview

David L. Glackin

Spaceborne remote-sensing instruments are used for applications ranging from global climate monitoring to combat-theater weather tracking to agricultural and forestry assessment. Aerospace has pioneered numerous remote-sensing technologies and continues to advance the field.

Although the first weather satellite, TIROS I, was launched in 1960, the field of satellite-based remote sensing of Earth really began to take form in the 1970s. The launches of Landsat-1 in 1972, Skylab in 1973, Nimbus-7 in 1978, and Seasat in 1978 set the stage for modern environmental remote sensing.

During these years, the Defense Meteorological Satellite Program (DMSP) provided many scientists and engineers at Aerospace the opportunity to investigate new phenomenology and instrumentation. For example, the first sensor to remotely monitor the density of the upper atmosphere above 80 kilometers was conceived and built at Aerospace. The first reported analysis of spaceborne imagery of the aurora was done at Aerospace using DMSP low-light visible imagery. When a snow/cloud discrimination sensor flew on DMSP in 1979, Aerospace demonstrated that the combination of visible and shortwave infrared imagery could be used not only to discriminate snow from clouds, but water clouds from ice clouds as well. Aerospace analyzed defense satellite data on the eruption of Mt. St. Helens in 1980 and tracked the volcanic plume using stereo observations from two satellites. And in the days before DMSP, Aerospace built the second ozone profiler ever to fly in space, which flew in 1962.

Today, Aerospace work in remote sensing supports not only the Department of Defense (DOD), but NASA, NOAA, and other governmental agencies as well. In the coming years, as these organizations seek to coordinate their remote-sensing efforts, Aerospace research and analysis will play an important role in determining what type of systems are developed and deployed.

Remote Sensing in Perspective

Since the pioneering work of the 1970s, the field of satellite environmental remote sensing has steadily evolved. Before 1990, only about half a dozen nations owned environmental satellites, but since then, the number has nearly quintupled. Earlier programs primarily involved civil and military systems of high cost and complexity; more recently, the focus has shifted to include missions involving smaller satellites, greater commercial involvement, and lower complexity and cost.


	A typical electro-optical sensor design. Aerospace creates end-to-end simulations to assist in sensor design, planning, and performance analysis.

The civil, commercial, and military communities all pursue environmental remote sensing activities, but these communities have different needs and objectives. Civil institutions tend to focus on problems such as monitoring and predicting global climate change, weather patterns, natural disasters, land and ocean resource usage, ozone depletion, and pollution. Commercial organizations typically invest in systems with higher spatial resolution whose imagery can support applications such as mapping, precision agriculture, urban planning, communications-equipment siting, roadway route selection, disaster assessment and emergency response, pipeline and power-line monitoring, real-estate visualization, and even virtual tourism. Military users typically concentrate on weather monitoring and prediction as it directly supports military operations. The military is also interested in high-resolution imagery, and has in fact become the primary customer for commercial imagery at resolutions of 1 meter or better.

Types of Instruments

Remote-sensing instruments fall into the general classes of passive and active electro-optical and microwave sensors. Passive devices collect and detect natural radiation, while active instruments emit radiation and measure the returning signals. Electro-optical devices operate in the ultraviolet, visible, and infrared spectral regions, while microwave (and submillimeter- and millimeter-wave) devices operate below the far infrared (see sidebar, The Remote-Sensing Spectrum).

Passive electro-optical instruments include multispectral imagers, hyperspectral imagers, atmospheric profilers or sounders, spectrometers, radiometers, polarimeters, CCD 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, synthetic-aperture radiometers, and submillimeter-wave radiometers. Active microwave instruments include radars, synthetic-aperture radars (SARs), altimeters, and scatterometers.

Passive Electro-optical

Passive electro-optical multispectral imagers observe Earth's natural thermal radiation or solar radiation that has been reflected and scattered back toward space. The scanning optics can move in a cross-track "whiskbroom" fashion, or the motion of the spacecraft can simply carry the field of view along track in a "pushbroom" fashion. The radiation captured by the primary mirror is transferred through a set of optics and bandpass filters to one or more focal plane arrays, where it is converted to electrical signals by a number of detectors. 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.

If the multispectral imagers are calibrated to quantitatively measure the incoming radiation (as indeed most are), they are termed imaging radiometers. Such instruments typically detect radiation in a few (less than 20) spectral bands. Multiple wavelengths are almost always required to retrieve the desired environmental phenomena. A single "panchromatic" wavelength can be used purely for imaging at higher spatial resolution across a broader spectral band. Multispectral imagers are used to study clouds, aerosols, volcanic plumes, sea-surface temperature, ocean color, vegetation, land cover, snow, ice, fires, and many other phenomena.

In contrast to multispectral imagers, hyperspectral imagers typically cover 100 to 200 spectral bands, producing simultaneous imagery in all of them. Moreover, these narrow bands are usually contiguous, typically extending from the visible through shortwave-infrared regions. This makes it easier to discriminate surface types by exploiting fine details in their spectral characteristics. Hyperspectral imagery is used for mineral and soil-type mapping, precision agriculture, forestry, and other applications. A few hyperspectral imagers operate in the thermal (mid- to long-wave) infrared, notably the Aerospace SEBASS (Spatially Enhanced Broadband Array Spectrograph System), an airborne instrument.

Profilers or sounders monitor several frequencies across a spectral band characteristic of a particular gas (e.g., the 15-micron band characteristic of carbon dioxide). Typically operating in the thermal infrared, they are most often used to measure the vertical profile (a mapping based on altitude) of atmospheric temperature, moisture, ozone, and trace gases.

Spectrometers exploit the spectral "fingerprints" of environmental species, providing much higher spectral resolution than multispectral imagers. They use a grating, prism, or more sophisticated method (such as Fourier transform spectrometry) to spread the incoming radiation into a continuous 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.

The distinction between the various classes of instruments is often blurred. For example, a sounder might use bandpass filters to observe discrete spectral bands, or it might employ a spectrometer to observe a continuous 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).

Non-imaging radiometers are typically used to study 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. These are two of the principal quantities that determine the net heating and cooling of Earth.

Polarimeters, which can be imaging or nonimaging devices, exploit the polarization signature of the environment. The electromagnetic vector that characterizes the radiation from Earth can be linearly (or elliptically) polarized, depending on the physics of reflection and scattering. 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.

Active Electro-Optical

A lidar sends a laser beam into Earth's environment and measures what is returned 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 (coherent) detection. With direct detection, the receiving telescope acts as a simple light bucket, which means that phase information is normally lost. With heterodyne detection, the returning photons are combined with the signal from a local oscillator laser, which generates an intermediate (lower) frequency that is easier to detect while maintaining the frequency and phase information.

Few lidars have ever flown in space, owing to limitations involving high power, high cost, and the availability of robust laser sources. Lidar remote sensing is primarily limited to aircraft (although the shuttle-based Lidar In-space Technology Experiment, or LITE, was quite successful).

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 Earth's surface to measure features such as ice topography and the vegetative canopy (e.g., the tops of trees for biomass studies).

This portable ground-based lidar system uses Rayleigh and Raman scattering of light to generate vertical profiles of atmospheric temperature and water vapor. It is used to verify calibration of the sounding channels on environmental satellites in orbit.

Differential absorption lidar (DIAL) transmits at two wavelengths, one near the center of a spectral absorption line of interest, the other just outside it. The difference in the returned signal can be used to derive species concentration, temperature, moisture, or other phenomena, depending on the spectral line selected. The differential technique requires no absolute calibration, so it's 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 kilometers in altitude), but molecular scattering can be used to cover the aerosol-sparse regions. Strong competition exists in the United States between two schools of thought that propose using direct or heterodyne detection. Although wind lidar has been studied in the United States since 1978, it appears that the first Doppler lidar in space will be launched by the European Space Agency in 2007.

Fluorescence lidar is tuned to a spectral frequency that is absorbed by the species of interest, then reradiated at a different frequency, which is detected 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. The potential for this type of lidar to fly in space is remote. It is being used by Aerospace in a portable ground-based lidar for ground verification of atmospheric profiles from microwave instruments on DMSP.

Passive Microwave

Passive microwave imaging radiometers (usually called microwave imagers) collect Earth's natural radiation 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). 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, snow cover and water content, sea ice cover, atmospheric water content, and cloud water content. Unlike visible imagers, microwave imagers can operate day or night through most types of weather. The natural microwave radiation from the environment is not dependent on the sun, and microwave radiation over broad ranges of frequencies is quite insensitive to water in the atmosphere.

Model of the Conical-scanning Microwave Imager/Sounder (center) with a model of the DMSP Special Sensor Microwave/Imager (right) and a microwave imager for the Tropical Rainfall Measurement Mission (left). CMIS, a multiband radiometer that will be deployed on NPOESS, integrates many features of heritage conical-scanning radiometers into a single radiometer. It will offer several new operational products (sea surface wind direction, soil moisture, and cloud base height) and quantifiable resolution and measurement range improvements over existing remotely sensed environmental products. Boeing Space Systems

Microwave profilers or sounders, like electro-optical sounders, operate in several frequencies around a spectral band characteristic of a target gas. They are often used to measure the vertical profiles of temperature and moisture in the atmosphere. The oxygen band near 60 gigahertz, 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 gigahertz is typically used for moisture sounding. The advantage of microwave over electro-optical sounding is that it can be done through most forms of weather and cloud cover.

Passive microwave imagers and sounders generally operate at frequencies ranging from 6 to 183 gigahertz. Higher frequencies have recently been used in so-called submillimeter-wave radiometers for measuring cloud ice content. Lower frequencies, around 1 gigahertz, 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 gigahertz because a large antenna would be required at 1 gigahertz to achieve acceptable resolution. This difficulty can be overcome through a technique known as aperture synthesis. 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 sparse 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. This technique has been proven in aircraft flight demonstrations.

Active Microwave

Active microwave instruments can be broadly divided into real-aperture and synthetic-aperture radars. They all transmit microwaves toward Earth and measure what is reflected and scattered back. Some are interferometric, meaning that they exploit the signals that are seen from two somewhat different locations, which is a powerful means of elevation measurement. This can be done using two antennas separated by a rigid boom, or using a single antenna on a moving spacecraft that acquires data at two slightly different times, or using similar antennas on two separate spacecraft.

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. Only one atmospheric radar is now flying in space (for measuring tropical rainfall), but others slated for launch include NASA's CloudSat mission, which will perform the first 3-D profiling of clouds. This mission is 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. 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 Earth's surface. 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 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. A fully polarimetric synthetic-aperture radar employs all four possible send/receive combinations. 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 of the sensors on NPOESS include: VIIRS (Visible/Infrared Imager/Radiometer Suite), which collects radiometric data of Earth's atmosphere, ocean, and land surfaces; CMIS (Conical-scanning Microwave Imager/Sounder), which collects global microwave radiometry and sounding data; CrIS (Crosstrack Infrared Sounder), which measures Earth's radiation to determine the vertical distribution of temperature, moisture, and pressure in the atmosphere; OMPS (Ozone Mapping and Profiler Suite), which collects data for calculating the distribution of ozone in the atmosphere; ATMS (Advanced Technology Microwave Sounder), which provides observations of temperature and moisture profiles at high temporal resolution; and ERBS (Earth Radiation Budget Sensor). NOAA

Aerospace Support

Traditionally, environmental remote sensing activities at Aerospace supported military programs such as DMSP. In the early 1990s, that began to change. Aerospace support to NOAA (the National Oceanic and Atmospheric Administration) grew to include the GOES-NEXT series of geosynchronous weather satellites, AWIPS (the Advanced Weather Interactive Processing System), and risk assessment of a proposed spaceborne global wind-sensing system. At the same time, Aerospace conducted a series of independent reviews for NASA programs, including the Shuttle Imaging Radar, the Total Ozone Mapping Spectrometer, and the NASA Scatterometer, designed for ocean wind measurement. In 1992, Aerospace developed the concept for a DMSP digital data archive that was implemented by NOAA and the National Geophysical Data Center in 1994.

The DOD's next-generation DMSP and NOAA's next-generation POES (Polar-orbiting Operational Environmental Satellite) programs were officially merged by presidential directive in 1994, creating a new triagency NOAA/DOD/NASA program called NPOESS (National Polar-orbiting Operational Environmental Satellite System). NASA's initial role was to provide technology transfer. Aerospace began work on NPOESS with a small team in 1992 when the transition was first announced, studying issues such as whether the needs of both NOAA and DOD could be addressed by shared instruments. This program provides a good example for comparing and contrasting the needs of the civil and military communities in terms of requirements, instrument design, research, and operations. Designing a single visible/infrared imager that satisfies the civil community's need for accurate radiometric calibration and long-term stability and the military's need for high-quality imagery—including nighttime visible imagery—has been a particular challenge that Aerospace has helped to address during the last decade. During that period, NPOESS has also become the follow-on to the Earth Observing System climate mission. NPOESS will be an important Aerospace program for many years to come, requiring support for everything from basic physics to ground systems.

As of 2004, Aerospace support in environmental remote sensing extends to the Earth Science and Technology Directorate of Caltech's Jet Propulsion Laboratory, NASA's Earth Science Technology Office, NASA's Goddard Space Flight Center and the U. S. Geological Survey on Landsat, NOAA's Office of Systems Development on the future of the geosynchronous weather satellite program, and NASA Goddard on the NPOESS Preparatory Project, which is a bridge between the Earth Observing System and NPOESS. Aerospace members serve on the federal Interagency Working Group on Earth Observation and the international Group on Earth Observation, assisting these bodies in their attempt to coordinate future remote-sensing satellites and data. Aerospace further supports the remote-sensing space policy community by keeping tabs on the remote-sensing plans of every nation.

Recent Developments at Aerospace

The prototype BASS (Broadband Array Spectrograph System) instrument being used to study cirrus clouds simultaneously with NOAA radars in background, as part of NOAA's Climate and Global Change Program.

In areas where the company has unique expertise, Aerospace constructs proof-of-concept instruments and collects data in field tests. For example, BASS (the Broadband Array Spectrograph System) is a patented infrared spectrometer for ground-based and airborne remote sensing. Under the aegis of NOAA's Climate and Global Change Program, BASS has been used to support efforts to combine infrared and radar reflectivity studies of cirrus clouds to understand their physical properties better. Clouds and aerosols are the primary sources of uncertainty in global climate-change models, so improved understanding of their physical properties will advance scientific understanding of global change.

Aerospace has advanced the field of hyperspectral remote sensing through an evolution of BASS called SEBASS (Spatially Enhanced BASS). As mentioned earlier, hyperspectral instruments typically span the visible through shortwave infrared. SEBASS, on the other hand, operates in the thermal infrared. Although a few other hyperspectral instruments also cover this range, SEBASS does so with greater sensitivity. Aerospace has developed several other instruments that stem from the original BASS design.

Aerospace is also working on a new remote-sensing technique known as SAIL (synthetic-aperture imaging ladar). This technique, still in its infancy, uses aperture synthesis to achieve unprecedented spatial resolution with a ladar (or laser radar). The groundbreaking work by Aerospace on the SAIL technique has the potential to one day afford extremely high resolution imaging of objects with ladar.

Conclusion

For more than 40 years, Aerospace has pioneered the design and development of systems for remote sensing of Earth. Aerospace researchers have worked on every major type of instrument, as well as user requirements, system architecture, modeling and simulation, image compression, image processing, and algorithms for understanding the data, in support of programs managed by DOD, NASA, JPL, NOAA, and others. Familiarity with these user communities puts Aerospace in a unique position to help coordinate their efforts and ensure that sensing systems keep pace with the customers' changing needs and goals.