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A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum (2015)

Chapter: 2 Active Earth Remote Sensing for Atmospheric Applications

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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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2

Active Earth Remote Sensing for Atmospheric Applications

INTRODUCTION

This chapter covers active sensing of the atmosphere using both ground-based and satellite-borne radars. Owing to the vast expanse of weather, hydrometeors, and atmospheric gases, in situ measurements are not sufficient to obtain all of the critical data necessary for forecasting, warning, and monitoring of the weather and atmosphere worldwide, especially at high altitudes. In order to collect the necessary data, active remote sensing of the atmosphere is conducted from both ground- and satellite-based radar systems.

Our climate is changing faster than ever, breaking the normal cycles established since before the beginning of human civilization.1 Recent extreme weather events in the United States have received significant attention on both traditional and social media sites (Figures 2.1 and 2.2). It has therefore become more imperative than ever to monitor our atmosphere with all of the remote sensing tools available in order to best mitigate and adapt to changes in our climate, as well as to develop enhanced models that can better predict future weather patterns. Active ground-based and satellite sensors are a critical part of this monitoring. Environmental

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1 C. Field and N. Diffenbaugh, Changes in ecologically critical terrestrial climate conditions, Science 341(6145):486-492, 2013; “Stanford climate scientists warn that the likely rate of change over the next century will be at least 10 times quicker than any climate shift in the past 65 million years” (B. Carey, “Climate change on pace to occur 10 times faster than any change recorded in past 65 million years, Stanford scientists say,” Stanford Report, August 1, 2013, http://news.stanford.edu/news/2013/august/climate-change-speed-080113.html).

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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FIGURE 2.1 Record temperatures and extreme weather such as the polar vortex event of winter of 2014. SOURCE: Courtesy of Mike Nelson, 7News KMGH-TV.

measurements available from remote sensing will help us to better quantify the environment and its changes and understand the interactions that affect our climate, including human activities.

The measurement of atmospheric variables is needed because they play an important role in heat transport, cloud genesis, winter storms formation, hurricane and cyclone formation, and other atmospheric events. Some of the fundamental atmospheric variables that can be retrieved and monitored by active sensors include vapor and liquid water content, wind vectors, cloud cover, rainfall rate, precipitation type, and ice cloud content.

To develop and enhance climate/weather models, it is necessary to have a global atmospheric database of geophysical parameters, which can then be used to evaluate risk management options and develop preparedness plans, thereby reducing the risks of disasters. By incorporating knowledge about projected climate changes, we

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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FIGURE 2.2 The 2014 polar vortex induced extreme events, an example of which is displayed in this photo of the Schuylkill River in Philadelphia. SOURCE: Shuvaev, “Ice Formations on the Schuylkill River in Philadelphia,” January 7, 2014, Wikipedia file: Frozen Schuylkill River, Philadelphia 2014.JPG, Creative Commons Attribution-ShareAlike 3.0 Unported license.

can enhance our mitigation/adaptation response and help make informed decisions to reduce vulnerability to extreme events. The database also supports the development of improved weather prediction models.

The sensors discussed in this chapter are an important part of the nation’s (and the world’s) infrastructure for monitoring the Earth system. In order to observe the intended phenomena, however, these sensors require access to various windows of the electromagnetic spectrum. Without this access they become unable to support their many societal and scientific applications. Recent efforts to minimize spectral usage while remaining bound by the physical constraints of electromagnetism are discussed, following an introduction to the layers of the atmosphere and the instruments used to observe them.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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FIGURE 2.3 Climate models incorporate air/ocean/land interactions and processes. SOURCE: Koshland Science Museum of the National Academy of Sciences, “How Do Climate Models Work?” Earth Lab Online Exhibit, https://www.koshland-science-museum.org/, Copyright © 2011 National Academy of Sciences. All rights reserved.

LAYERS OF THE ATMOSPHERE AND SCIENTIFIC APPLICATIONS

From a scientific perspective, the atmosphere is segmented into layers (troposphere, stratosphere, mesosphere, and thermosphere) based on temperature characteristics as a function of height, with temperature defined as the average random kinetic energy of the molecules in the particular atmospheric stratum. Based on expected values across the globe, the temperature profile of the so-called “standard atmosphere” is shown in Figure 2.4. Distinct temperature gradients exist in each layer, which can have a significant effect on the stability of the atmosphere and therefore the type of phenomena observed.

Troposphere

The troposphere is the layer closest to Earth’s surface where most species of plants and animals live. Owing to the abundance of moisture from evaporation of rivers, lakes, and oceans, and the general instability caused by the decrease in temperature as a function of height, the troposphere contains virtually all of what

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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FIGURE 2.4 Temperature profile of the standard atmosphere as a function of height. Layers of the atmosphere are denoted on the figure along with transition layers. SOURCE: NOAA, National Weather Service, “Standardized Temperature Profile” [image], page last modified March 5, 2013, http://www.srh.noaa.gov/jetstream/atmos/atmprofile.htm.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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is generally called “weather” (not to be confused with “space weather,” which occurs in the upper atmosphere). From severe storms and tornadoes throughout North America to hurricanes in the Atlantic Ocean, the troposphere is where these violent phenomena occur. In addition, droughts, floods, heat waves, and winter storms are manifest in the troposphere and can also have a significant impact on the environment.2 For these reasons, natural activity in the troposphere has arguably the most significant impact on life and property.3

In the troposphere, the majority of scientific applications focus on observations and prediction of weather events. Surface-based observations include meteorological sensor stations (wind, temperature, moisture, etc.) and advanced Doppler weather radars, which can characterize precipitation over hundreds of kilometers. These data are often fed into numerical weather prediction (NWP) models, which are used to forecast severe weather, such as tornadoes and hail, hours in advance. Of course, the accuracy of forecasts is constantly improving through intensive research; however, this improvement is becoming increasingly dependent on data assimilation techniques that rely on accurate products from Doppler radar among other data sources.4 Weather radar products include rainfall rate, hydrometeor classification, and tornado classification, for example. It should also be emphasized that many of these radar products are used for value-added commercial markets and to broadcast to satisfy the public.

Stratosphere

The layer just above the troposphere is the stratosphere, where temperature begins to increase with height due to absorption of solar radiation by atmospheric ozone. This increase in temperature causes general stability in this layer, resulting in layered (“stratiform”) clouds rather than the strong convective clouds seen in the troposphere. Due to this stability and the existence of the jet stream, long-range airline traffic generally takes advantage of flying in the lower stratosphere to increase fuel economy.

The stratosphere can also exhibit extreme wind shear, which causes turbulent

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2 S.A. Changnon, R.A. Pielke Jr., D. Changnon, R.T. Sylves, and R. Pulwarty, Human factors explain the increase losses from weather and climate extremes, Bulletin of the American Meteorological Society 81(3):437-442, 2000.

3 K.M. Simmons and D. Sutter, The Economic and Societal Impacts of Tornadoes, American Meteorological Society, Boston, Mass., 2011.

4 D.J. Stensrud, M. Xue, L.J. Wicker, K.E. Kelleher, M.P. Foster, J.T. Schaefer, R.S. Schneider, S.G. Benjamin, S.S. Weygandt, J.T. Ferree, and J.P. Tuell, Convective-scale warn-on-forecast system: A vision for 2020, Bulletin of the American Meteorological Society 90(10):1487-1499, 2009.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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instabilities.5 These phenomena have a major impact on the momentum budget and must be taken into account when modeling global atmospheric fields. General circulation models (GCMs) depend on measurements of stratospheric parameters, but these are very difficult to obtain using ground-based sensors (compared with measuring those in the troposphere) due to lower moisture level and cloud density. Radar remote sensing techniques are largely based on scattering from turbulent eddies in this region, and can be used to estimate the three-dimensional wind field throughout the stratosphere. This scattering mechanism, along with specific radar systems used to observe the stratosphere, are discussed in subsequent sections.

Mesosphere

Within the mesosphere, temperature generally decreases as a function of height—similar to the troposphere. The upper part of the mesosphere is the coldest region of the atmosphere, with temperatures reaching as low as minus 100°C. Although moisture levels are extremely low, these low temperatures can cause deposition of water vapor and the formation of ice clouds. Since these clouds are visible only at night, when the Sun is illuminating the clouds but not Earth’s surface, they are called “noctilucent clouds.” In addition to being the highest known clouds, ionization of this region can cause associated radar echoes that are unexpectedly intense. These echoes were discovered in the 1990s and were appropriately termed Polar Mesospheric Summer Echoes (PMSE) since seasonal circulation patterns cause the lowest temperatures at these altitudes to occur in the northern hemisphere summer.6 It is also important to note that climate change may be part of the cause of the increasing observations of noctilucent clouds after the Industrial Revolution due to a possible increase in moisture and a simultaneous decrease in temperature at these high altitudes; this is in contrast to warmer temperatures at lower altitudes.

The mesosphere is extremely difficult to observe since standard radiosonde balloons cannot reach these heights. Furthermore, radar techniques are limited to the upper reaches of the mesosphere where PMSE are observed. In the lower mesosphere, appropriately called the “gap” region, low atmospheric density and limited ionization contribute to a dearth of effective remote sensing techniques.

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5 W.K. Hocking, Measurement of turbulent eddy dissipation rates in the middle atmosphere by radar techniques: A review, Radio Science 20(6):1402-1422, 1985.

6 J.Y.N. Cho and J. Rottger, An updated review of polar mesosphere summer echoes: Observation, theory, and their relationship to noctilucent clouds and subvisible aerosols, Journal of Geophysical Research 102(D2):2001-2020, 1997.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Thermosphere

As seen in Figure 2.4, temperature in the thermosphere increases rapidly with height due to absorption of solar radiation, which also causes molecules in the thermosphere to become electrically charged. This charged region, called the ionosphere, scatters electromagnetic waves effectively, providing a physical mechanism for remote sensing. Ionosphere and thermosphere science is discussed in detail in the section “Ionosphere–Thermosphere Science” in Chapter 5.

ATMOSPHERIC SCATTERING/REFLECTION MECHANISMS

Electromagnetic wave scattering and/or reflection in the atmosphere are a consequence of several physical mechanisms. The present section provides a brief overview of these mechanisms along with the physical limitations they impose on remote sensing techniques.

Rayleigh/Mie Scattering

Several active remote sensing techniques are based on scattering from distinct objects in the atmosphere. As discussed in later sections, weather radars are designed so that their transmitted electromagnetic waves are scattered primarily by hydrometeors (e.g., raindrops, ice crystals, hailstones). By far the most prevalent hydrometeor in the lower atmosphere is rain. Cloud droplets are created by either condensation of water vapor directly onto cloud condensation nuclei (CCN), forming a liquid drop, or by a deposition process with subsequent melting. Once formed, the primary mechanism for increasing the size of droplets is through collision and coalescence. As the liquid drop falls, it interacts with the wind field, causing drag forces to deform the drop and eventually break it up as shown in Figure 2.5. Because of these chaotic processes, a wide distribution of drop sizes exists in the atmosphere. From cloud droplets to intense convective storms, liquid water drops range from approximately 5 microns to just over 8 millimeters in diameter.7 It is important to note that these physically driven sizes dictate the design of radars used to obverse the raindrops.

If raindrops are assumed to be spherical (which is not always true for very large drops), the scattering of electromagnetic waves is well understood. The simplest case is for wavelengths much larger than the diameter of the sphere. In this situation of Rayleigh scattering, the amount of energy scattered from the drop depends on the sixth power of the drop diameter. In other words, larger drops backscatter more

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7 K. Andsager, K.V. Beard, and N.F. Laird, Laboratory measurements of axis ratios for large raindrops, Journal of Atmospheric Science, 56: 2673-2683, 1999.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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FIGURE 2.5 Snapshots of a raindrop falling through the atmosphere. The deformation and eventual breakup due to drag forces are clearly observed. SOURCE: Reprinted by permission from Macmillan Publishers Ltd: Nature Physics, E. Villermaux and B. Bossa, Single-drop fragmentation determines size distribution of raindrops, Nature Physics 5(9):697-702, 2009, copyright 2009.

energy than do smaller drops, which is not always the case when the wavelength is closer to the drop diameter. For this reason, it is highly desirable to design radars that operate in the Rayleigh regime.

In severe weather events, it is possible to observe hydrometeors with sizes that place the scattering outside the Rayleigh regime. For example, hailstones can be quite large (several centimeters), and in this case the scattering can become resonant, causing oscillations in the magnitude of the scattered electromagnetic energy. This phenomenon is called Mie scattering, and it is usually not desirable to design a weather radar for operation under these conditions.

The choice of optimum frequencies is different for precipitation radar than for cloud radar. This is because the radar backscattering cross section of a spherical or quasi-spherical water droplet is governed by Rayleigh scattering if r/λ < 0.1 (where r is the radius of the droplet and λ is the electromagnetic wavelength), and then transitions into Mie scattering as r/λ approaches and exceeds 1. In the Rayleigh regime, the backscattering cross section is proportional to (r/λ)4, so it is advantageous to operate a radar such that r/λ is close to 0.1 for the largest size of droplet radii expected for the medium under consideration. At longer wavelengths the scattering cross section becomes too small for detection, while at shorter wavelengths (the Mie regime) scattering exhibits a resonance pattern as a function of (r/λ), leading to ambiguous estimates of water content or precipitation rate.

Bragg Scattering

Atmospheric turbulence can be caused by a variety of mechanical forcing mechanisms such as wind shear, thermally driven convection, etc. As a consequence of this chaotic process, a continuum of turbulent eddy sizes is created and exists

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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FIGURE 2.6 Typical scales of turbulence in the atmosphere. For profiling radars, it is advantageous to operate at a wavelength that corresponds to a Bragg scale in the inertial subrange of turbulence. SOURCE: W.K. Hocking, “Measurement of turbulent energy dissipation rates in the middle atmosphere by radar techniques: A review,” Radio Science, November 1985.

in the atmosphere.8 This turbulence mixes various temperature and moisture regions of the atmosphere, creating a more homogeneous fluid. Much theoretical and empirical research has been accomplished in this field over many decades, and a reasonable understanding has emerged of the generation mechanisms as well as turbulent scale limitations. In an important review article, Hocking summarizes the theory behind quantifying turbulent intensity.9 He also explains how turbulent scales vary as a function of height and the range of potential eddy sizes. These results are reproduced in Figure 2.6.

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8 T.E. Faber, Fluid Dynamics for Physicists, Cambridge University Press, Cambridge, United Kingdom and New York, N.Y., 1995.

9 Hocking, 1985.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

As is evident from the curve, the scales of isotropic turbulence (called the inertial subrange) gradually increase with height. An abrupt change occurs at the transition region between the troposphere and stratosphere due to the large temperature inversion at that altitude, which has a significant impact on atmospheric stability and therefore the generation of turbulence. The key point to recognize is that the atmosphere limits the scales of turbulence that can exist, and that these scales vary with altitude from centimeters in the troposphere to hundreds of meters in the mesosphere.

In the absence of hydrometeors in the atmosphere, radar remote sensing is still possible through a phenomenon called Bragg scattering. When turbulent eddies exist at a scale of half the wavelength of the electromagnetic wave, constructive Bragg scattering can occur, resulting in a measurable amount of backscattered energy. Of course, this is only possible when the turbulence mixes parts of the atmosphere with different temperatures and moisture levels, which largely control the radio refractive index in the lower atmosphere. The intensity of the backscattered signal is proportional to the intensity of the turbulence and the variations in the refractive index. It is this Bragg scattering mechanism that drives the design of profiling radars, also known as mesosphere-stratosphere-troposphere (MST) radars.

Fresnel Reflection (or Partial Reflection)

In the upper atmosphere, large-scale vertically stratified discontinuities in the refractive index can cause what is called Fresnel reflection or partial reflection. Unlike the lower atmosphere, these refractive index variations are primarily caused by discontinuities in electron density in the ionosphere. In addition, meteor ablation in the upper mesosphere–lower thermosphere can leave long plasma trails, which can also cause Fresnel reflections for particular wavelengths of electromagnetic waves.

For the ionosphere, efficient scales for Fresnel reflection dictate radar wavelengths on the order of hundreds of meters resulting in the use of the medium frequency (MF)10 or high frequency (HF) bands.11 For the case of meteor trail reflections, shorter wavelengths in the VHF band (for example, tens of meters) are the most effective. In all cases, it is important to note that the physics of the particular region of the atmosphere to be observed drives the design of all remote sensing techniques.

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10 See Figure 1.5 for definitions of the radio spectrum bands.

11 A.H. Manson, J.B. Gregory, and D.G. Stephenson, Winds and wave motions (70-100 km) as measured by a partial reflection radiowave system, Journal of Atmospheric and Terrestrial Physics 35:2055-2067, 1973.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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GROUND-BASED RADAR SYSTEMS USED TO OBSERVE THE ATMOSPHERE

The design of an atmospheric radar system begins with an understanding of the region of the atmosphere to be observed and the physics of the potential scattering/reflection mechanisms that may occur in that region. As described in the previous sections, the distinct layers of the atmosphere encompass a variety of phenomena and characteristics. Therefore, the radars used to observe these layers have correspondingly distinct designs. Once the scattering/reflection mechanism is understood, the actual design of the system can begin, involving major decisions about operation frequency, bandwidth, transmit power, etc. The following sections provide a brief summary of the main upward-looking radars used to observe the atmosphere, from the troposphere to the thermosphere.

Weather Radar

The term weather radar describes the suite of radars used to remotely observe precipitation; these radar designs are largely based on the Rayleigh scattering assumption. Almost all modern weather radars have Doppler capability, allowing for an estimate of radial velocity in addition to the backscattered intensity. Furthermore, recent improvements to operational robustness have made dual-polarization weather radars more commonplace. In fact, the extremely successful WSR-88D radar network (NEXRAD), operated by the U.S. National Weather Service,12 of which an example is shown in Figure 2.7, has been recently upgraded to dual polarization, making it the world’s largest network of such radars. With this capability, more accurate rainfall estimates are possible, in addition to the promise of hydrometeor classification. It should also be mentioned that weather radars often observe scattering from insects in the lower atmosphere even when there is no actual precipitation. As a result, it becomes possible to use this type of scattering as a tracer of the wind field under clear-air conditions, which is invaluable for short-term prediction of convection initiation.

Since severe storms are primarily confined to the troposphere, the altitude coverage of weather radars is also limited to this atmospheric layer. Using a pencil beam, weather radars are scanned for a full 360 degrees of azimuth and then sequentially positioned to several higher elevation angles in order to fill the volume of interest.13 This type of volume scanning is used by the NEXRAD network and has proven successful in providing the data forecasters need to protect lives and

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12 T.D. Crum and R.L. Alberty, The WSR-88D and WSR-88D operational support facility, Bulletin of the American Meteorological Society 74(9):1669-1687, 1993.

13 R.J. Doviak and D.S. Zrnic, Doppler Radar and Weather Observations, 2nd Edition, Dover, Mineoloa, N.Y., 2006.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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FIGURE 2.7 Photograph of a WSR-88D radar (NEXRAD) operated by the U.S. National Weather Service. SOURCE: NOAA.

property.14 In addition, these data are available in real time and are used freely by commercial companies, TV stations, and the general public.

Because of the expected range of hydrometeor sizes in storms, the optimal wavelength for a weather radar is in the S-band. For example, the NEXRAD network is allocated for primary use in the 2.7-2.9 GHz band. At this wavelength,

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14 K.M. Simmons and D. Sutter, WSR-88D radar, tornado warnings, and tornado casualties, Weather and Forecasting 20(3):301-310, 2005.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

Rayleigh scattering is a valid assumption that allows for simpler rainfall estimation and for little attenuation of the electromagnetic wave. Nevertheless, commercial and some research weather radars operate in the C- and X-bands (~5 GHz and 10 GHz, respectively). Often, these wavelengths are chosen for mobile applications, where minimizing the overall size of the radar antenna is important. Higher frequencies allow for smaller antennas while providing good angular resolution.

It should be emphasized that the commercial weather industry relies heavily on C-band radars and plays an important role in directly alerting the public of severe weather. TV meteorologists are usually the first authority to whom the public reaches out for weather information. Commercial C-band weather radars have a very similar design to other weather radars, such as NEXRAD, with comparable beamwidths, bandwidths, etc. The key different is the wavelength, which makes them more susceptible to atmospheric attenuation and resonance scattering effects. In addition, commercial weather radars typically use magnetron transmitters, which have less controlled spectral leakage compared to klystron and solid-state transmitters.

Range resolution is controlled by the bandwidth of the radar. For weather radar, the typical range resolution is 100 m, requiring approximately 1-2 MHz of bandwidth depending on the specific pulse shape used. Peak power levels can be as high as 1 MW for large systems with duty cycles of approximately 0.1 percent. As mentioned earlier, the beam shape is always a pencil beam requiring extensive scanning in both azimuth and elevation in order to provide complete volumetric coverage.

Cloud Radar

In many ways, a cloud radar is similar to a weather radar. The former is typically used to observe nonprecipitating clouds, which are made up of extremely small droplets.15 In order to obtain the required sensitivity and to avoid extreme attenuation by atmospheric gases, cloud radars normally operate in the Ka- and W-bands (35 GHz and 94 GHz, respectively). At these frequencies, Rayleigh scattering dominates the backscatter from clouds. A prime example of the use of cloud radars is the Department of Energy’s (DOE’s) Atmospheric Radiation Measurement (ARM) program, which boasts numerous cloud radars used for climate studies, among other scientific applications.

Although some cloud radars have scanning capability, many are fixed in the vertical position. For such a configuration, the goal is to follow the temporal evo-

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15 P. Kollias, E.E. Clothiaux, M.A. Miller, B.A. Albrecht, G.L. Stephens, and T.P. Ackerman, Millimeter-wavelength radars: New frontier in atmospheric cloud and precipitation research, Bulletin of the American Meteorological Society 88(10):1608-1624, 2007.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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lution of the clouds by allowing advection to provide the sampling. Using such a technique allows for a simpler pedestal design, but separating the temporal from the dynamical evolution of the cloud structure can be a challenge.

As mentioned, the Ka- and W-bands are typically used for cloud radars. Range (or height in the case of a vertically pointing radar) resolution is often better than for weather radars, so bandwidth requirements can approach approximately 5 MHz. Normal pulsed waveforms are common with peak power levels of 100 kW for Ka-band systems and near 1 kW for W-band systems. As with weather radars, cloud radars use a pencil beam and sometimes have dual-polarization capabilities.

Wind Profiling Radar

Measuring winds in the upper atmosphere is fundamental to our understanding of global circulation and for numerical modeling. Conventional means of measuring the winds include launching radiosondes, which can be used to infer the wind profile as the balloons rise. Due to advection, radiosondes drift horizontally, thereby providing a less than ideal profile of the wind and other atmospheric states. Nevertheless, radiosondes are launched around the world at the same time twice per day. This synchronized sampling provides global data with a 12-hour temporal resolution.

Wind profiling radars, also known as MST radars, were conceived as a technology to provide wind profiles in the upper atmosphere with a much better temporal resolution than radiosondes. Boundary layer radars (BLR) are much smaller (~1-2 m aperture) and are designed to scatter from turbulence in the lowest 1-3 km of the atmosphere. Both MST radars and BLRs are based on Bragg scattering from refractive index discontinuities in the atmosphere, mostly caused by turbulence.16 Given the expected scales of turbulence, wind profiling radars must operate in the VHF, UHF, and L-bands. In the boundary layer, the L-band is the wavelength of choice, while VHF and UHF wavelengths are used for higher-altitude coverage with MST radars. One of the most sophisticated MST radars in the world is located in Shigaraki, Japan, and operated by Kyoto University. The MU (middle and upper atmosphere) radar, shown in Figure 2.8, has an aperture with a diameter of approximately 100 meters, made up of hundreds of crossed-Yagi antennas.17,18 Phased-array beam steering is usually used with these large wind-profiling radars.

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16 See, for example, R.F. Woodman, Spectral moment estimation in MST radar, Radio Science 20(6):1185-1195, 1985.

17 S. Fukao, T. Sato, T. Tsuda, S. Kato, K. Wakasugi, and T. Makihira, The MU radar with an active phased array system: 1. Antenna and power amplifiers, Radio Science 20(6):1155-1168, 1985.

18 S. Fukao, T. Tsuda, T. Sato, S. Kato, K. Wakasugi, and T. Makihira, The MU radar with an active phased array system: 2. In-house equipment, Radio Science 20(6):1169-1176, 1985.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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FIGURE 2.8 The MU radar in Shigaraki, Japan. This example of an MST radar has an antenna field with an overall aperture diameter of approximately 100 m. SOURCE: Hiroyuki Hashiguchi, Kyoto University.

Wind profiling radars are designed to direct the radar beam near vertical, thereby providing a profile of both the horizontal and vertical wind components. The horizontal wind component is obtained by steering the beam slightly off vertical using a method called Doppler beam swinging. Range resolution is normally not better than 150 meters, requiring an approximate 1 MHz bandwidth. Given the weak echoes from clear-air turbulence, peak power levels are as high as 1 MW with approximately 0.2 percent duty cycles. As is the case with most atmospheric radars, wind profilers use a pencil beam, but in this case a typical beamwidth is 5-10 degrees.

High-Frequency/Medium-Frequency Radar

Studies of the mesosphere and thermosphere require robust remote sensing techniques. Standard radiosondes can reach altitudes of only 15-20 kilometers. MST radars can be designed to observe this region, but require extremely high transmit powers, with correspondingly high cost, resulting in a limited number of

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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FIGURE 2.9 The Saura MF Radar antenna field in Andoya, Norway. SOURCE: Chris Adami, ATRAD/IAP Saura MF Radar, Andoya, Norway.

such systems across the globe. Fortunately, a technique based on Fresnel reflection was developed in the 1970s that requires much less transmit power at a lower cost.19 These HF/MF radars operate in these bands but also at lower VHF frequencies (1-60 MHz), given their strong sensitivity to partial reflections from ionized layers in the mesosphere and thermosphere.

Given wavelengths of up to hundreds of meters, unique antennas have been developed using coaxial cables strung among a set of large poles. An example of such a system is shown in Figure 2.9. The radar transmission is directed vertically with a fairly large (~20 degrees) beamwidth. By receiving the reflected signal on several spatially separated antennas, cross-correlation techniques can be used to find the delay of the diffraction pattern on the ground, thereby providing a method to estimate the horizontal velocity. This so-called Spaced Antenna Drift method has been used for decades with HF/MF radars20 as well as with wind profiling radars.

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19 Manson et al., 1973.

20 Manson et al., 1973.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

Range resolution is not a huge design constraint with these radars, so the typical bandwidths are ~1 MHz. Peak transmit power is approximately 10-200 kW with duty cycles less than 1 percent.

Meteor Radar

As with HF/MF radars, meteor radars were developed in order to obtain wind measurements in the upper mesosphere and lower thermosphere. Although the physical mechanism by which meteor radars operate is the same as HF/MF radars (Fresnel reflection), the source phenomenon is quite different. As thousands of meteors come into the atmosphere every day, most burn or “ablate” in the 80-100 kilometer region. This ablation process leaves behind a meteor trail, which drifts with the prevailing winds. By transmitting an electromagnetic wave perpendicular to these almost linear trails, Fresnel reflection occurs and can be used to detect the trails and to estimate their radial velocity. The angular position is determined using interferometry, and when combined with the velocity information from numerous meteors, it can be used to estimate the three-dimensional wind field at these altitudes. These relatively low-cost “meteor radars” operate most effectively at 20-60 MHz, and can provide estimates of winds, meteor concentration, and atmospheric temperature.21

Since meteor trails can be created over a wide range of angles, and Fresnel reflection requires a perpendicular orientation, the transmit beam of a meteor radar is extremely wide (~100 degrees). At least three small receive antennas (typically crossed-Yagi design) are used for interferometric processing in order to estimate the angular position of the echo. When combined with range gating, the three-dimensional position can then be determined. The bandwidth requirement is typically less than 1 MHz. Peak power levels range from 8 kW to 40 kW, with duty cycles less than 1 percent.

Summary of Current Ground-Based Radar Systems

A summary of general atmospheric radar characteristics is provided in Table 2.1. As should be evident, these radars do not require significant bandwidth in comparison to extremely high-resolution scientific applications or broadband wireless needs. The unique aspect of atmospheric remote sensing is that the physics of the atmosphere drives the choice of the operating frequency. As discussed previously, for example, the range of possible hydrometeor sizes is limited by drag forces,

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21 S.K. Avery, J.P. Avery, T.A. Valentic, S.E. Palo, M.J. Leary, and R.L. Obert, A new meteor echo detection and collection system: Christmas Island mesospheric wind measurements, Radio Science 25(4):657-669, 1990.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

TABLE 2.1 Summary of Typical Atmospheric Radar Characteristics


Radar Type Characteristics

Weather Radar  

Scattering/reflection

Rayleigh, Mie

Altitude coverage

0-15 km, usually confined below the tropopause with volume coverage to ranges 0-450 km

Frequency

2.7-2.9 GHz, ~5 GHz, ~10.0 GHz in the S-, C-, and X-bands, respectively

Bandwidth

<1-2 MHz

Antenna

Parabolic dish, pencil beam, 1 degree beamwidth, dual-polarization

Peak power

20 kW-1 MW

Duty cycle

<0.1%

Cloud Radar

 

Scattering/reflection

Rayleigh, Mie if precipitating clouds are observed

Altitude coverage

0-30 km, includes high-level clouds in the stratosphere

Frequency

~35 GHz, ~94 GHz

Bandwidth

<5 MHz

Antenna

Parabolic dish, pencil beam, some scanning but usually vertically pointing, typical 1 degree beamwidth, dual-polarization

Peak power

100 kW (35 GHz), 1 kW (94 GHz)

Duty cycle

<0.1%

Wind Profiling Radar

 

Scattering/reflection

Bragg

Altitude coverage

0-400 km, upper height depends on sensitivity and ionospheric state

Frequency

~50 MHz, ~400 MHz (MST radars), ~915 MHz or ~1.3 GHz (boundary layer radars)

Bandwidth

<1 MHz

Antenna

Large phased array, pencil beam, typical 3-5 degree beamwidth, ± 10 degree zenith angles

Peak power

500 kW-1 MW

Duty cycle

<0.2%

HF/MF Radar

 

Scattering/reflection

Fresnel

Altitude coverage

70-100 km, depends on ionospheric state

Frequency

~1-60 MHz

Bandwidth

<1 MHz

Antenna

Various dipole designs, 20 degree beamwidth

Peak power

10-200 kW

Duty cycle

<1%

Meteor Radar

 

Scattering/reflection

Fresnel

Altitude coverage

80-100 km, altitudes of meteor ablation

Frequency

~20-60 MHz

Bandwidth

<1 MHz

Antenna

Crossed-Yagi, ~100 degree beamwidth for all-sky coverage, interferometer

Peak power

8-40 kW

Duty cycle

<1%

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

collision/coalescence processes, etc. Therefore, the radar wavelength of choice is dictated by these physical mechanisms.

Future Trends and Concepts—Ground-Based Radar

Recently, some innovative weather-radar ideas have been developed that are relevant to the spectrum usage challenge. The first is the so-called Multifunction Phased Array Radar (MPAR) program, which envisions replacing the aging radar infrastructure in the United States with phased-array radar technology.22 The Federal Aviation Administration (FAA) and the National Oceanic and Atmospheric Administration (NOAA) partnered to develop the idea of deploying a fleet of phased-array radars of a single design to accomplish the missions of several disparate radars currently operated by these two federal agencies. Significant advantages would be gained through such a concept, including rapid-update weather observations that are fundamentally important for mitigating severe weather. From an economic standpoint, an advanced radar network with a common design, replacing several systems with distinct parts, maintenance procedures, etc., would inherently have much lower maintenance and operation costs. Another extremely important advantage of an MPAR system relevant to this study would be its spectral efficiency. By the simple fact that several radars, all in different frequency bands (L-, C-, and S-bands), would be replaced with a radar in a single band (S-band), significant reduction in spectrum usage would be realized. With the rapid growth of broadband wireless and its ever-increasing need for bandwidth, this advantage of the MPAR concept cannot be overstated. Of course, it should also be stated that many commercial weather radars are currently operating at C-band wavelengths, and the MPAR program will not be replacing these radars.

Funded by the National Science Foundation, the Center for the Collaborative Adaptive Sensing of the Atmosphere (CASA) was established with the goal of predicting severe weather using advanced radar technology in conjunction with numerical weather prediction models.23 An important aspect of the CASA radar concept, called distributed collaborative adaptive sensing (DCAS), is to use a network of low-cost X-band radars, which provides the important advantage of coverage of the lower troposphere, the source of severe weather that directly impacts life and property. Longer-range radars (e.g., NEXRAD) are more sig-

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22 D.S. Zrnić, J.F. Kimpel, D.E. Forsyth, A. Shapiro, G. Crain, R. Ferek, J. Heimmer, W. Benner, T.J. McNellis, and R. J. Vogt, Agile-beam phased array radar for weather observations, Bulletin of the American Meteorological Society 88(11):1753-1766, 2007.

23 D. McLaughlin, D. Pepyne, V. Chandrasekar, B. Philips, J. Kurose, M. Zink, K. Droegemeier, S. Cruz-Pol, F. Junyent, J. Brotzge, D. Westbrook, et al., Short-wavelength technology and the potential for distributed networks of small radar systems, Bulletin of the American Meteorological Society 90(12):1797-1817, 2009.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

nificantly impacted by Earth’s curvature, where the radar beam propagates to progressively higher altitudes at farther ranges. Using a much larger number of shorter-range X-band radars, a DCAS network could observe much closer to the ground, while at the same time providing much higher spatial and temporal resolutions.

SATELLITE-BASED RADAR SYSTEMS USED TO OBSERVE THE ATMOSPHERE

Meteorological data are extremely important for weather forecasting, including the monitoring and prediction of extreme droughts, mudslides, hurricanes, and winter storms. To be of value, precise measurements over wide areas are required. Satellites offer a unique perspective from which to measure atmospheric information over the globe. Satellite sensors provide critical information about current conditions, storm formation, and the evolution of weather events. Their data are also essential for feeding climate and weather prediction models used to understand interactions within the entire biosphere—croplands, forests, lakes, oceans, and the like, including urban and coastal areas.

Radar remote sensing instruments operating from satellites are particularly effective in the observation of precipitation, clouds, and near-surface winds over the ocean, providing invaluable data of environmental parameters that are vital for a wide variety of scientific, commercial, and military applications and that enhance our ability to protect human life and property. Active sensors can provide measurements day or night, independent of solar illumination. Furthermore, because the microwave spectrum offers a wide range of penetration depths, the use of multiple-frequency observations can provide three-dimensional (3D) information about the microphysical properties of clouds and rain.

Meteorological Satellite Radar Systems

The Tropical Rain Mapping Mission (TRMM) precipitation radar (PR) is the first meteorological radar to be launched in space for mapping 3D rain distributions. Launched in 1997 into a low-inclination (35 degree) orbit, the PR uses a Ku-band (13.8 GHz) radar to measure rainfall over a 50 km swath with 4-5 km horizontal and 250 m vertical resolution. An example of the PR pass over a hurricane is given in Figure 2.10.

The CloudSat satellite, launched in 2006, a member of NASA’s A-Train platforms, carried a 94 GHz radar designed for vertically profiling clouds and precipitation along its nadir track. The mission goal is to provide the first global survey of the vertical structure of clouds and profiles of cloud liquid water and ice water

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

images

FIGURE 2.10 Examples of PR 3D imagery for a hurricane. SOURCE: NASA/Goddard Space Flight Center Scientific Visualization Studio; see NASA, “Hurricane Bonnie from TRMM and GOES with Cloud Tower: August 22, 1998,” December 31, 1998, http://svs.gsfc.nasa.gov/goto?211.

content, filling a recognized, critical gap in the measurements and understanding of clouds for weather and climate research.

The follow-on mission to the TRMM is the Global Precipitation Mission (GPM), launched in February 2014. The GPM dual-frequency precipitation radar (DPR) uses two frequencies, 13.6 GHz (KuPR) and 35.5 GHz (KaPR). The KaPR is useful for detecting light precipitation. Measurements at the two frequencies can observe both strong rain in the tropics and light rain and snow in high latitudes. They also deliver the ability to estimate the drop size distribution (DSD). This information cannot be acquired using only one frequency as in TRMM, hence the accuracy of precipitation volume estimation is significantly improved. The first DPR image released is shown in Figure 2.11. Thus, future missions will be multifrequency.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

images

FIGURE 2.11 Examples of DPR 3D imagery for a hurricane. SOURCE: JAXA/NASA.

Scientific and Operational Applications

TRMM’s observations are critical to the study of precipitation and associated storms and climate processes in the tropics. Scientists using the data have been able to determine the time and space varying characteristics of tropical rainfall, convective systems, and storms and how these are related to variations in the global water and energy cycles. The satellite also helps us build a long record of near-global precipitation characteristics, which is critical to understanding Earth’s natural mechanisms and cycles. Additionally, the availability of real-time TRMM data has allowed monitoring of tropical cyclones, assimilation of precipitation information into weather forecast models, and other contemporaneous hydrological applications. The continuation of the TRMM data set will be the objective of the GPM.

Satellite Ocean Wind Sensors

The International Space Station (ISS) RapidScat instrument is the third Ku-band wind scatterometer in the SeaWinds series. The QuikSCAT instrument operated from 1999 until 2009. RapidScat was launched to the ISS in September 2014 as a cost-effective replacement for QuikSCAT. It measures ocean winds to support weather prediction, monitor hurricanes, and perform basic research. QuikScat’s measurements were so essential to the prediction community that when QuikSCAT

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

stopped collecting wind data in late 2009, NASA’s Jet Propulsion Laboratory (JPL) created the RapidScat instrument from the test hardware that was originally built to test QuikSCAT. This hardware was then launched to the ISS for a fraction of the cost and time it would have taken to build and launch a new satellite. Recently, RapidSCAT monitored the very active 2014-2015 winter on the East Coast and the extended typhoon season in the Pacific.

Summary of Current and Future Satellite-Based Sensors

Tables 2.2 and 2.3 list several current and planned future satellite missions that use active sensors to remotely sense the atmosphere. The sensors typically obtain 2D or 3D images of atmospheric winds and precipitation. In addition to supporting basic scientific research, the data are operationally used in numerical weather forecasting and are especially vital during tropical cyclones for monitoring and predicting land-falling hurricanes and predicting storm intensity.24 These sensors provide data used to retrieve the motion of ice and rain within storms and to support climate studies and hurricane evolution studies and other scientific applications.

SPECTRUM USAGE REQUIREMENTS FOR SATELLITE-BASED SENSING

Atmospheric transmission windows impose restrictions on what frequencies can be used for remote sensing atmospheric parameters.25 In addition, some frequencies are desirable (or undesirable) owing to their sensitivity to specific physical parameters of interest such as ice, rain, and particulates. These considerations lead to the selection of the operational frequencies commonly used for active atmospheric sensing. As indicated in Tables 2.3 and 2.4, the typical bands used by current and future atmospheric sensors extend from the L-band (1.2 GHz) into the millimeter-wave bands such as the W-band (94 GHz).

The bandwidth necessary to satisfy spatial resolution requirements for current and planned cloud and precipitation radars are typically between 0.6 and 60 MHz for precipitation radars and 0.3 and 10 MHz for cloud radars.

Because of the size, weight, and power limitations of spacecraft-borne sensors, they must limit their transmit power. The echo signals are typically very small and

______________

24 National Academy of Public Administration, Forecast for the Future: Assuring the Capacity of the National Weather Service, Washington, D.C., 2013, http://www.napawash.org/wp-content/uploads/2013/05/ForecastfortheFuture-AssuringtheCapacityoftheNationalWeatherService.pdf.

25 International Telecommunication Union, “Frequency Bands and Required Bandwidths Used for Spaceborne Active Sensors Operating in the Earth Exploration-Satellite (Active) and Space Research (Active) Services,” REC. ITU-R RS.577-7, 2009.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

TABLE 2.2 Current Active Atmospheric Sensor Missions

Instrument Frequency Band (GHz) Launch Description and Comments
GPMa Core Observatoryb 13.6 (Ku- band) 35.5 (Ka-band) 35-36 GHz February 2014 Near-global liquid and solid precipitation to improve climate predictions, follow-on for TRMM. DPR active/passive sensor light to heavy rain. Simultaneous measurements at the Ka-/Ku-bands will provide new information on DSD (NASA/JAXA).
ATTREX (airborne) 1.25 (L-band) 2011 Measures tropopause layer water vapor with Dual-frequency Airborne Precipitation Radar (NASA).c
CloudSAT (CPR)d 94-GHz (W-band) nadir-looking 94-94.1 GHz 2006 Measures rain and clouds to study their impact on Earth’s radiation budget and climate change.
NASA/Canadian Space Agency (CSA) mission.
HS3 (airborne) — (Hurricane and Severe Storm Sentinel)e 13.47, 13.91 33.72, 35.56 (Ka/Ku-band), HIWRAPf Doppler radar 2012 Maps the 3D tropospheric and atmospheric winds, precipitation field and ocean surface wind field of tropical cyclones, hurricane development, monitoring, storm intensity prediction. Motion of ice and rain within storms (NASA).
TRMM (PR) 13.8 GHz (Ku-band) 13.4-14 GHz 1997 Measures rainfall in the tropics and subtropics for a better understanding of cloud formation, rain, floods, and droughts and how the winds drive ocean currents (NASA/JAXA).
METOP ASCAT series 5.4 GHz (C-band) 2007 Measure near-surface wind speed and direction over the global oceans. Monitor severe weather events such as hurricanes and typhoons (ESA).
ISS-RapidScat 13.6 (Ku-band) 2014 Diurnal cycle observation of global near-surface winds and rain over the ocean (NASA/JPL).

NOTE: ASCAT, Advanced Scatterometer; ATTREX, Airborne Tropical Tropopause Experiment; ESA, European Space Agency; HIWRAP, High-Altitude Imaging Wind and Rain Airborne Profiler; JAXA, Japan Aerospace Exploration Agency; METOP, Meteorological Operational satellite program.

a NASA, “Constellation Partners,” http://pmm.nasa.gov/GPM/constellation-partners, accessed June 2, 2015.

b NASA, “Global Precipitation Measurement: Core Observatory,” http://www.nasa.gov/sites/default/files/files/GPM_Mission_Brochure.pdf, accessed June 2, 2015.

c NASA, “Dual-Frequency Airborne Precipitation Radar (PR-2),” https://espo.nasa.gov/missions/attrex/instrument/PR-2, accessed June 2, 2015.

d Colorado State University, “The Cloud Profiling Radar (CPR),” http://cloudsat.atmos.colostate.edu/instrument, accessed June 2, 2015.

e NASA, “HS3 Mission Overview,” http://www.nasa.gov/mission_pages/hurricanes/missions/hs3/overview/#.UuQ_C2TD8YI, accessed June 2, 2015.

f H. Kramer, “HIWRAP (High-Altitude Imaging Wind and Rain Airborne Profiler),” Earth Observation Portal, https://directory.eoportal.org/web/eoportal/airborne-sensors/hiwrap, accessed June 2, 2015.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

TABLE 2.3 Future Active Atmospheric-Sensing Satellite Missions

Instrument Frequency Band (GHz) Launch
Earthcare (CPR)a 94.05 (W-band)b Late 2015 Clouds and aerosols (lidar/ radar) cloud profiling radar (CPR)—(ESA/JAXA/NICT), doppler providing sensitivity of -36 dBZ.c
Aerosol/Cloud/ Ecosystems (ACE), Doppler NASA radar 35.6 GHz (Ka-band) and 94.1 GHz (W-band) 2020+ Measures cloud ice and rain, will help to answer emerging science questions associated with aerosols, clouds, air quality, and global ocean ecosystems (NASA).d
Ocean Surface Vector Wind (OSVW) 5.4 GHz (C-band) and 13.2 GHz (Ku-band) TBD Measure near-surface wind speed and direction and rain rate over the global oceans. Monitor severe weather events such as cyclones (NASA/NOAA).e

NOTE: NICT, National Institute of Information and Communications.

a AXA, “Earth Cloud, Aerosol and Radiation Explorer (EarthCARE),” November 29, 2012, http://www.jaxa.jp/projects/sat/earthcare/index_e.html.

b P. Foster, J. Hartmann, H. Horie, and R. Wylde, “Performance Verification of the 94 GHz Quasi-Optical-Feed for EarthCARE’s Cloud Profiling Radar,” Proceedings of the 5th ESA Workshop on Millimeter Wave Technology and Applications and 31st ESA Antenna Workshop, May 18-20, 2009, Noordwijk, The Netherlands, ESA WPP-300, 2009.

c ESA, “EarthCARE,” https://earth.esa.int/web/guest/missions/esa-future-missions/earthcare, accessed June 3, 2015.

d NASA, “ACE Science Working Group (SWG) Workshop,” November 2014, http://dsm.gsfc.nasa.gov/ace/.

e P.S. Chang, Z. Jelenak, J.M. Sienkiewicz, R. Knabb, M.J. Brennan, D.G. Long, and M. Freeberg, Operational use and impact of satellite remotely sensed ocean surface vector winds in the marine warning and forecasting environment, Oceanography 22(2):194-207, 2009.

therefore vulnerable to interference. Active atmospheric sensors are designed to meet their performance requirements so long as RFI does not exceed the interference criteria specified in Recommendation ITU-R RS.1166, where I/N refers to the ration of interference to noise power. According to the said recommendation,

Performance criteria for active sensors is defined in terms of the precision of measurement of physical parameters and the availability of measurements free from harmful interference. Interference criteria are stated in terms of the interfering signal power not to be exceeded in a reference bandwidth for more than a given percentage of time.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

TABLE 2.4 Commonly Used Bands for Active Atmospheric Sensing by Sensor Type

Altimeter Wind Scatterometer Precipitation Radar Cloud Radar
(C) 5.25-5.57 GHz
(Ku) 13.25-13.75 GHz
(C) 5.4-5.5 GHz
(Ku) 13.25-13.75 GHz
(Ku) 13.25-13.75 GHz
(Ku) 17.2-17.3 GHz
(K) 24.05-24.25 GHz
(Ka) 35.5-36.0 GHz
(W) 94.0-94.1 GHz
(mm) 133.5-134.0 GHz
(mm) 237.9-238.0 GHz
Weather prediction, scientific studies Weather prediction, severe storms, scientific studies, hurricanes Hurricanes, severe storms, scientific studies Hurricanes, severe storms, scientific studies

SOURCE: Bryan Huneycutt, Jet Propulsion Laboratory.

TABLE 2.5 Interference Sensitivity for Precipitation and Cloud Radarsa

Sensor Type Interference Criteria Data Availability Criteria (%)
Performance Degradation I/N (dB) Systematic Random
Precipitation radar 7% increase in minimum rainfall rate –10 N/A 99.8
Cloud profile radar 10% degradation in minimum cloud reflectivity –10 99 95

NOTES: I/N, interference to noise power ratio.

a International Telecommunication Union, “Performance and Interference Criteria* for Active Space-borne Sensors,” Recommendation RS.1166-4, 2009, http://www.itu.int/rec/R-REC-RS.1166-4-200902-I.

Table 2.5 summarizes the maximum allowable interference levels for current precipitation radars based on Recommendation ITU-R RS.1166 for each operating frequency. Interference levels exceeding these values would degrade, or may entirely prevent, collection of useful measurements.

Table 2.6 summarizes the maximum allowable interference levels for current precipitation radars based on Recommendation ITU-R RS.1166 for each operating frequency.

ECONOMIC AND SOCIETAL VALUE

According to NOAA, “NOAA’s National Weather Service forecasts, warnings, and the associated responses result in a $3 billion savings in a typical hurricane season. Two-thirds of this savings, $2 billion, is attributed to the reduction in

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

TABLE 2.6 Maximum Interference Power Level for Specific Active Bands Used in Current Precipitation Radarsa


Frequency (GHz) Interference Power Level (dBW)

13.75-13.790 −90
13.790-13.793 −115
13.793-13.805 −150
13.805-13.808 −115
13.808-13.850 −90
13.85-13.86 −70
35.5-36.0 −152
94.0-94.1, 133.5-134, 237.9-238 (CR) −155 (over 300 kHz)

a International Telecommunication Union, “Performance and Interference Criteria* for Active Space-borne Sensors,” Recommendation RS.1166-4, 2009, http://www.itu.int/rec/R-REC-RS.1166-4-200902-I.

storm-related deaths, and one-third of this savings, $1 billion, to a reduction in property-related damage.”26

Although NASA’s missions are mostly scientific, not operational, its active sensors provide important meteorological and space weather data in near-real time that is also used to improve forecasting, monitoring, and mitigation planning.27

Table 2.7 lists some of the yearly savings that accrue to industries like agriculture and railways and to society in general given in the NOAA report. Active remote sensing contributes to these annual savings by providing data to weather services and continually improving the understanding of Earth’s processes.

FINDINGS AND RECOMMENDATIONS

Finding 2.1: Whether measured from the ground or space, active remote sensing of the atmosphere provides immense scientific and operational value to society. From saving lives and protecting property from severe storms to facilitating a deeper understanding of upper atmospheric winds and global circulation (as examples), active remote sensing cannot be replaced by any other observational technology.

Radar wind profiling of the atmosphere exploits scattered energy from temperature and moisture discontinuities caused by turbulence. Owing to the viscous characteristics of the atmosphere, however, these discontinuities exist only over cer-

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26 NOAA, Economic Statistics for NOAA, 5th ed., April 2006, http://www.publicaffairs.noaa.gov/pdf/economic-statistics-may2006.pdf.

27 National Academy of Public Administration, Forecast for the Future, 2013.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

TABLE 2.7 Estimated Financial Savings to the U.S. Economy to which Active Atmospheric Sensing Contributes


Yearly Savings Estimate Quote

$175 million, electric utilities U.S. electricity producers save $185 million annually using 24-hour temperature forecasts to improve the mix of generating units that are available to meet electricity demand. Incremental benefits are relevant in assessing the merits of investments that will improve forecast accuracy.a
$418 million per °F, electric utilities Errors in temperature and precipitation forecasting for even benign meteorological events such as local or regional heat or cold waves can cost U.S. utilities approximately $1.14 million per degree Fahrenheit daily as a result of an impaired ability to match energy supplies with demand.b
$312-$354 million, agriculture Benefits to U.S. agriculture by altering planting decisions based on improved El Niño forecasts have been estimated at $312 million to $354 million annually, throughout El Niño, normal, and La Niña years. Costs associated with errors in predicting the onset of regional climate changes could thus easily amount to hundreds of millions of dollars per year.c
$78-$177 million per 1°C, many industries The incremental benefit of an improvement in temperature forecast accuracy is estimated to be about $1.85 million per percentage point of improvement per year. For a 1°C improvement in accuracy, the benefit is about $78 million per year (or a $49 million benefit for a 1°F improvement). It is estimated that a perfect forecast would add $99 million to these savings.d
$13.4 million, railways For every $1 that railway companies spend in acquiring NOAA climate data, they receive a $16,000 savings in infrastructure costs that would be required to maintain their own climate database storage, archiving, and reporting system. After extrapolating these savings to the entire Class I freight railroad sector, the potential benefits are approximately $14 million.e

NOTE: All values have been changed to 2014 dollars.

a National Weather Service, “Value of a Weather-Ready Nation,” National Oceanic and Atmospheric Administration, last revised September 13, 2011, http://www.nws.noaa.gov/com/weatherreadynation/files/Weather-Econ-Stats.pdf.

b National Research Council, Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses, The National Academies Press, Washington, D.C., 2007.

c National Oceanic and Atmospheric Administration, Economic Statistics for NOAA, 5th Ed., Silver Spring, Md., April 2006.

d T. Teisberg, R. Weiher, and A. Khotanzad, The economic value of temperature forecasts in electricity generation, Bulletin of the American Meteorological Society 86:1765-1771, 2005.

e Centrec Consulting Group, LLC, Economic Value of Selected NOAA Products within the Railroad Sector, Report prepared for NOAA’s National Climatic Data Center, Savoy, Ill., June 2005.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×

tain length scales. This type of scattering (Bragg backscattering is one) can only be observed at particular radio frequency bands. Similar physically based constraints exist for other atmospheric remote sensing techniques, such as turbulence, rain, and clouds.

Finding 2.2: The frequency bands used for active remote sensing of the atmosphere are selected to optimize the detection of specific physical features and phenomena. In general, observations at approximately 50, 400, and 900 MHz and 3, 10, 14, 35, 90, 140, and 210 GHz (very roughly every octave in frequency) are needed.

Recommendation 2.1: Because of the immense impact on society and for the sake of atmospheric science, the current frequency allocations for active remote sensing of the atmosphere should be preserved and strongly protected.

Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Page 33
Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Page 34
Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Page 41
Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Page 43
Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Page 44
Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×
Page 46
Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×
Page 47
Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×
Page 48
Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
×
Page 49
Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Page 50
Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Suggested Citation:"2 Active Earth Remote Sensing for Atmospheric Applications." National Academies of Sciences, Engineering, and Medicine. 2015. A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum. Washington, DC: The National Academies Press. doi: 10.17226/21729.
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Active remote sensing is the principal tool used to study and to predict short- and long-term changes in the environment of Earth - the atmosphere, the oceans and the land surfaces - as well as the near space environment of Earth. All of these measurements are essential to understanding terrestrial weather, climate change, space weather hazards, and threats from asteroids. Active remote sensing measurements are of inestimable benefit to society, as we pursue the development of a technological civilization that is economically viable, and seek to maintain the quality of our life.

A Strategy for Active Remote Sensing Amid Increased Demand for Spectrum describes the threats, both current and future, to the effective use of the electromagnetic spectrum required for active remote sensing. This report offers specific recommendations for protecting and making effective use of the spectrum required for active remote sensing.

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