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

Summary

The Committee on a Survey of the Active Sensing Uses of the Radio Spectrum was tasked with: (1) documenting the importance of active remote sensing, particularly for the purpose of serving societal needs; (2) documenting the threats, both current and future, to the effective use of the electromagnetic spectrum required for active remote sensing; and (3) offering specific recommendations for protecting and making effective use of the spectrum required for active remote sensing.

Active remote sensing is defined as the use of a transmitter and at least one receiver to measure (sense) the transmission or scattering properties of a medium at radio frequencies. These measurements discern the physical state of the medium through which the signal passes or is scattered by. The media of interest encompass Earth’s atmosphere (including the ionosphere), oceans, and land surfaces, as well as extraterrestrial objects.

This report concentrates on active remote sensing at radio frequencies, which is the portion of the electromagnetic spectrum from near 0 Hz to 300 GHz. Measurements in this range have direct societal benefits. Active remote sensing measurements can be either transmission measurements, in which the transmit and receive antennas usually point at each other; or scattering measurements, where the transmitted signal reflects from the medium and is received by an antenna colocated with the transmitter (the backscatter mode), or by a non-colocated receiver (the bistatic mode). Active remote sensing is performed by ground-based, airborne, or satellite platforms, or combinations thereof.

Demand for spectrum is growing quickly, spurred by advanced, affordable electronics and mobile wireless technology. The proliferation of wireless technology

has also meant increasing interference to active remote sensing systems, particularly in the L- and C-bands.

Regulators are using reallocation, spectrum sharing, and higher spectral efficiency to try to make the desired spectrum available.

Wireless communication systems have already demonstrated the ability to share the spectrum. While it has not yet occurred, it should be possible for Earth active remote sensing systems to operate in existing communication bands, although limitations exist, to provide scientists with access to improved observations and thus an improved understanding of Earth.

Several letter-designation schemes are in common use for sub-bands within and adjacent to the radio bands and are used in this report: HF band (3-30 MHz); VHF band (30-300 MHz); UHF band (300 MHz to 1 GHz); L-band (1-2 GHz); S-band (2-4 GHz); C-band (4-8 GHz); X-band (8-12 GHz); K_u-band (12-18 GHz); K-band (18-27 GHz); K_a-band (27-40 GHz); V-band (40-75 GHz); W-band (75-110 GHz); and millimeter-wave band (110-300 GHz).¹

THE IMPORTANCE OF ACTIVE REMOTE SENSING

Active remote sensing is a 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 in the near-space environment of Earth. All of these measurements are essential to understanding terrestrial weather, climate change, solid Earth processes, space weather hazards, inventory and tracking of space debris, and threats from asteroids. Active remote sensing measurements are also of great 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.

Specific examples of the importance of different types of active remote sensing measurements to science and society include these:

• Active remote sensing of the atmosphere can save lives and protect property from severe storms, as well as provide a deeper understanding of upper-atmospheric winds and global circulation to scientists.
• Active microwave sensors provide unique measurements of ocean currents, waves, and wind speed and direction and are complementary to passive microwave and visible and infrared (IR) measurements. Applications include global weather prediction, storm and hurricane warning, wave forecasting, coastal storm surge, ship routing, commercial fishing, and coastal currents and wave monitoring. These applications are vital to the interests of the United States.

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¹ The committee has used the IEEE Standard Letter Designations for Radar-Frequency Bands.

• Spaceborne, airborne, and surface-operated active remote sensing measurements with radar, including real-aperture radars, synthetic-aperture radars (SARs), and scatterometers, provide essential data over land-use and ice critical to our understanding of global change and environmental management and weather forecasting, all of which are important for a wide range of scientific, commercial, and defense applications, such as urban planning, agriculture, forestry, water management, topography, sea ice mapping, earthquake and volcano studies, post-disaster assessment, and spatial intelligence.
• Active remote sensing of the ionosphere is essential for understanding the basic physics of this near-space region but also for predicting the impacts of space weather events on our electricity-dependent society.
• Active remote sensing through planetary radar astronomy continues to make important contributions to our understanding of the solar system, planning for space missions to extraterrestrial objects, and in particular for the tracking and characterization of near-Earth asteroids that may pose a threat to society.

However, many of these benefits are not easy to fully internalize in a market system, so the value of active sensing is very difficult to compare with commercial systems. For example, benefits from advances in weather prediction might be hard to internalize such that private entities would not invest sufficiently in the prediction systems. Also, basic research such as this develops knowledge, which is a public good, and is again hard to fully internalize in a market system. Furthermore, scientific discoveries can lead to many different types of social benefits.

Current and Future Threats to the Effective Use of Spectrum Required for Active Remote Sensing

In all cases the frequencies utilized by active remote sensing are determined by the physics of the phenomena that are being studied. The frequencies were carefully chosen to best reveal the underlying physics, and in most cases considerable expense has been incurred in facilities and technology to operate in the chosen frequency range. For each type of measurement, it may be very difficult to relocate operations into other frequency bands. Thus, given that ongoing active remote sensing measurements are essential to protect the future of society, there must be effective access for these measurements to the required spectrum.

There are primarily two spectrum issues that can impact active science sensors. Like passive sensors, active sensors can experience radio-frequency interference (RFI) from other radio services. Conversely, and unlike passive systems, active

systems also transmit signals and are hence subject to operational restrictions to ensure that they do not interfere with other services. With growing demand for and use of the spectrum growing rapidly, both of these spectrum issues are generating concerns about the successful operation of current and planned active science sensors.

Specific examples of current interference and potential future interference include these:

• In several cases, transmit restrictions imposed on active science sensors have significantly impeded the ability to collect the desired science data (as in the operational restrictions imposed on the European Space Agency’s [ESA’s] BIOMASS mission), degraded the science data (an example being the deep spectral waveform notches required on the GeoSAR sensor flown on aircraft), or significantly driven up costs (as for the NASA Soil Moisture Active-Passive mission). Conservative interference standards in some bands can make science operations difficult. Restrictions imposed in the lower frequency UHF and L-bands are increasing with time. (Finding 8.2)
• The RFI environment has been observed to be growing worse in some bands. Within the heavily used and well-studied L-band allocation of 1215-1300 MHz, the amount of RFI observed worldwide has steadily increased over time. This trend has been detected by a series of L-band SARs operated by the Japan Aerospace Exploration Agency (JAXA) spanning the years 1992-2011. ESA has reported an increase in RFI at the C-band. (Finding 8.4)
• Established, high-value science radar measurements at the C-band face near-term threats due to the planned expansion of commercial services in the Earth Exploration-Satellite Service-Active spectrum allocation. The proposed 5350-5470 MHz Radio Local Access Network (RLAN) service will severely limit science performed by the ESA’s Sentinel-1 Constellation (Copernicus Program) and the Canadian Radarsat-2 and RadarSat Constellation Mission (RCM) constellations. The broad-band, noise-like nature of RLAN emitters is difficult or impossible to mitigate. (Finding 8.7)
• There are multiple bands allocated to Earth-Exploration Satellite Service-Active where only portions of the band are being used or that are not being used at all. With constant pressure to accommodate new services, it may be more difficult to establish new science in these bands in the future. (Finding 8.10)
• For nearly four decades ocean-sensing radar systems have coexisted with the global communications and radar infrastructure. Recently there have been more instances of RFI to active sensors from communications and navigation systems at the C-band and lower frequencies. So far the impact

• of degraded sensor performance or loss of data has been manageable over most regions of the world through the application of aggressive RFI mitigation techniques. (Chapter 3 “Findings and Recommendations”)

• RFI has not been a significant impediment to planetary radar astronomy observations to date. However, as bandwidth requirements increase due to the need to image near-Earth asteroids at high spatial resolution, RFI could pose a significant problem in the future. To facilitate high-spatial-resolution imaging of small near-Earth asteroids, frequency assignments with bandwidths of 60 MHz to 120 MHz are required. The NASA JPL Goldstone radar currently has an assignment of 200 MHz centered at 8.600 GHz. (Finding 6.2)

It should be noted that whereas active science sensors routinely report interference from other non-science sources, science sensors appear to rarely interfere with other services. The only documented instance to come to the attention of the committee of an active science sensor actually interfering with the operations of another service was the radar on the NASA CloudSat mission, which can interfere with radio astronomy measurements (another science service).

One of the reasons for this lack of interference from active remote sensing users is the resistance of communications systems to interference from radar systems with narrow pulse waveforms and low duty cycles, which are typical characteristics of scientific and operational radars.

Current RFI mitigation techniques work best for interfering signals that have sparse spectral or temporal occupancy—for example, signals that are close to being a continuous wave or having short, widely separated pulses. The more that sources, or aggregates of sources, resemble broadband white noise, the more difficult the interference is to mitigate with known techniques. Consequently, active remote sensing is able to share more effectively with some services than with others, depending on the nature of the interfering signal. So far, current RFI mitigation techniques have able to significantly reduce the impact of interference on science in the UHF, L-, and C-bands, and few problems with RFI, generally, have been experienced with the science measurements made at frequencies above the C-band.

It should also be noted that one of the difficulties with characterizing the impact of RFI on active remote sensing space instruments is the incompleteness of information regarding current emitters world-wide, as well as the evolving nature of the RFI environment over time. There is currently a lack of good metrics for quantifying the degradation of science measurements for the full variety of active instrument types (e.g., scatterometers, altimeters, SARs, interferometers, and sounders). This makes it very difficult to accurately quantify how a given active sensor might be impacted by RFI, how the RFI might be mitigated, and how the spectrum might be shared.

Recommendations for the Protection and Effective Use of the Spectrum Required for Active Remote Sensing

The recommendations of how to protect and effectively use the spectrum required for active remote sensing fall into the following categories: (1) actions by the science community; (2) actions by federal agencies; (3) possible actions by the telecommunications industry; (4) opportunities for spectrum sharing; and (5) recommended increases in the spectrum allocated for scientific active remote sensing.

Actions by the Science Community

Merit alone will not assure that the spectrum required is available for the scientific community. Scientific interests must be actively engaged in the spectrum allocation and assignment process to assure that science needs are met. (Finding 7.2) This will require ongoing efforts to ensure active remote sensing is balanced with competing interests in the regulatory processes, and to make more information available about the value of active remote sensing:

• The science community should increase its participation in the International Telecommunications Union (ITU), the National Telecommunications and Information Administration (NTIA), and the Federal Communications Commission (FCC) spectrum management processes. This includes close monitoring of all spectrum management issues to provide early warning for areas of concern. It also requires regular filings in regulatory proceedings and meetings with decision makers. This will build credibility for the science community and ensure a seat at the table for spectrum-related decision making that impacts the science community. (Recommendation 7.1)
• For the spectrum management process to be effective, the science community, NASA, the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation (NSF), and the U.S. Department of Defense (DOD) should also articulate the value of the science-based uses of radiofrequency spectrum. Such value will include both economic values, through enabling commerce or reducing the adverse economic impacts of natural phenomenon, and noneconomic values that come from science research. (Recommendation 7.2)
• The next decadal surveys in solar and space physics and Earth science should address the future spectrum needs for their communities. (Recommendation 7.4)

Actions by Federal Agencies

Actions for federal agencies responsible for supporting the scientific use of active remote sensing, and for overseeing spectrum allocations, include these:

• NASA should lead an effort to significantly improve characterization of the RFI environment that affects active science measurements. This effort should also involve other agencies involved in active remote sensing, including NOAA, NSF, and perhaps DOD, as well as the agencies regulating these activities—the FCC and the NTIA—and include the use of (1) modeling, (2) dedicated ground-based and airborne characterization campaigns, and (3) data mining of currently operating science sensors. To the extent possible, this effort should be a collaborative one with other space and science agencies of the world. (Recommendation 8.1)
• NASA should lead a community effort to construct a set of metrics that relate to the various RFI environments encountered and the associated degradation in science performance for each major class of instruments employed in active remote sensing. (Recommendation 8.2)
• NASA and NSF should conduct a formal survey of the space physics research community to determine future spectrum needs. (Recommendation 5.3)
• NOAA should conduct a full assessment of the recent World Radiocommunication Conference (WRC) 2012 results regarding ground-based high-frequency radars to ensure that the planned build-out needs of the U.S. high-frequency over-the-horizon radar observing system can be adequately met. (Recommendation 3.1)
• Coastal ocean dynamics applications radar (CODAR) would benefit from allocated bandwidths larger than 25 kHz near 4.438-4.488 MHz. The FCC should reinstate an experimental licensing process for CODAR to allow for future engineering research progress and exploratory science advances. (Finding 3.2 and Recommendation 3.2)
• Radar systems meeting specific criteria for pulse repetition, maximum pulse width, and duty cycle should be permitted by the FCC or the NTIA to operate as secondary users in communications bands, where minimal interference to the communications operations would be expected to occur. (Recommendation 9.3)
• The Office of Science and Technology Policy should adjudicate the possibility of time and frequency sharing between ESA BIOMASS and DOD’s Space Object Tracking Radar (SOTR) system. (Recommendation 4.1)
• Given the importance of the educational CubeSat program for the development of the aerospace workforce, and for the development of small satellite

Possible Actions by the Telecommunications Industry

There are certain actions the telecommunications industry should consider, for their own benefit and for the benefit of active remote sensing users:

• The use of millimeter-wave frequencies for short-wave femtocell-sized² communications would significantly increase network capacity by an order of magnitude, thereby reducing pressure on the spectrum and therefore on the active remote sensing users, as well. (Finding 9.3)
• The wireless industry should consider pursuing the femtocell approach by developing towers, networks, and the like to add the use of millimeter-wave frequencies for communications in 6G and up communication standards. (Recommendation 9.2)

Opportunities for Spectrum Sharing

There are actions by the scientific community that would facilitate spectrum sharing:

• From the perspective of efficient spectrum usage, the active sensing community would benefit from the consolidation of the L-, C-, and S-band radar assets of NOAA and the FAA to a single multi-function radar at the S-band, as proposed by the Multifunction Phased Array Radar program. (Finding 9.1)
• The committee recommends an investigation of spatial frequency re-use techniques (e.g., 7-to-1 spatial frequency saving) to reduce the total S-band spectrum requirements. The existing L-band spectrum should be maintained for Earth imaging radar use. (Recommendation 9.1)

Recommended Increases in the Spectrum Allocated for Scientific Active Remote Sensing

Some modest increases in spectrum allocations for scientific active remote sensing would be highly beneficial:

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² Femtocells are discussed in Chapter 9.

• The FCC and the NTIA should support access to the frequency bands that best support the extraction of ocean-related information from ocean science remote sensing observations. (Recommendation 3.3)
• Coastal ocean dynamics applications radar (CODAR) would benefit from allocated bandwidths larger than 25 kHz near 4.438-4.488 MHz. The FCC should reinstate an experimental licensing process for CODAR to allow for future engineering research advances and exploratory science advances. (Finding 3.2 and Recommendation 3.2)
• If deemed worthwhile by the astronomy community, and if the NSF considers it appropriate, NSF should seek frequency assignments in the relevant bands for the proposed Green Bank and upgraded Arecibo radar systems to facilitate high-spatial-resolution imaging of small near-Earth asteroids. (Recommendation 6.1)