TECHNICAL COMMIttEES MICHAEL SPENCER anD FAWWAZ ULABY Spectrum Issues Faced by Active Remote Sensing Radio frequency interference and operational restrictions article [2]. Recognizing that active microwave sensors Message from FARS Technical Committee Chairs also face spectrum-related issues, NASA later commis- Sidharth Misra and Paolo de Matthaeis sioned the NRC to perform a similar study, “A Strategy One of the main objectives of the Frequency Allocations in for Active Remote Sensing Amid Increased Demand Remote Sensing (FARS) Technical Committee (TC) has been to for Radio Spectrum,” which was recently published in inform IEEE Geoscience and Remote Sensing Society members of July 2015 [3]. (In this article, the report will be abbre- the increasing spectrum challenges faced by the remote sensing viated as the NRC Active Sensing Report.) This report community. In the June 2014 issue of IEEE Geoscience and addresses the spectrum issues faced by active science Remote Sensing Magazine, we presented an overview of spec- sensors, primarily radars, and makes recommenda- trum allocations and radio frequency interference management tions to government, industry, and the remote sensing techniques for passive remote sensing. This article, prepared community going forward. The report considers mul- by FARS-TC members, summarizes the impact of interference on tiple types of active sensors including ground-based active remote sensing systems. If you are interested in contribut- operational weather radars, ionospheric sensing radar, ing or learning about issues such as these, please contact the and radar astronomy. This article focuses on spectrum chairs of the FARS-TC. topics related primarily to Earth remote sensing from aircraft and spacecraft. he scientific users of radio frequencies must con- THE USE OF THE RADIO SPECTRUM T tend with the fact that the spectrum is becoming BY ACTIVE SENSORS increasingly crowded, which is in large measure due Active remote sensing—with its unique ability to in- to the advent of advanced affordable electronics and vestigate geophysical phenomena by exploiting the mobile wireless technology. The growing demand for amplitude, range delay, Doppler shift, and phase bandwidth has sparked increased discussions in the changes in the reflected signal—is employed in a va- microwave remote sensing community of how to re- riety of earth science disciplines by a growing num- spond to this crowded spectrum environment and how ber of nations. These disciplines include atmospheric to deal with the consequent issues of radio frequency science, weather prediction, oceanography, climate interference (RFI). The National Research Council studies, cryospheric monitoring, terrestrial ecology, (NRC) published a study in 2010, “Spectrum Manage- hydrology, seismology, as well as disaster assessment ment for Science in the 21st Century” [1], that exam- applications. The choice of frequency for a given me- ined the increasing difficulties encountered by pas- dium to be sensed (i.e., land, water, or atmosphere) is sive microwave measurements in the presence of the dictated by the nature of the wave-medium interac- expanding worldwide commercial and governmen- tion associated with the target as well as the transmis- tal occupancy of the radio spectrum. The challenges sion properties of any intervening medium, such as faced by passive sensors also have been summarized the atmosphere for land remote sensing. Active Earth in a 2014 IEEE Geoscience and Remote Sensing Magazine remote sensing is currently employed at frequencies as low as a few megahertz and as high as hundreds of gigahertz and at many frequencies in between. For Digital Object Identifier 10.1109/MGRS.2016.2517410 Date of publication: 15 April 2016 example, low RF frequencies (i.e., long wavelengths) 40 0274-6638/16©2016IEEE IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE MARCH 2016 HiRes SAR (Dual-Use), Wave Structure, Topography, Land Use, Ice, Wind, Ocean Forest Geology, Soil Surface Motion, Altimetry, Monitoring, Moisture Snow/Ice, Rain Radars, Altimetry (Land and Ice Sounding ... ... ... ... Ice at High Resolution) 0.435 GHz 1.2, 3.2 GHz 8.5, 9.3–10 GHz 36 GHz 78 GHz 30 cm 3 cm 3 mm 0.3 mm 1 GHz 10 GHz 100 GHz 1,000 GHz Geology, Oceans, Ocean, Vegetation, Snow, Precipitation Cloud Profiling Sea Ice, Land Use, Wind, Ice, ... Rain, Wind Radars, ... 94, 134, 238 GHz Topography, Waves, 13.5 GHz 17 GHz 24 GHz Swells, Solid Earth, ... 5 GHz P Band L Band S Band C Band X Band Ku/K/Ka Band Millimeter Submillimeter FIGURE 1. The choice of frequencies for satellite active sensing is dictated by the physics of the relevant scattering mechanism. Representa- tive, but certainly not exhaustive, examples of the types of measurements used at each frequency are shown. [Figure used with permission from “A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum,” courtesy of the European Space Agency (ESA).] are best suited for applications that require good penetra- radiolocation radars. For active systems, it is also impor- tion through ice or vegetation. In contrast, high frequen- tant to differentiate between a spectrum allocation, which cies (i.e., short wavelengths) are needed for the detection is basically the divvying up of the spectrum for different of small microscopic cloud particles (Figure 1). uses, and a spectrum assignment, which is the actual permission to radiate at a specific transmit power in a giv- FREQUENCY ALLOCATIONS en band over a particular region The radio spectrum is used by many types of services, from of the earth. For active sensors, radio and television broadcasting to wireless phone com- having a spectrum allocation RFI refers to the munication; weather, military, and remote sensing radars; may not entitle a sensor to ra- unintended reception and radio and radar astronomy, among many others. Ra- diate if that sensor is thought of A signal transmit- dio regulations and frequency allocations are developed to create harmful interference ted by an unrelated at both national and international levels. At the interna- to other primary users of that tional level, regulations are formulated by the Radiocom- spectral band. source. munication Sector of the International Telecommunica- The following two constraints tions Union (ITU-R). Spectrum allocations for specific that active sensors often encoun- uses are established at the World Radiocommunication ter are associated with the spectrum allocation and assign- Conference, which is held every three to four years. Space- ment process that governs active sensors: based radar remote sensing operates under the Earth Ex- 1) The shared nature of the allocations can produce RFI ploration-Satellite Service (EESS/active), and the associat- that can degrade the performance of science sensors. ed spectrum allocations are shown in Table 1. Within the 2) Active science sensors may be denied an assignment or, United States, spectrum oversight of governmental users otherwise, restricted in their ability to transmit as desired. [such as NASA, the National Oceanic and Atmospheric Ad- ministration (NOAA), and the U.S. Department of Defense RADIO FREQUENCY INTERFERENCE (DoD)] is the responsibility of the National Telecommuni- RFI refers to the unintended reception of a signal transmit- cations and Information Administration (NTIA), whereas ted by an unrelated source. When an active sensor receives oversight of private sector users is provided by the U.S. such a signal, the intended science measurement may be Federal Communications Commission (FCC). It is impor- corrupted. An active sensor may also act as the source of in- tant to note that active sensors typically share allocations terference to a communication system, a passive sensing sys- with other services, such as communication systems and tem, or another radar system, which is discussed in the next MARCH 2016 IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE 41 TABLE 1. The EESS (ACTIVE) FREQUENCY ALLOCATIONS AND SOME SAMPLES OF CURRENT UTILIZATION BY RADAR SENSORS. (From “A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum.”) FREQUENCY BAND AS APPLICATION BANDWIDTHS BAND ALLOCATED IN ARTICLE 5 OF PRECIPITATION CLOUD PROFILE DESIGNATION THE RADIO REGULATIONS SCATTEROMETER ALTIMETER IMAGER RADAR RADAR P band 432–438 MHz 6 MHz L band 1,215–1,300 MHz 5–500 kHz 20–85 MHz S band 3,100–3,300 MHz 200 MHz 20–200 MHz C band 5,250–5,570 MHz 5–500 kHz 320 MHz 20–320 MHz X band 8,550–8,650 MHz 5–500 kHz 100 MHz 20–100 MHz X band 9,300–9,900 MHz 5–500 kHz 300 MHz 20–600 MHz Ku band 13.25–13.75 GHz 5–500 kHz 500 MHz 0.6–14 MHz Ku band 17.2–17.3 GHz 5–500 kHz 0.6–14 MHz K band 24.05–24.25 GHz 0.6–14 MHz Ka band 35.5–36 GHz 5–500 kHz 500 MHz 0.6–14 MHz W band 78–79 GHz 0.3–10 MHz W band 94–94.1 GHz 0.3–10 MHz mm band 133.5–134 GHz 0.3–10 MHz mm band 237.9–238 GHz 0.3–10 MHz grow over time. Examples cited in the report include 700 34 the ground-based European Incoherent Scatter Radar, which ceased operations at 900 MHz after many years 600 33 32 of ionospheric studies due to interference from recently 500 31 deployed telecom services; the interference observed 400 30 by UHF airborne radars (e.g., the Furgro GeoSAR and 29 NASA AirMOSS) due to the plethora of land-mobile 300 28 systems; and the steady increase of global L-band inter- Line Sector 27 200 26 ference observed by Japan’s series of L-band synthetic 100 25 aperture radars (SARs) over the past two decades (the 24 JERS-1 mission from 1992 to 1998, ALOS/PALSAR from 2006 to 2011, and the recently launched ALOS-2). A 1.5 4.5 7.5 –7.5 –4.5 –1.5 10.5 13.5 –13.5 –10.5 typical time spectrogram manifesting RFI at the L band Frequency (MHz) is shown in Figure 2.