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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses 4 Technical Aspects of Protection for the Scientific Use of the Radio Spectrum As long ago as 1960, the vulnerability of radio astronomy to interference was being documented by the International Radio Consultative Committee (CCIR) of the International Telecommunication Union (ITU).1 The threshold levels of interference to radio astronomy bands are published in Recommendation ITU-R RA.769, “Protection Criteria for Radioastronomical Measurements.”2 With the increasing commercial use of the spectrum, it is ever more important to implement ways to protect radio astronomy and other services from adjacent and neighboring band interference resulting from air-to-ground and space-to-ground transmissions. 4.1 INTERFERENCE DETRIMENTAL TO RADIO ASTRONOMY 4.1.1 Radio Astronomy Signals The threshold levels of interference detrimental to radio astronomy3 given in Recommendation ITU-R RA.769 are specified in both power flux density (pfd) and spectral power flux density (spfd) at the radio telescope site. They are based on a consideration of the effect of interference on measurements of the total power received in a single antenna. Several criteria are basic to this analysis: 1 The CCIR has now been replaced by the ITU Radiocommunication Sector (ITU-R). 2 These thresholds were originally published in CCIR Report 224: Documents of the Xth Plenary Assembly, Geneva, 1963, Vol. IV, p. 331. 3 For background information from technical papers regarding the detection of radio frequency interference, see the following: E.G. Njoku, P. Ashcroft, T.K. Chan, and L. Li, “Global Survey and Statistics of Radio-Frequency Interference in AMSR-E Land Observations,” IEEE Transactions on Geoscience and Remote Sensing, Vol. 43, No. 5, pp. 938-946, 2005; S.W. Ellingson and J.T. Johnson, “A Polarimetric Survey of Radio Frequency Interference in C- and X-Bands in the Continental United States Using WindSat Radiometry,” IEEE Transactions on Geoscience and Remote Sensing, Vol. 44, No. 3, pp. 540-548, March 2006; and J.T. Johnson, A.J. Gasiewski, B. Güner, G.A. Hampson, S.W. Ellingson, R. Krishnamachari, N. Niamsuwan, E. McIntyre, M. Klein, and V.Y. Leuski, “Airborne Radio Frequency Interference Studies at C-band Using a Digital Receiver,” IEEE Transactions on Geoscience and Remote Sensing, Vol. 44, No. 7, Pt. 2, pp. 1974-1985, 2006.
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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses The maximum level of interference that can be tolerated is that which increases the output power of the receiver by 10 percent of the root mean square (rms) noise level at the output after averaging for 2000 seconds. Because of the low level of cosmic signals, radio astronomers almost always work at levels close to the limits set by the system noise. This criterion can be thought of as allowing uncertainties in the data to be increased by 10 percent. Interference that is being considered is received in the antenna sidelobes. It is not realistic to set thresholds for interference received in the main beam of a large radio telescope; such interference occurs only transiently and is dealt with by an editing of the data. A value for the sidelobe gain of 0 dBi (i.e., equal to that of an isotropic radiator) is used in the calculations.4 The corresponding collecting area is λ2/4π, where λ is the wavelength. The choice of the 0 dBi level is discussed in Chapter 4 of the ITU Handbook on Radio Astronomy (2003 edition),5 with respect to the models of sidelobe gain for large antennas that are given in Recommendations ITU-R SA.509, S.580, and S.1428. The signal-to-noise ratio at the output of the receiver is measured after the data values have been averaged for a period of 2000 seconds. This value was typical of a long integration period in the 1960s when these calculations were first made; it continues to be used as a generally representative value. More recently there are observations, such as searches for prebiotic interstellar molecules, that push sensitivity to the limits of what is possible, with integration times on the order of days and even months. For an interfering signal with pfd FH, the interference-to-noise ratio can be calculated using the values of bandwidth, antenna noise temperature, and receiver noise temperature appropriate for each band. By equating the interference-to-noise ratio to 0.1, the threshold value FH is obtained. The corresponding value of the threshold as spfd across the band is SH = FH/Δf, where Δf is the bandwidth. In Tables 1 and 2 of Recommendation ITU-R RA.769, values of FH and SH are given for representative radio astronomy bands across the spectrum for both continuum and spectral line observations. For continuum observations, the bandwidth used is the width of the allocated radio astronomy band; for spectral line (multichannel) observations, a value for the channel bandwidth appropriate for observations within the particular band is used. These values apply to observations that measure the total power received in a single antenna. Threshold values for interferometers and synthesis arrays are somewhat less stringent and are considered in §4.3.5. However, such instruments are not suitable for observations 4 The ability of an antenna/receiver system to detect a signal (or equivalently, the susceptibility of being adversely affected by signals) is dependent on the inherent sensitivity of the receiver, as well as on the direction that the antenna happens to be pointing relative to the transmitter. However, even though the antenna may not be pointing directly toward the transmitter, it nonetheless has some ability to receive signals from essentially all directions, which is roughly equivalent to the reception achieved by a small wire antenna. Such a wire antenna is approximately able to receive signals equally from all directions and is therefore called an isotropic antenna (although a true isotropic antenna is not realizable in practice). Since the reception is approximately the same, the off-axis performance is generally assumed to be equivalent on average. A ratio of 1 converts to 0 dBi, where the “i” refers to the fact that the comparison is being made to an isotropic antenna (see Appendix H for a full discussion of the 0 dBi sidelobe gain). The use of 0 dBi means that the transmitter engineer need not consider the potential impact on a large variety of different radio telescope designs, the calculations are much simplified when gain and pointing direction are removed as variables, and the onus is on the radio astronomy design engineers and observers for dealing with regions of the radio astronomy antenna pattern near the main beam that are above isotropic. In some specific cases, for example, nongeosynchronous satellites, the 0 dBi model may not be adequate. 5 International Telecommunication Union, ITU Handbook on Radio Astronomy, 2nd Ed., Geneva, Switzerland, 2003.
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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses of extended objects, for which total-power observations are essential. Thus, the values in Tables 1 and 2 of Recommendation ITU-R RA.769 are the basic protection criteria for radio astronomy. 4.1.2 Remote Sensing Environmental remote sensing measurements are extremely vulnerable to interference because there is no established way to detect and reject data that are contaminated with low-level interference— that is, interference that cannot be differentiated from signals originating from background thermal emission. The propagation of undetected contaminated data into numerical weather- and climate-prediction models may have a significant destructive impact on the reliability and/or quality of weather forecasting in some cases. In other cases observations may be partially obscured or denied completely owing to strong out-of-band or weak in-band emissions affecting regional or broad-area measurements. Criteria for threshold levels of interference for passive remote sensing are defined by limiting the maximum permissible interference power within a reference bandwidth. This interference level is determined by fixing the unwanted signal level below 20 percent of ΔP = kΔTeB, where ΔTe is the sensitivity of passive radiometric sensors, k is Boltzmann’s constant, and B is the receiver bandwidth. Recommendation ITU-R SA.1028 provides the performance criteria for satellite passive remote sensing, and ITU-R SA.1029.1 provides the interference criteria that are compatible with those performance objectives by defining the maximum permissible interference level within a reference bandwidth which is not necessarily the same as any particular sensor’s bandwidth. The ITU further recommends that in shared frequency bands (except absorption bands), the availability of passive Earth Exploration-Satellite Service (EESS) sensor data shall exceed 95 percent from all locations in the sensor service area in the case where the loss occurs randomly, and that it shall exceed 99 percent from all locations in the case where the loss occurs systematically at the same locations. For three-dimensional measurements of atmospheric temperature of gas concentration or of water-vapor measurements, the availability of data shall exceed 99.99 percent. In bands that are allocated to the EESS and other passive services on an exclusive basis, the Radio Regulations state that “all emissions shall be prohibited (RR 5.340).” See §188.8.131.52, “Passive Sensors,” in this handbook for more information. For active microwave remote sensing, frequency and bandwidth requirements have also been studied in the ITU-R and can be found in Recommendation ITU-R SA.577.4. Performance criteria for active microwave remote sensors have been extensively studied and have been defined in terms of interfering power within a reference bandwidth. These recommendations can be found in ITU-R SA.1166.1. 4.1.3 Out-of-Band and Spurious Signals When considering the regulation of signals that may spill into science service bands, account should be taken of how such signals will appear to the scientific instruments in question. Radio astronomy observations of transient phenomena generally are vulnerable. Ultrahigh-energy cosmic rays can be observed in the very high frequency (VHF) radio band, which can be interfered with by human-made sources such as ocean-wave radars. Giant pulses from pulsars share a similar time-frequency signature with chirp radar.6 Nontransient observations can be interfered with by intermodulation and harmonic 6 See S.W. Ellingson and G.A. Hampson, “Mitigation of Radar Interference in L-Band Radio Astronomy,” Astrophysical Journal Supplement Series, Vol. 147, No. 1, pp. 167-176, July 2003.
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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses products from VHF broadcast signals (those generated from both the transmitting and the receiving end). These interfering signals are routinely detected throughout the L band and appear somewhat like spectral lines. They are not difficult to discriminate against, but their removal is time-consuming and often frustrating. In other words, interference that can be recognized as such can be excised, resulting only in a reduction in efficiency. But most unwanted signals that are not easily recognizable can masquerade as valid scientific data. Because it is not possible for radio astronomy to operate in frequency bands for which there are transmitting antennas within the line of sight, the sharing of primary radio astronomy bands with services using satellite downlink transmissions or aeronautical transmissions is avoided. The most serious cases of interference to radio astronomy during recent years have resulted from transmitters on satellites producing unwanted emissions that fall within radio astronomy bands. An example of interference from a geostationary orbit (GEO) satellite in a band adjacent to a radio astronomy band is provided by a European television broadcast satellite transmitting in the Fixed Satellite Service band 10.7-10.95 GHz. A measured spectrum showed that at 10.7 GHz, the upper edge of a primary radio astronomy band, the spfd from the satellite was approximately 39 dB greater than the corresponding threshold value for continuum observations in Table 1 of Recommendation ITU-R RA.769. The resulting radiation into the 10.6-10.7 GHz radio band makes that band completely unusable for observations by the 100 m radio telescope at Effelsberg, Germany: for further details see Chapter 6 in the ITU Handbook on Radio Astronomy (2003 edition). The Russian Global Navigation Satellite System (GLONASS) radiodetermination system provides an example of interference from non-geostationary orbit satellites (non-GSO), shown in Figure 4.1. The FIGURE 4.1 The effect of radio-frequency interference from an Iridium satellite on a radio astronomical observation, as shown by a comparison of two VLA images of the same OH/IR star at 1612 MHz. In the image on the left, no satellite is present. In the image on the right, an Iridium satellite is approximately 22 degrees from the star. Courtesy of Gregory B. Taylor, University of New Mexico.
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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses modulation scheme used by these satellites (digital phase-shift keying) results in a power spectrum of the form [sin(πx)/πx]2, where x is a measure of the frequency relative to the center frequency of the transmitter. This type of spectrum has extensive sidelobes, which fall off with frequency by only 6 dB per octave of x. For many years these sidelobes have caused serious interference to observations in the 1610.6-1613.8 MHz and 1660-1670 MHz radio astronomy bands. Note that the receivers of GLONASS and similar systems that use this type of modulation have bandwidths that accept only the central maximum of the transmitted spectrum, and the extended sidebands serve no useful purpose. Interference from satellites in GEO presents a special problem, because a constellation of interfering satellites distributed along the orbit could preclude the observation within a band of sky centered on the orbit. The apparent declination of the orbit varies by approximately 10° as seen from observatories at intermediate latitudes in the Northern and Southern Hemispheres of Earth (see Figure 1 in Annex 1 of Recommendation ITU-R RA.517, or see ITU-R RA.612). Thus, the whole sky can be observed if observations can be made to within 5° of the orbit from observatories in both hemispheres. In the sidelobe model in Recommendation ITU-R SA.509, the sidelobe gain at 5° from the main-beam axis is 15 dBi, so values of the detrimental thresholds for such observations are 15 dB lower than those based on a sidelobe gain of 0 dBi, as in the tables in Recommendation ITU-R RA.769. It is desirable that these lower detrimental thresholds be applicable to unwanted emissions from GEO satellites. For further discussion, see ITU-R RA.769 or Chapter 4 of the ITU Handbook on Radio Astronomy (2003 edition). 4.1.4 Occasional Unavoidable Interference Recognizing that interference to radio astronomy is difficult to avoid, the ITU recommends that the fraction of time during which transmissions from any one service into a radio astronomy band exceed the corresponding threshold level (ITU-R RA.769) should not exceed 2 percent. The corresponding fraction of time for the aggregate emission from all services within the radio astronomy band should not exceed 5 percent. These limits are specified in Recommendation ITU-R RA.1513, Annex 1 of which contains a discussion of the basis and application of the limits. 4.2 SEPARATION OF INCOMPATIBLE SERVICES 4.2.1 Geographical Separation The signal levels received by radio astronomers from cosmic objects are generally many tens of decibels weaker than the signal levels usually required for communications, broadcasting, radar, and other transmitting services. An important degree of protection from ground-based transmitters can be obtained by choosing observatory sites in locations of low population density and taking advantage of shielding by mountains or other terrain features. However, at frequencies above about 60 GHz, atmospheric absorption becomes important, and observatories must be located at high elevations. In these cases sites with effective terrain shielding are hard to find. Transmitters on aircraft, spacecraft, balloons, and high-altitude platform stations can remain within the line of sight over long distances. Choice of the site for an observatory is of little help in providing protection against them. It is useful to establish a coordination zone around an observatory for protection against terrestrial transmissions of a particular service. For example, coordination zones are used for cases in which the radio astronomy band is shared with the terrestrial mobile service (see Box 4.1). Such a zone can be defined by the requirement that, at the observatory, the sum total of all transmissions from outside the zone should not exceed a detrimental threshold for the frequency band concerned. Coordination zones
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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses BOX 4.1 Footnote US311 US311 Radio astronomy observations may be made in the bands 1350-1400 MHz, 1718.8-1722.2 MHz, and 4950-4990 MHz on an unprotected basis at the following radio astronomy observatories: Allen Telescope Array, Hat Creek, California: Rectangle between latitudes 40° 00′ N and 42° 00′ N and between longitudes 120° 15′ W and 122° 15′ W NASA Goldstone Deep Space Communications Complex, Goldstone, California: 80 kilometers (50 mile) radius centered on latitude 35° 18′ N, longitude 116° 54′ W National Astronomy and Ionosphere Center, Arecibo, Puerto Rico: Rectangle between latitudes 17° 30′ N and 19° 00′ N and between longitudes 65° 10′ W and 68° 00′ W National Radio Astronomy Observatory, Socorro, New Mexico: Rectangle between latitudes 32° 30′ N and 35° 30′ N between longitudes 106° 00′ W and 109° 00′ W National Radio Astronomy Observatory, Green Bank, West Virginia: Rectangle between latitudes 37° 30′ N and 39° 15′ N and between longitudes 78° 30′ W and 80° 30′ W National Radio Astronomy Observatory Very Long Baseline Array Stations: 80 kilometers (50 mile radius centered on: Latitude (North) Longitude (West) Pie Town, NM 34° 18′ 108° 07′ Kitt Peak, AZ 31° 57′ 111° 37′ Los Alamos, NM 35° 47′ 106° 15′ Fort Davis, TX 30° 38′ 103° 57′ North Liberty, IA 41° 46′ 91° 34′ Brewster, WA 48° 08′ 119° 41′ Owens Valley, CA 37° 14′ 118° 17′ Saint Croix, VI 17° 46′ 64° 35′ Mauna Kea, HI 19° 48′ 155° 27′ Hancock, NH 42° 56′ 71° 59′ Owens Valley Radio Observatory, Big Pine, California: Two contiguous rectangles, one between latitudes 36° 00′ N and 37° 00′ N and between longitudes 117° 40′ W and 118° 30′ W and the second between latitudes 37° 00′ N and 38° 00′ N and between longitudes 118° 00′ W and 118° 50′ W. “In the bands 1350-1400 MHz and 4950-4990 MHz, every practicable effort will be made to avoid the assignment of frequencies to stations in the fixed and mobile services that could interfere with radio astronomy observations within the geographic areas given above. In addition, every practicable effort will be made to avoid assignment of frequencies in these bands to stations in the aeronautical mobile service which operate outside of those geographic areas, but which may cause harmful interference to the listed observatories. Should such assignments result in harmful interference to these observatories, the situation will be remedied to the extent practicable.” SOURCE: Reprinted from FCC, Online Table of Frequency Allocations, 47 C.F.R. § 2.106, revised on November 29, 2006, accessed on December 14, 2006.
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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses can also be effective for protection in cases in which the potentially interfering transmissions are from a service with an allocation close to a radio astronomy band edge. Defining the boundaries of a zone requires knowledge of the radiated powers in the direction of the observatory and a determination of the transmission loss in various directions with respect to the observatory. The transmission loss for propagation over terrain is calculable from terrain profiles. Major observatories usually have gridded elevation data for their surrounding area and software for the calculation of propagation loss. In the case of terrestrial mobile transmitters, it is often useful to use the Monte Carlo method to determine how the flux density at the observatory varies with the size of the coordination zone, using the criteria in Recommendation ITU-R RA.1513 for the percentages of time for which flux density in excess of the detrimental thresholds can be allowed. If it is necessary that a transmitter be sited within the coordination zone, then coordination between the transmitting service and the observatory is required. Protection of the observatory may be possible by various means, such as placing a null in the pattern of the transmitter in the direction of the observatory or, if the allocated transmitter frequency is outside the radio astronomy band, by additional filtering at the transmitter. The calculation of separation distances for services in shared bands and the use of coordination zones are discussed in Annex 1 of Recommendation ITU-R RA.1031 and, for observations above 60 GHz, in ITU-R RA.1272. 4.2.2 Spectral Separation A second strategy for mitigating the detrimental effect of an emission on a passive service is that of spectral separation. In this strategy, the center frequencies of the active-service emissions are moved sufficiently far from the band of interest to the active service so as to reduce detrimental effects to an acceptable level. The required separation depends on the strength and spectrum of the emission as seen by the receiver, as well as on the design of the receiver. All emissions naturally have spectra that extend above and below the carrier frequency. Regardless of how this emission is filtered before transmission, some fraction of this power remains. The total power that is subsequently delivered to a band used by passive services then depends on the transmitting power and the characteristics of the transmitter’s filter. This design of the receiver determines the vulnerability of the receiver to out-of-band emissions through a separate mechanism: compression. Just as in the design of communications receivers, receivers for passive services represent a trade-off between sensitivity and linearity. The extremely weak nature of the signals of interest to scientific users constrains sensitivity requirements, with the result that a low-noise figure is essential. This consideration subsequently limits the type of filtering that can be done at the receiver. For example, any filter preceding the first amplifier must have sufficiently low insertion loss so as not to dominate the noise figure. This constraint on the filter’s insertion loss limits the order of the filter design, which in turn limits the rate of attenuation with increasing frequency separation. Thus, the need for high sensitivity in the passive scientific services limits the amount of suppression that can be achieved for out-of-band signals. Just as in a communications receiver, out-of-band signals stronger than a threshold determined by this suppression cause the receiver to enter a nonlinear mode of operation—that is, compression. At this point, received signals become distorted. In summary, there are two technical considerations in determining the separation between active and passive services required to limit interference to acceptable levels: (1) the actual out-of-band power spectral density delivered to the receiver, and (2) the receiver’s limited ability to maintain linearity in the presence of strong out-of-band emissions, owing to the need to achieve an extremely low noise figure.
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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses There is no substitute for a situation-specific engineering analysis to determine the actual potential for detrimental interference due to these mechanisms; nonetheless, the situation is clearly helped by preferentially avoiding the assignment of bands used by high-power transmitters near the band edges of passive services. 4.3 TECHNOLOGICAL SOLUTIONS In considering interference from transmitters in other bands, definitions of the following terms, which can be found in Appendix A, are important: necessary bandwidth, out-of-band emission (OOBE), spurious emission, unwanted emissions.7 In addition, OOBEs are usually defined as those that fall within ± 250 percent of the necessary bandwidth from the band center. With a few exceptions, spurious emissions are those at frequencies that are separated from the band center by more than ± 250 percent of the necessary bandwidth. Further discussion of these definitions can be found in Chapter 6 of the ITU Handbook on Radio Astronomy (2003 edition). Interference from transmitters in other bands can be generated by several mechanisms: Modulation sidebands can fall within a neighboring radio astronomy band when the transmitted spectrum is not adequately filtered. Two or more strong signals in a system that has a nonlinear response can generate beat frequencies at sums and differences of the frequencies and their harmonics. Harmonics can be generated by nonlinear responses within the transmitter, and filtering of the output to remove such unwanted responses may be absent or inadequate. Limits on spurious emissions are given in Appendix 3 of the ITU Radio Regulations, and also in Recommendation ITU-R SM.329 (spectrum-management series). However, these limits generally fall short of being able to protect radio astronomy to the required levels (Recommendation ITU-R RA.769) by several tens of decibels. Further discussion is given in Chapter 6 of the ITU Handbook on Radio Astronomy (2003 edition), including calculations based on these limits as applied to non-GSO and GEO satellites. 4.3.1 Advanced Modulation The extensive unwanted sidebands generated by types of digital phase-shift keying are mentioned in §4.1.2. One method of control of the sidebands of these and similar types of digital modulation is through the use of modulation techniques that control the rate of change of amplitude or phase at the transitions and thereby greatly reduce the level of unwanted frequency components that cause the sidebands. Several practical modulation systems of this type have been developed—for example, 7 For background information from technical papers regarding the mitigation of radio-frequency interference, see the following: American Geophysical Union, “Mitigation of Radio Frequency Interference in Radio Astronomy,” Radio Science, Vol. 40, No. 5, 2005, available at http://www.agu.org/journals/ss/RADFREQ1/, accessed November 15, 2006; F.H. Briggs and J. Kocz, “Overview of Technical Approaches to Radio Frequency Interference Mitigation,” Radio Science, Vol. 40, No. 5, pp. RS5S02.1-RS5S02.11, 2005; S.W. Ellingson, “RFI Mitigation and the SKA,” Experimental Astronomy, Vol. 17, Nos. 1-3, pp. 261-267, June 2004, reprinted in The Square Kilometre Array: An Engineering Perspective, P.J. Hall (ed.), Springer, Dordrecht, The Netherlands, 2005; and A.J. Boonstra, “Radio Frequency Interference Mitigation in Radio Astronomy,” Netherlands Foundation for Research in Astronomy, Delft, The Netherlands, June 16, 2005.
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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses Gaussian-filtered minimum shift keying. These are capable of reducing the power level in the sidebands by many tens of decibels. 4.3.2 Filtering in Radio Astronomy Receivers As explained in §4.2.2, the ability of passive services to employ filtering of sufficiently high order to mitigate the deleterious effects of emissions outside the radio astronomy band is severely limited by the need to achieve an extremely low noise figure. The requirements for a filter designed to mitigate out-of-band emission effectively must take into account (1) the strength of the interfering signal at the receiver, (2) the total received power level at which the receiver enters a state of compression, and (3) the potential for the generation of harmonic and intermodulation products due to any power remaining after filtering. It is not possible—or reasonable—to assign a single requirement for out-of-band suppression achieved by a filter for this application; situation-specific engineering analysis is required. It should be noted that in certain cases it is possible to do better by employing alternative filtering technologies. For example, it may be more effective to notch specific interferers than to develop filters with broad band-stop characteristics. Also, it is now possible to design filters using cooled or superconducting materials that allow greater out-of-band suppression to be achieved without compromising insertion loss (sensitivity), albeit at greatly increased cost. As stated above, a proper situation-specific engineering analysis is required to determine whether these alternative techniques can provide the necessary attenuation. 4.3.3 Filtering in Transmitters Band-pass filtering in transmitters to confine the transmitted power to the allocated band of the transmitting service can be an effective technique in reducing band-edge interference. In certain cases it introduces technical difficulties. For example, in very high power transmitters, as used in some terrestrial television stations, the power dissipated in an output filter is a serious consideration. In some satellite transmitters, such as those of the Iridium system, the transmitting antenna is a phased array in which each radiating element is driven by a separate power amplifier. Power limitations on satellites necessitate the use of power amplifiers in which linearity is sacrificed to some degree to obtain high efficiency. Since harmonics and intermodulation products can be generated by nonlinearity in the output stages, effective filtering requires a separate filter for each amplifier. The additional weight of such filters can make their implementation impractical. Filtering is more practical in satellites which use, for example, a parabolic reflector antenna in which the filtering is required in only one (or a small number) of signals going to the feed system. 4.3.4 Transmitter Beamshaping In the case of transmitters in fixed terrestrial locations, it is often possible to place a null in the radiation pattern in the direction of a radio astronomy observatory. This technique has been used on numerous occasions to protect radio telescopes at Green Bank, West Virginia, within the National Radio Quiet Zone. In the case of a simple antenna such as a vertical dipole, generating the null basically requires using two such dipoles, fed through a power divider, and phased so that the two signals cancel for propagation in the direction of the observatory.
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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses 4.3.5 Interferometric Excision For a radio telescope that consists of a combination of individual antennas, the response to interference is generally reduced relative to that of a system that measures the total power received in a single antenna. The simplest combination is a two-element interferometer: that is, a system with two spaced antennas connected to a receiving system that produces an output proportional to the voltage product of the two received signals. Synthesis arrays consist of arrays of antennas connected in pairs as interferometers. Two effects reduce the response to interference in interferometric systems. These are related to the fringe oscillations that occur as the relative phase of the signal from the two antennas varies, and, for wideband systems, to the decorrelation that results from the difference in the time delays of the interfering signals in reaching the two antennas. The treatment of interference in these cases is more complicated than for a single antenna. Results for two synthesis arrays and for very long baseline interferometry (VLBI) are plotted as a function of frequency in Recommendation ITU-R RA.769 and in the ITU Handbook on Radio Astronomy (2003 edition, Figure 4.2). These results are expressed in terms of the detrimental threshold levels for interference. The period of the fringe oscillations increases as the baseline (spacing) of the antennas, measured in wavelengths, is increased. The reduction in the interference response becomes effective when the fringe period is similar to or less than the time for which each data point is averaged. The reduction increases with frequency, since the fringe periods decrease. As the interference response is decreased, the corresponding detrimental threshold is increased. For example, for the Very Large Array (VLA) in its longest-spacing configuration, the detrimental threshold at meter wavelengths is approximately 12 dB higher than that for a single antenna, and at 43 GHz it is 35 dB higher. For VLBI, in which the antenna spacings are typically 100 km or more, the reduction of the fringe responses is effectively complete, but other effects of the interference result in detrimental thresholds no more than 40 to 50 dB higher than the corresponding single-antenna values. It is important to note that observations using interferometric techniques are applicable to measurements of discrete sources, and VLBI is only applicable to exceedingly small, bright objects. Total-power measurements using single antennas remain essential for measurements of extended sources and do not enjoy this advantage. Also, the ability of interferometric arrays to discriminate against interference decreases as the antennas become more closely spaced, as required for measurements of extended sources. 4.4 MITIGATION TECHNIQUES FOR PASSIVE REMOTE SENSING Increased occurrences of radio-frequency interference (RFI) have been noted by passive remote sensing in regions of the spectrum that are heavily used by other services. For example, airborne radiometers operating within the EESS-allocated 1400-1427 MHz band have incurred interference from radiolocation services operating adjacent to the band. In the C-band region (5-7 GHz), NASA’s Earth Observing System Advanced Microwave Scanning Radiometer on Aqua is the first spaceborne radiometer with channels near 6.8 GHz since the Nimbus 7 Scanning Multi-channel Microwave Radiometer that stopped operating in September 1987. There is no EESS allocation near 6.8 GHz, and subsequent use of this spectral region, primarily by the fixed service, mobiles service, and fixed-satellite service, in accordance with the national and international alloca-
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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses tions, has caused the band to be unusable for passive microwave measurements over land in much of the world’s developed regions without the application of RFI mitigation. RFI mitigation techniques are being explored for passive radiometric observations in RFI-affected areas. In general, these techniques require costly and extensive modifications to the sensor and retrieval algorithms. 4.4.1 Sub-banding The technique of sub-banding requires the sensor’s receiver passband to be split into multiple subbands. Calibrated brightness temperatures are derived for each sub-band and are intercompared to identify any sub-bands that may have brightness temperatures exceeding a predetermined variance from the group. A logic tree is followed based on the level of agreement in brightness temperatures among the sub-bands. In cases where one sub-band is determined to be an outlier, it can be thrown out and the others averaged to provide an estimate of brightness temperature with only degradation in noise performance (sensitivity). 4.4.2 Digitization and Signal Excision The digitization and signal-excision technique requires the signal passband to be digitally sampled. Initially this procedure was developed to be effective against RFI from pulsed emitters. After digital sampling, the time sequence of samples is checked for large deviations that are characteristic of coherent RFI. If the pulse repetition rate and pulse duration of the interferer are known, the input can be effectively blanked during the pulse period with little impact to the radiometric sensitivity and accuracy. 4.5 GOALS FOR ADDITIONAL PROTECTION 4.5.1 Bandwidths In general, a 1 to 2 percent bandwidth is the minimum practical allocation; a 5 percent bandwidth would be desirable for the continuum bands. This practice is strongly reinforced by the new and rapidly increasing requirements for bandwidth allocation at all frequencies, and it addresses the concerns for both EESS and radio astronomy applications. For example, this requirement can be met by the use of the same fractional bandwidth allocations for spectral lines (such as for complex molecular line studies in the Galaxy and for redshifted lines of distant galaxies) as well as for continuum astronomy, so long as the allocated bands occur reasonably frequently throughout the full spectrum. 4.5.2 Unwanted Emissions The EESS and Radio Astronomy Service communities share mutual concerns regarding unwanted emissions. In particular, passive EESS observations are prone to the same problems that face the radio astronomy community, although the two communities are usually looking in opposite directions (one looking to, the other from, Earth). Strong efforts must be made to protect radio astronomy bands from interference due to air- or space-to-ground transmissions in other bands.8 Passive services are particu- 8 See Appendix A for ITU definitions relating to interference.
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Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses larly sensitive to spurious, out-of-band, and harmonic emissions from other services. A major effort to modernize and upgrade engineering standards for active services is desirable, especially with regard to out-of-band emissions. Modernization of these standards would be useful to other services as well as to radio astronomy. This is particularly the case with airborne and satellite transmitters because of the potential clear line of sight to the radio telescope and devices that do not require licensing. 4.5.3 New Frequencies of Interest In the past 30 years, radio astronomy studies have demonstrated the presence of ever-more-complex molecules in interstellar space. These discoveries have been one of the most fascinating and puzzling developments in the field. The complexity of the largest molecules already exceeds that of simple amino acids. It is anticipated that in the future, still-more-complex molecules, and possibly amino acids, will be found. The identification of complex molecules can be made only by detecting of a number of radio lines. Consequently, observations may be necessary either in or adjacent to bands allocated to other services. If unwanted emissions are minimized, observations adjacent to the bands of other services may be possible.
Representative terms from entire chapter: