1、 Rec. ITU-R RA.1750 1 RECOMMENDATION ITU-R RA.1750 Mutual planning between the Earth exploration-satellite service (active) and the radio astronomy service in the 94 GHz and 130 GHz bands (2006) Scope This Recommendation describes measures to be taken by the Earth exploration-satellite service (EESS
2、) (active) and the radio astronomy service (RAS) to minimize the potential impact of 94 GHz and 130 GHz EESS (active) cloud-mapping radars upon RAS observations in adjacent bands. The ITU Radiocommunications Assembly, considering a) that current and future satellite-borne cloud radar mapping experim
3、ents of the Earth exploration-satellite service (EESS) (active) in the 94 and 130 GHz bands shared with the radio astronomy service (RAS) are expected to return important scientific results on global climate; b) that the RAS is expected to continue studying important scientific questions in the 94 a
4、nd 130 GHz bands shared with EESS (active); c) that at millimetre wavelengths, the directive antenna gain available both on a satellite and at RAS ground stations is very high, creating the possibility of very strong main beam-to-main beam coupling between a satellite transmitter antenna and an RAS
5、antenna; d) that in order to obtain adequate radar echoes from atmospheric phenomena, orbiting radars of the EESS (active) require very high EIRP that has the potential to cause physical damage to the sensitive RAS receivers in the case of main beam-to-beam coupling; e) that individual RAS instrumen
6、ts may consist of dozens or even hundreds of co-directed antennas, some or all of which may be co-located within the main beam of an EESS (active) satellite on an instantaneous basis, greatly multiplying the consequences of a main beam-to-main beam encounter; f) that at millimetre wavelengths, curre
7、nt technology does not permit the construction of high performance stop band filters with sufficiently low insertion loss within the wanted passband; g) that receivers used by the RAS at millimetre wavelengths must employ state-of-the-art technology in order to be sufficiently sensitive to carry out
8、 original astronomical research and that such technology currently allows very limited dynamic range with a relatively low saturation threshold; h) that because of the expected high EIRP of the cloud radar, main beam-to-sidelobe coupling between the satellite transmitter and the RAS station may caus
9、e saturation of the RAS receiver, potentially preventing observations at an RAS station for a significant fraction of the time that the active cloud radar satellite is above the local horizon; j) that current technology now permits RAS stations to be outfitted with multi-element focal plane array re
10、ceiver systems having full main beam sensitivity subtending 1 000 times the angular area of a single pixel receiver, 2 Rec. ITU-R RA.1750 further considering a) that of necessity, millimetre-wave RAS observatories operate at the frequencies shared with EESS (active) only under dry-clear conditions s
11、o that atmospheric attenuation gives no protection to the RAS station from the satellite radar; b) that mutual planning between operators of the EESS (active) and radio astronomers is essential in order to avoid damage to the RAS instrumentation, and in order to maintain the integrity both the RAS a
12、nd the EESS (active) data to the maximum extent possible; c) that the Space Frequency Coordination Group (SFCG) and the Scientific Committee on Frequency Allocations for Radio Astronomy and Space Science (IUCAF) developed a mutual planning procedure1between SFCG member agencies and the radio astrono
13、my observatories for space-borne cloud radars to be operated in the band at 9494.1 GHz, recommends 1 that as early as possible in the design cycle of such an EESS (active) cloud radar system, contact should be established between the EESS (active) designers and operators and with radio astronomy sit
14、es the international organization IUCAF may provide the initial link between the EESS operators and potentially affected radio astronomy observatories; 2 that close contact between radio astronomers and the operators of the EESS (active) system should be maintained throughout the design and operatio
15、nal life-cycles of all systems which are subject to sharing in the 94 and 130 GHz bands such that each party is apprised of pertinent developments within the other; 3 that the design and operation of systems operating in each service should be performed so as to account for sharing to the greatest p
16、racticable extent; 4 that the considerations relevant to sharing given in Annex 1 should be taken into account in the design and operations of such systems; 5 that the example provided in Annex 2 of the impact upon one instrument operating in the radio astronomy service from one satellite operating
17、in the EESS (active) should be taken into account in the design and operation of stations of both services. Annex 1 Considerations relevant to the design and operation of systems intended for sharing between EESS (active) and RAS in the 94 GHz and 130 GHz bands For the EESS (active): An active radar
18、 system should be designed according to best engineering practices to minimize unwanted emissions, and to minimise off-axis emission from the radar antenna into sidelobes. 1See Resolution 24-2 of the SFCG at: http:/www.sfcgonline.org/handbook/res/index.shtml Rec. ITU-R RA.1750 3 So far as is practic
19、able, an EESS (active) system should be designed and operated in such a way as to avoid transmitting through its main beam directly at stations of the RAS. Operators of an EESS (active) system should ensure that all operational help possible be given to RAS stations, such as providing timely orbital
20、 details of the satellite radar. For the RAS: RAS stations should be designed to be able to prevent their antennas from pointing directly at the orbiting radar, by flexible dynamic scheduling of observations or other means. RAS stations should provide the means to protect their receivers from physic
21、al damage if complete avoidance of main beam encounters is impracticable. To the extent reasonably possible, without compromising the capability of the RAS station, RAS receiver systems should be designed to have a high tolerance for damage from received high power transmissions, and to possess as h
22、igh a dynamic range as is feasible. RAS antennas should be designed with the lowest practicable sidelobe levels so as to permit observations to continue while the satellite radar is above the local horizon, although not directing its radar towards the RAS station. RAS data acquisition systems should
23、 be designed to log or flag instances of potential interference from the orbiting radar, based on known RAS and satellite operational parameters. RAS should continue to devote resources to extending the possibilities of real time or post-observation RFI mitigation techniques. Annex 2 An example of c
24、onsiderations relevant to sharing to be taken into account in the design and operations of systems of the RAS and EESS (active) in the 94 GHz band The CloudSat and implications for the Atacama Large Millimeter Array (ALMA) Abstract The Atacama Large Millimeter Array (ALMA) Observatory is under const
25、ruction in northern Chile, and will become the worlds prime observatory at mm and sub-mm wavelengths. CloudSat is a downward-looking 94 GHz satellite-borne radar, due for launch in 2005. The peak EIRP of the radar beam is some 4109W, which is sufficient to damage ALMA receivers on the ground if ALMA
26、 antennas and the orbiting radar ever look directly at each other. Although the likelihood of this happening is very small, ALMA does need to take some operational precautions to avoid receiver damage, and to flag data that will be contaminated by radar interference. 4 Rec. ITU-R RA.1750 The ALMA ra
27、dio astronomy facility ALMA is an international millimetre and sub-millimetre wavelength telescope, being an equal partnership between Europe, North America and Japan, in cooperation with the Republic of Chile. Important points to note about ALMA are: Up to 64 12 m antennas located at an elevation o
28、f 5 000 m in Llano de Chajnantor, Chile. Imaging instrument in all atmospheric windows between 10 mm and 350 microns (31.3 GHz to 950 GHz). Array configurations from approximately 150 m to 10 km. Spatial resolution of 10 milliarcseconds, 10 times better than the Hubble Space Telescope. The ability t
29、o image sources arcminutes to degrees across at one arcsecond resolution. Velocity resolution under 0.05 km/s. Fast and flexible imaging instrument. Largest and most sensitive instrument in the world at millimetre and submillimetre wavelengths. The cloud mapping radar mission CloudSat is the first s
30、paceborne radar to make use of the 94 GHz allocation. It is part of the “A-train” constellation, consisting of five satellites, most of which (apart from CloudSat) are passive sensing satellites. In order of orbital formation, the satellites are Aqua, Cloud Mapping Radar, Calipso, Parasol and Aura.
31、Parasol, Aqua and Aura are already in orbit, with CloudSat itself now to be launched in 2005. All satellites will have similar orbits, arranged specifically to have identical ground tracks, separated by only some seconds of time. Details of the scientific mission are as follows: CloudSat is a satell
32、ite experiment designed to measure the vertical structure of clouds from space. Once launched, CloudSat will orbit in formation as part of a constellation of satellites (the A-Train) that includes NASAs Aqua and Aura satellites, a NASA-CNES lidar satellite (CALIPSO), and a CNES satellite carrying a
33、polarimeter (PARASOL). A unique feature that CloudSat brings to this constellation is the ability to fly a precise orbit enabling the fields of view of the CloudSat radar to be overlapped with the CALIPSO lidar footprint and the other measurements of the constellation. The precision and near simulta
34、neity of this overlap creates a unique multisatellite observing system for studying the atmospheric processes essential to the hydrological cycle. The vertical profiles of cloud properties provided by CloudSat on the global scale fill a critical gap in the investigation of feedback mechanisms linkin
35、g clouds to climate. Measuring these profiles requires a combination of active and passive instruments, and this will be achieved by combining the radar data of CloudSat with data from other active and passive sensors of the constellation. Technical details of CloudSat The CloudSat radar operates at
36、 94.05 GHz, from a sun synchronous polar orbit with a height of 705 km; the ground tracks of the orbit (see below) repeat precisely every 16 days. Peak radar transmitter power is 1 800 W, degrading to 1500 W at the end of its lifetime, into an antenna with 63 dBi gain, giving approximately 4109 W ef
37、fective radiated power (EIRP). Polarization is linear. The radar points along the geodetic nadir within 0.07 degrees. The nominal footprint of the radar beam on the ground is 1.4 km in diameter, but the extent of the footprint to the first nulls in the pattern is 3 km. Pointing knowledge is 0.053 an
38、d pointing control is around 1 km. ALMA, with a diameter of 14 km, subtends an angle of 1.14 degrees as seen from the satellite. The actual ground track may have a cross-track error 10 km, caused by an unpredictable component of atmospheric drag perturbing the orbit. The radar pulse length is approx
39、imately 3.3 s, and the pulse repetition Rec. ITU-R RA.1750 5 frequency (PRF) varies from 3 700 to 4 300 Hz, giving mean radar transmitter power of about 25 W. The CloudSat antenna is an offset parabola and has a beamwidth of 0.108, with a boresight gain of 63 dBi. The distance to first null in the p
40、attern is 0.125. The first major sidelobe, 0.2 degrees from boresight, has a gain of 47 dBi. The antenna gain reaches 0 dBi at about 6.4, and beyond about 11 the antenna response is no greater than about 12 dBi. Because the radar beam is always directed downwards, along the local gravitational vecto
41、r, the only possibility of ALMA antennas looking directly into the radar beam is if they are pointed at the zenith, as the radar flies directly over the site. This is an unlikely occurrence. The CloudSat beam will occasionally be directed away from the nadir, for calibration purposes, but this measu
42、rement will always be carried out over ocean, so should not be an issue for ALMA. CloudSats orbit Because of the nadir-pointing radar beam, only the ground directly below the satellite, the sub-satellite point, is directly illuminated. The orbital period is 98.8 minutes, giving 15 orbits per day, an
43、d the inclination of the orbit is 98.2; this is a sun synchronous orbit, arranged so that the ground tracks repeat exactly every 16 days. At ALMA there will be 5 or 6 orbits per day which rise above the horizon. After the 16-day period, there will have been 233 orbits giving ground tracks covering t
44、he earth regularly every 170 km (at the equator) with of course an equal number of ascending and descending passes. CloudSat will have an orbit similar to the lead satellite of the A-Train, AQUA, which is already in orbit. In the following discussion the AQUA orbital parameters have been used as bei
45、ng representative of the future CloudSat. These AQUA elements are likely to remain representative, but may not correspond to the precise CloudSat orbit. The orbital elements for AQUA (July 26 2004) that is used in the simulations below are: 1 27424U 02022A 04207.86676596 .00000493 00000-0 11941-3 0
46、7961 2 27424 98.2212 147.2739 0001153 89.3378 270.7995 14.57121862118484 Most standard satellite tracking programs expect this format. Derived from this, the orbital period of AQUA is currently 98.83 min and the inclination of the orbit 98.221. The height above ground is approximately 705 km. The fo
47、llowing orbital analyses used a variety of different software. Figure 1 shows a map of the world with Aqua ground tracks plotted after 24 h of orbit. Note that, because of the inclination of the orbit (98.2), no ground tracks reach beyond latitudes 81.8. Both ascending and descending orbital tracks
48、are shown. 6 Rec. ITU-R RA.1750 Note the light circle delineating the typical extent of the terrestrial horizon visible from Aqua. Figure 2 shows a view of South America, with the ALMA coordinates marked, and showing the full set of 16-days of ground track. For these orbital parameters, these repres
49、ent ALL ground tracks from the satellite; the tracks repeat precisely every 16 days. As with Figs. 1 and 3, the ascending (S-N) orbits slant from lower right to upper left, while the descending (N-S) orbits slant from upper right to lower left. Rec. ITU-R RA.1750 7 Subsequent tracks repeat precisely every 16 days. The final CloudSat ground tracks will be similar, but not identical. 8 Rec. ITU-R RA.1750 Figure 3 plots in more detail the ground tracks that pass close to ALMA over any 16-day period. The central red dots mark antenna p
