1、 Rec. ITU-R RA.1750-0 1 RECOMMENDATION ITU-R RA.1750-0* 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
2、(EESS) (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 ex
3、periments of the Earth exploration-satellite service (EESS) (active) in the 94 GHz 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
4、 the 94 and 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 an
5、d an RAS antenna; d) that in order to obtain adequate radar echoes from atmospheric phenomena, orbiting radars of the EESS (active) require very high e.i.r.p. 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
6、RAS instruments 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 wavel
7、engths, current 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
8、 to carry out 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 e.i.r.p. of the cloud radar, main beam-to-sidelobe coupling between the satellite transmitter and the RAS
9、 station may cause 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; * Radiocommunication Study Group 7 made editorial amendments to this Recommendation in the
10、 year 2017 in accordance with Resolution ITU-R 1. 2 Rec. ITU-R RA.1750-0 j) that current technology now permits RAS stations to be outfitted with multi-element focal plane array receiver systems having full main beam sensitivity subtending 1 000 times the angular area of a single pixel receiver, fur
11、ther considering a) that of necessity, millimetre-wave RAS observatories operate at the frequencies shared with EESS (active) only under dry-clear conditions so that atmospheric attenuation gives no protection to the RAS station from the satellite radar; b) that mutual planning between operators of
12、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 and the EESS (active) data to the maximum extent possible; c) that the Space Frequency Coordination Group (SFCG) and the Scientific Committee
13、on Frequency Allocations for Radio Astronomy and Space Science (IUCAF) developed a mutual planning procedure1 between SFCG member agencies and the radio astronomy 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
14、 cycle of such an EESS (active) cloud radar system, contact should be established between the EESS (active) designers and operators and with radio astronomy sites the international organization IUCAF may provide the initial link between the EESS operators and potentially affected radio astronomy obs
15、ervatories; 2 that close contact between radio astronomers and the operators of the EESS (active) system should be maintained throughout the design and operational life-cycles of all systems which are subject to sharing in the 94 GHz and 130 GHz bands such that each party is apprised of pertinent de
16、velopments 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 practicable extent; 4 that the considerations relevant to sharing given in Annex 1 should be taken into account in the design and operati
17、ons 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 in the EESS (active) should be taken into account in the design and operation of stations of both services. 1 See Resolution 24-2 of the
18、 SFCG at: https:/www.sfcgonline.org/resolutions/RES SFCG 24-2 (94 GHz allocation use).pdf Rec. ITU-R RA.1750-0 3 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
19、radar 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. So far as is practicable, an EESS (active) system should be designed and operated in such a way as to avoid transmitting th
20、rough 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 details of the satellite radar. For the RAS: RAS stations should be designed to be able to prevent the
21、ir 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 physical damage if complete avoidance of main beam encounters is impracticable. To the extent reasonably poss
22、ible, 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 high a dynamic range as is feasible. RAS antennas should be designed with the lowest practicable sidelob
23、e 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 be designed to log or flag instances of potential interference from the orbiting radar, based on known
24、 RAS and satellite operational parameters. RAS should continue to devote resources to extending the possibilities of real time or post-observation RFI mitigation techniques. 4 Rec. ITU-R RA.1750-0 Annex 2 An example of considerations relevant to sharing to be taken into account in the design and ope
25、rations 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 construction in northern Chile, and will become the worlds prime observatory at mm an
26、d sub-mm wavelengths. CloudSat is a downward-looking 94 GHz satellite-borne radar, due for launch in 2005. The peak e.i.r.p. of the radar beam is some 4109 W, which is sufficient to damage ALMA receivers on the ground if ALMA antennas and the orbiting radar ever look directly at each other. Although
27、 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. The ALMA radio astronomy facility ALMA is an international millimetre and sub-millimetre wavelength telesco
28、pe, 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 of 5 000 m in Llano de Chajnantor, Chile. Imaging instrument in all atmospheric windows between 1
29、0 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 to image sources arcminutes to degrees across at one arcsecond resolution. Velocity resolution un
30、der 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 spaceborne radar to make use of the 94 GHz allocation. It is part of the “A-train” constellation,
31、 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. Parasol, Aqua and Aura are already in orbit, with CloudSat itself now to be launched in 2005. Al
32、l 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 satellite experiment designed to measure the vertical structure of clouds from space. Once launched, C
33、loudSat 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 polarimeter (PARASOL). A unique feature that CloudSat brings to this Rec. ITU-R RA.1750-0 5 cons
34、tellation 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 simultaneity of this overlap creates a unique multisatellite observing system fo
35、r 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 linking clouds to climate. Measuring these profiles requires a combination of a
36、ctive 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 94.05 GHz, from a sun synchronous polar orbit with a height of 705 km; t
37、he 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 effective radiated power (e.i.r.p.). Polarization is linear. The radar poin
38、ts along the geodetic nadir within 0.07. 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 and pointing control is around 1 km. ALMA, with a diameter of 14 km, subtends a
39、n angle of 1.14 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 approximately 3.3 s, and the pulse repetition frequency (PRF) varies from 3 700 to 4 300 Hz
40、, 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 pattern is 0.125. The first major sidelobe, 0.2 from boresight, has a gain of 47 dBi. The antenna gain reac
41、hes 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 vector, the only possibility of ALMA antennas looking directly into the radar beam is if they are pointed at the zenith
42、, 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 measurement will always be carried out over ocean, so should not be an issue for ALMA. CloudSats orbit Because of the n
43、adir-pointing radar beam, only the ground directly below the satellite, the sub-satellite point, is directly illuminated. The orbital period is 98.8 min, giving 15 orbits per day, and the inclination of the orbit is 98.2; this is a sun synchronous orbit, arranged so that the ground tracks repeat exa
44、ctly 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 the earth regularly every 170 km (at the equator) with of course an equal number of ascending and descending passes. Cl
45、oudSat 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 being representative of the future CloudSat. These AQUA elements are likely to remain representative, but may not corres
46、pond 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 0.00000493 00000-0 11941-3 0 7961 2 27424 98.2212 147.2739 0001153 89.3378 270.7995 14.57121862118484 6 Rec. ITU-R RA.1750-0 Most standard satell
47、ite 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 following orbital analyses used a variety of different software. Figure 1 shows a map of the wo
48、rld 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 are shown. Note the light circle delineating the typical extent of the terrestrial horizon vi
49、sible 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 represent 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.1750-0 7 Subsequent tracks repeat precisely every 16 days. The final CloudSat ground trac
