1、 Rec. ITU-R RS.1744 1 RECOMMENDATION ITU-R RS.1744 Technical and operational characteristics of ground-based meteorological aids systems operating in the frequency range 272-750 THz (Question ITU-R 235/7) (2006) Scope This Recommendation provides the operational and technical characteristics of repr
2、esentative MetAids systems operating in the optical frequency range 272-750 THz. The ITU Radiocommunication Assembly, considering a) that observations in the frequency range 272-750 THz (hereafter referred to as optical) provide data critical to operational meteorology and scientific research of the
3、 atmosphere and climate; b) that the spectrum in the optical frequency range is used for active and passive meteorological sensor systems as well as many other applications; c) that the technology for meteorological sensors using optical spectrum is continuously evolving to provide better accuracy a
4、nd resolution of measurement data; d) that frequencies in the optical frequency range are now being used for data links, range measuring devices, and other active systems on ground-based and space-based platforms, and as these systems are rapidly expanding and increasing in number, the interference
5、between optical meteorological sensors and other optical systems is likely to increase; e) that many applications of active and passive systems operating in the optical range are very similar to those being used at lower frequencies in the electromagnetic spectrum; f) that it is timely to consider t
6、he nature of protective measures and sharing considerations to ensure that ground-based optical meteorological sensors can continue to operate without interference, recommends 1 that operators of meteorological aids operating in the optical frequency range should take into account the possibility of
7、 interference from other optical transmitters in their choices of observatory sites and in the design of sensors; 2 that studies of interference to and from optical meteorological aids systems should take into account the technical and operational parameters provided in Annex 1. 2 Rec. ITU-R RS.1744
8、 Annex 1 1 Introduction Ground-based meteorological sensor systems using spectrum in the optical frequency range are operated typically in the range 272-750 THz by a variety of meteorological services and other organizations interested in meteorological and climate research. This Annex provides the
9、operational and technical characteristics of a representative set of meteorological sensors that transmit and receive signals at optical frequencies. 2 Laser ceilometers 2.1 Ceilometer technical characteristics A ceilometer contains a laser as the transmitting source, and a photodetector for the rec
10、eiver. A laser ceilometer senses and reports cloud levels in the atmosphere by using invisible laser radiation to detect cloud levels. They operate by transmitting a pulse of laser light into the atmosphere and sensing the light return as it is reflected back toward the ceilometer by objects in its
11、path. By timing the interval between the transmission and reception, the height of particles (such as water droplets or ice crystals in clouds) above the ceilometer is calculated and reported to the data collection package. Ceilometers are light detection and ranging (LIDAR) devices. Cloud height de
12、termination is based on electronic interpretation of backscattered returns, based on the LIDAR equation: ThhAcEhPr= e)(2)(20(1) where: Pr(h): instantaneous power received from height h (W) E0: effective pulse energy, compensated for optics attenuation (J) c: speed of light (m/s) A: receiver aperture
13、 (m2) h: origination height of the backscattered return (m) (h): volume backscatter coefficient at height h, the portion of light which is reflected back towards the ceilometer (m1sr1) (sr = steradian) T: atmospheric transmittance which accounts for the transmitted and backscattered power by extinct
14、ion at various heights between transceiver and height of backscatter; equal to 1 in a clear atmosphere (i.e. no attenuation); this term in the LIDAR equation allows for determining which backscattered returns are from cloud interaction and which are from other obstructions such as fog or precipitati
15、on. 2.2 Representative Ceilometer System A System A is capable of measuring cloud heights to approximately 3 700 m. It is employed with other weather monitoring equipment such as visibility, precipitation, and temperature and dew point sensors for support of aviation operations and weather forecast
16、activities. System A determines cloud height by emitting a pulsed laser into the atmosphere and measuring the time required for backscattered returns from particles in the atmosphere, if present, to reach Rec. ITU-R RS.1744 3 an adjacently mounted receiver. A laser pulse of nominal 904 nm wavelength
17、 (331.8 THz) and 150 ns duration is emitted once per measurement cycle. Receiver readings are then processed every 100 ns for 25.4 s to provide 254 stored values for each measurement cycle, representing a 15 m height resolution over 3 850 m. For each cycle, a spatial density profile is obtained for
18、the vertical atmosphere column directly above the ceilometer, from 0 to 3 850 m, which can be interpreted to yield cloud height and cloud layer data. The results of multiple cycles are averaged to minimize the effects of erroneous readings. 2.2.1 Transmitter assembly A Gallium Arsenide (GaAs) laser
19、diode emits 904 nm wavelength pulses at a repetition frequency of between 620 Hz and 1 120 Hz. The exact repetition frequency is processor controlled to yield a constant average power of 5 mW, with a nominal factory setting of 770 Hz. Each laser pulse is emitted with a span of 30. An 11.8 cm effecti
20、ve diameter lens with focal length of 36.7 cm is used to focus the incident beam. Maximum irradiance is 50 W/cm2, as measured with 7 mm diameter aperture. The transmitter assembly contains a light monitor for determination of output laser power and incoming sky light power. A downward pointing photo
21、diode is used to monitor output laser power. Interfering ambient light current, at peak magnitude, is much less than laser pulse current and thus does not affect the laser power derivation. Peak emitted laser power is 40 W. The laser power monitor output signal is input to the main processor board,
22、and used to limit average emitted power to 5 mW. An upward pointing photodiode, with a maximum deflection from vertical of 5.7, is used to monitor incoming light. Its signal is input to the optional solar shutter circuitry, discussed below, and the main processor for monitoring purposes. Sensitivity
23、 of the sky light monitor is approximately 0.4 A/W. Direct sunlight in a clear-atmosphere sky produces approximately 1 200 W/m2, with a typical current of 1.1 mA. A clear blue sky typically yields a sky light monitor current of 10 A; indoor conditions typically yield less then 1 A. Ceilometers of th
24、e design of System A that are installed in tropical regions from 30 N latitude to 30 S latitude are equipped with an optional solar shutter mounted on the transmitter assembly. The shutter protects the transmit laser from damage by direct sunlight. The shutter is set to close over the transmit lens
25、during times when direct sunlight can enter the lens system. Ceilometers equipped with solar shutters are also equipped with tropical receiver assemblies, which have a different filter and mounting block than that installed on the standard receiver assembly. 2.2.2 Receiver assembly An 11.8 cm effect
26、ive diameter lens with 8.4 cm focal length is used to focus backscatter returns from particles in the atmosphere onto a silicon avalanche diode. Sensitivity of the photodiode is temperature-dependant. This is compensated for by temperature-dependant control of a biasing voltage in the receiver circu
27、itry, which is factory-adjusted at room temperature to yield a nominal responsivity of 40 A/W. A 50 nm bandwidth interference filter is mounted on the receiver lens to block out background radiation noise. A special filter is installed on units equipped with the optional solar shutter. 2.3 Represent
28、ative Ceilometer System B The System B ceilometers principles of operation are identical to that of System A, with differences outlined in the following text. System B can be utilized to determine cloud heights and vertical visibilities to 7 300 m, and is capable of detecting three cloud layers simu
29、ltaneously. In addition to cloud layer detection, it can determine the presence of precipitation or other obstructions to vision. 4 Rec. ITU-R RS.1744 2.3.1 Transmitter assembly An Indium Gallium Arsenide (InGaAs) laser diode emits 905 5 nm (331.5 THz) wavelength pulses, with duration of 100 ns and
30、at a repetition frequency of 5.57 kHz. Peak emitted power is 16 W, yielding 8.9 mW average power. 2.3.2 Receiver assembly A 35 nm bandwidth interference filter, centred on 908 nm, is mounted on the System B receiver lens to block out background radiation noise. Responsivity is factory adjusted to 65
31、 A/W at 905 nm. TABLE 1 Ceilometer characteristics Parameter System A System B Transmitter laser and optics Peak power 40 W 10-20 W Duration (50% level) 135 ns (typical) 20-100 ns (typical) Energy (diameter = 118 mm) 6.6 Ws Repetition rate 620-1 120 Hz 5-10 kHz Source Gallium Arsenide (GaAs) Diode I
32、ndium Gallium Arsenide (InGaAs) Diode Wavelength 904 nm 855/905/910 nm at 25C Operating mode Pulsed Pulsed Transmitted pulse energy 6 J 10% 1-2 J 20% Average power 5 mW 5-10 mW (full range measurement) Maximum irradiance 50 W/cm2meas. with 7 mm aperture 170 760 W/cm2meas. with 7 mm aperture Optics s
33、ystem focal length 36.7 cm 35-40 cm Effective lens diameter 11.8 cm 6-15 cm Transmitter beam divergence 2.5 mrad maximum 0.4 - 0.7 mrad Lens transmittance 90% typical 96% typical Window transmittance 97% typical, clean 98% typical, clean Receiver optics Detector Silicon avalanche photodiode Silicon
34、avalanche photodiode Responsivity 40 A/W, at 904 nm 65 A/W, at 905 nm Surface diameter 0.8 mm 0.5 mm Interference filter 940 nm 908 nm typical centre wavelength Filter 50% band pass 880-940 nm typical 35 nm at 880-925 nm typical Filter transmissivity at 904 nm 85% typical, 60% minimum 80% typical, 7
35、0% minimum Focal length 15.0 cm Reception lens effective diameter 11.8 cm Field of view divergence 2.7 mrad 0.66 mrad Lens transmittance 90% typical 96% typical Window transmittance 97% typical, clean 98% typical, clean Rec. ITU-R RS.1744 5 TABLE 1 (end) Parameter System A System B Optical system Le
36、ns distance, transmitter receiver 30.1 cm Laser beam, entering Rx field of view 30 m Laser beam 90% within receiver field of view 300 m Performance measurement range 0 to 3 700 m 0 to 7 300-13 000 m Resolution 15 m 3-15 m Acquisition time 30 s, maximum (for 3 658 m range) 2-120 s System bandwidth (3
37、 dB) 10 MHz at low gain 3 MHz at high gain 3 MHz Tolerance of precipitation To 7.5 mm per hour, range-limited 3 Visibility sensors 3.1 Visibility sensor technical characteristics Visibility sensors are used to provide a means of automatically calculating the current visibility level, as well as an i
38、ndication of current day/night conditions. The conventional meteorological method for measuring visibility is to determine the maximum distance a black target can be seen against the fog/cloud background. Visibility sensors provide automated measurement of visibility. With a visibility sensor, the a
39、mbient meteorological optical range (visibility) is measured using the forward scatter technique. This technique involves transmitting a flash of xenon light through a section of the atmosphere (which scatters the light) and measuring the scattered light level to determine the loss. An extinction co
40、efficient is calculated from the amount of light received from the scattered xenon flash lamp light source. This coefficient is then translated into a value of visibility. The visibility sensor also computes and outputs a day or night indication as derived from an ambient light sensor. 3.2 Represent
41、ative visibility sensor systems The representative visibility sensor is capable of providing an extinction coefficient equivalent to visibilities up to and including 16 km. The day/night assembly indicates day or night condition according to the ambient light level and operates for ambient light lev
42、els up to 540 lux. The day/night sensor indicates day for illumination greater than 32 lux and indicates night for illumination less than 5 lux. The transition from indicating day to indicating night occurs once in the region from 32 to 5 lux (as illumination decreases), while the transition from in
43、dicating night to indicating day occurs once in the region from 5 to 32 lux (as illumination increases). The day/night sensor points in the same direction as the receiver. The visibility sensor has either one or two EMI filters (based on model number of unit) that is/are located in the electronics e
44、nclosure. 3.2.1 Transmitter assembly The transmitter assembly flashes a xenon bulb to produce visible light for scattering. Light is focused into the scatter volume by a fixed lens included with the transmitter assembly. 6 Rec. ITU-R RS.1744 3.2.2 Receiver assembly The receiver assembly detects the
45、transmitted xenon light after it is scattered by the atmosphere. The detector is a positive-intrinsic-negative (PIN) photodiode mounted in the receiver canister. Light is focused onto the diode by a fixed lens included with the receiver assembly. The photodiode converts the light energy into an elec
46、trical current for signal processing. The day/night assembly is a photometer that detects light via a photodiode mounted behind a clear window. The photodiode is positioned such that its field of view is 6 above the horizon. TABLE 2 Visibility sensor characteristics Parameter System A System B Sourc
47、e Xenon flash lamp Infrared LED Wavelength 400-1 100 nm 400-1 100 nm Pulse repetition rate 0.1-1 Hz 1 Hz Receiver sensor PIN photodiode Silicon photodiode Principal viewing direction Horizontal 20 below horizon Field of view 6 above the horizon 9 mrad Receiver bandwidth 400-700 nm 400-700 nm Optical
48、 sensor damage level Greater than direct sunlight Greater than direct sunlight Sensor visibility measurement range Up to 16 km Up to 75 km 4 Precipitation sensors 4.1 Technical characteristics Precipitation sensors, also known as forward scatter sensors, are employed to provide assessment of both pr
49、ecipitation occurrence (true or false) and, if present, the characteristics of that precipitation (rain, snow, etc.). They can also be used for measurement of visibility. Methods to measure precipitation parameters have focused on using optical and microwave technologies. Categorically, the measured parameters can be scaled based on the attenuation (or extinction), scattering, Doppler, or scintillation of energy sources from transmitter to receiver. The precipitation sensors outlined here take advantage of the scattering effect that occurs when an i
