1、REPORT ITU-R M.2013 WIND PROFILER RADARS (1997) 1 General subject matter 1.1 Introduction Wind profiler radars are radio systems which can be very helpful in weather forecasting applications. To be able to make use of the benefits of wind profiler radars, suitable radio frequency bands need to be id
2、entified for the accommodation of this type of system. One must note the existence of acoustic wind profilers (Doppler SODARs). They can be used to complement certain wind profiler radar measurements at very low altitudes. However, we stress that Doppler SODARs cannot be used as a substitute for win
3、d profiler radars. On the one hand, radar systems for weather forecasting purposes are to be accommodated in the frequency allocations of the radiolocation service and/or the meteorological aids service. Existing uses in these bands need to be protected and compatibility with the services in the adj
4、acent bands has to be assured. On the other hand, accommodation in the fiequency bands of other radio services could be considered, if this is acceptable fiom a frequency-sharing point of view. For the identification of the various compatibility and/or sharing options, a clear understanding of the c
5、oncept of wind profiler radar systems and their behaviour in the electromagnetic environment is needed. In the following paragraphs the need for wind profiler radars will be touched upon and a general system description will be given including the typical electromagnetic behaviour. 1.2 User requirem
6、ents for wind profiler radar data The development of weather forecasting presently requires frequent, closely spaced and high quality wind data with improved accuracy from near the Earths surface to high in the atmosphere. Wind data based principally on balloon borne instruments, satellite measureme
7、nts and automated aircraft reporting systems are insufficient to satisfj the needs of the increasingly high resolution atmospheric computer models as well as those on man-machine interactive forecasting systems. Without substantial increases in high resolution wind data, the capacity of these new mo
8、dels and interactive systems being deployed later this decade to improve weather forecasts and severe weather warnings will be greatly limited. Planetary numerical models of the atmosphere which produce three to ten day forecasts require upper air data from extensive areas of the globe. Especially i
9、n remote areas, wind profiler radars operating unattended may offer a means of obtaining essential high altitude data for these models from data sparse areas. Numerical models for forecasts fiom 3 to 48 h covering a continent or smaller area require data from a large vertical extent of the atmospher
10、e, typically from 200 m to 18 km, with vertical resolution of approximately 250 m depending on the application. The time resolution presently needed is for hourly data. For very short-term weather forecasting, air pollution monitoring, wind field analyses and forecasts of toxic. plume trajectories r
11、esulting fiom chemical or nuclear incidents, severe weather warnings for aviation, meteorological observations, airport operations and public protection, meteorologists need wind information with a very high temporal and spatial resolution, mainly in the lower atmosphere. The requirements are for co
12、ntinuous data acquisition, between the ground and 5 km, with a desirable resolution sometimes as low as 30 m. Measurements will usually be made in populated areas. STDmITU-R RECMN M-2013-ENGL 1997 4855232 0535482 Lb8 = 2 Rep. ITU-R M.2013 Wind profiler radars also play an important role in experimen
13、tal atmospheric research. Their ability to measure wind with a high temporal and spatial resolution makes them very well suited for the experimental verification of models, for boundary layer research and for the investigation of processes that are important for understanding the atmosphere, includi
14、ng climate evolution. At present meteorological organizations use balloon borne systems to measure profiles of wind, temperature and humidity from the ground to high in the atmosphere. While current wind profiler radars do not operationally measure all of these parameters, they do have several advan
15、tages in comparison to the balloon based systems in meeting the above- mentioned requirements: - they sample winds nearly continuously; - the winds are measured almost directly above the site; - the vertical air velocity can be measured; - they provide the temporal and spatial density soundings need
16、ed to compute derived fields in a much more timely manner; - - the cost per observation is lower; they operate unattended in nearly all weather conditions. In addition, it has been demonstrated that wind profiler radars can be adapted to measure temperature profiles when they are used in conjunction
17、 with a radio-acoustic sounding system (RASS). This opens the possibility to obtain denser and higher quality temperature profiles compared to present measurement techniques such as balloon tracking. No other measurement technique will present comparable advantages in the near future, including sate
18、llite borne sensors. The World Meteorological Organization has expressed the need to operate such radars as a matter of urgency, due to the necessity of better monitoring and forecasting of the Earths atmosphere. A standardization of operating frequency bands is most important for the weather servic
19、es in order to build an operational network in a practical and cost-effective manner. 1.3 System concept of wind profiler radars Wind profiler radars are vertically directed pulsed Doppler radars capable of analysing the back-scattered signals to determine the velocity of air along the beams. By ste
20、ering the beams typically 15“ from zenith, the horizontal and vertical components of the air motion can be obtained. Wind profiler radars depend on signals scattered from gradients in the radio refractive index associated with turbulent eddies with scales of one-half the radar wavelength (Bragg reso
21、nance). Hydrometeor scattering may also contribute or even dominate the returned signals, depending on the radar operating frequency. The goal of detecting the very weak clear air signals dictates the use of long coherent dwell times, low-noise system design, low antenna side lobes, and careful atte
22、ntion to siting, and potential interference. A related development, the RASS provides profiles of temperature, typically with no alteration of the radio emission characteristics of the wind profiler radar. The propagation velocity of a Bragg-matched acoustic signal, which is related to the air tempe
23、rature, is measured by the wind profiler radar using slightly different Doppler processing. The nature of the scattering mechanism requires wind profiler radars to function between 40 and 1400 MHz. As frequency increases over 1300 MHz, performance of the wind profiler radar decreases significantly.
24、The choice of operating frequency is influenced by the required altitude coverage and resolution. 1.4 Radiation aspects of wind profiler radar systems In practice, systems are built for three frequency bands, i.e. around 50 MHz, 400 MHz, and 1 O00 MHz, and these systems typically operate in two mode
25、s (see Note 1) which trade height coverage for resolution. Table 1 lists the range of characteristics of wind profiler radars in these three bands: NOTE 1 - Low mode means shorter pulse, lower altitude; high mode means longer pulse, higher altitude. STD-ITU-R RECMN M-2013-ENGL 1997 W 4855212 0535483
26、 OT4 Rep. ITU-R M.2013 3 Height range (km) Height resolution (m) Antenna type Antenna size (m2) Peak power (kW) Mean power (kW) Necessary bandwidth (MHz) TABLE 1 50 MHz 400 MHz 1-24 0.5-16 150-1 500 150- 1 200 Yagi, coaxial, Yagi, coaxial, co-linear Co-linear, co-linear 2 500-10000 30-1 50 5-60 5-50
27、 0.5-5 0.2-2.0 0.2-2.2 0.3-2.2 Range of operational wind profiler radars characteristics 1 O00 MHZ 0.5-3 30-1 50 dish, patch co-linear 3-15 0.5-5 0.05-0.5 0.7-7.3 1.4.1 Harmonization of operating frequencies Global harmonization of wind profiler radar operational frequencies and identification of sp
28、ectrum by a world radiocommunication conference is most important. This will enable cost-effective development and exploitation of wind profiler radars. Wind profiler radars are operated as pulse-modulated Doppler radars or in frequency modulated-CW- mode. FM CW-mode radars were not considered furth
29、er in this report because of lack of technical standard. Examples of spectrum produced by a pulse-modulated Doppler radar is show in Fig. 1. I I -110 I 1 3 85 3 90 395 400 405 410 415 420 425 Frequency (MHz) Pulse width: i .67 ps Puise repetition frequency: i0 kHz Rap 2013-01 STD-ITU-R RECMN M.20L3-
30、ENGL 3997 4855232 0535484 T30 4 Rep. ITU-R M.2013 Based on discussions with manufacturers, wind profiler radars (see Note 1) can be designed to operate on assigned frequencies in a range up to fl per cent. NOTE 1 - For example, the National Telecommunications and Information Administration (NTIAKJSA
31、) - “Manual of Regulations and Procedures for Federal Radio Frequency Management”. To accomplish this, small compromises would have to be made in the design of the RF power transmitter, antenna, circulator, and receiver noise figure. These would not be expected to degrade performance by more than 1
32、dB, an amount which could easily be compensated by a small increase in radiated power. 1.4.2 Antenna pattern The desired signal being a reflection fiom clear air is very weak. This requires both extreme sensitivity in the wind profiler radar receiver and a vertically directed antenna with low amplit
33、ude side lobes. The antenna pattern, especially at large off-axis angles fiom main beam, is important in analysing potential interference. As measurements for a 400 MHz wind profiler radar in Switzerland have shown, siting the profiler antenna in a sufficiently deep topographical depression can redu
34、ce most low-elevation side lobes by up to 20 dB. Terrain shielding is also effective for 50 MHz wind profiler radars as experience in Japan and Germany has shown. Investigations in Germany show that a specially designed fence around a profiler antenna can improve the low-angle side-lobe suppression
35、by 10 dB to 15 dB. 1.4.3 Polarization The polarization of signals received near the ground fiom a wind profiler radar changes randomly as a result of the scattering process. Similarly, the polarization of signals received directly from a side lobe is also likely to be random. The mean contribution o
36、f polarization decoupling to an improvement of sharing conditions is, therefore, only minimal (e.g. 3 dB). 1.4.4 Occupied bandwidth For the efficient use of the limited radio spectrum resource, all efforts must be made to reduce the occupied bandwidth as well as unwanted emissions to a minimum. The
37、spectrum produced by pulse-modulated emissions is mainly determined by the shape of the pulses, the choice of the transmitter chain and the output filtering employed. Control of the spectrum can be achieved by appropriate pulse shaping, phase modulation and amplifier linearity. Measurements accompli
38、shed in Germany with a 50 MHz wind profiler radar in this respect have shown that pulse shaping reduces the occupied bandwidth by a factor up to five compared to the bandwidth of rectangular pulses. It should be noted that puise shaping produces, as a side effect, a reduction of average transmitter
39、power and a reduction of the effective range. With reference to the 99% bandwidth (see Radio Regulations (RR) No. S 1.153) of the radars in Fig. 1, the spectrum represents good technical achievement at this time. Due to the fact that the bandwidth of a frequency modulated emission largely depends on
40、 the frequency deviation, it should be reduced as far as possible with regard to technical and operationailfunctional aspects of the spectrum. In this study it is assumed that the occupied bandwidth of a wind profiler radar completely falls within a proposed candidate band. 1.5 Sharing consideration
41、s between wind profiler radars and other systems in various services It may be possible to use bands other than those as identified in this report but such use must be preceded by studies which show compatibility. STD-ITU-R RECMN M*2013-ENGL 1997 4855212 0535485 977 m Rep. ITU-R M.2013 5 1.5.1 Land
42、mobile service Sharing between wind profiler radars and the land mobile service is possible with proper fiequency/distance (F/D) separation. Land mobile operations are mostly concentrated in urban areas. In some countries wind profiler radars may be located in remote areas. This will enhance sharing
43、 possibilities except in densely populated countries. Two cases must be considered concerning sharing with wind profiler radars. The two cases are sharing with base stations and sharing with vehicular and portable stations. Sharing with base stations is easier since they operate at known specific lo
44、cations, thus proper distance separation can be maintained. Vehicular and portable stations, however, operate intermittently throughout the entire area covered by the base station. 1.5.2 Aeronautical services In general, sharing with airborne systems requires large F/D separation and as a result sha
45、ring may be difficult. In addition, harmful interference to aircraft must be avoided in bands which provide critical communication such as aeronautical radionavigation. An aircraft flying close to the wind profiler radar may suffer harmful interference and may also cause strong reflections back to t
46、he wind profiler radar that will disrupt wind data for that length of time. Thus, the location of wind profiler radars intended for strictly meteorological use, should avoid the flight paths of aircraft. On the other hand, 1 O00 MHz wind profiler radars might be used in airport areas for measuring w
47、inds. In this case, the benefit of the observation may outweigh the temporary disturbance by aircraft. 1.5.3 Satellite and space services Sharing with satellite and space services requires large angular and frequency separation, and as a result sharing may be difficult. Usually satellite and space s
48、ervices receivers are very sensitive and may experience interference and overload from wind profiler radars depending on the position and antenna pattern. Wind profiler radars are also very sensitive, and may experience interference from satellite and space services. Harmful interference to safety-o
49、f-life satellite operation must be avoided. 1.5.4 Fixed service Sharing between wind profiler radars and the fixed service is possible with proper F/D separation. Fixed systems typically transmit point-to-point or point-to-multipoint over tropospheric propagation paths (including line-of-sight). Some fixed systems employ highly directional antennas. The transmission paths are well defined. Some fixed systems are transportable. 1.5.5 Radio astronomy service Radio astronomy stations are receiving stations that listen for radio waves of cosmic origin. Radio astronomy bands are used internat
