1、 Report ITU-R RS.2184(10/2010)Arrival time difference lightning detection systems in the meteorological aidsservice in operation below 20 kHzRS SeriesRemote sensing systemsii Rep. ITU-R RS.2184 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and eco
2、nomical use of the radio-frequency spectrum by all radiocommunication services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted. The regulatory and policy functions of the Radiocommunication Sector are performed b
3、y World and Regional Radiocommunication Conferences and Radiocommunication Assemblies supported by Study Groups. Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of Resolution ITU-R 1. Forms to be u
4、sed for the submission of patent statements and licensing declarations by patent holders are available from http:/www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found. S
5、eries of ITU-R Reports (Also available online at http:/www.itu.int/publ/R-REP/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F Fixed service M Mobile, radiodeterminati
6、on, amateur and related satellite services P Radiowave propagation RA Radio astronomy RS Remote sensing systems S Fixed-satellite service SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service systems SM Spectrum management Note: This IT
7、U-R Report was approved in English by the Study Group under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2011 ITU 2011 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without written permission of ITU. Rep. ITU-R RS.2184 1
8、REPORT ITU-R RS.2184 Arrival time difference lightning detection systems in the meteorological aids service in operation below 20 kHz (2010) TABLE OF CONTENTS Page 1 Summary . 2 2 Introduction 2 3 ATD network basics . 2 3.1 ATD network service area 4 3.2 ATD network detection output data . 4 3.3 Lig
9、htning detection method basics . 5 4 ATD outstations 6 4.1 Outstation configuration . 6 4.2 Sensor unit 6 4.3 Outstation functions 8 5 Flash location processor functions . 9 5.1 Overview of wave form processing 9 5.2 Wave form extraction . 9 5.3 Selection of reference station 10 5.4 Spectral correct
10、ions 10 5.5 Time difference extraction 11 5.6 Principles of flash location . 12 6 Characterization of interference . 13 6.1 Impact of interference . 14 6.2 Examples of interference received at various ATD network outstations are shown in Figs 9 and 10 . 14 6.3 An example of intermittent interference
11、 emanating from Norway and received at Lerwick, Keflavik and Camborne . 15 7 Interference mitigation . 16 7.1 Limits to usefulness of notch filters 16 2 Rep. ITU-R RS.2184 1 Summary This Report describes the general operational and technical characteristics of the arrival time difference (ATD) syste
12、m of the meteorological aids services operating in the frequency range below 20 kHz. 2 Introduction This ITU-R Report characterizes the technical properties and operational characteristics of the ATD system operating in the meteorological aids service in the frequency range below 20 kHz. 3 ATD netwo
13、rk basics The purpose of this section is to address the use by the meteorological aids services of frequencies below 20 kHz. Long-range lightning detection using observations near 10 kHz has been performed since 1939, originally with a very manpower-intensive system measuring the direction from whic
14、h signals were received, but since 1987 detection has been carried out with an automated Arrival time difference system (ATD) using the time differences of signals received to derive strike locations. The ATD system utilizes a network of “detector” out-stations to monitor spectral emissions of cloud
15、 to ground lightning strikes centred between about 5 and 20 kHz. At these frequencies the sky waves, reflected off the ionosphere, propagate for very large distances with relatively little attenuation and are preceded by a ground wave at shorter ranges. Thus, it is possible to receive the emissions
16、from the cloud to ground strokes at thousands of kilometres from the stroke location. A distributed network of ground based sensors can locate the origin of the lightning stroke, using the time differences between the arrivals of the lightning emission at the individual sensor sites. At June 2010, t
17、he network comprised 11 sensors distributed across Europe from Iceland to Cyprus, operating in collaboration with Finland, France, Germany, Iceland, Ireland, Portugal and Switzerland. The network configuration is shown in Fig. 1. A further sensor has been placed in La Reunion (in the Indian Ocean to
18、 the east of Madagascar) to evaluate improvements in location for Africa, but this is not yet processed as part of the operational system. In the immediate future new sensors are to be installed in Croatia, at some sites in Africa, South America, the Middle East and in western Asia, and in the long
19、term there should be opportunities to extend the system to provide global coverage. This and other intended sensor locations are shown in Fig. 2. Rep. ITU-R RS.2184 3 FIGURE 1 The current ATD system sensor network 30W 30E15W 0 15E30N40N50N60N70NKeflavikLerwickValentiaCamborneGibraltarAzoresExeterNor
20、derneyHelsinkiAkrotiriPayerneFIGURE 2 New and proposed locations of ATD system 20W 60E60W 20E40S20S020N40N100100W60N80NPossible ATDNET sensor locations in 2011,superimposed on ATDNET Climatology, number of flashes per 1 latitude and longitude box for January 20084 Rep. ITU-R RS.2184 3.1 ATD network
21、service area The ATD network can provide a 24-hour thunderstorm detection service over the areas shown in Fig. 3. The area of coverage is governed by the properties of sferic1propagation, with least loss of sensitivity with distance travelled occurring along sea tracks. In South America, Central Ame
22、rica the Caribbean and North and West Africa the system provides useful thunderstorm detection throughout the day. South Africa is well outside of the Service Area covered throughout the day, but work is under way in extending the ATD service area to cover the whole of Africa. FIGURE 3 ATD system ne
23、twork coverage 20W 100E60W 20E40S20S020N40N100W60N80N60EOctober 2007Dashed line October, November, December 2007Solid line, June, July, August 2007Estimated area of useful detection for ATDNET compared to LIS satellite climatologysuperimposed on ATDNET lightning climatology for October 20073.2 ATD n
24、etwork detection output data The typical output from the system is illustrated in Fig. 4, where the lightning locations have been detected at a time of year when there are few thunderstorms in Europe, with intense activity in Central Africa, the Caribbean and parts of South America. 1Sferic: A light
25、ning generated electromagnetic signal (abbreviation for radio-atmospheric). Rep. ITU-R RS.2184 5 FIGURE 4 Example of two hours lightning detection output around the world from a long-range lightning detection system based in Europe, the numbers on the left show the number of lightning strokes detect
26、ed in each 5 min, sensors operating at 13.733 kHz Now, with modern sensor systems and communications it is possible to monitor the radio frequency received at the sensor in real time. The optimal frequency for ATD measurements is around 9.76 kHz. Recent monitoring (2004) showed that it was not pract
27、ical to use this frequency at all the sensor sites (particularly at Gibraltar and at Reunion in the Indian Ocean), because of powerful transmissions near 10 kHz in some locations. This adversely impacted the performance and accuracy of the system. Due to this, the system now operates at 13.733 kHz a
28、s noted in Fig. 4. Note that, before this time, the lightning detection systems have coexisted with the existing services operating below 20 kHz without any problems. The data provided by the ATD system is used by meteorological organizations worldwide and contributes towards safety of life, both in
29、 terms of forecasting for public safety and safety in forecasting aviation operations, especially over the oceans, and large areas of land, where national lightning detection systems do not exist. As well as the dangers of the lightning strike itself, thunderstorms can result in intense precipitatio
30、n with consequent flooding, severe icing, wind shear, turbulence and gusting winds. Additionally it has the potential to give a service across Africa in support of disaster risk reduction initiatives. 3.3 Lightning detection method basics The system uses arrival time difference techniques for lightn
31、ing detection by measuring differences between sferic arrival times of VLF signals emitted by lightning strikes at network outstations. Spectral emissions from lightning strikes propagate within the VLF earth-ionosphere wave guide at ranges of up to many thousands of kilometres from the strike locat
32、ion. The ATD system is entirely passive and measures the vertical component of the VLF signal, which enables lightning strikes to be fixed at ranges of thousands of km. The horizontal ground component could be used to fix strikes up to a few hundreds of km, up to maximum of about 400 km. There is no
33、t much of a seasonal or diurnal propagation effect. The long-range propagation of the vertical component is hardly affected, but detection capabilities at shorter ranges can be somewhat less at 6 Rep. ITU-R RS.2184 night because of interference between the various modes of the sky waves. Propagation
34、 characteristics of the VLF lightning emission mean that the best frequencies for long-range detection are below 20 kHz. This is the frequency range with highest emission from cloud to ground lightning strokes and where the propagation characteristics become increasingly favourable. 4 ATD outstation
35、s The purpose of this section is to address the characteristics and processes in use of outstations within the ATD Network. 4.1 Outstation configuration The constituent items shown in Fig. 5 are described in 4.2 to 4.2.3. FIGURE 5 Outstation configuration ATD S F ER I C SinterfaceunitSuppressorjunct
36、ion box(SJB)GPS /IP SENSOR /IP SensorGPS antennaAnalogue dataSerial data (GPS)DELL PCSensorrodMonitordisplayCommunications linkNI PCI-6602timer card(1 Hz and 10 MHz)NI PCI-4451signal acq.card(sensor / )IP UPSMains powerDigital dataSTARLOC IIGPS andrubidiumunitExternal InternalStandard DellNIC with a
37、nRJ45 outputRouterCISCO 877ADSL enabled telephone lineKeyboardand mouse4.2 Sensor unit An outdoor-mounted whip antenna receives radio waves over a wide frequency range, up to several hundred kHz. The electric field variations are capacitively coupled to the sensor unit and converted to a differentia
38、l voltage. The sensor unit has a capacitively coupled essentially flat frequency response to signals over the frequency range 40 Hz-400 kHz. 4.2.1 Starloc II GPS and rubidium unit The Starloc unit uses a high precision rubidium oscillator calibrated from GPS time pulses to provide one pulse per seco
39、nd (1 PPs) and also a phase coherent 10 MHz reference signal. Rep. ITU-R RS.2184 7 4.2.2 Signal acquisition card and counter/timing card The signal acquisition card and counter/timing card includes the following features: simultaneous sampling on two channels; programmable gain, 20 to +60 dB; anti-a
40、lias protection; flat group delay. At present two input channels are connected to the same ATD sensor. One channel is operated with a higher gain than the other in order to maximize the dynamic range of the system. The software normally selects data from the high gain channel, but switches to the lo
41、w gain channel if the former is overloaded. The sampling rate is set to 109.864 kHz. Each ADC channel has a 16-bit A to D converter. Given that a signal of only 6 bits will still yield an acceptable waveform, the dynamic range is 210in amplitude, or 60 dB. By running one channel at a gain level 10 o
42、r 20 dB higher than the other the effective dynamic range of the system can be increased to 70-80 dB. This should allow both near and distant flashes to be reported with no change in hardware gain settings. In practice, overlap of sferic waveforms could become a problem if there is intense local act
43、ivity. The channels have built-in overload protection. A hardware gain setting of 10 dB in the high-gain channel is recommended; this gives a maximum signal of 3.2 V and allows the receiver to see sensor noise adequately. 0 dB in the low-gain setting gives a maximum of 10 V. 4.2.3 Summary of technic
44、al characteristics of ATD stations Technical characteristics of ATD BBB is presented in Table 1. TABLE 1 Technical characteristics of the ATD system Receiver (sensor unit) amplifier gain 12 dB if switched on by control software (normally the case) otherwise zero Centre frequency 13.733 kHz(1)Measure
45、ment bandwidth 3 kHz Total passband 6.87 to 20.6 kHz Antenna type and directivity V-pol, omnidirectional whip Software filter Broad-band high-pass filter (3 dB at 2.0 kHz), cascaded with low-pass filter (0.28 dB pass-band limit at 17.75 kHz) Software narrow-band pass filter 3 dB bandwidth 2.5 kHz 10
46、 dB bandwidth 4.3 kHz 20 dB bandwidth is 5.7 kHz Typical receiver noise floor(2)70.4 dBm/5 kHz Signal-to-noise ratio (S/N) Values of 2: TDV = Epoch variance + (100 s)2If ASNR 5: TDV = TDV + (500 s)2The 8 s is supposed to represent the combination of encoding/digitization errors and propagation effec
47、ts. The above replaces a rather complicated formula in the original ATD specification. For illustration purposes a diagram showing the mean absolute signal-to-noise ratio (ASN) with distance from lightning fix location during the day and night is shown in Fig. 7. FIGURE 7 Mean absolute signal-to-noi
48、se ratio (ASN) with distance from lightning fix location during the day and night for signals received at Norderney 500001 00246812101 500 2 000 2 500 3 000 3 500 4 000Distance from goodlog fix to Norderney (km)1416MeanNorderneygoodlogASN08-17UT21-02UT5.6 Principles of flash location A geometrical m
49、ethod is used first to get an approximate fix. We need to find the flash location that finds the best fit between the theoretical time differences, Tthand the measured time differences Tm. In other words, minimize: ()=Mi iimthTT122where there are M time differences, with variances i2. In theory this is a straightforward task: we use a suitable minimizer, combined with a routine for evaluating theoretical time differences, and expect that the final solution should not depend on the first guess position. Rep. ITU-R
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