Fullscreen HTML5Med Res (??MB)HTML4

Each day of eve.zy year, there are an average 44,000 thunderstorms and 8 million lightning flashes. A big storm will have many thousands of lightning strikes and these can cause great damage. Hence, it is important to be able to plot the course of thunderstorms. Plotting the course of As you read this article, a lightning monitoring system for the states of New South Wales and Victoria is in the process of being set up, in time for the next "lightning season" in the coming summer. Lightning monitoring re- By LEO SIMPSON TrT2 = a • R2 Fig.1: LPATS receivers record the precise time that they detect a lightning strike. When two receivers detect a lightning strike simultaneously, the time difference between the two is zero and the lightning can be assumed to be anywhere on a straight line equidistant from the two receivers. When receiver 1 detects the strike before receiver 2, the strike has occurred on a hyperbola which passes around receiver 1. 12 SrucoN CHIP ceivers placed up to 500km apart will be able to plot each lightning strike, virtually as it happens, with an accuracy of as little as 200 metres. The information provided by the lightning monitoring system will be of great importance to government bodies such as the various electricity commissions, Telecom, state railway authorities and defence establishments, as well oil refineries, large chemical plants and organisers of sporting events where large crowds of people are expected. There is already some monitoring of thunderstorms being done in Australia apart from that provided by weather radar. For example, the Northern Territory Power and Water Authority uses the Stormscope system to monitor the progress of thunderstorms in the Katherine region. If a large storm threatens the area and the 132kV transmission line in particular, additional gas turbines in the Katherine power station are brought on line, so that an interruption to power transmission from Darwin will not cause blackouts . And dui;ing the particularly damaging thunderstorm which hit Sydney last summer, Prospect Electricity (previously known as Prospect County Council) had prior warning of the storm's extent from a direction finding storm location system and thus had alerted all its line crews. Sydney Electricity (previously known as Sydney County Council) did not have this warning and thus did not learn the full extent of the storm damage until many hours later. The system to be installed for monitoring NSW and Victoria is a great deal more involved than the Stormscope system which works in conjunction with weather radar. The problem with weather radar is that while it can give a good indication of cumulo nimbus clouds and heavy rain, it does not detect lightning. Stormscope does, but not with any great accuracy. The new system is known as LPATS, which stands for Lightning Positioning & Tracking System. It is a Time of Arrival (TOA) system whereby the time when a lightning stroke is detected at a number of remote radio receivers is precisely recorded. Then, with the position of each the remote receiver being fixed and known, the position of the lightning strike can be calculated. Basic principle The principle of the Time of Arrival system is illustrated in Fig.1. Here we see two radio receivers which are lo,cated a considerable distance apart which may be up to 500 kilometres. Now consider a lightning strike which is recorded at exactly the same time by the two receivers. A moment's thought will reveal that the strike must have been somewhere on a straight line equidistant between the two receivers. This line is depicted in Fig.1 as (Ti-Tzl = O. Now consider another lightning stroke which is somewhat closer to receiver 1 than to receiver 2. Receiver 1 will detect the stroke at a time before receiver 2. Again, by a similar process of deduction, the lightning stroke must have occurred somewhere along a curved line shown as (T 1 -T 2 ) = a. Other lightning strikes can be shown to have occurred anywhere along a hyperbolic line which circles receiver 1 or receiver 2. Now if we add another receiver as shown in Fig.2, we get more information about the possible location of a lightning stroke. Receivers 1 and 2 give a "time difference line" of T1 -T 2 while receivers 2 and 3 give a time difference line of T 2 -T 3 . Receivers 1 and 3 give a third time difference line. The location where all three intersect is the position of the lightning strike. Or is it? In fact, there are some thunderstorms situations where three receivers are not enough to give a clear result so four receivers is the practical minimum in an LPATS network. And in practice, to give a degree of redundancy, five or six receivers are used. Timing In order to give precise location of lightning strikes, all of the receivers in the system must have the same very precise time reference. In the USA, the LORAN navigational system has been used but now the Navstar Global Positioning Satellite (GPS) system is the preferred reference. LPATS makes use of the civilian access standard positioning service of GPS which has an accuracy of up to 100 metres. This is used to establish the location of the receivers at installation. After that, in normal operation, the GPS signal is used to continually synchronise the 10MHz time clock. The receiver itself uses a simple whip antenna to pick up the lightning signal. An enclosed helix antenna is used to receive the GPS satellite timing signals. The detection receiver has a bandwidth of 2kHz to 500kHz and apparently uses an AM detector although the manufacturers, Atmospheric Research Systems, Inc. are coy about giving any details. However, the detection process is good enough to produce a good approximation of the waveform of the lightning strike. T1-T2 • a Lightning and thunderstorms can do tremendous damage in Australia. With a precise system for plotting thunderstorms and lightning strikes, the hazards can be minimised and any damage more quickly repaired. Fig.2: with three receivers, three "time difference" hyperbolas can be plotted although only two are shown here. 'l\vo hyperbolas will intersect at two points or (rarely) touch at one point. Three hyperbolas are needed to plot a single unambiguous location for every lightning strike. SEPTEMBER1991 13 Fig.3: how lightning is located by time of arrival: (1). The signal will be detected at each receiver at a different time relative to the event, dependent on the distance from the event. (2). Time is measured at each site with a resolution of 100 nanoseconds (±50 nanoseconds). (3). Each receiver has a 10MHz timebase which is typically synchronised 20 times a second from a precise source such as the Navstar GPS satellites. (4). A minimum of three receivers are required for a solution. Achievable accuracy is 1 microsecond and within 200 metres, dependent on the size of the network. This is fed to an 8-bit analog to digital converter with a 200 nanosecond sample rate. The waveform is then stored in memory with 100 microseconds of storage. Two characteristics of the strike waveform are important - the peak current of the strike and its risetime. The exact peak of the waveform is crucial because that is used tu define the time of the strike. If a preset signal threshold was used to define the time of strike, there would be timing errors because of the large range of magnitude of lightning strikes - they can range from a peak current of less than 1000 amps to more than 100,000 amps and they can have a duration of 500 milliseconds. Since the peak of the lightning strike is timed with an accuracy of ±0.1 microseconds, any inaccuracies which could occur due to the differing magnitudes of lightning strikes are eliminated. The digitising and storage process also allows other information such as the stroke polarity and total stroke energy to be determined. All this information about the time of the strike and its amplitude is sent by a serial 14 SILICON CHIP data link such as a phone or radio modem to a central computer which calculates the exact location of the strike. This information is stored for later analysis and is also available for immediate display on area maps by the users of the LPATS service. They can plot the progress of storms as they develop and, with experience, they can predict where they are heading and the likely amount of lightning damage. If necessary, vital equipment can be shut down or otherwise protected, sporting events can be cancelled, crowds evacuated from golf courses and so on. Noise rejection You might wonder how a receiver with a bandwidth of 2kHz to 500kHz would be able to discriminate between local radio interference noise and a lightning strike. After all, a lightning strike which may be 200km or more away from the receiver will not produce a very strong signal. Local radio interference can easily be much stronger. The answer is that the detection receiver really does not have to perform the discrimination process. Why? Because the only naturally occurring electromagnetic event that can be simultaneously detected by four or more LPATS receivers which are many hundreds of kilometres apart is a lightning strike. Hence, if less than four receivers in an LPATS system detect an electromagnetic discharge, it is not recorded as a lightning discharge by the central computer. This method has led to a high degree of detection accuracy. Naturally, each receiver needs to be sited away from strong sources of radio interference but apart from that, the installations are quite uncritical. In fact, if a receiver site does become noisy, its threshold of detection is automatically adjusted, under software ~ontrol. Accuracy In practice, the LPATS system can locate lightning strikes to within 200 metres at the centre of the network (depending on its overall size), ranging out to a kilometre for strikes well outside the region covered by the reC!')ivers. You might wonder if the ac- curacy could be improved, down to say 50 metres or less. In practice, the answer is no. For a start, the Navstar GPS enables positioning only within 100 metres; although the military capability of GPS enables targets to be located to within less than 10 metres! Second, there are inevitable errors, both random and systematic, which add up to give the ultimate positional accuracy for lightning strikes of within 200 metres. But there is a third reason why lightning strikes cannot be located with better accuracy and that has to do with the path of the strike itself. This is usually several thousand metres long and is rarely over a straight vertical path. So while•LPATS could perhaps locate the centre of a discharge to within better than 200 metres, the exact point where it hit the ground would still be unpredictable. In practice, where a lightning strike causes substantial damage, it will usually be fairly easy to locate the exact point, once the LPATS system has done its work. In practice too, the users of the information provided by an LPATS network will know precisely where any damage prone installation is, given the locality of a lightning strike. Suffice to say that the information on lightning strikes and thunderstorms from LPATS is far more precise than from any other lightning detection system previously developed, especially those based on direction finding antennas. A very good example of the efficacy of an LPATS system was given during a thunderstorm in the USA, on 13th June 1991. This took place during the 1991 US Open Golf Tournament at the Hazeltine National Golf Club. A spectator was killed by lightning during this storm and several people were injured. A subsequent inquiry into this tragedy was able to obtain archived data which showed the initial development of the storm, its path and even the strike which killed the player. Had the event's organisers had access to this information during the storm, it is likely that no-one would have been hurt. Acknowledgement Our thanks to Ken Ticehurst of Kattron Pty Ltd, Ourimbah NSW and to Dr Rodney Bent of Atmospheric 40.273N 9'J.675W 132 mi 170° _ 10381 Strokes in 1:07 18:21 USA NATIONAL LIGHTNING DATA - □ 12Clx120 Magnified Magnified - Normal Comm Connected Fig.4: this is a screen display from an LPATS network covering the United States. There is a facility to zoom in on thunderstorms & lightning strikes are plotted virtually as they occur. The system can locate lightning strikes to within 200 metres at the centre of the network, depending on its overall size. 11 5. 0 0 119 .00 12J.00 127 . 00 1J 1 .00 1J5.00 1J9,00 14J.00 147 .00 * -12.00 - 16 . 00 -12.00 Darwin ~1.0~ 1.0 1.0 -20.00 *~ Da~ r w C ::::, Ij:: < ...I -24 . 00 -28 . 00 - J 2.00 *Alice l. 0 ~ ~~rt 1.5 ~ 1.0_1.o -28.00 _J -J6.00 * -20.00 -24.00 1.0 Springs 1.0 L~ -16.00 -]2 .00 Adelaide -]6.00 - 40. 00 ~ ~ ~ ~~ ~ ~ ' - - ' - - ' - ~ - ' - - ~ ~ ~ ' - - ' - - ' - ~ . , . , _ ~ L . . . . L . ~' - ' - - ' - ~ ~ -40. 00 115. 00 11 9. 00 123.00 127 . 00 1]1 . 00 135 . 0 0 139.00 14J.00 147.00 LONGITUDE Fig.5: this is a plot of location accuracy for an LPATS network covering the whole of Australia. In this notional system, lightning receivers are located at Dampier, Darwin, Townsville, Adelaide, Perth and Alice Springs. Note that in spite of the huge distance between the receivers, there is a large area for which lightning strikes could be pinpointed to within a kilometre or better. Research Systems Inc for their assistance in preparing this article. Thanks also to Michael Nott of the Northern Territory Power and Water Authority for information on their Stormscope warning system. SC SEPTEMBER 1991 15