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The system can cope with some 100 strikes a second.
At the peak of a really active storm computing
restrictions mean that the display may be a minute
or so behind the action, but to all intents and
purposes, the system is a real-time display. The
benefits available from having this kind of up-to-the-minute
information can be enormous.
Bespoke software allows stored data to be replayed
at accelerated speed over any chosen time period.
Colour prints of the display map showing all the
strikes within a given period and area can be
produced, as can a list of strike information
within a given period and area.
The display software runs on IBM PC compatible
machines within the Microsoft Windows environment.
The time taken to locate trouble spots following
lightning damage is drastically reduced by knowing
where to look. By archiving the information over
a period of time the effectiveness of the protection
systems may be assessed, leading to better investment
policy decisions.
The program is menu driven. It is controlled using
a 'mouse' to position a cursor on the screen and
select a command or item. Flashes are displayed
within seconds of their occurrence on an outline
map of the United Kingdom and North West coast
of Continental Europe. Pan and zoom of the display
are provided. Overlays indicating the location
of the users plant or other geographic features
may be added by the user. The strike position
indicators are colour coded, representing the
time of the strike. Data on any specific strike
can be extracted by positioning cross hairs over
the strike indicator using the 'mouse' button.
A window opens which shows the details of the
strike including the latitude, longitude and Ordnance
Survey National Grid Reference of the strike position,
the uncertainty in the position, the current of
the strike, the date and time to the nearest hundredth
of a second. In addition a multiplicity counter
is reported for restrikes. A colour printer provides
hard copy of both screen display and details of
strikes within a given time period.
Damage and interference
to earth-based structures is caused by cloud-to-ground
strikes. These make up a third of all lightning
strikes: the remaining two thirds occur within
clouds or between clouds. It is important to differentiate
between these types, since often lightning may
be seen but no damage occurs. The EA Technology
system has been designed to locate only cloud-to-ground
strikes, because its primary purpose is assessing,
locating and predicting damage likely to have
occurred.
EA Technology's system uses radio direction finding
techniques to locate the lightning strikes at
the extra low frequency (ELF) of 1.1kHz with a
bandwidth of 350Hz. At this frequency the earth's
surface and the ionosphere act as conducting shells
separated by an insulating gap of between 50 and
100km.
This is less than half
the wavelength range of 235km to 325km which can
be 'seen' by the EA Technology direction finders.
The ELF waves generated in the gap and propagated
around the world in this waveguide are only the
'ground waves'.
There is no interfering 'sky wave' at the operating
frequencies and the bearings produced are more
accurate than conventional
systems.
Because inter and intra-cloud
strikes mainly produce horizontally polarised
radiation, a further advantage of using this frequency
is that they are not registered by the system
unless they are very close (within 30km) to a
direction finding station.
At each ground station the analogue signals from
the direction finding aerials are amplified, filtered
and converted into a bearing and strength value.
The data is then sent as a digital signal along
permanent land-lines to Capenhurst, where the
strike is logged and its position triangulated.
With data from several base stations to compare,
spurious signals caused by local interference
can be rejected and genuine thunder bolts triangulated
anywhere in mainland Britain; often with an accuracy
of less than 1 kilometre. Each incident is plotted,
and stored along with its time and other data,
on the computer mapping system.
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