Compact system detects lightning

Waltham, MA--There's more to lightning than meets the eye. Although cloud-to-ground lightning puts on quite a show, even in broad daylight, you typically can't see intracloud lightning during the day. That's important, because the number of intracloud flashes per minute reflects storm intensity, and intracloud lightning precedes cloud-to-ground lightning.

This system uses optical and electric-field sensors to observe intracloud and cloud-to-ground lightning, determine the intensity of approaching storms, and obtain an approximate measure of the distance to the storm. By doing so, the F-10 AllSky Lightning Detection System permits users to protect themselves, and vulnerable equipment, from lightning strikes.

To detect intracloud and cloud-to-ground lightning optically, the F-10 relies on a sensor that consists of a 45-degree conical mirror, a mirror on an annular disc that faces the conical mirror, and a biased photodiode positioned beneath the center of the annular disc. A hole in the conical mirror allows the diode to look at the sky directly above it. Ralph Markson, president of Airborne Research Associates, which manufactures the F-10, explains that engineers house the mirrors and photodiode inside a plastic cylinder. The clear plastic cylinder permits light to enter and reach the diode.

Basically, this sensor unit serves to gather the maximum possible amount of light and deliver it to the photodiode. When the photodiode generates an output signal with a rise time of 10 to 20 aeseconds, a one-shot latches for 20 mseconds. This action produces an output that can drive a small beeper.

Electric-field sensing enables the F-10 to distinguish between lighting and phenomena like strobe lights and welding arcs. To measure changes in the thunderstorm cloud's electrical field, Markson and his colleagues use a conventional flat-plate antenna. Induction on the antenna produces an input to a charge detector. If that charge exceeds a threshold level, which users can change using a varistor, it produces a usable output signal.

By adjusting the varistor, users effectively change the range at which the system can detect lightning-induced changes in electric field. At its most sensitive setting, the system can detect events 100 miles away; at its least sensitive, it detects lightning three to five miles away. Electric-field monitoring, says Markson, is always sensitive to the location of the flat plate antenna. If users place it under power lines, for example, the antenna cannot detect lightning at all.

In the F-10, signals from the optical detector and flat-plate antenna can both enter a coincidence circuit. If the circuit sees simultaneous inputs, it produces an output. If only one input arrives, the coincidence circuit does not allow an output signal, thus effectively acting as a filter to prevent nuisance alarms.

When users employ the F-10 in this combined detection mode, says Markson, "the F-10 becomes, in a sense, a synthetic weather radar." Each lightning flash generates an output from the electric-field and optical detectors, and from the coincidence circuit. The rate at which the F-10 detects flashes indicates the height of the convective clouds. Cloud height controls the storm's intensity and thus indicates the degree of danger it represents. Airborne Research has demonstrated that by detecting intracloud discharges the F-10 can provide 5 to 30 minutes of warning before a storm's initial ground strikes occur.

There are several ways to use the output of the F-10. Trained personnel can make all decisions to sound alarms or close down lightning-sensitive operations. But two versions of the F-10, called the F-10/RA and the F-10 SI, can use the system's outputs to fire audible alarms (such as sirens), place phone calls to specified personnel, or start standby generators.

Basic F-10 systems sell for $2,500. The F-10 RA and F-10/SI are priced at less than $4,000. Users include golf courses, park districts, and the Kennedy Space Center.

Additional details...David McLaughlin, Marketing Manager, Airborne Research Associates, 260 Bear Hill Road., Waltham, MA 02154.