BTD-300 Thunderstorm Detector FAQs

The manual states that no movement of animals should occur near the detector. My BTD-300 is often overflown by birds. Will this produce false alarms?

– Generally no. However, in rare instances some fast-flying birds can produce false alarms of distant lightning if they fly directly overhead within approximately 2m of the antennas. Birds landing on the antennas should not produce false alarms, although it is advised that an appropriate installation site is selected which avoids regular bird activity within a few metres of the antennas. If nearby birds become a concern then increasing the lightning strength threshold from its default setting (level 5) will effectively remove any bird signals without compromising on performance, for correctly sighted units.

 

Why does my BTD-300 report occasional flashes greater than 45 NM (83 km) away when no storms are in the region?

– Depending on your selected sensitivity settings, the BTD-300 may occasionally pick up very weak signals from natural or man-made sources within the 1-47 Hz listening frequency which are not related to thunderstorm activity in the region. Like we advise in this manual, although strong lightning flashes can sometimes be detected at ranges exceeding the maximum warning range (45 NM / 83 km), these events will not activate any warning and should not be used for the purpose of thunderstorm monitoring due to their increased ambiguity compared to events detected within the maximum operating range.

 

Data from a lightning location network shows more lightning strikes in the area than my BTD-300. Is my unit working correctly?

– Providing the BTD-300 is installed and operated in accordance with the instructions provided by the manual, the unit will detect nearly all of the lightning within range. Due to its high sensitivity to weak intra-cloud activity, the BTD-300 usually reports significantly more flashes than other detectors and networks. When comparing the BTD-300 output with that of a radio-detection based system such as a lightning location network it is important to remember that the BTD-300 reports lightning flashes, unlike nearly all networks which report lightning strokes (the supplier will be able to confirm whether their data are strokes or flashes).

These lightning strokes usually reported by networks represent the individual pulses of strong current which occur during a complete lightning discharge. The complete discharge is referred to as a lightning flash. Strokes from the same flash occur within half a second of each other (usually considerably shorter). This is why a lightning flash appears to flicker to an observer. Many strokes of the same flash follow the same path, but some form a new one within a few kilometres. Whilst some flashes only contain a single stroke, most comprise of two or three, although occasionally flashes may contain more than ten! This, combined with the location uncertainty of the network can show multiple nearby points on their lightning map at the same time for what was actually produced by the same lightning flash.

Since the BTD-300 reports the location of the flash, i.e. the complete discharge and not the individual components, it may appear to count less lightning than a network reporting lightning strokes, which have not been grouped into flashes by the supplier. From a thunderstorm warning perspective, there is no advantage to reporting strokes compared to flashes (networks usually only do so since it is the fundamental unit derived from the radio pulses they detect). It is lightning flash rates (not strokes) that normally define a storm’s overall electrical activity and these will be the same as that identified by a human observer.

If you are concerned that a correctly sited and calibrated BTD-300 is not detecting sufficient nearby (within 30 NM/56 km) flashes in your area, you could try decreasing the lightning ratio threshold from the default (level 5). If you are concerned more with improving detection of the most distant (30-45 NM / 56-83 km) flashes detectable by the BTD-300, you could try lowering the lightning strength threshold from the default (level 5). It is important to be aware however that lowering any threshold from the default (level 5) setting has the potential to increase false alarms. If in doubt, please contact Biral for advice on the most suitable sensitivity levels for your requirements.

 

Why does my BTD-300 sometimes produce a different distance to the same lightning flash compared to that derived from a lightning location network?

– If the BTD-300 is installed at a non-ideal site (as defined by the manual), it may need a site correction factor due to the distortion of the ambient electric field by nearby obstructions. This is achieved by identifying the correct distance to a storm and entering it into the system as described by the manual (section 5.3). This procedure only needs to be completed once for each non-ideal site. Once your BTD-300 is correctly configured there may still be occasional instances where the distance is noticeably different from that derived from a good quality lightning location network. Interestingly, both can be correct! This is because the network will locate the part of the flash producing the strongest radio signal (which depends on the radio frequency used and orientation of the channel). The BTD-300 will weight its distance estimation to the nearest location of the storm where significant charge neutralisation occurred. For most flashes these are approximately the same, although since lightning can have a significant horizontal extent (sometimes tens of kilometres), the portion of the lightning flash located by the radio detector network may be several kilometres from the closest approach of the flash to the BTD-300. The BTD-300 will therefore tend to report the flash as being closer. Weighting the thunderstorm distance to the closest region of significant electrical activity for horizontally extensive flashes has clear advantages from an early warning and safety perspective.

The BTD-300 is programmed with knowledge of the typical lightning strengths found throughout the world. However, there will be cases where a thunderstorm produces some extremely weak or strong lightning flashes which may be incorrectly ranged as being too far (weak flashes) or close (strong flashes). Even for these extreme events the deviation from the actual distance is usually small compared to other standalone lighting detectors, due to the unique operating principle of the BTD-300.

 

Why are some flashes given a range but no direction when the direction finder module is installed and operating correctly?

– There will sometimes be lightning flashes which do not generate sufficient low frequency radio signals for the direction finder module to be confident of their bearing, despite producing sufficient quasi-electrostatic field change for the main unit to provide a range. Typically these flashes are weak intra-cloud and distant from the detector. This situation highlights the advantage provided by the BTD-300 of being able to detect and range these weak flashes which would otherwise not have been detected (and certainly not ranged) by a conventional system using low frequency radio detection.

It is worth noting that the diversity of radio signal amplitudes produced by different lightning strokes means that a signal from lightning 1000 km away can have the same strength as a stroke only 30 km away! The same does not apply to its quasi-electrostatic signal used by the BTD-300, which therefore provides a substantially more reliable measure of lightning range than by using radio signal amplitude alone. Nonetheless, radio signals are the only practical method of lightning direction finding available for single site systems, so the direction finder module used by the BTD-300 must rely upon these signals.

 

My BTD-300 is covered in snow and ice. Will this affect its performance?

– The BTD-300 is designed to operate in harsh weather conditions and the antenna insulators are shielded and heated, so providing there is no contact between the snow and ice on the antennas with the metal support pole or other antennas, the unit should perform as expected.

Since the top of the snow represents the ground plane, the effective height of the antennas will be reduced according to the snow depth around the base of the unit. This is not expected to significantly affect performance (principally detection of distant flashes) until the snow depth exceeds approximately 60 cm (2 ft). For persistent snow depths in excess of 60 cm the BTD-300 will tend to overestimate lightning distance, so will require re-calibration, and have a lower detection efficiency for far away (>20 NM / 37 km) flashes. Snow depths exceeding 140 cm (~4.5 ft) will likely make contact with the tertiary antenna and prevent the system from detecting lightning (although it will still detect charged precipitation and strong electric field variability), so the area immediately surrounding the BTD-300 will need clearing of snow. The optional electronics enclosure heater available for the BTD-300 upon initial order is recommended for installation in cold environments where the minimum ambient temperature is lower than -20°C (-4°F).

Although lightning is relatively uncommon during snowstorms, it does occur during certain meteorological conditions so there is still a risk to be considered. For example, winter rain/hail showers in mid-latitudes can produce extremely powerful cloud-to-ground lightning, although the storm’s lightning flash rate is usually low compared to summer thunderstorms. Such winter storms tend to produce highly charged precipitation and strong electric fields, allowing the BTD-300 to warn of their potential overhead development.