Method and apparatus for lightning threat indication

Abstract

The invention describes a method of monitoring of increased risk of lightning on the basis of information about the increase of electric charge of water droplets, which are obtained by measuring the characteristics of scattered EM radiation. The change of these characteristics is related to the electric charge, which the droplets acquire. In order to normalize the optical characteristics as well as the concentration of droplets it is necessary that the measurements are carried out at two suitably selected wavelengths. The wavelengths can be combined, allowing the use of two or more wavelengths.

Claims

1. A method of detecting a lightning threat, comprising: setting, via a controller, a threshold for detecting a lightning threat for at least two wavelengths of electromagnetic radiation between 1 mm and 50 mm in length; transmitting the at least two wavelengths of electromagnetic radiation towards at least one cloud; monitoring, by the controller, a first signal of backscattered electromagnetic radiation of a first of the at least two wavelengths and a second signal of backscattered electromagnetic radiation of a second of the at least two wavelengths; comparing a relative change in a ratio of the first signal and the second signal to the threshold; determining that there is an absence of lightning threat if the relative change in the ratio is below the threshold; and determining that there is a lightning threat if the relative change in the ratio exceeds the threshold.

2. A method of detecting a lightning threat according to claim 1, wherein the threshold is set by: arbitrarily selecting a first candidate wavelength and a second candidate wavelength of electromagnetic radiation between 1 mm and 50 mm in length; assessing complex refractive indices of water droplets at each of the first candidate wavelength and the second candidate wavelength from tables; calculating a theoretical relative difference N between a ratio of a first expected signal of backscattered electromagnetic radiation for the first candidate wavelength and a second expected signal of backscattered electromagnetic radiation for the second candidate wavelength from water droplets without an electric charge, and a ratio of the first expected signal of backscattered electromagnetic radiation for the first candidate wavelength and the second expected signal of backscattered electromagnetic radiation for the second candidate wavelength from water droplets with an electric charge; and selecting another first candidate wavelength and second candidate wavelength based on a comparison of the theoretical relative difference N and 10%, wherein, during the setting, the threshold for detecting a lightning threat is set to a relative signal difference of 0.75 N.

3. An apparatus for detecting a lightning threat, comprising: a transmitter; at least one receiver; a control unit configured to control the transmitter and receive signals from the receiver; a data storage unit; and a computer system that is bidirectionally connected to the control unit and to the data storage unit, the computer system being configured to: set a threshold for detecting a lightning threat for at least two wavelengths of electromagnetic radiation between 1 mm and 50 mm in length; and monitor a first signal of backscattered electromagnetic radiation of a first of the at least two wavelengths and a second signal of backscattered electromagnetic radiation of a second of the at least two wavelengths; compare a relative change in a ratio of the first signal and the second signal to the threshold; determine that there is an absence of lightning threat if the relative change in the ratio is below the threshold; and determine that there is a lightning threat if the relative change in the ratio exceeds the threshold.

4. The apparatus for detecting a lightning threat according to claim 3, wherein the at least one receiver comprises at least two receivers, each of the at least two receivers being tuned to collect only a signal of a specific frequency.

5. The apparatus for detecting a lightning threat according to claim 3, wherein the transmitter comprises a microwave generator operating in pulse mode, wherein the microwave generator repeatedly alternates between a first frequency and a second frequency.

6. The apparatus for lightning threat indication according to claim 3, further comprising a circulator between a microwave generator and a detector, the circulator being configured to limit transmission of a signal from the microwave generator to an irradiator and limit transmission of a signal received via the irradiator to the detector, wherein direct transmission of a signal from the microwave generator to the detector via the circulator is inhibited.

7. The apparatus for lightning threat indication according to claim 3, wherein the at least one threshold of the apparatus for lightning threat indication is set by: arbitrarily selecting a first candidate wavelength and a second candidate wavelength of electromagnetic radiation between 1 mm and 50 mm in length; assessing complex refractive indices of water droplets at each of the first candidate wavelength and the second candidate wavelength from tables; calculating a theoretical relative difference N between a ratio of a first expected signal of backscattered electromagnetic radiation for the first candidate wavelength and a second expected signal of backscattered electromagnetic radiation for the second candidate wavelength from water droplets without an electric charge, and a ratio of the first expected signal of backscattered electromagnetic radiation and the second expected signal of backscattered electromagnetic radiation from water droplets with an electric charge; and selecting another first candidate wavelength and second candidate wavelength based on a comparison of the theoretical relative difference N and 10%, wherein, during the setting, the threshold for detecting a lightning threat is set to a relative signal difference of 0.75 N.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: Ratio of backscattering efficiencies of charged and neutral droplets as a function of size parameter x.

(2) FIG. 2: Plot of electric charge as a function of droplet radius based on the work of Pruppacher a Klett [3].

(3) FIG. 3: Ratio of the backscattering efficiencies of charged and uncharged droplets as a function of their radius.

(4) FIG. 4: A(; 1 mm:5 mm) as a function of droplet radius.

(5) FIG. 5: Possible configuration of the apparatus: a simple configuration of the apparatus with a transmitter and a receiver placed at the same location.

(6) FIG. 6: Example of realization of the apparatus for two frequencies (f.sub.1 and f.sub.2).

(7) FIG. 7: Example of realization for a microwave generator, in which the switching between the operational frequencies f.sub.1 and f.sub.2 is controlled by a controller.

(8) FIG. 8: Example of realization for a solution with a circulator.

DETAILED DESCRIPTION OF THE INVENTION

(9) The technology described in the summary of the invention is realized in practice by means of an apparatus that measures the intensity of backscattering at the two wavelengths. FIG. 4 depicts the calculated ratio A (1 mm:5 mm), for instance, as a function of droplet radius for the case where .sub.1=1 mm and .sub.2=5 mm. This ratio is about 1.6 for particles with radius lower than cca 5 m. The transition region can be observed for particles with radii of 5-10 m. For larger particles, the value of the function A asymptotically approaches 1. In the first region with droplet radii of less than 5 m, the increase of the signal ratio is of approximately 60%.

Example 1

(10) A simple configuration of the apparatus with a transmitter 1 and a receiver 2 located at the same location is depicted in FIG. 5. The transmitter 1 generates EM radiation of two frequencies (or wavelengths) and the generated radiation is directed to the cloud of water droplets. The device can be constructed as radar, in which the source of EM radiation is directed to the clouds. In a traditional configuration, the transmitter 1 and the receiver 2 are simple devices, shaped as a parabola, or they can be separate devices. The wave will be transmitted towards the cloud, in which scattering on individual droplets occurs. Scattered waves propagate in all directions. Those that travel back toward the transmitter 1 are designated as backscattering waves and can be detected by the receiver 2 which is located close to, or collocated with, transmitter 1. The detected signal is a superposition of components of scattered radiation of both wavelengths.

(11) A part of the radiation that is scattered on the droplets travels back and is registered by the receiver 2. The individual components of the intensities for the two wavelengths are recognizable by the computer system 4. The ratio of these intensities is monitored by the control unit 3. The data are stored in the data storage unit 5.

Example 2

(12) The second option is to use several detectors, each of which is tuned so that it collects only a signal of a specific frequency.

(13) FIG. 6 depicts an example of realization for two frequencies (f.sub.1 and f.sub.2), where two generators, microwave generator (1) 6 and microwave generator (2) 7, generate microwave radiation with .sub.1=1 mm and .sub.2=5 mm. The signals are sent to irradiator (1) 8 and irradiator (2) 9 and the radiation is then emitted to the atmosphere by means of parabolic reflector 10.

Example 3

(14) Another approach should be adopted in the case when the transmitter works in a pulse mode. Then it is convenient to generate and receive a signal of only one frequency, and subsequently to switch the device to another frequency mode, and after which the whole process is repeated cyclically. The receiver will process a rectangular signal, wherein the frequency will correspond to the frequency of the transmitter. Regardless of how the signal intensities at both wavelengths will be obtained, the ratio of these signals is given by the ratio of both components. If during the monitoring of the ratio A(.sub.1:.sub.2, a significant change is registered, this change will be interpreted as a result of electric charge in the clouds, and therefore also as a potential risk of lightning. FIG. 7 depicts the realization for microwave generator 14, in which the switching between operational frequencies f.sub.1 and f.sub.2 is controlled by controller 11.

Example 4

(15) FIG. 8 shows a solution with circulator 13 which releases a signal only in a certain direction, and so enables it to lead the signal to the irradiator 8 (AF1) and also to lead the detected signal from the atmosphere to the detector 12. The transition from the MW sourcegenerator 14 to the detector 12 is not possible.

INDUSTRIAL APPLICABILITY

(16) Lightning is particularly dangerous during landing and take-off of an aircraft, where it can cause loss of power or a complete failure of aircraft equipment resulting in catastrophe. To avoid this, airports close during increased lightning threats. However, the disablement of airports is extremely costly and therefore it is economically advantageous for airports to have a device for accurate indication of occurrence of dangerous lightning.

FIGURE LEGEND

(17) 1 transmitter 2 receiver 3 control unit 4 computer system 5 data storage unit 6 generator 1 7 generator 2 8 irradiator 1 9 irradiator 2 10 parabolic reflector 11 controller 12 detector 13 circulator 14 microwave generator