STORM WARNING METHOD AND APPARATUS

Abstract: A storm warning apparatus comprises a set of receiving aerials (10,12,14) which are constructed to pick up the E-field, and orthoganal components of the H-field of electromagnetic energy generated during lightning activity. The apparatus discriminates against spurious signals by means of a threshold and a slope detector (32,30). The direction of the lightning activity is calculated from the incoming E and H field strengths and the range is calculated from the relative attenuation of the H-field between a first and a second frequency. The calculations and sequence of events in computing the position of lightning activity are controlled by a microprocessor (52).

Description: STORM WARNING METHOD AND APPARATUS This invention relates to apparatus and methods for detecting regions of lightning activity. Thunderstorms, characterised by turbulence and electrical activity (lightning), constitute a great danger, particularly to aircraft. It is therefore desirable to locate thunderstorms as accurately as possible so that they can be tracked and avoided. Lightning associated with thunderstorms generates electromagnetic energy that propagates through the atmosphere. The detection, recognition, measurement and analysis of this energy provide a basis for storm tracking and avoidance. Lightning flashes are composed of a series of high current lightning strokes, each stroke being preceded by a lower current discharge called a leader. The duration of electrical activity associated with a lightning stroke varies, but in many instances can last as long as 100 microseconds. The initial rise time of electrical energy discharge associated with a lightning stroke almost never exceeds five microseconds. Following the first peak of a lightning stroke, lesser signals of sub-microsecond duration, but with faster rise times (of five microseconds or less), will occur. U.S. Patent No. 4,422,037 discloses an improved apparatus and method of detecting, tracking and displaying lightning activity. In accordance with the method and apparatus disclosed therein, sensors are provided which measure the time rate of change of the magnetic and electric flux densities of the electrical discharges generated by the lightning. The outputs of the sensors are integrated to provide a measure of electric (E) and magnetic (H) fields of the lightning. The total magnetic field value is calculated from sampled magnetic fields and the value is compared with a predetermined field strength to estimate in which of several range regions the lightning stroke occurred. The integrated magnetic field value is also compared to predetermined field strengths to predict in which range region the lightning stroke is likely to have occurred. If the regions predicted by the two methods are adjacent or coincident, then the ratio of the integrated magnetic to electric field values is used to determine the range from a 'look-up'table. If the two regions predicted are the near and distant regions, then the ratio of the total magnetic to total electric field value is calculated from the sampled data and used to determine the range from the 'look-up' tables. Due to ambiguities present in the plot of the ratio of magnetic to electric fields, the maximum effective range of such airborne systems in accordance with the invention disclosed in the aforementioned patent is approximately fifty kilometres at frequencies practical for use in airborne equipment. Lower frequencies could extend this maximum range somewhat, but would necessitate the use of larger sensors which are impractical for installation on smaller aircraft. Considering the high speeds of present day aircraft, this fifty kilometre limitation on range can be quite disadvantageous in that it may not provide sufficient advance warning. Thus the prior art provides apparatus for determining the location of lightning activity, the apparatus comprising receiving means for receiving separately the electric and magnetic field components generated by a lightning stroke, measuring means for measuring the field strengths of the electric and magnetic field components and processing circuitry, connected with the measuring means for determining the direction of the lightning activity as a function of the electric and magnetic field components. The present invention is characterised in that the measuring means determine the field strength of one of the field components at both a first and a second frequency and the processing means, in addition to determining the direction, determines the range of lightning activity as a function of the field strengths of the said one of the field components at the first and second frequencies. According to the invention there is also provided a method of determining the location of lightning activity comprising the steps of receiving separately the electric and magnetic field components generated by a lightning stroke, measuring the field strengths of the electric and magnetic field components and processing the measured field strengths in order to determine the direction of lightning activity, characterised by the steps of measuring field strength of one of the field components at both a first and a second frequency and processing the measured one of the field strengths in order to determine the range of the lightning activity as a function of the ratio of the one field strength at the first and second frequencies. A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: - Figure 1, consisting of Figures la, ib and ic, is a functional block diagram of the preferred embodiment of the storm detection apparatus of the present invention. Figure 2 is a schematic diagram of the preferred embodiment of one of the three wide band amplifiers and one of the three integrators of the apparatus of the present invention. Figure 3 is a schematic diagram of the recognition circuitry of the apparatus of the present invention. Figure 4 is a schematic diagram of the peak detector and pulse width monitor portion of the apparatus of the present invention. Figure 5 is a schematic diagram of one of three low frequency detectors of the apparatus of the present invention. Figure 6 depicts the signals of a lightning stroke in relation to a timing diagram for the apparatus of the present invention and Figure 7 is a functional block diagram of the electric and magnetic field sampling portion of the apparatus of the present invention. Referring generally to the figures, the apparatus includes receiving means for separately receiving the electric (E) and magnetic (H) field components of a lightning stroke. Processor means analyses the lightning fields to determine both range and direction to the lightning activity. Range is determined by measuring the magnitude of the magnetic (H) field at two different frequencies. The range is then computed as a ratio of the strength of the H field at a first frequency to the strength of the H field at a second frequency. In a preferred embodiment of the invention, the first frequency is lower than the second frequency. Display means shows the region of a thunderstorm by indicating the amount of lightning occurring in the region. The receiving means includes a pair of cross-loop sensors and an electric field sensor. The outputs are suitably amplified and integrated to provide a measure of the E and H fields of the lightning stroke. The processor means includes threshold circuitry and slope detection circuitry connected with the receiving means for recognising lightning stroke energy received by the receiving means and discriminating against interfering signals and background noise. When the rise time and amplitude of the lightning stroke component are above a predetermined minimum, first gating circuitry is triggered which provides a signal indicating that a bona fide lightning stroke has occurred. At the peak amplitude of the lightning stroke, a pulse width monitor circuit measures the time occurrence of the peak since the amplitude of the lightning stroke was above a predetermined minimum threshold. The processor means also includes first, or low, frequency detection circuitry connected with the receiving means for separately detecting the lightning fields received by the receiving means within a frequency band centered at the predetermined first frequency. The output of the receiving means is also connected with the second or high frequency detection circuitry which, in response to peak control signals from a peak detector, separately samples the lightning fields at the predetermined second frequency. The fast rise time signals are held at their peak values by track and hold circuits and then converted by analog to digital converters and stored in a memory. In the event of a bona fide lightning stroke the peak detector connected with the first gating circuitry transmits a narrow pulse to the pulse width circuit, an interrupter controller and analog to digital converters. The interrupter controller is actuated in response to the narrow pulse to interrupt the processor means after a predetermined time interval. When the predetermined time interval is over, that is when the lightning activity ends, an interrupt signal from the interrupt controller is transmitted to a programmable processor which in turn reads the contents of the memory into its buffer memory. At the same time the processor also reads the low frequency field values for the predetermined interval through a multiplexer and the memory circuitry. The high frequency sampled values read from the memory are related by the following equation: -HE-W SIN (pal) + HN-S COS () = Ez/(Z0 SIN (9)) where 9 is the azimuth angle to the lightning stroke and is the elevation angle and Z0 is the characteristic impedance of free space. The e angle for the preferred embodiment is 900 (H equal Oo at zenith). Several sample sets of the E and H field values entered in the above equation results in the simultaneous solution of two equations to solve for /. Because in general each lightning signal is not vertical, the azimuth angle 8 calculation varies among sample sets. By using an iterative calculation of solutions to the above equation, the angle e accuracy can be improved by the processing circuit means. The processing circuit means calculates the range to the lightning by utilizing the low frequency and the high frequency data stored in memory. The first high and low frequency magnetic field data are used in the equation below to estimate the range to the lightning stroke. The ratio of the measured magnetic fields at two frequencies H(M1) and H(U2) is given by: EMI5.1 Assuming the ratio of the dipole moment M(wl)/M(wZ) is a constant a1, the range to the lightning flash is given as follows: : EMI5.2 where HLOW is the low frequency total magnetic field in teslas HHIGH is the high frequency total magnetic field in teslas is the frequency of the high frequency measured in radians per second is is the low frequency of the measurement in radians per second c is the velocity of light in free space in metres per second R is the range to the lightning flash in metres After the computation of the direction and range to the lightning is completed, the processor circuitry means compares the new data location to the data at that location which has occurred within a predetermined time period. The processor transmits to a display the location of all lightning activity greater than a predetermined magnitude during the predetermined time period. Referring now to Figure 1, there is shown a block diagram of the preferred embodiment of the storm warning system of the present invention. As shown in Figure la, the system comprises a pair of cross loop magnetic field antennas 10 and 12 and an electric field antenna 14. Since the cross loop magnetic field antenna elements have their axes perpendicular to one another, antenna 10 will be referred to as the north/south magnetic field antenna while the antenna 12 will be referred to as the east/west magnetic field antenna. The three antenna elements are responsive to the time rate of change of magnetic and electric flux densities occurring as a result of electrical activity associated with lightning stroke. The signals received by the antenna elements are transmitted to wide band amplifiers, designated generally 16, 18 and 20, which provide an east/west magnetic field component, a north/south magnetic field component and a vertical electric field component respectively to integrators 22, 24 and 26. In the preferred embodiment, the antenna elements, the integrators and the wide band amplifiers operate over a frequency range from approximately 1.5 kilohertz to approximately 3 megahertz. The output of each of the integrators 22, 24 and 26 is provided in parallel to a corresponding sampling portion of the circuit and also to the recognition circuit The recognition circuit comprises slope detection circuitry designated generally 30 and threshold detection circuitry designated generally 32. The output of the integrator 26, containing the vertical electric field signal, is transmitted to slope detection circuitry 30 via line 27. If the rise time of this signal is less than a predetermined rise time, a positive signal is transmitted to the peak detector 36. Similarly, the electric field signal transmitted from the integrator 26 represents the magnitude of the total vertical electric field present at the antenna element 14. This signal is transmitted via line 27 to threshold detection circuitry 32 which provides a positive signal to the pulse width monitor 34 via line 35 if the signal exceeds a predetermined threshold level. The output of the integrator circuitry 26 is also transmitted via line 27 to the low frequency detector, generally designated 42. The magnetic field signals from integrators 24 and 22 are transmitted via lines 55 and 57 to low frequency detectors 40 and 33 respectively. If the voltage level of the electric field signal from the integrator 26 exceeds the predetermined threshold level of the threshold detector 33 and if the rise time of the vertical electric field signal from the integrator 26 is less than the predetermined rise time of the slope detector 30, then the peak--detector 36 is caused to provide a control signal output via line 43 to the pulse width monitor 34, the interrupt controller 38 and a NOR gate 642 (see Figure Ib). Low frequency detectors 33, 40 and 42 measure the root mean squared (RMS) value of the magnetic and electric field signals over a predetermined time interval which corresponds to the most likely period over which the low frequency components of the lightning stroke exist. The slope detector 30, the threshold detector 32 and the pulse width monitor 34 provide a means for recognising the low frequency magnetic and electric field signals associated with the lightning stroke in the presence of interfering and background noise signals. The north/south and east/west magnetic field signals are also transmitted over lines 55 and 57 to high frequency sample/hold circuits 44 and 46 respectively. The vertical electric field signal from integrator 26 is transmitted via line 27 to high frequency sample/hold circuit 48. Samples of the high frequency magnetic and electric field signals representing the peak of each change in the fields are taken in response to peak detection signals provided from peak detector 36 over line 43. Peak detection signals are generated by peak detector 36 in response to positive output signals from the slope detector 30. The pulse width monitor 34, in response to positive output signals from the threshold detector 32, measures the threshold to peak time of each sample amplitude by monitoring the output of threshold detector 32 on line 35. The pulse width monitor 34 provides threshold to peak time measurement signals, over multiple lines 15, to memory 628 (see Figure lib). Referring to Figure lb, the high frequency samples of the magnetic and electric field signals, which are output from high frequency sample and hold circuits 44, 46 and 48 on lines 45, 47 and 49 respectively, are converted to digital signals by analog to digital (A/D) converters 612a, 612b and 612c in response to the peak detector signals via line 43. As shown in Figure 1bathe signal output from high frequency sample and hold circuit 48 on line 49 passes through a multiplexer 636, the output of which is converted by A/D converter 612a via line 51. A/D converter 612a transmits a high signal to peak detector 36, pulse width monitor 34 (see Figure a) and end-of-convert circuit 613 via line 618 which disables peak detector 36 while the A/D converter 612a is actuated. The end-ofconvert circuit 613 transmits a high-going-low signal via line 615 at the end of the A/D conversion cycle. The digital signals from A/D converter 6l2a, along with the pulse width signals provided by the pulse width monitor 34 over lines 15, are stored in first-in first-out (FIFO) memory 628 by a store enable signal output from the end of convert circuit 613 over line 615. Similarly, the low frequency electric and magnetic field signals from low frequency detectors 40, 42 and 33 pass through multiplexer 636, A/D converter 612a and are stored in FIFO memory 628. Referring to Figure lc, the system further comprises processing circuitry means designated generally 52. When the predetermined integration interval corresponding to the existence of the lightning stroke is ended, the interrupt controller 38 (see Figure lb) transmits an interrupt control signal via line 54 to the processing circuitry means which, in response thereto, commands the memories 628, 630 and 632 of the processing circuitry 50, via multiple lines 622, 624 and 626 to transmit the high frequency samples of the magnetic and electric field values via data bus 712 to random access memory (RAM) 703. The processing circuitry 52 commands, over line 53, low frequency magnetic and electric field data to be held by the low frequency detector circuits 33, 40 and 42. Next, the processor 52 commands, via lines 620, 621 and 711, the A/D converter 612a to convert the low frequency signals of lines 66, 68 and 69 from analog to digital form. This information is utilized in accordance with the programming of the processing circuitry means to calculate the region of activity of the lightning stroke. The results are transmitted to the display means 62 which then displays the results. The display means 62 comprises a display processor and CRT display. The processing circuitry 52 contains a routine which enables it to compensate for movement of a host platform (such as an aircraft) when determining the region of lightning activity. This is done by monitoring the aircraft direction, via multiple lines 732, as provided by syncro-to-digital converter 740. The north/south magnetic field antenna 10 and the east/west magnetic field antenna 12 (see Figure la) each preferably consists of three turns of 50 ohm cable (preferably .082 inch diameter) each of which has gaps at the top of each turn. The two outputs of the shield loop are connected to a 1800 summer. The output of the 1800 summer is then filtered by a seven pole Butterworth filter. The electric field antenna 14 preferably consists of a thin copper sheet bonded to the inside of the antenna radome structure. The copper sheet is connected to a seven pole Butterworth filter which matches the antenna terminating impedance nominally to 50 ohms and filters signals above three megahertz. The antenna radome is preferably a six-inch long blade designed for a subsonic aircraft. An aircraft flying at 500 knots using such a radome will experience a drag force of approximately 13.61 kilograms at sea level and 6.8 kilograms above 6,100 metres. Referring now to Figure 2, there is shown a schematic diagram of the preferred embodiment of one of three wide band amplifiers 20, 18, 16 as well as integrators 26, 24 and 22. The input to the amplifier is a five pole Butterworth low pass filter. The two stage amplifier preferably provides a gain of 28 decibels (db). The maximum input signal to cause one db compression at the output of amplifiers is -10 db. For a lightning stroke at a distance of two kilometres, the output will be 10 db below the 1 db compression point and may be sampled without distortion. The amlifier is AC coupled to an integrator U3. The combined low frequency gain of the integrator and the wide band amplifier is 80 db. This offsets the antenna loss. The lightning signal at 1.5 kilohertz and at a distance of 10 kilometres will have an output signal substantially equal to 11.5 dbm. The low frequency signal is narrow band filtered, preferably to 200 Hz, by the combination of inductor L3 and capacitor C11. The output of the integrator U3 is also AC coupled by capacitor C10 to provide the high frequency pulses over lines 55, 57 and 27. The decay constant of the integrator is 45 microseconds. Referring now to Figure 6, there is shown a curve, generally designated 500, which represents the vertical electric field Eb signal output from integrator 26. The ordinate in each curve represents the voltage amplitude in millivolts and the abscissa represents time in microseconds. Inspection of the curve shows that the E field voltages associated with a single lightning flash (many strokes comprise a lightning flash) usually lasts approximately 0.5 second. The lightning flash is usually composed of a preliminary breakdown (see area 502 on curve 500), stepped-leaders (see area 504 on curve 500), return stroke (see area 506 on curve 500), continuing currents (see area 508 on curve 500), and interstroke intervals (see area 510 on curve 500). Research into the nature of electrical activity arising from lightning strokes indicates that almost all vertical electric field voltages ( ) for the return stroke will have an initial rise time (see area 507 on curve 500) of less than five microseconds; and that the flash will contain a plurality of randomly placed smaller pulses or peaks of submicrosecond duration and rise times of less than five microseconds. See for example positive slopes 509, 511, 512 and 514 on curve 500. The recognition circuitry, as shown in Figure 3, takes advantage of the aforementioned characteristics to cooperate with the threshold detector 32 and interrupt controller 38 to furnish control signals via lines 43 and 53 for low frquency detection and high frequency sampling the E and H field voltages. As shown in Figure 3, the vertical electric field voltage W is transmitted from integrator 26 via line 27 (see Figure 1) and then lines 425 and 428 through DC blocking capacitors C28 and C31 to the slope detector 30. Slope detector 30 comprises two parallel similar circuits designated generally 430 and 432. U17 is an operational amplifier with TTL output and, in the preferred embodiment, is a Signetics Model No.SE529 with two inputs, 435 and 436. Input 435 is connected to input 436 through a resistor R36 and input 436 is connected to ground through a capacitor C29. Operational amplifier U17 will provide a negative output voltage to NAND gate 446 whenever the field voltage N increases faster than capacitor C29 can charge through resistor R36. The product of resistor R36 and capacitor C29 determines the sensitivity of the slope detector 30. In the preferred embodiment, this product is set to be equivalent to a lightning signal change of 40 volts per microsecond and thus allows only the sub-microsecond lightning pulses to be detected. Input 436 is a positive voltage bias through resistor R38 and the ratio of resistors R36 and R38 sets the minimum signal level which the slope detector 30 will detect. The preferred embodiment is set at 15 millivolts, corresponding to a lightning range of 200 kilometres. Similarly, circuitry 432 comprises operational amplifier U18 (preferably Signetics Model No. SE529) which provides a negative output to NAND gate 446 whenever field voltage EV decreases faster than capacitor C32 can charge through resistor R40. Either a positive or negative voltage change of N having the correct rise time as functionally defined by circuitry 430 and 432 will provide a negative input to NAND gate 446. If no fast rise time occurs, the outputs of operational amplifiers U17 and U18 will be high. Examples of submicrosecond pulses which will trigger the above described circuitry are shown as slopes 507, 509, 511, 512 and 514 in Figure 6. When the slope of the incoming signal is sufficiently steep (either positive or negative) one of the operational amplifiers U17 or U18 will go low causing the output of NAND gate 446 to go high until the peak voltage on the pulse on curve 500 is reached and then the output of NAND gate 446 will go low. For example see Figure 6, curve 516, where the output of NAND gate 446 is shown as the series of pulses 518 coresponding to fast rise times of the W signal 500. The output of NAND gate 446 is transmitted through resistor R1 to NAND gate 454 as shown in Figure 4. Threshold detection NAND gate 448 transmits a high to the flip-flop 408 and NAND gate 410 via line 451. Flip-flop 408 input from line 451 is clocked to NAND gate 418 on the rising edge of the clock signal via line 415. The clock signal provided by the high speed clock 414 is generated in the preferred embodiment by a 74LS393 counter from the master clock 412. The high speed clock signal provided by line 415 has a frequency which is preferably equal to two megahertz. The high signal provided by NAND gate 448 is transmitted to flip-flop 408 output on clock 414 rising edge and is provided as a low to the set input of flip-flop 416 via line 417. A low signal via line 417 causes the flip-flop 416 output to be high. A high signal on line 421 enables the NAND gates 420 and 424. The low speed clock 413 is driven by the master clock 412 via line 411. A 74LS393 is used to divide the master clock to 250 kilohertz. The low speed clock signal is provided to low speed counter 426 via line 423 and NAND gate 424 when enabled by a high on line 421. Likewise, the output of high speed clock 414 is provided via NAND gate 420 to high speed counter 422 when enabled by the high signal on line 421. Outputs of the low speed counter 426 and high speed counter 422 are provided to lines 427, 429, 431 and 433 via latches 428 and 430. Output lines 427, 429, 431 and 433 are changed at each peak detect signal provided over line 461 which represents the time at which the peak of the pulse occurred since the moment it rose above a minimum threshold. As the data ready signal provided by line 618 goes from high to low, the end of convert circuit 434 provides a high reset pulse to latches 428 and 430. In the preferred embodiment, latches 430 and 428 are 74LS197. Also in the preferred embodiment, the high speed counter 422 and low speed counter 426 are both 74LS393 counters. Whenever slope detection circuitry causes a low-going-high signal at the input to the one shot 432 (this output goes high when the fast rise time pulse from the integrator 26 peaks) it generates a 250 nanosecond pulse on line 461. The falling edge of the pulse causes the data inputs to latches 428 and 430 to be set as outputs 427, 429, 431 and 433 respectively. This 250 nanosecond high signal on line 461 is one of the control signals generated by the peak detector 36 and is called the peak detection signal. The peak detection signals are shown on curve 516 of Figure 6. Note that they occur on the leading edge of the high going signal from NAND gate 454. The electric field from the integrator 26 is also transmitted via line 27 (see Figure la) to the low frequency detector 42 which comprises a true RMS detector and sample/hold (S/H) circuitry designated generally 478 (see Figure 5). A true RMS measurement of the HEW, HNS and N field voltages are made accurately in the preferred embodiment by a commercially available device, Analog Devices Model No. AD536. Alternatively, a peak detector of the low frequency signal could also be used. The design implements the auxiliary dB output of the AD536A as described in the Analog Devices Data Acquisition Products Catalog, 1978 edition, pages 229-234 which is incorporated herein by reference. The input to an RMS converter 900 is from t low frequency output line 901 (see Figure 2). The output of the RMS converter 900 is connected to a S/H circuit 902. The low signal from the processing circuitry means via line 53 is connected to S/H circuit 902. On the rising edge of the low signal, the S/H circuit 902 holds the integrated voltages while the processing circuitry means converts and stores them as digital data. In the preferred embodiment, the S/H circuit 902 is a Harris Corporation Model No. HA-2420-8 device. Referring again to Figure 4, the output of the one shot 432 is connected to a one shot 462. The Q output of the one shot 462 is connected via line 490 to a flip-flop 492. When the 1 millisecond interval is over and the Q output of the one shot 462 goes high, it sets the flip-flop 492 causing a high signal to be transmitted via line 54 to the processing circuit 52 as an interrupt signal (see Figure lc). At the same time, the Q of flip-flop 492 goes low and is transmitted via line 495 as an inhibit data measurement signal to the one shot 432. The low signal disables the one shot 432 while the processing circuit 52 interrogates the preprocessing circuit 50 for the sampled and integrated E and H field voltages. When the processing circuit 52 has finished interrogating the pre-processing circuit 50, it transmits a clear signal to the flip-flop 492 via line 459 which results in enabling the one shot 432 and disabling the high interrupt signal 54 to the processing circuit 52 from the flip-flop 492. Figure 7 shows the high frequency S/H 44, 46 and 48 and the preprocessing circuit 50 of Figure 1 in more detail. Capacitors 602, 604 and 606 block unwanted DC components from the signals EV, HNS and HEW respectively furnished over lines 27, 55 and 57 but pass the high frequency signals. The high frequency signals are then transmitted through buffer circuitry 608a, b and c (Analog Device Model No. AD 509 in the preferred embodiment) before being forwarded to track and hold circuits 610a, b and c. The track and hold circuits (Analog Devices Model No. HTC-0300 MM in the preferred embodiment), in response to the peak detection control signals received by analogue-to-digital (A/D) converters 612a, b and c, from the one shot 432 over line 43 (see Figures 1 and 7), hold the peak field voltage constant. The voltage held by the track and hold circuits 610a, b and c is the peak voltage occurring in the voltage 500 (see the slopes 507, 509, 511 and 512 in Figure 6). Each peak field voltage signal held by the track and hold circuits 610a, b and c is converted to a digital signal by the A/D converters 612a, b and c in response to the rising edge of the peak detection signal from line 43 and causes a high signal on line 634. In the preferred embodiment, the A/D converters 612a, b and c are Datel Intersil Devices, Model No. ADC817MM, which convert the analog signal into a twelve bit digital signal in 2.5 microseconds. When the conversion is complete, A/D converter 612a transmits a high signal over lines 634 and, on the falling edge, causes a NOR gate 646 to output a low-going-high signal. This causes the digital sampled peak field voltage and the voltage on lines 427, 429, 431 and 433 to be clocked into the first-ififirst-out (FIFO) memory 628 where they are stored until called for by the processing circuit 52. As described earlier, each peak signal on wave form 500 (rev) in Figure 6 which has a rise time less than or equal to five microseconds causes a peak detection signal which then causes a sample of the EV, HNS and HEW field signals to be taken and stored in memories 628, 632 and 630 during the one millisecond interval (signal 517 in Figure 6) generated by the Q output of one the shot 462 in Figure 4. In the preferred embodiment, memories 628, 630 and 632 are Advanced Micro Devices, Model No. AM 2841. When the A/D converters 612a generates a rising edge signal over line 634 to a NAND gate 616, the NAND gate 616 transmits a low data ready signal over line 618 to another NAND gate 454 (see Figure 4). This disables the one shot 462 from generating a new peak detection signal. If the data ready signal were not used to enable the one shot 462, a second closely occurring peak voltage signal on waveform W would generate a second peak detection signal causing the A/D conversion process of the first sample signal to be interrupted. The high signal over line 634 causes a capacitor 650 to charge to a high signal via a resistor 648 and a NOR gate 644. On the falling edge of the signal on line 634, a NOR gate 646 is enabled to generate a low signal over line 652 for a period of time determined by the product of a resistor 648 and a capacitor 650. The control signal on line 652 enables the transmission of the twelve bit A/D digital signals to FIFO memories 628, 630 and 632. The processing circuit 52 is described in block diagram form in Figure ic. The processing circuit comprises a controller 56, a random access memory (RAM) 703, read only memories (ROM) 704 and 706, an arithmetic processing unit 707, and an address data bus 712. The various portions of the processing circuit 52 are connected together by the address/data bus 712. The processing circuit 52 is programmable, the programs being stored in ROMs 704 and 706. After the processing circuit 52 has finished processing data and is ready to receive new data from a new lightning stroke, it transmits a data clear signal to the flip-flop 492 (see Figure 4) over line 459. Line 459 is shown connected to the RAM 703. When a lightning stroke is recognised by the recognition circuit shown in Figure 3, a one millisecond integration and sampling time is set by a low Q output of the one shot 462 as described earlier. When this one millisecond interval is over, the change in state of the output of one shot 462 sets the flip-flop 492 providing an interrupt signal over line 54 to the controller 56. The processing circuit 52 is now ready to read the data via the bus 712. The data to be read are the sampled high frequency EV and UNS and HEW field values stored in the FIFO memories, the pulse width data stored in FIFO 628 and the low frequency N and magnetic field values transmitted from low frequency detectors 33, 40 and 42. To accomplish this, the processing circuit 52 transmits sample data control signals over lines 622, 624 and 626 (Figure 7 and la) from the controller 56 via a decoder 623. The controller 56 provides an address to bus 712 and control signals on bus 710 to decoder 623 which in turn provides interrogation signals over lines 622, 624 and 626 to memories 628, 630 and 632 respectively. The memories 628, 630 and 632 are connected both with to the address/data bus 712 and control bus 710. Control lines 620 and 621 from random access memory 703 are connected with multiplexer 636 (see Figure 7). The multiplexer 636 is connected between the track and hold circuit 610c of the high frequency sample/hold circuit 48 and the A/D converter 612a. The low frequency vertical electric field signal from low frequency detector 42 (see Figure 5) is transmitted via line 69 to the multiplexer 636 while the low frequency magnetic field from the low frequency detectors 40 and 33 are transmitted via lines 66 and 68 to the multiplexer 636 in response to command signals transmitted over lines 620 and 621. The processing circuit 52 transmits an A/D convert command over line 711 to A/D converter 612a via NOR gate 642. At the completion of an A/D conversion, the digital data is transmitted to FIFO 628. Assuming that the circuitry of Figure 4 is cleared to accept data from a lightning stroke which is recognised by the circuitry of Figure 3, the one shot 462 generates a one millisecond pulse in a manner as described previously. As the one millisecond interval ends the flip-flop 492 is set and a high interrupt signal is transmitted therefrom, via line 54, to the controller 56. This signal initiates a sample data interrupt program stored in ROMs 704 and 706. The signals transmitted on lines 620 and 621 are changed by the processing circuitry means. The decoder 623 transmits a signal via line 622 to memory 628. A read sampled EV data signal, sent via the control bus 710 from the processing circuit 52, causes the sampled EV data and pulse width data stored in FIFO 628 to be transmitted via address/data bus 712 to RAM 703. The sample'data interrupt program transmits an address signal, via lines 620 and 621 to the multiplexer 636, and a command signal, via line 711, to read the low frequency vertical electric field value over line 69 into the A/D converter 612a where it is converted into digital form. Next the address over lines 620 and 621 is changed so that the multiplexer 636 reads the low frequency magnetic field value from line 68 into the A/D converter 612a where it is converted into digital form. Finally, the address over lines 620 and 621 is changed so that the multiplexer 636 reads the low frequency magnetic field value from line 66 into the A/D converter 612a where it is changed into digital form. The digital values of the low frequency electric and magnetic fields are transmitted to FIFO memory 628 on the falling edge of control line 634. Next, address signals are transmitted from the controller 56 via the bus 712 to the decoder 623. The decoder 623 transmits a signal via line 622 to the memory 628 and processing circuitry transmits a read sample HNS data signal via bus 710 to the FIFO memory 630. The data in the FIFO memory 630 is then read via address/data bus 712 into the random access memory 703. Finally, the signals on bus 712 are set by the controller 56 so that the decoder 623 transmits a signal via line 624 to the memory 630 which then reads the sample HEW. These also are stored in the RAM 703. When all the data has been read from the FIFO memories 628, 630 and 632, a sampled data program is called for. The sampled data program, also stored in ROMs 704 and 706, utilizes the sampled high frequency and low frequency E and H field values of the lightning stroke to calculate the direction and measure the range of the lightning activity. The direction is calculated in terms of an azimuth angle e existing between the location of the lightning and the antennas 10, 12 and 14. As described earlier, each fast rise time pulse present in the lightning stroke triggers the circuitry (described earlier) to sample E and H fields radiated by the lightning stroke. For each sample taken (where each sample corresponds to one pulse in the lightning stroke), three values EV HNS and HEW are measured by the three antenna elements. For one lightning stroke in the preferred embodiment, a maximum of 30 sets of three sampled field values are measured and stored during the first 1 millisecond. As described earlier, the sampled field values are related by the equation, HEW Sin() + HNS Cos(B) Ez ZO Sin(O) This equation can be solved by using at least two different threesomes of the sampled field values in the set. The sampled data program using the equation above (assuming sin (Q) equals one) calculates using at least two threesomes of sampled field values from the set of sampled field values stored for each lightning stroke. The sampled data program calculates a plurality of times for each possible combination of two different threesomes found in the set (a maximum of 30 in the preferred embodiment). Each time the program calculates # it checks its validity as described above. In general, all pairs of pl will not agree exactly because of nonvertical lightning strokes. The program estimates the correction to the calculated Q necessary to satisfy the above equation. The sample data program calculates the centrÏld value of the plurality of 's calculated. This is the direction angle to the lightning flash from the equipment. The centroid value is based upon data collected during a predetermined period of time. In the preferred embodiment, this predetermined time period is one millisecond and there are several one millisecond data collection time periods during the occurrence of a lightning stroke. Next, the program calculates the standard deviation of the set of calculated azimuth angles. The standard deviation will be used to display the lightning activity as a region of activity rather than an isolated point. The standard deviation along with the mean angle ss is stored in RAM 702. The sample data program looks for pulses with durations of longer than 1 microsecond but less than 10 microseconds by examining the data stored in RAM 703 associated with the pulse width measurement for each high frequency sample. Sample data pulse widths less than 1 microsecond represent lightning predischarge or leader signals and do not in general have the low frequency signals present. Once the sample data program calculates a sample pulse with a time duration of greater than two microseconds for the preferred embodiment, the low frequency signal stored in RAM 702 for the E field measured during this 1 millisecond period is compared to the low frequency measured during other 1 millisecond periods of this lightning flash (Figure 6 depicts the total lightning flash). For the largest low frequency E field signal detected, its corresponding low frequency H field signal is used in the following equation to determine range: EMI19.1 where Hlow is the total low frequency H field signal detected during the above described 1 millisecond period, Hhigh is the corresponding maximum high frequency H field signal for the same period and R is the range of the lightning activity. This equation has the advantage over prior art of not producing ambiguous range values. The value of the ratio of the magnetic field, H, to the electric field, E, varies with the range from the lightning stroke. The above equation can be stored in ROM 706 as a table of ratios relating to a specific range. The estimated range to the lightning flash is stored in the RAM 703. The sample data program translates the lightning stroke location into a fix earth position. The fix earth position is determined by a main program stored in the ROM 706 when the processor 52 is initially turned on. The following equations are used to translate the new data: XP = R.cos (/) YP 5 R.sin X = XP.cos (HEA) + YP.SIN (HEA) + RAXO Y = YP.cos (HEA) - XP.SIN (HEA) + RAYO where is the angle to the flash R is the range to the flash RAXO is the present X position of aircraft RAYO is the present Y position of aircraft HEA is the present aircraft heading The lightning data is stored in memory as a function of range (g) and angle (p). Following the calculation of location to each pulse in a fixed earth coordinate, it is desirable to know the region having the most lightning activity. All sample point locations are stored in RAM 708 in a buffer area defined as G1. A schedule routine program (more detail is provided later) removes sample points from G1 which are older than four minutes. The sample data program uses the data base of all lightning locations which occurred within the last four minutes to define activity region boundries as cells and saves the cell data in buffer G2 for display. The test for inclusion of a set of lightning samples for display is defined as follows: G1 = (PO : Pn) all new lightning points G2 = (PO : Pm) points which exhibit clustering which is defined as cells. New lightning location PitGl. is tested for inclusion in G2. Pity2 if dip That is, every lightning point whose distance from at least one other point is less than or equal to a constant dp is stored in RAM 708. Each cell defined in buffer G2 is stored with the centroid of the cell (i.e. the average X and Y positions of all lightning flashes grouped together as described above), the number of lightning flashes and the last time the cell acquired new lightning data. In order to save processing time, the processor first compares the buffer G2 cell locations with the new lightning flash data. If the new lightning flash data is not from an existing cell area, then a new cell is defined in G2. It should be understood from the above description of the sample data program thus far, that the method and apparatus of this invention provides circuitry and microprocessing power which uses a great deal of the detailed electromagnetic field information resulting from a lightning stroke to determine accurately the range and region of activity of the lightning strokes relative to the equipment. The method of translating the new lightning data to a fixed earth position and then grouping all lightning data could be alternatively changed to continuously translate the old data to the current aircraft position and group the old data with the new lightning data. For example, as previously stated, the method and apparatus of the present invention utilizes the ratio of the magnitude of the magnetic (H) field measured at a first frequency to the magnitude of the H field measured at a second frequency to determine range. Note that in the preferred embodiment, the first frequency is lower than the second frequency. However, range information can also be determined in accordance with the present invention when the first frequency is higher than the second frequency and as such is within the scope and contemplation of the present invention. Now that the direction and distance of the lightning activity relative to the observing equipment is calculated, it must be displayed. But first, the distance calculated thus far must be adjusted for the range scale factor which is controlled by a switch 736 in Figure Ic. This manual selection of scale factor is monitored via lines 739 and 738 by the schedule routine program stored in the ROM 704. The schedule routine program is executed by controller 56 every 10 milliseconds. A detailed description of the design and software programming techniques using the INTEL 8085A for a minimum system is found in INTEL Component Data Catalog. 1980 edition, pages 6-9 through 6-24 which is incorporated by reference as if fully set forth herein. The controller 56 is interrupted once every 10 milliseconds by the clock output provided by RAM 702 (the INTEL Component Data Catalog completely explains the use of the timer output connection). The schedule routine program first determines if 0, 200, 500, 700 and 900 milliseconds (10 interrupts from RAM 703 equals 100 milliseconds) has been completed. At the 0 millisecond time the range input lines are monitored over lines 738 and 739. This range scale factor is stored in RAM 703 and is used to offset the range calculated by the sample data program. The schedule routine program calculates the true range on every 900 millisecond count and updates the display. This is accomplished by translating the x and y fixed earth rectangular coordinates of the lightning cells of buffer G2 into the aircraft coordinates and the results provided to the display means 62. The x and y coordinates are translated into aircraft coordinates as follows: X = R cos () - RAXO Y = R sin (/) - RAYO XP = X cos () + Y sin (O) YP = Y cos (0) - X. S in (O) where R is the average range to a cell in buffer G2 i is the angle to the centroid of the cell in buffer G2 g is the heading of the aircraft. The display processor is programmable and the information provided to it can be displayed and the display updated in a variety of ways. Display systems including a display processor, a display under the control of the display processor and programs for use by the display processor are commercially available along with an interface for accepting the information to be displayed. One such display system suitable for use with the present system is a DIGITS Corporation Rainbow 2000 System. See in particular DIGITUS Corporation Users Manual which is hereby incorporated by reference as if fully set forth herein. One such program shows the equipment origin at the centre of the display with azimuth angle varying 3600 about the origin. The farther away the lightning stroke is the farther away from the origin is the indication. The uncertainty in precise distance is shown as a shaded circle based on the number of sample points included in the set of lightning cells G2. The number of lightning flashes grouped in the cell is used to colour the circle. The preferred embodiment does not display the cell if there are less than four lightning flashes in the cell. The cell is shown as three different colours for lightning flash numbers between four and sixteen, sixteen and thirty two, and greater than thirty two. It is clear from the design thus far, the range scale can be varied by the switch 736. The program also allows for variation in how often the display is updated with new information; how long old information is retained; and the use of colour or intensity to differentiate between intensity levels of the storm based on similarities between old and new information about the lightning activity. The ability to locate and track electrical activity by aircraft is extremely important. In an aircraft the lightning's activity is displayed relative to the heading of the aircraft. The nose of the aircraft points at zero degrees azimuth. However, the speed and direction of the aircraft constantly changes the location of the lightning activity relative to the aircraft. To compensate for this the processing means is programmed with the schedule routine program previously discussed in above paragrph, which updates the RAXO and RAYO coordinates of the aircraft in the fixed earth coordinates. Heading information from the aircraft's navigational system is transmitted via a plurality of lines 732 to a synchro-to-digital converter (S/D) 740 in Figure ic. The heading information is analog in nature which the synchro to digital device 740 converts to digital data when processing circuitry means transmits a command over line 734. In the preferred embodiment, Model No. HXDC10-L-3, made by ILC Data Device Company, is used as the synchro to digital device 740. The change in azimuth display, caused by the aircraft heading (HEA), is calculated by the schedule routine program on the 900 millisecond count by subtracting the previous heading from the latest. The change in RAXO and RAYO coordinates of the aircraft is calculated by using the aircraft heading azimuth angle (HEA): RAXO = Speed x sin (HEA) RAYO t Speed x cos (HEA) where speed is the known speed of the aircraft stored in ROM 704 (this may be input from the aircraft navigation system) with value set in accordance with a particular type of aircraft. For example, a Cessna 182 aircraft has a nominal cruising speed of 120 knots. The schedule routine program updates the display every second. Finally, the sample data interrupt and sample data programs will operate when triggered by randomly occurring lightning strokes. These will not always occur every tenth of a second when the heading is being updated, thus it is desirable to use the latest heading information possible, between sampled headings at 200, 500, 700 and 900 milliseconds. Consequently, the sample data program will use the latest heading information available when calculating the azimuth and range to the lightning flash.

Claims: CLAIMS: 1. Apparatus for determining the location of lightning activity9 the apparatus comprising receiving means for receiving separately the electric and magnetic field components generated by a lightning stroke, measuring means for measuring the field strength of the electric and magnetic field components and processing circuitry, connected with the measuring means for determining the direction of the lightning activity as a function of the electric and magnetic field components, characterised in that the measuring means determine the field strength of one of the field components at both a first and a second frequency and the processing means, in addition to determining the direction, determines the range of lightning activity as a function of the field strengths of the said one of the field components at the first and second frequencies. 2. Apparatus as claimed in claim 1, characterised in that the field strength of the magnetic field component is measured at the first and second frequencies. 3. Apparatus as claimed in claim 1 or 2, characterised in that gating means, comprising a slope detector, receives signals related to the lightning activity from the receiving means and enables the measuring means according to the rate of rise of the amplitude of a field generated by a lightning stroke. 4. Apparatus as claimed in any of the preceding claims, characterised in that gating means comprises a threshold detector which receives signals related to the lightning activity from the receiving means and enables the measuring means according to a field strength generated by a lightning stroke. 5. Apparatus as claimed in any of claims 2 to 4, characterised in that the functional relationship between the range of lightning activity and its magnetic field at the first and second frequencies is governed by the equation: EMI26.1 where: R is the range of the lightning activity is is the first angular frequency is is the second angular frequency c is the velocity of light Ho is the ratio of the total magnetic field at the first frequency to the total magnetic field at the second frequency and al is the ratio of the dipole moments at the first and second frequencies. 6. Apparatus as claimed in any of the preceding claims, characterised in that the first frequency is substantially equal to 1.5 kilohertz and the second frequency is substantially equal to 500 kilohertz. 7. A method of determining the location of lightning activity comprising the steps of: receiving separately the electric and magnetic field components generated by a lightning stroke; measuring the field strengths of the electric and magnetic field components and processing the measured field strengths in order to determine the direction of lightning activity; characterised by the steps of: measuring field strength of one of the field components at both a first and a second frequency and processing the measured one of the field strengths in order to determine the range of the lightning activity as a function of the ratio of the one field strength at the first and second frequencies. 8. A method as claimed in claim 7, characterised in that the magnetic field strength is measured at a first and a second frequency in order to determine the range of the lightning activity. 9. A method according to claim 7 or 8 characterised by the steps of: detecting the rate of the rise in a field strength and initiating the measuring of the field strengths of the electric and magnetic field components only on detection of a field strength risinmg at greater than a predetermined rate. 10. A method according to any of claims 7 to 9, characterised by the steps of: detecting the electric field strength and initiating the measuring of the field strengths of the electric and magnetic field components only on detection of a field strength above a predetermined threshold level. 11. A method as claimed in any of claims 8 to 10, characterised in that the functional relationship between the range of lightning activity and its magnetic field at the first and second frequencies is governed by the equation: EMI28.1 where: R is the range of the lightning activity is is the first angular frequency #2 is the second angular frequency c is the velocity of light Ho is the ratio of the total magnetic field strength at the first frequency to the total magnetic field strength at the second frequency and al is the ratio of the dipole moments at the first and second frequencies. 12. A method as claimed in any of claims 7 to 11, characterised in that the first frequency is substantially equal to 1.5 kilohertz and the second frequency is substantially equal to 500 kilohertz.

Inventors: COLEMAN, ERNEST WILSON
Applicants: ERNCO INDUSTRIES INCORPORATED
Application year: 1985
Main classification: G01W1
IPC codes: G01W1, G01R29