Storm warning method and apparatus

An apparatus and method of detecting, tracking and displaying lightning activity is disclosed. A lightning stroke has associated therewith electric and magnetic field components characterized by maximum rise times and minimum power levels. The field signals comprise a plurality of sub pulses also. An electric field antenna and a pair of magnetic field antennas are disposed to receive the field components associated with lightning activity. Control circuitry cooperating with rise time and threshold measuring which operates on the field signals received by the antennas generates control signals including integration and sampling control signals for integrating the electric and magnetic field signals over a predetermined time interval (preferable one hundred microseconds) and for sampling and holding the field signals at each of the sub pulse peaks. Preprocessing circuitry upon command from a programmable microprocessor A to D converts the integrated and sampled field components where they are stored as digital data in FIFO memories. In response to control signals from the control circuitry the microprocessor transfers the digital data from the FIFO memories to its own memory whereupon it determines the azimuth and elevation angles to the lightning activity based on the sampled field data and determines the range based on the ratio of the magnetic to electric field components using the integrated values of the magnetic and electric fields. The angle and range information is transmitted to a display processor and display where it can be displayed in a variety of formats. Where the apparatus is mounted in an aircraft, the speed of the aircraft and changes in heading are factors into the determination and display of the angles and range.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus and method for displaying regions of 
lightning activity. 
Thunder storms characterized by turbulence and electrical activity 
(lightning) create great dangers particularly to air travel. It is 
therefore desirable to locate thunder storm activity as accurately as 
possible so that thunder storms can be tracked, predicted and avoided. 
Lightning associated with the mature stages of thunder storms generates 
electrical signals that propagate through the atmosphere. The detection, 
recognition, accurate measurement and analysis of these electrical signals 
provide a basis for storm tracking, avoidance, etc. 
Lightning flashes are composed of a series of high current lightning 
strokes, each stroke being preceeded by a lower current discharge called a 
leader. The duration of electrical activity associated with a lightning 
stroke varies but in many instances last as much as a hundred 
microseconds. The initial rise time of electrical signals associated with 
a lightning stroke almost never exceeds five microseconds. Following the 
first peak of the electrical signals of a lightning stroke, lesser signals 
of sub-microsecond duration but with fast rise times (of five microseconds 
or less) will occur. 
U.S. Pat. No. 4,023,408 discloses a storm mapping system which detects 
electrical activity caused by weather phenomenon such as lightning 
strokes. The system is intended to operate on the far field (or radiation 
field) pattern generated by the lightning stroke. According to the 
disclosure, the far field pattern is characterized mainly by a low 
frequency spectrum with maximum amplitude signals occurring between seven 
and seventy three kilohertz (KHZ). A trio of antenna sensors, an electric 
field antenna and two crossed magnetic field antennas, are used and each 
is connected to a tuned receiver on a center frequency of fifty KHz. The 
crossed loop magnetic field antennas are used to locate the lightning 
signals in azimuth angle by comparing the relative magnitude of the 
signals induced in the cross loop sensors to the electric field antenna in 
a conventional manner. The magnetic field signals are time correlated with 
the electric field signals before integration. This provides some measure 
of avoiding unwanted noise like signals. Integration of the correlated 
signals is formed for 0.5 milliseconds but only after the vector sum of 
the magnetic field sensor signals is found to exceed a predetermined 
threshold value. The algebraic sum of the magnetic field sensor signals is 
amplified and then squared. This signal is used to divide the integrator 
output signals thereby reducing the magnitude of larger correlated 
integrated signals below the magnitude of smaller ones. These inverted 
signals then drive a display such as a CRT display to show larger signals 
closer to the observation point and smaller signals farther away. 
This system has been used on aircraft and appears to work well, but it 
depends heavily on the magnitude of correlated electric and magnetic field 
signals to provide a measure of the range of the signal from the 
observation point of the equipment. Accordingly, the accuracy of range 
estimates may be affected by the variation in the severity of the thunder 
storms. Also, some of the detailed characteristics of lightning stroke 
signals are not utilized to discriminate between interfering signals and 
true lightning electrical signals. 
The Ruhnke U.S. Pat. No. 3,715,660, discloses an apparatus for determining 
the distance to lightning strokes. It does not measure or calculate the 
direction of the storm. Like U.S. Pat. No. 4,023,408 it discloses the use 
of crossed magnetic field sensors and an electric field sensor. 
Discrimination of lightning signals over background and interfering 
signals is provided by filtering the output of the antenna elements at one 
kilohertz. The square root of the outputs of the magnetic field sensors 
are compared with the absolute value of the electric field element to 
produce a number which is related to the ratio of the magnetic to electric 
fields. This ratio is related to range according to FIG. 8 of the subject 
application. The inventor, Ruhnke, first described this curve in a NOAA 
Technical Report ERL 195-APCL 16. 
As disclosed in the Ruhnke patent, range is calculated based on the 
.vertline.H/E.vertline. ratio curve of FIG. 8 of the subject application. 
However, the curve shows that ambiguities in range occur for some 
.vertline.H/E.vertline. values since .vertline.H/E.vertline. decreases 
after peaking at about 50 Km. No mention is made of how to resolve the 
ambiguity. Similarly, the sole discriminate for background noise relies on 
a one kilohertz filter. The full information contained in the details 
characterizing lightning strokes are not utilized. 
Within a few years of Doctor Ruhnke's effort, Doctor E. Krider and 
associates built a magnitude direction finder utilizing the initial few 
microseconds of a lightning stroke which provided accurate directions to 
the channel basis lightning discharges. Tests on a number of storms at 
distances of ten to one hundred kilometers indicated angular resolution in 
the range of one to two degrees. Another important observation by Dr. 
Krider was that the first few microseconds of a wide band magnetic 
waveform are due to the radiation field term and the general field 
equation and that the lightning channel near the ground tends to be 
straight and vertical which minimizes polarization errors. Dr. Krider's 
instrument did not actively address ranging. 
SUMMARY OF THE INVENTION 
The present invention comprises an apparatus and method for displaying the 
location of regions of recently occurring lightning activity. The 
invention comprises a receiving means for separately receiving the 
electric (E) and magnetic (H) field components of lightning signals over a 
wide range of frequencies. In the preferred embodiment, the receiving 
means includes a pair of cross-loop sensors and an electric field sensor. 
These sensors measure the time rate of change of the magnetic and electric 
flux densities. The outputs are suitably amplified and integrated to 
provide a measure of the E and H fields of the lightning signals. 
Recognition circuitry means connected to the receiving means recognizes 
lightning signals received by the receiving means and discriminates 
against interfering signals and background noise. The recognition 
circuitry means, in the preferred embodiment, comprises rise time 
circuitry means and threshold circuitry means which responds to the rise 
time of the electric field signals and the amplitude of the magnetic field 
components respectively. Alternatively, the rise time of the magnetic 
field components can be utilized. When a positive rise time signal and a 
positive threshold signal is present at the same time, first gating 
circuitry is triggered which provides a signal indicating that a bonafide 
lightning strike has occurred. In the preferred embodiment, only rise 
times of less than five microseconds will provide a positive signal. 
Control circuitry means connected to the recognition circuitry means 
provides control signals to the apparatus. It provides integration control 
signals, sampling control signals, and an interrupt signal. 
Integration circuitry means connected to the receiving means and to the 
control circuitry means, separately integrates the total E field and H 
field received from the receiving means. The integration is performed over 
a predetermined time interval in response to integration control signals 
received from the control circuitry means. In the preferred embodiment, 
when the rise time and theshold of the received lightning signals indicate 
that a valid lightning stroke is present at the E and H field sensors, 
switching circuitry is activated in response to the integration control 
signals to allow the output of the receiving means to be integrated over 
the predetermined time interval. In the preferred embodiment, this 
predetermined time interval is one hundred microseconds. 
The output of the receiving means is also transmitted to the sampling means 
which in response to sampling control signals from the control circuitry 
means samples the E and H field values of the receiving means. These 
samples are provided each time the electric field value peaks after a rise 
time which is less than the predetermined rise time, in this case five 
microseconds. These fast rise time signals are held at the peak value by 
track and hold circuitry and then converted by A to D converters and 
stored in a first-in first-out (F IF O) memory. 
When the predetermined integration interval is over, that is, when the 
lightning stroke activity ends, an interrupt signal from the control 
circuitry means is transmitted to a programmable processing means which in 
turn reads the contents of the F IF O memory into its own read only memory 
(ROM). At the same time, the processing circuitry means also reads the 
integrated E and H field values for the predetermined interval through a 
multiplexer circuit and one of the F IF O memories. The sampled E and H 
field values read from the F IF O's are related by the following equation: 
EQU -H.sub.x sin (.phi.)+H.sub.y cos (.phi.)=E.sub.z /(Z.sub.o sin (.theta.)) 
where .phi. is the azimuth angle to the lightning stroke and .theta. is the 
elevation angle. Two 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 .theta. and .phi.. Because in general each lightning stroke 
produces a plurality of sampled E and H field values, at least one set of 
values for .theta. and .phi. can be solved for by the processing circuitry 
means. 
The total magnetic field value is calculated from the sampled magnetic 
fields and the value compared with a predetermined field strengths to 
estimate which of several range regions the lightning stroke occurred in, 
that is, in the near, mid, or distant range region. Similarly, the 
integrated H field value is also compared to predetermined field strengths 
to predict which range region the lightning stroke occurred in. If the 
regions predicted by the two methods are adjacent regions or the same 
region then the ratio of the integrated H/E field values are 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 H to the total E field value is calculated from the 
sample data and used to determine the range from the look-up tables. 
A standard deviation value for the set of elevation and azimuth angles 
calculated from the sampled field values is calculated. The standard 
deviation value and the range value to a lightning stroke (expressed in 
rectagular coordinates) is transmitted to a programmable display means. 
The display means then displays the lightning stroke activity as a region 
of activity on a display, in accordance with its own programming. 
When the apparatus of the present invention is installed in an aircraft, 
means for adjusting and updating the measurement of lightning stroke 
location for aircraft movement (speed and heading) is provided.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a block diagram of the preferred embodiment of the present 
invention. The apparatus 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 antenna element 12 is 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 because of electrical 
activity associated with lightning strikes. The signals received by the 
antenna elements are transmitted to integrators designated generally 16, 
18 and 20 which provide an east/west magnetic field component, a 
north/south magnetic field component and a vertical electrical field 
component respectively to wide band amplifiers 22, 24 and 26. In the 
preferred embodiment, the antenna elements, the integrators and the wide 
band amplifiers operate over the frequency range of one kilohertz (KHz) to 
fifteen megahertz (MHz) but other bandwidths larger or smaller could be 
used. 
The outputs of the wide band ampifiers 22 through 26 are provided in 
parallel to a sampling portion of the circuitry of the apparatus and to 
the recognition circuitry portion of the apparatus. The recognition 
circuitry portion of the apparatus comprises slope detection circuitry 
designated generally 30, absolute summer circuitry designated generally 32 
and threshold detection circuitry designated generally 36. The output of 
wide band amplifier 26 containing the vertical electric field signal is 
transmitted via line 27 to slope detection circuitry 30. If the rise time 
of this signal is less than a predetermined rise time a positive signal is 
transmitted to timer circuitry designated generally 34 via line 31. 
Similarly, the magnetic field signals transmitted from wide band 
amplifiers 22 and 24 respectively are summed vectorially in the absolute 
summer circuitry 32. The output of this circuitry represents the magnitude 
of the total magnetic field present at the antenna elements 10 and 12. 
This signal is transmitted via line 33 to threshold detection circuitry 36 
which provides a positive signal to timer circuitry 34 via line 35 if the 
signal exceeds a predetermined threshold level. The output of the absolute 
summer circuitry is also transmitted via line 39 to integrator circuitry 
40 while the electric field signal from wide band amplifier 36 is also 
transmitted via line 41 to integrator 42. 
If the voltage level of the magnetic field signal from circuitry 32 exceeds 
the predetermined threshold level of circuitry 36 and if the rise time of 
the vertical electric field signal from wideband amplifier 26 is less than 
the predetermined rise time of slope detection circuitry 30 then the time 
circuitry 34 is caused to provide integration control signals via line 53 
to the integrators 40 and 42. Integrators 40 and 42 then integrate the 
magnetic and electric field signals over a predetermined time interval 
which corresponds to the most likely period over which the lightning 
stroke exists. The slope detection circuitry 30, the absolute summer 
circuitry 32, and the threshold detection circuitry 36, provide a means 
for recognizing the magnetic and electric field signals associated with 
the lightning strike in the presence of interferring and background noise 
signals. 
The north/south and east/west magnetic field signals are also transmitted 
over lines 55 and 57 to sampling circuitry 44 and 46 respectively. The 
vertical electric field signal from wide band amplifier 26 is transmitted 
via line 59 to sampling circuitry 48. Samples of the magnetic field and 
electric field signals representing the peak of each change in the fields 
are taken in response to peak detection signals provided from timer 
circuitry 34 over lines 43, 45 and 47. Peak detection signals are 
generated by timer circuitry 34 in response to positive output signals 
from the slope detection circuitry 30. 
The samples of the magnetic and electric field signals are converted to 
digital signals by A/D converters in response to the peak detection 
signals. The digital signals are stored in first-in first-out (F IF O) 
memories. Similarly, the integrated electric field and magnetic field 
signals from integrators 40 and 42 pass through A/D converters and are 
stored in F IF O memory. Together the A/D converters and F IF O memories 
are shown as preprocessing circuity 50 in FIG. 1. The peak detection 
signal for the A/D converters are transmitted thereto via line 49. 
The apparatus further comprises processing circuitry means designated 
generally 52. When the predetermined integration interval corresponding to 
the existence of the lightning stroke is ended the timer circuitry 34 
transmits an interrupt control signal via line 54 to the processing 
circuitry means which in response thereto commands the first-in first-out 
memories of preprocessing circuitry 50 over multiple lines 51 to transmit 
the samples of the magnetic and electric field values and the integrated 
magnetic and electric field values to a memory in processing circuitry 
means 52. 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 a display means which 
then displays the results on a display. The display means comprises a 
display processor 60 and display 62. In the preferred embodiment, the 
processing circuitry means 52 is enabled to compensate for movement of a 
host platform (such as an aircraft) when determining the region of 
lightning activity. 
Referring now to FIG. 2, further details of the integrators 16 through 20 
are provided. In the preferred embodiment, the north/south magnetic field 
antenna 10 and the east/west magnetic field antenna 12 are commercially 
available antennas from EG&G Company, Models Nos. CML-7 (R). The electric 
field antenna element 14 is an EGG Model No. FPD-2 B(R). The antenna 
elements 10 through 14 are AC coupled through capacitors 202 for the 
magnetic field elements and capacitor 204 for the electric field antenna 
14. The integrators 16 through 20 comprise amplifiers 17, 19 and 21 
(Analog Device Model No. AD509) in parallel with filter circuitry 
designated generally 206 and 208 for magnetic field antennas 10 and 12 
respectively and designated generally 210 for electric field antenna 
element 14. The filter circuitry filters out unwanted high frequency 
components detected by the antenna elements and outside the frequency band 
of interest. Further details on the design of magnetic antenna sensors and 
wide band integrators suitable for the present invention can be found in a 
paper by E. Philip Krider and R. Karl Noggle entitled "Broad Band Antenna 
Systems for Lightning Magnetic Fields", Journal of Applied Meteorology, 
Vol. 14, March 1975 hereby incorporated by reference as if specifically 
set forth herein. A similar circuit is reported by Fisher and Uman, in 
"Measured Electric Field Rise Times for First and Subsequent Lightning 
Return Strokes", Journal of Geophysical Research, Vol. 77, Jan. 1972 which 
is hereby incorporated by reference as if specifically set forth heren. 
Selection of impedance matching devices are described in these two 
references and further information may be found in numerous articles on 
lightning measurement techniques. The outputs from the integrators 16 
through 20 are then transmitted over lines 212, 214 and 216 respectively 
to single pole three throw switches 218, 220 and 222 (Analog Device Model 
Nos. AD7502). The output of the switches provide the weak amplified 
north/south and east/west magnetic field components and the vertical 
electric field components. The functioning of the switches will be 
described later in connection with the processing circuitry means. 
FIG. 3 shows in some detail a transistorized wide band amplifier 300 of 
conventional design. It comprises at least four amplifier stages which are 
directly coupled. The wide band amplifier operates over a frequency range 
from one KHz to 15 MHz and has a maximum gain of approximately six 
thousand. The selection of the parameters of the wide band amplifier 
depends on the type of sensors used, the electric environment and 
subsequent processing circuitry. The wide band amplifier shown in detail 
in FIG. 3 is suitable for use as amplifiers 22, 24 and 26 in FIG. 1. 
Alternatively, wide band amplifiers 22, 24 and 26 can be provided using 
Motorola wide band amplifiers (Model No. MWA 110,-120,-130) in a cascaded 
circuit design. See Motorola RF Data Maual, 1980 edition, pages 16-33 
through 16-40 hereby incorporated by reference as if specifically set 
forth herein. 
Before describing FIG. 4 which shows the absolute summer circuitry 32, the 
slope detection circuitry 30, the level detection circuitry 36, timer 
circuitry 34 and integrators 40 and 42 in detail, it is instructive to 
discuss the shape of the expected field voltage pulses received from the 
wide band amplifiers 22, 24 and 26. In FIG. 5 a series of even numbered 
curved designated generally 500-506 are shown representing the vertical 
electric field E.sub.V from wide band amplifier 26, the H.sub.NS field 
voltage from wide band amplifier 24, the H.sub.EW field value from wide 
band amplifier 22 and the absolute magnitude of the combined magnetic 
field, respectively H. The ordinate in each curve represents voltage 
amplitude in millivolts while the abscissa represents time in 
microseconds. Inspection of the curves show that the H and E field 
voltages associated with a single lightning stroke (many strokes comprises 
a lightning flash) usually last about one hundred microseconds. Much 
research in the nature of electrical signals arising from lightning 
strokes indicates that almost all vertical electric field voltages 
(E.sub.V) will have an initial rise time (see 507 on curve 500) of less 
than five microseconds, and that the pulse 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 
508, 510, 512 etc. on curve 500 in FIG. 5. The recognition circuitry of 
FIG. 4 takes advantage of these characteristics to cooperate with the 
control circuitry portion of FIG. 4 to furnish control signals for 
integrating and sampling the E and H field voltages. 
Referring now to FIGS. 1 and 4, the absolute summer circuitry designated 
generally 32 connected to the H.sub.EW and H.sub.NS wide band amplifiers 
22 and 24 on lines 402 and 404 respectively comprises a pair of first 
circuitry portions designated generally 406 and 408 and an output 
operational amplifier 409. Capacitors 410 and 412 block unwanted DC 
components from the first circuitry portions 406 and 408 but pass the 
pulses magnetic field voltages H.sub.NS and H.sub.EW. 
First circuitry portion 406 (which is identical to circuitry portion 408) 
comprises an operational amplifier 414 and switching circuitry designated 
generally 416. If the H.sub.NS field voltage is negative then operational 
amplifier 414 switches resistors 405 and 413 into the circuit at the 
output of 409. This has the effect of inverting the H.sub.NS voltage. If 
H.sub.NS is positive the switching circuitry 416 by-passes the resistors 
405 and 413 and the output is through resistor 407. In this manner only 
the absolute magnitudes of the H.sub.NS and H.sub.EW field voltages are 
delivered to operational amplifier 409 over single input line 418. 
The combined magnetic field voltage from operational amplifier 409 is then 
transmitted to operational amplifier 420 via the line 33 (see also FIG. 
1). Operational amplifier 420 is biased to a predetermined negative 
threshold voltage. Unless the combined magnetic field voltage exceeds the 
magnitude of the threshold voltage, operational amplifier 420 provides a 
low output. The biased operational amplifier 420 provides the threshold 
detection function of threshold detection circuitry 36. Curve 506 in FIG. 
5 represents the total magnetic field signal transmitted from operational 
amplifier 409. 
The vertical electric field voltage E.sub.V is transmitted from wide band 
amplifier 26 via line 27 (see FIG. 1) and then lines 425 and 426 through 
DC blocking capacitors 427 and 428 to the slope detection circuitry 
designated generally 30. Slope detection circuitry 30 comprises two 
parallel similar circuits designated generally 430 and 432. Circuitry 430 
comprises operational amplifier 434 (Analog Device Model No. AD 509) with 
two inputs 435 and 436. Input 435 is connected to input 436 via resistor 
437 and input 436 is connected to ground through capacitor 438. 
Operational amplifier 434 will provide a positive output voltage to 
inverter 446 whenever the field voltage E.sub.V increases faster than 
capacitor 438 can charge through resistor 437. The product of resistor 437 
and capacitor 438 determines the sensitivity of the slope detection 
circuitry. For the preferred embodiment, this product is set to 62 
microvolts per second thus allows only the submicrosecond lightning pulses 
to be detected. Input 436 has a positive voltage bias through resistor 439 
and the ratio of resistors 437 and 439 sets the minimum signal level which 
the slope detection circuit will detect. The preferred embodiment is set 
at 2 millivolts. 
Similarly, circuitry 432 comprises operational amplifier 440 (Analog Device 
Model No. AD509) and it will provide a positive output to inverter 448 
whenever field voltage E.sub.V decreases faster than capacitor 442 can 
charge through resistor 444. Either a positive or negative voltage change 
of E.sub.V having the correct rise time as functionally defined by 
circuitry 430 and 432 will provide a positive input to inverter 446 or 
448. If no fast rise time occurs, the outputs of operational amplifiers 
434 and 440 will be low triggering a high output from inverters 446 and 
448. Examples of submicrosecond pulses which will trigger the above 
described circuitry is shown as slopes 507, 508, 510 and 512 in FIG. 5. 
The outputs of inverters 446 and 448 are furnished as inputs to NAND gate 
450. In the absence of any fast rise time pulses from wide band amplifier 
26, the outputs of the inverters are high and NAND gate 450 output is low. 
When a slope of the proper rise time (either positive or negative) occurs 
on the E.sub.V voltage pulse 500 one of inverters 446 or 448 will go low 
causing the output of NAND gate 450 to go high until the peak voltage on 
the pulse on curve 500 is reached and then the output of NAND gate 450 
will go low. See FIG. 5, curve 514, where the output of NAND gate 450 is 
shown as a series of pulses 516 corresponding to fast rise time on the 
E.sub.v signal 500. 
The output of NAND gate 450 is transmitted via line 452 to NAND gate 454 
and line 456 to NAND gate 458. A flip flop 460 is provided and its Q 
output is transmitted to NAND gates 454 and 458 as a high gating signal. 
NAND gate 458 also receives the output of operational amplifier 420 from 
threshold detection circuitry 32 via line 35 as a third input. 
When the processing circuitry means 52, (see FIG. 1) clears flip flop 460 
with a data clear signal via line 459a (after it has finished taking data 
from preprocessing circuitry 50) a high signal is transmitted from 460 Q 
to NAND gates 454 and 458 enabling them. When a low signal is transmitted 
to NAND gate 450 (indicating a fast rise time) it transmits a high to NAND 
gate 454 and 458. If at the same time, threshold detection operational 
amplifier 450 transmits a high to NAND gate 458, NAND gate 458 will 
transmit a low signal to one shot 462 which upon receiving the low signal 
will generate or transmit a one hundred microsecond long low signal at its 
Q output 463. See signal 517 of FIG. 5 for the 100 microsecond low signal. 
This low signal is transmitted via line 464 to a low enable input of one 
shot 468. 
Whenever slope detection circuitry 30 provides a low signal input to NAND 
gate 450 in response to a fast rise time pulse on signal 500, NAND gate 
450 transmits a high signal to NAND 454. If the Q output from flip flop 
460 is also high then NAND gate 454 will transmit a low signal to one shot 
468 as long as the output from gate 454 remains high. (This output remains 
high until the fast rise time pulse from wideband amplifier 26 peaks and 
then it returns low.) As the signal from NAND gate 450 returns low, the 
output of NAND gate 454 will go high as this will trigger one shot 468 (as 
long as the low enable input is low) to generate a 250 nanosecond pulse on 
line 465 on the leading edge of the high signal from NAND gate 454. This 
250 nanosecond high signal is one of the control signals generated by 
timer circuitry 34 and is called the peak detection signal. Peak detection 
signals occur even when a positive threshold signal from operational 
amplifier 420 is absent as long as one shot 468 is enabled by the one 
hundred microsecond pulse from 462Q. The peak detection signals are shown 
on curve 518 of FIG. 5. Note that they occur on the falling edge of the 
NAND gate 450 high signal which is the same as the leading edge of the 
high going signal from NAND gate 454. 
The total magnetic field from absolute summer circuitry 32 is also 
transmitted via line 39 (see FIG. 1) to integrator 40 which comprises an 
input switch 471 and integrator circuitry designated generally 478 in 
parallel with a second switch 481. Integrator circuitry 478 comprises an 
operational amplifier 479 in parallel with a capacitor 480. The output of 
switch 471 is connected to the input of the parallel arrangement of 
integrator circuitry 47 and second switch 481. The vertical E field 
component from wideband amplifier 26 is also transmitted through DC 
blocking capacitor 474 over line 41 to an integrator 42 which is identical 
in design to the integrator 40. Input switches 471 are normally open but 
they are switched to closed by the one hundred microsecond low signal from 
one shot 462 Q along lines 475 and 476. When the switches are closed, the 
E.sub.V and H field voltages are transmitted to integrator circuitry 478. 
The second switch 481 in each circuit is normally closed and provides an 
alternate path to the integration circuitry 478. However, the low one 
hundred microsecond pulse 517 from one shot 462 is also connected to one 
shot 485. Upon receiving it, one shot 485 on the falling edge of the low 
signal 517 generates and transmits a high two hundred microsecond pulse 
(see diagram 520 of FIG. 5) over lines 486 and 487 to switches 481 to open 
them. 
When input switches 471 are open, closed second switches 481 provide a 
means for keeping integrator circuitry 478 from saturating on input noise. 
Switches 481 open when switches 471 close but switches 481 remain open for 
100 microseconds after switches 471 open to allow the H and E.sub.v 
integrated signals to be transmitted to preprocessing circuitry 50 in FIG. 
1. The one hundred microsecond 462 Q signal to switches 471 and 485 Q 
signal to switches 481 are control signals controlling the integration of 
the E and H field voltages over the one hundred microsecond interval. 
Finally, the Q output of one shot 462 is connected via line 490 to flip 
flop 492. When the one hundred microsecond interval is over and 462 Q goes 
high it sets flip flop 492 causing a high signal to be transmitted via 
line 54 to the processing circuitry means 52 as an interrupt signal (see 
diagram 522 in FIG. 5). At the same time, flip flop 492 Q goes low an is 
transmitted via line 495 as an inhibit data measurement signal to flip 
flop 460 setting it and causing 460 Q to go low. The low signal disables 
NAND gates 454 and 458 while the processor circuitry means 52 interrogates 
preprocessing circuitry 50 for the sampled and integrated E and H field 
voltages (see diagram 524 in FIG. 5 for the inhibit data measurement 
signal). When the processing circuitry means 52 has finished interrogating 
the preprocessing circuitry 50, it transmits a clear signal to flip flops 
460 via line 459a (see diagram 530 in FIG. 5) and 492 via line 459b (see 
diagram 526 in FIG. 5) which results in enabling gates 454 and 458 and 
disabling the high interrupt signal 54 to the processing circuitry means 
52 from flip flop 492. 
All of the operational amplifiers shown in FIG. 4 are Analog Device Model 
No. AD509 amplifiers. Switches 471 and 481 are Analog Device Model No. 
AD7513 switches. The remaining logic and circuit components are easily 
recognized and commercially available components. 
FIG. 6 shows the electric (E.sub.V) and magnetic (H.sub.NS and E.sub.EW) 
field sampling circuitry designated generally 44, 46 and 48 and 
preprocessing circuitry 50 of FIG. 1 in more detail. Capacitors 602, 604 
and 606 block unwanted DC components from the signals E.sub.V, H.sub.NS 
and H.sub.EW respectively furnished over lines 59, 55 and 57 but pass the 
pulsed field voltages. The field voltages are then transmitted through 
delay circuitry 608 a, b and c (Harry J. White Co. Model No. MD600201K) 
before being forwarded to track and hold circuitry 610 a, b and c. The 
track and hold circuitry (Model No. HTC-0300 MM, made by Analog Devices) 
in response to the peak detection control signals from one shot 468 over 
line 465 holds the peak field voltage constant. The voltage held by track 
and hold circuitry 610 a, b and c is the peak voltage occurring after a 
five microsecond or less rise time in the E.sub.V voltage 500. (see the 
slopes 507, 508, 510 and 512). There is some delay of the peak voltage as 
it passes through the circuitry of FIG. 4 before a peak voltage detection 
control signal is generated. To accommodate this delay, delay circuitry 
608 a, b and c is added. In the preferred embodiment, about six hundred 
nanoseconds of delay is required. 
Each peak field voltage signal held by track and hold circuitry 610 a, b 
and c is converted to a digital signal by A to D (A/D) converters 612 a, b 
and c in response to the falling edge of the peak detection signal from 
line 465 and causes a low signal on lines 634 a,b and c and 638 a,b and c. 
In the preferred embodiment, the A/D converters 612 a,b and c are 
Datel-Intersil devices, Model No. ADC817MM, which converts the signal into 
a twelve bit digital signal in 2.5 microseconds when the conversion is 
complete. A/D converters 612 a,b and c then transmits a high signal over 
lines 634 a,b and c which causes the digital sampled peak field voltage to 
be clocked into the first-in first-out (FIFO) memories 614 a,b and c where 
they are stored until called for by the processing circuitry means 52. As 
described earlier, each peak signal on waveform 500 (E.sub.V ) in FIG. 5 
which has a rise time less than equal to five microseconds causes a peak 
detection signal which then causes a sample of the E.sub.V, H.sub.NS and 
H.sub.EW field signals to be taken and stored in FIFOs 614 a,b and c 
during the one hundred microsecond interval (signal 517 in FIG. 5) 
generated by one shot 462 Q in FIG. 4. In the preferred embodiment, FIFOs 
614 a,b and c are Advanced Micro Devices, Model No. AM 2812A. 
When the A/D converters generate rising edge signals over lines 634 a,b and 
c and transmit the twelve bit digital signals to FIFO memories 614a,b and 
c, they also transmit high data ready signals to AND gate 616 over lines 
638a,b and c. AND gate 618 transmits a high data ready signal over line 
618 to one shot 468. This enables one shot 468 to generate a new peak 
detection signal. If the data ready signal were not used to enable 468 a 
second closely occurring peak voltage signal on waveform E.sub.V would 
generate a second peak detection signal causing the A/D conversion process 
of the first sample signal to be interrupted. 
An alternate embodiment for implementing the integration of the total H and 
E field voltages requires modifications to FIG. 4 and is described in 
block diagram form in FIGS. 9a,b and c. A true RMS measurement of the 
H.sub.EW, H.sub.NS and E.sub.V field voltages are made accurately by a 
commercially available device Analog Devices Model No. AD536. The 
preferred embodiment implements the auxiliary dB output of the AD536A as 
described in the Analog Devices Data Acquisition Products Catalog, 1978 
edition, pages 229-234 hereby incorporated by reference as if specifically 
set forth herein. Input to the RMS converters 900a,b and c are from 
switches 901a,b and c which are identical to the switches 471 described in 
FIG. 4. One shot 462Q in FIG. 4 transmits a low 100 microsecond pulse to 
switches 901a,b and c over lines 904a,b and c closing them and allowing 
RMS converters 900a,b and c to integrate the H.sub.EW, H.sub.NS and 
E.sub.V field voltages. At the same time high signal 462Q opens switches 
903a,b and c via lines 906a,b and c. Opening switches 903a,b and c sets 
the RMS converters 900a,b and c to a known voltage level (ground for 
preferred embodiment) before starting the 100 microsecond integration. The 
outputs of RMS converters 900a,b and c connect to Sample and Hold (S/H) 
circuitries 902a,b and c. The 100 microsecond low signal from 462Q is also 
connected to S/H circuitries 902a,b and c via lines 900a,b and c. On the 
rising edge of the low signal, the (S/H) circuitries 902a,b and c hold the 
integrated voltages while the processing circuitry means converters and 
stores them as digital data. The S/H circuitries 902a,b and c for the 
preferred embodiment are Harris Corporation Model No. HA-2420-8 devices. 
With the alternate embodiments of FIG. 9a, the total H field is not 
available as an analog voltage. The threshold detection signal for 
amplifier 420 is provided by connecting the E.sub.V field voltage thereto 
via line 911 which is connected to line 27. See FIG. 9b. Finally, in FIG. 
9c, output lines 912a,b and c from S/H circuitries 902a, b and c are 
provided to the multiplexer circuitry 636 of FIG. 6. The lines 912a,b and 
c will now represent the dB RMS voltage of the H.sub.EW, H.sub.NS and 
E.sub.V fields. The multiplexing and A/D conversion of these signals are 
similar to that described hereinafter for the integrated E and H fields on 
lines 66 and 68 in connection with the description of FIGS. 6 and 7. 
The processing circuitry means 52 is described in block diagram form in 
FIG. 7. The processing means comprises the controller 56, random access 
memory 702, read only memory 704, random access memory 706, arithmetic 
processing unit 707, and an address data bus 710. The various portions of 
the processing circuitry means 52 are connected together by the 
address/data bus 710. 
The processing circuitry means is programmable and the programs are stored 
in ROM 704. After the processing circuitry means has finished processing 
data and is ready to receive new data from a new lightning strike it 
transmits a data clear signal (see diagram 530 in FIG. 5) to flip flops 
460 and 492 in FIG. 4 over lines 459a and b. Lines 459a and 459b are shown 
connected to the random access memory 706. When a lightning strike is 
recognized by the circuitry of FIG. 4, a one hundred microsecond 
integration and sampling time is set by the low output of one shot 462Q as 
described earlier. When this one hundred microsecond interval is over the 
change in state of the output of one shot 462 sets flip flop 492 providing 
an interrupt signal 522 over line 54 to the controller 56. 
The processing circuitry means is now ready to read the data via bus 712 
through the read only memory 704. The data to be read are the sampled 
E.sub.V and H.sub.NS and H.sub.EW field values stored in the FIFOs 614 and 
the integrated E.sub.V and magnetic field values transmitted from 
integrators 40 and 42 (or, alternatively, S/H circuitries 902a,b and c). 
To accomplish this the processing circuitry means transmits sample data 
control signals (see diagrams 526, 528 in FIG. 5) over lines 51 (FIG. 1) 
which comprise even numbered control lines 716 through 624 (FIGS. 6 and 7) 
from the read only memory 704. Signals 720 and 722 are transmitted to 
decoder 620 of FIG. 6 which in turn provides interrogation signals over 
lines 622,624 and 626 to tri-state buffers 628,630 and 632 (Intel Device 
No. M8212) respectively. The tri-state buffers 628 through 632 are 
connected to FIFO 614a, b and c respectively. 
Control lines 716 and 718 from read only memory 704 are connected to 
multiplexer 636 of FIG. 6. Multiplexer 636 (Analog Device No. AD7502) is 
connected between track and hold circuitry 610 c of H.sub.EW sampling 
circuitry 46 and A/D converter 612c. The integrated vertical electric 
field from integrater 42 of FIG. 1 is transmitted via line 66 to 
multiplexer 636 while the integrated magnetic field from integrator 40 is 
transmitted via line 68 to multiplexer 636 in response to command signals 
transmitted over lines 716 and 718. (See also FIGS. 9a through 9c). The 
processing circuitry means transmits an A/D convert command over line 724 
to A/D converter 612c via OR gate 642. At the completion of an A/D 
conversion the digital data is transmitted to FIFO 614c. 
Assuming that the circuitry of FIG. 4 is cleared to accept data from a 
lightning strike, when a lightning strike occurs that is recognized by the 
circuitry of FIG. 4, a one hundred microsecond pulse is transmitted from 
one shot 462 in a manner as described previously. As the one hundred 
microsecond interval ends 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 ROM 704. The sample 
data interrupt program transmits an address signal via lines 716 and 718 
to multiplexer 736 and a command signal via line 724 to read the 
integrated vertical electric field value over line 66 into A/D converter 
612c where it is converted into digital form. Next the address over lines 
716 and 718 is changed so that multiplexer 636 reads the integrated 
magnetic field value from line 68 into A/D converter 612c where it is 
converted into digital form. (In FIGS. 9a-9c, three separate addresses on 
lines 716 and 718 are required to read the integrated H.sub.NS, H.sub.EW 
and E.sub.V fields on lines 904a,b and c). From A/D converter 612c the 
digital values of the electric and magnetic fields are transmitted to FIFO 
614c on the rising edge of control line 634c. Next control signals are 
transmitted from read only memory 704 via lines 720 and 722 to decoder 
620. Decoder 620 transmits a signal via line 624 to tri-state buffer 630 
and processing circuitry means transmits a read sampled H.sub.NS data 
signal over line 728 to FIFO 614b. The data in FIFO 614b is then read via 
line 712 into the read only memory 704 and is stored in RAM 706. When all 
the data has been read from FIFO 614b, the signals via lines 720 and 722 
are changed by the processing circuitry means so that decoder 620 
transmits a signal via line 626 to tri-state buffer 632. A read sampled 
H.sub.EW data signal over line 726 from the processing circuitry means 52 
causes the sampled H.sub.EW data stored in FIFO 614c to be transmitted via 
line 712 into read only memory 704 and stored in RAM 706. Finally, the 
signals via lines 720 and 722 from read only memory 704 are changed and 
the decoder 620 transmits a signal via line 622 to tri-state buffer 628 
which then reads the sampled E.sub.V data in FIFO 614a along with the 
integrated E.sub.V and H field values on command from processing circuitry 
means via line 730. These also are stored in RAM 706. When all the data 
has been read from the FIFO 614a,b and c a Sampled Data Program is called 
for. 
The Sample Data Program also stored in ROM 704 utilizes the sampled and 
integrated E and H field values of the lightning stroke to calculate the 
direction and to measure the range of the lightning activity. 
The direction is calculated in terms of an elevation angle .theta. existing 
between the location of the lightning stroke and the antennas 10, 12 and 
14; and an azimuth angle .phi. existing between the location of the 
lightning and the same antennas. As described earlier, each fast risetime 
pulse present in the lightning stroke triggers the circuitry described 
earlier to sample the 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 E.sub.V, H.sub.NS and H.sub.EW 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 100 microseconds. 
As described earlier, the sampled field values are related by the equation, 
##EQU1## 
This equation in two unknowns can be solved by using two different 
threesomes of the sampled field values in the set. For example, 
##EQU2## 
Since the Sin .theta. must be less than 1, the bracketed expression above 
on the right side of the equation opposite .theta. must also be less than 
1. If it is not the calculated .theta. and .phi. values are invalid. The 
Sample Data Program using the equations above for .theta. and .phi. 
calculates .theta. and .phi. using two threesomes of sampled field values 
from the set of sampled field values stored for each lightning stroke. The 
Sample Data Program calculates .theta. and .phi. a plurality of times for 
each possible combination of two different threesomes found in the set 
(maximum of 30 for preferred embodiment). Each time the Program calculates 
a .theta. and .phi. pair it checks its validity as described above. In 
general, all pairs of .theta. and .phi. will not agree exactly. 
The Sample Data Program calculates the centroid value from the plurality of 
.theta. and .phi.'s calculated. This is the direction angle to the 
lightning stroke from the equipment. Next, the Program calculates the 
standard deviation of the set of calculated elevation and azimuth angles, 
.theta. and .phi.. Later it will be seen that the centroid value of 
.theta. will be used to convert the range to the projected distance on the 
ground to the lightning stroke. The standard deviation will be used to 
display the lightning activity as a region of activity rather than an 
isolated point. The standard deviation is stored in RAM 702. 
The absolute value of the ratio of the magnetic field, H, to the electric 
field, E, (.vertline.H/E.vertline.) varies in a predictable way with the 
range from the lightning stroke. See FIG. 8. FIG. 8 is separated into 
three range regions: the near region 800 from 0 to 10 kilometers; the mid 
region 802 from 10 kilometers to 50 kilometers; and the far region 804 
from 50 kilometers and greater. Note that .vertline.H/E.vertline. peaks in 
value at about 50 kilometers and that it is possible to have range 
ambiguities for values of .vertline.H/E.vertline. near the peak, i.e., for 
a given value of .vertline.H/E.vertline. near its peak value two possible 
ranges are possible, one in the far range 804 and one in the mid range 
802. 
The Sample Data Program, stored in ROM 704, first calculates the mean value 
of the H.sub.NS field components for the lightning stroke in question from 
the sampled components of H.sub.NS that had been stored in FIFO 614b and 
stored in RAM 706 of the processing means. Then in similar fashion 
H.sub.EW is calculated (from the sampled values of H.sub.EW that had been 
stored in FIFO 614c). A total value, P, for the magnetic field is 
calculated by adding H.sub.NS to H.sub.EW. 
A magnetic or electric field radiated from a lightning stroke is assumed to 
have an initial amount of energy associated therewith. This energy 
attenuates as the E and H fields travel farther from the source of the 
lightning stoke. For example, at ten kilometers from the stroke the 
expected energy of the E and H fields is the initial energy minus the 
amount of attenuation occuring in ten kilometers. This energy can be 
assigned a value, K1. At 50 kilometers the expected value is K2. To 
determine a rough estimate, TA, of the range of the lightning strike and 
to help resolve the ambiguity present in the curve of FIG. 8, the Sample 
Data Program compares P with K1 and K2. If P is greater than K1 then the 
lightning stroke is close and the near range of curve 800 is used. If P is 
greater than K2 then the mid range is used; otherwise, the lightning 
stroke is in the far range. 
The Sample Data Program compares the total power in the H field from the 
lightning stroke (as calculated from the samples of the H field stored in 
FIFO's 614b and 614c) with first set of predetermined range values K1 and 
K2 as described above. 
This estimate is called TA. In a similar manner, the Sample Data Program 
takes the integrated H field value (as integrated by integrator 40; 
multiplexed by multiplexer 636; converted by A/D converter 612c; and 
stored in FIFO 614c until transmitted to memory 54 of the processing 
means) and compares it with a second set of predetermined range values K3 
and K4. This comparison is used as before to estimate which range region 
the associated lightning stroke occured in. This estimate is called TH. 
The constants K1 and K2 differ from K3 and K4 because the K1 and K2 values 
are used with the peak sampling circuitry 44,46 and 48. The field samples 
measured thereby are high frequency samples of the ligntning and thus are 
weaker at greater distances from the strike than the integrated field 
values which are mainly a measurement of the low frequency component of 
lightning. Hence, the constants K3 and K4 differ from K1 and K2. 
The two estimates of range (TA and TH) are examined by the Sample Data 
Program to see if they agree. If the two estimates of range differ widely, 
that is, if one estimates is for the far range region and one, near range 
region, then the integrated vertical electric field (as inputted from 
integrator 42 in FIG. 1 through multiplexer 636, A/D converter 612c and 
FIFO 614c in FIG. 6) is used to estimate range by comparison with a third 
set of predetermined range values K5 and K6. This estimate is called TE. 
Values of K5 and K6 are determined by predicting the signal strength of an 
average lightning strike with the high frequency energy value as removed 
by the integrator 42. If the two estimates of range region TA and TH based 
on H field values are not widely different or agree then the Sample Data 
Program will form the .vertline.H/E.vertline. ratio, which will provide an 
accurate determination of range from FIG. 8, by using only the integrated 
values of the H and E.sub.V fields formed by integrators 40 and 42 
respectively. 
In the case where the estimates of range TA and TH differ widely, the 
estimate of range region based on the integrated E.sub.V field TE is 
compared separately with the TA estimate and with the TH estimate. If TE 
agrees with either TH or TA an error is presumed in the data measured by 
the circuitry. If TE is different from TA or TH then the 
.vertline.H/E.vertline. ratio will be formed using the field power 
calculated from the sampled values of the field. Of course, the Sample 
Data Program must first calculate the mean value of E from the sample 
values of E that were stored in FIFO 614a before being inputted to memory 
54. 
When the embodiment of FIGS. 9a through 9c is used the db value of the RMS 
field strengths for H.sub.NS, H.sub.EW and E.sub.V are used in place of 
the integrated E and H field values provided via lines 66 and 68. 
The near, mid and far range regions versus .vertline.H/E.vertline., even 
numbers 800 through 804 respectively of FIG. 8, are stored as a set of 
three tables in memory. The .vertline.H/E.vertline. value formed by the 
Sample Data Program using either the integrated field values or the 
sampled field values is compared with the appropriate table to determine 
the corresponding range, R, of the lightning activity. This value of R is 
the true distance from the measuring apparatus to the lightning stroke. 
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 lighting 
strike to determine accurately the range and region of activity of the 
lightning strikes relative to the equipment. 
It is desirable to know the ground distance, D, between the observing 
equipment and the lightning activity. D is related to R by the Sin of the 
elevation angle .theta., that is, 
EQU D=R Sin .theta. 
The Sample Data Program will convert R to D using the centroid value of 
.theta. calculated earlier by the Program. However, D is set equal to R 
where R is determined from the table corresponding to the far range region 
since for lightning strikes that far away (greater than 50 kilometers) 
.theta. is small and Sin .theta. approximately equals one. 
Now that the direction and distance of the storm 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 thumb wheel switch 736 in FIG. 7. This manual selection of 
scale factor is monitored via lines 739 and 738 by the Schedule Routine 
Program stored in ROM 704. The Schedule Routine Program is executed by 
controller 56 every 10 milliseconds. A detailed description of the design 
using the INTEL 8085A for a minimum system is found in INTEL Component 
Data Catalog, 1980 edition, pages 6-9 through 6-24, hereby incorporated by 
reference as if specifically set forth herein. The controller 56 is 
interrupted once every 10 milliseconds by RAM 702 (the INTEL Component 
Data Catalog completely explains the sue of the timer output connection). 
The Schedule Routine Program first determines if 0, 200, 500, 700 and 900 
milliseconds (10 interrupts from RAM 702 equals 100 milliseconds) has been 
completed. At the 0 millisecond time the range input lines are monitored 
over lines 738 and 739 and a signal transmitted over lines 744 and 746 
which are used to adjust the switches 218, 220 and 222 in FIG. 2 to a 
desired input signal level that does not over saturate the circuitry or 
increases the apparatus sensitivity to distant lightning strikes. This 
range scale factor is stored in RAM 702 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 transmitting the x and y rectangular 
coordinates of the lightning and the standard deviation to the display 
processor 60 for display on the display means 62. The x and y coordinates 
are related to D and the centroid value for .phi. as follows: 
EQU x=D.multidot.Sin .phi. 
EQU y=D.multidot.Cos .phi. 
x and y is stored in RAM 702. 
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 invention is a DIGITUS Corporation Rainbow 2000 System. See in 
particular DIGITUS Corporation users Manual which is hereby incorporated 
by reference as if specifically set forth herein. 
One such program shows the equipment at the origin of the display with 
azimuth angle varying 360.degree. about the origin. Distance is shown 
increasing along radials from 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 circle based on the value of 
the standard deviation of the set of .theta.'s and .phi.'s. It is clear 
from the design thus far, the range scale can be varied by the thumb wheel 
736. 
The programs also allows for variations in how often the display is updated 
with new information; how long old information is retained; and the use of 
color 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 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 aircraft 
flight constantly changes the location of the lightning activity relative 
to the aircraft. To compensate for this the processing means is programmed 
with a Schedule Routine Program previously discussed in above paragraph, 
which updates the x and y coordinates of lightning activity previously 
displayed. Heading information from the aircraft's navigational system is 
transmitted via a plurality of lines 732 to synchro to digital converter 
(S/D) 740 in FIG. 7. The heading information is analog in nature which the 
synchro to digital device 740 converts to digital data when processing 
circuitry means transmitts a command over line 734. Model No. HXDC 10-L-3 
made by ILC Data Device Company, can be used as the S/D. 
The change in azimuth display, .phi., is calculated by the Schedule Routine 
Program on the 900 millisecond count by subtracting the previous heading 
from the newest. The change in x and y coordinates of previously directed, 
analyzed and displayed lightning activity (x and y) is calculated by using 
the calculated centroid value of .phi.c: 
EQU x=Speed.multidot.Sin .phi.c 
EQU y=Speed.multidot.Cos .phi.c 
where speed is the known speed of the aircraft stored in ROM 704 with value 
set in accordance to type of aircraft. For example, a Cessna 182 as a 
nominal cruising speed of 120 NM. In the preferred embodiment the display 
is updated every second. 
However, the Sample Data Interrupt and Sample Data Programs will operate 
when triggered by randomly occuring lightning strokes. These will not 
always occur every tenth of a second when heading is being updated. It is 
desirable to use the latest heading information possible. Between display 
updates (every second) the Schedule Routine Program samples heading at 
200, 500, 700 and 900 milliseconds, The Sample Data Program will use the 
latest heading information available when first calculating, .theta., 
.phi. and R and then will update the heading to correct the x and y 
coordinates before transmitting them to the display. As an example, 
suppose a lightning strike occurs at 450 milliseconds after the last 
display update by the Schedule Routine Program. The latest heading 
information available to the Sample Data Program is the heading 
information at 200 milliseconds. This information will be used by the 
Sample Data Program to calculate .theta., .phi. etc. However, it takes the 
Sample Data Program more than 50 milliseconds to acquire and calculate the 
information. This means that by the time the information (x and y 
coordinates) are ready for transmission to the display processor, the 
Schedule Routine Program has provided a new heading at 500 milliseconds. 
So, the Sample Data Program updates the newly measured and calculated x 
and y coordinates for the new heading using the following equations: 
EQU .phi.=Heading (500 milliseconds)- Heading (200 milliseconds) 
EQU x=Speed.multidot.Sin .phi. 
EQU y=Speed.multidot.Cos .phi. 
EQU x=D.multidot.Sin .phi.+x 
EQU y=D.multidot.Cos .phi.+y 
The display means will display x, y and the standard deviation of all 
calculated lightning strikes which occured since last one second update. 
While the present invention has been disclosed in connection with the 
preferred embodiment thereof, it should be understood that there may be 
other embodiments which fall within the spirit and scope of the invention 
as defined by the following claims