Development of Low Frequency Lightning Receiver

Dissertation Report on

Development of Low Frequency Lightning Receiver for Weather Monitoring Application Submitted in partial fulfilment for the award of the degree of

MASTER OF ENGINEERING in

ELECTRONICS & TELECOMMUNICATION ENGINEERING

by Miheer P. Mayekar

Under the Guidance of Dr. Bijith Markarkandy Associate Professor

Department of Electronics and Telecommunications (P. G.) (Academic Year: 2013-14)

Dissertation Report on

Development of Low Frequency Lightning Receiver for Weather Monitoring Application Submitted in partial fulfilment for the award of the degree of

MASTER OF ENGINEERING in

ELECTRONICS & TELECOMMUNICATION ENGINEERING

by Miheer P. Mayekar

Under the Guidance of Dr. Bijith Markarkandy Associate Professor

Department of Electronics and Telecommunications (P. G.) (Academic Year: 2013-14)

Spine/Side view of the Report –

M. E. (EXTC) MIHEER P. MAYEKAR 2013-2014

DISSERTATION APPROVAL CERTIFICATE

This is to certify that the dissertation work entitled “Development of Low Frequency Lightning Receiver for Weather Monitoring Application” for M.E. in Electronics and Telecommunication Engineering submitted to University of Mumbai by “Miheer P. Mayekar”, a bona fide student of Thakur college of Engineering and Technology, Kandivali, Mumbai has been approved for the award of Master of Engineering Degree in Electronics and Telecommunication Engineering.

Examiners:

Internal Examiner

External Examiner

Name:

Name:

Designation:

Designation:

Date:

Date:

Department of Electronics and Telecommunication Engineering Thakur College of Engineering and Technology Kandivali, Mumbai

CERTIFICATE

This is to certify that “Miheer P. Mayekar” has satisfactorily carried out the dissertation work entitled “Development of Low Frequency Lightning Receiver for Weather Monitoring Application” for the degree of Master of Engineering in Electronics and Telecommunication Engineering under University of Mumbai.

Dr. Bijith Markarkandy Project Guide

Dr. B. K. Mishra Principal

Associate Professor

and Head of

Electronics and Telecommunication Engineering Department

Electronics and Telecommunication Engineering Department (PG)

Thakur College of Engineering & Technology Date

Thakur College of Engineering & Technology Date

Principal Thakur College of Engineering and Technology Kandivali, Mumbai

SYNOPSIS OF PROJECT WORK

Name of dissertation:

Development of Low Frequency Lightning Receiver for Weather Monitoring Application

Student‟s Name:

Miheer P. Mayekar

Class:

M.E.(Electronics and Telecommunication)

College:

Thakur College of Engineering and Technology

Semester:

IV

University Registration Number:

Thakur/103

Date Of Registration:

13/09/2012

Name of Guide:

Dr. Bijith Markarkandy

Semester

University Seat Number

Result ( marks )

Result ( CGPA)

I

2960

374/600

6.36

II

3109

398/600

6.73

Abstract Lightning discharge is an impulsive event which radiates electromagnetic (EM) energy over an extremely wide bandwidth, from a few Hz to many tens of MHz. However, because of the time scales and spatial extent of the radiating current lightning discharges radiate the bulk of their electromagnetic energy in the Very Low Frequency (VLF, 3–30 kHz) and Extremely Low Frequency (ELF, 3–3000 Hz) band with a peak component at VLF. VLF and ELF energy radiated near the ground propagates in a guided fashion by taking multiple reflections from ionosphere and ground, which forms what, is known as Earth Ionosphere Waveguide (EIWG). This guided propagation occurs with low attenuation rates at VLF and ELF frequencies (a few dB per 1000 km), allowing VLF and ELF radiations to be observed literally around the world from a single source lightning discharge. Because of this fact lightning signals in this frequency range are very useful in forecasting and real time tracking of the thunderstorms and other lightning producing natural calamities before they are detectable by weather radars. The signals generated by lightning discharge in the ELF and VLF band have a more or less constant power spectral density in this frequency range. Thus, we need the receiver to have a flat frequency response over this range rather than one, for example, proportional to frequency. If we use a receiver with low input impedance, the increase in induced electromotive force with frequency in the low impedance magnetic antenna is counteracted by the increase in inductive reactance of the antenna, making the current into the receiver flat with frequency. The problem is to design a low-impedance amplifier with a good noise figure to detect lightning signals in the VLF band when connected to an inductive source. It has been found that a common–base input stage gives good results, much better than, for example, terminating the loop with a resistor of the same impedance even if followed by an ideal noise-free amplifier. In this work, a receiver for detecting magnetic fields generated by Cloud to Ground lightning discharges in the frequency range from 3KHz to 30 KHz is designed and a part of an analog section, comprising of the input transformer, Low Noise Amplifier (LNA) and some signal conditioning circuits including Variable Gain Amplifier (VGA), High Pass Filter (HPF) for power line harmonics removal, output amplifier, output matching transformer, and a calibration circuit for testing, is developed. Current receiver can be modified to record fine structure of the lightning waveform in time domain by extending its bandwidth below and above VLF band. By choosing appropriate transformer turns ratio , a trade-off can be maintained between the lower cut off frequency and noise figure requirement of the receiver. This work is completed under the external guidance of Society for Applied Microwave Electronics Engineering & Research (SAMEER), Mumbai. i

Acknowledgements I want to commence this section with a quote, “If you have learned much (scholarly wisdom), do not take credit for yourself; it is for this reason that you have been formed” [16]. First, I want to thank Dr. Bijith Markarkandy (Internal guide) for giving me enough freedom and having faith in me throughout the project phase which allowed me to implement my own ideas. I am sincerely thankful to him for his guidance and effort in improving my understanding of this project. I am thankful to Dr. B. K. Mishra (Principal) for his encouraging comments and valuable suggestions which helped me in maintaining quality in my work. I am extremely thankful to Mr. Anil Kulkarni (External guide, Scientist – F, H.O.D., IMS Dept., SAMEER, Mumbai.) for introducing me to this research opportunity rich project topic and his encouraging attitude which helped me in nurturing my creativity and building self-confidence. I learned a lot from long technical as well as nontechnical discussions with him. I am also thankful to Dr. A. L. Das (Director, SAMEER, Mumbai) for providing me the platform to perform my M.E. project at SAMEER, Mumbai. I also want to thank Dr.Vinitkumar Dongre (M.E. Coordinator & Head of the Department - EXTC) from Thakur College of Engineering & Technology and Mr. Ajay Khandare & Mr. Sivaraju Rajasekhar from SAMEER, Mumbai for their continuous assistance and valuable suggestions which aid in smooth and unnecessary delay free work. At the end, a big thank to all those who helped me by working at the backstage, giving hassle free and enjoyable work experience.

Miheer P. Mayekar. ii

Publications Conference papers [1] M. Mayekar, A. Kulkarni, and B. Marakarkandy, “Designing and simulation of low frequency cloud to ground lightning receiver for severe weather monitoring application,” to be published in Procedia Computer Science, accepted in International Conference on Advanced Computing Technologies and Applications (ICACTA-2015).

Research Convention Participation [1]

Participated and won overall championship for University of Mumbai in “AVISHKAR-2013-14” the state level Inter-University Research Convention at North Maharashtra University, Jalgaon on project “Development of LF lightning receiver for weather monitoring application” in P.G. level during January 16-18, 2014.

[2]

Participated and won second prize in “AVISHKAR-2013-14”, the University level Inter-Collegiate Research Convention at International Hall, V. V. Bhavan, Churchgate, Mumbai on project “Development of LF lightning receiver for weather monitoring application” in P.G. level on January 7, 2014.

iii

Contents

Abstract......................................................................................................................... i Acknowledgements .....................................................................................................ii Publications ............................................................................................................... iii Contents ...................................................................................................................... iv List of Figures............................................................................................................vii List of Tables ............................................................................................................... x Abbreviations and Symbols ........................................................................................ xi Chapter 1.

Introduction ...................................................................................... 1

1.1

Understanding Lightning ................................................................................... 2

1.2

Motivation .......................................................................................................... 5

1.3

Objectives .......................................................................................................... 6

1.4

Structure of the Thesis ....................................................................................... 6

Chapter 2.

Related Theory ................................................................................. 7

2.1

Introduction ........................................................................................................ 8

2.2

Lightning detection systems ............................................................................ 10

2.3

2.2.1

Space-borne lightning detection .............................................................. 11

2.2.2

Ground based lightning detection ............................................................ 11

Summary .......................................................................................................... 21

Chapter 3.

Literature Survey ........................................................................... 22

3.1

Introduction ...................................................................................................... 23

3.2

Receiver analysis ............................................................................................. 23

iv

3.2.1

Loop antenna model ................................................................................. 23

3.2.2

Transformer model................................................................................... 27

3.2.3

Amplifier model ....................................................................................... 30

3.3

Propagation of lightning signals ...................................................................... 39

3.4

Sferics detection ............................................................................................... 40

3.5

3.4.1

Power-line hum mitigation ...................................................................... 42

3.4.2

Narrow band transmitters mitigation ....................................................... 50

3.4.3

Preprocessing steps .................................................................................. 52

Summary .......................................................................................................... 55

Chapter 4.

Design Methodology....................................................................... 57

4.1

Lightning node/sensor ...................................................................................... 58

4.2

Block diagram .................................................................................................. 58 4.2.1

4.3

Analog section ................................................................................................. 59 4.3.1

4.4

4.5

Description ............................................................................................... 58

Description ............................................................................................... 60

Analog section circuit design ........................................................................... 63 4.4.1

PCB-1 circuit description ......................................................................... 63

4.4.2

PCB-2 circuit description ......................................................................... 68

Summary .......................................................................................................... 74

Chapter 5.

Results and Discussions ................................................................. 75

5.1

Results .............................................................................................................. 76

5.2

Discussions ...................................................................................................... 79

Chapter 6. 6.1

Conclusion and Future Scope ....................................................... 81

Conclusions ...................................................................................................... 82 v

6.2

Future Scope .................................................................................................... 82

References ................................................................................................................... 85

vi

List of Figures

Figure 1-1 Charge distribution within cloud and on ground and three categories of lightning flashes [3] ....................................................................................................... 3 Figure 1-2 Propagation of lightning signals and their detection [1] .............................. 4 Figure 1-3 Electric (E) field spectra of a negative CG lightning‟s first stroke [3] ........ 4 Figure 1-4 A ground based lightning detection network ............................................... 5 Figure 2-1Various phases of a negative CG lightning flash [4] .................................... 9 Figure 2-2 Timing structure of the entire CG flash [10]. ............................................... 9 Figure 2-3 Ten second time frequency spectrum showing sferics from 0-20 kHz [4] 10 Figure 2-4 Example of a time domain waveform of a sferic [4] ................................. 10 Figure 2-5 Orthogonal loop antenna structure ............................................................. 12 Figure 2-6 B field orientation around the CG lightning channel and through the loop antenna[11] .................................................................................................................. 13 Figure 2-7 Possible lightning strikes around the antenna [11] .................................... 14 Figure 2-8 Determination of bearing angle θ [11]. ...................................................... 14 Figure 2-9 Geometry of TOA location technique [12] ................................................ 15 Figure 2-10 Hyperbolic intersection [6] ...................................................................... 16 Figure 2-11 Ambiguous location, 3-sensor hyperbolic intersection [6] ...................... 16 Figure 2-12 Overestimation of the flash multiplicity in an angle based flash grouping algorithm [8]. ............................................................................................................... 18 Figure 2-13 Location based flash grouping algorithm. Strokes 1, 3, and 4 constitute one flash; stroke 2 is regarded as a separate flash [13]. ............................................... 19 Figure 2-14 Confidence ellipse: unfavourable sensor geometry [6]. ........................... 20 Figure 2-15 Confidence ellipse: favourable sensor geometry [6]. ............................... 20 vii

Figure 3-1 Receiver system design [14] ...................................................................... 24 Figure 3-2 Loop antenna model [15] ........................................................................... 24 Figure 3-3 High impedance amplifier with current feedback [15] .............................. 32 Figure 3-4 Low impedance configurations [15] .......................................................... 33 Figure 3-5 Low impedance amplifier with second stage [15]. .................................... 34 Figure 3-6 Low impedance amplifier, second stage feedback [15]. ............................ 36 Figure 3-7 Differential input amplifier [15]. ............................................................... 37 Figure 3-8 Spectrogram of unfiltered data, showing the broadband sferics together with various artificial and man-made noise sources [3] .............................................. 41 Figure 3-9 Example of power line hum affected chorus emissions, which can be seen as a series of brief ~1 second long rising emissions clearly visible in the right panel which is the filtered version of raw data[16] ............................................................... 42 Figure 3-10 Fundamental power line harmonic

for a 60 second period [3] ........... 42

Figure 3-11 Block diagram of the broadband saturation method for reducing narrowband transmitter interference [3] ...................................................................... 51 Figure 3-12 Stages of sferic processing [4] ................................................................. 53 Figure 3-13Time of arrival dependence on threshold setting [4] ................................ 55 Figure 4-1 Lightning node/sensor block diagram ........................................................ 58 Figure 4-2 Orthogonal square loop antenna schematic................................................ 58 Figure 4-3 Analog section block diagram [24]. ........................................................... 60 Figure 4-4 Test and calibration setup using a dummy loop instead of antenna. The

signal is provided by a function generator and should have a high output

impedance [5]............................................................................................................... 60 Figure 4-5 PCB – 1 (Amplifier Board) circuit design. ................................................ 64 Figure 4-6 PCB – 1 (Amplifier Board) circuit design. ................................................ 65

viii

Figure 4-7 PCB – 1 (Amplifier board) front side. ....................................................... 66 Figure 4-8 PCB – 1 (Amplifier board) back side. ........................................................ 66 Figure 4-9 PCB – 2 (Anti-aliasing filter board) circuit design. ................................... 69 Figure 4-10 Cascading filter stages for higher order filters [25] ................................. 70 Figure 4-11 Single 2-nd order Butterworth LPF [25] .................................................. 71 Figure 4-12 Anti-aliasing filter‟s frequency response ................................................. 72 Figure 4-13 Simplified diagram of the level shifter U9 [25] ....................................... 73 Figure 5-1 Simulated frequency response of the PCB – 1 till op amp U4A output. ... 76 Figure 5-2 Measured frequency response of the PCB – 1 till op amp U4A output. .... 77 Figure 5-3 Simulated frequency response of the complete PCB – 1. .......................... 77 Figure 5-4 Measured frequency response of the complete PCB – 1............................ 78 Figure 5-5 Simulated input referred noise voltage of the complete PCB – 1. ............. 78 Figure 5-6 Orthogonal square loop antenna [30]. ........................................................ 80 Figure 6-1Measured radiation spectrum from negative first stroke [3]. ...................... 83

ix

List of Tables

Table 3-1 Constants for various magnetic loop antenna shapes [14] .......................... 25 Table 3-2 Expected signal levels at the receiver input ................................................ 40 Table 4-1 Filter coefficients [25]. ................................................................................ 71

x

Abbreviations and Symbols VLF

Very Low Frequency

ELF

Extremely Low Frequency

CG

Cloud to Ground

IC

Intra Cloud

EIWG

Earth Ionosphere Wave Guide

DE

Detection Efficiency

LA

Location Accuracy

LNA

Low Noise Amplifier

VGA

Variable Gain Amplifier

BJT

Bipolar Junction Transistor

FET

Field Effect Transistor

Op Amp

Operational Amplifier

AAF

Anti-Aliasing Filter

HPF

High Pass Filter

LPF

Low Pass Filter

ADC

Analog to Digital Converter

FPGA

Field Programmable Gate Array

GPS

Global Positioning System

MDF

Magnetic Direction Finding

TOA

Time Of Arrival

xi

Chapter 1. Introduction

1

1.1 Understanding Lightning Lightning is both beautiful and dangerous. The bright imagery in the sky that entertains us is a direct threat to air and ground based operations, and is a reflection of other destructive forces associated with thunderstorms and severe weather such as cyclones, tornadoes etc. [1]. All thunderstorms go through stages of growth, development, electrification and dissipation. Thunderstorms often begin to develop early in the day when the sun heats the air near the ground and pockets of warmer air start to rise in the atmosphere. When these pockets of air reach a certain level in the atmosphere, cumulus clouds start to form. Continued heating causes these clouds to grow vertically into the atmosphere. These "towering cumulus" clouds may be one of the first signs of a developing thunderstorm. The final stage of development occurs as the top of the cloud becomes anvil-shaped. As a thunderstorm cloud grows, precipitation forms within the cloud. A well-developed thunderstorm cloud contains mostly small ice crystals in the upper levels of the cloud, a mixture of small ice crystals and small hail in the middle levels of the cloud, and a mixture of rain and melting hail in the lower levels of the cloud. Air movements and collisions between the various types of precipitation in the middle of the cloud cause the precipitation particles to become charged. The lighter ice crystals become positively charged and are carried upward into the upper part of the storm by rising air. The heavier hail becomes negatively charged and is either suspended by the rising air or falls toward the lower part of the storm. These collisions and air movements cause the top of the thunderstorm cloud to become positively charged and the middle and lower part of the storm to become negatively charged. In addition, a small positive charge develops near the bottom of the thunderstorm cloud. The negative charge in the middle of thunderstorm cloud causes the ground underneath to become positively charged, and the positively charged anvil causes the ground under the anvil to become negatively charged [2]. Lightning is a giant spark of electricity in the atmosphere or between the atmosphere and the ground. In the initial stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground; however, when the differences in charges becomes too great, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning. Lightning can occur between opposite charges within the thunderstorm cloud (Intra Cloud Lightning (IC)) or between opposite charges in the cloud and on the ground (Cloud-To-Ground Lightning). Cloud-to-ground (CG) lightning is divided in two different types of flashes depending on the charge in the cloud where the lightning originates [2]. If the ground flash originates in the negatively charged layer it is called a negative CG flash and electrons are moved from the cloud to the ground. If the ground flash originates in the positively charged layer it is called a positive cloud-to-ground flash and positive charge is moved from the cloud to the ground. In 2

other words, electrons travel from the ground to the cloud. Figure 1-1 illustrates the charge distribution within cloud and on ground and three categories of lightning flashes [3].

Figure 1-1 Charge distribution within cloud and on ground and three categories of lightning flashes [3]

Lightning radiates an electromagnetic pulse which contains energy over a wide bandwidth, spanning from just a few hertz up to tens of megahertz. Due to the submillisecond to millisecond time scales and several kilo-meter spatial scales associated with the CG lightning current, most of the energy in the radiated spectrum is contained in the Extremely Low Frequency (ELF 3-3000 Hz) and Very Low Frequency (VLF 3-30 kHz) bands [4]. The electromagnetic pulses from lightning at ELF/VLF frequencies are known as radio atmospherics, more often referred to as sferics, and are the primary focus of this thesis. At ELF and VLF frequencies, electromagnetic waves are reflected by the ground and by the conducting layer of the atmosphere known as the ionosphere, and they can thus be efficiently guided around the Earth. In this “Earth-ionosphere” waveguide, sferics propagate with low loss (typically 2-3 dB/1000 km) and can therefore be detected at great distances from their source locations as indicated in figure 1-2. By observing sferics at several different locations, the source locations of the individual lightning discharges can be determined [4]. Information about the source lightning can also be derived from the characteristics of the sferic waveforms observed at each receiver. Figure 1-3 illustrates the measured electric (E) field spectra of a negative CG lightning‟s first stroke. As seen from the figure 1-3 there is a peak in the VLF band, from a few KHz to about 30 KHz [3]. Sensitivity is the most important specification for the receiver designing. Sensitivity gives the information on the minimum detectable signal by the receiver. Major noise sources, present in the receiver that limits its sensitivity are thermal noise, shot noise and flicker noise. Flicker noise or ⁄ is inversely proportional to frequency. It is also 3

Figure 1-2 Propagation of lightning signals and their detection [1]

referred to as pink noise and present in all transistors and some resistors when they have DC current flowing through them. A common way to compare the flicker noise in a device is to measure the noise corner frequency. This frequency is the point when the flicker noise equals the other white noise of the device from thermal and shot noise sources. Above this point the thermal and/or shot noise dominates, while at frequencies below this point the flicker noise dominates. When designing circuits for low frequency applications, it is especially important to choose devices with a low flicker noise corner frequency, and if possible, the flicker noise corner should be below the frequencies of interest [5].

Figure 1-3 Electric (E) field spectra of a negative CG lightning’s first stroke [3]

4

1.2 Motivation Figure 1-4 illustrates the structure of a ground based lightning detection network. Node/Sensor

User/Display Software

Central Processing System

(CPS) Wireless

link

Figure 1-4 A ground based lightning detection network

The Node/Sensor will capture electromagnetic radiations emitted by lightning. Several such Nodes/Sensors distributed on a certain geographical area detect lightning signals and send the digitized data to a Central Processing System (CPS). CPS then process this data to calculate certain parameters of lightning signals e.g. source location, peak current, polarity etc. CPS will then send this information to the User/Display software [8]. Typical users of lightning information are air traffic managers, weather monitoring and severe weather warning service providers and power utility staff [6]. Main aim of the lightning detection network is to determine the location, intensity, and movement of thunderstorms and other severe weather conditions in real time [1]. Many lightning detection networks currently exist that detect and locate lightning flashes for a broad range of commercial and scientific applications, including air traffic control, insurance claims, climate modelling, and the investigation of secondary atmospheric and magnetospheric electrical phenomena. These lightning detection networks have varying degrees of coverage area and location accuracy. Commercial ground-based lightning detection networks that excel at locating return strokes in cloud-to-ground lightning use radio detection in the LF (30 - 300 kHz) band to provide very accurate location data, with a typical accuracy of ~0.5 km, but they require a dense network of receivers separated by ~400 km and are therefore primarily limited to monitoring the land areas within the network [3]. Weather radars are also used for the detection of the presence of thunderstorms. Due to the fact that, the microwave frequency weather radar beams are blocked by the mountainous terrain because of their line of sight propagation, weather radars are less effective for 5

thunderstorm detection over longer distances [7]. Many developed countries have radar network to track storms but the major portion of the globe is not covered by such observation systems. The temporal and spatial resolutions provided by various satellite systems are also not sufficient to study the small scale thunderstorm systems. Also, these methods are highly expensive and need enormous computational power and infrastructural facilities [9]. ELF/VLF emissions from lightning can propagate over mountainous terrain over long distance making networks of ELF/VLF sensors particularly useful in long range thunderstorm detection and tracking.

1.3 Objectives Following are the main objectives of this project: 



Development of an analog front end (analog section) of the low frequency lightning receiver, which is a part of a node/sensor in a lightning detection network explained in section 1.2, to capture CG lightning signal‟s magnetic field in the frequency range of 3 – 30 KHz with the sensitivity as low as possible. Development of digital signal processing algorithm to separate noise from lightning signals at node/sensor level.

1.4 Structure of the Thesis This thesis presents the development of low frequency lightning receiver, its testing and development of steps involved in lightning signal‟s preprocessing as follows. Chapter 1 sheds light on the various phases of the cloud to ground lightning. It also provides an overview of the existing lightning detection systems and various lightning parameters they provide. Chapter 3 provides basic background on the loop antenna, transformer and the low noise amplifier along with their design parameters. It also explains the calculations involved in the received lightning signals‟ strength and the various steps involved in the lightning signals‟ preprocessing. Chapter 4 describes the design methodology and working of various sections of the lightning receiver. Chapter 5 explains the simulation as well as test results of the lightning receiver. Finally chapter 6 provides the conclusion and future scope of the work.

6

Chapter 2. Related Theory

7

2.1 Introduction As illustrated in figure 2-1, a negative CG flash is initiated when a conductive channel known as a stepped leader begins to work its way down from the cloud after a preliminary breakdown within the negatively charged layer. The stepped leader approaches the ground in a series of discrete steps that are 10‟s to 100‟s of meters in length. As the stepped leader advances downward the electric field between the end of the stepped leader and the ground becomes high enough that conductive leaders begin to reach upwards from the ground until the stepped leader and a conductive leader are only 10 to 100 meters apart. Once within this “striking distance”, attachment occurs and a conductive channel is created between the negatively charged layer of the cloud and the ground. At this point the first return stroke of the flash occurs and a large electric current flows from the ground to the cloud generating an electromagnetic impulse, i.e., a radio atmospheric or a sferic. The current typically has a peak value of around 30 kilo amperes (kA) but intense return strokes may have currents in excess of several hundred kA. Also, due to the time duration of the primary current and the typical lengths (~7 km) of the conductive channel, the peak intensity of the radiated field of the sfreic lies in the 1-10 kHz range. After the first return stroke the layer of charge may be depleted resulting in the termination of the flash. However, if additional charge is available, processes known as J and K occur that redistribute the remaining charge in the cloud. The J-process is characterized by a steady state electric field change over a period of tens of milliseconds and K-processes are characterized by small relatively rapid electric field variations at intervals of 2-20 milliseconds. The conducting channel from the first return stroke is still partially ionized following the first return and a dart leader re-ionizes the channel leading the way for a second return stroke. This process may occur over and over again resulting in dozens of return strokes in a given lightning flash with intervals between return strokes of tens of milliseconds. Subsequent return strokes after the first one typically have peak currents that are about half that of the first return stroke. Various stages of a two stroke negative CG lightning flash are shown in figure 2-1 and figure 2-2 illustrates the timing structure of the entire CG lightning flash [4]. A similar process occurs for positive CG discharges. Positive CG flashes typically have higher peak currents than negative CG flashes but also make up a smaller percentage of all lightning flashes, although the ground flashes in some storms are predominantly positive flashes. IC lightning flashes are the most common type of lightning discharge, with their occurrence within typical thunderstorms typically exceeding CG lightning flashes by a ratio of about 3 to 1. IC lightning flashes occur between the negatively charged and the positively charged layers of thunderclouds and can have currents of similar magnitude to CG lightning discharges [4].

8

Figure 2-1Various phases of a negative CG lightning flash [4]

Figure 2-2 Timing structure of the entire CG flash [10].

Figure 2-3 shows a frequency time spectrogram covering the range from 0-20 kHz containing the lightning signals that occupy the ELF/VLF frequency band. Each vertical line in the figure 2-3 is a sferic and, even at this resolution, hundreds are visible during the 10 seconds shown. Each of this sferics originate at lightning discharges occurring in thunderstorms located all around the world, propagating to the receiver location through the earth-ionosphere waveguide [4].

9

Figure 2-3 Ten second time frequency spectrum showing sferics from 0-20 kHz [4]

A typical sferic pulse in the time domain looks much like the relatively large amplitude sferic shown in figure 2-4. Sferics generally consist of a VLF impulse lasting less than 1 ms and are sometimes followed by a lingering ELF component known as an ELF “slow tail”, which typically lasts for an additional 1-3 ms (a “slow tail” is visible in figure 2-4). The oscillatory nature of the initial VLF portion is due to the superposition of different waveguide modes after multiple reflections between the earth and the ionosphere. As mentioned before, the bulk of the energy of a sferic lies in the ELF/VLF range with a typical peak occurring in the 2.5 – 10 kHz range [4].

Figure 2-4 Example of a time domain waveform of a sferic [4]

2.2 Lightning detection systems In remote areas where conventional weather radar and surface observations are not available, tracking of thunderstorms and assessing cyclone intensification are important challenges in weather prediction for civilian and military purposes. Thunderstorms over the ocean represent a threat to airborne carriers and ocean shipping and are mostly beyond the range of weather radars. Although today‟s operational geostationary satellites provide continuous visible and infrared imagery, 10

cirrus anvils often obscure convective activity. Convective clouds that produce lightning have significant updrafts, thus increasing the threat of turbulence and icing. Lightning measurements can be very useful in these situations. This section provides a brief overview of both ground and space based approaches capable of providing real time global information about lightning and thunderstorm [1].

2.2.1 Space-borne lightning detection The first targeted detection of lightning from a space-borne platform was realized with the Optical Transient Detector (OTD) which was followed by the Lightning Imaging Sensor (LIS) launched aboard the Tropical Rainfall Measuring Mission (TRMM) satellite in 1997. The TRMM satellite is located at 350 km altitude and its orbit has an inclination of 35°. The LIS sensor detects total lightning by registering optical transients, which occur as a result of the scattering within the tops of the clouds of luminous radiation produced by lightning channels. With a 600 km field of view, LIS can view a particular spot on the earth for 90 seconds at a time and reports optical transients detected by a CCD imager with 2 ms resolution. The lowest level of data provided by LIS is called an “event”, which is a transient detected on a single pixel. Events that are adjacent to each other are placed in to units called “groups”. A group roughly corresponds to an optical event from a lightning stroke. LIS “flashes” are sets of groups that are separated by not more than 330 ms in time and 5.5 km in distance. Over an extended period of time LIS can create density maps for lightning occurrence over the entire earth. A limitation of this data is that they currently do not separate out CG and IC lightning incidence. The next generation series of Geostationary Operational Environmental Satellite (GOES-R) is planned to carry a Geostationary Lightning Mapper (GLM) based on the pioneering work by NASA, which will monitor lightning continuously over a wide field of view. Until these instruments are in orbit, tested and calibrated, ground based lightning detection remains the only method to provide continuous lightning observations [1]. VHF emissions from lightning have also been observed from space. Much of the literature on such observations has derived from the Fast On-Orbit Rapid Recording of Transient Events (FORTE) satellite built by LANL. FORTE was designed as a more specialized follow on platform to study lightning associated signals that had previously been observed by an instrument called Blackbeard. The FORTE satellite combined optical and VHF observations. Among its many research objectives, FORTE was used as part of a demonstration of the possibility of performing multiplesatellite geo-location of VHF lightning emissions from space [1].

2.2.2 Ground based lightning detection When propagation distances between a lightning discharge and remote electromagnetic sensor are less than about 1000 km, significant energy in both the VLF and LF band can propagate as a ground wave. At greater distances energy in the 11

VLF range can propagate effectively in the waveguide defined by the earth‟s surface below and by the ionosphere above, specifically its lowest layer, i.e., the D region. Out to distances of 3000 – 4000 km, most of the energy is carried in signals that can be accounted for using the first two “ionospheric hops”. At even greater distances propagation is more efficiently characterized using modal analysis. Given these characteristics, long range lightning detection systems have the potential to provide cost effective and accurate monitoring of convective storms over large synoptic scale regions [1]. All ground based long range lightning detection systems use the ELF and VLF portion of the electromagnetic spectrum, utilizing the low attenuation through the earth-ionosphere waveguide at these frequencies. VLF lightning detection systems can be divided in to two classes: systems that use arrival time information and systems that use magnetic direction finding (MDF) either solely or in conjunction with arrival time information. There are two existing long range technologies that geo locate lightning strikes using exclusively timing information of VLF measurements of individual sferics. The first approach, which is referred to as simply the arrival time difference (ATD) technique, determines the arrival time difference between sensors by cross correlating recorded sferic waveforms. The second approach calculates a time of arrival (TOA) at each receiver by calculating an averaged group delay arrival time, a value referred to as the time of group arrival (TOGA) [3]. Following are some lightning related parameters that ground based lightning detection systems calculate and provide to the users of lightning information. 2.2.2.1 Location Before the development of weather radars, a variety of sferics detection systems were the primary means of identifying and mapping thunderstorms at medium and long ranges. Following are some techniques that can be used to detect and locate CG lightning. i Magnetic direction finding (MDF) MDF lightning location technique is used to compute the ground strike locations of the return strokes of CG lightning striking the ground. A sensor in a MDF system uses two orthogonal loop antenna pointing in east-west (E-W) and north-south (N-S) direction as shown in figure 2-5. Loop pointing N - S

Loop pointing E - W

Figure 2-5 Orthogonal loop antenna structure

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A distant CG lightning strike produces a horizontal magnetic field (B) along with a vertical electric field (E) that passes through the antenna as illustrated in figure 2-6. In figure 2-6 an upward moving return stroke current such as would come from a negative CG discharge is assumed. For a positive CG strike, the current would point downward, and the signal from the loop antenna would have the opposite polarity. Therefore electric fields need to be recorded together with magnetic fields to be able to determine the polarity of the return stroke. It is also assumed that the lightning channel is straight and vertical. According to faraday‟s law, the voltage across the open ends of the loop antenna ( ) will be equal to the time rate of change of the flux through the antenna as shown in figure 2-6, which is given by [11], ∬

̂ 2.1

[ ]

Figure 2-6 B field orientation around the CG lightning channel and through the loop antenna[11]

Where, is the magnetic flux through a single loop, N is the number of turns of wire (each with same ), θ is the angle between the normal of the magnetic loop ̂ and the magnetic field Bdirection. It is assumed that B is uniform across the area of the antenna so that it can be taken out of the integral of equation 2-1. This voltage signal can be integrated to give a signal that is proportional to B. An important point to take from this figure is that the output signal from the antenna will depend on the location of the strike with respect to the plane of the antenna (the cosθ term). This is shown on figure 2-7 [11]. 13

Figure 2-7 Possible lightning strikes around the antenna [11]

Figure 2-7 illustrates the top view of single loop antenna. Lightning strikes are located north of the antenna in (a), east in (b) and south in (c). You'd measure a large positive signal coming from the loop in (a), zero signal in (b) because the B field doesn't pass through the antenna (B and the normal vector are perpendicular so their dot product is zero), and a strong negative signal in (c). A negative cloud-to-ground discharge (upward pointing current) is assumed in each of these examples [11]. Next we'll look at how the bearing angle to a lightning strike can be determined using the signals from two orthogonal loops. We want to be able to determine θ using measurements from a N-S loop and E-W loop antennas. We'll look at the output from the NS loop first.

Figure 2-8 Determination of bearing angle θ [11].

From figure 2-8 b, the output signal from N-S antenna is proportional to the cosine of the bearing angle and from figure 2-8 c, the output signal from E-W antenna is proportional to the sine of the bearing angle. The bearing angle then can be determined by taking the inverse tangent of the ratio of the two loop antenna signals. After the bearing angle estimation at multiple sensor locations, triangulation will give the location of the lightning strike point on ground [11]. 14

ii Time of Arrival (TOA) TOA technique uses four or more sensors to measure the arrival time of the lightning stroke. The onset (arrival time) of the signal at a sensor is the sum of the time of occurrence of the lightning event (onset of the return stroke for CG lightning) and the travel time from the event location to the sensor location. TOA system basically solve the following equation, 2.2 Or more specifically 2.3 Where is the velocity, is the difference between the arrival time at location and the source time , and is the distance between the measurement location and source location [12]. Geometry of TOA location technique is given in figure 2-9.

Figure 2-9 Geometry of TOA location technique [12]

From the Pythagoras theorem, we have √

2.4

Measuring at 4 or more locations is sufficient to determine the four unknowns and . For simplicity consider the 2-dimensional case in which the source and measurement locations lie in the same z-plane. For this case, and equation 2.4 becomes √

2.5

The three unknowns can be determined from measurements of at three different locations. The manner in which the arrival time measurements locate the source can be determined graphically from the fact that the differences in the arrival

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times at a pair of stations i, j constrain the source to lie on the hyperboloid of revolution about the base line between the two station [12]. Each pair of sensors yields a hyperbolic curve describing the set of possible latitude/longitude locations (locus of points) which satisfy the difference in arrival time ( ) between the sensors in the pair (S1, S2) as shown in figure 2-10.

Figure 2-10 Hyperbolic intersection [6]

Given two such curves produced by three sensors (S1, S2, S3), it is possible to determine a position from the intersection of these curves, as shown in figure 2-10, as well as the time at which the discharge occurs. However, under some geometrical condition, curves produced by only three sensors result in two intersections, leading to an ambiguous location as shown in figure 2-11. This problem is avoided if four or more sensors detect the discharge [6].

Figure 2-11 Ambiguous location, 3-sensor hyperbolic intersection [6]

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2.2.2.2 Peak Current During the initial onset of a return stroke, i.e. up to the time of the initial peak current, the waveform of the distant lightning (radiation) electric field can be approximated by the simple “transmission line model” [13], 2.6 Where is the vertical electric field on the ground (assumed perfectly conducting) at time t, the permeability of free space, the upward velocity of the stroke (assumed 8 1.5×10 m/s) near the ground, the current at the base of the channel, the speed of light, and D the horizontal distance to the lightning flash. An upward propagating, positive current produces a downward directed electric field. By scaling down the equation 2.6 by the speed of light we will get the expression for the horizontal magnetic radiation field B. If the lightning source location and the stroke velocity are known, then with the help of sensors, having sufficient bandwidth to measure the peak field (signal strength) without distortion, we can estimate the peak current ( ) in a return stroke from a remote measurement of the electric and/or magnetic field. The simple model described in equation 2.6 assumes propagation over a perfect flat, conducting surface. While measuring fields at distances of hundreds of kilometres, this assumption is not adequate. As a first step in more accurate peak current estimation procedure, propagation effects are taken in to account to produce a range normalized value of the signal strength (RNSS) for each reporting sensor using the following signal propagation model [8] ( )

2.7

Where SS is the raw signal strength, is range in kilometers, is the normalization range which is set to 100 kilometers, is the attenuation exponent, is the e-folding length for attenuation, and is a constant. As stated in [8], attenuation exponent used by the National Lightning Detection Network (NLDN), = 1.13, was determined empirically by assuming that was infinite. The actual value of used by the NLDN is 105 km [8]. RNSS is the measured signal normalized as if the lightning stroke had occurred 100 km from the sensor. The values of RNSS for all reporting sensors within 625 km (to avoid polarity reversals due to ionospheric skip) are averaged, and then the stroke average RNSS is converted to an estimate of peak current. For the case of = 1.13, this conversion has the form [8] 2.8 Where is in kilo amperes. The unphysical zero intercept in the equation 2.8 reflect the fact that this equation will not detect strokes with peak currents below 5 kA. To detect smaller signal strengths, an improved conversion is given by [8] 17

2.9 2.2.2.3 Flash multiplicity and polarity As explained in [13], the U.S. National Lightning Detection Network (NLDN) location algorithm groups strokes into flashes and determines the flash multiplicity and polarity. Prior to 1995, each Direction Finder (DF) sensor counted all strokes that occurred within 2.5° of the first stroke for a period of one second after the first stroke and the flash multiplicity was simply the largest number of strokes detected by any DF. This method has a tendency of overestimation of the true multiplicity when concurrent flashes occurred at similar azimuths relative to any of the sensors. This problem is illustrated in figure 2-12.

Figure 2-12 Overestimation of the flash multiplicity in an angle based flash grouping algorithm [8].

In an updated version of the algorithm, NLDN uses a spatial and temporal clustering algorithm to group strokes in to flashes as illustrated in figure 2-13. Strokes are included in any active flash for a period of one second, which is the NLDN set flash duration limit, after the first stroke as long as the additional strokes are within 10 km of the first stroke and the time interval from the previous stroke is less than 500 ms. In a rare case when a stroke becomes a candidate for more than one flash, it is assigned to the flash with the closest first stroke. The maximum multiplicity in the real time data stream for NLDN is set to 15 strokes, any strokes after that are considered the beginning of a new flash. Subsequent strokes having opposite polarity as that of the first stroke are counted in the multiplicity. The flash location provided by the NLDN in real time is the location of the first return stroke and the flash polarity is the same as that of the first stroke [13]. 18

Figure 2-13 Location based flash grouping algorithm. Strokes 1, 3, and 4 constitute one flash; stroke 2 is regarded as a separate flash [13].

2.2.2.4 Network performance parameters Network detection efficiency and location accuracy are the widely used parameters for the analysis of ground based lightning detection network‟s performance. Since the ground based lightning detection networks are discrete in nature, these parameters will necessarily vary with position. Both of these parameters are functions of the source amplitudes and could change abruptly if there are any outages in nearby sensors or the data communications subsystem. Variations in these parameters can be mitigated by having a large sized network with inherent redundancy [13]. i Detection efficiency (DE) The ability of a lightning detection network to detect and report lightning discharges (IC or CG) is referred to as the network detection efficiency, which is computed as the percentage of actual discharges that are reported by the network. Network detection efficiency can be computed for CG flashes, strokes and IC discharges [6]. Typically, a stroke remains undetected if its peak field is below the trigger threshold of a sensor or if the return stroke selection criteria is not met by the waveform, i.e., the pre-trigger, rise time, width, multiple peak structure, and bipolar shape criteria that are applied to each waveform [13]. Reliability of the network depends on the spacing, gains, and threshold of the sensors in a network. If the separation between the sensors in a network is more than their “nominal range,” i.e., the distance at which small return strokes can no longer be detected, then due to the failure of communications or power at a single site, a significant fraction of the events can remain undetected by the network. Networks containing small number of sensors experience more loss. Since the first strokes radiated fields tend to be larger than for subsequent strokes, the overall flash detection efficiency is assumed by U. S. NLDN to be the same as that for first strokes [13]. ii Location accuracy (LA) Random and systematic errors in the time and direction measurements will produce random and systematic errors in the source locations and onset times. The basic 19

procedure in identifying and correcting the systematic angle or site errors in magnetic direction measurements, is to search for a pattern in the residual errors (plotted as a function of angle) using an ensemble of lightning flashes that are uniformly distributed over the network and then to add angle corrections to these measurements that make the errors a minimum. Lightning signal propagation over complex terrain and variations in the conductivity of the surface; generate systematic errors in the arrival time measurements also [13]. By giving spatial dimension of the associated confidence region for each stroke, effects of random and systematic errors on position can be quantified. For normally distributed errors, the confidence region will have elliptical shape and by giving the length of the semi major and semi minor axes and the orientation of error ellipse, the location accuracy can be characterized. The computed (optimum) stroke location is present on the centre of the ellipse having a certain region, within which the probability of the stroke occurrence is 50%. The confidence ellipse can be described in terms of its semi-major axis, its eccentricity (ratio of the semi-major axis to the semi-minor axis), and the orientation of the semimajor axis (in degrees relative to north). Figure 2-14 and 2-15 illustrates some examples of the confidence ellipses along with all the parameters that describe them.

Figure 2-14 Confidence ellipse: unfavourable sensor geometry [6].

Figure 2-15 Confidence ellipse: favourable sensor geometry [6].

As illustrated in figure 2-14, if only two sensors detect a stroke occurred at a great distance from them, the ellipse is quite eccentric, with its major axis oriented on a line between the two sensors. As shown in figure 2-15, if several sensors detect a stroke at various angles, the ellipse becomes smaller and more circular [6]. The location error is the length of the semi-major axis of the 50% confidence region, assuming that the random errors in TOA have a standard deviation of 1.5 µs, that the random errors in 20

direction are 0.9° and that the only sensors reporting are those within 550 km of the stroke [13].

2.3 Summary Due to the time duration of the primary current and the typical lengths (~7 km) of the conductive channel, the peak intensity of the radiated field of the sferic lies in the 110 kHz range [4]. Due to the limitations of the space based lightning detection networks and weather radars in long range lightning detection, ground based long range lightning detection networks are still very important. Ground based lightning detection networks provide various lightning parameters to their users e.g. location, peak current, flash multiplicity and polarity. Performance of the ground based lightning detection networks can be judged by using two parameters detection efficiency and location accuracy.

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Chapter 3. Literature Survey

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3.1 Introduction Electromagnetic (EM) signals generated by cloud to ground (CG) lightning have a more or less constant power spectral density from 30 Hz to 30 KHz. Thus, we need the receiver to have a flat frequency response over this range rather than one, for example, proportional to frequency. If we use a receiver with low input impedance, the increase in induced electromotive force in the antenna with frequency is counteracted by the increase in inductive reactance of the antenna, making the current into the receiver flat with frequency. The problem is to design a low-impedance amplifier with a good noise figure when connected to an inductive source. It has been found that a common–base input stage gives good results, much better than, for example, terminating the loop with a resistor of the same impedance even if followed by an ideal noise-free amplifier [14]. Magnetic field receivers are used to sense low frequency [