Abstract:
An evaluation system and retest method uses essential coupling physics of lightning, monitored system geometries, and on-line sensor readings for generating and comparing monitored retest indices to critical retest indices using a retest chain algorithm using pre-launch analyses to then provide real-time retest decisions for spacecraft launch operations following major lightning storms.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention was made with Government support under contract No. F04701-93-C-0094 by the Department of the Air Force. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of lightning-induced transients monitoring. More particularly the invention relates to sensory input processing for the detection of potentially damaging lightning strikes on various systems. 
     BACKGROUND OF THE INVENTION 
     Launching space vehicles into space from locations of high frequency of lightning strikes, such as in Florida, presents a recurring problem and concern of the potential lightning induced damage to the space vehicles and payload systems prior to launch. This concern is especially acute for launches taking place during the high lightning season in the summer months. Those involved in the actual launch activities must account for the chaotic atmosphere particularly when a nearby lightning strike has been reported during the critical time path just before a launch. Currently, the assessment of the potential damage to the launch and space vehicle mainly depends on limited and primitive means. The entire launch vehicle including the payload, space vehicle, and an upper stage, if required, are subject to precautions offering modest protection against lightning, often without accurate knowledge of the threat. The decision to proceed with normal launch activities or to switch to retest and re-certification procedures is a difficult one which is subject to conjecture and often results in over-testing or under-testing. As more and more sensitive devices are used in the space vehicles and in order to achieve a balance between too much retest and too little retest, a more reliable lightning retest criterion to help make a real-time decision is needed. The purpose of such a criterion is, on the one hand, to avoid unnecessary and costly tests and delays, and on the other hand, to avoid launching damaged or degraded vehicles and payloads into space. 
     In order to effectively protect against lightning-induced electromagnetic effects, it is prudent to monitor and then protect selected lines and devices. There are two types of known lightning monitoring systems. One type of monitoring system is a remote off-line sensing system that measures the lightning current in the distant lightning channel. Another type of monitoring system is a proximal on-line monitoring system that measures direct parameters that stress the launch system and payload. One on-line monitoring system is the transient pulse monitoring system and has been used during Air Force launches. The proximal on-line monitoring system supports real-time launch operations. Ideally, the on-line monitoring system should be able to provide readings at as many sensor locations as needed. The initial placement of the sensors is guided by analyses for severe field points, that is, major entry points for lightning energies along critical and sensitive penetration paths. The present transient pulse monitoring system can only provide readings at six locations external to the space vehicle. High and low limits for sensors are used to start printed recorders or to alert the launch director based on external stress estimates that need to be refined to correlate with the internal stress at the circuit level. Flow-down analyses are needed to propagate the external stress to the internal circuits, followed by circuit coupling analysis and susceptibility analysis. Ideally, some internal circuit lines can be monitored to provide direct readings at the circuit level eliminating some of the simplifying assumptions and uncertainties in the mathematical models of the analyses. The present transient pulse monitoring system provides three types of stress readings including sheath current readings on the umbilical cable reading, electric field readings on selected surfaces, and magnetic field readings above the umbilical cable. Each type of reading presents a different mode of entry for the lightning energy coupled into the space vehicle system. These readings must be coordinated to give a combined indication of the stress at the circuit level. 
     The present Launch Range-operated cloud-to-ground lightning surveillance system is a remote sensing system. While it is valuable for general weather forecast purposes, it provides only indirect cloud-to-ground data of unspecified effects on the system circuits. To translate the distant environmental lightning data into system-specific stress data, many contractors at the present time rely on analyses that are quasi-static and back-of-the-envelope types of estimates for simple, now-obsolete launch configurations. Other existing monitoring systems are even more primitive having low fidelity and resulting in low confidence level information about lightning-induced transients that have resulted in many unnecessary retests and delays. Recent events have made many contractors aware of these limitations, and have moved them to use the proximal on-line monitoring system, and to re-examine their retest criteria. 
     The existing proximal on-line lightning monitoring systems use either analog or digital technology for detection and data processing. Currently two types of direct on-line lightning monitoring systems have been fielded. An analog type system, such as the transient pulse monitoring system, performs analog peak detection during continuous monitoring of lightning-induced transients. The digital monitoring system provides detailed waveform information of the transients. When deployed alone, the analog monitoring system gives no actual waveforms, while the digital monitoring system may miss significant events due to sampling limitations. The analog monitoring offers continuous front-end monitoring but lacks the actual waveform details that are needed and used by many contractors. It should be advantageous and cost-effective to combine these analog and digital systems into one integrated hybrid system that would avoid these shortfalls. 
     A correct determination of a Go and No-Go evaluation is vital to the launch operation that has obvious consequences on mission success. An efficient and effective retest algorithm is desired. The present-day algorithms used by contractors are based on the Boolean logic, indirect parameters, and inadequate measurements that often result in over-testing or under-testing. Over-testing leads to unnecessary delays, higher cost, and schedule impacts, while under-testing is dangerous because damage may not be discovered, resulting in the launch of a defective vehicle into space where no repair is feasible. Under-testing is most dangerous in that damage will not be discovered and damaged vehicles may be launched. 
     The miniaturization of electronic devices makes the launch system more sensitive and vulnerable to lightning-induced electromagnetic transients. Therefore, many systems have implemented procedures, often called lightning retest criteria or lightning damage search criteria for their launch processing operations following major lightning storms. Many existing lightning retest criteria and lightning damage search criteria are based on indirect parameters. This results in simplistic, misleading and erroneous retest decisions. During the critical moments shortly before launch, little time is allowed for detailed and elaborate analysis of the situation that may arise due to lightning events. However, many complicated analyses are generic processes that can be performed well in advance of a launch, and the analytical results with appropriate system configurations can be incorporated into the lightning effects evaluation algorithm in the form of lookup data tables. 
     Instead of waiting for nature to deliver all types of lightning over a long period of time, parametric studies using mathematical models can be carried out to analyze many possible scenarios. Three kinds of analyses are needed to support the formulation of a dependable lightning retest evaluation method. The three kinds of analyses are electromagnetic (EM) field coupling analysis, circuit coupling analysis, and susceptibility analysis. These analyses can be performed during pre-launch periods. 
     Field coupling analysis is an EM field coupling analysis that is based on the scattering theory in terms of Maxwell equations and is used to characterize the induced electromagnetic transients on the segments in the model space due to lightning-generated electromagnetic fields. Because of the complexity of the structures and geometry, a method of moments computer code or a finite difference computer code is needed to S incorporate essential geometry and physics into the analysis. Analysis results provide the stress drivers from any selected location for the stress evaluation of identified circuits and devices in a later circuit analysis. Each structure is segmented to represent locations where the responses can be monitored. 
     Circuit analysis is a coupling analysis that determines the most likely entry point of lightning energy. Critical circuits or devices for a particular space system must be first identified. The stresses induced by lightning on the circuits and devices in the system will be evaluated. Circuit models for these circuits will be built for a circuit analysis code such as the industry standard code named SPICE using the location drivers from the EM field coupling analysis. The SPICE code evolved from the research project at UC Berkeley and is available commercially in many versions. The SPICE code remains a powerful circuit analysis tool for electrical circuits. The outputs of the circuit analysis are stress energies coupled into the circuits. Stress energy ratios for different location drivers provide initial individual sensor channel weights. The weights with drivers of various severity will also yield a stress profile as a stress curve in energy versus a sensor reading for a particular circuit at a given location. 
     Susceptibility analysis of the space system will determine the susceptibility of the system to energy stresses. A Wunch-Bell type of damage mechanism is usually used in the damage assessment. The Wunch-Bell model for damage and its applications can be found in the public literature and is well known by those skilled in the art of lightning testing. This susceptibility analysis will yield threshold energies that cause damage to the circuits. The susceptibility analysis aids in the identification of sensitive circuits or devices in the system. Electric field sensor readings inside known layers of enclosures can be compared to a Fourier transform, in the time domain, of an analytical Electric field prediction for determining a certain threat level. Other points can also be found for less severe threat levels to obtain the shape for the Electric sensor stress curve. Without performing the susceptibility analysis for the circuits, this stress curve only defines a generic shape for an allowable electric field stress level. This generic stress curve lacks the details and the characteristics of the selected circuits and, therefore, the susceptibility analysis should be circuit specific for generating suitable strength levels. The threshold energy for the most sensitive circuits or devices will provide a strength line. Certain safety margins, e.g. 6 dB, are required for many electromagnetic compatibility considerations and can be built-in by moving the strength line downwards on the retest plot thereby providing safety margins desired for the decision-making process. 
     As micro-technology advances, miniature probes and sensors may become available to allow closer placement of the probes and sensors to circuits and devices of interest reducing the need for circuit analyses. Because there is no a priori knowledge about the next lightning event and the general topology of launch platforms, it is necessary to deploy multiple sensors to cover all possible entry points of lightning energies. Multiple sensor deployment increases confidence levels of detection by confirming significant events and by discarding spurious events through correlated multiple sensor inputs. 
     A lightning retest criterion should include all the important parameters that affect the system under lightning monitoring. The first parameter is the stress induced by lightning on the system, and the second parameter is the intrinsic strength of the system to withstand the stress without suffering damage. Lightning, by virtue of transient electromagnetic field scattering from the system, will first induce electromagnetic stresses on the external surface of the system such as the payload faring and umbilical cable sheath. The external stress, in terms of induced electromagnetic fields or currents on the surface of the system will then propagate through available paths to the circuits and devices in the interior, and discharge undesirable energy there causing damage. A validated transient pulse monitoring system as installed for a particular payload will give the actual external stress, and analyses will indicate the effect of the external stress over a wide range of lightning threats. The stress at circuit levels is the true reason for damage concerns, and stress is stated in terms of stress energy and voltage waveforms by circuit coupling analyses. 
     However, stress alone will not indicate whether or not the circuit will be damaged. The strength, that is, the tolerance, of the circuit is determined by the susceptibility analysis. A comparison of the stress and strength in the susceptibility analysis yields an indication of how well the circuit will tolerate a given stress. A retest criterion can then be formulated with desired safety margins. Even if the stress is very severe, a circuit that has strength to withstand that stress should not be damaged. The strength of a device or circuit is an intrinsic characteristic of such a device or circuit. A good example of the strength of a device or circuit is the burn-out damage threshold. Abundant experimental data is available for various types of devices such as diodes, transistors, digital circuits, and other basic electronic components. This data has been fitted to the Wunch-Bell-Jenkins-Durgin empirical failure models. A conservative sample burn-out damage energy threshold is one micro-joule for many popular sensitive electronic devices. 
     The historical account has shown that the present monitoring systems are inadequate in detecting lightning-induced transients, and the retest criteria produce decisions subject to conjecture and results in over-testing or under-testing. As more and more sensitive devices are used in launch and space vehicles and other electronic systems, a more reliable lightning effects monitoring and retest evaluation method is needed. The inadequacies and other disadvantages of the present monitoring systems can be solved or reduced using the present invention. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an evaluation method of determining when a lightning event warrants a costly retest of the target system. 
     Another object of the invention is to provide a hybrid on-line monitoring system that combines analog and digital technologies for the monitoring of lightning-induced electromagnetic transients that can possibly damage the target system being monitored. 
     The invention is directed to assisting in the evaluations of the effects that lightning-induced electromagnetic transients have on launch systems during space launch operations. The invention is a lightning-induced electromagnetic effects evaluation and monitoring system and method that combines the advantages of both analog and digital technologies, while avoiding the shortfalls of each if either is applied alone. This monitoring system will be driven by a retest algorithm that incorporates the correct interaction physics and essential launch geometries, by utilizing multiple sensor data that is statistically weighed and evolves in a dynamic manner. The retest algorithm output will be an evaluation of the lightning-induced threat from which a real-time recommendation on Go and No-Go decision can be made. The on-line monitoring system comprises a multiple-sensor suite, a characterizer, a responder, and a detailer. The sensor suite consists of multiple sensors of various types such as current probes, electric and magnetic field sensors and voltage probes. These sensors detect the lightning-induced electromagnetic transients and provide the sensory inputs to the rest of the monitoring system. The characterizer is made up of a number of peak detectors that take the signals from the sensors and perform necessary functions to condition and characterize the sensor signals. Only those signals exceeding a preset threshold will be sent to trigger the detailer where they will be digitized and displayed in suitable forms. These characterized sensor signals are then communicated to the responder that consists of A/D converters and a microprocessor to capture data and a computer to further analyze the data using the retest algorithm. 
     The software of the system consists of commercial data handling and analysis software and the novel retest algorithm for real-time threat evaluations for launch decisions. The retest algorithm integrates results from extensive electromagnetic (EM) field coupling analyses, circuit analyses, and susceptibility analyses, and can be updated using a database of data collected from the multiple sensor suite. The retest algorithm utilizes innovative retest indices of monitor readings to correlate the multiple sensor data that can be compared to a critical retest index of a predefined damage strength threshold to identify the subsystem most likely to be affected for repair and retest purposes to arrive at a Go or No-Go determination recommendation. 
     Two sets of indices are used. A set of monitoring retest indices MRI indicate monitored stress. A set of critical retest indices CRI indicate critical stress levels that when exceeded by monitored stresses presents a potential retest situation. The susceptibility analysis and circuit analysis provide the stress curves and the strength lines, with or without safety margins, for sensitive circuits forming a stress curve graph having intercepts indicating the individual critical retest indices CRIi. The composite critical retest index CRI is preferably computed from the average of the individual CRIi. Critical retest indices CRIi are determined and used to indicate the threshold events at which potential damage can occur. This determination is based on the stress curves and the circuit damage threshold energy levels, with or without safety margins. Each CRIi is obtained at the point where the stress curve crosses a horizontal threshold strength level. A safety margin may be used to lower the strength level to define the intercept with the stress curve at a safer level. The monitor retest indices MRIi represent the actual stress experienced by selected circuits for each of the N monitor sensor readings, where the sensor index i is equal to one to N. The retest monitoring indices MRI are computed from monitor readings Mi and channel weights Wi indicating what fraction of energy is coupled to the respective circuit. The channel weights Wi are derived from the circuit coupling analysis. A composite monitor retest index MRI is preferably calculated by the sum of the individual MRIi. The retest algorithm then performs a comparison of the composite MRI and CRI indices for a retest recommendation. Equal weights for damage are assumed once the energy from a strike reaches the circuit. Depending on a particular space vehicle, various stress curves with threshold strength levels can be obtained for various types of circuits and devices. The combined composite retest index MRI of the monitor readings is determined for a particular event and is preferably compared to a composite critical retest index CRI. Further, individual MRIi indices can be compared to respective individual CRIi indices for each sensor and respective circuit for respective retest recommendations. The CRIi indices are determined from pre-launch analyses whereas the MRIi indices are computed during a lightning event. 
     The retest algorithm is applicable to space vehicle or structures using monitored sensor data. For space vehicles, pre-launch, coupling analysis is performed for respective launch vehicular configurations. That is, susceptibility and circuit analyses are performed in advance of on-line monitoring. Circuit analysis results and databases are used in the retest algorithm to update the individual channel weight Wi as new data is accumulated. The real-time on-line monitoring system provide monitor readings Mi that are immediate inputs to the retest algorithm for computing in real time the monitor retest indices MRIi using channel weights Wi computed from the circuit analysis. The retest algorithm adopts a balanced intermediate approach where fractional weighted indices Wi from the circuit analysis are used to compute the individual MRIi for of all identified coupling modes of lightning coupling reflecting the physical inter-relationship of the vehicle and the lightning points of entry. 
     The lightning retest criterion contains essential physics of lightning and geometry of the system. It also correlates all sensors readings into a simple rule of comparison using the concept of retest indices. Therefore, the retest algorithm provides an instantaneous means for real-time decisions following lightning storms. The retest criterion is based on the family of stress curves that are expressed in terms of stress energy obtained from analyses and monitor readings from a high-fidelity on-line lightning monitoring system. The measurements will be taken by the hybrid on-line lightning monitoring system that uses both analog and digital sensory inputs. The lightning retest criterion is well suited for space launch processing operations, but can be applied to other systems. There are broad potential markets for the monitoring system wherever lightning is a concern, such as communication systems, control and command centers, railroad stations, as well as factories, storage or office buildings. 
     The retest algorithm for a lightning retest criterion uses pre-launch analysis and high-fidelity on-line measurements. The retest algorithm correlates multi-sensor data with correct interaction physics for complex launch pad structures and geometries. The retest algorithm correlates multi-sensor data to achieve cost and risk reduction for space launch operations. Using the data collected by the hybrid on-line monitoring system, it provides a credible, real-time retest decision-making tool for evaluating electromagnetic effects induced by major lightning storms. Pre-launch analyses such as EM field coupling analysis, the circuit analysis, and the susceptibility analysis, are updated and coordinated with real-time multiple sensor measurements on the launch pad. This information is combined in a weighted manner in the retest algorithm. 
     The hybrid on-line lightning effects monitoring system combines the desirable features of both analog and digital systems. The system provides continuous analog monitoring and digital waveform recording. This monitoring system is designed to be generic enough for all space vehicles at all launch pads, yet is flexible enough to accommodate specific needs of a particular launch configuration. After implementation, the monitoring system can be operated and maintained by the Launch Range Authority. The comprehensive suite of sensors for a baseline design in the monitoring system will keep a vigil watch for on-line transients because of the importance not to miss any event usable for damage assessments, while only selected channels are digitized to give waveform information for verification of significant events. The number, type, and placement of sensors are determined from analyses and field experiences. Most of the current and voltage probes and electromagnetic field sensors are placed at fixed, identified entry points while the remainder of the sensors is deployed throughout the launch pad for launch-specific and reference purposes. The real-time on-line measurements are used in two ways. Inputs to the monitoring system are used for the real-time updates of the individual monitor retest indices MRIi. The inputs to the database are used for iterative updates of the individual channel weights for later use. In this way, the dynamic updated characterization of the space vehicle susceptibility and on-line monitoring is achieved. 
     The monitoring system and method offers continuous monitoring of lightning-induced electromagnetic effects on selected locations, with actual selectable waveform detail information at pre-set levels, offers the correlation of multiple channel on-line data, offers the combination of correct interaction physics with realistic system geometry, offers minimal real-time user inputs and data processing, offers provision of flexibility for different system configurations and future expansion, offers unification of compartmentalized subsystem circuits, and offers utilization of real-time decisions with any desired safety margin. These features facilitate both short-term and long-term cost and risk reductions of space launch operations. The retest algorithm is almost instantaneous, and decisions can be made in real time when launch operations are in progress. The space vehicle contractors will also have the information for different types of circuits, such as individual MRIi and CRIi enabling the contractors to identify particular circuits or devices needing retest or repair. These and other advantages will become more apparent from the following detailed description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the lightning effects monitoring system. 
     FIG. 2 is a maximum predicted stress and strength energy chart. 
     FIG. 3 is a flow chart of the retest algorithm for detecting and evaluating lightning-induced EM effects. 
     FIG. 4 is a table of a MathCad file for setting retest criterion for applying the retest algorithm. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to FIG. 1, a lightning monitoring system comprises a suite of input sensors  10  providing sensory signals  11  to a characterizer  12  connected to a responder  14  both of which are connected to a detailer  16 . These sensors  10  detect the lightning-induced electromagnetic transients and provide the sensory input signals to the lightning monitoring system. The responder  14  communicates with a computer  18  in a control station for reporting analysis results to the users of the system. A retest algorithm can be executed by the computer  18 . The retest algorithm utilizes the benefits of the on-line monitoring data for determining stress indexed to strength levels by indices for rapid retest determinations. The retest algorithm reduces the launching of damaged space vehicles and to avoid conducting unnecessary retests. Space vehicle specific information, that is, strength levels and stress curves, can be read in simple (x,y) paired formats and stored in the computer  18 . 
     The sensor suite  10  is a set of multiple sensors including current probe sensors, electric field sensors, and magnetic field sensors and voltage probe sensors. Sensory input signals are received by a buffer  20  communicating the input signals to a first peak amplitude detector  22 , an integrator  24 , and a differentiator  26  in the characterizer  12 . The characterizer  12  includes a plurality of peak detectors  22 ,  28 , and  30 . The integrator  24  and the differentiator  26  condition and characterize the sensor signals  11  and communicate respective integrated input signals and differentiated input signal to the respective second peak detector  28  and the respective third peak detector  30 . The detectors  22 ,  28 , and  30  provide respective positive and negative peak signals to a comparator  32  providing respective comparator positive and negative outputs to respective sample and hold amplifiers  33 ,  34 , and  35  providing respective positive and negative compared input signals to an analog to digital converter  40  in the responder  14 . These characterized sensor signals are then communicated to the responder  14  through the A/D converter  40  to a microprocessor  42  communicating monitored data to the computer  18  executing the retest algorithm. Only those sensor inputs from the buffer  20 , integrator  24 , and differentiator  26  exceeding a preset threshold will trigger the detailer  16  where these signals will be displayed in suitable forms. 
     The responder  14  comprises a microprocessor  42  that controls the A/D converter  40  with selecting address signals on address lines and receives converted digital data from A/D converter  40  over data lines. The data lines communicate digital data indicating peak values of the sensor input, integrated input, and differentiated input from the sample and hold amplifiers  33 ,  34 , and  35 , respectively. The microprocessor  42  provides a reset signal  44  to the detectors  22 ,  28 , and  30 , comparator  32  and provides a sample and hold signal  46  to the sample and hold amplifiers  33 ,  34 , and  35  for controlling the comparison and conversion of the compared signals converted into digital data on the data lines. A sample and hold signal  46  from the microprocessor  42  samples and holds the input signal voltages to the sample and hold amplifiers  33 ,  34 , and  35  for a period of time with a certain droop rate. Once a sample and hold cycle is completed, the microprocessor  42  sends a reset signal  44  to the peak detectors  22 ,  28 ,  30 , and the comparator  32  to reset the peak detectors and to start a new cycle of detecting, comparing, sampling and holding, and digitizing. 
     The comparator  32  generates a pulse count signal  48  communicated to the detailer  16 . Only those input signals exceeding a preset threshold in the comparator  32  will trigger the detailer  16  and be communicated to the detailer  16  for display. A triggering circuit  50  is triggered by the pulse count signal  48  from the comparator  32  and the digital outputs  51  from the A/D converter  40 , routes waveform data from the converter  40  to a digital scope  52  for display. The detailer  16  is used for detailing the positive and negative sensory input signals, integrated input signals and differentiated input signals into suitable forms for storage and display on the digital scope  52 . The digitized waveform  53  has digital amplitude valued across a predetermined time period where as the peak value communicated on the data lines to the microprocessor  42  is a single peak amplitude value. The waveform data  53  on the scope  52  will be communicated to the microprocessor  42  for further processing and for later data distribution to the computer  18  in the control station. 
     Referring to FIG. 2, the retest algorithm is used in connection with stress curve graphs for each respective sensory input in relation to a circuit or device of a target monitored system, each of which may further comprise several subcircuits and subdevices. The sensor stress curve graph includes a stress curve  70  across monitor readings for a respective circuit or device for a respective sensor input normalized by an scalar value to a predicted maximum value  72  of the monitor readings. In the preferred form, the scalar value is ten for a normalized index scale from zero to ten. The stress curve  70  is intersected by a safety margin strength level  74  which is less that an inherent circuit strength level  76  by a safety margin. 
     Due to the fact that lightning energy can enter the system in different ways, and the fact that the on-line monitoring system will deploy multiple sensors for higher fidelity and correlation, retest indices are generated and used to account for different coupling mechanisms as recorded by multiple sensor readings, and to normalize the reading to stress curves having the same base scale. A selected circuit for each sensor will be assigned a respective critical retest index CRIi value  78  where the stress curve  70  intersects the safety margin strength level  74  for the selected circuit and where i indicates a sensor and a respective selected circuit. During a lightning event, a monitor retest index MRIi is generated for each sensor and compared to a respective CRIi to determine if the event warrants a retest of the circuits. The stress curve graph is used in connection with the operation of the retest algorithm of FIG.  3  and an exemplar mathcad process of FIG.  4 . 
     Referring to all of the Figures, and more particularly to FIGS. 3 and 4, when applying the stress curve for a retest decision, the stress in the x-axis must be the actual stress value as experienced at the selected location. Therefore, an end-to-end calibration and validation of the on-line monitoring system has to be performed at the start  110  before the retest criterion can be used with confidence. As a user logs on to the detection system, the retest algorithm is started  110  by the computer  18 . The retest algorithm offers a method to best utilize the full benefits of the on-line monitoring data from the microprocessor  42 . A retest criterion is based on reliable data from the on-line measurement system. The retest algorithm is used for reducing the launch of damaged space vehicles into space and avoids conducting unnecessary retests on the ground. As time goes on and more data become available, the lightning retest criteria based on the retest algorithm will dramatically evolve. Any software based on the retest algorithm can complement any other risk assessment procedure. The retest algorithm can also be made to be system-specific by entering specific data such as threshold levels and stress profiles. 
     At the start  110 , the user will be prompted to specify the number N of sensors desired indicated by the sensor index i where i equals one to N. The user may specify sensor placements from a list of recommended locations, and other relevant operational information such as audio alarms and print-out functions. An electromagnetic field coupling analysis  112 , a circuit coupling analysis  114 , and a susceptivity analysis  116  are performed during pre-launch activities prior to monitoring a lightning event  118 . Real time monitor data  120  is used to update database information  122  that is used by the circuit analysis  114  that generates individual channel weights Wi that relate the amount of energy coupled to a receptive circuit from monitor sensory readings from a respective sensor of the sensor suite  10 . The on-line monitoring readings  120  can also be used to verify the scattering model in the field coupling analysis  112 , to provide increased confidence in the future ability to predict and understand the lightning-induced stress. During the electromagnetic field coupling analysis  114 , maximum monitor readings MAXi  131  of the monitor sensor inputs  11  are firstly related to a full-scale lightning threat. During operation, and after a lightning event  118 , the real-time monitor data Mi is collected  120  and monitor retest indices MRIi are computed  133  and are used to then compute a composite monitor retest index MRI that indicate monitored stress on the circuits being monitored. Minimal real-time data processing is required to compute the MRIi and composite MRI. The on-line monitoring system database  122  is created to aid in the generation of updated individual channel weights Wi  132  and can be later further dynamically updated in post-launch activities. During the EM-field analysis  112 , sensor locations are selected and current densities for magnetic fields and charge densities for electric fields are computed. The weights WI are initially computed as the ratio of the current or charge density for a respective sensor at a respective sensor location divided by the total current or charge densities of all of the sensors. After a lightning event, the weights WI can be dynamically recomputed based on actual measurement data stored in the database  122 . The monitor data Mi  120  are used in real-time to calculate  133  the individual monitor retest index MRIi on the arbitrary normalized scaled value from which the composite monitor retest index MRI  134  is computed. The normalization to the factor of ten is somehow arbitrary, yet without loss of generality. The composite monitor index MRI is equal the sum of [scalar x Mi/MAXi]Wi for each indexed sensor. 
     The coupling analysis  114  is used to generate  140  stress curves  70  for each sensor i of the sensor in suite  10 . During the susceptibility analysis  116 , inherent strength levels  76  and safety strength levels  74  are generated  142  and intercept points on the stress curves  70  intercepted by the safety strength levels  74  are used to generate  144  individual critical retest indices CRIi  135  that can be used to generate a composite critical retest index CRI  146 . The individual sensor channel weights Wi  132  are generated using a lightning channel model in coupling analysis  112  and circuit analyses  114 . The individual channel weights Wi and the critical retest indices CRIi  135  can be routinely updated in a dynamic mode with probability and efficiency results from the database  112 . The composite monitor retest index MRI  150  is compared to the composite critical retest index CRI  146  to determine if a retest  152  of the circuits is indicated. When a retest  152  is performed, the database  122  can be updated for updating the weights Wi for improving subsequent retest decisions  150  for significant lightning events  118 . 
     The susceptibility analysis  116  and the circuit analyses  114  are performed during pre-launch activities to provide the stress curves  70  and the strength lines  76  and  74  on the stress curve graphs for the respective sensor and circuit. The stress curve  70  is for a respective sensor, such as an exemplar current probe sensor, with and without the safety margin, for example of 6 dB. From the intercepts of the stress curves  70  and the strength line  74  and  76  for the individual critical retest indices CRIi  135  are found. The composite critical retest index is preferably equal to the arithmetic average of the individual critical retest indices CRIi, that is, the CRI is equal to the sum of the individual CRIi derived by N for i equal one to N. The composite index MRI is preferably compared  150  to a composite index CRI as a single comparison to determine if a retest  152  of the circuits is indicated. However, the comparison  150  may also determine if a retest  152  is desirable when any one of the MRIi is greater than the respective CRIi for each of the sensors. Alternatively, a composite stress curve can be generated by averaging all stress curves  70  of all the individual channels i in the circuit analysis  114 . Averaging the safety strength levels  74  also provides a composite safety strength level. In this case of generating a composite stress curve  70  and a composite safety strength level  74 , the composite critical retest index CRI  135  can then be generated  144  from the intercept on the composite stress curve where intercepted by the composite safety strength level  74 . The final decision step in the retest algorithm is the retest decision  150  that is preferably a simple single comparison of the composite MRI and CRI values for indicating if the circuits should be retested. This is especially desirable at the time of a critical usage of the circuits, for example, just before a launch of space vehicle housing containing the monitored circuits. 
     The weights Wi are dictated by the lightning coupling mechanism and are given by expert analysts based on the circuit analyses and are updated in an evolving dynamic manner as more data are accumulated. Monitoring reading Mi are the actual monitor readings that are continuously updated. MAXi values are predicted maximum monitor reading values for the full-threat case known from pre-launch analyses. Each point on each stress curve is related to a MRII value on the normalized x-axis. The CRIi values can be determined from a stress curve graph having a stress curve  70  intersection by strength levels  74  or  76 . The MRIi values can be compared to the respective CRIi value to determine if a particular circuit should be retested. 
     Referring to all of the Figures and more particularly to FIG. 4, the retest algorithm was applied to a three-sensor suite example using a MathCad program. A current probe sensor and two E-field sensors are positioned relative to the space vehicle. The maximum monitor readings from the full-threat case analysis were found to be maxi=1930 Amps, 300 kV/m and 350 kV/m, respectively. The circuit analyses  114  initially indicated weights of W1 equaling 0.34 for the current probe sensor, and weights W2 and W3 equaling 0.33 for each of E-field sensors. The monitor readings Mi from the on-line monitoring system where M1=879 Amps, M2=2.2 kV/m, and M3=14.4 kV/m, providing individual monitor retest indices of MRI1=1.548, MRI2=0.024, and MRI3=1.358, respectively. These channel weights Wi and monitor retest indices MRIi are used to calculate the composite MRI and CRI with safety margins of 0 dB, 6 dB, 12 dB, and 20 dB. No retest was recommended for safety margins less than 12 dB. For safety margin equal to 12 dB, the composite MRI was slightly larger than composite CRI and the retest recommendation was marginal. After a lightning event and after updating the database  122  using on-line measured data from an actual launch, the weights  132  were updated to be W1=0.63, W2=0.11 and W3=0.26, and the MRIi  133  became M1=2.864, M2=0.008 and M3=1.07 providing a composite MRI  134  of 3.94 which was definitely larger than the composite CRI of 2.27 at the 12 dB safety margin. The retest recommendation with the updated weights was more definite. 
     For a retest criterion to be an effective tool for real-time decision-making, the decision should be based on a balance between too much and too little retest. The retest algorithm is dynamic in that the database can be updated from various lightning events to accumulate actual data. Small-scale model testing, triggered lightning tests, in-situ measurements and comparison to existing databases can be done. Also, the retest criterion can be verified by injecting known pulses into the algorithm stream, and by monitoring the responsiveness of the retest criterion to various stimuli. As time goes on, and more data on the retests and damage is accumulated, this data can be gradually incorporated into the retest criterion to gain more confidence in the tool for decision-making. 
     The retest algorithm uses a set of sophisticated analyses and measurements to reduce the retest no-retest decision to a simple comparison of two quantities, the MRI and CRI, based on lightning physics, stress and strength data. An ability to make an instantaneous and well-founded decision can greatly streamline the launch operation, while reducing the risk, which will ultimately result in great long-term cost savings. Many launch directors will desire an immediate and clear-cut decision of a retest or no retest. Most of the inputs to the algorithm can be obtained in pre-launch and post-launch activities. Therefore, only minimal real-time data processing is required. All stress curve and strength information for each sensor channel with safety margins will also be available for examination. The retest algorithm can also be applied separately to sensors monitoring space vehicle circuits as well as to those for launch vehicle circuits, giving each contractor information about the status of the systems. Stress and strength information can also be given to identify particular circuits which are most likely to be damaged. In the special case when there is only one stress available from one monitoring sensor where N equals one, the above retest criterion is still applicable, although with a somewhat reduced level of confidence. Those skilled in the art can make enhancements, improvements, and modifications to enhance the invention. However, those enhancements, improvements, and modifications may nonetheless fall within the spirit and scope of the following claims.