A Modeling Approach for Developing System Performance Requirements John M. Green Naval Postgraduate School jmgreen@nps.edu
Issues to be Addressed • The concept of system performance and how to measure effectiveness has been the topic of numerous papers of over the years. • Typically the focus is on one system characteristic such as reliability (R) or operational availability (A 0 ) though the Air Force Weapons System Effectiveness Industry Advisory Committee (WSEIAC) recommended that both are required along with system capability (C). • This is in recognition that performance measures are extremely useful to the system engineer in five key areas: 1. Establishing requirements; 2. Assessing successful mission completion; 3. Isolating problems to gross areas; 4. Ranking problems relative to their potential to impact the mission; and 5. Providing a rational basis for evaluating and selecting between proposed problem solutions and their resulting configurations. 2
Goal • This presentation will present a top-down modeling approach based on functional flow block diagrams that shows how the system engineer can develop an overall system performance measure that is inclusive of R, A 0 , and C. • It starts with the system concept and allows the system engineer to allocate performance at each layer of analysis, from system to components, ultimately providing detailed performance requirements which will provide a basis for evaluating candidate solutions. 3
Prediction • Effectiveness calculations are about prediction • Objective of prediction is twofold: 1. System effectiveness predictions form a basis for judging the adequacy of system capabilities 2. Cost-effectiveness predictions form a rational basis for management decisions. 4
Outline • Three key studies • Overview of the approach • An example • Summary • References 5
Three Key Studies – WSEIAC (Weapons System Effectiveness Industry Advisory Committee) Study (1964) – MORS C2 Measures of Effectiveness Study (1986) – Paper by John Marshall (1991) – Other support work listed in references 6
# 1: WSEIAC Study • Developed for the Air Force in 1964 and follows AFSC-375 series • Looked at two approaches: 1. Immediately commit resources to an intuitively plausible (re)design and surmount the problems as they arise, or 2. Explore in the “minds eye” the consequences of the (proposed) system characteristics in relation to mission objectives before irrevocably committing resources to any specific approach • It is a framework for evaluating effectiveness 7
System Effectiveness • Concluded that system effectiveness can be defined as a measure of the extent to which a system may be expected to achieve a set of specific mission requirements. • System effectiveness is a function of three primary components: availability (A), dependability (D), and capability (C). • Definition allows one to determine the effectiveness of any system type in the hierarchy of systems 8
Definitions • Availability (A) – a measure of the condition of the system at the start of a mission, when the mission is called for at some random point in time. • Dependability (D) – a measure of the system condition during the performance of the mission given its condition (availability) at the start of the mission. • Capability (C) – a measure of the results of the mission given the condition of the system during the mission (dependability) 9
Mathematical Formulation c (0) 1 A a a C 1 2 0 c (0) 2 System state (up/down) Capability at t 0 d 11 = probability of operational at end given operational d d at start 11 12 D d 12 = probability of fail at end given operational at start d 21 = probability of operational at end given fail at start d d 21 22 d 22 = probability of fail at end given fail at start d d c (0) 11 12 1 E a a 1 2 d d c (0) 21 22 2 10
#2: MORS C2 Study DECISION PROBLEM MAKER FORMULATION Environment PROBLEM STATEMENT C2 SYSTEM Force IMPLEMENT BOUNDING RESULTS SYSTEM ELEMENTS System C2 PROCESS DEFINITION FUNCTIONS Subsystem INTEGRATION OF SYSTEM Dimensional ELEMENTS AND FUNCTIONS Parameters SYNTHESIS OF STATICS AND DYNAMICS SPECIFICATION OF MEASURES MOPs (CRITERIA) MOP, MOE, MOFE MEASURES FOR FUNCTIONS DATA GENERATION MOEs EX, EXP, SIM, SUBJECTIVE VALUES OF MEASURES MOFEs AGGREGATION OF MEASURES ANALYSIS RESULTS MOP f p p p p MODULAR COMMAND AND CONTROL , , ,......., EVALUATION STRUCT URE (MCES) 1 2 3 n 11
#3: John Marshall Paper • Marshall developed a Threat Activity Probability mathematical relationship P ta between D, A, C, and S Threat Detect/Control/ Engage P dce =P cd* P cl based on the work of Ball and Habayeb Threat Attack P ca • Related concept to an Ship Susceptability Ship Vulnerability operational characteristics P h =P ta* P dce* P ca P k/h curve Ship Killability Cumulative – Initial Curve is based solely on P k =P h* P k/h Probability Available P Performance 1.0 Curve the physics involved Ship Survivability – Subsequent shape of the P s =1-P k curve is defined by variance P in system design, operational usage, and environmental Range conditions t 1 R Derived Performance Curve 12
A FFBD Example http://en.wikipedia.org/wiki/Functional_flow_block_diagram 13
Aggregating Processes P P P P Start Process A Process B Process C Outcome t A B C Process A P P P P P Start Outcome t A B A B Process B Process A P P P ( P P P ) t c A B A B Start Process C Outcome Process B 14
Overview of the Approach 1. Establish the intended purpose of the system 2. Establish those system characteristics which contribute to the designed ability of the system to accomplishment of the system purpose. 3. Measure/compute the numerical value that describes the degree to which each of these characteristics affects the accomplishment of the system purpose 4. Combine all computed/measured values into a form suitable to obtain a system operational value. 15
A SIMPLIFIED EXAMPLE 16
From the Ship’s Perspective Prime Directive: Defend Ship against Cruise Missile Threats Actively Defend Ship Process Functions: Detect Control Engage Search Track TEWA Launch Functions Allocated to: Search Radar Auto Auto Track TEWA OR Weapons OR OR Track Radar Manual Manual Track Assign OR Optical Search 17
Sensor Operational Objectives • Required functions to be performed: – Detection, Tracking, Classification, ID, Ranging • Target characteristics and separation • Coverage volume or area and background • Atmospheric and weather conditions 18
Baseline Network P d IFF R MIN Max wait for P d Radar IFF response 1.0 R T open fire T R max MAX T max MIN + P d C2 Fire Control IR 1.0 1.0 + R T T R max T min T max T FC IFF Raleigh C2 Uniform Radar Swerling FC Fixed Time Delay IR Exponential IFF Wait Fixed Time Delay 19
System Reaction Time Distribution Reaction Time Distribution 1.00 Cumulative Probability 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 0 5 10 15 20 25 Range (NM) Shown without effects of D, A, or S 20
Sensor Operational Objectives into Effectiveness Performance Parameters (C) Reliability Parameters • Detection range • Dependability • Tracking range • Availability • Classification range • • Survivability ID range • Pd • SNR minimum • Spatial resolution • Sensitivity • Total FOV (look angle) • False alarm time • Frame time • Physical characteristics 21
Parameters Drive IR Sensor Design • Optics • Signal processing • Detectors • Display and recording 22
In Summary • Presented a top-down modeling approach based on functional flow block diagrams that shows how the system engineer can develop an overall system performance measure that is inclusive of R, A 0 , and C. • It started with the system concept which allows the system engineer to allocate performance at each layer of analysis, from system to components, ultimately providing detailed performance requirements which will provide a basis for evaluating candidate solutions. • Approach can be useful to the system engineer in five key areas: 1. Establishing requirements; 2. Assessing successful mission completion; 3. Isolating problems to gross areas; 4. Ranking problems relative to their potential to impact the mission; and 5. Providing a rational basis for evaluating and selecting between proposed problem solutions and their resulting configurations 23
Functional and Non-functional Performance 24
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References • Ball, Robert. The Fundamentals of Aircraft Combat Survivability Analysis and Design , AIAA Press, 1985 • Bard, Jonathon. An Analytical Model of the Reaction Time of a Naval Platform . IEEE Vol. SMC-11, No. 10 Oct. 1981, pp. 723-726 • DARCOM-P 706-101 CH. 24 • Habayeb, A. R. Systems Effectiveness , Pergamon Press, 1987 • Hitchens, Derek. • Friddell, Harold G. and Herbert G. Jacks. System Operational Effectiveness (Reliability, Performance, Maintainability) , 5 th National Symposium on Reliability and Quality Control, January 1959 • Marshall, John. Effectiveness, Suitability & Performance , 59 th MORS, 12 June 1991 26
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