Showing papers in "Transportation Research Part E-logistics and Transportation Review in 2007"
TL;DR: Based on a three-layer emergency logistics co-distribution conceptual framework, the proposed methodology involves two recursive mechanisms: (1) disaster-affected area grouping, and (2) relief codistribution as mentioned in this paper.
Abstract: Quick response to the urgent relief needs right after natural disasters through efficient emergency logistics distribution is vital to the alleviation of disaster impact in the affected areas, which remains challenging in the field of logistics and related study areas. This paper presents a hybrid fuzzy clustering-optimization approach to the operation of emergency logistics co-distribution responding to the urgent relief demands in the crucial rescue period. Based on a proposed three-layer emergency logistics co-distribution conceptual framework, the proposed methodology involves two recursive mechanisms: (1) disaster-affected area grouping, and (2) relief co-distribution. Numerical studies with a real large-scale earthquake disaster occurring in Taiwan are conducted, and the corresponding results indicate the applicability of the proposed method and its potential advantages. We hope that this study can not only make the proposed emergency logistics system available with more benefits to the development of emergency logistics systems for the urgent needs of disaster areas around the world but also stimulate more excellent researches concerning emergency logistics management.
650 citations
TL;DR: In this article, the authors developed a decision-making tool that can be used by government agencies in planning for flood emergency logistics. But the decision variables include the structure of rescue organizations, locations of rescue resource storehouses, allocations of rescue resources under capacity restrictions, and distributions of resources.
Abstract: This paper aims to develop a decision-making tool that can be used by government agencies in planning for flood emergency logistics. In this article, the flood emergency logistics problem with uncertainty is formulated as two stochastic programming models that allow for the determination of a rescue resource distribution system for urban flood disasters. The decision variables include the structure of rescue organizations, locations of rescue resource storehouses, allocations of rescue resources under capacity restrictions, and distributions of rescue resources. By applying the data processing and network analysis functions of the geographic information system, flooding potential maps can estimate the possible locations of rescue demand points and the required amount of rescue equipment. The proposed models are solved using a sample average approximation scheme. Finally, a real example of planning for flood emergency logistics is presented to highlight the significance of the proposed model as well as the efficacy of the proposed solution strategy.
516 citations
TL;DR: In this paper, the authors constructed a relief distribution model using the multi-objective programming method for designing relief delivery systems in a real case, and the model features three objectives: minimizing the total cost, minimizing the travel time, and maximizing the minimal satisfaction during the planning period.
Abstract: The fatal earthquake on September 21, 1999 caused significant damages to Taiwan, which made the national government focus on strengthening relief systems regarding natural disasters. Disaster prevention, protection, and reconstruction are the major areas of focus to reduce human suffering and damage from disasters. A key point is the ability to enhance the distribution of relief materials effectively. In this study, we construct a relief-distribution model using the multi-objective programming method for designing relief delivery systems in a real case. The model features three objectives: minimizing the total cost, minimizing the total travel time, and maximizing the minimal satisfaction during the planning period. The first two objectives pursue the efficiency goal, whereas the third pursue fairness – making best effort to ensure relief commodity delivery to all demand points. Results of an empirical study are presented and suggestions are given for future research.
514 citations
TL;DR: In this article, the authors investigate the effect of a transportation disruption on supply chain performance using system dynamics simulation, comparing a traditional supply chain and a vendor managed inventory system (VMI) when a transportation disruptions occurs between two echelons in a 5-echelon supply chain.
Abstract: This paper investigates the effect of a transportation disruption on supply chain performance using system dynamics simulation, comparing a traditional supply chain and a vendor managed inventory system (VMI) when a transportation disruption occurs between 2 echelons in a 5-echelon supply chain. The greatest impact occurs when transportation is disrupted between the tier 1 supplier and warehouse. In the traditional structure the retailer, warehouse, and tier 1 supplier experience the greatest inventory fluctuations and the highest goods in transit to their facilities. These impacts are less severe for the VMI structure, although unfilled orders are approximately the same for each.
448 citations
TL;DR: In this paper, the authors present the results of a large-scale survey on the potential benefits of and impediments for horizontal cooperation in Flanders, finding a reliable party to lead the cooperation and constructing a fair allocation mechanism for the benefits are the impediments that respondents agree with most.
Abstract: This paper presents the results of a large-scale survey on the potential benefits of and impediments for horizontal cooperation in Flanders. The main findings are that in general Logistics Service Providers strongly believe in the potential benefits of horizontal cooperation to increase their profitability or to improve the quality of their services. The impediments for cooperation that are perceived or expected by the non-cooperating Logistics Service Providers prove to be experienced by the cooperating Logistics Service Providers. Finding a reliable party to lead the cooperation and constructing a fair allocation mechanism for the benefits are the impediments that respondents agree with most.
405 citations
TL;DR: In this article, a meta-heuristic of ant colony optimization (ACO) for solving the logistics problem arising in disaster relief activities is presented. But, the problem is not solved in an iterative manner and the results indicate that this algorithm performs well in terms of solution quality and run time.
Abstract: This paper presents a meta-heuristic of ant colony optimization (ACO) for solving the logistics problem arising in disaster relief activities The logistics planning involves dispatching commodities to distribution centers in the affected areas and evacuating the wounded people to medical centers The proposed method decomposes the original emergency logistics problem into two phases of decision making, ie, the vehicle route construction, and the multi-commodity dispatch The sub-problems are solved in an iterative manner The first phase builds stochastic vehicle paths under the guidance of pheromone trails while a network flow based solver is developed in the second phase for the assignment between different types of vehicle flows and commodities The performance of the algorithm is tested on a number of randomly generated networks and the results indicate that this algorithm performs well in terms of solution quality and run time
365 citations
TL;DR: In this paper, the design of container liner shipping service networks by explicitly taking into account empty container repositioning is addressed, and a genetic algorithm-based heuristic is developed for the problem.
Abstract: This paper addresses the design of container liner shipping service networks by explicitly taking into account empty container repositioning. Two key and interrelated issues, those of deploying ships and containers are usually treated separately by most existing studies on shipping network design. In this paper, both issues are considered simultaneously. The problem is formulated as a two-stage problem. A genetic algorithm-based heuristic is developed for the problem. Through a number of numerical experiments that were conducted it was shown that the problem with the consideration of empty container repositioning provides a more insightful solution than the one without.
336 citations
TL;DR: In this article, revealed preference (RP) data was used for airport and airline choice collected in the US in 2001, and significant effects relating to factors such as airfare, access time, flight time and airline and airport allegiance were investigated.
Abstract: The majority of studies of air travel choice behavior make use of revealed preference (RP) data, generally in the form of survey data collected from departing passengers. While the use of RP data has certain methodological advantages over the use of stated preference (SP) data, major issues arise because of the often low quality of the data relating to the un-chosen alternatives, in terms of explanatory variables as well as availability. As such, studies using RP survey data often fail to recover a meaningful fare coefficient, and are generally not able to offer a treatment of the effects of airline allegiance. In this paper, we make use of SP data for airport and airline choice collected in the US in 2001. The analysis retrieves significant effects relating to factors such as airfare, access time, flight time and airline and airport allegiance, illustrating the advantages of SP data in this context. Additionally, the analysis explores the use of non-linear transforms of the explanatory variables, as well as the treatment of continuous variations in choice behavior across respondents.
216 citations
TL;DR: In this article, the authors categorize the challenges of emergency management logistics into four distinct areas: 1) defining emergency logistics, which allow for an efficient flow of goods and services from manufacturer/retailer to consumer; 2) an inability to control the timeliness of relief supply distribution; 3) challenges involved in providing resource management for emergency logistics.
Abstract: The author notes that emergency logistics management has emerged as a "worldwide-noticeable theme" for both artificial and natural (e.g., earthquakes, tsunamis, hurricanes) disasters. He categorizes the challenges of emergency management logistics into four distinct areas. These include: 1) defining emergency logistics. Unlike business logistics, which allow for an efficient flow of goods and services from manufacturer/retailer to consumer, emergency management logistics involves the efficient flow of relief services and information from the point of origin (i.e., relief distribution centers) to a destination point near affected areas where people are living under emergency conditions; 2) an inability to control the timeliness of relief supply distribution. The author particularly notes the critical 3-day period following a disaster, when in bound logistics (i.e., supplies to relief distribution centers) and outbound logistics (i.e., supplies from distribution centers to affected areas) face the greatest challenges from operational problems; 3) challenges involved in providing resource management for emergency logistics. This notes the operational uncertainties (i.e., controlling the inventory of supplies ready for transport) and communications challenges found in emergency situations; 4) the demand for nearly inaccessible, yet crucial, real-time relief data. The author notes that information provided by on-the-spot rescuers and reporters may not be as accurate as needed, due to the limited availability of information immediately following a disaster and the decentralized distribution of that information. The author adds that the uniqueness of emergency logistics point to an urgent need for more research, especially as the operational scope of emergency logistics expands beyond the domestic to include international concerns.
194 citations
TL;DR: In this paper, a model formulation and solution for simultaneous mobilization destination, traffic assignment, and departure schedule for multi-priority groups (SMDTS-MPG) for real-time emergency response in no-notice disasters is presented.
Abstract: This paper presents a model formulation and solution for simultaneous mobilization destination, traffic assignment, and departure schedule for multi-priority groups (SMDTS-MPG) for real-time emergency response in no-notice disasters. The presented approach addresses the decision context in which multiple emergency responses and evacuation flow groups with different destinations and varying priorities coexist in the same traffic network, within which simultaneous mobilization strategies must consider this requirement. The proposed modeling technique and the cell transmission model (CTM)-based linear-programming model provides a mechanism to accomplish this goal in an intuitive and coherent manner. The proposed matrix formulation of the SMDTS-MPG model permits the rapid deployment of the model to large networks. Model formulation and numerical examples are presented in detail in this paper.
165 citations
TL;DR: In this paper, the authors enhance the mixed logit model to capture additional alternative-specific unobserved variation not subject to the constant variance condition, which is independent of sources revealed through random parameters.
Abstract: Developments in simulation methods, and the computational power that is now available, have enabled open-form discrete choice models such as mixed logit to be estimated with relative ease. The random parameter (RP) form has been used to identify preference heterogeneity, which can be mapped to specific individuals through re-parameterisation of the mean and/or variance of each RP’s distribution. However this formulation depends on the selection of random parameters to reveal such heterogeneity, with any residual heterogeneity forced into the constant variance condition of the extreme value type 1 distribution of the classical multinomial logit model. In this paper we enhance the mixed logit model to capture additional alternative-specific unobserved variation not subject to the constant variance condition, which is independent of sources revealed through random parameters. An empirical example is presented to illustrate the additional information obtained from this model.
TL;DR: This proposed approach enables the decision maker not only to optimally allocate resources across the transit network but to achieve targets for societal variables that represent the environment in which the bus services are provided.
Abstract: In this paper, the provision of bus services along different routes that comprise a public transit network is assessed taking into consideration, the service providers, the users and the societal perspectives. This model is based on Data Envelopment Analysis (DEA) [Charnes, A., Cooper W.W., Rhodes, E., 1978. Measuring the efficiency of decision-making units. European Journal of Operational Research 2, 429–444] and derives from the Network Model in DEA [Fare, R., Grosskopf, S., 2000. Network DEA. Socio-Economic Planning Sciences 34, 35–49] and Goal Programming in DEA [Athanassopoulos, A., 1995. Goal programming and data envelopment analysis (GoDEA) for target-based multi-level planning: allocating central grants to the Greek local authorities. European Journal of Operational Research 87, 535–550]. This proposed approach enables the decision maker not only to optimally allocate resources across the transit network but to achieve targets for societal variables that represent the environment in which the bus services are provided.
TL;DR: In this article, an n-stage multi-customer supply chain inventory model is proposed, where there is a company that can supply products to several customers, and the model is formulated as the same cycle time for all companies in the supply chain.
Abstract: In this paper, we propose an n -stage-multi-customer supply chain inventory model where there is a company that can supply products to several customers. We formulate the model for the simplest inventory coordination mechanism which is referred to as the same cycle time for all companies in the supply chain. We conclude that it is possible to use an algebraic approach to optimize the supply chain model without the use of differential calculus.
TL;DR: A stated preference experiment on a sample of Italian manufacturing firms shows that the marginal utilities of attributes are not constant over the whole range of values presented, and that significant improvements can be made by introducing non-linearities at points specified by individual respondents.
Abstract: The paper applies the choice model incorporating attribute cut-offs proposed by [Swait, J.D., 2001. A non-compensatory choice model incorporating attribute cutoffs. Transportation Research: Part B 35 (10), 903–928] to evaluate shippers’ preferences for freight service attributes. A stated preference experiment on a sample of Italian manufacturing firms shows that the marginal utilities of attributes are not constant over the whole range of values presented, and that significant improvements can be made by introducing non-linearities at points specified by individual respondents. It is found that the conventional model overestimates the importance of the attributes, though their relative ranking is maintained. The proposed choice model provides useful information on the difficulty of achieving a modal transfer of freight from road to truck-rail intermodal transport.
TL;DR: This work considers the carrier's optimal bid generation problem in combinatorial auctions for transportation procurement and employs both column generation and Lagrangian based techniques for solving the carrier optimization model and presents numerical results.
Abstract: We consider the carrier’s optimal bid generation problem in combinatorial auctions for transportation procurement. Bidders (carriers) employ vehicle routing models to identify sets of lanes (origin-destination pairs) based on the actual routes that a fleet of trucks will follow in order to maximize profit. Routes are constructed by optimally trading off repositioning costs of vehicles and the rewards associated with servicing lanes. The carrier optimization represents simultaneous generation and selection of routes and can incorporate any existing commitment. We employ both column generation and Lagrangian based techniques for solving the carrier optimization model and present numerical results.
TL;DR: The results of the three numerical examples show that the synchronized cycles policy works better than independent optimization as well as restricting buyers to adopt a common order cycle.
Abstract: This paper proposes a coordinated single-vendor multi-buyer supply chain model by synchronizing delivery and production cycles. The synchronization is achieved by scheduling the actual delivery days of the buyers and coordinating them with the vendor’s production cycle whilst allowing the buyers to choose their own lot sizes and order cycles. A mathematical model for our proposed coordination is developed and analyzed. The results of our three numerical examples show that the synchronized cycles policy works better than independent optimization as well as restricting buyers to adopt a common order cycle.
TL;DR: Results of numerical experiments indicate that the proposed tabu search heuristic is able to obtain near optimal solutions for large-size problem instances in reasonable computation time.
Abstract: A real-world planned maintenance scheduling problem that exists at several business units within United Technologies Corporation (UTC) is addressed in this paper. The scheduling problem is formulated as a multiple tour maximum collection problem with time-dependent rewards and an adaptive memory tabu search heuristic is developed to solve it. The effectiveness of the proposed solution approach is examined using real-world problem instances supplied by UTC. Relevant upper bounds are derived for the application. Results of numerical experiments indicate that the proposed tabu search heuristic is able to obtain near optimal solutions for large-size (i.e., actual) problem instances in reasonable computation time.
TL;DR: In this paper, the determinants of adoption of vendor managed inventory programs (VMI) have been empirically examined in three industries, and it was shown that the competitiveness of the supplier's market and buyer-supplier cooperation are positively associated with VMI adoption.
Abstract: This study empirically examines the determinants of adoption of Vendor Managed Inventory programs that have recently gained popularity in many industries. To achieve this goal, survey scales are adapted and developed for buyer and supplier market competitiveness, product demand, buyer operational uncertainty, and buyer–supplier cooperation. Based on the analysis of responses from purchasing managers in three industries, structural equation modeling results suggest that the competitiveness of the supplier’s market and buyer–supplier cooperation are positively associated with VMI adoption, while operational uncertainty for the buyer is negatively associated with VMI adoption. Managerial implications and limitations of the study are also noted.
TL;DR: In this paper, the authors examined international air passenger and cargo flows within and among Asia, Europe, and America, and the degree of air traffic density for major cities worldwide, using a basic gravity model composed of GDP, population, distance, and several dummy variables.
Abstract: This paper examines international air passenger and cargo flows within and among Asia, Europe, and America, and the degree of air traffic density for major cities worldwide, using a basic gravity model composed of GDP, population, distance, and several dummy variables. The results reveal that many cities are strengthening their position as international air transportation hubs, especially: Tokyo, Hong Kong, Singapore, London, Paris, Frankfurt, Amsterdam, New York, and Miami. Finally, the results show that the air traffic density of three cities, Seoul, Hong Kong, and Amsterdam, is growing at an extraordinary rate.
TL;DR: In this paper, the authors apply three game-theoretic models to analyze airlines' choices in duopoly markets: one short-haul market and one longhaul market, and also do sensitivity analysis to explore how the equilibrium results may change when air travel demand is higher, as it may happen in the future.
Abstract: We are interested in how airlines make decisions on aircraft size and service frequency in a competitive environment. We apply three game-theoretic models to analyze airlines’ choices in duopoly markets: one short-haul market and one long-haul market. We study how airlines’ choices in a competitive environment may vary with flight distance, and also do sensitivity analysis to explore how the equilibrium results may change when air travel demand is higher, as it may happen in the future. Our research considers the competition factor in airlines’ decisions on both aircraft size and service frequency, and the impact of these decisions on both the cost and demand sides of airlines’ business. Different from previous studies, our research is based on cost, market share and demand models derived from empirical studies.
TL;DR: The hypercube is a spatially distributed queuing model based on Markovian analysis approximations, used to analyze the configuration and operation of server-to-customer emergency systems, adapted to analyze emergency medical systems on highways, which operate within particular dispatching policies.
Abstract: The hypercube is a spatially distributed queuing model based on Markovian analysis approximations, used to analyze the configuration and operation of server-to-customer emergency systems. In the present study we adapted the model to analyze emergency medical systems (EMS) on highways, which operate within particular dispatching policies. The study takes into consideration that: the emergency calls are of different types; the servers are distinct (e.g., rescue ambulances, medical vehicles); only certain servers in the system can service calls in a given region (partial backup); and, depending on the type of call, one or more identical or distinct servers are immediately dispatched to service such calls (multiple dispatch). We also consider that the arriving calls take place either along the highway or at the home location of a server – in which case the server does not need to travel to the call location. Finally, we analyzed the computational results of applying such an approach to the case study of an EMS operating on Brazilian highways.
TL;DR: In this article, a Simulated Annealing algorithm has been developed to solve the problem of congestion in the M/M/1 queuing system in a hub-and-spoke network.
Abstract: Hub-and-spoke networks are employed in cargo transportation. This paper presents a model for hub location in these kinds of networks. Hubs are considered capacity limited. For that reason, costs due to congestion in hubs are introduced into the model. A Simulated Annealing algorithm has been developed to solve the model. The algorithm includes a balanced module, the purpose of which is to reduce congestion. Each hub is modelled as an M/M/1 queuing system. The algorithm has been tested in randomly created networks. The results show that when a situation of congestion occurs, the proposed algorithm enables better solutions to be found, thereby improving the standard of service.
TL;DR: In this article, the authors investigated the parameter settings of a real-time vehicle-dispatching system for consolidating milk runs, and the results of the experiments suggest that the system should have an initial vehicle dispatching module and an inter-route improvement module.
Abstract: This study investigates the parameter settings of a real-time vehicle-dispatching system for consolidating milk runs. Seven modules are used to implement the real-time system and the parameters are determined by a comprehensive experimental design. The real-time vehicle-dispatching system will be demonstrated in different milk run scenarios. The results of the experiments suggest that the system should have an initial vehicle dispatching module and an inter-route improvement module. It is also recommended that the Best Fit algorithm for initial vehicle dispatch and the 2-Exchange algorithm for inter-route improvement are most suitable for the real-time system.
TL;DR: In this paper, the authors investigate the implications of bounding the attribute processing task by attribute elimination through not attending to one or more attributes, and compare the value of travel time savings distribution under alternative attribute processing regimes.
Abstract: In this paper the authors investigate the implications of bounding the attribute processing task by attribute elimination through not attending to one or more attributes. Using a sample of car non-commuters in Sydney the authors estimate a mixed logit model in which all attributes are assumed to be attended to, and models which assume that certain attribute(s) are not attended to, based on supplementary information provided by respondents. The supplementary information is accounted for in a deterministic and a stochastic way; the latter in recognition of the analyst’s lack of full information on why a specific attribute processing (AP) strategy was adopted by each sampled individual. The authors compare the value of travel time savings distribution under alternative attribute processing regimes. As expected, there are noticeable variations in the mean and standard deviation willingness to pay (WTP) across the three AP strategies. (a)
TL;DR: In this paper, the authors consider the problem of determining the vendor's credit period, the buyer's retail price, and order quantity while still maximizing profits in a channel with trade credit and quantity discounts for freight cost.
Abstract: In this paper we consider vendor-buyer channels subject to trade credit and quantity discounts for freight cost. We deal with the problems of determining the vendor’s credit period, the buyer’s retail price and order quantity while still maximizing profits. We focus on how channel coordination can be achieved using trade credit and how trade credit can be affected by quantity discounts for freight cost. We show that profits for both parties increase under channel coordination when the credit period is kept within an appropriate range. This range becomes wider as the discount for freight rates increases.
TL;DR: In this article, the authors present the use of game theoretic models to analyse the cost allocation problem in the context of repairable spare parts pooling and show that cost allocation policy influences the companies in making their inventory decisions.
Abstract: In this paper we present the use of game theoretic models to analyse the cost allocation problem in the context of repairable spare parts pooling. Two situations are studied. In the first situation, all pooling members fully cooperate without having self-interest. We use the core concept from cooperative game theory as the basis for designing a fair cost allocation. In the second situation, competition exists as each member has an interest in maximizing his own benefit. Using the concept of Nash equilibrium, we show that the cost allocation policy influences the companies in making their inventory decisions.
TL;DR: In this paper, the authors set up a theoretical framework for the valuation of the Asian-style options traded in the freight derivatives market, and suggested approximate dynamics in the settlement period for the FFA that leads to closed-form option pricing formulas for Asian call and put options written on the spot freight rate indices.
Abstract: In this paper we set up the theoretical framework for the valuation of the Asian-style options traded in the freight derivatives market. Assuming lognormal spot freight dynamics, we show that Forward Freight Agreements (FFA) are also lognormal prior to the settlement period, but that this lognormality subsequently breaks down. We suggest approximate dynamics in the settlement period for the FFA that leads to closed-form option pricing formulas for Asian call and put options written on the spot freight rate indices in the Black [Black, F., 1976. The pricing of commodity contracts. Journal of Financial Economics 3, 167–179] framework. In a Monte Carlo experiment we show that our formula gives very accurate prices, in particular for forward-starting freight options.
TL;DR: In this article, the authors studied the economic development of the Swedish railway and explore if and how the deregulation has affected cost efficiency, and conclude that vertical separation raises costs, and also that the introduction of competition lowers costs.
Abstract: Vertical separation between infrastructure service provision and the operation of trains is one important element in the Swedish deregulation process. Another is the introduction of various forms of competition. In this paper, we study the economic development of the Swedish railway and explore if and how the deregulation has affected cost efficiency. We use a longitudinal econometric approach in our study and conclude that vertical separation raises costs, and also that the introduction of competition lowers costs. The combined effect seems to be an improvement in cost efficiency as an impact of the deregulation process. The study also gives some results on lagged relationships between output and costs in railways with mixed passenger and freight services and provides a methodological approach for causal research on the relationship between railway deregulation and costs.
TL;DR: In this article, the authors developed and estimated a nested logit model of airport-airline choice that incorporates the "two-step" decision process of air travelers, where a traveler first eliminates certain choice alternatives that do not satisfy his/her minimum acceptable standards (first step), and then chooses the utility-maximizing alternative from the set of screened choice alternatives (second step).
Abstract: This paper develops and estimates a nested logit model of airport–airline choice that incorporates the “two-step” decision process of air travelers. The model assumes that a traveler first eliminates certain choice alternatives that do not satisfy his/her minimum acceptable standards (first step), and then chooses the utility-maximizing alternative from the set of screened choice alternatives (second step). The model is calibrated by using the survey data collected in the USA (central Iowa). The results imply that the “two-step” choice model may fit the observed data significantly better than the conventional “one-step” choice models.
TL;DR: In this article, a static electric power supply chain network equilibrium model with known demands was developed and the equivalence between the model and a transportation network equilibria with fixed demands over an appropriately constructed supernetwork was established.
Abstract: In this paper, we develop a static electric power supply chain network equilibrium model with known demands and establish the equivalence between the model and a transportation network equilibrium model with fixed demands over an appropriately constructed supernetwork. This equivalence yields a new interpretation of electric power supply chain network equilibria in path flows. We then exploit this equivalence to propose a dynamic electric power supply chain network model in which the demand varies over time using an evolutionary variational inequality formulation. Finally, we demonstrate how numerical dynamic electric power supply chain network problems can be solved utilizing recently obtained theoretical results in the unification of evolutionary variational inequalities and projected dynamical systems.