Reliability/Availability missions

Dependability A measure of the degree to which an item is operable and capable of performing its required function at any (random) time during a specified mission profile, given the availability at the start of the mission. In other words, it is a measure of RAM of a system, where RAM stands for reliability, availability, and maintainability. Each of these three terms has been defined separately in this clause. 

RAM Here RAM stands for reliability, availability and maintainability. These three factors collectively referred to as RAM are major attributes towards system design with great impact on sustainment, life cycle costs as well as system s ability to perform the intended mission and affect overall mission success. 

A large number of explosives are already available for use. However, selection of an explosive is made in the light of its stability, reliability, safety, application and mission requirements. At the time of development of a new explosive, it is essential to keep in mind factors such as indigenous availability of starting materials, ease of method of preparation, purity of explosive and its cost, in addition to its impact on the environment and potential for demilitarization. In this Chapter the following aspects of explosives will be discussed ... 

Liquid propulsion is characterized by its high state of development, relatively complicated systems design, capability for repeated operation, long firing times, and of course the propellants employed. Their use has been based on a number of selection criteria, such as the operational mission, performance required, reliability, minimum weight, logistics, economics, availability, maintainability, mobility, and others. 

Given the failure rate of a component and an operational time interval (mission time), one can calculate the reliability of that component. If the component is repairable and the restore rate is estimated, tiie steady state availability of the component can be calculated. If the failure rates, proof test interval, proof test coverage, and component lifetime are known, one can calculate the average probability of fatiure. 

The first steps of control system design consist essentially in finding the hardware architecture. The designer has to determine the places where sensors or actuators can be implemented, then for each of them, to determine the number and the type of instruments among those available. The choice is made in order to obtain hardware organisations that can properly perform the mission according to the objectives in terms of financial cost, reliability and safety criteria. 

Pthk is seen as a logical AND of aU sensors included in this path. The dependabdity metric used to evaluate WSN reliability is the availability of the safety function for this point over a given mission time and is then defined by... 

The design for a constellation of satellites to serve communications needs, includes choices for the number of satellites, their orbital parameters, the satellite G/T and EIRP, etc. These mission analysis and design topics involve trades of many factors such as total communications capacity, link margins, space segment and Earth segment costs, reliability, interconnectivity, availability and cost of launch vehicles, mission lifetime, and system operations (Wertz and Larson, 1991). 

Availability A measure of the degree to which an item is in the operable and committable state at the start of the mission, when the mission is called for at an unknown state. Availability can be conceived as the probability that the system is operating properly when it is requested for use. In terms of failure, availability is the probability that a system has not failed or is undergoing a repair action when it needs to be used. From this, one can infer that if a system has a high availability then it should also have a high reliability. So these two terms go hand in hand. The relationship between availability, maintainability, and reliability has been shown in Fig. VII/1.3.1-1. 

Nickel-cadmium (Ni-Cd) batteries were widely deployed by earlier communication satellites and spacecrafts from I960 to 1995. When advanced designs of nickel-hydrogen (Ni-Hj) batteries were available in early 2000, most of the satellites and spacecrafts preferred to use Ni-H2 batteries. These batteries will be described in detail with an emphasis on reliability and improved electrical performance. For short mission satellites and LEO communication satellites, both the Ni-Cd and Ni-Hj batteries are still being used. 

On the basis of the tabulated data it can be stated that the increase in cost factor is minimum with the DSR redundant system. These conclusions are valid for this spacecraft power system, and these conclusions indicate a trend in the increase in weight and cost factor as a function of mission duration using various redundant systems. In summary, performance data available from the power systems deployed by various spacecraft and satellites do indicate that redundant systems tend to yield higher reliability as a function of mission duration or length. Furthermore, the power system reliability does decrease with the increase in mission length regardless of redundant system deployed. 

Kuo et al. also consider reliability prognosis. The authors do this for an aircraft engine as an example (Kuo et al. 2001). They developed three criteria for determination the replacement age of a critical item for a maintained system 1) minimum replacement cost-rate, 2) maximum availability, and 3) lower bound on mission reliability. Four methods are used to obtain solutions for multiple criteria decision making (Kuo et al. 2001). However these methods do not take into account SIL requirements, which are mandatory and defined by international standards as mentioned above. The time of replacement of an element is defined by decision-making technic. Suggested in this paper reliability prognosis method allows to... 

Systems definition It is supposed that a redundant system contains n units. The instantaneous availability of which are independent but follow the same exponential distribution A(t) = e where t = the failure rate of each unit. Note that the probability that a unit or system without maintenance is able to perform its mission at time t equals to the probability that it is able to perform its mission for a stated period of time [0, t]—that is the instantaneous availability equals to the reliability). The system is workable when at least k units are in operation without failures. Maintenance interval is constant Tq. And fault detection rate is supposed to be FDR fault isolation rate equals FIR repair rate equals RR. Maintenance time is negligible. 

In practical applications, different ways can be used to describe the reliability of an element, such as mean time to failure (MTTF), availability, failure probability over a given time, or failme probability during a specified mission. The relevant specification depends very much on the application. However, all measures are based on probabilities or probability densities over time. These functions over time can then be interpreted using any of the above measures. Therefore, this paper will use fault probabilities as a generic way to measure reliability ... 

The main constraint on the availability of a satellite comes from the method of handling faults, while respecting the reliability objectives. A full automatic processing allows continuation of the service or restart after a very brief interruption, but this is more risky than a process limited to shifting to a safe mode, thereby stopping the mission, until diagnosis and recovery by operators on the ground is effected. 

The voyage corresponds to a discontinnous operation of the system limited to brief periods, some of which are critical in the sense that a failure of the trajectory control leads almost inevitably to the definitive loss of the missioa Consequently, there is a strong need for irrstantaneous availability at critical moments (known in advance). In general, this irtstantaneous availability need does not form an exphdt requirement, but is covered and induced by the need of mission success (reliability). 

Similar to the voyage of an exploration probe, an orbital transport vehicle is not characterized by a service for a certain period, but by an objective, which reqrrires the operation of the command and control system at certain moments These moments are less critical than for an exploration probe in the sense that, in general, if a failure occurs, other additional attempts can be undertaken. There is no strong need for availabihty for orbital transport vehicles, at least in terms of the mission (depending on the solutions adopted, the dependability goals of the mission, mainly reliability and safety, can naturally lead to the availability reqrrirements on some sub-systems). 

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