Formal testing for separation assurance

In order to address the rapidly increasing load of air traffic operations, innovative algorithms and software systems must be developed for the next generation air traffic control. Extensive verification of such novel algorithms is key for their adoption by industry. Separation assurance algorithms aim at predicting if two aircraft will get closer to each other than a minimum safe distance; if loss of separation is predicted, they also propose a change of course for the aircraft to resolve this potential conflict. In this paper, we report on our work towards developing an advanced testing framework for separation assurance. Our framework supports automated test case generation and testing, and defines test oracles that capture algorithm requirements. We discuss three different approaches to test-case generation, their application to a separation assurance prototype, and their respective strengths and weaknesses. We also present an approach for statistical analysis of the large numbers of test results obtained from our framework.     

Congenital infection by human parvovirus B19 ascites-anaemia

A case of intrauterine infection by human parvovirus B19 (HPV B19) manifested as ascites during pregnancy is presented. Ascites was diagnosed by ultrasound at 27 weeks' gestation. A caesarean section was performed at 37 weeks'. owing to affected mobility of the fetus. A pale, female infant with low haemoglobin and bradycardia was delivered. Polymerace Chain Reaction (PCR) lab tests revealed that the mother and the fetus were infected by HPV B19. The neonate was born with low haemoglobin (Hb = 10 g/dl) and with ascites; it was discharged in good general condition 50 days after delivery.     

Sorption behavior of cesium on various soils under different pH levels

In the present study we investigated the sorption behavior of Cs in four different soils (sandyloam, loam, clayloam and clay) by using batch experiment. Cs sorption characteristics of the studied soils were examined at 4 mgL(-1) Cs concentration, at various pH levels, at room temperature and with 0.01 M CaCl(2) as a background electrolyte. Among different soils the decrease of k(d) (distribution coefficient) of cesium, at all pH levels, followed the sequence sandyloam > loam > clayloam > clay, indicating that the particle size fractions and especially the clay content plays predominant role on sorption of Cs. The effect of pH on cesium sorption displays a similar pattern for all soils, depending on soil type. At acid pH levels less cesium was sorbed, due to a greater competition with other cations for available sorption sites. The maximum sorption of Cs was observed at pH 8, where the negative charge density on the surface of the absorbents was the highest. For all soils was observed significantly lower Cs sorption at pH 10.     

Preface: Special Issue on Nasa Formal Methods Symposium 2009

This journal issue contains seven articles representing extended versions of the best papers selected from the proceedings of the First NASA Formal Methods Symposium (NFM 2009). The NASA Formal Methods Symposium is a new annual event that focuses on formal techniques, their theory, current capabilities, and limitations, as well as their application to aerospace, robotics, and other safety-critical systems.     

Optimal balking strategies in single-server queues with general service and vacation times

In many service systems arising in OR/MS applications, the servers may be temporarily unavailable, a fact that affects the sojourn time of a customer and his willingness to join. Several studies that explore the balking behavior of customers in Markovian models with vacations have recently appeared in the literature. In the present paper, we study the balking behavior of customers in the single-server queue with generally distributed service and vacation times. Arriving customers decide whether to enter the system or balk, based on a linear reward-cost structure that incorporates their desire for service, as well as their unwillingness to wait. We identify equilibrium strategies and socially optimal strategies under two distinct information assumptions. Specifically, in a first case, the customers make individual decisions without knowing the system state. In a second case, they are informed about the server’s current status. We examine the influence of the information level on the customers’ strategic response and we compare the resulting equilibrium and socially optimal strategies.     

Learning to divide and conquer: applying the L* algorithm to automate assume-guarantee reasoning

Assume-guarantee reasoning enables a “divide-and-conquer” approach to the verification of large systems that checks system components separately while using assumptions about each component’s environment. Developing appropriate assumptions used to be a difficult and manual process. Over the past five years, we have developed a framework for performing assume-guarantee verification of systems in an incremental and fully automated fashion. The framework uses an off-the-shelf learning algorithm to compute the assumptions. The assumptions are initially approximate and become more precise by means of counterexamples obtained by model checking components separately. The framework supports different assume-guarantee rules, both symmetric and asymmetric. Moreover, we have recently introduced alphabet refinement, which extends the assumption learning process to also infer assumption alphabets. This refinement technique starts with assumption alphabets that are a subset of the minimal interface between a component and its environment, and adds actions to it as necessary until a given property is shown to hold or to be violated in the system. We have applied the learning framework to a number of case studies that show that compositional verification by learning assumptions can be significantly more scalable than non-compositional verification. J.M. Cobleigh currently employed by The MathWorks, Inc., 3 Apple Hill Drive, Natick, MA 01760, USA.