An experiment in software reliability

The results of a software reliability experiment conducted in a controlled laboratory setting are reported. The experiment was undertaken to gather data on software failures and is one in a series of experiments being pursued by the Fault Tolerant Systems Branch of NASA Langley Research Center to find a means of credibly performing reliability evaluations of flight control software. The experiment tests a small sample of implementations of radar tracking software having ultra-reliability requirements and uses n-version programming for error detection, and repetitive run modeling for failure and fault rate estimation. The experiment results agree with those of Nagel and Skrivan in that the program error rates suggest an approximate log-linear pattern and the individual faults occurred with significantly different error rates. Additional analysis of the experimental data raises new questions concerning the phenomenon of interacting faults. This phenomenon may provide one explanation for software reliability decay

An empirical study of flight control software reliability

The results of a laboratory experiment in flight control software reliability are reported. The experiment tests a small sample of implementations of a pitch axis control law for a PA28 aircraft with over 14 million pitch commands with varying levels of additive input and feedback noise. The testing which uses the method of n-version programming for error detection surfaced four software faults in one implementation of the control law. The small number of detected faults precluded the conduct of the error burst analyses. The pitch axis problem provides data for use in constructing a model in the prediction of the reliability of software in systems with feedback. The study is undertaken to find means to perform reliability evaluations of flight control software

Effect of pixel dimensions on SAR picture quality

In an SAR mapping system, the product of looks per pixel and number of pixels in the scene is kept constant. Assuming that the returns from all resolution cells obey Rayleigh statistics, the expression for pixel SNR incorporating both speckle and additive white Gaussian noise was derived. It is shown that it is possible to use fine resolution and leave the large-area estimate slightly but not much worse than if a larger pixel size had been initially decided upon

The Improvement of Efficiency in the Numerical Computation of Orbit Trajectories

An analysis, system design, programming, and evaluation of results are described for numerical computation of orbit trajectories. Evaluation of generalized methods, interaction of different formulations for satellite motion, transformation of equations of motion and integrator loads, and development of efficient integrators are also considered

Deep, wide-field, multi-band imaging of z approximately equal to 0.4 clusters and their environs

The existence of an excess population of blue galaxies in the cores of distant, rich clusters of galaxies, commonly referred to as the 'Butcher-Oemler' effect is now well established. Spectroscopy of clusters at z = 0.2-0.4 has confirmed that the luminous blue populations comprise as much as 20 percent of these clusters. This fraction is much higher that the 2 percent blue fraction found for nearby rich clusters, such as Coma, indicating that rapid galaxy evolution has occurred on a relatively short time scale. Spectroscopy has also shown that the 'blue' galaxies can basically be divided into three classes: 'starburst' galaxies with large (O II) equivalent widths, 'post-starburst' E+A galaxies (i.e. galaxies with strong Balmer lines shortward of 4000A but elliptical-like colors, and normal spiral/irregulars. Unfortunately, it is difficult to obtain enough spectra of individual galaxies in these intermediate redshift clusters to say anything statistically meaningful. Thus, limited information is available about the relative numbers of these three classes of 'blue' galaxies and the associated E/SO population in these intermediate redshift clusters. More statistically meaningful results can be derived from deep imaging of these clusters. However, the best published data to date (e.g. MacLaren et al. 1988; Dressler & Gunn 1992) are limited to the cluster cores and do not sample the galaxy luminosity functions very deeply at the bluest wavelengths. Furthermore, only limited spectro-energy distribution data is available below 4000A in the observed cluster rest frame providing limited sensitivity to 'recent' star formation activity. To improve this situation, we are currently obtaining deep, wide-field UBRI images of all known rich clusters at z approx. equals 0.4. Our main objective is to obtain the necessary color information to distinguish between the E+SO, 'E+A', and spiral/irregular galaxy populations throughout the cluster/supercluster complex. At this redshift, UBRI correspond to rest-frame 2500A/UVR bandpasses. The rest-frame UVR system provides a powerful 'blue' galaxy discriminate given the expected color distribution. Moreover, since 'hot' stars peak near 2500A, that bandpass is a powerful probe of recent star formation activity in all classes of galaxies. In particular, it is sensitive to ellipticals with 'UV excess' populations (MacLaren et al. 1988)

Examining the importance of Aberdeenshire (UK) coastal waters for North Sea bottlenose dolphins (Tursiops truncates)

Using land- and vessel-based surveys, data on the relative abundance, distribution and habitat use of bottlenose dolphins (Tursiops truncatus) in Aberdeenshire waters were collected between 1999 and 2001. Bottlenose dolphins were present throughout the year, with peak abundance during the months of March to May. The occurrence of calves was seasonal, With the proportion of calves highest during the spring months. Foraging behaviour was recorded mainly in the vicinity of Aberdeen harbour. Dolphins photographed in Aberdeenshire waters were successfully matched and confirmed as Moray Firth animals. The results of the present study suggest that Moray Firth bottlenose dolphins utilize Aberdeenshire waters more frequently than previously reported. Aberdeen harbour is apparently an important feeding area, and Aberdeenshire waters are regularly used by another-calf pairs. This has important management implications since this area of coastline does not currently form part of the designated Special Area of Conservation (SAC) for this population

Proceedings or transactions?

J.R. Pierce, Editor of the Proceedings suggested that certain views regarding publications in the Transactions or Proceedings presented to the membership. It is felt best to present these ideas for comment as prepared by him in their original form

Optical Channels: Practical Limits with Photon Counting

n optical communication, ideal amplification of the received signal leads to a limiting signaling rate of 1 nat per photon. This is much inferior to the optimum limit ofkTjoules/nat, which we can theoretically approach by counting photons. Practically, the rates we can attain by photon counting will be limited by how elaborate codes we can instrument rather than by thermal photons

Comparison of Three-Phase Modulation with Two-Phase and Four-Phase Modulation

For a constant added noise density at high signal-to-noise ratios and for the same error rate, three-phase PSK requires about 0.75 dB less energy per bit than two-phase or four-phase PSK. However, at very low signal-to-noise ratios, three-phase PSK requires about 0.74 dB more energy per bit of channel capacity than two-phase or four-phase PSK