Regolith X-Ray Imaging Spectrometer (REXIS) Aboard the OSIRIS-REx Asteroid Sample Return Mission

The Regolith X-ray Imaging Spectrometer (REXIS) is the student collaboration experiment proposed and built by an MIT-Harvard team, launched aboard NASA’s OSIRIS-REx asteroid sample return mission. REXIS complements the scientific investigations of other OSIRIS-REx instruments by determining the relative abundances of key elements present on the asteroid’s surface by measuring the X-ray fluorescence spectrum (stimulated by the natural solar X-ray flux) over the range of energies 0.5 to 7 keV. REXIS consists of two components: a main imaging spectrometer with a coded aperture mask and a separate solar X-ray monitor to account for the Sun’s variability. In addition to element abundance ratios (relative to Si) pinpointing the asteroid’s most likely meteorite association, REXIS also maps elemental abundance variability across the asteroid’s surface using the asteroid’s rotation as well as the spacecraft’s orbital motion. Image reconstruction at the highest resolution is facilitated by the coded aperture mask. Through this operation, REXIS will be the first application of X-ray coded aperture imaging to planetary surface mapping, making this student-built instrument a pathfinder toward future planetary exploration. To date, 60 students at the undergraduate and graduate levels have been involved with the REXIS project, with the hands-on experience translating to a dozen Master’s and Ph.D. theses and other student publications.     

Massive Stars: Input Physics and Stellar Models

We present a general overview of the structure and evolution of massive stars of masses ≥12 M _⊙ during their pre-supernova stages. We think it is worth reviewing this topic owing to the crucial role of massive stars in astrophysics, especially in the evolution of galaxies and the universe. We have performed several test computations with the aim to analyze and discuss many physical uncertainties still encountered in massive-star evolution. In particular, we explore the effects of mass loss, convection, rotation, ¹²C(α,γ)¹⁶O reaction and initial metallicity. We also compare and analyze the similarities and differences among various works and ours. Finally, we present useful comments on the nucleosynthesis from massive stars concerning the s-process and the yields for ²⁶Al and ⁶⁰Fe.     

Superclustering and the large-scale structure of the universe

The spatial distribution of a complete redshift sample of nearby rich clusters of galaxies is studied. Strong superclustering, extending to very large scales (100h⁻¹Mpc), is observed in the cluster distribution. In particular, we determined and discuss below the following studies:I. The spatial correlation function of rich clusters of galaxies.II. The construction of a complete catalog of superclusters and a study of their growth (percolation).III. The finding of large-scale superclusters surrounding the giant galaxy void in Bootes.IV. The finding of a 300 Mpc void of rich clusters of galaxies.All the above studies provide evidence for the existence of large-scale structure in the universe, and reveal some of its characteristics. These findings are of importance to models of the formation of galaxies and structure in the universe.     

Multipath interferometry technique for airborne array calibration

A new technique is developed to compensate multiple-wavelength distortion in airborne antenna arrays. This approach exploits the phase information in microwave reflections from arbitrary terrain. To handle reflections incident over a broad angle, a range-Doppler preprocessor is used in each element channel to resolve wavefronts incident simultaneously from different directions. The phase information for each direction of arrival is compared between elements and processed by optimal estimators to determine the phase corrections needed to compensate the distortion. To develop the estimators, a statistical model of the complex baseband terrain reflections is developed. This is in turn used to generate conditional probability densities involving the range Doppler observations and the parameters to be estimated. These densities are subsequently used to develop minimum variance and maximum likelihood estimators. The new estimators use additional information that has not been exploited by previous techniques and therefore provide enhanced performance     

A General Model for Switching Converters

A low frequency, small signal model, valid for all types of converters, both in the heavy and the light mode of conduction, is developed in block diagram form. The model provides a clear insight into the behavior of such converters and enhances the respective contributions of input and output filters.     

High Dynamic GPS Receiver Using Maximum Likelihood Estimationand Frequency Tracking

A new high dynamic global positioning system (GPS) receiver ispresented and its performance characterized by analysis,simulation, and demonstration. The demonstration receiver is abreadboard model capable of tracking a single simulated satellitesignal in pseudorange and range rate. Pseudorange and range rateestimates are made once every 20 ms, using a maximum likelihoodestimator, and are tracked by means of a third-order fadingmemory filter in a feedback configuration. The receiver trackspseudorange with rms errors of under 1 m when subjected tosimulated 50 g, 40 g/s circular trajectories. The tracking thresholdis approximately 28 dB·Hz, which provides 12 dB margin relativethe the minimum specified signal strength, assuming 3.5 dB systemnoise figure and 0 dB antenna gain.     

Ionospheric response of equatorial and low latitude F-region during the intense geomagnetic storm on 24–25 August 2005

In this investigation, we present and discuss the response of the ionospheric F-region in the South American and East Asian sectors during an intense geomagnetic storm in August 2005. The geomagnetic storm studied reached a minimum Dst of −216 nT at 12:00 UT on 24 August. In this work ionospheric sounding data obtained of 24, 25, and 26 August 2005 at Palmas (PAL; 10.2° S, 48.2° W; dip latitude 6.6° S), São José dos Campos (SJC, 23.2° S, 45.9° W; dip latitude 17.6° S), Brazil, Ho Chi Minh City, (HCM; 10.5° N, 106.3° E; dip latitude 2.9° N), Vietnam, Okinawa (OKI; 26.3° N, 127.8° E; dip latitude 21.2° N), Japan, are presented. Also, the GPS observations obtained at different stations in the equatorial and low-latitude regions in the Brazilian sector are presented. On the night of 24–25 August 2005, the h′F variations show traveling ionospheric disturbances associated with Joule heating in the auroral zone from SJC to PAL. The foF2 variations show a positive storm phase on the night of 24–25 August at PAL and SJC during the recovery phase. Also, the GPS-VTEC observations at several stations in the Brazilian sector show a fairly similar positive storm phase on 24 August. During the fast decrease of Dst (between 10:00 and 11:00 UT) on 24 August, there is a prompt penetration of electric field of magnetospheric origin that result in abrupt increase (∼12:00 UT) in foF2 at PAL, SJC (Brazil) and OKI (Japan) and in VTEC at IMPZ, BOMJ, PARA and SMAR (Brazil). OKI showed strong oscillations of the F-region on the night 24 August resulted to the propagation of traveling atmospheric disturbances (TADs) by Joule heating in the auroral region. These effects result a strong positive observed at OKI station. During the daytime on 25 August, in the recovery phase, the foF2 observations showed positive ionospheric storm at HCM station. Some differences in the latitudinal response of the F-region is also observed in the South American and East Asian sectors.     

The gaussian form of the variation-of-parameter equations formulated in equinoctial elements—Applications: Airdrag and radiation pressure

In this paper the Gaussian equations are derived for the nonsingular equinoctial elements, as they are required for dissipative perturbations of a satellite orbit. As examples, airdrag and solar radiation pressure are considered and the first-order variation of the elements and time after one revolution are given explicitly.     

Response of Hainan GPS ionospheric scintillations to the different strong magnetic storm conditions

Using the GPS ionospheric scintillation data at Hainan station (19.5°N, 109.1°E) in the eastern Asia equatorial regions and relevant ionospheric and geomagnetic data from July 2003 to June 2005, we investigate the response of L-band ionospheric scintillation activity over this region to different strong magnetic storm conditions (Dst < −100 nT) during the descending phase of the solar cycle. These strong storms and corresponding scintillations mainly took place in winter and summer seasons. When the main phase developed rapidly and reached the maximum near 20–21 LT (LT = UT + 8) after sunset, scintillations might occur in the following recovery phase. When the main phase maximum occurred shortly after midnight near 01–02 LT, following the strong scintillations in the pre-midnight main phase, scintillations might also occur in the post-midnight recovery phase. When the main phase maximum took place after 03 LT to the early morning hours no any scintillation could be observed in the latter of the night. Moreover, when the main phase maximum occurred during the daytime hours, scintillations could also hardly be observed in the following nighttime recovery phase, which might last until the end of recovery phase. Occasionally, scintillations also took place in the initial phase of the storm. During those scintillations associated with the nighttime magnetic storms, the height of F layer base (h’F) was evidently increased. However, the increase of F layer base height does not always cause the occurrence of scintillations, which indicates the complex interaction of various disturbance processes in ionosphere and thermosphere systems during the storms.     

UWB radar detection of targets in foliage using alpha-stableclutter models

We address the problem of detection of targets obscured by a forest canopy using an ultrawideband (UWB) radar. The forest clutter observed in the radar imagery is a highly impulsive random process that is more accurately modeled with the recently proposed class of alpha-stable processes as compared with Gaussian, Weibull, and K-distribution models. With this more accurate model, segmentation is performed on the imagery into forest and clear regions. Further, a region-adaptive symmetric alpha stable (SαS) constant false-alarm rate (CFAR) detector is introduced and its performance is compared with the Weibull and Gaussian CFAR detectors. The results on real data show that the SαS CFAR performs better than the Weibull and Gaussian CFAR detectors in detecting obscured targets