Analysis of the linear amplifier/analog-digital converter interfacein a digital microwave receiver

An analysis of the relationship between a linear amplifier chain and an analog-to-digital converter (ADC) in a digital microwave receiver, with respect to sensitivity and dynamic range issues, is presented. The effects of gain, third-order intermodulation products and ADC characteristics on the performance of the receiver are illustrated and design criteria for the linear amplifier chain (given a specified ADC) are developed. A computer program is included which calculates theoretical receiver performance based on gain and third-order intermodulation product selections. Experimental results are also presented and compared with theoretical values     

Biological degradation and composition of sweet potato biomass - errata

Many challenges are presented by biological degradation in a bioregenerative Controlled Ecological Life Support System as envisioned by the U.S. National Aeronautics and Space Administration. In studies conducted with biodegradative microorganisms indigenous to sweetpotato fields, it was determined that a particle size of 75 microns and incubation temperature of 30°C were optimal for degradation. The composition of the biomass and characterization of plant nutrient solution indicated the presence of potential energy sources to drive microbial transformations of plant waste. Selected indigenous soil isolates with ligno-cellulolytic or sulfate-reducing ability were utilized in biological studies and demonstrated diversity in their ability to reduce sulfate in solution and to utilize alternative carbon sources: a lignin analog 4-hydroxy, 3-methoxy cinnamic acid, cellulose, arabinose, glucose, sucrose, mannitol, galactose, ascorbic acid.     

Molecular analysis of mutagenesis by high LET radiation.

Mutation induction by high linear energy transfer [LET] alpha particles and gamma-rays was scored in the human hamster hybrid [AL] cells. Southern blotting technique was used to analyse the molecular changes in the DNA from both the HGPRT- and S1- mutants. Dose dependent mutagenesis in the AL cells irradiated with the charged particles was higher by almost 20 fold at the S1 than the corresponding HGPRT locus. Southern analysis of the mutants induced by the high LET particles showed mostly multilocus deletion at both the HGPRT and S1 genes.     

The impact of alternative mission models on the future orbital debris environment

In reviewing discussions of future directions for space activity, it becomes obvious that there are a large number of groups formulating a wide diversity of plans for the future use of space. These plan alternatives are being made to account for user needs, technology development constraints, economic constraints, and launch support, and each of the plans will have direct or indirect effects on the orbital debris environment in terms of mass to orbit, deposition of operational debris, and control of accidental breakups. Thus it is important to develop the ability to project future debris states for a range of possible space traffic scenarios. The impact that these possible traffic environments would have on space operations forms the basis for studies of alternative options for the usage of space. In this paper, the effects on the orbital debris environment of a base-line mission model and two alternatives are investigated, using a numerical debris environment simulation code under development at JSC.     

Calculations and observations of solar particle enhancements to the radiation environment at aircraft altitudes.

Solar particle events can give greatly enhanced radiation at aircraft altitudes, but are both difficult to predict and to calculate retrospectively. This enhanced radiation can give significant dose to aircrew and greatly increase the rate of single event effects in avionics. Validation of calculations is required but only very few events have been measured in flight. The CREAM detector on Concorde detected the event of 29 September 1989 and also four periods of enhancement during the events of 19-24 October 1989. Instantaneous rates were enhanced by up to a factor ten compared with quiet-time cosmic rays, while flight-averages were enhanced by up to a factor six. Calculations are described for increases in radiation at aircraft altitudes using solar particle spectra in conjunction with Monte Carlo radiation transport codes. In order to obtain solar particle spectra with sufficient accuracy over the required energy range it is necessary to combine space data with measurements from a wide range of geomagnetically dispersed, ground-level neutron monitors. Such spectra have been obtained for 29 September 1989 and 24 October 1989 and these are used to calculate enhancements that are compared with the data from CREAM on Concorde. The effect of cut-off rigidity suppression by geomagnetic activity is shown to be significant. For the largest event on record on 23 February 1956, there are no space data but there are data from a number of ground-level cosmic-ray detectors. Predictions for all events show very steep dependencies on both latitude and altitude. At high latitude and altitude (17 km) calculated increases with respect to cosmic rays are a factor 70 and 500 respectively for 29 September 1989 and 23 February 1956. The levels of radiation for high latitude, subsonic routes are calculated, using London to Los Angeles as an example, and can exceed 1 mSv, which is significantly higher than for Concorde routes from Europe to New York. The sensitivity of the calculations to spectral fitting, geomagnetic activity and other assumptions demonstrates the requirement for widespread carriage of radiation monitors on aircraft.     

Changes in gravitational force induce alterations in gene expression that can be monitored in the live, developing zebrafish heart.

Little is known about the effect of microgravity on gene expression, particularly in vivo during embryonic development. Using transgenic zebrafish that express the gfp gene under the influence of a beta-actin promoter, we examined the affect of simulated-microgravity on GFP expression in the heart. Zebrafish embryos, at the 18-20 somite-stage, were exposed to simulated-microgravity for 24 hours. The intensity of GFP fluorescence associated with the heart was then determined using fluorescence microscopy. Our measurements indicated that simulated-microgravity induced a 23.9% increase in GFP-associated fluorescence in the heart. In contrast, the caudal notochord showed a 17.5% increase and the embryo as a whole showed only an 8.5% increase in GFP-associated fluorescence. This suggests that there are specific effects on the heart causing the more dramatic increase. These studies indicate that microgravity can influence gene expression and demonstrate the usefulness of this in vivo model of 'reporter-gene' expression for studying the effects of microgravity.     

Escaping near-relativistic electron beams from the solar corona

Processes in the solar corona are prodigious accelerators of energetic ions, and electrons. The angular distribution, composition, and spectra of energetic particles observed near Earth gives information on the acceleration mechanisms. A class of energetic particle observations particularly useful in understanding the solar acceleration is the near-relativistic impulsive beam-like electron events. During five years of operation the Advanced Composition Explorer (ACE) has measured well over 400 electron events. Approximately 25% of these electron events are impulsive beam-like events that are released onto interplanetary field lines predominantly from western solar longitudes. We extend our initial 3 year study during the rise to solar maximum (Haggerty and Roelof, 2002; Simnett et al., 2002) to a five year statistical analysis of these beam-like energetic electron events in relationship to optical flares, microwave emission, soft X-ray emission, metric and decametric type-III radio bursts, and coronal mass ejections.     

Productivity and food value of Amaranthus cruentus under non-lethal salt stress.

Stress effects from the accumulation of metal salts may pose a problem for plants in closed biological systems such as spacecraft. This work examined the effects of salinity on growth, photosynthesis and carbon allocation in the crop plant, Amaranthus. Plants were germinated and grown in modified Hoagland's solution with NaCl concentrations of 0 to 1.0%. Plants received salt treatments at various times in development to assess effects on particular life history phases. For Amaranthus cruentus, germination, vegetative growth, flowering, seed development and yield were normal at salinities from 0 to 0.2%. Inhibition of these phases increased from 0.2 to 0.4% salinity and was total above 0.5%. 1.0% salinity was lethal to all developmental phases. Onset of growth phases were not affected by salinity. Plants could not be adapted by gradually increasing salinity over days or weeks. Water uptake increased, while photosynthetic CO2 uptake decreased with increasing salinity on a dry weight basis during vegetative growth. Respiration was not affected by salinity. After flowering, respiration and photosynthesis decreased markedly, such that 1.0% NaCl inhibited photosynthesis completely. Protein levels were unchanged with increasing salinity. Leaf starch levels were lower at salinities of 0.5% and above, while stem starch levels were not affected by these salinities. The evidence supports salt inhibition arising from changes in primary biochemical processes rather than from effects on water relations. While not addressing the toxic effects of specific ions, it suggests that moderate salinity per se need not be a problem in space systems.     

Adaptive pulse compression via MMSE estimation

Radar pulse compression involves the extraction of an estimate of the range profile illuminated by a radar in the presence of noise. A problem inherent to pulse compression is the masking of small targets by large nearby targets due to the range sidelobes that result from standard matched filtering. This paper presents a new approach based upon a minimum mean-square error (MMSE) formulation in which the pulse compression filter for each individual range cell is adaptively estimated from the received signal in order to mitigate the masking interference resulting from matched filtering in the vicinity of large targets. The proposed method is compared with the standard matched filter and least-squares (LS) estimation and is shown to be superior over a variety of stressing scenarios.     

Near infrared studies of the massive X-ray binary 2S 0114+65

We present near-infrared spectroscopy of the massive X-ray binary 2S 0114+650. These observations covering the spectral range 1.08–2.35 μm span the region where Paschen and Brackett series recombination lines of hydrogen are expected to be seen, namely, Paβ , Brγ and Br 10–17 lines. The absence of any of these lines in emission supports earlier inferences that the optical component in 2S 0114+650 is unlikely to be a Be star but rather a B type supergiant. Near-IR photometry gives J = 8.78, H = 8.53 and K = 7.96; these values show marginal variations from earlier reported measurements.