Terrestrial Standards Research Articles

Application of the IEEE 802.11 Protocol to Inter-Satellite Communications

This paper is concerned with the application of the terrestrial wireless standard IEEE 802.11 for inter-satellite communication in autonomous distributed spacecraft constellations and formations. In low-altitude orbital multi-satellite communication systems the network topology is predictable and periodic but signals suffer variable propagation delays, low signal-to-noise ratios, and large Doppler shifts. Therefore it is investigated how changes of orbital parameters influence the use of terrestrial wireless standards for inter-satellite links in space. Optimization approaches such as tuning of the physical layer parameters and enhancing the data link layer are presented. The integration of smart antennas with the Medium Access Control protocol is also considered aiming at increasing the capacity and throughput while minimizing interference and collision in close proximity links. Finally, simulation results are presented to demonstrate the effectiveness of the proposed approaches for satellite formations.

Human health and performance risk management—an approach for exploration missions

During long duration exploration missions, maintaining human health and performance will be essential to enabling success. Therefore, NASA has developed standards through the Health and Medical Technical Authority to insure human health and performance during exploration. Human health standards are the first step in defining acceptable risk for human space flight and take into consideration both short-term (mission) and long-term (lifetime) health risk. These standards are based on the best medical evidence from terrestrial standards; analog spaceflight environments; and spaceflight experience. Standards drive the development of focused program requirements to mitigate risks associated with specific missions. Program requirements include vehicle design as well as health care systems including medical, environmental and countermeasures. NASA has also developed the risk mitigation analysis tool (RMAT), a process to evaluate the effectiveness of risk mitigation strategies. The RMAT facilitates documentation and analysis of the effectiveness of mitigation strategies, enables NASA to baseline a risk mitigation approach based on the best evidence, and provides the traceability from research and technology development projects to specific mission deliverables.

Oxygen and carbon isotope ratios in the martian atmosphere

Oxygen and carbon isotope ratios in the martian CO 2 are key values to study evolution of volatiles on Mars. The major problems in spectroscopic determinations of these ratios on Mars are uncertainties associated with: (1) equivalent widths of the observed absorption lines, (2) line strengths in spectroscopic databases, and (3) thermal structure of the martian atmosphere during the observation. We have made special efforts to reduce all these uncertainties. We observed Mars using the Fourier Transform Spectrometer at the Canada–France–Hawaii Telescope. While the oxygen and carbon isotope ratios on Mars were byproducts in the previous observations, our observation was specifically aimed at these isotope ratios. We covered a range of 6022 to 6308 cm −1 with the highest resolving power of ν / δ ν = 3.5 × 10 5 and a signal-to-noise ratio of 180 in the middle of the spectrum. The chosen spectral range involves 475 lines of the main isotope, 184 lines of 13CO 2, 181 lines of CO 18O, and 119 lines of CO 17O. (Lines with strengths exceeding 10 −27 cm at 218 K are considered here.) Due to the high spectral resolution, most of the lines are not blended. Uncertainties of retrieved isotope abundances are in inverse proportion to resolving power, signal-to-noise ratio, and square root of the number of lines. Laboratory studies of the CO 2 isotope spectra in the range of our observation achieved an accuracy of ∼ 1 % in the line strengths. Detailed observations of temperature profiles using MGS/TES and data on temperature variations with local time from two GCMs are used to simulate each absorption line at various heights in each part of the instrument field of view and then sum up the results. Thermal radiation of Mars' surface and atmosphere is negligible in the chosen spectral range, and this reduces errors associated with uncertainties in the thermal structure on Mars. Using a combination of all these factors, the highest accuracy has been achieved in measuring the CO 2 isotope ratios: 13C/ 12C = 0.978 ± 0.020 and 18O/ 16O = 1.018 ± 0.018 times the terrestrial standards. Heavy isotopes in the atmosphere are enriched by nonthermal escape and sputtering, and depleted by fractionation with solid-phase reservoirs. The retrieved ratios show that isotope fractionation between CO 2 and oxygen and carbon reservoirs in the solid phase is almost balanced by nonthermal escape and sputtering of O and C from Mars.

Analytical Developments for High-Precision Measurements of W Isotopes in Iron Meteorites

A procedure was developed to accurately measure the W isotopic compositions of iron meteorites with a precision of better than +/-0.1 epsilon on epsilon182W and epsilon184W (normalized to 186W/183W). Purification of W was achieved through a two-step, ion-exchange procedure. In most cases, the yield is better than 80%, and purified W solutions are clear of matrix elements and direct isobars of W. The final W solutions were analyzed using a Micromass Isoprobe multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Tests performed on mixtures of terrestrial standards and meteorite samples demonstrate that the method is accurate and that epsilon182W variations as small as approximately 0.1 epsilon can be detected. Analyses of three different aliquots of the Gibeon (IVA) iron meteorite obtained over a period of 6 months show identical epsilon182W values with a weighted mean of 3.38 +/- 0.05, consistent with literature data for IVA iron meteorites, and indicating that the metal-silicate differentiation event in its parent body was either contemporaneous with or slightly postdated (by up to approximately 2.5 My) the formation of refractory inclusions. We demonstrate our ability to measure epsilon184W accurately and precisely (within +/-0.1 epsilon), which is useful for characterizing cosmogenic and nucleosynthetic effects that may be present in iron meteorites. We also report for the first time measurements of epsilon180W, albeit with large error bars (<+/-4 epsilon, in most cases).

Solar Nebula Heterogeneity in p-Process Samarium and Neodymium Isotopes

Bulk carbonaceous chondrites display a deficit of approximately 100 parts per million (ppm) in 144Sm with respect to other meteorites and terrestrial standards, leading to a decrease in their 142Nd/144Nd ratios by approximately 11 ppm. The data require that samarium and neodymium isotopes produced by the p process associated with photodisintegration reactions in supernovae were heterogeneously distributed in the solar nebula. Other samarium and neodymium isotopes produced by rapid neutron capture (r process) in supernovae and by slow neutron capture (s process) in red giants were homogeneously distributed. The supernovae sources supplying the p- and r-process nuclides to the solar nebula were thus disconnected or only weakly connected.

Photochemical Mass-Independent Sulfur Isotopes in Achondritic Meteorites

Sulfides from four achondrite meteorite groups are enriched in 33S (up to 0.040 per mil) as compared with primitive chondrites and terrestrial standards. Stellar nucleosynthesis and cosmic ray spallation are ruled out as causes of the anomaly, but photochemical reactions in the early solar nebula could produce the isotopic composition. The large 33S excess present in oldhamite from the Norton County aubrite (0.161 per mil) suggests that refractory sulfide minerals condensed from a nebular gas with an enhanced carbon-oxygen ratio, but otherwise solar composition is the carrier. The presence of a mass-independent sulfur effect in meteorites argues for a similar process that could account for oxygen isotopic anomalies observed in refractory inclusions in primitive chondrites.

Comparative Planetary Atmospheres of the Galilean Satellites

We know that each of the Galilean satellites of Jupiter has a tenuous atmosphere by terrestrial standards. Io’s SO2 equatorial atmosphere and, perhaps, Callisto’s atmosphere inferred from its large ionospheric densities are measured in nanobars, whereas the atmospheres of Europa and Ganymede produced by ion sputtering of the water ice surfaces only reach picobar pressures and are comprised mostly of O2. Io’s polar atmosphere is probably an order of magnitude less dense than its equatorial atmosphere. Europa and Ganymede have O2 atmospheres with column densities in the range of (1-10)×1014cm−2. These atmospheres, on the basis of their inferred production and loss rates, are estimated to have short residence times of ~2-3 days.

Mn-Cr systematics of pallasites

I report here on a comprehensive ion microprobe study of Mn-Cr systematics of six pallasites. Four pallasites show excesses of radiogenic Cr-53 (Cr-53*), which correlate well with the Mn/Cr ratios. For these pallasites, the inferred initial (Mn-53/Mn-55). ratios are similar to 1 x 10(-5), which indicates that pallasites formed very early in the solar system. Eagle Station and Imilac show hints of Cr-53*, but the excesses are not resolved from terrestrial standards. Comparisons of Mn-Cr results with those of Pd-Ag indicate that these two short-lived radionuclide chronometers are self-consistent in pallasites. Inferred (Mn-53/Mn-55)(0) ratios in this study are generally higher than those previously observed by TIMS (1-3 x 10(-6)). It is not clear what mechanism caused such a discrepancy. Nevertheless, it should be noted that ion probe depends on the zoning profiles to give high Mn/Cr ratios at the edges of the olivine crystals. Thus, it dates the closure time of diffusive fractionation of Mn and Cr in pallasite olivines.

Hydrogen Isotopes of Glassy and Phyllosilicate Spherules in Al Rais (CR) and Orgueil (CI) Chondrites

Abstract The hydroxyl in phyllosilicate minerals is the most common occurrence of water in primitive meteorites. Direct hydrogen isotopic analysis of this water component using an ion microprobe has been made in some glassy or phyllosilicate spherules from the Al Rais (CR) and Orgueil (CI) chondrites. The spherules from Al Rais show large deuterium excesses (δD = +200 – +800‰) relative to terrestrial standards, whereas deuterium‐enrichments in the spherules from Orgueil are much smaller (δD = +40 – +130‰). The phyllosilicate spherules are products of aqueous alteration of glassy precursors. In Al Rais the phyllosilicate spherules have relatively higher δD values than the glassy ones, indicating that water introduced during aqueous alteration was deuterium‐enriched. The deuterium‐enrichments in the phyllosilicate spherules from Orgueil could result from isotopic exchange under thermodynamic conditions within the solar nebula. The much larger δD excesses of the Al Rais spherules, however, cannot be attributed to the similar process; instead, an interstellar origin needs to be invoked.

182Hf-182W isotope systematics of chondrites, eucrites, and martian meteorites: Chronology of core formation and early mantle differentiation in Vesta and Mars

Abstract The timescale of accretion and differentiation of asteroids and the terrestrial planets can be constrained using the extinct 182Hf-182W isotope system. We present new Hf-W data for seven carbonaceous chondrites, five eucrites, and three shergottites. The W isotope data for the carbonaceous chondrites agree with the previously revised 182W/184W of chondrites, and the combined chondrite data yield an improved ϵW value for chondrites of −1.9 ± 0.1 relative to the terrestrial standard. New Hf-W data for the eucrites, in combination with published results, indicate that mantle differentiation in the eucrite parent body (Vesta) occurred at 4563.2 ± 1.4 Ma and suggest that core formation took place 0.9 ± 0.3 Myr before mantle differentiation. Core formation in asteroids within the first ∼5 Myr of the solar system is consistent with the timescales deduced from W isotope data of iron meteorites. New W isotope data for the three basaltic shergottites EETA 79001, DaG 476, and SAU 051, in combination with published 182W and 142Nd data for Martian meteorites reveal the preservation of three early formed mantle reservoirs in Mars. One reservoir (Shergottite group), represented by Zagami, ALH77005, Shergotty, EETA 79001, and possibly SAU 051, is characterized by chondritic 142Nd abundances and elevated ϵW values of ∼0.4. The 182W excess of this mantle reservoir results from core formation. Another mantle reservoir (NC group) is sampled by Nakhla, Lafayette, and Chassigny and shows coupled 142Nd-182W excesses of 0.5–1 and 2–3 ϵ units, respectively. Formation of this mantle reservoir occurred 10–20 Myr after CAI condensation. Since the end of core formation is constrained to 7–15 Myr, a time difference between early silicate mantle differentiation and core formation is not resolvable for Mars. A third early formed mantle reservoir (DaG group) is represented by DaG 476 (and possibly SAU 051) and shows elevated 142Nd/144Nd ratios of 0.5–0.7 ϵ units and ϵW values that are indistinguishable from the Shergottite group. The time of separation of this third reservoir can be constrained to 50–150 Myr after the start of the solar system. Preservation of these early formed mantle reservoirs indicates limited convective mixing in the Martian mantle as early as ∼15 Myr after CAI condensation and suggests that since this time no giant impact occurred on Mars that could have led to mantle homogenization. Given that core formation in planetesimals was completed within the first ∼5 Myr of the solar system, it is most likely that Mars and Earth accreted from pre-differentiated planetesimals. The metal cores of Mars and Earth, however, cannot have formed by simply combining cores from these pre-differentiated planetesimals. The 182W/184W ratios of the Martian and terrestrial mantles require late effective removal of radiogenic 182W, strongly suggesting the existence of magma oceans on both planets. Large impacts were probably the main heat source that generated magma oceans and led to the formation metallic cores in the terrestrial planets. In contrast, decay of short-lived 26Al and 60Fe were important heat sources for melting and core formation in asteroids.

Constraints on the structure of the martian interior determined from the chemical and isotopic systematics of SNC meteorites

Abstract— The crystallization ages of martian (SNC) meteorites give evidence that martian volcanism has continued until recent times‐perhaps until the present. These meteorites also indicate that the mantle source regions of this volcanism are modestly to extremely depleted by terrestrial standards. These 2 observations produce a conundrum. How is it that such depleted source regions have produced basaltic magma for such a long time?This contribution attempts to quantify the radiogenic heat production in 2 distinct martian mantle source regions: those of the shergottites and nakhlites. Compared to the depleted upper mantle of the Earth (MORB), the nakhlite source region is depleted by about a factor of 2, and the shergottite source region is depleted by a factor of 6. According to current geophysical models, the nakhlite source contains the minimum amount of radioactive heat production to sustain whole‐mantle convection and basalt generation over geologic time. A corollary of this conclusion is that the shergottite source contains much too little radioactivity to produce recent (&lt;200 Ma) basalts. A model martian interior with a deep nakhlite mantle that is insulated by a shallow shergottite mantle may allow basalt production from both source regions if the divide between the nakhlite‐shergottite mantles acts as a thermal boundary layer. Similarities between lunar and martian isotopic reservoirs indicate that the Moon and Mars may have experienced similar styles of differentiation.

Automated isotopic measurements of micron-sized dust: application to meteoritic presolar silicon carbide

We report the development of a new analytical system allowing the fully automated measurement of isotopic ratios in micrometer-sized particles by secondary ion mass spectrometry (SIMS) in a Cameca ims-6f ion microprobe. Scanning ion images and image processing algorithms are used to locate individual particles dispersed on sample substrates. The primary ion beam is electrostatically deflected to and focused onto each particle in turn, followed by a peak-jumping isotopic measurement. Automatic measurements of terrestrial standards indicate similar analytical uncertainties to traditional manual particle analyses (e.g., ∼3‰/amu for Si isotopic ratios). We also present an initial application of the measurement system to obtain Si and C isotopic ratios for ∼3300 presolar SiC grains from the Murchison CM2 carbonaceous chondrite. Three rare presolar Si 3N 4 grains were also identified and analyzed. Most of the analyzed grains were extracted from the host meteorite using a new chemical dissolution procedure. The isotopic data are broadly consistent with previous observations of presolar SiC in the same size range (∼0.5–4 μm). Members of the previously identified SiC AB, X, Y, and Z subgroups were identified, as was a highly unusual grain with an extreme 30Si enrichment, a modest 29Si enrichment, and isotopically light C. The stellar source responsible for this grain is likely to have been a supernova. Minor differences in isotopic distributions between the present work and prior data can be partially explained by terrestrial contamination and grain aggregation on sample mounts, though some of the differences are probably intrinsic to the samples. We use the large new SiC database to explore the relationships between three previously identified isotopic subgroups—mainstream, Y, and Z grains—all believed to originate in asymptotic giant branch stars. The isotopic data for Z grains suggest that their parent stars experienced strong CNO-cycle nucleosynthesis during the early asymptotic giant branch phase, consistent with either cool bottom processing in low-mass (M < 2.3M ⊙) parent stars or hot-bottom burning in intermediate-mass stars (M > 4M ⊙). The data provide evidence for a sharp threshold in metallicity, above which SiC grains form with much higher 12C/ 13C ratios than below. Above this threshold, the fraction of grains with relatively high 12C/ 13C decreases exponentially with increasing 29Si/ 28Si ratio. This result indicates a sharp increase in the maximum mass of SiC parent stars with decreasing metallicity, in contrast to expectations from Galactic chemical evolution theory.

Thermoremanent magnetization of the Martian lithosphere

This paper investigates the thermoremanent magnetization (TRM) of the Martian lithosphere assuming that the upper part of the lithosphere acquired TRM in the early history of the planet and in the presence of the core field (the primary magnetization), whereas the lower part has been gradually magnetized by the magnetic field of the upper part as it has cooled below the Curie temperature (secondary magnetization). The secondary magnetization is relatively weak if the magnetic properties of the lower lithosphere are taken to be similar to that of the extrusive basalt near the oceanic ridge axes on Earth. However, if the Martian lower lithosphere is more magnetic, e.g., by an order of magnitude, the secondary magnetization can be significant, although still weaker than the primary magnetization of the upper part. Such a highly magnetic lower lithosphere does not seem plausible by terrestrial standards, and it remains to be confirmed whether it is geologically plausible for Mars. I also examine a lithospheric model with a strongly magnetic layer beneath the crust, 25 times more magnetic than the extrusive basalt, representing a possible ilmenite‐rich layer with a considerable amount of hematite‐ilmenite lamellae. I assume that the layer was not magnetized by the core field because it was hotter than the Curie temperature of the lamellae when the core dynamo was active. It acquired secondary magnetization later in the presence of the magnetic field of the crust. Despite adopting an extraordinarily high magnetic layer which seems implausible by terrestrial standards, the secondary magnetization of the layer has minor contributions to the observed magnetic anomalies. In all models considered the primary magnetization is ∼20–30 A/m and has the dominant contribution to the observed magnetic anomalies. It is worth noting that the radial components of the primary and secondary magnetization are parallel, whereas the tangential components are in opposite directions. Because the strong magnetic anomalies in the south likely arise from the tangential magnetization, considering the secondary magnetization requires even stronger primary magnetization.

The Hell Creek Formation and the Cretaceous-Tertiary Boundary in the Northern Great Plains: An Integrated Continental Record of the End of the Cretaceous

The Hell Creek Formation and the Cretaceous–Tertiary Boundary in the Northern Great Plains: An Integrated Continental Record of the End of the Cretaceous, J.H. Hartman, K.R. Johnson, and D.J. Nichols, (eds.), 2002, Geological Society of America Special Paper 361, Boulder, USA, 520 p. (Softcover, GSA Members, US$ 96.00, Non-Members, US $120.00) ISBN: 0-8137-2361-2. These days, who publishes a large volume devoted to a single geological formation? The approach seems positively nineteenth century; however in this case, the work is timely, appropriate, and important. The Hell Creek Formation (HCF) is the well-known and oft-cited nexus of terrestrial Cretaceous-Tertiary (K/T) boundary research. It contains the whole package: palynomorphs, a megaflora, a K/T boundary with all the trimmings (Iridium, microtektites, the boundary couplet, soot), extraordinarily abundant vertebrates (including those K/T poster children, dinosaurs) and, by terrestrial standards, something approaching continuous sedimentation. It breathes promise of containing the keys to all questions K/T-ish; and indeed, it has been implicated by innumerable authors to sustain their takes on what transpired 65 Ma. The only problem is that those viewpoints have historically been contradictory and mutually exclusive. Special Paper 361 is, as it claims, “an integrated continental record of the end of the Cretaceous.” The key word is “integrated.” The central geological theme underscores and reaffirms the importance of the geosciences to the apparently biological questions that are the focus of so many extinction analyses. And while on the face of it some of the geological contributions appear not to bear directly upon more glamorous extinction …

Possible mineral sources of magnetic anomalies on Mars

At the end of the last millennium, numerous (USSR and U.S.) spacecraft were sent to Mars to begin exploration of that planet. Mars Global Surveyor (1997 onward) is one of the successful missions that provided a detailed map of the topography and high resolution images of the surface, and also discovered unusually large remanent magnetism of the crust (up to 20 times larger than terrestrial standards).

Galaxies and the magnetization of intergalactic space

There is increasing evidence that widespread magnetic fields in the intergalactic medium (IGM) originate in galaxies. Up to now the field strengths have been easiest to estimate where the ambient intergalactic (i.g.) nonrelativistic gas density is highest, i.e., in galaxy clusters, the gravitational “sinks” of large scale matter flows. Intracluster gas densities reach 10−3–10−2 cm−3, and magnetic field strength estimates range from 1 to 40 μG. For much of the intracluster volume the plasma β∼1, a consequence of the intracluster magnetic field measurements, and x-ray-determined densities and temperatures of the thermal intracluster (ic) gas. While these ic gas densities represent an impressively good vacuum by terrestrial standards, they are still about 1000× more than in the general IGM. The logical next question is: Are there significant magnetic fields in the wider IGM beyond clusters? Recent, preliminary estimates in i.g. zones beyond, but not far from clusters are 10−6–10−7 G. Integrating this field strength over a typical intergalaxy separation volume gives an average magnetic energy density within the “walls and filaments” of intergalactic space of ∼10−15 B−6.5 ergs cm−3. Interestingly, for Big∼3×10−7 G, and plausible values of nig∼5×10−6 cm−3, and Tig∼108 K—the intergalactic plasma β may be within a factor 10 of unity, roughly comparable to β within the ∼103 times denser medium in galaxy clusters. It is now widely accepted that the only plausible energy source for these intergalactic fields is gravitational, e.g., not the aggregate thermonuclear energy from stars. Two sufficiently large gravitational energy reservoirs are (1) the kinetic energy from large scale intergalactic matter flows, and (2) the gravitational binding energy of supermassive black holes that have been found to be nearly ubiquitous in the nuclei of medium-to-large galaxies. This paper focuses mostly on (2), and also discusses why the largest radio sources are the best independent calibrators, and calorimeters of magnetic energy release into i.g. space from individual galaxy systems.

A Versatile Optical Intersatellite Terminal Design

This paper presents an Optical Intersatellite Link (OISL) terminal design that has the flexibility to meet the requirements of a wide range of potential customers. The program concentrated on the development of the terminal Optical Front End (OFE). The OFE acts as the interface between a fiber-optic communication sub-system and free-space. It finds and tracks the partner satellite, and then maintains a full duplex optical communication link with that partner; 1550 nm is the chosen communication wavelength because of its compatibility with established terrestrial standards. However, the use of all metal, all reflective optics permits the choice of other wavelengths. This paper reviews the terminal architecture and the design of the key OFE components. At the heart of the OFE resides the Afocal Optical Assembly (AOA), positioned between the terminal's telescope and the fiber-optic subsystem. The AOA is essentially a mini-OFE unto itself. It is the common element in all variations of the terminal design. For this reason, the construction and testing of a flight-representative AOA was a significant achievement.

Exobiological Implications of a Possible Ammonia–Water Ocean inside Titan

Models of Titan's thermal history indicate that an ocean consisting of an ammonia–water solution, as much as 200 km deep, is presently concealed beneath a crust of water ice. Conditions within this hypothetical ocean are discussed here and compared with conditions in terrestrial biomes. These conditions, while extreme by terrestrial standards, are such that life could indeed survive. This is in stark contrast with conditions on the surface where even the simplest prebiotic reactions have half-lives of the order 10 7 years, and free water would only infrequently be available to participate in chemical reactions. The plausible existence of life within Titan's subterranean ocean is discussed in terms of the effects that gaseous waste products of such indigenous organisms could have on the chemical and isotopic composition of the atmosphere. The possibility that all of Titan's current atmospheric CH 4 and N 2 is of biogenic origin is explored from a consideration of the potential productivity of oceanic microbes. While microbial life appears to be possible, it is unlikely that the Cassini/Huygens mission will be able to distinguish any unambiguous biomarkers. The existence and possible depth of the ocean may be constrained from Cassini flyby data, but detection of life in that ocean calls for a closer examination than the Huygens lander will afford us.

Isotopic Composition of Lithium in Carbonaceous Chondrites Measured by a Modified Type Surface Ionization Mass Spectrometer.

The isotopic ratios of lithium, 7Li/6Li, have been measured by a modified type surface ionization mass spectrometer whose ion source was newly designed. It needs no chemical preparation prior to the ionization, and makes an analysis of small portions of sample possible. We mainly analyzed the Murchison meteorite and compared with terrestrial laboratory standard samples, spodumene and lepidolite. Many kinds of isotope anomalies for Murchison have been reported, and these anomalies are considered to be originated from presolar events. Spallation reactions are thought to be one of the processes that produced lithium isotopes, and to synthesize 7Li and 6Li isotopes as almost the same quantities. If lithium isotopes produced by such reactions in the presolar stage have been maintained in meteorites, there may be an isotopic anomaly of lithium whose ratio, 7Li/6Li, is extremely lower than that for terrestrial standards. The mean isotopic ratio, 7Li/6Li, for one laboratory standard, spodumene, is 10.519±0.0004, and that for another one, lepidolite, is 11.329±0.0009. The discrepancy between these lithium isotopic ratios is estimated to be due to the different fractionation processes of these minerals. The isotopic ratios for Murchison are within a range from 10.056 to 11.356. This range of the scattering for Murchison is within those for previous works. The lithium isotopic ratios for Murchison analyzed so far gave no significant difference from those for terrestrial standards.

The HDTV Broadcast Standard: Subjective Evaluation of Proponent Systems

Six high-definition television (HDTV) systems were tested to establish a proposed HDTV terrestrial transmission standard. The tests occurred in two rounds and were conducted at the Advanced Television Test Center (ATTC); and at Cable Labs in Alexandria, VA; and at the Advanced Television Evaluation Laboratory (ATEL) in Ottawa, Ont., Canada. The testing process played an important role in accelerating the development of HDTV technology in North America, culminating in the development of the digital Grand Alliance HDTV System, the adopted standard. This paper summarizes the results of subjective image quality tests (Basic Received Quality, Impairment/Interference, and Receiver Scan Conversion) of the proponent HDTV systems from both rounds of testing, thereby permitting the reader to compare the relative performance of competing systems and gain some insight into the testing process conducted at ATEL.