ALBERT

Description: The objectives of this program are as follows: modelling of dynamics of composite tubular space structure truss members; and utilization of ultrasonic waves as probes for material and defect characterization. This discussion is presented in viewgraph format.

Description: Through the Surveyor 3 and 7, and Apollo 11-17 missions a knowledge of the mechanical properties of Lunar regolith were gained. These properties, including material cohesion, friction, in-situ density, grain-size distribution and shape, and porosity, were determined by indirect means of trenching, penetration, and vane shear testing. Several of these properties were shown to be significantly different from those of terrestrial soils, such as an interlocking cohesion and tensile strength formed in the absence of moisture and particle cementation. To characterize the strength and deformation properties of Lunar regolith experiments have been conducted on a lunar soil simulant at various initial densities, fabric arrangements, and composition. These experiments included conventional triaxial compression and extension, direct tension, and combined tension-shear. Experiments have been conducted at low levels of effective confining stress. External conditions such as membrane induced confining stresses, end platten friction and material self weight have been shown to have a dramatic effect on the strength properties at low levels of confining stress. The solution has been to treat these external conditions and the specimen as a full-fledged boundary value problem rather than the idealized elemental cube of mechanics. Centrifuge modeling allows for the study of Lunar soil-structure interaction problems. In recent years centrifuge modeling has become an important tool for modeling processes that are dominated by gravity and for verifying analysis procedures and studying deformation and failure modes. Centrifuge modeling is well established for terrestrial enginering and applies equally as well to Lunar engineering. A brief review of the experiments is presented in graphic and outline form.

Description: In the interest of aviation safety, NASA and the FAA are jointly conducting research to determine the applicability of airborne, coherent Doppler radar techniques to detect early microburst in wind shear conditions during aircraft takeoff and landing. Researchers have developed a computer model of the radar which predicts its response when viewing a simulated microburst against the simulated clutter background of an airport, the so-called radar microburst ground clutter model. Studies employing this model revealed that Doppler radar can accurately detect microburst ahead of the aircraft in time for pilot evasive response, but flight experiments will be required for complete performance evaluation of the system. An experimental X band radar is being developed for future flight experiments to verify the simulation modeling results. A description of the experimental radar, recording equipment, and its installation on the NASA 515 aircraft is presented. The flight experiments to be conducted are also described.

Description: A new technique is described for calibration of complex multipolarization SAR imagery. Scatterer reciprocity and lack of correlation between like- and cross-polarized radar echoes for natural targets are used to remove cross-polarized contamination in the radar data channels without the use of known ground targets. If known targets are available, all data channels can be calibrated relative to one another and absolutely as well. The method is verified with airborne SAR data.

Description: The SAR (synthetic-aperture radar) data-processing algorithm to be used for the Magellan mission is described. Radar system design, SAR data characteristics, and hardware (H/W) constraints, which are critical to the processing algorithm design, are highlighted. Data flow and the H/W architecture are given to show the real-time data processing capability. Simulation results obtained from processing the synthetic point-target echos are presented to demonstrate the performance of the processing algorithm.

Description: Nonthermal radio emissions from earth, Jupiter, Saturn, and Uranus are reviewed. The dominant source of emission at each planet appears to be AKR-like auroral emission in the X-mode. O-mode emissions are substantially responsible. There is a remarkably constant scaling factor relating the total solar wind input power into each planetary system and the AKR-like auroral emissions.

Description: This article presents the results of a fit of a model of the Martian satellite orbits to earthbased and spacecraft-based observations. An assessment of the orbit accuracies is given and the orbits are compared with those obtained by previous investigators.

Description: The petrology and chronology of early lunar crust is examined using the least equivocal of the available petrographic and age data on lunar rock samples, and the possible processes which produced the lunar crust are discussed. The results suggest that the lunar anorthositic crust was formed by about 120 Ma after the primary accretion of the moon at 4.56 Ga. At least some members of the diverse Mg-suites of rocks, such as norites, troctolites, and dunites, crystallized within a very few 100s of Ma after 4.56 Ga. A trace-element-rich material (KREEP) was formed by about 4.3 Ga ago, and this residue was subsequently reworked in melting and impact processes such that most samples which contain it have ages around 3.9-4.0 Ga. The findings also suggest that the onset of ferrous mare basalt volcanism began about 4.33 Ga, much earlier than was once assumed, and was still in process before the end of the most intense period of bombardment (3.9-4.0 Ga ago).

Description: The analysis and low-power testing of a square multiflare horn designed for 1-megawatt CW operation is described. Design considerations for the five-port input section are discussed. The required aperture modes are determined from radiation pattern considerations, and analysis of the multiflare section is carried out using mode matching. Measurements demonstrate that a circularly symmetric beam is produced with relatively low sidelobes.

Description: The cross polarization characteristics of rectantular patch antennas are studied experimentally. Data are presented showing the dependence of the copolarization to cross-polarization ratio on the aspect ratio in both the E and H planes. Three substrate thicknesses are included and the variation with resonant frequency is examined.

Description: Synthesis of dual-shaped offset reflector antennas to control the exit aperture distribution of amplitude and phase has received considerable attention in recent years. For a given feed illumination and desired aperture field distribution, an exact formulation of the problem of simultaneously synthesizing the shapes of the sub and main reflectors was presented recently by Galindo-Israel et al. (1987) in terms of a set of nonlinear first-order differential equations. In this paper, a numerical approach to solving these equations is discussed which circumvents some of the difficulties encountered by Galindo-Israel et al., particularly for small values of theta.

Description: The findings made by the Voyager 2 Neptune encounter are reviewed. Data on the bowshock, magnetic field, magnetosphere, rings, plasma sheet, aurora, moons, and dust of Neptune are discussed. Findings made concerning Triton are summarized.

Description: The technique described uses the radar return from natural targets and at least one trihedral corner reflector to calibrate compressed polarimetric radar data for relative amplitude, relative phase, absolute amplitude, and system crosstalk. The crosstalk method is based on the theoretical result that, for natural targets with azimuthal symmetry, the copolarized and cross-polarized components of the scattering matrix are uncorrelated. This method does not require any external calibration targets to be deployed. Since compressed data are used, it is necessary to model the transmitting and receiving systems as reciprocal. The method is used to estimate the crosstalk parameters of the NASA/JPL aircraft for different types of terrain and for two frequencies. For the C-band systems the crosstalk is less than -20 dB for all ranges in the images. The crosstalk of the L-band system is a function of range, however, and may be as poor as -10 dB in the near range, leading to a noticeable distortion of the polarization signatures.

Description: A technique for estimation of the Doppler centroid of an SAR in the presence of large uncertainty in antenna boresight pointing is described. Also investigated is the image degradation resulting from data processing that uses an ambiguous centroid. Two approaches for resolving ambiguities in Doppler centroid estimation (DCE) are presented: the range cross-correlation technique and the multiple-PRF (pulse repetition frequency) technique. Because other design factors control the PRF selection for SAR, a generalized algorithm is derived for PRFs not containing a common divisor. An example using the SIR-C parameters illustrates that this algorithm is capable of resolving the C-band DCE ambiguities for antenna pointing uncertainties of about 2-3 deg.

Description: A theoretical analysis of polarization filtering for the bistatic case is developed for optimum discrimination between two types of targets. The resulting method is half analytical and half numerical. Because it is based on the Stokes matrix representation, the targets of interest can be extended targets. The scattered field from such targets is partially polarized. This method is then applied to the monostatic case with numerical examples relying on the JPL (Jet Propulsion Laboratory) full-polarimetric L-band radar data. A matched filter to maximize the power ratio between urban and natural targets is developed. The results show that the same filter is optimal for both ocean and forest targets as natural targets.

Description: The unique characteristics of a spaceborne SAR (synthetic aperture radar) operating in a squint mode include large range walk and large variation in the Doppler centroid as a function of range. A pointing control technique to reduce the Doppler drift and a new processing algorithm to accommodate large range walk are presented. Simulations of the new algorithm for squint angles up to 20 deg and look angles up to 44 deg for the Earth Observing System (Eos) L-band SAR configuration demonstrate that it is capable of maintaining the resolution broadening within 20 percent and the ISLR within a fraction of a decibel of the theoretical value.

Description: The composition of lunar regolith and its attendant properties are discussed. Tables are provided listing lunar minerals, the abundance of plagioclase feldspar, pyroxene, olivine, and ilmenite in lunar materials, typical compositions of common lunar minerals, and cumulative grain-size distribution for a large number of lunar soils. Also provided are charts on the chemistry of breccias, the chemistry of lunar glass, and the comparative chemistry of surface soils for the Apollo sites. Lunar agglutinates, constructional particles made of lithic, mineral, and glass fragments welded together by a glassy matrix containing extremely fine-grained metallic iron and formed by micrometeoric impacts at the lunar surface, are discussed. Crystalline, igneous rock fragments, breccias, and lunar glass are examined. Volatiles implanted in lunar materials and regolith maturity are also addressed.

Description: Theoretical models of planetary-ring dynamics are discussed in a detailed analytical review and illustrated with graphs and diagrams. The streamline concept is introduced, and the phenomena associated with the transport of angular momentum are described. Particular attention is then given to (1) broad rings like those of Saturn (shepherding, density-wave excitation, gaps, bending-wave excitation, multiringlet structures, inner-edge shepherding, and the possibility of polar rings around Neptune), (2) narrow rings like those of Uranus (shepherding, ring shapes, and a self-gravity model of rigid precession), and (3) ring arcs like those seen in stellar-occultation observations of Neptune.

Description: A wavelength that lies within a spectral interval of reduced solar emission (a Fraunhofer line) can carry optical communications with reduced interference from direct or reflected background sunlight. Suitable Fraunhofer lines are located within the tuning range of good candidate lasers. The laser should be tunable dynamically to track Doppler shifts in the sunlight incident on any solar system body that may appear in the background as viewed by the receiver. A Fraunhofer filter used with a direct-detection receiver should be tuned to match the Doppler shifts of the source and background. The required tuning calculated here for various situations is also required if, instead, one uses a heterodyne receiver with limited post-detection bandwidth.

Description: Images of Neptune obtained by the narrow-angle camera of the Voyager 2 spacecraft reveal large-scale cloud features that persist for several months or longer. The features' periods of rotation about the planetary axis range from 15.8 to 18.4 hours. The atmosphere equatorward of -53 deg rotates with periods longer than the 16.05-hour period deduced from Voyager's planetary radio astronomy experiment (presumably the planet's internal rotation period). The wind speeds computed with respect to this radio period range from 20 meters per second eastward to 325 meters per second westward. Thus, the cloud-top wind speeds are roughly the same for all the planets ranging from Venus to Neptune, even though the solar energy inputs to the atmospheres vary by a factor of 1000.

Description: The theory of turbulent plumes maintained above steady line sources of buoyancy is worked out in detail within the limitations of Taylor's entrainment assumption. It is applied to the structure of a pure plume injected into a stably stratified atmosphere. Volcanic basalt eruptions that develop from long, narrow vents create line source plumes, which rise well above the magmatic fire fountains playing near the ground level. The eruption of Laki in 1783 may provide an example of this style of eruption. Flood basalts are more ancient examples. Evidence of enormous fissure eruptions that occurred in the past on Mars and Venus also exists. Owing to the different properties of the atmospheres on these two planets from those on the earth, heights of line source plumes are expected to vary in the ratios 1:6:0.6 (earth:Mars:Venus). It is very unlikely that the observed increase of sulfur dioxide above the Venusian cloud deck in 1978 could have been due to a line source volcanic eruption, even if it had been a flood basalt eruption.

Description: An analytical and numerical study of the stability of polar rings around Neptune is presented. The stability proofs are based on various methods used to study gas disks in galaxies. It is shown that stable polar rings can exist despite energy dissipation by collisions between particles. Also, four equilibrium orientations which pass nearly over the pole of Neptune are found, two of which are stable in the presence of dissipation.

Description: Issues related to the establishment of lunar observatories are briefly addressed. The advantages of placing an observatory in a crater at one of the poles, where permanent darkness exists, are pointed out, and the methods required to emplace and operate such an observatory are considered. Planning for the installation of the first set of observatory instruments is discussed.

Description: The authors investigate microwave imaging of metallic objects using a diversity method and interpret and predict the reconstructed image from an approach based on analysis of the scattering mechanism and a back-projection algorithm used in image retrieval. The connection between the various scattering mechanisms and the reconstructed images is discussed, what the images represent is interpreted, and a prediction is made as to what the image will look like over given spectral and angular windows. A brief description is given of the microwave diversity imaging system and the formulation of the microwave diversity imaging based on the physical optics approximation. The scattering mechanism of a complex shaped metallic object is then briefly reviewed and an alternate approach to interpreting the reconstructed image based on the understanding of the scattering mechanism and the reconstruction algorithm is given. Several numerical and experimental examples are included to support this interpretation approach.

Description: An examination is presented of three techniques used for the efficient computation of fields diffracted by a subreflector that has been shaped by geometrical optics synthesis. It is found that these techniques, which are based on the geometrical theory of diffraction (GTD), produce errors in the computed fields that are specific to shaped reflectors. These errors are examined for a reflector system shaped to produce maximum gain from a tapered feed illumination. The discrepancies are directly related to the caustic being located near an observation point of the GTD calculations. The errors found are localized, and they increase in magnitude as the caustic approaches the main reflector. In a general offset geometry, the location of the caustic may be located arbitrarily close to the main reflector given a prescribed output aperture distribution. For the specific case considered here-the common situation of shaping to produce maximum gain-the caustic is located near the edge of the main reflector and on the reflection shadow boundary. A local correction is derived which creates a uniform solution through the caustic and across the reflection shadow boundary. Away from this point the calculation receeds to the standard GTD solution.

Description: The effect of filtering on the signal-to-noise ratio (SNR) of a coherently demodulated band-limited signal is determined in the presence of worst-case amplitude ripple. The problem is formulated as an optimizaton in the Hilbert space L2. The form of the worst-case amplitude ripple is specified, and the degradation in the SNR is derived in closed form. It is shown that, when the maximum passband amplitude ripple is 2Delta (peak-to-peak), the SNR is degraded by at most (1-Delta-squared), even when the ripple is unknown or uncompensated. For example, an SNR loss of less than 0.01 dB due to amplitude ripple can be assured by keeping the amplitude ripple under 0.42 dB.

Description: The phenomena of the Saturn system discovered by the Voyager missions are addressed. The characteristics of the rings, including grooves, spikes, and warps, are described, and the discovery of bombarded rings and young rings is discussed. The unique and unexplained properties of Enceladus are summarized, and the possibility that some moons of Saturn have been reassembled is addressed. The interaction of moons with the rings is examined, and the possibility that Titan's atmosphere may contain complex molecules that are the precursors of life is considered.

Description: Spectral data from earth observations have indicated the presence of N2 and CH4 on Triton. This paper outlines the use of the 1-D radiative-convective model developed for Titan to calculate the current pressure of N2 and CH4 on Triton. The production of haze material is obtained by scaling down from the Titan value. Results and predictions for the Voyager Triton encounter are as follows: A N2-CH4 atmosphere on Triton is thermodynamically self consistent and would have a surface pressure of approximately 50 millibar; due to the chemically produced haze, Triton has a hot atmosphere with a temperature of approximately 130 K; Triton's troposphere is a region of saturation of the major constituent of the atmosphere, N2.

Description: The process by which ambient gases can be implanted into silicates by shocks was investigated by analyzing the noble-gas content of several experimentally and naturally shocked silicate samples. The retentivity of shock-implanted gas during stepwise heating in the laboratory was defined in terms of two parameters, namely, the activation energy for diffusion and the extraction temperature at which 50 percent of the gas is released, both of which correlate with the shock pressure. The experiments indicate that, with increasing shock pressure, gas implantation occurs through an increasing production of microcracks/defects in the silicate lattice. The degree of annealing of these defects control the degree of diffusive loss of implanted gas.

Description: Radar cross section (RCS) reduction by absorber covering is experimentally studied by employing microwave diversity imaging. Experimental results show that broadband absorber covering is not effective at reducing the co-polarized (the transmitting and receiving antennas have opposite sense of circular polarization) RCS of a plate when the incident wave approximates the edge-on direction but is effective at reducing the cross-polarized (both the transmitting and receiving antennas have the same sense of circular polarization) RCS for all incident directions. The surface current absorber covering is effective at reducing the nonspecular energy and multiple bounces regardless of the polarization status of the measurement.

Description: The long-term modulation of Saturn's nonthermal radio emission in the kilometric wavelength range has been studied based upon data obtained by Voyagers 1 and 2. A comparison of the ballistic and hydrodynamic propagation of solar wind features from the spacecraft to Saturn allows the uncertainty inherent in the projection to be determined. The results confirm the previous suggestion that momentum, ram pressure, and kinetic energy flux are the primary solar wind parameters that drive the nonthermal radio emission. It is suggested that, under certain conditions and for limited periods of time, the magnetic properties and time derivatives of the solar wind have increased importance.

Description: It has been observed from the plasma experiments on the Pioneer Venus Orbiter that the altitude of the upper boundary of the ionosphere decreases in response to increasing solar wind dynamic pressure. However, at pressures above about 2.5 x 10 to the -8th dynes/sq cm, the further decrease in the ionopause height is rather small. Following the model of Cloutier et al. (1969), it is suggested that during high solar wind conditions, when the ionopause is formed at lower altitudes, the solar wind induces vertical and horizontal flows which sweep away the ionospheric plasma that is produced locally by photoionization. As a result, a disturbed photodynamical ionosphere is formed which has the scale height of the ionizable neutral constituent. It is shown that such a photodynamical ionosphere is observed at the subsolar ionopause under these conditions. As a consequence of this interaction, the ionopause altitude is observed to follow the small-scale height of the ionizable species, atomic oxygen, showing only small changes with solar wind pressure.

Description: Asteroids appear in light of telescopic and meteority studies to be the most accessible repositories of early solar system history available. In the cooler regions of the outer asteroid belt, apparently unaffected by severe heating, the C, P, and D populations appear to harbor significant inventories of volatiles; the larger primordial belt population may have had an even greater percentage of volatile-rich, low-albedo asteroids, constituting a potent asteroid for veneering early terrestrial planet atmospheres. The volatile-rich asteroids contain carbon, structurally bound and adsorbed water, as well as remnants of interstellar material predating the solar system.

Description: The development and implementation of the Voyager missions are reviewed. The interplanetary missions preceding Voyager are discussed, focusing on the technological development leading up to the Voyager spacecraft. The main results from Voyager observations of Jupiter, Saturn, and Uranus are outlined. Also, consideration is given to the prospects for observations of Neptune.

Description: Isotopic analysis of nesquehonite recovered from the surface of the LEW 85320 H5 ordinary chondrite shows that the delta C-13 and delta O-18 values of the two generations of bicarbonate (Antarctic and Texas) are different: delta C-13 = + 7.9 per mil and + 4.2 per mil; delta O-18 = + 17.9 per mil and + 12.1 per mil, respectively. Carbon isotopic compositions are consistent with equilibrium formation from atmospheric carbon dioxide at - 2 + or - 4 C (Antarctic) and + 16 + or - 4 C (Texas). Oxygen isotopic data imply that the water required for nesquehonite precipitation was derived from atmospheric water vapor or glacial meltwater which had locally exchanged with silicates, either in the meteorite or in underlying bedrock. Although carbonates with similar delta C-13 values have been identified in the SNC meteorites EETA 79001 and Nakhla, petrographic and temperature constraints argue against their simply being terrestrial weathering products.

Description: The gain of circularly polarized (CP) array antennas realized by proper phasing of sequentially rotated linearly polarized (LP) elements is compared to that of arrays using CP elements and demonstrated by calculations for microstrip patch elements. When element spacing is large and array size is small, the advantages of LP elements are offset by the significant reduction in gain due to high cross polarized lobes in the diagonal planes. For large arrays of closely spaced elements, this gain loss reduces to a negligible amount. However, for spacings above a critical value of about 0.7 wavelengths, unacceptably high gain losses will be incurred.

Description: The use of an imaging radar polarimeter data for unsupervised classification of scattering behavior is described by comparing the polarization properties of each pixel in a image to that of simple classes of scattering such as even number of reflections, odd number of reflections, and diffuse scattering. For example, when this algorithm is applied to data acquired over the San Francisco Bay area in California, it classifies scattering by the ocean as being similar to that predicted by the class of odd number of reflections, scattering by the urban area as being similar to that predicted by the class of even number of reflections, and scattering by the Golden Gate Park as being similar to that predicted by the diffuse scattering class. It also classifies the scattering by a lighthouse in the ocean and boats on the ocean surface as being similar to that predicted by the even number of reflections class, making it easy to identify these objects against the background of the surrounding ocean. The algorithm is also applied to forested areas and shows that scattering from clear-cut areas and agricultural fields is mostly similar to that predicted by the odd number of reflections class, while the scattering from tree-covered areas generally is classified as being a mixture of pixels exhibiting the characteristics of all three classes, although each pixel is identified with only a single class.

Description: A primary concern in the formation of a multinational Mars-exploration program is that of technology transfer, which would be most acute in the U.S./U.S.S.R. case but would exist in lesser degree among other nations. Another concern is that of management-complexity, which could inflate total costs and substantially counteract the anticipated benefits of spreading program costs among a number of nations. It is presently suggested that these problems can be substantially reduced by having each nation become responsible for one (or more) complete flight system. Each nation's role in mission operations must be clear, and the cleanliness of interfaces among flight systems must carry over into mission operations.

Description: The electromagnetic properties of a structure that is both chiral and periodic are investigated using coupled-mode equations. The periodicity is described by a sinusoidal perturbation of the permittivity, permeability, and chiral admittance. The coupled-mode equations are derived from physical considerations and used to examine bandgap structure and reflected and transmitted fields. Chirality is observed predominantly in transmission, whereas periodicity is present in both reflection and transmission.

Description: A method is presented for analyzing a finite planar array of circular microstrip patches fed by coaxial probes. The self- and mutual impedances between array elements are calculated using the method of moments with the dyadic Green's function for a dielectric layer on a ground plane. The patch circuits are determined by using the reaction integral equation. The active input impedance as well as the active element pattern of the array are computed from a knowledge of the resultant patch currents. The calculated results for two-element and eight-element linear arrays are in good agreement with experimental data. The active reflection coefficient and element pattern for the center and edge elements of a two-dimensional array as a function of scan angle are also presented.

Description: The issue of reducing the cost of phased array vehicle antennas through the use of a lens feeding arrangement instead of phase shifters at each element is addressed. In particular, the economic viability of a mobile satellite system (MSAT) is largely dependent on the efficient use of the allocated scarce spectrum and orbit as well as the satellite power. the type of vehicle antenna used will play a critical role in achieving this efficiency. A standard design approach for an electronically steered array uses phase shifters at each element to provide beam steering. A method for reducing the required number of phase shifters by using an R-KR lens feed network is outlined. The authors briefly discuss the phase shifter approach to beam steering, examine various lens feed techniques, and describe the R-KR lens approach. The lens feed network architecture is examined, a computer model for simulation of the array is presented, and the results of analysis of a suggested design for the MSAT application are given. In addition, satellite acquisition and tracking considerations are investigated.

Description: Slot timing recovery in a direct-detection optical PPM communication system can be achieved by processing the photodetector output waveform with a nonlinear device whose output forms the input to a phase-locked loop. The choice of a simple transition detector as the nonlinearity is shown to give satisfactory synchronization performance. The rms phase error of the recovered slot clock and the effect of slot timing jitter on the bit error probability were directly measured. The experimental system consisted of an AlGaAs laser diode (wavelength = 834 nm) and a silicon avalanche photodiode photodetector. The system used Q = 4 PPM signaling and operated at a source data rate of 25 Mbits/s. The mathematical model developed to compute the rms phase error of the recovered clock is shown to be in good agreement with results of actual measurements of phase errors. The use of the recovered slot clock in the receiver resulted in no significant degradation in receiver sensitivity compared to a system with perfect slot timing. The system achieved a bit error probability of 10 to the -6th at a received optical signal energy of 55 detected photons per information bit.

Description: Observations of the stratospheric IR emission structure on Saturn are presented. The high-spatial-resolution global images show a variety of new features, including a narrow equatorial belt of enhanced emission at 7.8 micron, a prominent symmetrical north polar hotspot at all three wavelengths, and a midlatitude structure which is asymmetrically brightened at the east limb. The results confirm the polar brightening and reversal in position predicted by recent models for seasonal thermal variations of Saturn's stratosphere.

Description: An overview of the Voyager 2 encounter with Neptune is presented, including a brief discussion of the trajectory, the planned observations, and highlights of the results described in the 11 companion papers. Neptune's blue atmosphere has storm systems reminiscent of those in Jupiter's atmosphere. An optically thin methane ice cloud exists near the 1.5-bar pressure level, and an optically thick cloud exists below 3 bars. Neptune's magnetic field is highly tilted and offset from the planet's center; it rotates with a period of 16.11 hours. Two narrow and two broad rings circle the planet; the outermost of these rings has three optically thicker arc segments. Six new moons were discovered in circular prograde orbits, all well inside Triton's retrograde orbit. Triton has a highly reflective and geologically young surface, a thin nitrogen atmosphere, and at least two active geyser-like plumes.

Description: Recent models of the internal structure of Pluto and Charon, made possible by analysis of the Pluto/Charon mutual events are reviewed. At a mean density of just over 2 g/cu cm and a predicted rock mass fraction of roughly 0.7, the Pluto/Charon system is significantly rockier than the icy satellites of the giant planets, a contrast which may reflect its formation in a CO-rich outer solar nebula rather than a circumplanetary nebula. Pluto and Charon may in fact be so rocky that they lost volatiles early in their history (possibly during a Charon-forming impact event), although this is still an open issue.

Description: Preliminary results from electric field measurements in the environment of Mars using the plasma-wave system on board Phobos 2 are reported. Electron-plasma oscillations observed upstream of the bow shock correspond to a solar-wind density of 2/cu cm. The shock-foot boundary was crossed up to three times on each orbit. The shock ramp was detected at altitudes between 0.45 and 0.75 Mars radii R(M) above the planetary surface. The density increased by about a factor of two at the ramp. The shock position, although variable, seems to be consistent with previous measurements. The downstream magnetosheath contained broadband electric-field noise below the plasma frequency. The boundary of th obstacle, or plasmapause, was crossed at altitudes of the order of 0.28 R(M); the cold plasma density was highly variable within the planetopause and reached the unexpected value of 700/cu cm on the third orbit, at 0.25 R(M) altitude. Bursts of waves with frequencies below the electron cyclotron frequency occur within the planetopause.

Description: The magnetic fields of Mars have been measured from Phobos 2 with high temporal resolution in the tail and down to an 850-km altitude. During four successive highly elliptical orbits, the position of the bow shock as well as that of a transition layer, the 'planetopause', were identified. Subsequent circular orbits at 6000-km altitude provided the first high-resolution data in the planetary tail and indicate that the interplanetary magnetic field mainly controls the magnetic tail. Magnetic turbulence was also detected when the spacecraft crossed the orbit of Phobos, indicating the possible existence of a torus near the orbit of this moon.

Description: The moment method solution to the problem of a reactively loaded circular patch is presented. Using the reaction integral equation in conjuction with the method of moments, parameters of the Thevenin's equivalent network for the loaded patch are obtained. From the equivalent network parameters an expression for the imput impedance of the loaded patch is derived. A design procedure for a circularly polarized disk antenna is presented. Computed results are compared with the experimental data.

Description: The formation of the ice-rock cores of the giant planets by density wave-assisted accretion is outlined. The process could be rapid (100,000-1,000,000 yr) and completed within the probable lifetime of the solar nebula. The mechanism works for both Jupiter and Saturn and does not require a large excess of mass over that believed present in their cores.

Description: This paper identifies and documents several well-preserved examples of Martian strike-slip faults and examines their relationships to wrinkle-ridges. The strike-slip faulting predates or overlaps periods of wrinkle-ridge growth southeast of Valles Marineris, and some wrinkle ridges may have nucleated and grown as a result of strike-slip displacements along the echelon fault arrays. Lateral displacements of several km inferred along these arrays may be related to tectonism in Tharsis.

Description: The use of commercially available 1-bit LSI correlator chips whose main function is the correlation of sequences of plus ones and minus ones (rather than zeros and ones) as building blocks in the design of multibit correlators is investigated. A radix-2 number system in which the bit values are + or - 1 (no zero) is discussed. The development of a simple conversion algorithm to find the + or - 1 representation of a number given in 2's complement is presented. Some of the issues raised by this conversion algorithm are discussed. It is shown that, if the analog signal is available, there is a simple step to take before the analog-to-digital conversion in order to improve the overall precision. The construction of multibit correlators based on the + or - 1 system is compared to alternative approaches.

Description: Results are presented on the long-term behavior of the main peak electron density in the Venus ionosphere during the solar cycle 21, based on 104 radio occultation measurements of the vertical electron density profile in the dayside ionosphere of Venus carried out aboard the Pioneer Venus Orbiter spacecraft (along with published data on 11 Venera 9-10 measurements). The equation representing the electron temperature at h = 140 km is presented. The results imply that the electron temperature at h = 140 km decreased by about 25 percent from solar maximum to solar minimum, compared to a decrease of 50-75 percent above 200 km found by Kliore and Mullen (1989).

Description: Theoretical models of early Martian atmospheric evolution describe the maintenance of a dense CO2 atmosphere and a warm, wet climate until the end of the heavy-bombardment phase of impacting. However, the presence of very young, earthlike fluvial valleys on the northern flank of Alba Patera conflicts with this scenario. Whereas the widespread ancient Martian valleys generally have morphologies indicative of sapping erosion by the slow outflow of subsurface water, the local Alba valleys were probably formed by surface-runoff processes. Because subsurface water flow might be maintained by hydrothermal energy inputs and because surface-runoff valleys developed late in Martian history, it is not necessary to invoke drastically different planet-wide climatic conditions to explain valley development on Mars. The Alba fluvial valleys can be explained by hydrothermal activity or outflow-channel discharges that locally modified the atmosphere, including precipitation and local overland flow on low-permeability volcanic ash.

Description: On the Mars rover sample-return mission, the rover vehicle will collect and select samples from different locations on the Martian surface to be brought back to earth for laboratory studies. It is anticipated that an in situ energy-dispersive X-ray fluorescence (XRF) spectrometer will be on board the rover. On such a mission, sample selection is of higher priority than in situ quantitative chemical anlaysis. With this in mind, a pattern recognition technique is proposed as a simple, direct, and speedy alternative to detailed chemical analysis of the XRF spectra. The validity and efficacy of the pattern recognition technique are demonstrated by the analyses of laboratory XRF spectra obtained from a series of geological samples, in the form both of standardized pressed pellets and as unprepared rocks. It is found that pattern recognition techniques applied to the raw XRF spectra can provide for the same discrimination among samples as a knowledge of their actual chemical composition.

Description: The present photoelectric lightcurve observations of 44 Nysa at one aspect, and 64 Angelina at both polar aspects, indicate the presence of an opposition spike or brightening of approximately 0.25 mag within several deg of zero-phase angle. Since the three curves are identical within the bounds of observational scatter, the opposition spike cannot be judged anomalous within the asteroid taxonomic class E. The general similarity of the observed phase curves to those of the Uranian satellites and the rings of Saturn supports the status of the spike as an ordinary property of moderate-to-high albedo atmosphereless surfaces.

Description: In order to ascertain the features of organic compound-production in planetary atmospheres under the effects of plasmas and shocks, various mixtures of N2, CH4, and H2 modeling the atmosphere of Titan were subjected to discrete sparks, laser-induced plasmas, and UV radiation. The experimental results obtained suggest that UV photolysis from the plasma is an important organic compound synthesis process, as confirmed by the photolysis of gas samples that were exposed to the light but not to the shock waves emitted by the sparks. The thermodynamic equilibrium theory is therefore incomplete in the absence of photolysis.

Description: An account is given of the results of photometric light curve observations in the V band for asteroids, yielding estimates of the mean and maximum reduced magnitudes for each object. On the basis of fits of the H-G magnitude relation for 33 objects, the mean values of the slope parameter are examined for different taxonomic classes; these values are then applied to the analysis of less complete data sets. While in the case of the moderate albedo S and M class asteroids the H-G relation appears to fit the available data, the relation for dark asteroids appears to predict more of an opposition than is characteristically present.

Description: Voyager imaging, radio occultation, and stellar occultation data for the regular structure of Saturn's inner Cassini Division are presently analyzed. The regular optical depth variation observed by the radio occultation experiment scan and the feature noted in Voyager images is the same structure, namely the gravitational wakes of two 10-km radius satellites orbiting within the division. The structure is azimuthally symmetric, and is judged to rule out the possibility that large moonlets may be responsible for the observed structure.

Description: TEM and IR spectroscopy investigations of the interplanetary dust particles (IDPs) collected in the stratosphere have shown the majority of IDPs in the layer-lattice silicate and pyroxene classes to not have been heated to temperatures above 600 C during atmospheric entry. This implies that they arrive at the upper atmosphere with low geocentric encounter velocities, and limits the possible encounter trajectories for these particles to relatively circular prograde orbits. On this basis, it is judged unlikely that these IDPs are from earth-crossing comets or asteroids; collected IDPs dominated by olivine include a larger portion of above-600 C-heated particles, suggesting their capture from more eccentric orbits.

Description: The present examination of the west Deuteronilus Mensae region of Mars notes the changes in fretted terrain across the gradational boundary from uplands to lowlands to include a reduction of canyon wall slopes and depths, so that the fretted terrain north of the gradational boundary appears matted, but not obscured. The two process-classes that may account for the lateral overlap are the erosion of stratified upland terrain, and the deposition of plains materials onto the sloping upland margin and fretted terrain. A variety of plains-emplacement mechanisms is considered.

Description: The compositional contrast between the giant-planet satellites and the significantly rockier Pluto/Charon system is indicative of different formation mechanisms; cosmic abundance calculations, in conjunction with an assumption of the Pluto/Charon system's direct formation from solar nebula condensates, strongly suggest that most of the carbon in the outer solar nebula was in CO form, in keeping with both the inheritance from the dense molecular clouds in the interstellar medium, and/or the Lewis and Prinn (1980) kinetic-inhibition model of solar nebula chemistry. Laboratory studies of carbonaceous chondrites and Comet Halley flyby studies suggest that condensed organic material, rather than elemental carbon, is the most likely candidate for the small percentage of the carbon-bearing solid in the outer solar nebula.

Description: An attempt is made to provide a constraint on the combined mass of Janus and Epimetheus from an analysis of Voyager I and Voyager 2 data and ground-based observations obtained during the 1966 and 1980 ring plane crossings. The results of the analysis presented here suggest that the total mass is 2.59 + or - 0.26 x 10 to the 21st g, the mass ratio is 3.61 + or - 0.01, and Janus' density is 0.67 + or - 0.10 g/cu cm. The low density of Janus is attributed to its porosity rather than composition.

Description: A coordinated multinational Mars Surveyor Program involving a series of robotic missions is proposed in order to select worthwhile human landing sites for discoveries, for safe operations, and for testing and proving technologies for making human flight to Mars possible. Some characteristics motivating exploration of the planet are briefly discussed, including the possibility of life, geological features, and meteorological conditions. The necessity for preliminary exploration of Mars by robots prior to human exploration is discussed, and the rationale behind a multinational approach for a Mars Surveyor Program is presented.

Description: Arecibo high-resolution (1.5 to 2 km) radar data of Venus for the area extending from Beta Regio to western Eisila Regio provide strong evidence that the mountains in Beta and Eisila Regiones and plains in and adjacent to Guinevere Planitia are of volcanic origin. Recognized styles of volcanism include large volcanic edifices on the Beta and Eisila rises related to regional structural trends, plains with multiple source vents and a mottled appearance due to the ponding of volcanic flows, and plains with bright features surrounded by extensive quasi-circular radar-dark halos. The high density of volcanic vents in the plains suggests that heat loss by abundant and widely distributed plains volcanism may be more significant than previously recognized. The low density of impact craters greater than 15 km in diameter in this region compared to the average density for the higher northern latitudes suggests that the plains have a younger age.

Description: In February and March 1989 the Phobos 2 spacecraft took 37 television images of Phobos from a distance of 190-1100 km. The data are being used to update the three-dimensional model of Phobos, to provide improved determinations of its density and orbital dynamics, and to study its surface color, composition, and texture. Preliminary findings are presented here which include different integrated photometric behavior in visible and near-infrared bands, observation of a region immediately west of Stickney which is relatively free of large grooves, the prevalence of bright rims on grooves and younger craters, and low bulk density.

Description: The nearly ubiquitous presence of a negative Eu anomaly in the mare basalts has been suggested to indicate prior separation and flotation of plagioclase from the basalt source region during its crystallization from a lunar magma ocean (LMO). Are there any mare basalts derived from a mantle source which did not experience prior plagioclase separation? Crystal chemical rationale for REE substitution in pyroxene suggests that the combination of REE size and charge, M2 site characteristics of pyroxene, fO2, magma chemistry, and temperature may account for the negative Eu anomaly in the source region of some types of primitive, low TiO2 mare basalts. This origin for the negative Eu anomaly does not preclude the possibility of the LMO as many mare basalts still require prior plagioclase crystallization and separation and/or hybridization involving a KREEP component.

Description: Pioneer Venus ion composition measurements are used to study the Venus ionosphere during solar minimum. It is suggested that the topside electron density profile at Venus during solar minimum has two distinct regimes. One beween 140 and 180 km is dominated by O2(+) ions which are in photochemical equilibrium. The other regime is above 180 km and is dominated by O(+) ions which are disturbed by the solar wind induced plasma transport. For Pioneer Venus, Mariner 10, and Venera 9 and 10 data, it is found that Venus exhibits a photodynamical type of ionopause during solar minimum.

Description: Voyager 2's IR observations of Neptune encompass thermal emissions and broadband radiometer measurements of reflected solar radiation. Temperature maps were obtained for the planet between 80 deg S and 30 deg N for two atmospheric layers, one in the lower stratosphere and the other in the troposphere. The relatively warm pole and equator, with cooler midlatitudes, are qualitatively similar to Uranus, despite the two planets' very different obliquities and internal heat fluxes. Powerful wavelike longitudinal thermal structure is noted, of which some appears to be associated with the Great Dark Spot; a localized cold region uncorrelated with any visible feature is found in the lower stratosphere.

Description: Detection of very intense short radio bursts from Neptune was possible as early as 30 days before closest approach and at least 22 days after closest approach. The bursts lay at frequencies in the range 100 to 1300 kilohertz, were narrowband and strongly polarized, and presumably originated in southern polar regions of the planet. Episodes of smooth emissions in the frequency range from 20 to 865 kilohertz were detected during an interval of at least 10 days around closest approach. The bursts and the smooth emissions can be described in terms of rotation in a period of 16.11 + or - 0.05 hours. The bursts came at regular intervals throughout the encounter, including episodes both before and after closest approach. The smooth emissions showed a half-cycle phase shift between the five episodes before and after closest approach. This experiment detected the foreshock of Neptune's magnetosphere and the impacts of dust at the times of ring-plane crossings and also near the time of closest approach. Finally, there is no evidence for Neptunian electrostatic discharges.

Description: Based on the success of several two-dimensional (latitude, longitude) linear barotropic instability models at matching some of the observed characteristics of the cloud level, polar region of the Venus atmosphere, a more realistic, linear, three-dimensional (height, latitude and longitude) model has been developed to further test the hypothesis that the observed features can be described by linear instability theory. The approach taken is to vary the model input parameters to see whether it is possible to produce modes that resemble the observations of wave activity and to compare those input parameters with other observations of the mean state. Sensitivity studies show that in addition to a well-documented dependence on the mean zonal wind, the growth and propagation of unstable modes depends on the latitude variation of the mean temperature (and hence static stability). These studies have led to the specification of a model basic state wind and temperature field that produces modes which are matched to observations of spatial structure, preferred wavenumber and phase speed of the polar disturbances. Wavenumber 2 is found to have the shortest growth time and unlike the two-dimensional results, wavenumbers 1-3 share a nearly common period of about 3 days. The derived basic state has a temperature field that is quite similar to Pioneer Venus observations; however, in some regions the model basic state wind field departs from cyclostrophic values based on temperature observations.

Description: It is appropriate to note that 1988 marks the 100th anniversary of the birth of the reflector antenna. It was in 1888 that Heinrich Hertz constructed the first one, a parabolic cylinder made of sheet zinc bent to shape and supported by a wooden frame. Hertz demonstrated the existence of the electromagnetic waves that had been predicted theoretically by James Clerk Maxwell some 22 years earlier. In the 100 years since Hertz's pioneering work the field of electromagnetics has grown explosively: one of the technologies is that of remote sensing of planet Earth by means of electromagnetic waves, using both passive and active sensors located on an Earth Science Geostationary Platform (ESEP). For these purposes some exquisitely sensitive instruments were developed, capable of reaching to the fringes of the known universe, and relying on large reflector antennas to collect the minute signals and direct them to appropriate receiving devices. These antennas are electrically large, with diameters of 3000 to 10,000 wavelengths and with gains approaching 80 to 90 dB. Some of the reflector antennas proposed for ESGP are also electrically large. For example, at 220 GHz a 4-meter reflector is nearly 3000 wavelengths in diameter, and is electrically quite comparable with a number of the millimeter wave radiotelescopes that are being built around the world. Its surface must meet stringent requirements on rms smoothness, and ability to resist deformation. Here, however, the environmental forces at work are different. There are no varying forces due to wind and gravity, but inertial forces due to mechanical scanning must be reckoned with. With this form of beam scanning, minimizing momentum transfer to the space platform is a problem that demands an answer. Finally, reflector surface distortion due to thermal gradients caused by the solar flux probably represents the most challenging problem to be solved if these Large Space Antennas are to achieve the gain and resolution required of them.

Description: During a period spanning more than 5 years, low elevation radar measurements of rain were systematically obtained in the mid-Atlantic coast of the U.S. Drop size distribution measurements with a disdrometer were also acquired on the same rain days. The drop size data were utilized to convert the radar reflectivity factors to estimated rain rates for the respective rain days of operation. Applying high level algorithms to the rain data, core values of rain intensities were identified (peak rain rates), and families of rain rate isopleths analyzed. In particular, equicircle diameters of the family of isopleths enveloping peak rain intensities were statistically characterized. The presented results represents the analysis of two rain days, 12 radar scans, corresponding to 430 culled rain rate isopleths from an available data base of 22,000 contours, approximately 100 scans encompassing 17 rain days. The results presented show trends of the average rain rate vs. contour scale dimensions, and cumulative distributions of rain cell dimensions which belong to core families of precipitation.

Description: Propagation effects in the aeronautical mobile-satellite service differ from those in the fixed-satellite service and other mobile-satellite services because: small antennas are used on aircraft, and the aircraft body may affect the performance of the antenna; high aircraft speeds cause large Doppler spreads; aircraft terminals must accommodate a large dynamic range in transmission and reception; and due to their high speeds, banking maneuvers, and three-dimensional operation, aircraft routinely require exceptionally high integrity of communications, making even short-term propagation effects very important. Data and models specifically required to characterize the path impairments are discussed, which include: tropospheric effects, including gaseous attenuation, cloud and rain attenuation, fog attenuation, refraction and scintillation; surface reflection (multipath) effects; ionospheric effects such as scintillation; and environmental effects (aircraft motion, sea state, land surface type). Aeronautical mobile-satellite systems may operate on a worldwide basis, including propagation paths at low elevation angles. Several measurements of multipath parameters over land and sea were conducted. In some cases, laboratory simulations are used to compare measured data and verify model parameters. The received signals is considered in terms of its possible components: a direct wave subject to atmospheric effects, and a reflected wave, which generally contains mostly a diffuse component.

Description: In 1992, an Engineering Test Satellite 6 is scheduled to be launched by an H-2 rocket. The missions of ETS-6 are to establish basic technologies of inter-satellite communications using S-band, millimeter waves and optical beams and of fixed and mobile satellite communications using multibeam antenna on board the satellite. A plan of the experiments is introduced.

Description: The JPL Channel Simulator lab was modified to allow full duplex links and to allow the use of field propagation data for link fading. This capability will be used to test equipment for the joint AUSSAT/NASA mobile satellite experiment in July 1989.

Description: Propagation experiments for maritime, aeronautical, and land mobile satellite communications were performed using Engineering Test Satellite-Five (ETS-5). The propagation experiments are one of major mission of Experimental Mobile Satellite System (EMSS) which is aimed for establishing basic technology for future general mobile satellite communication systems. A brief introduction is presented for the experimental results on propagation problems of ETS-5/EMSS.

Description: Land-mobile satellite propagation measurements were implemented at L Band (1.5 GHz) in South-Eastern Australia during an 11 day period in October 1988. Transmissions (CW) from both the Japanese ETS-5 and INMARSAT Pacific geostationary satellites were accessed. Previous measurements in this series were performed at both L Band (1.5 GHz) and UHF (870 MHz) in Central Maryland, North-Central Colorado, and the southern United States. The objectives of the Australian campaign were to expand the data base acquired in the U.S. to another continent, to validate a U.S. derived empirical model for estimating the fade distribution, to establish the effects of directive antennas, to assess the isolation between co- and cross-polarized transmissions, to derive estimates of fade as well as non-fade durations, and to evaluate diversity reception. All these objectives were met.

Description: Results from the most recent MSAT-X field experiments, the Tower-3 Experiment and the JPL/FAA/INMARSAT MARECS-B2 Satellite Experiment, are presented. Results that distinguish the propagation environment of the tower set-up are given and explained. The configuration and flight variables of the aeronautical experiment which used an FAA aircraft and an INMARSAT satellite are described. Results that highlight the disturbances on the aeronautical satellite channel are presented. The roles of satellite-induced signal variations and of multipatch are identified and their impact on the link is discussed.

Description: Future communication systems will move toward the 20 to 30 GHz frequency range for wider bandwidth and reduced interference, and will use small earth terminals. The ESA satellite OLYMPUS is scheduled for launch this June and will be ready for use in October. OLYMPUS has 12, 20, and 30 GHz beacons. Virginia Tech and Michigan Tech are working with NASA/JPL on an OLYMPUS experiment and hardware development program. OLYMPUS beacons provide coverage of the east coast of the US sufficient for attenuation measurements. The planned hardware for the OLYMPUS experiments is illustrated. The components of the experiment are summarized. A block diagram level overview of a typical channel is given and one of the four RF front ends is shown.

Description: ACTS counters fading by resource sharing between the users. It provides a large margin only for those terminals which are at risk by unfavorable atmospheric conditions. ACTS, as an experimental satellite, provides a 5 dB clear weather margin and 10 dB additional margin via rate reduction and encoding. For the uplink, this margin may be increased by exercising uplink power control. Some of the challenges faced by the radiowave propagation community are listed. The issue of needs for the satellite are listed, both general and specific.

Description: Some information is given on the ITALSAT millimetric waves propagation experiment, which is to be conducted with the ITALSAT satellite, whose launch is foreseen for the middle of 1990. The purpose of the experiment is one of experimenting with advanced technologies and techniques employing the 20/30 GHz bands in wideband telecommunications. Among the most qualified features of this system are the multispot antenna and the exchange function performed directly onboard. Details of the experiment are given.

Description: Ground based radiometer measurements at 20.6, 31.65, and 90.0 GHz were analyzed to provide attenuation statistics, thus extending the data base of previous NAPEX studies. Using data from colocated radiosondes, comparisons of measurements and calculations of brightness temperatures are presented. The oxygen absorption model of Rosenkranz and the water vapor absorption models of Liebe and of Waters are used. Data from July 1987 at San Nicolas Island, California and from December 1987, August and November 1988 at Denver, Colorado, are included. Joint attenuation statistics at 20.6 and 31.65 GHz are presented for two locations of the Colorado Research Network for December 1987 and August 1988.

Description: The need to develop accurate models for secondary statistics of fading land mobile satellite signals has motivated a study of fading signal autocorrelations and multipath spectrum. Results of autocorrelations and power spectral densities from measured data are presented and comparisons to multipath spectrum models are made.

Description: Beam-waveguide (BWG) antennas provide multiple frequency band operations and other operational benefits for large ground-station antennas. Present design practices use diffraction analyses that ignore the presence of the BWG enclosure and may be inaccurate at lower frequency bands for ground-station antennas operating over multiple frequency bands. Introduced here is a new analysis approach that considers the presence of the BWG enclosure. Results based on the new analysis have revealed new understandings of the performance degradation mechanisms in a BWG antenna and have provided direction for potential design improvements.

Description: The benefits of improved radiometric tracking data have been studied for planetary approach within the inner Solar System using the Mars Rover Sample Return trajectory as a model. It was found that the benefit of improved data to approach and encounter navigation was highly dependent on the a priori uncertainties assumed for several non-estimated parameters, including those for frame-tie, Earth orientation, troposphere delay, and station locations. With these errors at their current levels, navigational performance was found to be insensitive to enhancements in data accuracy. However, when expected improvements in these errors are modeled, performance with current-accuracy data significantly improves, with substantial further improvements possible with enhancements in data accuracy.

Description: It is demonstrated how the use of interleaving/deinterleaving in trellis-coded modulation (TCM) systems can reduce the signal-to-noise ratio loss due to imperfect carrier demodulation references. Both the discrete carrier (phase-locked loop) and suppressed carrier (Costas loop) cases are considered and the differences between the two are clearly demonstrated by numerical results. These results are of great importance for future communication links to the Deep Space Network (DSN), especially from high Earth orbiters, which may be bandwidth limited.

Description: Thermal measurements of microwave transmitter feedhorn windows were performed using an imaging infrared radiometer. The measurement technique is described and results are presented for windows made of 0.001-in. Kapton (trademark of Dupont Chemical Co.) and 0.1-in. HTP-6 (Space Shuttle tile material). Measured and calculated temperatures agree well.

Description: The design and testing of the prototype horn for the proposed 1-megawatt radar are described. The unique features of this square horn include a multiflare design in which flare angle changes rather than corrugations are used to generate the required higher-order modes. A five-port combining section is used at the input. The design of this section and the multiflare section are described. Measured radiation patterns are in good agreement with theoretical patterns.

Description: Aperture efficiency measurements made during 1988 on the three 70-m stations (DSS-14, DSS-43, and DSS-63) at X-band (8420 MHz) and S-band (2295 MHz) have been analyzed and reduced to yield best estimates of antenna gain versus elevation. The analysis has been carried out by fitting the gain data to a theoretical expression based on the Ruze formula. Newly derived flux density and source-size correction factors for the natural radio calibration sources used in the measurements have been used in the reduction of the data. Peak gains measured at the three stations were 74.18 (plus or minus 0.10) dBi at X-band, and 63.34 (plus or minus 0.03) dBi at S-band, with corresponding peak aperture efficiencies of 0.687 (plus or minus 0.015) and 0.762 (plus or minus 0.006), respectively. The values quoted assume no atmosphere is present, and the estimated absolute accuracy of the gain measurements is approximately plus or minus 0.2 dB at X-band and plus or minus 0.1 dB at S-band (1-sigma values).

Description: The characteristics of the Synthetic Aperture Radar (SAR) antenna aboard the ESA Remote Sensing Satellite (ERS-1) are presented. The antenna is folded into a dense package for launch and is deployed in orbit. The design requirements and constraints, their impact on the design, and the resulting features of the mechanisms are discussed.

Description: The various formulations of Maxwell's equations are reviewed with emphasis on those formulations which most readily form analogies with Navier's equations. Analogies involving scalar and vector potentials and electric and magnetic field components are presented. Formulations allowing for media with dielectric and conducting properties are emphasized. It is demonstrated that many problems in electromagnetism can be solved using the NASTRAN finite element code. Several fundamental problems involving time harmonic solutions of Maxwell's equations with known analytic solutions are solved using NASTRAN to demonstrate convergence and mesh requirements. Mesh requirements are studied as a function of frequency, conductivity, and dielectric properties. Applications in both low frequency and high frequency are highlighted. The low frequency problems demonstrate the ability to solve problems involving media inhomogeneity and unbounded domains. The high frequency applications demonstrate the ability to handle problems with large boundary to wavelength ratios.

Description: The objective of any radar experiment is to determine as much as possible about the entities which scatter the radiation. This review discusses many of the various parameters which can be deduced in a radar experiment, and also critically examines the procedures used to deduce them. Methods for determining the mean wind velocity, the RMS fluctuating velocities, turbulence parameters, and the shapes of the scatterers are considered. Complications with these determinations are discussed. It is seen throughout that a detailed understanding of the shape and cause of the scatterers is important in order to make better determinations of these various quantities. Finally, some other parameters, which are less easily acquired, are considered. For example, it is noted that momentum fluxes due to buoyancy waves and turbulence can be determined, and on occasions radars can be used to determine stratospheric diffusion coefficients and even temperature profiles in the atmosphere.

Description: Fundamental methods of signal processing used in normal mesosphere stratosphere troposphere (MST) radar observations are described. Complex time series of received signals obtained in each range gate are converted into Doppler spectra, from which the mean Doppler shift, spectral width and signal-to-noise ratio (SNR) are estimated. These spectral parameters are further utilized to study characteristics of scatterers and atmospheric motions.

Description: The correlation and spectral analysis methods for uniformly sampled stationary random signals, estimation of their spectral moments, and problems arising due to nonstationary are reviewed. Some of these methods are already in routine use in atmospheric radar experiments. Other methods based on the maximum entropy principle and time series models have been used in analyzing data, but are just beginning to receive attention in the analysis of radar signals. These methods are also briefly discussed.

Description: Two concepts of fundamental importance are reviewed: the autocorrelation function and the frequency power spectrum. In addition, some turbulence concepts, the relationship between radar signals and atmospheric medium statistics, partial reflection, and the characteristics of noise and clutter interference are discussed.

Description: Discussed here is a kind of radar called atmospheric radar, which has as its target clear air echoes from the earth's atmosphere produced by fluctuations of the atmospheric index of refraction. Topics reviewed include the vertical structure of the atmosphere, the radio refractive index and its fluctuations, the radar equation (a relation between transmitted and received power), radar equations for distributed targets and spectral echoes, near field correction, pulsed waveforms, the Doppler principle, and velocity field measurements.

Description: The principle of pulse modulation used in the case of coherent scatter radars (MST radars) is discussed. Coherent detection and the corresponding system configuration is delineated. Antenna requirements and design are outlined and the phase-coherent transmitter/receiver system is described. Transmit/receive duplexers, transmitters, receivers, and quadrature detectors are explained. The radar controller, integrator, decoder and correlator design as well as the data transfer and the control and monitoring by the host computer are delineated. Typical operation parameters of some well-known radars are summarized.

Description: Radio telecommunication links are used for communication with deep space probes. These links consist of sinusoidal carrier signals at radio frequencies (RF) modulated with information sent between the spacecraft and the earth. This carrier signal is a very pure and stable sinusoid, typically derived from an atomic frequency standard whose frequency and phase are used to measure the radial velocity of the probe and from this and other data types derive its trajectory. This same observable can be used to search for space-time distortions cased by low frequency (0.1 to 100 MHz) gravitation radiation. How such a system works, what its sensitivity limitations are, and what potential future improvements can be made are discussed.

Description: Future space missions such as the Earth Science Geostationary Platform (ESGP) will require highly accurate antennas with apertures that cannot be launched fully formed. The operational orbits are often inaccessible to manned flight and will involve expendable launch vehicles such as the Delta or Titan. There is therefore a need for completely deployable antenna reflectors of large size capable of efficiently handling millimeter wave electromagnetic radiation. The parameters for the type of mission are illustrated. The logarithmic plot of frequency versus aperture diameter shows the regions of interest for a large variety of space antenna applications, ranging from a 1500-meter-diameter radio telescope for low frequencies to a 20-meter-diameter infrared telescope. For the ESGP, a major application is the microwave radiometry at high frequencies for atmospheric sounding. Almost all existing large antenna reflectors for space employ a mesh-type reflecting surface. Examples are shown and discussed which deal with the various structural concepts for mesh antennas. Fortunately, those concepts are appropriate for creating the very large apertures required at the lower frequencies for good resolution. The emphasis is on the structural concepts and technologies that are appropriate to fully automated deployment of dish-type antennas with solid reflector surfaces. First the structural requirements are discussed. Existing concepts for fully deployable antennas are then described and assessed relative to the requirements. Finally, several analyses are presented that evaluate the effects of beam steering and segmented reflector design on the accuracy of the antenna.