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NASA NTRS2004

Amplitude Scintillation due to Atmospheric Turbulence for the Deep Space Network Ka-Band Downlink

Fast amplitude variations due to atmospheric scintillation are the main concerns for the Deep Space Network (DSN) Ka-band downlink under clear weather conditions. A theoretical study of the amplitude scintillation variances for a finite aperture antenna is presented. Amplitude variances for weak scattering scenarios are examined using turbulence theory to describe atmospheric irregularities. We first apply the Kolmogorov turbulent spectrum to a point receiver for three different turbulent profile models, especially for an exponential model varying with altitude. These analytic solutions then are extended to a receiver with a finite aperture antenna for the three profile models. Smoothing effects of antenna aperture are expressed by gain factors. A group of scaling factor relations is derived to show the dependences of amplitude variances on signal wavelength, antenna size, and elevation angle. Finally, we use these analytic solutions to estimate the scintillation intensity for a DSN Goldstone 34-m receiving station. We find that the (rms) amplitude fluctuation is 0.13 dB at 20-deg elevation angle for an exponential model, while the fluctuation is 0.05 dB at 90 deg. These results will aid us in telecommunication system design and signal-fading prediction. They also provide a theoretical basis for further comparison with other measurements at Ka-band.

Ho, C.↗

NASA NTRS2004

Wavefront Control Testbed (WCT) Experiment Results

The Wavefront Control Testbed (WCT) was created to develop and test wavefront sensing and control algorithms and software for the segmented James Webb Space Telescope (JWST). Last year, we changed the system configuration from three sparse aperture segments to a filled aperture with three pie shaped segments. With this upgrade we have performed experiments on fine phasing with line-of-sight and segment-to-segment jitter, dispersed fringe visibility and grism angle;. high dynamic range tilt sensing; coarse phasing with large aberrations, and sampled sub-aperture testing. This paper reviews the results of these experiments.

Burns, Laura A.↗

NASA NTRS2004

Progress in NEXT Ion Optics Modeling

Results are presented from an ion optics simulation code applied to the NEXT ion thruster geometry. The error in the potential field solver of the code is characterized, and methods and requirements for reducing this error are given. Results from a study on electron backstreaming using the improved field solver are given and shown to compare much better to experimental results than previous studies. Results are also presented on a study of the beamlet behavior in the outer radial apertures of the NEXT thruster. The low beamlet currents in this region allow over-focusing of the beam, causing direct impingement of ions on the accelerator grid aperture wall. Different possibilities for reducing this direct impingement are analyzed, with the conclusion that, of the methods studied, decreasing the screen grid aperture diameter eliminates direct impingement most effectively.

Emhoff, Jerold W.↗

NASA NTRS2004

Formation Flying: The Future of Remote Sensing from Space

Over the next two decades a revolution is likely to occur in how remote sensing of Earth, other planets or bodies, and a range of phenomena in the universe is performed from space. In particular, current launch vehicle fairing volume and mass constraints will continue to restrict the size of monolithic telescope apertures which can be launched to little or no greater size than that of the Hubble Space Telescope, the largest aperture currently flying in space. Systems under formulation today, such as the James Webb Space Telescope will be able to increase aperture size and, hence, imaging resolution, by deploying segmented optics. However, this approach is limited as well, by our ability to control such segments to optical tolerances over long distances with highly uncertain structural dynamics connecting them. Consequently, for orders of magnitude improved resolution as required for imaging black holes, imaging planets, or performing asteroseismology, the only viable approach will be to fly a collection of spacecraft in formation to synthesize a virtual segmented telescope or interferometer with very large baselines. This paper provides some basic definitions in the area of formation flying, describes some of the strategic science missions planned in the National Aeronautics and Space Administration, and identifies some of the critical technologies needed to enable some of the most challenging space missions ever conceived which have realistic hopes of flying.

Leitner, Jesse↗

NASA NTRS2000

Ground-to-Ground Optical Communications Demonstration

A bidirectional horizontal-path optical link was demonstrated between Strawberry Peak (SP), Lake Arrowhead, California, and the JPL Table Mountain Facility (TMF), Wrightwood, California, during June and November of 1998. The 0.6-m telescope at TMF was used to broadcast a 4-beam 780-nm beacon to SP. The JPL-patented Optical Communications Demonstrator (OCD) at SP received the beacon, performed ne tracking to compensate for the atmosphere-induced beacon motion and retransmitted a 844-nm communications laser beam modulated at 40 to 500 Mb/s back to TMF. Characteristics of the horizontal-path atmospheric channel as well as performance of the optical communications link were evaluated. The normalized variance of the irradiance fluctuations or scintillation index delta2/I at either end was determined. At TMF where a single 844-nm beam was received by a 0.6-m aperture, the measured delta2/I covered a wide range from 0.07 to 1.08. A single 780-nm beam delta2/I measured at SP using a 0.09-m aperture yielded values ranging from 0.66 to 1.03, while a combination of four beams reduced the scintillation index due to incoherent averaging to 0.22 to 0.40. This reduction reduced the dynamic range of the fluctuations from 17 to 21 dB to 13 to 14 dB as compared with the OCD tracking sensor dynamic range of 10 dB. Predictions of these values also were made based on existing theories and are compared. Generally speaking, the theoretical bounds were reasonable. Discussions on the probability density function (PDF) of the intensity fluctuations are presented and compared with the measurements made. The lognormal PDF was found to agree for the weak scintillation regime as expected. The present measurements support evidence presented by earlier measurements made using the same horizontal path, which suggests that the aperture averaging effect is better than theoretically predicted.

Biswas, A.↗

NASA NTRS1997

Super-Low-Mass Spaceborne SAR Array Concepts

For Earth remote sensing applications, a synthetic aperture radar (SAR) typically employs an antenna with a fairly long along-track aperture in order to achieve the desired performance. At orbital velocities, the antenna along-track dimension is driven by a careful trade off between resolution, swath width, and available data rate and is independent of wavelength. 10 to 20 m long antennas have been flown or proposed in previous spaceborne SAR designs. To maintain an acceptable electrical flatness across this long aperture, very massive antenna support structures, weighing several hundred kilograms or more, have been used to date. For example, the fixed-beam L-band SeaSat antenna, which used a microstrip array with honeycomb substrate, had a mass of 250 Kg (including deployment mechanism). The beam-scanning L/C/X-band shuttle-based SIR-C antenna has a mass of l,800 Kg. These massive antenna systems generally require a launch vehicle with large stowage volume and heavy-payload-lift capability. To achieve good launch volume efficiency and to reduce payload weight, three super-low-mass array concepts are proposed and described here. With these new concepts, the mass of the future Earth remote sensing SAR antennas is expected to be less than 100 Kg.

Huang, J.↗

NASA NTRS1998

A Dual-Band Dual-Polarized Array for Spaceborne SAR

Future synthetic aperture radar antennas will be significantly more sophisticated than their earlier counterparts, requiring enhanced electrical capabilities such as operation at multiple frequencies with multiple polarization ability, as well as desirable non-electrical features such as light weight, easy and reliable deployability, and low cost. The present paper describes the results of a prototype SAR array developed with these considerations in mind. This paper will describe the development of a dual-frequency array operating at L and C bands, with dual linear polarization capability at both bands. Unlike most earlier dual-band arrays, this array shares the same radiating aperture for both bands and both polarizations, resulting the smallest possible aperture area for a given gain specification. A critical constraint in this project was a requirement for an extremely light weight package, leading to the use of foam substrates with thin dielectric membranes for metalizations of the radiating elements and feed networks.

Pozar, David M.↗

NASA NTRS2006

Searching for Radial Velocity Variations in eta Carinae

A hot companion of eta Carinae has been detected using high resolution spectra (905 - 1180 A) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite (see poster by Sonneborn et al.). Analysis of the far-UV spectrum shows that eta Car B is a luminous hot star. The N II 1084-86 emission feature indicates that the star may be nitrogen rich. The FUV continuum and the S IV 1073 P-Cygni wind line suggest that the effective temperature of eta Car B is at least 25,000 K. FUV spectra of eta Carinae were obtained with the FUSE satellite at 9 epochs between 2000 February and 2005 July. The data consists of 12 observations taken with the LWRS aperture (30x30 arcsec), three with the HIRS aperture (1.25x20 arcsec), and one MRDS aperture (4x20 arcsec). In this paper we discuss the analysis of these spectra to search for radial velocity variations associated with the 5.54-year binary orbit of Eta Car AB.

Iping, R. C.↗

NASA NTRS2006

Novel In-Space Manufacturing Concepts for the Development of Large Space Telescopes

There is a continuous demand for larger, lighter, and higher quality telescopes. Over the past several decades, we have seen the evolution from launchable 2 meter-class telescopes (such as Hubble), to today s demand for deployable 6 meter-class telescopes (such as JWST), to tomorrow s need for up to 150 meter-class telescopes. As the apertures continue to grow, it will become much more difficult and expensive to launch assembled telescope structures. To address this issue, we are seeing the emergence of new novel structural concepts, such as inflatable structures and membrane optics. While these structural concepts do show promise, it is very difficult to achieve and maintain high surface figure quality. Another potential solution to develop large space telescopes is to move the fabrication facility into space and launch the raw materials. In this paper we present initial in-space manufacturing concepts to enable the development of large telescopes. This includes novel approaches for the fabrication of both the optical elements and the telescope support structure. We will also discuss potential optical designs for large space telescopes and describe their relation to the fabrication methods. These concepts are being developed to meet the demanding requirements of DARPA s LASSO (Large Aperture Space Surveillance Optic) program which currently requires a 150 meter optical aperture with a 17 degree field of view.

Mooney, James T.↗

NASA NTRS2002

Development of inflatable antenna structures

Self-rigidizable spring-tape-reinforced (STR) booms were used to develop structural systems for large-aperture space inflatable radar antennas. The lightweight planar frames built with STR booms can be compactly stowed for launch, deployed by inflation pressure in space, and used to tension the RF membrane apertures of the antennas. Engineering models of a Ka-band reflectarray antenna and a L-band synthetic-aperture radar antenna were developed and have successfully demonstrated stowage and deployment, as well as RF performance. A design improvement was also being implemented to enhance long-term configuration stability and widen the application range of the STR booms.

inflatable antenna structures deployment↗

NASA NTRS2007

A Scanning Hartmann Focus Test for the EUVI Telescopes aboard STEREO

The Solar TErrestrial RElations Observatory (STEREO), the third mission in NASA s Solar Terrestrial Probes program, was launched in 2006 on a two year mission to study solar phenomena like coronal mass ejections. STEREO consists of two nearly identical satellites, each carrying a suite of instruments that provide, among other data, simultaneous images of the Sun. One of these telescopes is the Extreme Ultraviolet Instrument (EUVI). There are two EUVI telescopes, one on each STEREO satellite (EUVI-A and EUVI-B). EUVI is a normal incidence, 98mm diameter, Ritchey-Chretien telescope designed to obtain wide field of view (approx.1deg) images of the Sun at short wavelengths (approx.20nm) using a CCD detector. The telescope entrance aperture is divided into four quadrants by a mask near the secondary mirror spider veins. A mechanism that rotates another mask allows only one of these sub-apertures to accept light from the Sun during an observation. The EUVI is thus four co-aligned, off-axis telescopes. Each off-axis segment on the primary and secondary mirrors has a different extreme ultraviolet coating stack. Furthermore, the aperture select mechanism is synchronized with a filter wheel mechanism near the CCD detector. The EUVI contains no focus mechanism. Models predict that the difference in on-orbit operating temperature and ambient clean room conditions yield a "best focus" difference between integration and operation of approx. 0.2mm.

Ohl, R.↗

NASA NTRS2007

A Scanning Hartmann Focus Test for the EUVI Telescopes aboard STEREO

The Solar TErrestrial RElations Observatory (STEREO), the third mission in NASA's Solar Terrestrial Probes program, was launched in 2006 on a two year mission to study solar phenomena. STEREO consists of two nearly identical satellites, each carrying an Extreme Ultraviolet Imager (EUVI) telescope as part of the Sun Earth Connection Coronal and Heliospheric Investigation instrument suite. EUVI is a normal incidence, 98mm diameter, Ritchey-Chretien telescope designed to obtain wide field of view images of the Sun at short wavelengths (17.1-30.4nm) using a CCD detector. The telescope entrance aperture is divided into four quadrants by a mask near the secondary mirror spider veins. A mechanism that rotates another mask allows only one of these sub-apertures to accept light over an exposure. The EUVI contains no focus mechanism. Mechanical models predict a difference in telescope focus between ambient integration conditions and on-orbit operation. We describe an independent check of the ambient, ultraviolet, absolute focus setting of the EUVI telescopes after they were integrated with their respective spacecraft. A scanning Hartmann-like test design resulted from constraints implied by the EUVI aperture select mechanism. This inexpensive test was simultaneously coordinated with other NASA integration and test activities in a high-vibration, clean room environment. The total focus test error was required to be better than +/-0.05 mm. We describe the alignment and test procedure, sources of statistical and systematic error, and then the focus determination results using various algorithms. The results are consistent with other tests of focus alignment and indicate that the EUVI telescopes meet the ambient focus offset requirements. STEREO is functioning well on-orbit and the EUVI telescopes meet their on-orbit image quality requirements.

Ohl, Ray↗

NASA NTRS1996

Schlieren system and method for moving objects

A system and method are provided for recording density changes in a flow field surrounding a moving object. A mask having an aperture for regulating the passage of images therethrough is placed in front of an image recording medium. An optical system is placed in front of the mask. A transition having a light field-of-view and a dark field-of-view is located beyond the test object. The optical system focuses an image of the transition at the mask such that the aperture causes a band of light to be defined on the image recording medium. The optical system further focuses an image of the object through the aperture of the mask so that the image of the object appears on the image recording medium. Relative motion is minimized between the mask and the transition. Relative motion is also minimized between the image recording medium and the image of the object. In this way, the image of the object and density changes in a flow field surrounding the object are recorded on the image recording medium when the object crosses the transition in front of the optical system.

Weinstein, Leonard M.↗

NASA NTRS1996

Optical detector calibrator system

An optical detector calibrator system simulates a source of optical radiation to which a detector to be calibrated is responsive. A light source selected to emit radiation in a range of wavelengths corresponding to the spectral signature of the source is disposed within a housing containing a microprocessor for controlling the light source and other system elements. An adjustable iris and a multiple aperture filter wheel are provided for controlling the intensity of radiation emitted from the housing by the light source to adjust the simulated distance between the light source and the detector to be calibrated. The geared iris has an aperture whose size is adjustable by means of a first stepper motor controlled by the microprocessor. The multiple aperture filter wheel contains neutral density filters of different attenuation levels which are selectively positioned in the path of the emitted radiation by a second stepper motor that is also controlled by the microprocessor. An operator can select a number of detector tests including range, maximum and minimum sensitivity, and basic functionality. During the range test, the geared iris and filter wheel are repeatedly adjusted by the microprocessor as necessary to simulate an incrementally increasing simulated source distance. A light source calibration subsystem is incorporated in the system which insures that the intensity of the light source is maintained at a constant level over time.

Strobel, James P.↗

NASA NTRS2007

Sample mounts for microcrystal crystallography

Sample mounts (10) for mounting microcrystals of biological macromolecules for X-ray crystallography are prepared by using patterned thin polyimide films (12) that have curvature imparted thereto, for example, by being attached to a curved outer surface of a small metal rod (16). The patterned film (12) preferably includes a tapered tip end (24) for holding a crystal. Preferably, a small sample aperture is disposed in the film for reception of the crystal. A second, larger aperture can also be provided that is connected to the sample aperture by a drainage channel, allowing removal of excess liquid and easier manipulation in viscous solutions. The curvature imparted to the film (12) increases the film's rigidity and allows a convenient scoop-like action for retrieving crystals. The polyimide contributes minimally to background and absorption, and can be treated to obtain desired hydrophobicity or hydrophilicity.

Thorne, Robert E.↗

NASA NTRS2007

High-Capacity Communications from Martian Distances

High capacity communications from Martian distances, required for the envisioned human exploration and desirable for data-intensive science missions, is challenging. NASA s Deep Space Network currently requires large antennas to close RF telemetry links operating at kilobit-per-second data rates. To accommodate higher rate communications, NASA is considering means to achieve greater effective aperture at its ground stations. This report, focusing on the return link from Mars to Earth, demonstrates that without excessive research and development expenditure, operational Mars-to-Earth RF communications systems can achieve data rates up to 1 Gbps by 2020 using technology that today is at technology readiness level (TRL) 4-5. Advanced technology to achieve the needed increase in spacecraft power and transmit aperture is feasible at an only moderate increase in spacecraft mass and technology risk. In addition, both power-efficient, near-capacity coding and modulation and greater aperture from the DSN array will be required. In accord with these results and conclusions, investment in the following technologies is recommended:(1) lightweight (1 kg/sq m density) spacecraft antenna systems; (2) a Ka-band receive ground array consisting of relatively small (10-15 m) antennas; (3) coding and modulation technology that reduces spacecraft power by at least 3 dB; and (4) efficient generation of kilowatt-level spacecraft RF power.

Williams, W. Dan↗

NASA NTRS2008

Advanced Imaging for Space Science

Future NASA interferometric missions will realize high-resolution with less mass and volume compared to filled-apertures thus saving in cost over comparable filled-aperture systems. However, interferometeric aperture systems give reduced sensitivity requiring longer integration times to achieve a desired signal-to-noise ratio but is likely the only cost effective path forward for high-resolution space imaging.

Lyon, Richard G.↗

NASA NTRS1999

Continuum Modeling of the Dynamics of Externally Injection-locked Coupled Oscillator Arrays

Mutually injection-locked arrays of electronic oscillators provide a novel means of controlling the aperture phase of a phased-array antenna, thus achieving the advantages of spatial power combining while retaining the ability to steer the radiated beam. In a number of design concepts, one or more of the oscillators are injection locked to a signal from an external master-oscillator. The behavior of such a system has been analyzed by numerical solution of a system of nonlinear differential equations which, due to its complexity, yields limited insight into the relationship between the injection signals and the aperture phase. In this paper, we develop a continuum model, which results in a single partial differential equation for the aperture phase as a function of time. Solution of the equation is effected by means of the Laplace transformation and yields detailed information concerning the dynamics of the array under the influence of the external injection signals.

injection locked↗