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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

Wavefront Compensation Segmented Mirror Sensing and Control

The primary mirror of very large submillimeter-wave telescopes will necessarily be segmented into many separate mirror panels. These panels must be continuously co-phased to keep the telescope wavefront error less than a small fraction of a wavelength, to ten microns RMS (root mean square) or less. This performance must be maintained continuously across the full aperture of the telescope, in all pointing conditions, and in a variable thermal environment. A wavefront compensation segmented mirror sensing and control system, consisting of optical edge sensors, Wavefront Compensation Estimator/Controller Soft ware, and segment position actuators is proposed. Optical edge sensors are placed two per each segment-to-segment edge to continuously measure changes in segment state. Segment position actuators (three per segment) are used to move the panels. A computer control system uses the edge sensor measurements to estimate the state of all of the segments and to predict the wavefront error; segment actuator commands are computed that minimize the wavefront error. Translational or rotational motions of one segment relative to the other cause lateral displacement of the light beam, which is measured by the imaging sensor. For high accuracy, the collimator uses a shaped mask, such as one or more slits, so that the light beam forms a pattern on the sensor that permits sensing accuracy of better than 0.1 micron in two axes: in the z or local surface normal direction, and in the y direction parallel to the mirror surface and perpendicular to the beam direction. Using a co-aligned pair of sensors, with the location of the detector and collimated light source interchanged, four degrees of freedom can be sensed: transverse x and y displacements, as well as two bending angles (pitch and yaw). In this approach, each optical edge sensor head has a collimator and an imager, placing one sensor head on each side of a segment gap, with two parallel light beams crossing the gap. Two sets of optical edge sensors are used per segment-to-segment edge, separated by a finite distance along the segment edge, for four optical heads, each with an imager and a collimator. By orienting the beam direction of one edge sensor pair to be +45 away from the segment edge direction, and the other sensor pair to be oriented -45 away from the segment edge direction, all six degrees of freedom of relative motion between the segments can be measured with some redundancy. The software resides in a computer that receives each of the optical edge sensor signals, as well as telescope pointing commands. It feeds back the edge sensor signals to keep the primary mirror figure within specification. It uses a feed-forward control to compensate for global effects such as decollimation of the primary and secondary mirrors due to gravity sag as the telescope pointing changes to track science objects. Three segment position actuators will be provided per segment to enable controlled motions in the piston, tip, and tilt degrees of freedom. These actuators are driven by the software, providing the optical changes needed to keep the telescope phased.

Redding, David C.↗
NASA NTRS1989

Stress distribution and topography of Tellus Regio, Venus

The Tellus Regio area of Venus represents a subset of a narrow latitude band where Pioneer Venus Orbiter (PVO) altimetry data, line-of-sight (LOS) gravity data, and Venera 15/16 radar images have all been obtained with good resolution. Tellus Regio also has a wide variety of surface morphologic features, elevations ranging up to 2.5 km, and a relatively low LOS gravity anomaly. This area was therefore chosen in order to examine the theoretical stress distributions resulting from various models of compensation of the observed topography. These surface stress distributions are then compared with the surface morphology revealed in the Venera 15/16 radar images. Conclusions drawn from these comparisons will enable constraints to be put on various tectonic parameters relevant to Tellus Regio. The stress distribution is calculated as a function of the topography, the equipotential anomaly, and the assumed model parameters. The topography data is obtained from the PVO altimetry. The equipotential anomaly is estimated from the PVO LOS gravity data. The PVO LOS gravity represents the spacecraft accelerations due to mass anomalies within the planet. These accelerations are measured at various altitudes and angles to the local vertical and therefore do not lend themselves to a straightforward conversion. A minimum variance estimator of the LOS gravity data is calculated, taking into account the various spacecraft altitudes and LOS angles and using the measured PVO topography as an a priori constraint. This results in an estimated equivalent surface mass distribution, from which the equipotential anomaly is determined.

Williams, David R.↗
NASA NTRS1978

On isostatic geoid anomalies

In regions of slowly varying lateral density changes, the gravity and geoid anomalies may be expressed as power series expansions in topography. Geoid anomalies in isostatically compensated regions can be directly related to the local dipole moment of the density-depth distribution. This relationship is used to obtain theoretical geoid anomalies for different models of isostatic compensation. The classical Pratt and Airy models give geoid height-elevation relationships differing in functional form but predicting geoid anomalies of comparable magnitude. The thermal cooling model explaining ocean floor subsidence away from mid-ocean ridges predicts a linear age-geoid height relationship of 0.16 m/m.y. Geos 3 altimetry profiles were examined to test these theoretical relationships. A profile over the mid-Atlantic ridge is closely matched by the geoid curve derived from the thermal cooling model. The observed geoid anomaly over the Atlantic margin of North America can be explained by Airy compensation. The relation between geoid anomaly and bathymetry across the Bermuda Swell is consistent with Pratt compensation with a 100-km depth of compensation.

Haxby, W. F.↗
NASA NTRS2018

Extraction of Doppler Observables from Open-Loop Recordings for the Juno Radio Science Investigation

The goal of the Juno Gravity Science investigation is to estimate the gravitational field of Jupiter by measurement of the spacecraft velocity during periods of closest approach. Velocity is measured by the Doppler shift of dual X- and Ka-band radio links between the Juno spacecraft, in orbit around Jupiter, and the DSS-25 antenna of the Deep Space Network (DSN). During times of closest-approach, Juno experiences large dynamic ranges caused by the orbital dynamics and spin signatures caused by the spin-stabilized spacecraft that are detectable by the receivers at DSS-25. Open-loop recordings of received voltages are processed to compute Doppler observables utilized in the estimation of the gravity field. Presented is a method to process open-loop data collected by the DSN to compensate for the spin signature of the spacecraft, removal of artifacts from Doppler observables caused by the high dynamic environment, and improve performance of the digital phase-locked loop utilized in the data processing.

Buccino, Dustin R.↗
NASA NTRS1980

A harmonic analysis of lunar gravity

An improved model of lunar global gravity has been obtained by fitting a sixteenth-degree harmonic series to a combination of Doppler tracking data from Apollo missions 8, 12, 15, and 16, and Lunar Orbiters 1, 2, 3, 4, and 5, and laser ranging data to the lunar surface. To compensate for the irregular selenographic distribution of these data, the solution algorithm has also incorporated a semi-empirical a priori covariance function. Maps of the free-air gravity disturbance and its formal error are presented, as are free-air anomaly and Bouguer anomaly maps. The lunar gravitational variance spectrum has the form V(G; n) = O(n to the -4th power), as do the corresponding terrestrial and martian spectra. The variance spectra of the Bouguer corrections (topography converted to equivalent gravity) for these bodies have the same basic form as the observed gravity; and, in fact, the spectral ratios are nearly constant throughout the observed spectral range for each body. Despite this spectral compatibility, the correlation between gravity and topography is generally quite poor on a global scale.

Bills, B. G.↗
NASA NTRS1975

The isostatic state of Martian topography

As is evidenced by the gravity data, the regional topography of Mars falls into two disctinct age groups. The older group is isostatically compensated at relatively shallow depth. All of our analyses indicate that the younger group is only partly compensated and that the depth of partial compensation is also shallow. The young topography is composed primarily of the Tharsis plateau and the adjacent low areas of Chryse and Amazonis. They appear to have all undergone the same amount of partial compensation, suggesting a comtemporaneous origin. All of the topography is consistent with a model of viscous relaxation in an environment of approximate thermal steady state. The minimum viscosity necessary to explain the partial compensation is 10 to the 27th power P, a value typical of a number of lunar studies. Alternatively, the strength required to support the uncompensated fraction of the topography is similar to strengths determined for the earth and moon.

Phillips, R. J.↗
NASA NTRS1981

Venus gravity - A high-resolution map

The Doppler data from the radio tracking of the Pioneer Venus Orbiter (PVO) have been used in a two-stage analysis to develop a high-resolution map of the gravitational potential of Venus, represented by a central mass and a surface mass density. The two-stage procedure invokes a Kalman filter-smoother to determine the orbit of the spacecraft, and a stabilized linear inverter to estimate the surface mass density. The resultant gravity map is highly correlated with the topographic map derived from the PVO radar altimeter data. However, the magnitudes of the gravity variations are smaller than would be expected if the topography were uncompensated, indicating that at least partial compensation has taken place.

Reasenberg, R. D.↗
NASA NTRS1982

Venus gravity - Analysis of Beta Regio

Radio tracking data acquired over Beta Regio were analyzed to obtain a surface mass distribution from which a detailed vertical gravity field was derived. In addition, a corresponding vertical gravity field was evaluated solely from the topography of the Beta region. A comparison of these two maps confirms the strong correlation between gravity and topography which was previously seen in line-of-sight gravity maps. It also demonstrates that the observed gravity is a significant fraction of that predicted from the topography alone. The effective depth of complete isostatic compensation for the Beta region is estimated to be 330 km, which is somewhat deeper than that found for other areas of Venus.

Esposito, P. B.↗
NASA NTRS1990

The inclusion of higher degree and order gravity terms in the design of a repeat ground track orbit

Mean Orbit Elements can be chosen such that a desired ground track repeat criteria is obtained when propagating with the secular effects of a central body gravity field. If the proper initial conditions are chosen that correspond to these Mean Elements, the orbit can also be propagated with the full gravity field and form another ground track repeat pattern. Further, if the effects of other perturbative forces are compensated for, such that the ground track remains near this pattern, the central body effects can again be considered repetitive. The process of arriving at these conclusions, along with the supporting numerical simulations, are presented.

Vincent, Mark A.↗
NASA NTRS1993

Implications of new gravity data for Baikal Rift zone structure

Newly available, 2D Bouguer gravity anomaly data from the Baikal Rift zone, Siberia, indicate that this discrete, intracontinental rift system is regionally compensated by an elastic plate about 50 km thick. However, spectral and spatial domain analyses and isostatic anomaly calculations show that simple elastic plate theory does not offer an adequate explanation for compensation in the rift zone, probably because of significant lateral variations in plate strength and the presence of subsurface loads. Our results and other geophysical observations support the interpretation that the Baikal Rift zone is colder than either the East African or Rio Grande rift.

Ruppel, C.↗
NASA NTRS1994

The isostatic state of Mead crater

We have analyzed high-resolution Magellan Doppler tracking data over Mead crater, using both line-of-sight and spherical harmonic methods, and have found a negative gravity anomaly of about 4-5 mgal (at spacecraft altitude, 182 km). This is consistent with no isostatic compensation of the present topography; the uncertainty in the analysis allows perhaps as much as 30% compensation at shallow dpeths (approximately 25 km). This is similar to observations of large craters on Earth, which are not generally compensated, but contrasts with at least some lunar basins which are inferred to have large Moho uplifts and corresponding positive Bouguer anomalies. An uncompensated load of this size requires a lithosphere with an effective elastic lithosphere thickness greater than 30 km. In order for the crust-mantle boundary not to have participated in the deformation associated with the collapse of the transient cavity during the creation of the crater, the yield strength near the top of the mantle must have been significantly higher on Earth and Venus than on the Moon at the time of basin formation. This might be due to increased strength against frictional sliding at the higher confining pressures within the larger planets. Alternatively, the thinner crusts of Earth and Venus compared to that of the Moon may result in higher creep strength of the upper mantle at shallower depths.

Banerdt, W. B.↗
NASA NTRS1993

Accurate attitude determination of the LACE satellite

The Low-power Atmospheric Compensation Experiment (LACE) satellite, launched in February 1990 by the Naval Research Laboratory, uses a magnetic damper on a gravity gradient boom and a momentum wheel with its axis perpendicular to the plane of the orbit to stabilize and maintain its attitude. Satellite attitude is determined using three types of sensors: a conical Earth scanner, a set of sun sensors, and a magnetometer. The Ultraviolet Plume Instrument (UVPI), on board LACE, consists of two intensified CCD cameras and a gimbal led pointing mirror. The primary purpose of the UVPI is to image rocket plumes from space in the ultraviolet and visible wavelengths. Secondary objectives include imaging stars, atmospheric phenomena, and ground targets. The problem facing the UVPI experimenters is that the sensitivity of the LACF satellite attitude sensors is not always adequate to correctly point the UVPI cameras. Our solution is to point the UVPI cameras at known targets and use the information thus gained to improve attitude measurements. This paper describes the three methods developed to determine improved attitude values using the UVPI for both real-time operations and post observation analysis.

Miglin, M. F.↗
NASA NTRS2012

Gravity Recovery and Interior Laboratory (GRAIL) Mission: Mission Status and Initial Science Results

The Gravity Recovery and Interior Laboratory (GRAIL) Mission is a component of the NASA Discovery Program. GRAIL is a twin-spacecraft lunar gravity mission that has two primary objectives: to determine the structure of the lunar interior, from crust to core; and to advance understanding of the thermal evolution of the Moon. GRAIL launched successfully from the Cape Canaveral Air Force Station on September 10, 2011, executed a low-energy trajectory to the Moon, and inserted the twin spacecraft into lunar orbit on December 31, 2011 and January 1, 2012. A series of maneuvers brought both spacecraft into low-altitude (55-km), near-circular, polar lunar orbits, from which they perform high-precision satellite-to-satellite ranging using a Ka-band payload along with an S-band link for time synchronization. Precise measurements of distance changes between the spacecraft are used to map the lunar gravity field. GRAIL completed its primary mapping mission on May 29, 2012, collecting and transmitting to Earth >99.99% of the possible data. Spacecraft and instrument performance were nominal and has led to the production of a high-resolution and high-accuracy global gravity field, improved over all previous models by two orders of magnitude on the nearside and nearly three orders of magnitude over the farside. The field is being used to understand the thickness, density and porosity of the lunar crust, the mechanics of formation and compensation states of lunar impact basins, and the structure of the mantle and core. GRAIL s three month-long-extended mission will initiate on August 30, 2012 and will consist of global gravity field mapping from an average altitude of 22 km.

Neumann, Gregory A.↗
NASA NTRS1985

Geophysical interpretation of Venus gravity data

The investigation of the subsurface mass distribution of Venus through the analysis of the data from Pioneer Venus Orbiter (PVO) is presented. The Doppler tracking data was used to map the gravitational potential, which was compared to the topographic data from the PVO radar (ORAD). In order to obtain an unbiased comparison, the topography obtained from the PVO-ORAD was filtered to introduce distortions which are the same as those of our gravity models. The last major software package that was required in order to determine the spectral admittance Z (lambda) was used. This package solves the forward problem: given the topography and its density, and assuming no compensation, find the resulting spacecraft acceleration along a given nominal trajectory. The filtered topography is obtained by processing these accelerations in the same way (i.e., with the same geophysical inverter) as the Doppler-rate data that we use to estimate the gravity maps.

Reasenberg, R. D.↗
NASA NTRS1988

Galileo Earth-Venus trajectory correction maneuver design

As a result of delays in the Space Shuttle program and a re-evaluation of safety concerns which resulted in cancellation of the Shuttle Centaur Upper Stage program, the Galileo mission to Jupiter has undergone a substantial redesign effort. Use of the Inertial Upper Stage (IUS) for injection has necessitated a complicated Venus-Earth-Earth gravity assist trajectory in order to reach Jupiter. One flyby of Venus and two flybys of Earth are required to compensate for the lower injection energy available from the IUS. As the spacecraft was not originally designed for the environment within 1 AU of the Sun, modifications to the spacecraft and operating procedures have been developed. This paper describes some of these changes and how they have affected the design and implementation of trajectory correction maneuvers on the Earth-Venus leg of the mission. A strategy of biasing the IUS target at injection to control the statistical velocity distribution of a subsequent trajectory correction maneuver is discussed.

Wilson, M. G.↗
NASA NTRS1988

Comparisons of global topographic/isostatic models to the Earth's observed gravity field

The Earth's gravitational potential, as described by a spherical harmonic expansion to degree 180, was compared to the potential implied by the topography and its isostatic compensation using five different hypothesis. Initially, series expressions for the Airy/Heiskanen topographic isostatic model were developed to the third order in terms of (h/R), where h is equivalent rock topography and R is a mean Earth radius. Using actual topographic developments for the Earth, it was found that the second and third terms of the expansion contributed 30 and 3 percents, of the first of the expansion. With these new equations it is possible to compute depths (D) of compensation, by degree, using 3 different criteria. The results show that the average depth implied by criterion I is 60 km while it is about 33 km for criteria 2 and 3 with smaller compensation depths at the higher degrees. Another model examined was related to the Vening-Meinesz regional hypothesis implemented in the spectral domain. Finally, oceanic and continental response functions were derived for the global data sets and comparisons made to locally determined values.

Rummel, Reiner↗
NASA NTRS2024

Enhancing the Payload Development Process for Lunar Gateway and Lunar Surface Science & Exploration: Space Biology Beyond Low-Earth-Orbit Instrumentation and Science Series (BLISS) Science Working Group 2023-2024 Annual Report

Space biology BLEO research is inherently driven by the differences between the LEO and BLEO environments, which can be broadly characterized by the five-hazard “RIDGE” paradigm (Radiation, Isolation, Distance, Gravity, Environment, e.g., similar to Figure 2 in (1)). Thus, the envisioned goals over the next decade will include using the cislunar and lunar surface environments to (A) characterize deep-space environments including biological effects of radiation and other stressors, (B) gain experience from isolation of very small groups in very small enclosures, (C) learn to compensate for distance from Earth via in situ resource utilization (ISRU) and bioregenerative life support, (D) gain assurance that all aspects of deep-space exploration can proceed in altered or artificial gravity environments, (E) develop essential adaptation scenarios for the built (e.g., low pressure) and external (e.g., temperature extremes, dust) environments.

Biology↗
NASA NTRS1985

Impact of GRM: New evidence from the Soviet Union

Gravity information released by the Soviet Union allows the quantitative assessment of how the geopotential research mission (GRM) mission might effect the ability to use global gravity data for continental tectonic interpretation. The information is of an isostatic response spectra for eight individual tectonic units in the USSR. The regions examined include the Caroathians, Caucasus, Urals, Pamirs, Tien-Shan, Altal, Chersky Ridge, and East Siberian Platform. The 1 deg x 1 deg gravity data are used to calculate the admittances are used in two different sorts of tectonic studies of mountain belts in the USSR: (1) interpretation of isostatic responses in terms of plate models of compensation for mountainous terrain. Using geologic information concerning time of the orogeny, lithospheric plates involved, and polarity of subduction in collision zones, they convert the best-fitting flexural rigidity to an elastic plate thickness for the lithospheric plate inferred to underlie the mountains; the isostatic admittance functions is an attempt to directly model gravity and topography data for a few select regions in the Soviet Union. By knowing the value of the expected correlation between topography and gravity from the admittances, the Artemjev's map in mountainous areas can be calibrated, and the maps are converted back to Bouguer gravity. This procedure is applied to the Caucasus and southern Urals.

Mcnutt, M.↗