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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS2017

Illumination Conditions at the Lunar Poles: Implications for Future Exploration

We produced 400 x 400 km Digital Terrain Models (DTMs) of the lunar poles from Lunar Orbiter Laser Altimeter (LOLA) ranging measurements. To achieve consistent, high-resolution DTMs of 20 m/pixel the individual ranging profiles were adjusted to remove small track-to-track o sets. We used these LOLADTMs to simulate illumination conditions at surface level for 50 x 50 km regions centered on the poles. Illumination was derived in one-hour increments from 01 January, 2017 to 01 January, 2037 to cover the lunar precessional cycle of 18.6 years and to determine illumination conditions over several future mission cycles. We identified three regions receiving high levels of illumination at each pole, e.g. the equator-facing crater rims of Hinshelwood, Peary and Whipple for the north pole and the rim of Shackleton crater, and two locations on a ridge between Shackleton and de Gerlache crater for the south pole. Their average illumination levels range from 69.5% to 82.9%, with the highest illumination levels found at the north pole on the rim of Whipple crater. A more detailed study was carried out for these sites as targets for a lander and/or rover equipped with solar arrays. For this purpose we assumed a lander with a structural height of two meters above the ground (height of the solar panels). Here average illumination levels range from 77.1% to 88.0%, with the maximum found at the ridge between Shackleton and de Gerlache crater on the south pole. Distances, sizes and slopes of nearby Permanently Shadowed Regions (PSRs) as a prime science target were also assessed in this case.

LOLA↗
NASA NTRS1972

Satellite-satellite tracking for estimating geopotential coefficients.

Results of simulation studies of tracking of low-altitude satellites by a high-altitude satellite using range measurements between satellites. The high-altitude satellite is tracked by ground-based range trackers. The low-altitude satellites are tracked only by the high-altitude satellites. Orbital elements for one tracking satellite and four tracked low-altitude satellites are estimated, along with a set of low degree and order geopotential coefficients. Estimation of geopotential coefficients of low degree and order is shown to be considerably more accurate using satellite-to-satellite tracking than using ground-based tracking. The use of data from one high-altitude satellite tracking four low-altitude satellites in a range of inclinations is shown to produce geopotential coefficient estimates with smaller errors than presently exist in individual coefficients or would be obtained using ground-based tracking of the same satellites.

Martin, C. F.↗
NASA NTRS1989

Geodynamics applications of spaceborne laser ranging

The Geodynamics Laser Ranging System (GLRS) is a spaceborne laser ranging instrument being developoed by NASA as a facility instrument for the Earth Observing System (EOS). GLRS is to be used to study regional and local crustal movements. It is designed to make highly precise range measurements to retroreflector targets located in geophysically interesting sites. Using a two-color ranging scheme, absolute range accuracies of several mm are expected. Simulations based on this accuracy and the EOS orbital parameters show that length of the intersite baseline between retroreflectors can be determined to several mm accuracy at distances from a few km to several hundred km with several passes of GLRS range data collected over a few-day interval. Short-arc techniques are used to minimize the effects of gravity field and other force model uncertainties. Relative heights can be determined to sub-cm accuracy over comparable distances.

Cohen, Steven C.↗
NASA NTRS1993

Eighth International Workshop on Laser Ranging Instrumentation

The Eighth International Workshop for Laser Ranging Instrumentation was held in Annapolis, Maryland in May 1992, and was sponsored by the NASA Goddard Space Flight Center in Greenbelt, Maryland. The workshop is held once every 2 to 3 years under differing institutional sponsorship and provides a forum for participants to exchange information on the latest developments in satellite and lunar laser ranging hardware, software, science applications, and data analysis techniques. The satellite laser ranging (SLR) technique provides sub-centimeter precision range measurements to artificial satellites and the Moon. The data has application to a wide range of Earth and lunar science issues including precise orbit determination, terrestrial reference frames, geodesy, geodynamics, oceanography, time transfer, lunar dynamics, gravity and relativity.

Degnan, John J.↗
NASA NTRS1990

Flight data acquisition methodology for validation of passive ranging algorithms for obstacle avoidance

The automation of low-altitude rotorcraft flight depends on the ability to detect, locate, and navigate around obstacles lying in the rotorcraft's intended flightpath. Computer vision techniques provide a passive method of obstacle detection and range estimation, for obstacle avoidance. Several algorithms based on computer vision methods have been developed for this purpose using laboratory data; however, further development and validation of candidate algorithms require data collected from rotorcraft flight. A data base containing low-altitude imagery augmented with the rotorcraft and sensor parameters required for passive range estimation is not readily available. Here, the emphasis is on the methodology used to develop such a data base from flight-test data consisting of imagery, rotorcraft and sensor parameters, and ground-truth range measurements. As part of the data preparation, a technique for obtaining the sensor calibration parameters is described. The data base will enable the further development of algorithms for computer vision-based obstacle detection and passive range estimation, as well as provide a benchmark for verification of range estimates against ground-truth measurements.

Smith, Phillip N.↗
NASA NTRS1989

Observations on electromagnetic bias in radar altimeter sea surface measurements

Because the relative radar cross section of the sea surface increases below mean sea level and decreases above it, the range measurements of satellite radar altimeters are biased toward the wave troughs. Published and unpublished direct measurements of this electromagnetic (EM) bias are examined as well as the predictions of theoretical developments. The EM bias is predominantly a function of the radar frequency used, averaging 1.2 percent of the wave height of Ka band and 3.3 percent of the wave height at X band. The airborne measurements present a consistent picture of the variation of the relative radar cross section as a function of deviation from mean sea level. A technique to measure EM bias at the Ku and C band operating frequencies of the Topex satellite altimeter is described.

Walsh, Edward J.↗
NASA NTRS1993

A new method of time difference measurement: The time difference method by dual phase coincidence points detection

In the high accurate measurement of periodic signals, the greatest common factor frequency and its characteristics have special functions. A method of time difference measurement - the time difference method by dual 'phase coincidence points' detection is described. This method utilizes the characteristics of the greatest common factor frequency to measure time or phase difference between periodic signals. It can suit a very wide frequency range. Measurement precision and potential accuracy of several picoseconds were demonstrated with this new method. The instrument based on this method is very simple, and the demand for the common oscillator is low. This method and instrument can be used widely.

Zhou, Wei↗
NASA NTRS1980

Some isotopic abundances in the cosmic radiation

Cosmic ray nuclei with Z not less than 10 have been observed in a detector which measures charge with a double scintillator-Cherenkov array and mass by combining the Cherenkov signal with residual range measured in nuclear emulsions. Results are presented for the isotopic analyses of Al, Ca, Sc, Mn, Fe, Co, Ni and Cu in the energy range 300-800 MeV/amu.

Young, J. S.↗
NASA NTRS2004

Signal Design for Improved Ranging Among Multiple Transceivers

"Ultra-BOC" (where "BOC" signifies "binary offset carrier") is the name of an improved generic design of microwave signals to be used by a group of spacecraft flying in formation to measure ranges and bearings among themselves and to exchange telemetry needed for these measurements. Ultra-BOC could also be applied on Earth for diverse purposes -- for example, measuring relative positions of vehicles on highways for traffic-control purposes and determining the relative alignments of machines operating in mines and of construction machines and structures at construction sites. Ultra-BOC provides for rapid and robust acquisition of signals, even when signal-to-noise ratios are low. The design further provides that each spacecraft or other platform constantly strives to acquire and track the signals from the other platforms while simultaneously transmitting signals that provide full range, bearing, and telemetry service to the other platforms. In Ultra-BOC, unlike in other signal designs that have been considered for the same purposes, it is not necessary to maneuver the spacecraft or other platforms to obtain the data needed for resolving integer-carrier-cycle phase ambiguities.

Young, Lawrence↗
NASA NTRS1973

Remote measurement of atmospheric temperatures by Raman lidar

The Raman shifted return of a lidar system was utilized to make atmospheric temperature measurements. The measurements were made along a horizontal path at temperatures between -20 and +30 C and at ranges of about 100 meters. The temperature data were acquired by recording the intensity ratio of two portions of the rotational Raman spectrum which were simultaneously sampled from a preset range. Measurements were made to an accuracy of + or - 3 C with 1 minute temporal resolution.

Salzman, J. A.↗
NASA NTRS1973

Remote measurement of atmospheric temperatures by Raman lidar.

The Raman shifted return of a lidar system has been utilized to make atmospheric temperature measurements. The measurements were made along a horizontal path at temperatures between -20 and +30 C and at ranges of about 100 meters. The temperature data were acquired by recording the intensity ratio of two portions of the rotational Raman spectrum which were simultaneously sampled from a preset range. Measurements were made to an accuracy of plus or minus 3 C with 1-minute temporal resolution.

Salzman, J. A.↗
NASA NTRS1973

A determination of meteor mass distribution from meteor echoes

Meteor counts from observations at four magnitudes in the range of about +9 to +15 in the visual scale were used to determine the exponent of the mass distribution law under certain assumptions. Since for these data no range measures were available, and since the pattern of the transmitting/receiving antennas was very broad, the same range and radiant distribution were assumed for all counts within the same half hour during which individual counts at different magnitudes were obtained. These echo counts, covering in five successive days 24 hr with overlapping periods, yielded for the exponent a value of 1.95 + or - 0.02, close to other determinations. Individual values, however, presented a considerable scatter, probably reflecting changes in radiant and mass distribution over the year. From the same data, it appeared that the mass exponent changed during the day, yielding generally a higher value for the evening hours.

Forti, G.↗
NASA NTRS1979

The Passive Microwave Remote Sensing of Soil Moisture: the Effect of Tilled Row Structure

The tilled rowstructure is known to be one of the important factors affecting the observations of the microwave emission from a natural surface. Measurements of this effect were carried out with both I and X band radiometers mounted on a mobile truck on a bare 40 m x 45 m row tilled field. The soil moisture content during the measurements ranged from approximately 10 percent to approximately 30 percent by dry weight. The results of these measurements showed that the variations of the antenna temperatures with incident angle theta changed with the azimuthal angle a measured from the row direction. A numerical calculation based on a composite surface roughness was made and found to predict the observed features within the model's limit of accuracy. It was concluded that the difference between the horizontally and vertically polarized temperatures was due to the change in the local angle of field emission within the antenna field of view caused by the large scale row structure.

Wang, J. R.↗
NASA NTRS1981

Effects of the gaseous and liquid water content of the atmosphere on range delay and Doppler frequency

When high precision is required for range measurement on Earth space paths, it is necessary to correct as accurately as possible for excess range delays due to the dry air, water vapor, and liquid water content of the atmosphere. Calculations based on representative values of atmospheric parameters are useful for illustrating the order of magnitude of the expected delays. Range delay, time delay, and phase delay are simply and directly related. Doppler frequency variations or noise are proportional to the time rate of change of excess range delay. Tropospheric effects were examined as part of an overall consideration of the capability of precision two way ranging and Doppler systems.

Flock, W. L.↗
NASA NTRS2000

An Integrated MEMS Sensor Cluster System for Aerospace Applications

Efforts to reduce viscous drag on airfoils could results in a considerable saving for the operation of flight vehicles including those of space transportation. This reduction of viscous drag effort requires measurement and active control of boundary layer flow property on an airfoil. Measurement of viscous drag of the boundary layer flow over an airfoil with minimal flow disturbance is achievable with newly developed MEMS sensor clusters. These sensor clusters provide information that can be used to actively control actuators to obtain desired flow properties or design a vehicle to satisfy particular boundary layer flow criteria. A series of MEMS sensor clusters has been developed with a data acquisition and control module for local measurements of shear stress, pressure, and temperature on an airfoil. The sensor cluster consists of two shear stress sensors, two pressure sensors, and two temperature sensors on a surface area of 1.24 mm x 1.86 mm. Each sensor is 300 microns square and is placed on a flexible polyimide sheet. The shear stress sensor is a polysilicon hot-film resistor, which is insulated by a vacuum cavity of 200 x 200 x 2 microns. The pressure sensors are silicon piezoresistive type, and the temperature sensors are also hot film polysilicon resistors. The total size of the cluster including sensors and electrical leads is 1 Omm x 1 Omm x 0.1 mm. A typical sensitivity of shear stress sensor is 150 mV/Pascal, the pressure sensors are an absolute type with a measurement range from 9 to 36 psia with 0.8mV/V/psi sensitivity, and the temperature sensors have a measurement resolution of 0.1 degree C. The sensor clusters are interfaced to a data acquisition and control module that consists of two custom ASICs (Application Specific Integrated Circuits) and a micro-controller. The data acquisition and control module transfers data to a host PC that configures and controls a total of three sensor clusters. Functionality of the entire system has been tested in the laboratory, and preliminary test results are presented.

Kahng, Seun↗
NASA NTRS2013

Tuning and Robustness Analysis for the Orion Absolute Navigation System

The Orion Multi-Purpose Crew Vehicle (MPCV) is currently under development as NASA's next-generation spacecraft for exploration missions beyond Low Earth Orbit. The MPCV is set to perform an orbital test flight, termed Exploration Flight Test 1 (EFT-1), some time in late 2014. The navigation system for the Orion spacecraft is being designed in a Multi-Organizational Design Environment (MODE) team including contractor and NASA personnel. The system uses an Extended Kalman Filter to process measurements and determine the state. The design of the navigation system has undergone several iterations and modifications since its inception, and continues as a work-in-progress. This paper seeks to show the efforts made to-date in tuning the filter for the EFT-1 mission and instilling appropriate robustness into the system to meet the requirements of manned space ight. Filter performance is affected by many factors: data rates, sensor measurement errors, tuning, and others. This paper focuses mainly on the error characterization and tuning portion. Traditional efforts at tuning a navigation filter have centered around the observation/measurement noise and Gaussian process noise of the Extended Kalman Filter. While the Orion MODE team must certainly address those factors, the team is also looking at residual edit thresholds and measurement underweighting as tuning tools. Tuning analysis is presented with open loop Monte-Carlo simulation results showing statistical errors bounded by the 3-sigma filter uncertainty covariance. The Orion filter design uses 24 Exponentially Correlated Random Variable (ECRV) parameters to estimate the accel/gyro misalignment and nonorthogonality. By design, the time constant and noise terms of these ECRV parameters were set to manufacturer specifications and not used as tuning parameters. They are included in the filter as a more analytically correct method of modeling uncertainties than ad-hoc tuning of the process noise. Tuning is explored for the powered-flight ascent phase, where measurements are scarce and unmodelled vehicle accelerations dominate. On orbit, there are important trade-off cases between process and measurement noise. On entry, there are considerations about trading performance accuracy for robustness. Process Noise is divided into powered flight and coasting ight and can be adjusted for each phase and mode of the Orion EFT-1 mission. Measurement noise is used for the integrated velocity measurements during pad alignment. It is also used for Global Positioning System (GPS) pseudorange and delta- range measurements during the rest of the flight. The robustness effort has been focused on maintaining filter convergence and performance in the presence of unmodeled error sources. These include unmodeled forces on the vehicle and uncorrected errors on the sensor measurements. Orion uses a single-frequency, non-keyed GPS receiver, so the effects due to signal distortion in Earth's ionosphere and troposphere are present in the raw measurements. Results are presented showing the efforts to compensate for these errors as well as characterize the residual effect for measurement noise tuning. Another robustness tool in use is tuning the residual edit thresholds. The trade-off between noise tuning and edit thresholds is explored in the context of robustness to errors in dynamics models and sensor measurements. Measurement underweighting is also presented as a method of additional robustness when processing highly accurate measurements in the presence of large filter uncertainties.

Holt, Greg N.↗
NASA NTRS1972

Tropospheric range error parameters: Further studies

Improved parameters are presented for predicting the tropospheric effect on electromagnetic range measurements from surface meteorological data. Parameters are given for computing the dry component of the zenith radio range effect from surface pressure alone with an rms error of 1 to 2 mm, or the total range effect from the dry and wet components of the surface refractivity, N, and a two-part quartic profile model. The parameters were obtained from meteorological balloon data with improved procedures, including the conversion of the geopotential heights of the balloon data to actual or geometric heights before using the data. The revised values of the parameter k show more latitude variation than is accounted for by the variation of g. This excess variation of k indicates a small latitude variation in the mean molecular weight of air and yields information about the latitude-varying water vapor content of air.

Hopfield, H. S.↗