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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Innovative operating modes and techniques for the spaceborne imaging radar-C instrument

The operation of the spaceborne imaging radar-C (SIR-C) is discussed. The SIR-C instrument has been designed to obtain simultaneous multifrequency and simultaneous multipolarization radar images from a low earth orbit. It is a multiparameter imaging radar which will be flown during at least two different seasons. The instrument has been designed to operate in innovative modes such as the squint alignment mode, the extended aperture mode, the scansar mode, and the interferometry mode. The instrument has been designed to demonstrate innovative engineering techniques such as beam nulling for echo tracking, pulse-repetition frquency hopping for Doppler centroid tracking, generating the frequency step chirp for radar parameter flexibility, block floating point quantizing for data rate compression, and elevation beamwidth broadening for increasing the swath illumination.

Huneycutt, Bryan L.↗
NASA NTRS1989

Wind measurements by electromagnetic probes

The operation and performance characteristics of the Marshall Space Flight Center's Radar Wind Profiler, designed to provide measurement of the wind in the troposphere, are discussed. The Radar Wind Profiler uses a technology similar to that used in conventional Doppler radar systems, except the frequency is generally lower, antenna is larger, and dwell time is much longer. Its primary function is to monitor the vertical wind profile prior to launch of the Space Shuttle at more frequency intervals and nearer to launch time than is presently possible with the conventional balloon systems. A new wind profile will be obtained on the order of every 15 min based on an average of five wind profiles measured every 3 min at a height interval of 150 m to 20 km. The most significant features of the Radar Wind Profiler are the continuity in time and reliability.

Susko, Michael↗
NASA NTRS1991

Trajectory control sensor engineering model detailed test objective

The concept employed in an existing Trajectory Control Sensor (TCS) breadboard is being developed into an engineering model to be considered for flight on the Shuttle as a Detailed Test Objective (DTO). The sensor design addresses the needs of Shuttle/SSF docking/berthing by providing relative range and range rate to 1500 meters as well as the perceived needs of AR&C by relative attitude measurement over the last 100 meters. Range measurement is determined using a four-tone ranging technique. The Doppler shift on the highest frequency tone will be used to provide direct measurement of range rate. Bearing rate and attitude rates will be determined through back differencing of bearing and attitude, respectively. The target consists of an isosceles triangle configuration of three optical retroreflectors, roughly one meter and one-half meter in size. After target acquisition, the sensor continually updates the positions of the three retros at a rate of about one hertz. The engineering model is expected to weigh about 25 pounds, consume 25-30 watts, and have an envelope of about 1.25 cubic feet. The following concerns were addressed during the presentation: are there any concerns with differentiating attitude and bearing to get attitude and bearing rates? Since the docking scenario has low data bandwidth, back differencing is a sufficient approximation of a perfect differentiator for this application. Could range data be obtained if there were no retroreflectors on the target vehicle? Possibly, but only at close range. It would be dependent on target characteristics.

Dekome, Kent↗
NASA NTRS1992

Differenced Doppler tracking in interplanetary navigation - The Magellan cruise reconstruction

The Magellan mission was the first in which Doppler and differenced Doppler tracking data were acquired at X-band uplink/downlink frequencies from the Deep Space Network (DSN). This paper describes a reconstruction of the Venus approach-phase navigation investigating the capabilities of these data types, which are up to an order of magnitude more precise than similar data acquired at S-band frequencies in previous missions. Comparisons of orbit solutions obtained with Doppler, Doppler plus differenced Doppler, and Doppler plus delta-differential one-way Range (DeltaDOR) data indicate that both differenced Doppler and DeltaDOR yield improvements of factors of 3 to 5 over the accuracy of the Doppler-only solutions obtained during Venus approach operations, in which the Doppler data were deweighted from their inherent accuracy due to mismodeling problems.

Ryne, Mark S.↗
NASA NTRS1995

High spatial resolution atmospheric wind measurements with the edge technique

The edge technique is a new and powerful method for measuring small frequency shifts such as the Doppler shift of an atmospheric backscattered signal from a pulsed laser. The edge technique can be used for high spatial resolution, high accuracy ground and airborne wind measurements as well as high accuracy spaceborne wind measurements. We have recently made our first ground based wind measurements. These have a spatial resolution of 15 m and an accuracy of 25 cm/s and these measurements are presented in this paper. This is a unique capability and provides valuable information for studies of turbulent processes in the lower atmosphere. It could also be used for high sensitivity detection of wind shear and microbursts in the vicinity of airports. In addition, global wind measurements can be made with the edge technique from space with an accuracy of 1 m/s and a vertical resolution as high as 150 m in the boundary layer and 1 km through the troposphere. Such a system could make eyesafe wind measurements using well developed diode pumped solid state laser technology at 1.06 micron. Multi-pulse averaging would provide a spatially representative wind measurement.

Korb, C. Laurence↗
NASA NTRS2008

Radiometric Spacecraft Tracking for Deep Space Navigation

Interplanetary spacecraft navigation relies on three types of terrestrial tracking observables.1) Ranging measures the distance between the observing site and the probe. 2) The line-of-sight velocity of the probe is inferred from Doppler-shift by measuring the frequency shift of the received signal with respect to the unshifted frequency. 3) Differential angular coordinates of the probe with respect to natural radio sources are nominally obtained via a differential delay technique of (Delta) DOR (Delta Differential One-way Ranging). The accuracy of spacecraft coordinate determination depends on the measurement uncertainties associated with each of these three techniques. We evaluate the corresponding sources of error and present a detailed error budget.

ranging↗
NASA NTRS2011

Measurement of CPAS Main Parachute Rate of Descent

The Crew Exploration Vehicle Parachute Assembly System (CPAS) is being designed to land the Orion Crew Module (CM) at a safe rate of descent at splashdown. Flight test performance must be measured to a high degree of accuracy to ensure this requirement is met with the most efficient design possible. Although the design includes three CPAS Main parachutes, the requirement is that the system must not exceed 33 ft/s under two Main parachutes, should one of the Main parachutes fail. Therefore, several tests were conducted with clusters of two Mains. All of the steady-state rate of descent data are normalized to standard sea level conditions and checked against the limit. As the Orion design gains weight, the system is approaching this limit to within measurement precision. Parachute "breathing," cluster interactions, and atmospheric anomalies can cause the rate of descent to vary widely and lead to challenges in characterizing parachute terminal performance. An early test had contradictory rate of descent results from optical trajectory and Differential Global Positioning Systems (DGPS). A thorough analysis of the data sources and error propagation was conducted to determine the uncertainty in the trajectory. It was discovered that the Time Space Position Information (TSPI) from the optical tracking provided accurate position data. However, the velocity from TPSI must be computed via numerical differentiation, which is prone to large error. DGPS obtains position through pseudo-range calculations from multiple satellites and velocity through Doppler shift of the carrier frequency. Because the velocity from DGPS is a direct measurement, it is more accurate than TSPI velocity. To remedy the situation, a commercial off-the-shelf product that combines GPS and an Inertial Measurement Unit (IMU) was purchased to significantly improve rate of descent measurements. This had the added benefit of solving GPS dropouts during aircraft extraction. Statistical probability distributions for CPAS Main parachute rate of descent and drag coefficient were computed and plotted. Using test data, a terminal rate of descent at splashdown can be estimated as a function of canopy loading.

Ray, Eric S.↗
NASA NTRS2016

Wave Telescope Technique for MMS Magnetometer

Multipoint measurements are a powerful method in studying wavefields in space plasmas.The wave telescope technique is tested against magnetic field fluctuations in the terrestrial magnetosheath measured by the four Magnetospheric Multiscale (MMS) spacecraft on a spatial scale of about 20 km.The dispersion relation diagram and the wave vector distribution are determined for the first time in the ion-kinetic range. Moreover, the dispersion relation diagram is determined in a proxy plasma restframe by regarding the low-frequency dispersion relation as a Doppler relation and compensating for the apparent phase velocity. Fluctuations are highly compressible, and the wave vectors have an angle of about 60 from the mean magnetic field. We interpret that the measured fluctuations represent akinetic-drift mirror mode in the magnetosheath which is dispersive and in a turbulent state accompanied by a sideband formation.

Magnetometer↗
NASA NTRS2019

Adaptive Antenna Pattern Notching of Interference in Synthetic Aperture Radar Data Using Digital Beamforming

Radio Frequency Interference (RFI) is a growing problem in Synthetic Aperture Radar (SAR)systems as scientific motivations push the radars to lower frequencies and as more wireless services share the frequency spectrum. New, advanced SAR instruments, such as NASA's EcoSAR, DBSAR and DLR's Tandem-L mission, employ a multichannel architecture capable of Digital Beamforming (DBF). Radars with DBF are capable of notching the antenna pattern in specific directions, which can be utilized to suppress RFI on board or in post-processing. A well-researched beamformer for this purpose is the Minimum Variance Distortionless Response (MVDR) Beamformer. However, the number of interferers that can be removed through notching is limited by the number of receive channels. It is therefore essential to adaptively change the antenna pattern notching throughout the image in time and frequency for the best results with a given number of receive channels. In this paper, we present several methods to achieve this notching by making use of the spatial SAR signal distribution in range time, range frequency, azimuth time and azimuth Doppler that is inherent to the SAR imaging geometry. Because this distribution is time-variable and yet predictable, it can be used to improve the angle of arrival estimation of the RFI and the adaptive notching. The presented methods can be applied to a Digital Beamforming (DBF) SAR signal with multiple channels in elevation and/or in azimuth. Simulations show that the proposed methods increase the ability to notch out-of-swath interference from multiple directions and lessen the impact on in-swath interference. The improvement of the interferometric coherence for a single-pass interferogram acquired by NASA's EcoSAR system (capable of DBF in elevation) is demonstrated. The removal of periodic RFI artifacts is achieved.

RFI↗
NASA NTRS2020

Design of a Direct-Detection Wind and Aerosol Lidar for Mars Orbit

The present knowledge of the Mars atmosphere is greatly limited by a lack of global measurements of winds and aerosols. Hence, measurements of height-resolved wind and aerosol profiles are a priority for new Mars orbiting missions. We have designed a direct-detection lidar (MARLI) to provide global measurements of dust, winds and water ice profiles from Mars orbit. From a 400-km polar orbit, the instrument is designed to provide wind and backscatter measurements with a vertical resolution of 2 km and with resolution of 2° in latitude along track. The instrument uses a single-frequency, seeded Nd:YAG laser that emits 4 mJ pulses at 1064 nm at a 250 Hz pulse rate. The receiver utilizes a 50-cm diameter telescope and a double edge Fabry-Pérot etalon as a frequency discriminator to measure the Doppler shift of the aerosol-backscatter profiles. The receiver also includes a polarization-sensitive channel to detect the cross-polarized backscatter profiles from water ice. The receiver uses a sensitive 4 × 4 pixel HgCdTe avalanche photodiode array as a detector for all signals. Here we describe the measurement concept, instrument design, and calculate its performance for several cases of Mars atmospheric conditions. The calculations show that under a range of atmospheric conditions MARLI is capable of measuring wind speed profiles with random error of 2–4 m/s within the first three scale heights, enabling vertically resolved mapping of transport processes in this important region of the atmosphere.

Cremons, Daniel R.↗
NASA NTRS1986

Improving range resolution with a frequency-hopping technique

Range resolution of a conventional pulsed Doppler radar is determined by the scattering volume defined by the transmitted pulse shape. To increase the resolution, the length of the pulse must be reduced. Reducing the pulse length also reduces the transmitted power and hense the signal to noise ratio unless the peak power capability of the transmitter is greatly increased. Improved range resolution may also be attained through the use of various pulse coding methods, but such methods are sometimes difficult to implement from a hardware standpoint. The frequency-hopping (F-H) technique described increases the range resolution of pulse Doppler MST (mesosphere stratosphere troposphere) radar without the need for extensive modifications to the radar transmitter. This technique consists of sending a repeated sequence of pulses, each pulse in the sequence being transmitted at a unique radio frequency that is under the control of a microcomputer. This technique is discussed along with other radar parameters.

Stitt, G. R.↗
NASA NTRS1997

Gravitational Wave Search with the Clock Mission

Doppler tracking of distant spacecraft is the only method currently available to search for gravitational waves in the low-frequency (approx. 0.0001-0.1 Hz) band. In this technique the Doppler system measures the relative dimensionless velocity 2(delta)v/c = (delta)f/f(sub o) between the earth and the spacecraft as a function of time, where (delta)f is the frequency perturbation and f(sub o) is the nominal frequency of the radio link. A gravitational wave of amplitude h incident on this system causes small frequency perturbations, of order h in (delta)f/f(sub o), replicated three times in the observed record (Estabrook and Wahlquist 1975). All experiments to date and those planned for the near future involve only 'two-way' Doppler-i.e., uplink signal coherently transponded by the spacecraft with Doppler measured using a frequency standard common to the transmit and receive chains of the ground station. If, as on the proposed Clock Mission, there is an additional frequency standard on the spacecraft and a suitable earth-spacecraft radio system, some noise sources can be isolated and removed from the data (Vessot and Levine 1978). Supposing that the Clock Mission spacecraft is transferred into a suitable interplanetary orbit, I discuss here how the on-board frequency standard could be employed with an all-Ka-band radio system using the very high stability Deep Space Network station DSS 25 being instrumented for Cassini. With this configuration, the Clock Mission could search for gravitational waves at a sensitivity limited by the frequency standards, rather than plasma or tropospheric scintillation effects, whenever the sun-earth-spacecraft angle is greater than 90 degrees.

Armstrong, J. W.↗
NASA NTRS1997

Accuracy Study of a 2-Component Point Doppler Velocimeter (PDV)

A two-component Point Doppler Velocimeter (PDV) which has recently been developed is described, and a series of velocity measurements which have been obtained to quantify the accuracy of the PDV system are summarized. This PDV system uses molecular iodine vapor cells as frequency discriminating filters to determine the Doppler shift of laser light which is scattered off of seed particles in a flow. The majority of results which have been obtained to date are for the mean velocity of a rotating wheel, although preliminary data are described for fully-developed turbulent pipe flow. Accuracy of the present wheel velocity data is approximately +/- 1 % of full scale, while linearity of a single channel is on the order of +/- 0.5 % (i.e., +/- 0.6 m/sec and +/- 0.3 m/sec, out of 57 m/sec, respectively). The observed linearity of these results is on the order of the accuracy to which the speed of the rotating wheel has been set for individual data readings. The absolute accuracy of the rotating wheel data is shown to be consistent with the level of repeatability of the cell calibrations. The preliminary turbulent pipe flow data show consistent turbulence intensity values, and mean axial velocity profiles generally agree with pitot probe data. However, there is at present an offset error in the radial velocity which is on the order of 5-10 % of the mean axial velocity.

Kuhlman, John↗
NASA NTRS1988

Far-infrared heterodyne receivers

The development of open resonator mixer structures and laser local oscillators has made heterodyne spectroscopy at far infrared (FIR) wavelengths between 150 and 400 microns a reality. Several laser based receivers are now part of the instrument complement flown aboard the Kuiper Airborne Observatory. Lasers are eminently practical as FIR local oscillators whenever there is close frequency coincidence between a strong laser transition and the Doppler-shifted astronomical line. While it is desirable to have continuous frequency coverage in a spectrometer, it should be recognized that most astronomers will focus their interest on the few spectral lines deemed optimum for probing the cosmos. Ultimately, the development of a reliable thin film technology for the new high temperature oxide superconductors may favor SIS-type devices for all wavelengths in future space based FIR receivers.

Betz, Albert↗
NASA NTRS1994

Multi-Spacecraft Coherent Doppler and Ranging for Interplanetary Navigation

Future interplanetary mission concepts are increasingly focusing on multi-spacecraft missions and on small sample return missions which may involve the rendezvous between a spacecraft which brings a sample from the surface of a solar system body and a spacecraft which will return the sample to the Earth. These types of missions place tight requirements on the knowledge of the relative positions of the spacecraft. Historically spacecraft positions have been determined by the use of radio metric data (Doppler and range) between the spacecraft and terrestrial receiving stations. However, the various systems available place tight requirements on the stability of onboard frequency standards on each spacecraft and on the precision of the data extraction hardware on the spacecraft. An alternative is to extend the coherent ground to spacecraft link through one spacecraft to the other and then to the ground or back through the first spacecraft and to the ground receiver. Although this might appear initially to complicate the separation of the dynamics of the two spacecraft, this paper will show that in actual application, judicious selection of spacecraft transponder frequency ratios and the use of coherent Doppler and ranging and the derived observable, DRVID (Differenced Range vs. Integrated Doppler) can allow for the generation of observable equations which are dominated by the spacecraft to spacecraft link.

interplanetary↗
NASA NTRS1994

RINGSAT

Radio frequency interference (RFI) presents a serious problem to naval communications. Unauthorized transmissions on the same frequencies that FLTSAT or LEASAT use cause a loss of one or more transponders on a satellite. The problem is not correctable until the emitter is located and turned off, or until it runs off on its own. The present geolocation system cannot deal with the RFI problem adequately. This causes the Navy to lose operational capability and money. The proposed Radio Frequency Interference Navy Geolocation Satellite (RINGSAT) would locate the source of the interference within 3 hours, an outstanding improvement over current equipment. Furthermore, RINGSAT can be built quickly, at minimal cost, and can be used concurrently for other military missions. Currently, geolocation to identify the RFI is done at Naval Space Command. Using single satellite doppler, time delay of arrival, and frequency delay of arrival techniques, it is possible to locate the interfering emitter. However, for the first quarter of fiscal year 1993, about 50 different channels used by the Navy were nonoperational for over 500 hours. Almost 30 percent of the interference occurrences lasted for over 12 hours. In only one quarter of the cases was the interference turned off in less than two_hours either by our detection or on its own.

DOWDY↗
NASA NTRS1975

Bow shock and magnetosheath waves at Mercury

Mariner 10 measurements at the Mercury bow shock provide examples where the magnetic field is approximately parallel or perpendicular to the bow shock normal. Upstream of a broad irregular parallel shock, left hand circularly polarized waves are observed which cut off very sharply at approximately 4 Hz. Upstream of a perpendicular shock, right hand circularly polarized waves are observed which persist up to the Nyquist frequency of 12 Ha. Determination of the wave propagation vector as a function of frequency helps conclusively identify the waves as whistler mode waves propagating from the shock. The magnetosheath downstream of the parallel shock is disturbed more than that downstream of the perpendicular shock particularly below 1 Hz. In the latter case regular left hand polarized waves observed slightly above the proton gyrofrequency are identified as ion cyclotron waves with wavelength approximately 300 km which are Doppler shifted up to their observed frequency.

Fairfield, D. H.↗