SEARCH · Search NASA
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Viking S-band Doppler RMS phase fluctuations (noise) and comparisons of Viking Doppler noise to Viking differenced S-X range measurements are used to construct a mean equatorial electron density model for 1976. Using Pioneer Doppler noise results (at high heliographic latitudes, also from 1976), an equivalent nonequatorial electron density model is approximated.
Observations of Viking differenced S-band/X-band (S-X) range are shown to correlate strongly with Viking Doppler noise. A ratio of proportionality between downlink S-band plasma-induced range error and two-way Doppler noise is calculated. A new parameter (similar to the parameter epsilon which defines the ratio of local electron density fluctuations to mean electron density) is defined as a function of observed data sample interval (Tau) where the time-scale of the observations is 15 Tau. This parameter is interpreted to yield the ratio of net observed phase (or electron density) fluctuations to integrated electron density (in RMS meters/meter). Using this parameter and the thin phase-changing screen approximation, a value for the scale size L is calculated. To be consistent with Doppler noise observations, it is seen necessary for L to be proportional to closest approach distance a, and a strong function of the observed data sample interval, and hence the time-scale of the observations.
The Doppler difference method as applied to track the GEOS 3 spacecraft is discussed. In this method a pair of 2 GHz ground tracking stations simultaneously track a spacecraft beacon to generate an observable signal in which bias and instability of the carrier frequency cancel. The baselines are formed by the tracking sites at Bermuda, Rosman, and Merritt Island. Measurements were made to evaluate the effectiveness of the Doppler differencing procedure in tracking a beacon target with the high dynamic rate of the GEOS 3 orbit. Results indicate the precision of the differenced data to be at a level comparable to the conventional precise two way Doppler tracking.
In the study described, a scanning dual-channel cross-beam laser Doppler velocimeter (developed for measuring simultaneously the two orthogonal components of flow velocity and turbulence intensity) was used to measure the instantaneous velocity characteristics of a model helicopter rotor and in a vortex wake survey experiment on a Boeing 747 aircraft model. Particular attention is given to a special purpose dual-loop frequency tracker developed to track and demodulate 10 microsecond wide pulse burst Doppler signals with a study cycle of 25% (or narrower signals at a higher duty cycle) generated by the laser Doppler velocimeter.
A description of both the mean and the fluctuating components of the flow, and of the Reynolds stress as observed using a dual forward scattering laser-Doppler velocimeter is presented. A detailed description of the instrument and of the data analysis techniques were included in order to fully document the data. A detailed comparison was made between the laser-Doppler results and those presented in Part 1, and an assessment was made of the ability of the laser-Doppler velocimeter to measure the details of the flows involved.
A microwave backscatter technique is presented that has the ability to sense the dominant surface wavelength of a random rough surface. The purpose of this technique is to perform this measurement from an aircraft or spacecraft, wherein the horizontal velocity of the radar is an important parameter of the measurement system. Attention will be directed at water surface conditions for which a dominant wavelength can be defined, then the spatial variations of reflectivity will have a two dimensional spectrum that is sufficiently close to that of waves to be useful. The measurement concept is based on the relative motion between the water waves and a nadir looking radar, and the fact that while the instantaneous Doppler frequency at the receiver returned by any elementary group of scatterers on a water wave is monotonically changing, the difference in the Doppler frequency between any two scattering 'patches' stays approximately constant as these waves travel parallel to the major axis of an elliptical antenna footprint. The results of a theoretical analysis and a laboratory experiment with a continuous wave (CW) radar that encompasses several of the largest waves in the illuminated area show how the structure in the Doppler spectrum of the backscattered signal is related to the surface spectrum and its parameters in an especially direct and simple way when an incoherent envelope detector is the receiver.
Ability of microwave Doppler radar to measure velocity of slow moving nearby target is enhanced by pulsed 90 degrees phase shifter in radar transmission line between circulator and antenna. Because of phase shifting, Doppler frequency is detected as modulation on carrier instead of baseband signal. Carrier is amplified and filtered before demodulation, resulting in strong, clean demodulated Doppler for measurement and display.
Gravitational radiation from galactic and extragalactic astrophysical sources will induce spatial strains in the solar system, strains which can be measured directly by the Doppler radio link to distant spacecraft. Current noise sources in Pioneer and Voyager Doppler data are delineated and a comparison is made with expected signal levels from gravitational wave sources. The main conclusion is that it is possible to detect gravitational radiation with current DSN hydrogen maser systems stable in fractional frequency + or - 2 x 10 to the minus 14th power over 1000 sec. In the future, however, a serious Doppler observational program in gravitational wave astronomy will require frequency systems stable to at least 10 to the minus 16th power, but at the same time the current single frequency S-band uplink transmission will have to be replaced by a dual frequency capability.
Based on comparisons between instantaneous and light time delay (LTD) formulations for S band range and Doppler observables, expanded models for instantaneous S band observables are recommended for inclusion in the ENTREE program. In a worst case situation using Guam tracking for a representative shuttle entry trajectory, the proposed ENTREE model differs from the LTD model by approximately + or - 0.2 ft in range and + or - 0.02 Hz in Doppler. In contrast, the maximum differences between the existing ENTREE model and the LTD model are approximately (-80, +60) ft in range and (-4, -1) Hz in Doppler.
A microwave radar signal is generated for transmission through an antenna. Before transmission, the signal is phase modulated by 0 deg or 90 deg amounts during each alternate half-cycles of an intermediate frequency (IF) clock signal. After transmission and return, the signal is again phase modulated the same amounts during each alternate half-cycles. The return phase modulated signal is mixed with a leakage signal component of the microwave signal, leaving an IF Doppler. The IF Doppler signal may then be amplified, removing any requirement that direct current level signals be amplified and also removing the effect of detector noise from the Doppler signal.
Analysis of data collected during the participation of the MSFC 10.6 micron pulsed Doppler lidar system (DLS) in the Joint Airport Weather Studies (JAWS) experiment is concluded. Observations from more than one Doppler lidar are combined to derive the three-dimensional Cartesian wind field. A comparison of radial velocity estimates at low elevations made by the MSFC lidar and the NCAR 5.5 cm Doppler radar revealed a substantial r.m.s. difference of 3 mls, and a mean difference of lmls. Detailed measurements of the horizontal wind fields and tropospheric backscattering were acquired at MSFC. These data are useful for defining future flight experiments, instrument design, and satellite DLS development.
The wide angle Michelson Doppler imaging interferometer (WAMDII) is a specialized type of optical Michelson interferometer working at sufficiently long path difference to measure Doppler shifts and to infer Doppler line widths of naturally occurring upper atmospheric Gaussian line emissions. The instrument is intended to measure vertical profiles of atmospheric winds and temperatures within the altitude range of 85 km to 300 km. The WAMDII consists of a Michelson interferometer followed by a camera lens and an 85 x 106 charge coupled device photodiode array. Narrow band filters in a filter wheel are used to isolate individual line emissions and the lens forms an image of the emitting region on the charge coupled device array.
It is pointed out that more definitive shock velocity observations near the sun are needed for an improved determination of the extent of shock deceleration from the sun to earth. Woo and Armstrong (1981) have demonstrated the use of radio scattering and scintillation observations, using spacecraft signals, for measuring interplanetary shock waves near the sun. Woo and Armstrong provided the first near-sun profiles of solar wind speed and electron density fluctuation for a shock wave produced by a solar flare. The present investigation has the objective to demonstrate the use of Doppler or phase scintillations for monitoring and observing interplanetary shocks. It is also shown that Doppler noise, a parameter which is routinely observed and recorded by the NASA Deep Space Network, is essentially a measure of Doppler scintillations.
A method for analyzing multiple Doppler radar data is briefly described. A feature of this method is a new automated objective unfolding scheme which unfolds Doppler velocities for all the radars simultanceously rather than one radar at a time. The method has been used to study a region of widespread precipitation trailing a squall line (anvil), using tripple Doppler radar data to calculate horizontal winds, divergence, and vertical air velocity over an area 120 km by 120 km by 9 km deep. The method yields vertical profiles of divergence which verify the extended VAD method results of Matejka and Srivastava (1981) for the same anvil, and provides a more detailed picture of the variation of winds, divergence, and vertical air velocity in the anvil behind the squall line.
The use of coherent Doppler lidar measurements for analysis of wind fields near airports was investigated during the Joint Airport Weather Studies (JAWS) project of NASA and NOAA. Periodic lidar scans were made with two 10.6-micron CO2-pulsed Doppler lidars spaced 15 km apart in order to calculate the Cartesian wind fields near Marshall Space Flight Center (MSFC). Analyses were also carried out for the flow behind gust fronts, with the desired flow fields corresponding both to surface winds measured by a Portable Automated Mesonet (PAM), and numerical models of Great Plains thunderstorm outflows. Comparison of low elevation scans carried out using the lidar instruments and a CP-4 Doppler radar showed distinct differences due to a bias toward weaker velocities in the radar measurements. The rms difference between the radar and the lidar measurements was 3.1 m/s. The experimental results were consistent with those of a previous study comparing Wave Propagation Laboratory (WPL) lidar with the CP-3 5.5-cm radar operated by the National Center for Atmospheric Research (NCAR).
The wide angle Michelson Doppler imaging interferometer (WAMDII) is a specialized type of optical Michelson interferometer working at sufficiently long path difference to measure Doppler shifts and to infer Doppler line widths of naturally occurring upper atmospheric Gaussian line emissions. The instrument is intended to measure vertical profiles of atmospheric winds and temperatures within the altitude range of 85 km to 300 km. The WAMDII consists of a Michelson interferometer followed by a camera lens and an 85 x 106 charge coupled device photodiode array. Narrow band filters in a filter wheel are used to isolate individual line emissions and the lens forms an image of the emitting region on the charge coupled device array.
Doppler lidar and multiple Doppler radar data were obtained in a convectively mixed planetary boundary layer. The lidar measurements were possible due to scattering from existing aerosols; radar reflecting chaff was released in the atmosphere to make it visible to the multiple Doppler radar network. The data were analyzed to obtain detailed horizontal wind structures. The divergence of the horizontal wind was calculated and the anelastic continuity equation integrated to obtain vertical air motions. Differences between the areally averaged quantities and the grid point values provided a measure of the fluctuations in the wind components or the turbulent wind fluctuations. Vertical profiles of the mean winds and quantities related to the turbulent kinetic energy components and the turbulent momentum transfers were also calculated.
Results of a study assessing the feasibility of a satellite-based Doppler lidar for global wind measurements are reported. In particular, attention is given to the feasibility of simulating such measurements from an airborne platform. A brief discussion of the proposed Doppler lidar system is presented along with a description of the computer simulation and airborne measurements. The measurements will be performed using a pulsed CO2 Doppler lidar in a scan configuration similar to that expected for the satellite system. Computer simulations will be used in addition to measurements to assess the accuracy of the scan technique and to determine the scalability of the measurements.