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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 37 records · Page 2
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NASA NTRS2002

JASON-1 Precise Orbit Determination (POD) Through the Combination and Comparison of GPS, SLR, DORIS and Altimeter Crossover Data

Jason-1, launched on December 7, 2001, is continuing the time series of centimeter level ocean topography observations as the follow-on to the highly successful TOPEX/POSEIDON (T/P) radar altimeter satellite. The precision orbit determination (POD) is a critical component to meeting the ocean topography goals of the mission. T/P has demonstrated that the time variation of ocean topography can be determined with an accuracy of a few centimeters, thanks to the availability of highly accurate orbits based primarily on SLR+DORIS tracking. The Jason-1 mission is intended to continue measurement of the ocean surface with the same, if not better accuracy. Fortunately, Jason-1 POD can rely on four independent tracking data types available including near continuous tracking data from the dual frequency codeless BlackJack GPS receiver. Orbit solutions computed using individual and various combinations of GPS, SLR, DORIS and altimeter crossover data types have been determined from over 100 days of Jason-1 tracking data. The performance of the orbit solutions and tracking data has been evaluated. Orbit solution evaluation and comparison has provided insight into possible areas of refinement. Several aspects of the POD process are examined to obtain orbit improvements including measurement modeling, force modeling and solution strategy. The results of these analyses will be presented.

Luthcke, S. B.↗
NASA NTRS1978

Evaluation program for secondary spacecraft cells: Initial evaluation tests of General Electric Company 40.0 ampere-hour nickel-cadmium spacecraft cells for the tracking data relay satellite system

Five cells provided by NASA's Goddard Space Flight Center were evaluated at room temperature and pressure (25 C plus or minus 2 C) with discharges at the 2 hour rate. Measurements of the cell containers following test, indicated an average increase of .006 inches at the plate thickness. Average end of charge voltages and pressures, and capacity output in ampere hours were determined. Three cells exceeded the voltage requirements of 1.52 volts during both c/10 charges at 20 C. All cells exceeded the voltage requirement of 1.52 volts during the 0 C overcharge test, although their end charges were below 1.50 volts. The pressure requirement of 65 psia was exceeded by both pressure transducer cells during c/10 charges at 25 C and 20 C and also during the 0 C overcharge test. The cells with pressure transducers reached a pressure of 20 psia before reaching the voltage limit of 1.550 volts during the pressure versus capacity test, and exhibited a pressure decay of 2 psia during the last 30 minutes of the 1 hour open circuit stand. Average capacity was 51.3 ampere hours.

Harkness, J. D.↗
NASA NTRS1978

Ray tracing evaluation of a technique for correcting the refraction errors in satellite tracking data

Errors may be introduced in satellite laser ranging data by atmospheric refractivity. Ray tracing data have indicated that horizontal refractivity gradients may introduce nearly 3-cm rms error when satellites are near 10-degree elevation. A correction formula to compensate for the horizontal gradients has been developed. Its accuracy is evaluated by comparing it to refractivity profiles. It is found that if both spherical and gradient correction formulas are employed in conjunction with meteorological measurements, a range resolution of one cm or less is feasible for satellite elevation angles above 10 degrees.

Gardner, C. S.↗
NASA NTRS2002

Gravity Fields from CHAMP Mission Data

The CHAMP mission, launched in July 2000, is the first in a series of missions that will revolutionize our ability to model the Earth s geopotential. The CHAMP spacecraft is equipped for precision tracking by the Global Positioning System (GPS) and Satellite Laser Ranging (SLR) along with a precision accelerometer to provide measurements of the surface forces. Preliminary satellite-only geopotential solutions with only 30 days of CHAMP data are, by some criteria, as strong as solutions made from tracking data collected over the previous 30 years of the space age. Compared to EGM96, CHAMP makes notable contributions in regions where the terrestrial data (surface gravimetry and altimetry) were weak, for example in the polar regions, in the Amazon and the Himalayas. The CHAMP data allow us to separate the geoid from the dynamic ocean topography (DOT) up to at least degree 25 rather than just under degree 20 as in EGM96. We report on satellite-only and combination models that incorporate up to 100 days of CHAMP data as well as other satellite data. We report on our updated processing of the CHAMP tracking and accelerometer data and evaluate the performance of the geopotential models using a variety of tests.

Lemoine, Frank G.↗
NASA NTRS2012

Evaluation of Atmospheric Loading and Improved Troposphere Modelling

Forward modeling of non-tidal atmospheric loading displacements at geodetic tracking stations have not routinely been included in Doppler Orbitography and Radiopositionning Integrated by Satellite (DORIS) or Satellite Laser Ranging (SLR) station analyses for either POD applications or reference frame determination. The displacements which are computed from 6-hourly models such as the ECMWF and can amount to 3-10 mm in the east, north and up components depending on the tracking station locations. We evaluate the application of atmospheric loading in a number ways using the NASA GSFC GEODYN software: First we assess the impact on SLR & DORIS-determined orbits such as Jason-2, where we evaluate the impact on the tracking data RMS of fit and how the total orbits are changed with the application of this correction. Preliminary results show an RMS radial change of 0.5 mm for Jason-2 over 54 cycles and a total change in the Z-centering of the orbit of 3 mm peak-to-peak over one year. We also evaluate the effects on other DORIS-satellites such as Cryosat-2, Envisat and the SPOT satellites. In the second step, we produce two SINEX time series based on data from available DORIS satellites and assess the differences in WRMS, scale and Helmert translation parameters. Troposphere refraction is obviously an important correction for radiometric data types such as DORIS. We evaluate recent improvements in DORIS processing at GSFC including the application of the Vienna Mapping Function (VMF1) grids with a-priori hydrostatic (VZHDs) and wet (VZWDs) zenith delays. We reduce the gridded VZHD at the stations height using pressure and temperature derived from GPT (strategy 1) and Saastamoinen. We discuss the validation of the VMF1 implementation and its application to the Jason-2 POD processing, compared to corrections using the Niell mapping function and the GMF. Using one year of data, we also assess the impact of the new troposphere corrections on the DORIS-only solutions, most importantly on the scale of the weekly solutions.

Zelensky, Nikita P.↗
NASA NTRS1972

GEOS-2 C-band radar system project. Marine study using C-band radars

One of the secondary objectives of the GEOS-2 C-band Systems Project is to study the feasibility of using geodetic satellites to both evaluate shipborne instrumentation and to determine ship positions in broad ocean areas. The purpose of this task is to determine whether shipborne C-band radar tracking, in conjunction with ground based tracking, is sufficiently accurate to provide instrumentation evaluation and ship position estimates. Data from several Apollo tracking ships, in particular the USNS Vanguard, was made available for this effort. A series of tests, using the USNS Vanguard, were carried out in the Port Canaveral and Bahama Acoustic Transponder Array Areas. The major portion of the analyses reported are the result of preliminary investigations using the data from these tests.

Source record↗
NASA NTRS2023

Comparative Accuracies of Models for Drag Prediction During Geomagnetically Disturbed Periods: A First Principles Model Versus Empirical Models

We examine the accuracy of density prediction by the first principles model Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) developed by the National Center for Atmospheric Research and compare it to the accuracy of three empirical models: Jacchia 71, the Naval Research Laboratory Mass Spectrometer Incoherent Scatter Extended 2000 (NRLMSIS), Jacchia 1971, and Jacchia-Bowman 2008. Comparisons are made for three large storms: the October 2003 storm, the March 2013 storm, and the March 2015 storm. To evaluate the accuracy of these models we use tracking data for nine space objects in low Earth orbit. Additionally, we evaluate the accuracy of the TIEGCM and NRLMSIS with data from high precision accelerometers on the Challenging Minisatellite Payload (CHAMP) and Gravity field and Circulation Explorer (GOCE) satellites. The goal is to assess the use of a first principles model as a potential tool for forecasting satellite drag during large magnetic storms. For the storms considered, we found the TIEGCM, JB2008, and NRLMSIS models to be substantially more accurate than the Jacchia 71 model. The accuracies of the TIEGCM and JB2008 models were similar, but overall, the TIEGCM was more accurate. We found smaller differences for TIEGCM versus CHAMP than for NRLMIS for the Halloween Storm, and smaller differences than results published for JB2008 and the assimilative model HASDM. The empirical models are at present more practical for operational purposes, but the TIEGCM, developed as a research model, with a greater focus on operational use offers the potential for improved utility during stressing conditions.

R. L. Walterscheid↗
NASA NTRS1976

Tracker implementation for the orbiter Ku-band communications antenna

Possible implementations and recommendations for the Space Shuttle Ku-Band integrated communications/radar antenna tracking system were evaluated. Communication aspects involving the Tracking Data Relay Satellite (TDRS)/Orbiter Ku-Band link are emphasized. Detailed analysis of antenna sizes, gains and signal-to-noise ratios shows the desirability of using maximum size 36-inch diameter dish and a triple channel monopulse. The use of the original baselined 20 inch dish is found to result in excessive acquisition time since the despread signal would be used in the tracking loop. An evaluation of scan procedures which includes vehicle dynamics, designation error, time for acquisition and probability of acquisition shows that the conical scan is preferred since the time for lock-on for relatively slow look angle rates will be significantly shorter than the raster scan. Significant improvement in spherical coverage may be obtained by reorienting the antenna gimbal to obtain maximum blockage overlap.

Rudnicki, J. F.↗
NASA NTRS1969

Multipath performance of a TDRS system employing wideband FM VHF signals

An approximate theoretical analysis is presented for the effects of specular reflection multipath on the performance of a one-way tracking and data relay satellite-to-user link. The analysis pertains to the wideband FM system employing a sinusoidal subcarrier to achieve spectrum spreading. Bounds on multipath effects are derived for receivers with and without limiters and for data modulated on the carrier or the subcarrier. For data modulation, performance is evaluated in terms of an additive lowpass signal at the data detection filter. Doppler and range tracking performance is evaluated in terms of root-mean-square (RMS) error of carrier frequency in a carrier PLL and rms phase jitter in a subcarrier PLL.

Sohn, S. J.↗
NASA NTRS1997

Active Microwave Remote Sensing Observations of Weddell Sea Ice

Since July 1991, the European Space Agency's ERS-1 and ERS-2 satellites have acquired radar data of the Weddell Sea, Antarctica. The Active Microwave Instrument on board ERS has two modes; SAR and Scatterometer. Two receiving stations enable direct downlink and recording of high bit-rate, high resolution SAR image data of this region. When not in an imaging mode, when direct SAR downlink is not possible, or when a receiving station is inoperable, the latter mode allows normalized radar cross-section data to be acquired. These low bit-rate ERS scatterometer data are tape recorded, downlinked and processed off-line. Recent advances in image generation from Scatterometer backscatter measurements enable complementary medium-scale resolution images to be made during periods when SAR images cannot be acquired. Together, these combined C-band microwave image data have for the first time enabled uninterrupted night and day coverage of the Weddell Sea region at both high (25 m) and medium-scale (-20 km) resolutions. C-band ERS-1 radar data are analyzed in conjunction with field data from two simultaneous field experiments in 1992. Satellite radar signature data are compared with shipborne radar data to extract a regional and seasonal signature database for recognition of ice types in the images. Performance of automated sea-ice tracking algorithms is tested on Antarctic data to evaluate their success. Examples demonstrate that both winter and summer ice can be effectively tracked. The kinematics of the main ice zones within the Weddell Sea are illustrated, together with the complementary time-dependencies in their radar signatures. Time-series of satellite images are used to illustrate the development of the Weddell Sea ice cover from its austral summer minimum (February) to its winter maximum (September). The combination of time-dependent microwave signatures and ice dynamics tracking enable various drift regimes to be defined which relate closely to the circulation of the sea ice in response to current and wind forcing and iceberg barriers. These are closely related to continental-shelf or central basin regimes, in which tidal forcing or barotropic circulation patterns appear to influence the sea-ice motion, respectively. These regimes provide valuable information about the regions of most prolific ice growth and influence of ice conditions upon air-sea-ice exchange processes in the Weddell Sea.

Drinkwater, Mark R.↗
NASA NTRS1993

TOPEX/Poseidon precision orbit determination production and expert system

TOPEX/Poseidon (T/P) is a joint mission between NASA and the Centre National d'Etudes Spatiales (CNES), the French Space Agency. The TOPEX/Poseidon Precision Orbit Determination Production System (PODPS) was developed at Goddard Space Flight Center (NASA/GSFC) to produce the absolute orbital reference required to support the fundamental ocean science goals of this satellite altimeter mission within NASA. The orbital trajectory for T/P is required to have a RMS accuracy of 13 centimeters in its radial component. This requirement is based on the effective use of the satellite altimetry for the isolation of absolute long-wavelength ocean topography important for monitoring global changes in the ocean circulation system. This orbit modeling requirement is at an unprecedented accuracy level for this type of satellite. In order to routinely produce and evaluate these orbits, GSFC has developed a production and supporting expert system. The PODPS is a menu driven system allowing routine importation and processing of tracking data for orbit determination, and an evaluation of the quality of the orbit so produced through a progressive series of tests. Phase 1 of the expert system grades the orbit and displays test results. Later phases undergoing implementation, will prescribe corrective actions when unsatisfactory results are seen. This paper describes the design and implementation of this orbit determination production system and the basis for its orbit accuracy assessment within the expert system.

Putney, Barbara↗
NASA NTRS1990

Evaluation of orbit determination using dual-TDRS tracking

This paper describes the results of a study to evaluate the orbit determinatioin of Tracking and Data Relay Satellite System (TDRSS) user spacecraft within the dual-Tracking and Data Relay Satellite (TDRS) environment. Dense TDRSS tracking of the Earth Radiation Budget Satellite (ERBS) was acquired for the period August 16 through 22, 1989. This tracking information was processed to evaluate the orbit determination consistency achieved using the Goddard Trajectory Determination System batch least-squares estimator. The effects of the use of the second operational relay spacecraft, of refinements in orbit determination models (geopotentials, polar motion, solid earth tidal gravitational perturbations, ionospheric refraction corrections), and of methods for providing relay spacecraft spacecraft position information were also studied.

Oza, D. H.↗