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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 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Improved DORIS accuracy for precise orbit determination and geodesy

In 2001 and 2002, 3 more DORIS satellites were launched. Since then, all DORIS results have been significantly improved. For precise orbit determination, 20 cm are now available in real-time with DIODE and 1.5 to 2 cm in post-processing. For geodesy, 1 cm precision can now be achieved regularly every week, making now DORIS an active part of a Global Observing System for Geodesy through the IDS.

IDS↗
NASA NTRS2003

Centimetric sea surface height accuracy using the Wide-Swath Ocean Altimeter

In this paper, we present error sources and predicted performance of the Wide-Swath Ocean altimeter, an instrument which has been proposed as an experiment for the NASA/CNES Ocean Surface Topography Mission. The data obtained by this instrument will allow the detailed study of ocean mesoscale phenomena, with a space-time resolution which cannot be achieved by a single conventional nadir altimeter.

ocean circulation↗
NASA NTRS2002

The wide swath ocean altimeter: radar interferometry for global ocean mapping with centimetric accuracy

We have developed an instrument concept that combines a conventional nadir altimeter with a radar interferometer to meet the above requirements. In this paper, we describe the overall mission concept and the interferometric radar design. We also describe several new technology developments that facilitate the inclusion of this instrument on a small, inexpensive spacecraft bus. Those include ultra-light, deployable reflectarray antennas for the radar interferometer; a novel five frequency feed horn for the radiometer and altimeter; a lightweight, low power integrated three frequency radiometer; and a field programmable gate array-based onboard data processor. Finally, we discuss recent algorithm developments for the onboard date processing, and present the expected instatements performance improvements over previously reported results.

interferometry↗
NASA NTRS2006

Towards the GEOSAT Follow-On Precise Orbit Determination Goals of High Accuracy and Near-Real-Time Processing

The US Navy's GEOSAT Follow-On spacecraft (GFO) primary mission objective is to map the oceans using a radar altimeter. Satellite laser ranging data, especially in combination with altimeter crossover data, offer the only means of determining high-quality precise orbits. Two tuned gravity models, PGS7727 and PGS7777b, were created at NASA GSFC for GFO that reduce the predicted radial orbit through degree 70 to 13.7 and 10.0 mm. A macromodel was developed to model the nonconservative forces and the SLR spacecraft measurement offset was adjusted to remove a mean bias. Using these improved models, satellite-ranging data, altimeter crossover data, and Doppler data are used to compute both daily medium precision orbits with a latency of less than 24 hours. Final precise orbits are also computed using these tracking data and exported with a latency of three to four weeks to NOAA for use on the GFO Geophysical Data Records (GDR s). The estimated orbit precision of the daily orbits is between 10 and 20 cm, whereas the precise orbits have a precision of 5 cm.

Lemoine, Frank G.↗
NASA NTRS1994

High-accuracy discrete positioning device

An article (30) is controllably and precisely positioned at one of three discrete locations defined by a linkage. The positioning apparatus includes two independently driven cranks (34, 42), with a link (50) pivotably connected between the two cranks (34, 42). Another connector (44) is pivotably connected between one of the cranks (34 or 42) and the article (30) to be positioned. The cranks (34, 42) are rotationally adjusted so that the pivot points (52, 54) of the link (50) are collinear with the axes of rotation of the cranks (40, 48), thereby defining one of the three discrete locations. Additional cranks and links can be provided to define additional discrete locations.

Brooks, John J.↗
NASA NTRS2006

Accuracy of High-Rate GPS for Seismology

We built a device for translating a GPS antenna on a positioning table to simulate the ground motions caused by an earthquake. The earthquake simulator is accurate to better than 0.1 mm in position, and provides the "ground truth" displacements for assessing the technique of high-rate GPS. We found that the root-mean-square error of the 1-Hz GPS position estimates over the 15-min duration of the simulated seismic event was 2.5 mm, with approximately 96% of the observations in error by less than 5 mm, and is independent of GPS antenna motion. The error spectrum of the GPS estimates is approximately flicker noise, with a 50% decorrelation time for the position error of approx.1.6 s. We that, for the particular event simulated, the spectrum of dependent error in the GPS measurements. surface deformations exceeds the GPS error spectrum within a finite band. More studies are required to determine whether a generally optimal bandwidth exists for a target group of seismic events.

Elosegui, P.↗