Engineering topics

Liebrecht, P.

Publications and source records attributed to Liebrecht, P..

3 records

NASA NTRS1990

Error analysis for earth orientation recovery from GPS data

The use of GPS navigation satellites to study earth-orientation parameters in real-time is examined analytically with simulations of network geometries. The Orbit Analysis covariance-analysis program is employed to simulate the block-II constellation of 18 GPS satellites, and attention is given to the budget for tracking errors. Simultaneous solutions are derived for earth orientation given specific satellite orbits, ground clocks, and station positions with tropospheric scaling at each station. Media effects and measurement noise are found to be the main causes of uncertainty in earth-orientation determination. A program similar to the Polaris network using single-difference carrier-phase observations can provide earth-orientation parameters with accuracies similar to those for the VLBI program. The GPS concept offers faster data turnaround and lower costs in addition to more accurate determinations of UT1 and pole position.

Zelensky, N.↗

NASA NTRS1988

VLBI tracking of the TDRS

The use of radio interferometry is being investigated by NASA/Goddard Space Flight Center as a means to track the TDRS geostationary satellites. This technique offers the advantages of very high accuracy using a ground network confined to the continental U.S. and minimal (or no) impact on spacecraft systems as the downlink transmissions can be used passively. A demonstration experiment was performed using very long baseline interferometry (VLBI). The TDRS-E was observed at S-band over a 28-hour definitive period together with dual X/S-band calibration observations of quasars. An orbit was determined for TDRS-E which agrees within estimated error bars with the orbit solution from NASA's operational tracking system but with about half the uncertainty. It was found that, for S-band tracking, the ionosphere is the largest source of orbit error. Use of Ku-band tracking instead would permit geostationary orbits accurate at the 10-m level.

Ray, J.↗

NASA NTRS1983

Crustal dynamics project session 4 validation and intercomparison experiments 1979-1980 report

As part of the Crustal Dynamics Project, an experiment was performed to verify the ability of Satellite Laser Ranging (SLR), Very Long Baseline interferometry (VLBI) and Doppler Satellite Positioning System (Doppler) techniques to estimate the baseline distances between several locations. The Goddard Space Flight Center (GSFC) lasers were in operation at all five sites available to them. The ten baselines involved were analyzed using monthly orbits and various methods of selecting data. The standard deviation of the monthly SLR baseline lengths was at the 7 cm level. The GSFC VLBI (Mark III) data was obtained during three separate experiments. November 1979 at Haystack and Owens Valley, and April and July 1980 at Haystack, Owens Valley, and Fort Davis. Repeatability of the VLBI in determining baseline lengths was calculated to be at the 2 cm level. Jet Propulsion Laboratory (JPL) VLBI (Mark II) data was acquired on the Owens Valley to Goldstone baseline on ten occasions between August 1979 and November 1980. The repeatability of these baseline length determinations was calculated to be at the 5 cm level. National Geodetic Survey (NGS) Doppler data was acquired at all five sites in January 1980. Repeatability of the Doppler determined baseline lengths results were calculated at approximately 30 cm. An intercomparison between baseline distances and associated parameters was made utilizing SLR, VLBI, and Doppler results on all available baselines. The VLBI and SLR length determinations were compared on four baselines with a resultant mean difference of -1 cm and a maximum difference of 12 cm. The SLR and Doppler length determinations were compared on ten baselines with a resultant mean difference of about 30 cm and a maximum difference of about 60 cm. The VLBI and Doppler lengths from seven baselines showed a resultant mean difference of about 30 cm and maximum difference of about 1 meter. The intercomparison of baseline orientation parameters were consistent with past analysis.

Liebrecht, P.↗