Engineering topics

C. Thomas

Publications and source records attributed to C. Thomas.

6 records

NASA NTRS1992

The need for GPS standardization

A desirable and necessary step for improvement of the accuracy of Global Positioning System (GPS) time comparisons is the establishment of common GPS standards. For this reason, the CCDS proposed the creation of a special group of experts with the objective of recommending procedures and models for operational time transfer by GPS common-view method. Since the announcement of the implementation of Selective Availability at the end of last spring, action has become much more urgent and this CCDS Group on GPS Time Transfer Standards has now been set up. It operates under the auspices of the permanent CCDS Working Group on TAI and works in close cooperation with the Sub-Committee on Time of the Civil GPS Service Interface Committee (CGSIC). Taking as an example the implementation of SA during the first week of July 1991, this paper illustrates the need to develop urgently at least two standardized procedures in GPS receiver software: monitoring GPS tracks with a common time scale and retaining broadcast ephemeris parameters throughout the duration of a track. Other matters requiring action are the adoption of common models for atmospheric delay, a common approach to hardware design and agreement about short-term data processing. Several examples of such deficiencies in standardization are presented.

W. Lewandowski↗

NASA NTRS1991

Precise GPS ephemerides from DMA and NGS tested by time transfer

It was shown that the use of the Defense Mapping Agency's (DMA) precise ephemerides brings a significant improvement to the accuracy of GPS time transfer. At present a new set of precise ephemerides produced by the National Geodetic Survey (NGS) has been made available to the timing community. This study demonstrates that both types of precise ephemerides improve long-distance GPS time transfer and remove the effects of Selective Availability (SA) degradation of broadcast ephemerides. The issue of overcoming SA is also discussed in terms of the routine availability of precise ephemerides.

W. Lewandowski↗

NASA NTRS1991

Accuracy of GPS time transfer verified by closure around the world

The precision of time transfer over intercontinental distances by the GPS common-view method, using measurements of ionospheric delays, precise ephemerides provided by the DMA and a consistent set of antenna coordinates, reaches 3-4 ns for a single 13-min measurement, and decreases to 2 ns when averaging several measurements over a period of one day. It is thought that even this level of precision can be bettered by improving the ionospheric measurements, the ephemeride,_ of the satellites and the antenna coordinates. In the same conditions, an estimation of the accuracy is attained by using three intercontinental links encircling the Earth to establish a closure condition: The three independent time links should add to zero. We have computed such a closure condition over a period of thirteen months using data recorded at the Paris Observatory in Paris (France), at the Communications Research Laboratory in Tokyo (Japan) and at the National Institute for Standards and Technology in Boulder, Colorado (USA). The closure condition is verified to within a few nanoseconds but a bias, varying with time, can be detected.

W. Lewandowski↗

NASA NTRS1991

Comparison of GLONASS and GPS time transfers between two west European time laboratories and VNIIFTRI

The University of Leeds built a GPS/GLONASS receiver about five years ago and since then has provided continuous information about GLONASS time and its comparison with GPS time. For the last two years VNllFTRl and some other Soviet time laboratories have used Soviet-built GLONASS navigation receivers for time comparisons. Since June 1991, VNIIFTRI has been operating a GPS time receiver on loan from the BIPM. This offered, for the first time, an opportunity for direct comparison of time transfers using GPS and GLONASS. This experiment shows that even with relatively imprecise data recording and processing, in terms of time metrology, GLONASS can provide continental time transfer at a level of several tens of nanoseconds.

P. Daly↗

NASA NTRS1976

Biostack 3 experiment MA-107

The Biostack III experiment onboard the Apollo spacecraft during the Apollo Soyuz Test Project complemented the Biostack I and II experiments of the Apollo 16 and 17 missions. The objectives of these experiments were to study the biological effects of individual heavy cosmic particles of high energy loss (HZE) not available on earth, to study the influence of additional space flight factors, to obtain knowledge on the mechanism by which HZE particles damage biological materials, to get information on the spectrum of charge and energy of the cosmic ions in the spacecraft, and to estimate the radiation hazards to man in space.

H. Bucker↗

NASA NTRSDate not supplied

Preliminary Results Cycling GEOS-JEDI with GSI-based Background Errors

The first phase of transitioning the NASA GMAO GEOS atmospheric data assimilation capabilities to JEDI involves the replacement of the Grid-point Statistical Interpolation (GSI) with a corresponding JEDI analysis. This includes taking JEDI's Unified Observation Operator (UFO), its underlying dependencies, and the JEDI solver that enables a hybrid 4DEnVar strategy similar to what is used in the current GEOS-GSI system. Variational analysis involves at least two main components associated with the observation and background cost function terms. The first is directly related to the UFO, which is being carefully validated in a joint collaboration between GMAO and NCEP to demonstrate consistency with corresponding observations usage in GSI. The second component is the background term, which in a hybrid system involves the ability to set up both a climatologically-based term and an ensemble-based term. JEDI provides the means to implement both terms through its BUMP component. Use of BUMP would require a complete re-tune of both climatological and ensemble, which is a non-trivial exercise we would prefer to avoid. As an alternative, the work here studies the results of interfacing the GSI-background error capability (GSIBEC) into JEDI through SABER. With this, the exact same background error covariance formulation used in GSI can be employed in JEDI without need for re-tuning. This brief summary covers the work done to interface GSIBEC into JEDI and shows preliminary results where the background error covariances of the control (GEOS-GSI) and experiment (GEOS-JEDI) are identical in corresponding cycling experiments. The cycling exercise is obviously preliminary and so much can be expected from GEOS-JEDI when compared to GEOS-GSI. There is still a number of features that need closer attention and although in some cases in principle ready to cycle have been intentionally either turned off or not fully exercised (e.g., VarBC is applied but not cycled). Other features are still pending implementation, one such example is the implementation of the Tangent Linear Normal Mode Constraint. Still, results are quite encouraging as hopefully the discussion here illustrates.

Ricardo Todling↗