Sixth DoD Astrometry Forum

Sixth DoD Astrometry Forum - Abstracts

The International Celestial Reference Frame: Construction, Maintenance, and Interaction with EOP/Nutation
(Chopo Ma, NASA/Goddard Space Flight Center)
The ICRF and ICRF Ext.1 are a set of accurate positions of extragalactic radio sources, largely quasars, derived from dual-frequency VLBI data, initially from 1979-95 and later extended from 1995-99. The ICRF replaced the FK5 as the defining realization of the International Celestial Reference System as of 1 January 1998 according to a resolution of the IAU. The limiting accuracy of the individual source positions is 250 microarcsec and the axes are maintained at the 20 microarcsec level. The modeling and estimation process used to create the ICRF was the state of the art in 1995, and ICRF Ext.1 was analyzed consistent with ICRF. The orientation of the ICRF was made consistent with prior realizations of the International Earth Rotation Service (IERS) and is consistent with the dynamical reference frame of planetary ephemerides and the FK5 frame at their respective levels of accuracy. The existence of nonzero nutation at J2000 was not corrected although the time series of nutation offsets would have allowed an accurate re-orientation. The ICRF is now the celestial reference frame for the definition and monitoring of Earth orientation parameters (EOP) and nutation. Further extensions will be made with suitable VLBI data as they become available.
Other useful links: hpiers.obspm.fr/webiers/results/icrf/README.html, maia.usno.navy.mil/rorf/rorf.html

Relativity Fundamentals for Time Scales and Astrometry
(Robert Nelson, Satellite Engineering Research Corporation)

The conceptual foundations of the special and general theories of relativity will be briefly described. The principal relativistic effects in inertial, rotating, and accelerated reference frames important to time measurements and the calculation of ephemerides will be summarized, including time dilation, the red shift, the Sagnac effect, and the speed of propagation of light. The magnitudes of these effects will be illustrated by the Global Positioning System, timekeeping systems onboard satellites in highly inclined elliptical orbits, and astronomical timescales.

Building Star Catalogs for Spacecraft Applications (Leonard J. Berg, Boeing)

Release of the Hipparcos and Tycho-2 star catalogs represented a huge step forward for the spacecraft application engineer. However, difficulties still remain when using these and other references to create star catalogs used to design, simulate, initialize, calibrate, and maintain spacecraft attitude determination processes. This presentation will outline the basic methods used to develop spacecraft application catalogs and help focus attention on the potential for improvement in resources used to support this effort.

USNO Almanacs (John Bangert, USNO)

The concept of an almanac has changed drastically in the 151+ years since the establishment of the U.S. Nautical Almanac Office. Today, the Astronomical Applications Department of the U.S. Naval Observatory (USNO) produces or co-produces a variety of practical astronomical data products and services that provide almanac information. These products are tailored for different applications, and are available in a variety of forms: as printed publications, small computer applications, source code libraries, and Internet services. This presentation will review the current set of almanac products available from USNO and present plans for their evolution.

Earth Orientation Products from USNO (Jim Ray, USNO)

Errors in predicted Earth orientation parameter (EOP) values can be a significant source of systematic error in predicted GPS orbits and other real-time navigation applications. An EOP error of 1 milliarcsecond (mas) corresponds to a net rotation of the GPS constellation by up to ~13 cm at altitude. During the past three years, the IERS Product Center for Rapid Service and Predictions, based at the U.S. Naval Observatory, has worked closely with the International GPS Service (IGS) to develop improved EOP prediction products. These depend critically on the IGS Rapid observational products (delivered daily with 17 hours delay) and are available for the IGS Predicted and new Ultra-rapid products which support real-time GPS applications. The goal is orbital accuracies about 20 cm or better for the most demanding uses, such as atmospheric soundings for weather models. To meet these demands, the IERS Bulletin A files are now updated daily with results available at the Web site http://maia.usno.navy.mil.

By moving from weekly to the current daily updates, together with numerous algorithmic improvements, the IERS Bulletin A now provides real-time users with accuracies no worse than 0.8 mas for polar motion and 0.17 ms (2.6 mas) for UT1. Prediction of UT1 variations remains problematic. Recently, predicted atmospheric angular momentum changes, a byproduct of the National Centers for Environmental Prediction weather models, were incorporated into the Bulletin A combination as a new data type. This step is certain to improve the near-term UT1 predictions but it remains to be seen by how much. In addition, twice-daily updates may be implemented in the near future using results from the IGS Ultra-rapid products.

Automated Celestial Systems for Attitude & Position Determination (George Kaplan, USNO)

The vulnerabilities of the GPS system are now well quantified and amelioration of the risk of GPS denial has become a major focus of R&D and operations planning within DoD. Hardware solutions have centered around increased use of inertial navigation systems (INS) and embedded GPS-INS (EGI) combinations. Prudent navigation practice (and Navy policy) requires both a primary and a secondary means of navigation, with the secondary independent of the primary. Since INS is basically a form of automated dead reckoning, INS and EGI solutions do not provide a truly independent navigation reference. Celestial navigation remains the only alternative to GPS that is linked to a global external reference frame. Augmentation of INS and EGI with absolute vehicle attitude determination based on automated measurement of stellar angles would provide the needed link to an independent, precise, and well-defined reference system. Although such astro-inertial systems are now in limited operational use, with good success, the automated star trackers they contain are based on outdated, gimbaled technology. New strapdown star tracker systems with silicon array detectors, currently used in space applications, would provide a cheaper, more reliable navigation system with a significantly reduced footprint. With reduced costs and enhanced reliability, such systems may be practical on many platforms not previously considered, including surface ships and a variety of aircraft.