Engineering topics
Gordon, David
Publications and source records attributed to Gordon, David.
Improving the S/X Celestial Reference Frame in the South
We believe that the S/X celestial reference frame(CRF) can be improved in the far-south by a factor of 2 in density and a factor of 2.5 in precision. We have started a collaboration to meet these goals. We have increased the data rates on existing IV Sastrometric sessions in the south from 256 Mbps to 1 Gbps. We will use this sensitivity to detect weaker sources and to improve the precision of sources in the southern S/X CRF, while simultaneously increasing the number of sources, in particular the overlap with other frames such as K-and Ka-band in the radio and the Gaia frame in the optical. VLBI observations in the southern celestial hemisphere have always been more difficult both because there are fewer radio telescopes in the south than in the north, and because there are fewer known reference sources in the south. There have been many efforts in recent years to increase the number of known reference sources in the south, in particular the LBA calibrator Survey (LCS), which has already produced a significant improvement at X-band. The ICRF-3 is expected to make significant improvements in the south, however the south has not yet reached parity with the north and much work remains to be done. Therefore dedicated astrometric and imaging observations have already begun to improve the southern CRF at S/Xbands.
GSFC VLBI Analysis Center
This report presents the activities of the GSFC VLBI Analysis Center during 2012. The GSFC VLBI Analysis Center analyzes all IVS sessions, makes regular IVS submissions of data and analysis products, and performs research and software development aimed at improving the VLBI technique.
Use of GPS TEC Maps for Calibrating Single Band VLBI Sessions
GPS TEC ionosphere maps were first applied to a series of K and Q band VLBA astrometry sessions to try to eliminate a declination bias in estimated source positions. Their usage has been expanded to calibrate X-band only VLBI observations as well. At K-band, approx.60% of the declination bias appears to be removed with the application of GPS ionosphere calibrations. At X-band however, it appears that up to 90% or more of the declination bias is removed, with a corresponding increase in RA and declination uncertainties of approx.0.5 mas. GPS ionosphere calibrations may be very useful for improving the estimated positions of the X-only and S-only sources in the VCS and RDV sessions.
Coordinating, Scheduling, Processing and Analyzing IYA09
The IVS scheduled a special astrometric VLBI session for the International Year of Astronomy 2009 (IYA09) commemorating 400 years of optical astronomy and 40 years of VLBI. The IYA09 session is the most ambitious geodetic session to date in terms of network size, number of sources, and number of observations. We describe the process of designing, coordinating, scheduling, pre-session station checkout, correlating, and analyzing this session.
RDV77 VLBA Hardware/Software Correlator Comparisons
Results of a hardware vs. software correlation of the RDV77 session are presented. Group delays are found to agree (WRMS differences) at an average level of 4.2 psec and with a noise floor of 2.5 psec. These RDV77 comparisons agree well with several previous correlator comparison studies.
Time-Dependent Selection of an Optimal Set of Sources to Define a Stable Celestial Reference Frame
Temporal statistical position stability is required for VLBI sources to define a stable Celestial Reference Frame (CRF) and has been studied in many recent papers. This study analyzes the sources from the latest realization of the International Celestial Reference Frame (ICRF2) with the Allan variance, in addition to taking into account the apparent linear motions of the sources. Focusing on the 295 defining sources shows how they are a good compromise of different criteria, such as statistical stability and sky distribution, as well as having a sufficient number of sources, despite the fact that the most stable sources of the entire ICRF2 are mostly in the Northern Hemisphere. Nevertheless, the selection of a stable set is not unique: studying different solutions (GSF005a and AUG24 from GSFC and OPA from the Paris Observatory) over different time periods (1989.5 to 2009.5 and 1999.5 to 2009.5) leads to selections that can differ in up to 20% of the sources. Observing, recording, and network improvement are some of the causes, showing better stability for the CRF over the last decade than the last twenty years. But this may also be explained by the assumption of stationarity that is not necessarily right for some sources.
Interaction Between the Celestial and the Terrestrial Reference Frames
Effects of International Celestial Reference Frame (ICRF2) on the Terrestrial Reference Frames (TRF), CRF and EOP's, The ICRF2 became official on Jan. 1, 2010. It includes positions of 3414 compact radio astronomical sources observed with VLBI, a fivefold increase from the first ICRF. Numerous new VLBI models were used and the most unstable sources were treated as arc parameters to avoid distortions of the frame. The ICRF2 has a noise floor of 40 micro-arc-seconds and an axis stability of 10 micro-arc-seconds. It was aligned with the ICRS using 138 stable sources common to ICRF2 and ICRF-Ext2. Maintenance of ICRF2 is to be made using 295 defining sources chosen for their historical positional stability, minimal source structure, and sky distribution. Their stability and their more uniform sky distribution eliminate the two largest weaknesses of ICRF I. The switchover to ICRF2 has some small effects on the TRF, CRF and Earth Orientation Parameters (EOP). A CRF based on ICRF2 shows a relative rotation of 40 micro-arc-seconds, mostly about the Y-axis. Small shifts are also seen in the EOP's, the largest being 11 micro-arc-seconds in X-pole. Some small but insignificant differences are also seen in the TRF. These results will be presented and discussed.
GSFC VLBI Analysis center
This report presents the activities of the GSFC VLBI Analysis Center during 2004. The GSFC Analysis Center analyzes all IVS sessions, makes regular IVS submissions of data and analysis products, and performs research and software development activities aimed at improving the VLBI technique.
Extending the ICRF to Higher Radio Frequencies: 24 and 43 GHz Astrometry
We present imaging results and source structure analysis of extragalactic radio sources observed using the Very Long Baseline Array (VLBA) at 24 GHz and 43 GHz as part of an ongoing NASA, USNO, NRAO and Bordeaux Observatory collaboration to extend the International Celestial Reference Frame (ICRF) to higher radio frequencies. The K/Q-band image database now includes images of 108 sources at 43 GHz (Q-band) and images of 230 sources at 24 GHz (K-band). Preliminary analysis of the observations taken to date shows that the sources are generally more compact as one goes from the ICRF frequency of 8.4 GHz to 24 GHz. This result is consistent with the standard theory of compact extragalactic radio sources and suggests that reference frames defined at these higher radio frequencies will be less susceptible to the effects of intrinsic source structure than those defined at lower frequencies.
Limitations to Dual Frequency Ionosphere Corrections for Frequency Switched K-Q-Band Observations with the VLBA
A series of VLBA experiments were carried out at K and Q bands for astrometry and imaging within the KQ VLBI Survey Collaboration. The paired K and Q observations of each source are separated by approximately 3 minutes of time. We investigate the delay effect of the ionosphere between K and Q bands involving the interscan separation. This differential delay effect is intermixed with the differential fluctuation effect of the troposphere.
Structure Corrections in Modeling VLBI Delays for RDV Data
Since 1997, bimonthly S- and X-band observing sessions have been carried out employing the VLBA (Very Long Baseline Array) and as many as ten additional antennas. Maps of the extended structures have been generated for the 160 sources observed in ten of these experiments (approximately 200,000 observations) taking place during 1997 and 1998. This paper reports the results of the first massive application of such structure maps to correct the modeled VLBI (Very Long Baseline Interferometry) delay in astrometric data analysis. For high-accuracy celestial reference frame work, proper choice of a reference point within each extended source is crucial. Here the reference point is taken at the point of maximum emitted flux. Overall, the weighted delay residuals (approximately equal to 30 ps) are reduced by 8 ps in quadrature upon introducing source maps to model the structure delays of the sources. Residuals of some sources with extended or fast-varying structures improve by as much as 40 ps. Scatter of 'arc positions' about a time-linear model decreases substantially for most sources. Based on our results, it is also concluded that source structure is presently not the dominant error source in astrometric/geodetic VLBI.
RDV Analysis and Mark 4/VLBA Comparison Results
The processing of the RDV sessions and problems encountered in their analysis axe briefly discussed. Results of a VLBA/Mark 4 correlator comparison are presented. Comparison of group delays in session RDV22 processed by two different correlating/fringing systems shows good agreement, with an RMS of approx. 12 psec. Group delay formal errors may be underestimated in the RDV processing.
Towards a Future ICRF Realization
The data and analysis for the ICRF were completed in 1995 to define a frame to which the Hipparcos optical catalog could be fixed. Additional observations on most of the 608 sources in the overall ICRF catalog have been acquired using a small portion of geodetic observing time as well as astrometric sessions concentrating on the Southern Hemisphere. Positions of new sources have been determined, including approximately 1200 from a VLBA phase calibrator survey. A future ICRF realization will require improved geophysical modeling, sophisticated treatment of position variations and/or source structure, optimized data selection and weighting, and re-identification of defining sources. The motivation for the next realization could be significant improvement in accuracy and density or preparation for optical extragalactic catalogs with microarcsecond precision.
GSFC VLBI Analysis Center
This report presents the activities of the Goddard Space Flight Center's Very Long Base Interferometry (VLBI) Analysis Center during the period from March 1, 1999 through December 31, 2000. The center's primary software development, analysis, and research activities axe reported, and the responsible staff members are described. Plans for 2001 are also presented.
GSFC VLBI Analysis Center Annual Report
The GSFC VLBI group, located at NASA's Goddard Space Flight Center in Greenbelt, MD, is a part of the NASA Space Geodesy Program. Since its inception in the mid 1970's, this group has been involved with and been a leader in most aspects of geodetic and astrometric VLBI. Current major activities include coordination of the international geodetic observing program; coordination and analysis of the CORE program; VLBI technique development; and all types of data processing, analysis, and research activities.