Abstract

Astrometric observations of distant active galactic nuclei (AGN) have been used to construct quasi-intertial global reference frames, most notably the International Celestial Reference Frame (ICRF) which now forms the basis for all astrometry including deep space navigation. The ICRF frame was defined using X- (8.4 GHz) and S-band (2.3 GHz) observations collected over the past 20+ years. There are several motivations for extending this work to higher radio frequencies, namely, to construct a more stable frame based on more compact sources, to provide calibrators for phase referencing, and to support spacecraft navigation at higher frequencies.

As a first step toward these goals, in 2002, we began a series of survey observations using the Very Long Baseline Array (VLBA) of ten radio telescopes at K-band (24 GHz) and Q-band (43 GHz). Each session covers the full 24 hours of right ascension and covers declinations down to the VLBA's southern limit (approx. -30 deg). Analysis of the first three sessions has produced a full sky catalog of 108 sources with median formal position uncertainties for K-band of 210 and 280 µas in RA cos(dec) and Dec, respectively. For Q-band the result is 292 and 404 µas in Ra cos(dec) and dec, respectively. Group delay residuals were excellent at ~20 psec WRMS. An external comparison of K-band frame to the S/X-band ICRF shows systematic errors at about 2 times larger than the formal precision. These systematic differences include a zonal trends vs. declination. We suspect that these errors are due in part to the North-South geometric weakness of the array and in part to mismodelling of troposphere.

Extending the ICRF to higher radio frequencies: Astrometry at 24 and 43 GHz

C.S. Jacobs (JPL), P. Charlot (Obs. Bordeaux), D. Gordon (GSFC) , G.E. Lanyi (JPL), C. Ma (GSFC), C.J. Naudet (JPL), O.J. Sovers (RSA/JPL), L.D. Zhang (JPL) and the KQ VLBI Survey Collaboration

Acknowledgement: The research described in this paper was performed by a team from the KQ VLBI survey collaboration with members from the Jet Propulsion Laboratory of the California Institute of Technology, Goddard Space Flight Center, U.S. Naval Observatory, (all under under a contract with the National Aeronautics and Space Administration), and at the National Radio Astronomical Observatory, and Bordeaux Observatory.

Fig. 1 - Photos credit NRAO/NSF/AUI (http://www.aoc.nrao.edu/vlba/html/vlbahome/thesites.html)

VLBA Array of Ten 25m antennas

ObservationsThe results presented here represent progress to date based on the first three sessions which took place on 15 May 02, 25 August 02 and 26 Dec 02. The 108 sources observed were selected for their expected compactness and strong flux (typically > 0.4 Jy). Each source was observed with 3-5 snapshots each which used the entire array for 2 min at K-band and 2 min at Q-band. This strategy was chosen to allow good uv coverage for simultaneous imaging while still permitting sub-mas global astrometry from group delays measured over a 400 MHz spanned bandwidth. Recorded bandwidth was 128 Mbps.

As may be seen in fig. 1 above the array has a greater East-West extent compared to its North-South extent. As a result the array produces more precise right ascensions than declinations by roughly factor of 1.5. The array is able to measure sources down to about -30 deg declination.

Results

The first three sessions have been combined to produce a global reference frame consisting of right ascension and declinations. Because these sources are at extreme extra-galactic distances (typically redshift z ~1) we have assumed that proper motions and parallaxes are negligible.

Fig 2. Distribution of 108 K-band Sources Observed.There are several things to note: 1) The VLBA observed only down to about -30 deg declination. 2) Declination precision systematically Decreases as one moves south. This may be seen from the color coding which indicates the declination formal precision according to the above given legend. 3) The dashed yellow line indicates the galactic plane. 4) The dashed light blue line indicates the ecliptic plane. 5) Q-band results are similar but the precisions are roughly 1.7 times larger due to fewer measurements.

Fig 3a. and 3b. RA and Dec differences: K-band vs. S/X ICRF.In order to evaluate the accuracy of our reference frame we compared our results to a recent S/X solution (DDOR_2002) which includes about 3 million group delay measurements acquired from 1978 to 2002. This S/X frame is an unpublished extension of the ICRF extension 1 frame and is consistent with that frame at the 100 µas level. The differences seen here are 293 and 574 µas wrms in Ra cos(dec) and Dec, respectively. The differences are about 2 times larger than the formal precisions. Thus these differences are an indication of systematic errors.

Fig 4a. and 4b. RA and Dec differences: K-band vs. Q-band.The histograms below show K-band vs. Q-band mean differences of 159 and 141 µas in RA and Dec, respectively. K and Q-band differences give evidence for systematic errors from sources such as plasma and/or instrumentation. The scatter is roughly consitent with formal errors.

Fig 5. Zonal Error: ∆RA vs. Declination.The figures above show K-band minus S/X-band differences for RA vs. Declination. A 3-D rotation (< 150 µas per component) was removed to account for unmodelled differences in the conventional orientation of each frame.

In Figure 5a. the minimal 1.5 sources have been constrained in order to fix the orienation of the frame. A residual zonal trend can be discerned. Removal of this trend reduces the RA cos(dec) wrms scatter from 294 to 248 µas. Note that the least squares fits plotted above account for the correlations between source coordinates (i.e. full covariance).

In figure 5b we have constrained four widely spaced K-band sources to their S/X positions. This effectively constrains the large scale zonal errors and reduces the scatter down to 168 µas. Some hints of zonal errors remain.

Further study:

The results shown represent a work in progress. In particular, more data is on the way and various aspects of the modelling have yet to be fully explored.

Data: We have been granted time for four more VLBA sessions in 2003. Data for the first of these sessions was acquired in May and will soon be added to the global analysis. Three more sessions are planned for later in the year.

Troposphere: Preliminary studies show that 3 hour troposphere estimation intervals improve the results compared to 1 hour intervals. Troposphere modelling is of interest because experience gained over the years from analyzing S/X-band data suggests that zonal trends vs. declination may be due in part caused by troposphere mismodelling.

GPS calibrations: About half the VLBA sites have nearby GPS antennas. Troposphere estimates from this GPS data could be applied to calibrate the VLBI data. GPS is also used for estimating global ionosphere models which could be used to calibrate our data sets.

Source Structure: Images of the sources have been made (see Fey et al paper this session). Code has been written to correct the astrometric data for extended structures. Preliminary results from applying these corrections show very slight improvements (2 psec) in both the K and Q-band group delay residuals. Estimated source positions changed typically by less than 100 µas.

Fig 6. RA inter-source correlations: Fixing 1.5 vs. 4 source positions.The figures above show RA inter-source correlations vs. the arclength separating a given pair of sources. The color coding indicates declination band (orange,red,green,blue,purple,black =lowest…,highest dec). Figure 6a has the minimally necessary one and half source constraint needed to fix the 3-D orientation. Figure 6b fixes 4 sources which effectively constrains the large scale zonal behavior and greatly reduces inter-source correlations.

As made clear by fig. 6a., when only 1.5 sources are fixed the RA parameters are not well separated. In fig. 6b. fixing 4 widely spaced sources allows a clean separation of RA parameters at the cost of imposing outside (S/X-band) information on the K-band solution. This takes away the K-band solution’s ability to stand as a completely independent frame.

Fig 5a: 1.5 sources fixed Fig. 5b: 4 sources fixed

Fig 6a: 1.5 sources fixed Fig. 6b: 4 sources fixed

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