TP015 Phase Referenced Imaging Results

We have used the ionosphere corrected data to make phase referenced images of the ``target'' sources in TP015. Figures 8 and 9 show such images for two of the sources at S band. Four images are shown of each source, differing only in the type of ionospheric calibration applied, as labeled above the plots. All images were made with IMAGR using a robustness of 4, which is close to natural weighting. It is clear that all of the ionospheric calibrations are better than no calibration. It is also clear that the S/X based calibration is best.

Often the goal of phase referenced observations is to measure positions. Figures 10 and 11 show positions derived for the source in each of the phase referenced images at S and X band respectively (the X band images were made with robustness of 1). The cross is the phase center, at the location of the position from the geodetic catalog. Recall that these positions have formal errors of less than 0.1 mas based on massive fits to many geodesy observations. The derived positions from the images are based on least squares fits of a gaussian to the peak in the image (JMFIT in AIPS). The relative flux density of each fitted gaussian is indicated by the size of the circle marking the source location. In the S band plots, the points marked with ``NON'' are based on data with no ionospheric calibration. In one case (0201+113), the image with no ionospheric calibration did not have a single peak that could be identified as the source. The ``SX'' points are based on the S/X calibration excluding the 2 stations (SC and MK) for which SATLOC data was unavailable or was unreliable, and the ``SXA'' data includes all stations. The ``JP'' and ``JPL'' points are the 8 and 10 station results with the JPL global model used for calibration. The ``GPS'' points are based on SATLOC calibration and 8 station data. In the X band plot, ``NO'' is the point with no calibration, ``SX'' and ``JP'' are the 10 station SX and JPL calibrated points and the ``SA8'' is the 8 station result using SATLOC calibration.

Figure 8: Images of 0202+149 made at S band with different ionospheric phase corrections, as labeled.

Figure 9: Images of 0235+164 made at S band with different ionospheric phase corrections, as labeled.

Figure 10: Positions of sources measured with least squares fits to image peaks in S band phase referenced images made with various ionospheric corrections. See the text for details.

Figure 11: Positions of sources measured with least squares fits to image peaks in X band phase referenced images made with various ionospheric corrections. See the text for details.

In most cases, the uncalibrated images give worse positions than the calibrated images. The recovered flux density is also lower. In one case, the phase referencing did not work at all without ionospheric calibration. In no case was the phase referencing perfect. The imaging results are subject to other model errors besides the ionosphere. The dominant such errors are likely to be from the troposphere which we have made no effort to remove. One reason that the geodetic positions are likely to be much better than our phase referenced positions is that serious efforts to remove tropospheric delay offsets and all other significant effects (EOP, clocks ...) are part of the usual geodetic data processing.

One somewhat disturbing feature of the position measurement results is that the spread of positions is much larger at S band than at X band. This is likely related to the larger beam size at S band and such an effect is reasonable when there are significant uncorrected errors. But the S/X results should have the ionosphere removed quite effectively and the troposphere should affect both S and X band the same in terms of delay, which is what matters for a position measurement. A frequency dependent spread might be expected if the longest baselines are sufficiently poorly calibrated that the source could be anywhere over roughly a beam area. But we don't believe we are in that regime.