The ionosphere also adds a delay and the correlator model has no ionospheric term. Typically this zenith path delay is about 0.7 cm at 43 GHz and scales as the inverse-square of the frequency. An approximate correction for the ionosphere can be made using a VLBAUTIL procedure in AIPS.
I have written a program that tries large numbers of randomly generated geodetic blocks and finds nearly optimum blocks that can be inserted in observing schedules. A library of geodetic blocks can be found in this website. There is a block for start times every 10 minutes; the start times are given in Pie Town local sidereal time (PT_LST). Each block requires about 45 minutes of observing time on the VLBA. The files are named "geoblock_hhmm.out", where hhmm is the start time. These blocks assume an on-source observing time of one minute (DWELL=1). For example, geoblock_0610.out would be for a 06h 10m PT_LST start time and can be retrieved with the following URL:
Since these blocks observe sources down to low elevations, it is important that they start very close in time to the indicated PT_LST time. If you use these, be sure to check that the source elevations (listed in the files) are close to what SCHED gives!
The library of geodetic blocks used a list of ICRF "QSOs" which is needed by the VLBA SCHED program. This file is called "icrf_calibrators.inp" and is also available on this web site. These QSOs were selected to have more than 1000 ICRF observations, little source structure, and positions accurate to better than 1 mas. They generally work well at 12 GHz, and most should also work at higher observing frequencies.
|Accuracy of atmospheric zenith delays from VLBA observations of geodetic blocks as a function of time dedicated to the observations. The accuracies are based on simulations and assume multi-band delays with uncertainties of 3 cm of path length (0.1 nsec delay). For blocks shorter than about 30 minutes, it is hard to find a wide range of source elevations at all 10 VLBA stations, and the accuracy degrades rapidly. For blocks longer than 30 minutes the accuracy increases slowly as the inverse square-root of time.|
1) Apply a "manual phase-calibration", using a single QSO observation to all the others. This should be done for a scan that has strong signal at all stations.
2) Use FRING to solve for multi-band delays.
3) Solve for the atmospheric delays with DELZN or some other program
4) Apply the corrections with CLCOR.
I strong recommend checking that this procedure works by re FRING'ing after correction and resolving with DELZN. The new corrections should be very close to zero.