RDV Processing Using Fourfit

David Gordon, NVI/NASA GSFC Roger Cappallo, MIT Haystack Observatory Mike Titus, MIT Haystack Observatory

Beginning in early 2011, it has been possible to fringe the VLBA difx correlated RDV sessions using the Haystack HOPS/Fourfit program. Previous VLBA correlated sessions were fringed using the NRAO AIPS program. The AIPS data was considered excellent and has had a very favourable impact on the TRF, CRF, and EOP from VLBI. Nonetheless, Fourfit processing presents several important advantages, such as the ability to fringe all channels within a band coherently, and the ability to use the many HOPS diagnostic tools that were designed specifically for geodetic processing. To develop this new ability, RDV's 85-89 were Fourfit fringed at Haystack Observatory. This involved considerable data and software debugging and various fixes to the difx2mark4 and Fourfit pograms. These 5 sessions were also AIPS processed at GSFC. In comparison, the Fourfit processing shows an approximately two-fold improvement in sensitivity, better detection of weak sources, fewer subambiguity problems, and the ability to use the difx-extracted phase cals. We will describe the new data processing path and will present a comparison of the AIPS vs. Fourfit processing of RDV's 85-89.