VLBI uses globally distributed radio telescopes to measure and accurately time tag the signals of extragalactic radio sources. The recorded signals are transferred to a central processing unit to determine the fundamental observable for geodetic VLBI, the difference in signal arrival times (delays). Since the delays are determined by the cross-correlation of the globally recorded signals, the central processing unit is commonly referred to as the correlator. 

In 2022, ten correlators are actively involved in VLBI correlation as an essential component of the International VLBI Service for Geodesy and Astrometry (IVS) worldwide. Six of them process signals of the new VLBI system, which is referred to as the VLBI Global Observing System (VGOS). One of the VGOS-capable correlators is located in Vienna and run by our research group Higher Geodesy of TU Wien. For more information on VLBI sessions carried out in 2022 and their corresponding correlators, you can look at the multi-agency schedule files hosted by NASA (master files).

The signals observed by VGOS telescopes are characterized by enormously high recording rates (up to 8 Gpbs) and unprecedented broadband observations (2-14 GHz). Furthermore, a critical goal of VGOS is to provide geodetic products from 24-hour sessions on an operational basis continuously seven days per week. These circumstances make the correlation and transfer of the signals to the correlator the bottleneck due to the demands on data rates, storage, computing cores, and operator expertise.

At TU Wien, we use the Vienna Scientific Cluster 4 (VSC-4) to correlate the vast amount of VGOS signals. Here we have access to: 

• 10 nodes with 2 × Intel Platinum 8174 (24 cores, 3,1 GHz, 96 GByte main memory),
• 1 PByte storage with General Parallel File-System (GPFS), and
• 10 Gbps links to GEANT for data transmission.  

 The following software packages are installed and maintained at the VSC-4 for VGOS correlation:

• data transmission: jive5ab, etd/etc, tsunami,
• correlation: Distributed FX (DiFX),
• post-processing: Haystack Post-Processing System (HOPS) and nuSolve. 

Currently, there are three people actively involved in data transmission and correlation:

• Frederic Jaron (data transmission, correlation, and post-processing),
• Jakob Gruber (data transmission, correlation, and post-processing), and
• Leo Baldreich (data transmission)

Our biennial reports can be found here.

For more questions or information, please do not hesitate to ask us via correlation@tuwien.ac.at.