VLBI_UDP: An application for transporting VLBI data using the UDP protocol

This paper describes the VLBI_UDP application, which has been designed to transport astronomical data recorded via a technique known as Very Long Baseline Interferometry (VLBI), using the UDP protocol. The design of the VLBI_UDP application is discussed together with modifications that provide additional features such as file access and packet dropping. Network tests of the application show that the application is stable and correctly deals with packets lost on the network. Correlation tests using data with deliberately dropped packets show that the correlation process is resilient under data losses approaching 20% and that the resulting correlation amplitude decreases as 1.2 times the packet-loss rate. The results also indicate that while the correlator is able to cope with at least 2 consecutive missing headers, it needs to see at least 2 correct headers, otherwise synchronisation is lost.