Mark 5C & Mark 6 Recorders
The VLBA's data transmission system comprises the recorder units at the stations, playback units at the correlator, and the magnetic disk modules that are shipped between those units. The Mark 5C recording system was developed jointly by NRAO, Haystack Observatory, and Conduant Corporation. It closely resembles the Mark 5A version used previously by the VLBA, and the Mark 5B used at some other observatories. In particular, identical disk modules are used. However, Mark 5C is a packet-based system, which allows a more straightforward functionality than its predecessors. It simply records the payload of each 10G Ethernet packet originating in the RDBE, without imposing any special recording format. All formatting of the observed data — most essentially, the precision time tag — is internal to the packet payloads, which are transmitted directly from recorder to playback by the Mark 5C system. Mark 5B formatting is used internally by the RDBE's PFB personality, but the newer VLBI Data Interchange Format (VDIF) is used in the DDC personality.
Each Mark 5C unit accommodates two removable modules, each in turn comprising eight commercial disk drives. Currently, these modules are recorded sequentially at a maximum rate of 2 Gbps.
The VLBA has also begun a transition of its data recording systems from the Mark 5C to the Mark 6 system. Mark 6 was developed by Haystack Observatory, and current documentation is available at their Mark 6 website. It is specified to record data at up to 16 Gbps, but inputs available from the current RDBE configuration limit its application to 4 Gbps. There is, thus, substantial headroom to support recording of data from newer, higher-capacity digital signal processing units that may be developed. About 40% of the open-skies observing time on a Shared Risk basis will be available in this observing semester (2019B) at 4 Gbps recording rate and at VLBA stations only. Details of the availability of this capability, and updates on the details for future observing semesters, are and will be specified in the current and future Calls for Proposals. Mark 5C recording will be discontinued at a future date to be determined.
