Notes for V2C telecon, Sept 26, 2013 ------------------------------------ In attendance: Dirk Behrend, Brian Corey, John Gipson, Chopo Ma, Dan MacMillan, Zinovy Malkin, Arthur Niell, Bill Petrachenko, Gino Tuccari Mixed-mode issues ----------------- The intended topics of this telecon were the identification of items still missing for a smooth running of VGOS (VGOS gaps) and issues related to mixed-mode observing. We started with mixed mode observing since I thought the topic would be shorter but it ended up taking the full telecon time so a discussion of VGOS gaps was postponed to next telecon. This is a summary of the mixed-mode discussion. Why is mixed-mode observing important? -------------------------------------- In this context "mixed-mode" refers to sessions in which stations with VGOS broadband systems co-observe with legacy S/X stations. These types of sessions are essential for the transition from S/X to broadband as they enable the generation of strong geodetic ties between the new VGOS stations and the legacy S/X network. A session of this type was observed in January 2013 when Westford and GGAO12 (using broadband systems) were added to a standard R1 session in tag-along mode. What are the issues related to mixed-mode observing? ---------------------------------------------------- The use of smaller 12m antennas for VGOS was made possible by the anticipated use of high record rates, e.g. 16 or even 32 Gbps. In contrast R1 sessions are run with 1-bit sampled 8 MHz channels for a total record rate of 256 Mbps. As a result, mixed broadband-S/X baselines in the R1 session required unacceptably long integration times to reach SNR targets resulting in degraded geodetic performance. In addition, there are many incompatibilities between the stardard R1 mode that uses individual BBC's and the broadband mode that uses a polyphase filter bank (PFB), e.g. - 1-bit vs 2-bit sampling - BW (8 MHz for R1 and 32 MHz for PFB), - LO offsets (10 KHz for R1 and 400 Khz for PFB), - USB vs LSB - circular vs linear pol. - real vs complex (next version of RDBE firmware) In principal these incompatibilities can all be corrected for in a software correlator without introducing significant loss. At present though all required features have not yet been fully debugged. Possible solutions to the excess integration time problem --------------------------------------------------------- Since integration time (for a target SNR) is inversely proportional to record rate, the most obvious solution to the integration time problem is to increase the record rate of the mixed-mode sessions. This can easily be achieved using DBBCs that are already in place at most (or all) of the frequently used IVS S/X stations. The IVS coordinating centre (CC) and network coordinator recently began a program to qualify, through simultaneous analog/DBBC observations, the use of the DBBCs (in digital down converter (DDC) mode) for IVS geodetic operations. The first qualification observation is planned for Medicina in Oct. Within the foreseeable future it is expected that all frequently used sites will be operating routinely with DBBCs. Since DDC mode immitates the operation of an analog system exactly, this option is ideal for the transition period when some S/X antennas use analog systems and some use DBE's. In addition, DBBC DDC's have channel BWs up to 32 MHz; when this is combined with 2-bit Nyquist sampling and USB/LSB, an output data rate of 4 Gbps can be achieved, although it is probably not possible to record the full data rate at many stations. This essentially solves the mixed-mode integration time problem. The DBBC DDC option can also be set up in a mode that is full compatible with the RDBE polyphase filter bank (PFB) mode used at broadband sites and hence is perfectly suited for mixed mode. The DBBCs can further be used in a PFB mode that is also compatible with the RDBE PFB mode. Action items ------------ 1. Qualify DBBC's in DDC mode for use at all frequently used S/X sites and introduce into regular S/X operations as soon as possible. This has already begun. 2. Determine a method for qualifying S/X sites in PFB mode [Brian] and begin a program for carrying out the qualification. [Is this still required if the DBBC DDC mode is compatible with the RDBE PFB mode?] 3. Develop Field System control of DBE PFB modes [Ed]. 4. Schedule a PFB-based mixed-mode R&D session for 2014 including as many stations as possible [Arthur]. Next telecon: Oct 24 - focused on VGOS gaps. -------------