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. Three methods were considered: 1. The analog systems could be used in different modes or upgraded to achieve higher bandwidth and record rate. This options was not however considered seriously since the analog systems are already essentially obsolete. 2. Digital back ends (DBEs) could be introduced at S/X stations and used in digital down converter (DDC) mode. Since DDC mode immitates the operation of an analog system exactly, this option is ideal for the period when some S/X antennas use analog systems and some use DBE's. In addition, DBBC DDC's have channel BWs up to 16 MHz; when this is combined with 2-bit sampling, a quadrupling (to 1 Gbps) of the R1 data rate can easily be achieved. This is a good first step in relieving the mixed-mode integration time problem. As it turns out, all of the frequently used stations already have DBBCs [I think that this is correct.] and the IVS coordinating centre (CC) and the network coordinator have begun a program to qualify, through simultaneous analog/DBBC observations, the use of the DBBCs (in DDC mode) for IVS geodetic operations. The first qualification observation is for Medicina in Oct. Within the foreseeable future all frequently used sites should be operating with DBEs. On the down side, the DBBC DDC option has limited data rate potential and is incompatible in many ways with the RDBE polyphase filter bank (PFB) mode used at the existing broadband sites. 3. Digital back ends (DBEs) could be introduced at S/X stations and used in PFB mode. This has the advantage of maximum compatibility with the VGOS broadband system S/X mode and greatest data rate increase potential; hence it is the recommended option for future mixed-mode operations. However, the use of PFB's has a number of significant differences from the current analog mode and hence is not considered practical for normal S/X operations during the period when DBEs are being sequentially introduced into the legacy S/X network. In the long run, it is proposed that all S/X sessions be carried out in PFB mode to maintain consistency between regular S/X and mixed-mode sessions. 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. 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. -------------