Notes for V2C telecon, Oct. 24, 2013 ------------------------------------ In attendance: Chris Beaudoin, Dirk Behrend, Johannes Boehm, Ruediger Haas, Chopo Ma, Dan MacMillan, Zinovy Malkin, Arthur Niell, Miroslav Pantaleev, Bill Petrachenko, Mamoru Sekido, Gino Tuccari As with the previous telecon, the intention of the agenda was to have a brief discussion of mixed mode issues and then to focus on aspects of VGOS development that still need to be done. However, mixed mode issues took a larger fraction of the telecon than expected so the VGOS 'gaps' will be rescheduled for the start of the next telecon. Mixed-mode discussion --------------------- The main intention here was to get confirmation from the group that the use, at legacy stations, of DBBC's in 32 MHz DDC mode for 'mixed mode' observing sessions would solve (or at least greatly reduce) the sensitivity deficit problem. It was pointed out by Gino that BW = 16 MHz is the current standard widest bandwidth for DBBC's and that 32 MHz is available only as an engineering version. The version was motivated by a desire for higher sensitivity astronomy applications and has been tested in actual observations but is not yet fully supported. With four CORE boards installed, a system can handle up to 16 32 MHz USB/LSB channels - enough for mixed mode observing. To the best of his knowledge all geodetic sites have DBBC's with four boards. To make this operational, some channel selection firmware also still needs to be developed for the FILA10G board but this should not be a large task. Eventually it was proposed that a short mixed-mode fringe test should be undertaken and that this should be preceeded by a zero baseline test. It was proposed that Effelsberg might be a good candidate site for a zero baseline test since it has both an RDBE and a DBBC and Gino has a good working relation with the staff there. Gino agreed to organize the zero baseline test but pointed out that it could not be carried until earliest the end of this year or start of next year since some development was still required. Status Reports -------------- Gino: DBBC3 development continues with new prototypes. A FILA40G board has been received from Onsala. The 4 GHz astronomy DBBC3 version is progressing quite well but the 14 GHz geodetic version is more challenging. For the geodetic version, work is progressing on the firmware for arbitrary frequency selection of bands. So far there are no commercial versions of the high speed sampler available only engineering samples. FILA10G firmware is progressing well with all functions including corner turning and Mk5 emulation mode. The broadband feed has been tested in spectral sections using narrow band LNA's but a final decision has not been made on the broadband LNA. The feed requires only one single ended LNA per polarization, much like the the QRFH. It was suggested that someone from Yebes should be invited to the telecons to report on broadband feed/LNA progress at Yebes. It is believed that the Yebes feed requires four differential LNA's. Arthur: Continues to process May data. Guidelines need to be put in place so that bands will be separated by multiples of 32 MHz. This is important for efficiency of 'fringing'. There seems to be an unexplained 300 ns offset in the low frequency band, i.e. the one not on fibre. Its possible to fit all four bands in each polarization separately but there are problems if the polarizations are joined. This worked successfully last October but Roger has been making changes that might have caused some confusion. No observations are planned for the near future but in January there will be a UT1 demo using GGAO12 and Kokee - both I believe with RDBE's. Chris: At Westford the upgrade is progressing well. Hardware has be bought, cable harnesses made and completion is expected in 4-5 weeks. The new down-link system will be sub-banded. Because of the long cable length (compared with GGAO12) both sub-bands will use fibre. The nominal sub-bands are 2-7 GHz and 6-14 GHz but more RFI data is required to make a more informed decision. RFI: It was noticed that pointing improved at GGAO when the SLR radar was down during the shut-down period. There has been no new communication with La Plata re a standard RFI measuring system. There was a telecon while the DORIS group visited GGAO. Neither Bill nor Chris attended although Brian did. A DORIS beacon will be made available for RFI studies at GGAO in summer 2014. The beacon frequency will be offset so it doesn't interfere with normal DORIS operations. A test plan is required. VEX2: Arthur reported progress at the VLBI technology meeting in Korea. All committee members were there with Ed via telecon. A first draft is expected soon. Ed should be asked for more details. Site ties: Dan asked Ruediger if there was any progress since his paper at last year's EVGA. Ruediger reported that two gimbled RTK (flat) GPS antennas have recently been mounted on the 20m antenna for determining the reference point with GPS. In addition there is an older non-gimbled antenna behind the sub-reflector. The antennas are permanent and have been calibrated both with a robot and in an anachoic chamber. No data have been taken yet. CRF: Zinovy asked whether anything had been done about increasing coverage of southern sources as proposed in his EVGA presentation. Chopo pointed out that the AUSTRAL network observed 10-12 times per year at least partly for better astrometry of southern sources. Zinovy reported that in recent CRF schedules sources were concetrated in the declination zone -30 to -70. He thought this might degrade tropo estimates. Chopo suggested he could send a note outlining the problem so it could be looked into. Action items: ------------- 1. Gino will begin to organize a zero baseline mixed-mode test. 2. Bill will invite someone from Yebes to report on their receiver/LNA development. 3. Bill will contact Hayo to see if the La Plata group is still interested in the RFI monitoring collaboration. 4. Bill, Chris, Brian: Develop a test plan for the DORIS beacon. Next telecon: Nov. 21 - focused on VGOS gaps. -------------