V2C Telecon Notes, Thursday, Jan.24, 2013 ----------------------------------------- In attendance: Arthur Niell, Bill Petrachenko, Brian Corey, Gino Tuccari, Jing Sun and Johannes Boehm, Zinovy Malkin, Ruediger Haas, Miroslav Pantaleev, Chopo Ma. 1. Broadband developments Preparations are under way for the R&D on Tuesday (Jan.29). The purpose of the R&D is to gain experience with mixed mode (broadband and S/X) observations. Westford and GGAO12 will operate in broadband mode and seven other stations will operate in S/X mode. For the broadband stations one band will be set to S-band, two bands to X-band (two are needed to cover the full 720-MHz of X-band) and one band will be located strategically in between. Having two bands set to X-band is somewhat inefficient for broadband. In the future, the use of 1-GHz VLBI2010 bands will make it possible to allocate a single band to X-band so that more efficient sequences are possible. In the meantime, the inefficiency will be overcome with excess SNR, which is anyway a side effect of the mixed mode observations since S/X is inherently less sensitive. The sensitivity discrepancy can be balanced somewhat if S/X stations convert to digital back ends and if both circular polarizations are acquired. The schedule is in the style of an R1 observation with 14-15 scans per hour. There is good news and not so good news for the system at GGAO. The good news is that the SEFD of the H-pol signal is in the range of 1900 to 2000 J. This exceeds the VLBI2010 target of SEFD=2500 J providing proof that a practical 12-m antenna system can be built to satisfy the broadband VLBI2010 sensitivity spec. The not so good news is that the sensitivity of the V-pol is not as good, but it is important to point out that the reason for this degradation is well understood. As the receiver is designed, digitally controlled attenuators are installed in the H-pol and V-pol signal path (and also in the pcal and noise cal signal paths). These are required to compensate for a lack of dynamic range in the down-link fiber connection. [The dynamic range of the LNA is 120-dB which needs to be matched by dynamic range of 140-dB in follow on electronics to avoid degradation. Unfortunately, the dynamic range of the down-link fiber is only 100-dB so the programmable attenuators are required to compensate for variations in input signal strength caused by for example variations in ground pick-up, atmosphere temperature, and (potentially largest of all) RFI.] In the short term it was possible, in the H-pol signal path, to replace the digital attenuator with a fixed attenuator with best compromise value. Unfortunately the V-pol attenuator is not easily accessible and will not be changed until after the R&D session. The malfunctioning digital attenuators have been sent back to the manufacturer for failure analysis. Two DBBC2's are nearly complete, one for Shanghai (Feb) and one for Warkworth, NZ (March). Implementation of field system support for the DBBC2 has been completed and very flexible firmware has been developed to support VDIF on the FILA10G. DBBC3 development is progressing. There is a 4-GHz EVN version and a 14-GHz IVS version. The EVN version is fully defined and PCB's are under construction. The two versions use different numbers of serial lines but are based on the same sampler chip. So far only proprietary samples of the chip are available and the commercial version is not out yet. The development of a full broadband receiver based on the DBBC3 continues. A commercial feed from a project in Torino is being tested. So far the receiver is mechanically defined but the dewar development is waiting for the final selection of feed. The receiver includes LNA's only and no analog down-conversion electronics since RF sampling with the DBBC3 will be employed. The first version of the receiver will be developed for Noto where a third mirror is required to adapt the feed optics to the antenna optics. Chris pointed out that he learned at URSI that a new foundry is available for LNA's using the Indium Phosphate process. Details were not clear but it appears that this foundry is at Chalmers. 2. RFI Work continues on the analysis of the V2PEG RFI survey. An abstract has been submitted to EVGA to report on preliminary results and conclusions. Work continues at GGAO to study barriers to attenuate DORIS and SLR radar signals. Scaffolding and fixtures are in preparation but as yet no measurements or results are available. 3. Source structure An informal source structure meeting is planned for the evening of Tuesday March 5 at the EVGA. Unfortunately for me (Arthur and Chopo too) that time conflicts with the already scheduled V2PEG meeting. Since there is no flexibility in the timing of the source structure meeting I will look into the possibility of rescheduling the V2PEG. 4. Automation Alan Whitney has prepared a new revision of VEX2 which is being reviewed by Walter Brisken and Ed Himwich. 5. Schedules The Vienna group is not currently working actively on optimizing mixed schedules but has submitted a proposal for future work. They are planning to use the R&D experiments this year to test mixed scheduling strategies and would like to provide as much support as possible for all IVS scheduling. There was a question from Arthur as to who will be preparing schedules for the mixed mode R&D's. It was decided that face to face discussions with John Gipson will be a good way to resolve these issues. He will be visiting Vienna in the near future. 6. Antenna deformations and Site ties There will be an IERS workshop on site ties held in Paris in May. Zuheir Altamimi and Perguido Sarti are the moderators. Depending on funding, Chopo, Dan, Jim Long, Chris and Arthur are considering going. Chris has proposed a novel ground-based method for tying the techniques together and was encouraged to present it at the workshop. 7. Atmospere Johannes has filled in some atmosphere related information on the V2C Wiki and encourages people to take a look. 8. TecSpec Post Mortem DBE comparison tables have been distributed. Miroslav will provide an updated version of the comparison document including a efficiency plot for the QRFH and Eleven Feeds. 9. Wiki MIT will allow up to about 1 Gbyte memory for each Wiki. Due to the memory limitation, data from Haystack/Goddard broadband tests will be stored on CDDIS. Next V2C telecon: March 28.