_                   
                                                         |_|                  
      V   V   SSSS   OOO   PPPP                 \__      |_|      __/         
      V   V  S      O   O  P   P                   --____/ \____--            
      V   V   SSS   O   O  PPPP                    _ _ _ --- _ _ _            
       V V       S  O   O  P                      |_|_|_|  @|_|_|_|           
        V    SSSS    OOO   P                             o-o                  
                                                          /                   
      ***  N    E    W    S  ***                        <)                    

PHASE TRANSFER ANTENNA CONCEPT VERIFICATION EXPERIMENT

The VSOP tracking stations serve dual purposes, transmitting the (suitably doppler-shifted) reference signal, and receiving the VLBI data stream (with its 96-bits of synchronization code and house-keeping telemetry every 5ms). During studies for future Space VLBI missions, it was recognized that the phase transfer and telemetry data services can be independent of VLBI data, and a separate ground station from that used to receive VLBI data can be used for reference signal transmission and telemetry reception. This "segmented architecture" uses a 10~15m antenna for VLBI data reception only, while a separate nearby 3m phase transfer antenna (PTA) is used for time transfer and telemetry. There are both advantages and disadvantages to this approach: existing 10~15m class radio telescopes could be readily reconfigured to work as the data-reception component of a tracking station, with the relatively inexpensive PTA installed nearby, however, there were concerns that the segmented architecture might introduce data and time correction problems, as the phase transfer antenna is separate from the VLBI data antenna, resulting in correlation being more difficult.

To test the concept, an experiment was conducted with VSOP at Goldstone. The team, led by Jim Springett of NeoComm Systems, used the regular 11m tracking station for time transfer, while receiving the VLBI data at the DSS-13 34m antenna. The two antennas are 12.5 km apart, and a relative delay of 210.2 microseconds was accurately measured during a separate VSOP tracking pass. During the VSOP observation w059a2, data was recorded on S2 tapes in the normal mode for 30 minutes, and then in the two-station mode for 60 minutes. The observation was correlated at Penticton, where correlator models used the standard time correction files for the pass, plus a 210 microsecond clock offset for two-station mode data. Space-ground fringes were found throughout the whole pass, and exhibited regularity and smoothness between the two experiment modes, as if the single 11m antenna had been used for the entire test pass. The experiment therefore conclusively demonstrated that the segmented architecture works well, with no problems evident with time correction or correlation.

VISITORS

The VSOP website, /astro/halca, currently features a photo of the government Minister of Education, Culture, Sports, Science and Technology being shown the HALCA model by VSOP Project Manager Prof. H. Hirosawa during her recent visit to ISAS. The model will also be displayed at the annual ISAS Open Day, to be held this year on July 27th, during which in excess of 10,000 visitors will have the chance to view the model and other VSOP and VSOP-2 related materials.

PUBLICATIONS

A number of papers relating to VSOP observations appear in the proceedings of the 6th EVN symposium, held last month in Bonn. The proceedings can be accessed on-line at http://www.mpifr-bonn.mpg.de/div/vlbi/evn2002/book/.

                Editors: Phil Edwards and Hirax Hirabayashi