_                    
                                                         |_|                   
      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  ***                        <)                     
 

Previous Issue Number 63 16th May 1997 Following Issue

FRINGES!!

First fringes have been found to HALCA! Observations of PKS1519-273 at 1.6 GHz were made on the 7th of May by HALCA, the Usuda 64m and the Kashima 34m. Data from the satellite were down-linked to the Usuda 10m tracking station antenna. The data were recorded in VSOPT format and correlated at the Mitaka correlator. The Usuda-Kashima baseline was correlated within several days of the observations, but the baselines to HALCA required the determination of the reconstructed satellite orbit and the creation of the time correction file for the tracking station time tags. (A predicted orbit is used during observations for the tracking stations to uplink the doppler-corrected reference frequency, however correlation requires a more accurate `reconstructed' orbit from range and range-rate data taken around the epoch of the observation.)

Fringe searching on the space baselines started around mid-day on Tuesday 13th May, and fringes appeared late that evening. One 16 MHz channel was used and the first results for the Usuda--HALCA baseline, obtained with an incomplete knowledge of the phase behaviour, revealed fringes with a delay of -4.4 microseconds, and a rate of 1.44 nano-seconds per second with a signal-to-noise ratio from a 200 second coherent integration of 13.

Fringe searches from the same network from observations in late March have not yielded fringes. Upgrades to the tracking station hardware were made and a better understanding of the time delays in the whole system were obtained during the period of long eclipses, and these improvements appear to be the main reason for the success of this observation.

The official press release was made this afternoon at Monbusho (the Ministry of Education, Science, Sports and Culture), with Project Manager H. Hirosawa and Project Scientist H. Hirabayashi from ISAS, and Prof. M. Inoue from NAO in attendance to describe the results and answer questions.

After the jubilation of the 13th and 14th, there was momentary concern as the satellite switched itself into safe-hold mode during a maneuver. This unfortunately resulted in the cancellation of VLBA and S2 fringe-finding experiments. The reason for the safe-hold was quickly found, and HALCA has been taken out of safe-hold mode, with fringe-finding tests to resume next week. Efforts will continue to be concentrated at 1.6 GHz for the few weeks, before moving to 5 GHz and 22 GHz in turn.

PKS1519-273 had the highest brightness temperature of the sources observed at 2.3 GHz in the TDRSS experiment, but just as importantly for these fringe-finding observations, was able to be observed on short projected baselines: 30 Mega-wavelengths, or about 6000 km.

It is just over three months after the `big bang' of the launch, and we have now entered the (re-)combination phase, with the satellite orbiting the Earth (like a classical electron around a proton), and the Universe rapidly becoming transparent to space VLBI observations!


Editors: Phil Edwards and Hirax Hirabayashi