A proposed real-time VLBI system 
                                     for 
               the determination of the orbit of the lunar probe

                                 Qian Zhihan
               Shanghai Observatory, Chinese Academy of Sciences


   In the end of this century and early next century,  several  lunar
  exploration projects will be carried out in the world. In order to track and
  navigate the lunar probes, it is very  important  to  timely  and  precisely 
  determine the orbits of the probes on the way from the Earth to the Moon and 
  their positions and paths after landing on the Moon. Doppler and ranging are  
  two conventional tracking methods and can  precisely  determine  the  radial
  motions of the probes. But, they are  not  sensitivity  for  the  transverse 
  motions of  the  probes.  Differential  VLBI  can  determine  their  angalar 
  positions with the accuracy of the  order  of  0.001"  magnitude  even  more 
  precise. But, the conventional VLBI system is  a non-real-time  facility. In 
  general, the final results will be obtained a few days later or  more  after 
  the arrival of the recorded tapes at the data processing center.  Therefore,  
  it is necessary to develop a real-time VLBI system  for  the  spacecraft and 
  probe navigation. This presentation briefly  describes  the  features  of  a 
  proposed real-time VLBI system for the application  of  the determination of
  the orbit of the lunar probe. The main points of the system are as follows:

    1. The rate of the data transmission from each station to the correlator:  
       > 100 Mb/s;

    2. The size of the memory of the correlator: 64 Mbytes/memory, 3 memories;

    3. 3-station correlator. 
   
  If the new real-time VLBI system will also be used for the astrometric  VLBI 
  observations, not only for spacecraft and probe tracking,  it  is  better to
  develop a 5-station real-time VLBI system. If the proposal will be realized, 
  the problem of the compatibility between the different VLBI systems will  be 
  solved which is a puzzled problem for the VLBI users, recently.