Ground Support of RadioAstron
Control and Tracking Stations
RadioAstron satellite control (command transmission, telemetry, orbit measurements) will be conducted from Evpatoria and Bear Lake control stations. Communication sessions are planned to be on every satellite orbit when the spacecraft is near the apogee position where it is seen from control stations on every orbit longer than four hours. Session duration may take from two to four hours depending on the program to be carried out during the following satellite orbit.
Coarse orbit measurements will be carried out from control stations. Each measurement takes ten minutes. For more accurate orbit determination the Doppler shift in phase transfer link will be measured by all tracking stations receiving science data stream transmitted from spacecraft.
Tracking stations will provide three main functions: data downlink (15.000 GHz) to receive digitized astronomical signals, phase transfer up-down link for phase synchronization (uplink frequency 7.207500 GHz, downlink frequency 8.400 GHz; high time and frequency stability is being provided by using hydrogen maser frequency standard), and measurements of radial velocity of the satellite through the Doppler shift of the downlink frequency 8.400 GHz. There will be one tracking station in Russia (Puschino), and two tracking stations abroad: Tidbinbilla (Australia), provided by NASA, and Green Bank (NRAO). Spacecraft must be in contact with one of these tracking stations whenever the interferometric observation is going.
Principal diagram of the design of the tracking station at the Pushchino can be see here. Electronic equipments installed at the radio telescope RT-22 (Pushchino): feed (large diameter circle), view from the above; feed with front end equipment, view from the bottom; electronic equipment installed inside of the antenna section of the radio telescope RT-22.
Coarse range and range rate measurements made at control stations are processed at the Ballistic Center together with measurements obtained at all tracking stations through high rate communication system.
Orbit measurements at tracking stations give only radial component of spacecraft velocity but the measurements are going continuously during observing sessions. RMS error of these measurements is equal to 0.1 mm/s.
Navigation information will be distributed between the ballistic centers and control groups included in the process of orbit reconstruction. The information is following:
The accuracy of orbit prediction depends on the kind of trajectory measurements that were used for orbit reconstruction. Anticipated RMS accuracy will be:
|For planning orbit (for spacecraft operations, 30 days in advance)|
|1000 m||50 cm/s|
|For reconstructed orbit (for data processing)|
|100-300 m||2 mm/s||5*10-5 mm/s2|
Ground Radio Telescopes
Because of the low sensitivity of 10-m antenna ant the large orbit of RadioAstron satellite, only large ground radio telescopes will allow to get useful scientific data for most observations with RadioAstron. However 25-m ground antennas also may be of some use on intermediate baselines.
The table 6 summarizes estimates of necessary observing time on large radio telescopes:
|Radio Telecope||Diameter (m)||P||L||C||K||Observing time in % *)|
*) Real observation time ought to confirm by The Radio Observatories before the launch.
Small radio telescopes can be also included in observing program. A minimum reasonable configuration of ground network is one large and one small radio telescope. In some observations with RadioAstron (for the sources near the orbit plane or at the observations near the perigee) more than 10 radio telescopes are expected to be used to provide high dynamic range in the resulting images.
Two type of recording systems will be used in RadioAstron project: Canadian S2 Recording Terminal (S2-RT), NRAO VLBA, and Russian hard disks. All types of recording systems will be installed at the tracking stations and ground radio telescopes in Russia and other countries.