There are many constraints for observations with RadioAstron. They are concerned with Attitude Control System of the spacecraft, satellite visibility from tracking stations, limits on the satellite autonomous operations during the orbit, and needs for auxiliary operations with the spacecraft.
The Sun must be located in restricted angular area relative to the spacecraft. It means that observing source have to be located in the hemisphere opposite to the Sun direction. Moon and Earth also introduce certain constraints.
All these constraints are considered in the Users Scheduling Software. Among the operational constraints the most important one is the number of programmed slews per orbit, which determines the maximum number of radio sources to be observed during one orbit.
There are several auxiliary operations with the spacecraft, which typically have to be done every orbit:
Main time unit in RadioAstron observing scenario is one satellite orbit (9.5 days). During communication session between spacecraft and control station, commands will be transmitted to on-board memory to provide all spacecraft operations during the following orbit. Communication session with the spacecraft have to be conducted when spacecraft is located near the apogee of its orbit. In this portion of the orbit spacecraft moves slowly, and some loss of observing time is not very critical.
Boresighting and orbit measurements (coarse) may be done during the control session. The main anticipated scenario is the observation of the 1-3 radio sources throughout the whole orbit. Such observation may be interrupted by auxiliary operations, which were mentioned in the previous paragraph.
The sensitivity of an interferometer is expressed in terms of the minimum correlated flux density that can be detected. During integration time, the observed fringe amplitude will be decreased by a factor "f" that reflects the coherence of the VLBI system.
A realistic criterion for the detection of a fringe on a given baseline is that it must be present at the "8-sigma" level. In the table 7 we give RMS values of correlated flux densities for all RadioAstron frequency bands in the best case (of using the EVLA system as ground telescope).
|Frequency band (GHz)||0.327
|Ground Radio Telescope 130-m (EVLA)|
|System temperature (K)||70||26||21||54|
|GRT sensitivity (Jy)||29||11||11||22|
|Instant BW per pol. (MHz)||4||32||32||32|
|Coherence factor f||1.0||1.0||0.9||0.8|
|Min. cor. flux (mJy, RMS)||10||1.3||1.4||3.2|
In these calculations the parameters for RadioAstron space radio telescope were assumed which are presented in a chapter Space Radio Telescope with integration time equal to 300 seconds.
The following calibration operations can be fulfilled in RadioAstron: