Observations

Observing Constraints

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:

• communication session: 2-4 hours,
• repointing: 0.4 hour,
• boresighting 0.5 hour,
• coarse orbit measurements 0.2 hour two times,
• transfer to another tracking station 0.1 hour,
• unloading of the gyros 0.3 hour.

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.

Sensitivity

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).

Table 7

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.

Calibration

The following calibration operations can be fulfilled in RadioAstron:

• System Temperature Measurements by periodically injecting a known level of broadband noise into the SRT front end for every on-board radio astronomy receiver.
  Two noise levels are available high level or low level. High or low level of injected noise power may be chosen by commands, transmitted directly from control station during the communication session, or by commands picked out from on-board memory in accordance with the preset program.

• Boresighting - determination of pointing offsets of SRT antenna by total power measurements while moving SRT around a very bright radio source. The results will be used for orientation correction. The entire scanning procedure can be performed by specifying a few commands to the attitude control system. Boresighting shall be done when spacecraft is in contact with the control station.Prospective technique for 22 GHz boresighting is auto correlation of strong water masers; to obtain the results in this mode one must use High Rate Data link.

• Phase Calibration by injecting a periodical sequence of ultra short pulses at RF or IF for 22 GHz. Pulse spectrum includes the lines with 1 MHz interval. The calibration signal level is a few percents relative to system temperature of every receiver. The pulse generator can be switched on and off by the commands transmitted from control station or picked out of on-board program memory.

• Fringe verification - interferometric observation of chosen bright and compact source which must produce fringes on the given space-ground baseline projection. Positive fringe verification checks the whole ground-space interferometer, but the procedure includes data correlation, and it would take long time. Therefore fringe verification will be done mainly during in-orbit checkout period.

• Basic Information
• Spacecraft Design
• Orbit
• Space Radio Telescope
• Multi-Frequency Synthesis in K-band
• Data and Phase Links
• Ground Support of the RadioAstron
• Observations
• Data Processing
• Status of the Mission
• Progress of the RadioAstron Project in Pictures