This Protocol determines functional constraints on the Attitude Control System of Radioastron Spacecraft (RS). It also includes constraints on the operations of ground radio telescopes, tracking and control stations. The document determines also necessary condition to conduct science observations, calibrations, and boresighting with RS. A principal draft of the spacecraft is given on Fig.1. The X-axis of body-axis coordinate system is pointed on a radio source during observing session, orientation of Y and Z-axis is determined by listed below restrictions.

1.1 Initial parameters:

1.2. Argument of perigee and longitude of ascending node are now still under consideration. Argument of perigee measured relative to the ecliptic plane shall be in the a range of 270 - 360 deg. Longitude of ascending node has no restrictions.

2.1. SC service systems.

2.1.1. Cooling and thermal control system (CTCS):an angle between normal of thermal radiator plane (Z-axis) and vector "SC-Sun" must exceed 90 deg..

2.1.2. Power generating system - solar-cell batteries:an angle between a vector "SC-Sun" and the semi-plane XOZ (Z<0) must not exceed 30 deg..

2.1.3. Star sensors of Attitude Control System (ACS).

2.1.3.1. AIS consists of three sensors. The axes of the first two sensors are in the plane YOZ; they have an angle of 35 degrees with Y and -Y axis being inclined to the direction of -Z axis. The third sensor has its axis in the plane XOZ with the angle of 15 deg from -X to -Z.

2.1.3.2. The angle between any two sensor's axis and interfering celestial body must exceed:

These values may be corrected during future development.

2.1.3.3. An angle between a vector "SC-Sun" and the semi-plane XOZ (Z<0) must not exceed 30 deg..

2.1.4. Solar and star sensors of standard 18KS System

2.1.4.1. Field of view of the solar sensor is limited by an angle +/- 23 degrees relative to the XOZ-plane; the area is limited also by an angle of 30 degrees relative to the half-plane YOZ (Z<0) to the direction of +X, and by an angle of 90 degrees to the direction of –X2.

2.1.4.2. The center of field of view of the every from the two star sensors can be directed in the plane in a range of +/-36 degrees relative to the main axis lying in the same plane which contains X-axis as well. The main axis projection of the first star sensor onto the YOZ-plane is inclined from the -Y axis to -Z axis by an angle of 20 degrees; the main axis projection of the second star sensor onto the YOZ plane is inclined from +Y axis to the -Z axis by an angle of 20 degrees. The plane containing both these axes is inclined from the YOZ plane by an angle of 15 degrees to the -X direction.

2.1.4.3. Canopus, Sirius, and Vega can be used as reference stars for 18KS star sensors. All stars G2V brighter than +1.0 magnitude in the internal photometric system also can be used. The list of reference stars will be prepared later.2.1.4.4. The pointing of X-axis in the given direction is possible with the 18KS system when there is at least one star in the range of +/- 36 degrees as described in 2.1.4.2, and the Sun is in the range described in 2.1.4.1.

2.1.5. High-gain communication antenna (HGA)

2.1.5.1. HGA pointing is executed by the rotation around two perpendicular axes: Yrot and Zrot. Yrot-axis is fixed relative the SC body, it is parallel to the XOZ plane and is inclined by 30 degree angle from Z-axis to X-axis.Zrot-axis in its initial position is parallel to the Y-axis of the spacecraft.HGA axis is perpendicular to Yrot and Zrot axes, and it is directed to the -X direction.The permitted range of rotation around Yrot and Zrot axes is +/-90 degrees. The range may be changed during the system development and testing.2.1.5.2. During communication session the vector SC-TS (tracking station) must be in the HGA deflection sector, which is a hemisphere with the axis belonging to XOZ plane, this axis is turned by mounting angle 30 deg. from -X to +Z.

Line of sight angular velocity projection onto the Yrot and Zrot axes must not exceed 0.45 deg/s for distances less than 20000 km, and 0.2 deg/s for greater distances.

2.1.5.3. Positive rotation round the Yrot and Zrot axes is counter clock rotation as observed from the positive end of these axes.

2.2. Constraints from on-board scientific payload.

2.2.1 Active cooling system of LNAs - micro cryogenic system (MCS)

2.2.1.1. An angle between +Z-axis and direction to the Sun center must exceed 90 degrees.

2.2.1.2. Sun must not illuminate the surface of the "cold plate" radiator. Radiator is located at the surface of cut cone with axis coinciding with +X. The radiator occupies only part of the cone surface limited by an angle of +/-60 deg. relative +Z; cone halfopening angle is 30 degrees.

2.2.1.3. An angle between direction "SA-Sun(center)" and XOZ plane must not exceed 30 degrees.

2.2.1.4. An angle between +Z axis and the direction to the Earth center must exceed 30 degrees for geocentric distances less than 20000 km. At greater distances this constraint is not valid.

2.2.2. Radio astronomy receivers and space ratio telescope (SRT).

2.2.2.1. An angle between direction to the Sun center and +X axis must exceed 60 degrees.

2.2.2.2. An angle between direction to the Moon center and +X axis must exceed 5 degrees.

2.2.2.3. An angle between the direction to the nearest edge of the Earth and +X axis must exceed 5 degrees.

2.2.3. Additional star sensor - Astrocorrector (installed with its axis parallel to the +X axis).

2.2.3.1. An angle between the star sensor axis and the direction to the Sun center must exceed 30 degrees.

2.2.3.2. An angle between the star sensor axis and the direction to the Moon center must exceed 20 degrees.

2.2.3.3. An angle between the star sensor axis and the direction to the nearest edge of the illuminated Earth must exceed 30 degrees.

2.2.4. Operations in the Moon's and Earth's shadow.

Permitted duration of operations of science payload will be defined later.

3.1. Constraints on the pointing of the Control Telemetry Antennas (CTA).

Elevation is an angle between the direction to the SC (or radio source) and local horizon.

3.1.1. The elevation of the SC must be greater than 7 degrees.

3.1.2. An angle between CTA axis and sun must exceed 10 degrees.

3.1.3. An angle between CTA axis and moon must exceed 5 degrees.

3.1.4. Geocentric coordinates of Control Telemetry Stations are presented in Table 1*. Ussuriisk Control Station is considered as a main resource.

In a case of contingency control station in Evpatoria will be available.

3.2. Constraints on the pointing of the Tracking Stations Antennas (TSA).

3.2.1. The elevation of the SC must be greater than 7 degrees.

3.2.2. An angle between TSA axis and sun must exceed 10 degrees.

3.2.3. An angle between TSA axis and moon must exceed 5 degrees.

3.2.4. Geocentric coordinates of Tracking Stations are presented in Table 2*.

3.3. Constraints on the Ground Radio Telescopes (GRT).

3.3.1. Ground radio telescopes are conventionally segregated into two groups: big radio telescopes (diameter greater than 32 m), and small radio telescopes (diameter less than 32 m).

3.3.2. Elevation of radio source must be greater than 10 degrees.

3.3.3. An angle Sun-GRT-Source must exceed 10 degrees.

3.3.4. An angle Moon-GRT-Source must exceed 5 degrees.

3.3.5. Geocentric coordinates of GRTs are presented in Table 3*.

Reorientation is a single turn (slew) of the Spacecraft around arbitrary (but certain for the given case) axis by a definite angle. After the reorientation SRT axis shall be directed to the chosen radio source.

The following constraints should be valid for the reorientation:

4.1. An angle between the direction to the center of Sun and +Z-axis must be greater than 90 degrees.

4.2. An angle between the direction to the center of Sun and +X-axis must be greater than 60 degrees.

4.3. Moon and Earth position constraints (see 2.2.2.2 and 2.2.2.3) are valid when any radio astronomy receiver is on. Otherwise these constraints may be disregarded.

4.4. The Sun must not illuminate the surface of the TCS radiator of the "cold" plate (see 2.2.1.2).

4.5. Constraints on the permitted angles between the center of the Sun and the axes of ACS star sensors will be determined later.

4.6. Maximum allowed angle between the normal to the plane of solar panels an direction to the Sun will be determined later.

4.7. To satisfy the constraints 4.1 - 4.3 more than single slew of the spacecraft may be needed.

Notice: During repointing from calibration to observing source the Communication antenna shall be directed to the tracking station to provide radio contact. This is also desirable during the repointing from one to another source.

It is presumed that constraints presented in paragraphs 2 and 3 are satisfied during observations.

5.1 Science observations.

The following conditions must be fulfilled during science observations:

5.1.1. The visibility of the investigated sources from the Spacecraft.

5.1.2. The visibility of the source from at least three GRTs, two of them being big, and one may be small.

5.1.3. Spacecraft visibility from one of the tracking stations

Notice: science observations may be interrupted by

1. orbit measurements;
2. command session;
3. transfer from one tracking station to another;
4. gyros unloading;
5. Earth and Moon shadows.

5.2. Boresighting

5.2.1. Boresighting modes are determined by The Protocol of Boresighting (N121-141-92)

5.2.2. To conduct boresighting it is necessary to provide reference source visibility from the SC, and SC visibility from Control Telemetry Antenna. Boresighting at 22.3 GHz requests SC contact with Tracking Station as well.

5.2.3.Boresighting may be done not in every orbit. Time interval between boresightings will be determined after in-orbit-checkout

5.2.4.The list of reference sources for boresighting is presented in Tables 4-7*.

5.3. Calibration (Fringe verification).From operational point of view Calibration is science observations of chosen radio sources which shall have notable correlated flux density at the given baseline projection.

5.3.1. To conduct the calibration the conditions presented in 5.1.1-5.1.3 and 5.3.2 must be satisfied.

5.3.2. Calibration source visibility from GRT located near tracking station. This condition is essential only for the usage of near-real-time correlator.

5.3.3.The list of calibration sources is presented in Table 8*.

Notice: number of calibrations per orbit will be determined by PI; typical figure is presumed to be one calibration per orbit.

*) Notice: it will be placed on web site later (V.Yakimov)