Venus Express antenna selection during quadrature operation
Schematic diagram showing antenna selection during quadrature operation.
The Venus Express spacecraft design is based upon that of Mars Express, with as few changes as possible while taking account of the different planetary environments in which the two spacecraft operate. The thermal requirements of Venus Express require that only two faces can be exposed to the Sun on a continual basis: +X and +Z. To prevent overheating of components, the spacecraft is 'flipped over' during Earth communication periods to keep the Sun off of the sensitive –X face. Since the conditions requiring this manoeuvre only occur when the spacecraft is inside the inner quadrature arc, and closer to Earth than at other times during the orbit, the Mars Express design could be simply modified with the addition of a second, smaller main antenna (HGA2), located behind the main HGA1 Earth communication antenna, and pointing in the opposite direction to it.
In the diagram above, consider the spacecraft at the noon position, moving counter clockwise. In keeping with the Venus Express thermal requirements, only two faces can be exposed to the Sun on a continual basis: +X and +Z. The main communication antenna, HGA1, is on the +X face and that has to point towards Earth. At noon, pointing HGA at Earth means the Sun will illuminate part of the +X and +Z faces – this is acceptable since it satisfies the spacecraft's thermal requirements.
When the spacecraft is further along the orbit – at quadrature, where the angle between the Sun-spacecraft-Earth is 90 degrees - the Sun illumination still satisfies the thermal requirements. At this position the +X face is perpendicular to the line to the Sun, so there is no illumination on that face. The +Z face faces the Sun, so all the exposure is on that face.
Continuing along the orbit, if the spacecraft continued to point HGA1 towards Earth the Sun would start to illuminate the –X face. This would violate the spacecraft's thermal requirements and therefore needs to be avoided. When the spacecraft arrives at the quadrature point, before the Sun starts illuminating the –X face when Earth pointing, the spacecraft is 'flipped over' and the HGA2 antenna takes over as the Earth communication antenna. This configuration is maintained until the spacecraft arrives at the outer quadrature position, when the spacecraft is again 'flipped' in order to avoid the Sun illuminating the -X face, and HGA1 is once again the communication antenna.