See Attitude and Orbit Control System
The apogee is the point in the orbit of a body, such as, the Moon or an artificial satellite that is furthest from the Earth. The path that these bodies take as they orbit the Earth is in the shape of an ellipse, rather than a perfect circle. (Also, see Perigee)
Attitude refers to the orientation of the spacecraft in space.
Attitude and Orbit Control System
The Attitude and Orbit Control System (AOCS) monitors and controls the spacecraft's orientation (attitude) and its orbit. It includes: a computer, Star Trackers, Sun Acquisition Sensors and reaction wheels. One function of this system is to make sure that the spacecraft reaches and maintains its planned operational orbit. When a spacecraft is launched the launcher hauls it through Earth's atmosphere and then releases it on the general trajectory it needs to get into orbit. Once separated from the launcher (see separation ring), the AOCS can command the small thrusters onboard the spacecraft to bring it into its operational orbit. This system also ensures the spacecraft is pointing in the right direction, so that the onboard scientific instruments are looking towards the desired astronomical target and that the telecommunication antenna can transmit to ground-stations on Earth. In addition, this system steers, or slews, the spacecraft so that the scientific instruments can view different parts of the sky and keeps it stable during observations. (See Reaction Control System, Reaction wheels, Star Tracker, and Sun Acquisition Sensor).
Gamma rays and X-rays are extremely difficult to focus using traditional lenses and mirrors, if not impossible in the case of gamma-rays. Therefore, the three instruments that detect high-energy radiation onboard INTEGRAL rely on a technique known as coded-mask imaging. A coded-mask telescope is a pinhole camera with not just one but many pinholes. INTEGRAL's IBIS, SPI and JEM-X don't have a system of lenses and mirrors. Instead, they have a mask that is made up of a pattern of holes. The high-energy radiation can pass through the holes and onto the detector, but is blocked by the opaque parts of the mask. The mask creates a shadow pattern, so when gamma rays pass through the holes from a different direction the shadow pattern falls onto a different place of the detector. These overlapping patterns are then disentangled by computer software that knows the pattern of the mask's shadow, and converted into a map or image of the gamma-ray sources in the sky.
On Earth, air and water transport heat from one place to another, distributing the temperature evenly. In the harsh environment of space there is no air or water, or any other medium to transport heat. As a spacecraft orbits the Earth it is exposed to extreme differences in temperature depending on whether it is illuminated by the Sun, or Earth or not. The detectors of INTEGRAL's spectrometer work best within a particular low temperature range and therefore need to be protected from the extreme heat or cold. On the INTEGRAL spacecraft this is achieved using electric heaters and coolers (see Heaters, and Thermal control system).
Data handling system
This system is an onboard computer that manages all of the spacecraft's activity and includes a clock and a telemetry system. It interprets and distributes commands sent by the team of Ground Operation Controllers and collects and processes telemetry data ready to send back to Earth (see Telecommand, and Telemetry).
Last Update: 26 September 2011