Exactly where they want it to be

Engineers keep the ISS on orbit

BY ED MEMI

The International Space Station has been safely orbiting the Earth these past six years thanks in part to the Boeing Guidance, Navigation and Control team. In November, the ISS team celebrated its sixth year of on-orbit operations with expedition crewmembers.

The 29-person Boeing GN&C team supports NASA in several areas including daily system monitoring, troubleshooting on-orbit anomalies, designing controls to support operations and upcoming assembly flights, and improving the system software.

The ISS has two navigation and control systems working in tandem, one in the Russian Zvezda Service module and one in the U.S. Destiny Laboratory module and S0 (center) Truss element. They operate simultaneously with one designated as the master system but with both exchanging data continuously for fault detection and redundancy. Each can navigate and control the station's orientation or attitude, but only the Russian segment has thrusters that can raise the altitude of the space station to overcome the decaying effects of atmospheric drag, or to avoid orbital debris.

Navigation consists of determining the precise location and attitude of the ISS at all times. This information is used not only by the attitude control system, but also by several other "customers" that require pointing information from sources such as solar arrays, communication antennas and heat rejection radiators, and payloads such as cameras.

Global Positioning System (GPS) receivers provide attitude corrections and precise information regarding the station's position and velocity. On the U.S. segment, rate gyroscopes monitor the vehicle's orientation.

The ISS requires periodic reboosts to maintain the right orbit. However, for a Space Shuttle, Soyuz or Progress docking, the ISS will travel to a lower altitude, so that the launch vehicle will have less distance to travel and can carry a heavier payload.

The attitude control function keeps the ISS properly oriented. "Usually the ISS orientation has the U.S. segment pointed in the direction of travel, and the 'floors' of the U.S. and Russian modules are pointed towards the Earth," said Mike Begley, Boeing lead GN&C engineer.

The ISS is rotated to point its solar arrays directly at the sun. While this power-rich attitude is fixed with respect to the sun and stars, from the Earth's point of view, the ISS is rolling in orbit.

Both the Russian and U.S. segments can maintain attitude control. When the Russian modules are in control, they use attitude thrusters, which burn propellant. When the U.S. modules are in control, Control Moment Gyros are used. CMGs are 220-pound (100 kilogram) wheels that rotate at 6,600 revolutions per minute to balance the effects of gravity and aerodynamic torques, keeping the ISS at an equilibrium attitude without using propellant.

Boeing GN&C also tailors the control system to support new station configurations that will occur once ISS assembly resumes in 2005. Unlike most spacecraft, the U.S. and Russian control systems were designed to be robust enough to fly more than 100 varying ISS configurations as the station is assembled.

"With each new stage in the assembly, we add a major element that requires new controller inputs to keep the station at the equilibrium point. The software can be modified or scaled to respond to these changes," Begley said.

Boeing GN&C must also maintain the existing flight software, such as adding new functionality or improvements. The GPS system recently received a NASA code update that resulted in increased data coverage and less need for hands-on operator involvement.

The GN&C team now is busy supporting Shuttle return-to-flight analyses. In addition to its standard controller design work, the team is also working to support flight attitudes that reduce the risk to the Shuttle from micrometeoroids and provide the capability to perform Shuttle thermal protection system repairs.

edmund.g.memi@boeing.com