GODDARD NAVIGATES THE INTERNATIONAL SPACE STATION

Over its lifetime, the International Space Station (ISS) will travel more than a billion miles, and NASA Goddard will help steer the way. Goddard is providing the Global Positioning System (GPS) receiver firmware (software directly built into the circuitry of a microchip) to be used as part of the navigation equipment on ISS in a partnership with industry and the Space Station Program office at the Johnson Space Center.

"Space Station is orbiting Earth at more that 17,000 miles per hour, so you can't just fly a typical sporting goods store GPS receiver," said Jim Simpson, lead test engineer for the GPS system within the Guidance, Navigation and Control Center (GNCC) at Goddard. "Due to its speed, the ISS GPS receiver has to acquire GPS satellites much more rapidly than ground or airborne systems. In addition to identifying the ISS location, our GPS receiver also determines the attitude (pointing) of the ISS, something GPS was not originally designed to accommodate."

"The Space Station's speed significantly changes the GPS satellite transmission frequency due to the Doppler effect," added Dr. Charles Campbell, also of the GNCC. "The Doppler is much greater than what standard GPS receivers experience - finding the correct frequency for a standard GPS receiver is like finding a dollar bill hidden somewhere in an average room. For the ISS receiver, it's as if the dollar is hidden in a house with one tenth of the time to find it."

Chief architect and programmer for the GPS attitude determination software, Dr. Glenn Lightsey, formerly of the GNCC and now at the University of Texas at Austin comments, "GPS development has reached the point of maturity to provide not only reliable timing and navigation information to the scientists and spacecraft operators but also knowledge of pointing in one sensor. This technology milestone is a culmination of almost a decade of research and development. This innovation has other far-reaching implications in the cost and operation of all future spacecraft operating near Earth."

"Goddard's GNCC has a long tradition of providing the science and space communities with state-of-the-art GPS systems. Together with industry and academia, we are truly giving the space community better and cheaper means of meeting mission success which was not possible just 5 years ago," said Frank Bauer, Chief of the Guidance, Navigation, and Control Center. He also adds, "We are very excited by the challenges presented to us by future formation flying missions which involve orbits that fly well above the GPS constellation, where the GPS signals are few and very hard to detect."

The Global Positioning System is a twelve billion dollar constellation of 24 navigation satellites placed into orbits approximately 12,560 miles high by the United States Air Force. GPS receivers determine accurate locations on the ground, in the air and in low-Earth orbits when they receive transmissions from the GPS satellites and compute the distance to them. Except for the poles, at least five GPS satellites are visible from any given place on the globe; distances to four are needed to determine precise location.

Even with the extensive customization provided by Goddard and its partners, the GPS system will be approximately three to four times less expensive than other navigation aids traditionally used for space flight, such as star trackers. "A star tracker is the Ferrari of navigation," said Simpson. "Each used to be custom-built for its spacecraft, and it provides a very accurate source of attitude knowledge. However, not all applications require that level of precision. GPS provides very good results for a lot less money. It's like commuting with a minivan instead of a high-end Lexus."

ISS is currently being piloted by the Russians, who are navigating with a satellite constellation similar to GPS called GLONASS. This GLONASS receiver can track both GLONASS and GPS signals but is not designed to determine spacecraft attitude. The Russians are also using a star tracker, sun sensors, horizon sensors, and magnetometers. When the four GPS antennas are delivered by the Space Shuttle to the ISS in early 2002 on Assembly Flight 8a, the ISS will transition to GPS navigation, with GLONASS as backup. The GPS receiver assembly has already been installed on the ISS, and the antennas are on the center section of a 300-foot long truss that will connect to massive solar panels; the section is designated Integrated Truss Structure S0. The GPS receiver is part of a larger navigation assembly called the Space Integrated Global Positioning System/Inertial Navigation System (SIGI), which includes laser gyroscopes that measure changes in orientation. Since the gyroscopes just measure the change in orientation, the GPS receiver is used to find the initial location as a reference. The GPS receiver also provides location updates because the gyros tend to drift, or accumulate small errors, over time.

Goddard provided the software necessary to customize the GPS receiver for use on the Space Station, which included the code to compute orientation, called attitude determination, and the command and data handling code. Goddard also performed extensive testing to ensure the system was reliable enough for human space flight. Trimble Navigation, Sunnyvale, California, provided the GPS receiver hardware, which is normally used on military aircraft. Both organizations are subcontracted to Honeywell, Inc., Clearwater, Florida, which provided the entire SIGI assembly to the Space Station Program office at JSC.

For more information about the SIGI, refer to:

http://www.futureshuttle.com/conference/Avionics/Majure.htm

For more information about the GPS, including a tutorial on how it works, refer to:

http://www.trimble.com/gps/index.htm

For more information about the ISS, refer to:

http://spaceflight.nasa.gov/station/index.html