NASA tests unique GPS technology on shuttles

Software to give astronauts more navigational controlBY JENNIFER JONES""

N ASA has used recent shuttle missions to test Global Positioning System (GPS) technology that was specially developed for the agency and could give astronauts more direct control in navigating shuttles for missions scheduled before the next century.

Software used in last month's Space Shuttle Discovery mission as well as in four previous shuttle missions involved experimental use of GPS receivers which rely on satellites to pinpoint within a few feet the exact position of the shuttle or in other uses an aircraft or individual on the ground.

NASA is testing the GPS technology in hopes of moving to a navigational system solely tied to GPS by 1999. Currently NASA relies on an antiquated ground-based navigational system maintained by the Defense Department called Tactical Air Navigation System (TacAN). But GPS will require less hands-on management from ground-based crews and will give astronauts on board shuttles more control over guiding shuttles through space.

"There are some significant benefits to be derived from GPS " said Mike Brieden NASA's program manager in charge of GPS implementation on shuttle vehicles. "Through the use of GPS the shuttle will become much more autonomous because GPS will alleviate the requirement to constantly update ground-tracking systems."

Tom Peterson Lockheed Martin Space Mission Systems & Services program manager for the Space Shuttle Orbiter Avionic Software (SSOAS) program said "One of the things that is most apparent to everyone is that what we are using on the shuttles is a system that was designed 20 years ago. By bringing in something like GPS we are really taking the NASA work force toward what the rest of the aviation world is doing - things like putting commercially available GPS on aircraft. It is really in step with the direction of technology."

Shuttle software is constantly upgraded under SSOAS a $256 million contract held by Lockheed Martin.Moving to a GPS-based navigational system however has been a technical challenge especially because the shuttles fly close to the GPS satellites not miles below them as do most other aircraft. "GPS sort of assumes that you are going to be on the surface of the Earth or at least close to it " such as flying in an aircraft just tens of thousands of feet in the air Peterson said. "NASA is not operating in any of the usual locations which use GPS. The shuttles are not exactly up there with the satellites but they are far closer than someone on Earth."

So buying GPS software off the shelf is out of the question. "From a GPS perspective this is not just something you can pick up at Radio Shack and apply to a space shuttle " Brieden said. Instead there must be several customizations made to ensure that a shuttle's position and velocity are correctly updated by a GPS receiver.

"We cannot afford to make a mistake in output " he said.

Space shuttle personnel therefore are currently not using GPS data for operations but instead are collecting the information and analyzing its accuracy through the use of IBM Corp.'s 75M 486 ThinkPad laptop computers which are carried on board.

"The receivers have been flying and they are collecting data. But that data is not being included into the navigation systems yet " said Andrew Klausman a Lockheed Martin staff engineer on space shuttle flight software.

GPS holds promise for NASA not only in terms of navigational benefits but it also will allow savings in terms of ground system maintenance. DOD plans to decommission TacAN near the turn of the century. "DOD wants to cancel maintenance of the TacAN system and if we did not shift from our reliance on TacAN we would end up maintaining that system " Brieden said.

-- Jones is a free-lance reporter in Falls Church Va.


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