Station resupply ship passes first demonstration day
BY STEPHEN CLARK
Posted: March 29, 2008
Europe's revolutionary Jules Verne cargo ship pulled within 2.2 miles of the international space station Saturday, proving the craft's long-range rendezvous systems are ready for next week's docking with the complex.
With ground controllers looking over the shoulder of Jules Verne's futuristic autopilot system, the ship automatically flew toward the station through waypoints at various distances from the complex.
One of four chains of electronics governing Jules Verne's propulsion system suffered a minor glitch early Saturday. The problem occurred during a series of engine burns designed to position the ATV at an "interface point" 24.2 miles behind and three miles below the space station.
Engineers determined the cause of the fault was a temperature measurement slightly exceeding preset tolerance limits inside one of Jules Verne's 28 maneuvering jets.
The temperature difference was just one degree Celsius from the limit, according to a senior ESA official.
"These thermal excursions are false alarms, but nevertheless trigger the (fault detection, isolation and recovery system) which always knocks out the propulsion chain associated with the thruster," said Bob Chesson, head of ESA's human spaceflight and exploration operations.
The ATV's four main engines and thrusters are controlled by four chains, each overseeing a quarter of the propulsion system. A similar problem in the hours after Jules Verne's March 9 launch cut off a propulsion chain for more than a day until engineers reset tolerance limits inside the system.
Engineers will again tweak the limits associated with the specific glitch encountered Saturday to make sure it does not occur again closer to the space station. Officials characterized the issue as minor, and it is not expected to impact plans for further rendezvous demonstrations and docking next week.
"We are resetting all the limits in line with our operational experience (plus) some margin so that we do not get close to triggering false alarms, but keep the limits tight enough to detect real problems," Chesson said.
After overcoming the propulsion system anomaly, Jules Verne began moving toward the outpost from the interface point at 1419 GMT (10:19 a.m. EDT).
A few minutes later, the ATV began using its relative GPS navigation system, which uses precise position data from antennas attached to Jules Verne and the space station. The ATV's computers processed the GPS data as planned to automatically plot a course toward the station.
The station sent its GPS position through Jules Verne's S-band proximity link communications system, which allows information and commands to flow between the two spacecraft during the rendezvous sequence.
"The relative GPS accuracy is really good and we had no issues with the proximity link. Two less things to worry about," Chesson said.
Jules Verne arrived at the S2 hold point 2.2 miles behind the station at 1557 GMT (11:57 a.m. EDT), where it stopped its approach for nearly 90 minutes of tests. The ATV turned on its external tracking lights and activated the Russian-built Kurs rendezvous radar, which will provide supplemental data during the docking.
Stationed at a control panel in the Zvezda service module, Russian cosmonaut Yuri Malenchenko sent hold and retreat commands to Jules Verne as it was parked off the stern of the complex. The crew tests further proved the functionality of the proximity communications system, officials said.
Cameras on the station's exterior captured several views of Jules Verne during Saturday's demonstrations. The 40,000-pound spacecraft appeared as a gleaming star above the Earth's limb.
"For the flight control team, the sight of the ATV thrusters firing was particularly exciting and brought the whole thing to life," Chesson said. "It was fantastic to be reunited with Jules Verne and to see it performing so perfectly, which is very promising for the days to come."
At about 1730 GMT (1:30 p.m. EDT), engineers at the ATV control center in Toulouse, France, ordered Jules Verne to escape the station's vicinity, demonstrating the ground's ability to intervene if something goes wrong during the final approach.
"The ATV control center monitors it all the time and we are able, if things go wrong, to interrupt in quite a significant way. If we spot a problem, for instance, we can initiate an escape maneuver and go off and do a thorough checkout of the spacecraft and even patch the software if necessary to allow us to make another attempt at rendezvous. There is a lot that the control center can do," Chesson said.
The escape command kicked off another string of nearly a dozen orbital maneuvers to set up for demo day two, slated to begin at 1226 GMT (8:26 a.m. EDT) Monday.
But officials must first review the results from Saturday's activities. Engineers will compile a detailed report to present to managers Sunday to make sure the ATV performed as expected. If no problems are discovered, the managers will approve plans to conduct demo day two as scheduled. "This will be a very exciting time, but we've really got to be sure that the vehicle is behaving exactly as we want," said John Ellwood, ATV project manager.
An early look at data from demo day one indicates no serious problems stand in the way of proceeding with Monday's ambitious rendezvous tests.
"The first analysis from the (report) are looking pretty good," said Alberto Novelli, ESA's mission director at the ATV control center. "We are going to spend some hours now analyzing them further. We are quite confident."
Monday's regimen of tests is planned to be a docking dress rehearsal. Plans call for Jules Verne to move within 36 feet of Zvezda's aft docking port using a high-tech suite of optical sensors on the forward end of the ship. After retreating to a distance of 62 feet, Jules Verne will fly away from the station after Malenchenko sends an escape command.
Another day of reviews are planned after demo day two before officials give a "go" for Jules Verne's docking with the station, currently scheduled for Thursday at about 1438 GMT (10:38 a.m. EDT).