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Autopilot in orbit: How does ATV reach the space station?

Posted: March 28, 2012

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Europe's automated resupply freighter will approach and dock with the International Space Station on Wednesday, delivering food, spare parts, propellant and other cargo to the outpost's six residents.

The Ariane 5 rocket launched Friday with the Automated Transfer Vehicle. Credit: ESA/Stephane Corvaja
The bus-sized Automated Transfer Vehicle, christened Edoardo Amaldi after an Italian space pioneer, is scheduled to dock with the space station's Russian service module at 2233 GMT (6:33 p.m. EDT).

The spacecraft is the largest vehicle resupplying the space station since the retirement of the space shuttle.

Since launching Friday at 0434 GMT (12:34 a.m. EDT), the Automated Transfer Vehicle has fired its engines to tweak its approach the space station. It also deployed an antenna boom to communicate with the complex during the rendezvous.

The ATV will initially use relative GPS navigation to track down the space station, comparing satellite fixes between itself and the complex. It also has a proximity radio communications link with the space station.

European Space Agency astronaut Andre Kuipers and Russian cosmonaut Oleg Kononenko will man a control console inside the station's Zvezda service module. The station crew would use the device to command the freighter to retreat, abort, or escape the vicinity of the complex.

Crew intervention becomes feasible when the ATV reaches the S3 hold point about 817 feet behind the station at 2110 GMT (5:10 p.m. EDT).

The automated cargo ship will hold there for about 36 minutes as the ATV activates its terminal phase rendezvous sensors and engineers thoroughly check the craft's health from a control center in Toulouse, France.

The craft will press closer to the station using precise navigation data derived from the ship's two videometers.

The videometers, working simultaneously with one in standby mode, fire pulses of laser light toward the station one-to-ten times per second.

Acting as space mirrors, 26 reflectors positioned on the back end of the station's Zvezda service module will beam the laser light back to the sensors on Edoardo Amaldi, creating unique light patterns captured on the ATV's cameras. The craft's advanced computers will use the patterns to autonomously determine its orientation, closing rate and distance from the space station.

Two other instruments known as telegoniometers will serve as watchdogs during the final rendezvous, ready to take over if something went wrong with the primary system.

The telegoniometers, similar to police radar guns, emit laser light at a different wavelength toward the Zvezda reflectors up to 10,000 times per second. The light's travel time between Edoardo Amaldi and the station allow the craft determine its range, while the direction of the station is given by the angles of two built-in mirrors rotating to the aim the laser at its target.

File photo of Europe's second ATV cargo ship arriving at the space station in 2011. Credit: NASA
Another hold in the approach is programmed at a distance of 62 feet for ATV engineers to review the progress of the rendezvous. Edoardo Amaldi is expected to stop at the S4 hold point at 2210 GMT (6:10 p.m. EDT) and stay there about 13 minutes.

A final halting of the ATV's rendezvous is planned at the so-called S41 point at 2225 GMT (10:40 a.m. EST) approximately 36 feet from the back end of the station.

If systems remain ready for docking, the 20-ton spacecraft will resume its approach for docking to the Zvezda service module at 2233 GMT (6:33 p.m. EDT).

The ATV is also outfitted with a Russian Kurs radar docking system as a backup.

Plans call for the space station crew to open hatches leading into Edoardo Amaldi on Thursday or Friday. It will take more than 80 hours to unload the supplies and restow the craft's pressurized module with trash.

Engineers packed eight supply racks into Edoardo Amaldi's pressurized section, two more than flew aboard previous ATV missions in 2008 and 2011. The spacecraft's total dry cargo load is more than 4,800 pounds, including fresh food, clothing, crew personal items, experiments and spare parts.

The rear section of the Edoardo Amaldi spacecraft contains propellant and gas tanks with 12,000 pounds of rocket fuel and oxidizer. About 1,900 pounds of propellant will be pumped into tanks inside the space station.

The ATV will deliver 220 pounds of oxygen and air and replenish the station with 75 gallons of potable water.

Edoardo Amaldi will stay at the complex until September, when it will depart and dive into the atmosphere, burning up and spreading debris over the remote Pacific Ocean.