Earlier this month, 352 kilometres above the Earth, over three orbital passes, the proximity communication link - indispensable for the first Automated Transfer Vehicle (ATV) rendezvous with the International Space Station (ISS) - was able to transmit "loud and clear" signals from the Station to two ground stations in Spain during a downlink test.
"The test went well despite a sandstorm at Maspalomas ground station on Gran Canaria island in Spain. It proves that the ATV communication system, designed for Proximity Operations between ISS and ATV, is working fine", said Detlef Otto, who is in charge of the ATV communication system for ESA and who directed the test.
The main hardware of this proximity communication system is a self-contained electronic box, called the PCE, for Proximity Communication Equipment. This 55-kg box, which looks like a mini-bar refrigerator with a lot of cable connections, is already on board the ISS. For the downlink test, the PCE, which has been inside the Russian Zvezda module since its arrival with a Progress cargo spaceship in March 2005, transmitted through the two dedicated antennas on the outside of the ISS. Even at a speed of 28 000 km/h, the signal was clearly acquired by two 15-metre S-band parabolic antennas on Earth which played the role of the ATV receivers.
When the first ATV, called Jules Verne, is launched in to orbit next year, and navigates autonomously towards the Station, the two-way proximity communication radio link will be established between the ISS and ATV at a distance of between 30 and 100 kilometres, just prior the rendezvous operations. At the same time, a second communications system via TDRSS (NASA Tracking and Data Relay Satellites System) will be used for remote-commands and telemetry transmissions.
The two-way proximity communication is especially important for the Jules Verne mission, which marks the first ever rendezvous and docking for a European spacecraft. Jules Verne will demonstrate that it can automatically - and safely - handle any contingency plans. This demonstration flight must test and prove that the 20.7-tonne cargo ship can interrupt the rendezvous - at any time - by stopping its motion and flying away from the Station.
In the attached phase between ATV and ISS, the proximity link chains are nominally powered off. At the end of the 6-month re-supply mission of ATV, just before undocking, the TDRSS links are switched on again and at least for the first ATV flight, the proximity communication link will also be used for departure. Therefore, one day prior to undocking, it is again necessary to re-install the PCE and check it out with the attached ATV.
The two 19-cm diameter reception antennas are already installed on the first ATV, one directly mounted on the side of the European re-supply spaceship, the other on a deployable boom. At ESA's test facilities in Noordwijk, the Netherlands, Jules Verne – which is about 98 percent assembled – is going through a challenging year long campaign of crucial environmental and functional tests, ahead of the inaugural launch on an Ariane 5 launcher scheduled for May 2007.
In September 2004, the Expedition 9 crew, ISS commander Gennady Padalka and ISS Flight Engineer Mike Fincke, installed the first three antennas during a five and a half hour spacewalk, to make ISS ready to communicate with Jules Verne. These antennas allow S-band digital radio communication at a rate of 20 kBaud between ATV and the Zvezda module, where the ATV will dock, as part of ATV automatic rendezvous and docking procedures.
As soon as the radio communication link is established between the unmanned ATV and ISS, selected ATV telemetry will be displayed to the crew. The crewmembers are not involved in the auto-piloting of ATV, but they will carefully monitor its performance from the Russian Zvezda Service Module using a video camera and an independent range finder and range rate measuring system.
Through the proximity communication link, they have the capability to interrupt the approach - at any moment - if they consider their safety is at stake. The astronauts could initiate the Collision Avoidance Manoeuvre on their own to move the 20.7-tonne spaceship away from the Station, in the remote case of a major anomaly or malfunction showing up on their consoles. This communication link will be also crucial for the relative Global Positioning navigation (GPS) in real time between the navigating ATV and ISS to prepare for the later acquisition of high precision optical link used at close distance (less than 300 m) for relative navigation.
Tilting the whole ISS
For the test in early March, at each orbital pass over Spain the whole 183-tonne ISS had to be tilted by 90 degrees to have the Russian module aft section - where the transmitting antennas are located - pointing vertically towards the Earth over the ground station reception antennas. Normally these antennas would point horizontally towards the approaching ATV, but since during this test the two ground stations played the role of ATV, the Stations antennas had to point downwards towards the Earth. The whole test, which required the Station to be tilted back and forth three times throughout the three orbits, used 40 kg of ISS onboard propellant to rotate the Station back and forth.
"Completion of this test marks another very important milestone in preparing the International Space Station for the arrival of Jules Verne. Planning and preparing for this test took the cooperative efforts of technical specialists from ESA, RSC-Energia and NASA. NASA welcomed the opportunity to involve the ESA ATV team into ongoing ISS operations", said Adam Baker, Operations Lead at NASA's International Liason Office at Johnson Space Center (JSC), Houston.
The two astronauts on board the Space Station were not involved with the test since the sequence was fully automatic and was initiated and supervised remotely from the ground. But a few days earlier, Flight Engineer Valery Tokarev had to retrieve the PCE box from storage on ISS, install it in the Russian Zvezda module and connect it to the electric circuitry of the Station. The whole operation, which will be repeated for each ATV rendezvous, takes about one day of work for a crewmember. To return the PCS to storage takes about half this time. At present, US Commander Bill McArthur and Russian Flight Engineer Valery Tokarev make up the 6-month Expedition 12 crew which is due to return to Earth early in April 2006.
Two European Ground Stations
Four ESA engineers from European Space Operations Centre (ESOC) in Darmstadt, Germany handled the communication test on the ground. Although the commands to switch the PCE on and off were conducted via an on board computer programme sent to the Station through the Russian Mission Control Centre in Moscow , with the coordination of NASA's Mission Control Centre in Houston who is responsible for the safety of the whole ISS. About 20 people located in the three centres and at the ground stations participated in the communication test.
The test, which involved about a 10-minute radio transmission on each orbital pass from the ISS to the ground stations, took place over the Canary Islands and Madrid, Spain. Because of its East-West motion at the speed of 28 000 km/h, the Station flew first over the Maspalomas station, located in the southern part of the Gran Canaria island and then, over the ESA Villafranca ground station, located approximately 31 km from the centre of Madrid. During the three orbital passes over the two ground stations, the total communication time reached 40 minutes.
Established in 1975, after an international agreement between ESA and the Spanish government, Villafranca is part of the worldwide ESOC station network. The Maspalomas station is part of the Instituto Nacional de Tecnica Aerospacial (INTA). ESA is using this facility in cooperation with INTA.
These two ground stations will again be used for each future ATV mission. Five days before each ATV launch, a similar test will be conducted to check if the PCE has been properly installed and is operational. This test is part of the launch commit criteria. In other words, if the test reveals that the PCE is not working properly, the ATV launch will have to be postponed.
During the entire mission, another ATV communication system will process telemetry and commands in both directions through the NASA Tracking and Data Relay Satellites System, which is used for the International Space Station. TDRSS communications will start after the launch, four minutes before separation of ATV from the Ariane 5 launcher and continue up to the rendezvous phase with ISS. ESA is also using Artemis, the European data relay satellite, as a backup during ATV free-flight, and as the primary communication system when docked to the Station.
The PCE was tested a first time in June 2005, but the test - because of its complexity involving two transmitters and two antennas over a 15 minute pass, and because of the interferences from a Progress spaceship docked to the Station at that time – did not show conclusively if the system was working properly or not. Even after the present successful result, the PCE will again be tested in orbit next year to check a new software upgrade made in Russia by RSC-Energia.