The contract, valued at 402 million euros ($441 million), will help guarantee the promised data continuity of the multibillion-euro Copernicus effort, which includes 15 satellites performing a range of Earth observation missions.
The Sentinel-1A satellite was launched in 2014 and the identical Sentinel-1B is scheduled for launch in April aboard a Europeanized Russian Soyuz rocket. Each is designed to operate for at least seven years in polar low Earth orbit, with sufficient fuel to last for 12 years.
The Sentinel-1C and -1D satellites placed under contract Dec. 15 will be launched starting in 2021 and are nearly identical to the previous pair, with some new elements.
Both will carry Automatic Identification System terminals to enable European coastal authorities to monitor sea traffic when ships are beyond the reach of terrestrial radars. The European Maritime Safety Agency (EMSA) will be using the terminals but it remains unclear whether the European Commission will finance a data delivery service.
The two new satellites also have upgraded navigation terminals to use both the U.S. GPS and European Galileo satellite systems.
In what European officials said is a feature never before tested in orbit, the two satellites will be equipped with a mechanism that will automatically separate the radar antenna from the satellite’s body when the spacecraft begins to enter Earth’s atmosphere.
Guido Levrini, Copernicus program manager at the 22-nation ESA, said the first two Sentinel-1 models were designed before global space safety and debris mitigation guidelines were adopted. ESA has publicly committed itself to following these guidelines with all future satellites, even when doing so will add substantial cost to the spacecraft.
Addressing the Sentinel-1C/1D contract signing here, Levrini said the system separating the 900-kilogram, 12.3-meter-long, 90-centimeter-wide radar antenna from the satellite is triggered automatically when it senses the sudden heat rise accompanying initial atmospheric re-entry.
“These satellites are not going to be in orbit before 2020 and we did not want to stick with a design that goes against debris mitigation guidelines that will be 10 years old by then,” Levrini said. “We were nonetheless concerned that we would be forced into a satellite redesign to meet this requirement, but the solution that was found is very clever and did not cause a new design or any major cost increase.”
Nonbinding space safety guidelines call for satellite owners to perform a controlled re-entry of their hardware — meaning guiding it to re-enter over the ocean — if there is a more than 1-in-10,000 chance of injuring someone.
The Sentinel-1 satellites use titanium fuel tanks that are more likely to survive re-entry because they are mainly shielded by the radar antenna. Jettisoning the antenna makes it more likely that the tanks will burn up.
Airbus Defence and Space has a patent pending on the separation technology and plans to test the system at a specially designed heat chamber in Friedrichshafen, Germany, Airbus said.
Airbus is providing the radar antenna’s structure and other elements of the satellite under a contract to Thales Alenia Space valued at 143 million euros, Airbus said.
European Commission rules prevent contract commitments stretching over more than a single seven-year funding cycle, which made it impossible to contract with Thales for all four Sentinel-1 satellites.
Splitting the order into two parts nonetheless did not result in a substantial increase in cost despite the gap of eight years, said Volker Liebig, ESA’s director of Earth observation.
The comparison with the first two-satellite order — for 420 million euros in 2007, against 402 million euros for the latest order — is made more difficult because this time around ESA had to purchase two laser-optical communications terminals from Germany’s Tesat Spacecom. For the earlier order, the German aerospace center, DLR, had contributed the terminals as part of a German government contribution, Liebig said.
“We’re now in late 2015 for a contract that will be continuing to 2020, and when you look at the entire picture we are not paying much more than we would have for recurring models ordered all at once,” Liebig said.
Mauro Facchini, head of the European Commission’s Copernicus unit, agreed.
“Of course we would have preferred to order all four at once,” Facchini said. “But in the end the cost was not much more than it would have been.”