The European Space Agency’s fleet of Sentinel satellites are busy checking into urban ground movements, using San Fransico as a case study. Their findings reveal the city’s Millennium Tower skyscraper is slowly sinking. The news comes as a new Sentinel satellite prepares for launch early next year.



Watching From Above:

The Sentinel spacecraft form part of the Global Monitoring for Environment and Security (GMES), or Copernicus, program for Europe.

A partnership between ESA and the European Commission, Copernicus use a fleet of satellites to study Earth and its environment. The spacecraft also have applications for aiding in the enforcement of civil security within European countries.

Each Sentinel mission is based on a constellation of two satellites to fulfill revisit and coverage requirements, providing robust datasets for Copernicus Services.

In an ESA release on Friday, the agency provided results from the Sentinel-1 satellite pair – which have radar imaging payloads – have shown that the Millennium Tower skyscraper in the center of San Francisco is sinking by a few centimeters a year.

Studying this city is helping scientists to improve the monitoring of urban ground movements, particularly for subsidence hotspots in Europe.

Completed in 2009, the 58-storey Millennium Tower has recently been showing signs of sinking and tilting.

“Although the cause has not been pinpointed, it is believed that the movements are connected to the supporting piles not firmly resting on bedrock,” added ESA.

Working with ESA, the team from Norut, PPO.labs and Geological Survey of Norway have also mapped other areas in the wider San Francisco Bay Area that are moving. These include buildings along the earthquake-prone Hayward Fault, as well as subsidence of the newly reclaimed land in the San Rafael Bay.

“An uplift of the land was detected around the city of Pleasanton, possibly from the replenishment of groundwater following a four-year drought that ended in 2015,” noted the ESA release.

The San Fransico case study is important, as European cities experience similar subsidence – such as Oslo. The studies of San Francisco and Oslo allow for targeted case studies to a nationwide or even continental-scale land deformation service.

“Experience and knowledge gained within the ESA’s Scientific Exploitation of Operational Missions program give us strong confidence that Sentinel-1 will be a highly versatile and reliable platform for operational deformation monitoring in Norway, and worldwide,” noted John Dehls from the Geological Survey of Norway.

Sentinel is the space segment of Copernicus, consisting of six different types of satellite to study different aspects of the planet.

Sentinel-1 satellites carry radar imaging payloads, Sentinel-2 spacecraft are used for optical imaging, Sentinel-3 vehicles perform an oceanography mission, Sentinel-4 will study Earth’s atmospheric composition from geostationary orbit while Sentinel-5 will carry out the same mission in low Earth orbit.

Sentinel-6, which is a successor to the Franco-American Jason spacecraft, will carry radar altimeters to measure variations in the height of sea surfaces.

Sentinel 1, 2, 3 and 6 are free-flying satellites, while Sentinel-4 and 5 will be payloads hosted aboard the next-generation Meteosat and METOP spacecraft respectively. At present, one satellite each of the Sentinel-1, 2 and 3 series are in orbit, with the remaining series due for introduction in the early 2020s.

An interim satellite, Sentinel-5 Precursor (Sentinel-5P) will be launched in 2017 to restore ESA’s ability to monitor atmospheric composition in the troposphere, which was lost after Envisat ceased operating in 2012.

Sentinel-1A was the first member of the constellation to be launched, riding a Soyuz to orbit in March 2014. It was joined by Sentinel-2A, which launched aboard a Vega in June 2015 and Sentinel-3A which was launched by a Rokot from the Plesetsk Cosmodrome this February.

Sentinel-1B – a 2,164 kilogram (4,771 lb) satellite which was constructed by Thales Alenia Space – was launched by a Soyuz ST-A rocket earlier this year from the Centre Spatial Guyanais, in Kourou, French Guiana.

Based around the Piattaforma Italiana Multi-Applicativa (PRISMA) satellite bus, the spacecraft is designed for a seven-year mission in sun-synchronous low Earth orbit although it carries sufficient propellant and resources to operate for up to twelve years.

The satellite is equipped with a synthetic aperture radar operating in the C-band, which can image with a resolution of up to five by five meters (16 by 16 feet).

The results from the San Fransico study how the satellite’s systems are already providing vital data to scientists on the ground.

“The Copernicus Sentinel-1 mission is, for the first time, making it possible to launch operational national deformation mapping services,” said Dag Anders Moldestad from the Norwegian Space Centre.

The next satellite to join the fleet will hook up with its sister – Sentinel 2A – next year. Sentinel 2B has now been placed into its container for transportation to the launch site.

“It’s a European satellite, built by more than 60 contractors from 15 countries,” noted Michael Menking from Airbus, the prime contractor leading the industrial consortium. Twenty-nine of the companies were small- or medium-sized.

Like its sister, Sentinel 2B will be launched by Arianespace’s Vega rocket, with the mission set to begin in March.

“Sentinel-2B will be the fifth satellite launched for Copernicus,” added Josef Aschbacher, Director of ESA’s Earth Observation Programs. “The Sentinels already in orbit are providing massive amounts of data: 6.5 petabytes so far.

The satellite is expected to arrive in South America in January ahead of integration processing with the Vega.

(Images via ESA).