By Professor Iain H Woodhouse
On 23 December 2021, I was tidying up some loose ends at work, getting ready for Christmas with the family. Meanwhile, 693km above me, a soldering process that had been used five years earlier to repair an electrical component on the Sentinel-1B satellite failed. The result was catastrophic for the mission – the C-band synthetic aperture radar (SAR) instrument lost power and was inoperable. Despite valiant efforts by the European Space Agency (ESA) technical team to resuscitate the instrument, on 3 August 2022, they finally declared Sentinel-1B unrecoverable. The agency’s formal Anomaly Review Board announced that it was impossible to recover the power supply unit – this marked the official end of Sentinel-1B.
Background to Sentinel-1B
Sentinel 1 is the imaging radar component of the EU/ESA Copernicus programme. The plan was always to have two identical satellites in orbit to ensure weekly global coverage of the land surface (and sampled coverage of the oceans). ESA launched Sentinel-1A in April 2014 and then Sentinel-1B two years later. The two identical satellites were established under Europe’s Copernicus Earth observation program. With both satellites in orbit, they were able to capture imagery of the entire planet with a maximum repeat frequency of six days, and for high coverage areas such as Europe, it was every couple of days.
The importance of SAR
The reason commonly cited for why radar is important is that radar imagers can see through clouds (so every image is guaranteed cloud free) and can see in the dark (so both sides of the orbit provide imagery, rather than just the sunlight side for sunlight-dependent optical satellites). But radar also offers sensing capability that is not available from other sensors. Sentinel-1, in particular, has demonstrated a consistent capacity to pick out objects on the ocean, including ships, wind turbines and icebergs, as well as artificial structures on the land surface (buildings, bridges, etc.). Sentinel-1 is also good for tracking changes in agricultural crops and is particularly good at mapping areas of flooding. So, the loss of coverage from Sentinel-1B impacts all of these applications.
How does this affect Earth Blox users?
In brief, it means fewer radar images and some gaps in global coverage. The images below show what the coverage was (both ascending and descending) for a month in 2021 (10 July - 12 August 2021) prior to the 1B malfunction and the coverage of the same period this year (2022).
The comparison is quite striking. In Europe, where the higher latitude and the use of both orbit passes ensures regular coverage, the impact is only a drop in frequency of coverage (essentially by a factor of 2). For some parts of the world, the risk is a loss of coverage altogether. The operational global (land) coverage from Sentinel-1 is normally shared between the two satellites, so the failure of one of them is a big loss. Radars use a lot of power to transmit their outgoing signal, and the data collected is usually enormous, so you need time in the orbit to download the data. Both factors influence how much time a radar satellite can spend collecting data. ESA has adjusted the Sentinel-1A observation plan to try to cover as much as possible, but unfortunately, the Sentinel-1A mission was almost at capacity already, so its ability to fill the S1B gaps is limited. It is largely areas of desert that are currently being omitted. Russia is another notable blank, but this looks like it is a political gap, not operational.
The areas that are being covered are getting images at least every two weeks. For some critical services (such as sea ice monitoring), these gaps are getting filled by Radarsat and other commercial radar image providers.
Looking forward to the Sentinel-1C launch date
The good news is that Sentinel-1C is on the substitute bench and has been getting warmed up since the first sign of trouble back in December. The current plan is that it will be launched from Europe’s Spaceport in French Guiana in the second quarter of 2023. But it takes about three months to get the radar imaging calibrated and fully operational, so even if it remains on schedule, it might still be October 2023 before we start getting a steady stream of images.
This means we will all have a nervous 12 months. With an operational design lifetime of seven years, Sentinel-1A is already 12 months beyond retirement age, so everyone will be keeping their fingers crossed that it is an uneventful and boring time. Fortunately, ESA has an excellent record of building and operating radar satellites that outlast their design lifetimes. ERS-1 reached nine years, despite a design lifetime of five, while Envisat reached ten years despite a design lifetime of five years. And ERS-2 was still collecting SAR images 16 years after launch (1995-2011).
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Professor Iain H Woodhouse
Iain H Woodhouse is Knowledge and Outreach Lead at Earth Blox and Professor of Applied Earth Observation at the University of Edinburgh. He specialises in active remote sensing, with over 27 years experience in academia and industry, and more than 100 publications. Iain has advised multiple UK government agencies on EO strategy and is currently Chair of the UK Space Agency’s EO Advisory Committee.