History

The concept of cloud radar and lidar in space

The idea of putting radar and lidar in space for learning about clouds and aerosols, and their role in the climate system, originated in the early 1990s. Spaceborne cloud radar was made possible by the pioneering work of Lhermite (1987), who built the first 94-GHz cloud radar: only at this high frequency can a radar be sufficiently sensitive to detect clouds while having the small antenna and low power required to work from a satellite. The first synergistic use of radar and lidar to infer the size of ice particles was by Intrieri et al. (1993), an idea further developed by Donovan and van Lammeren (2001) and Okamoto et al. (2003).  NASA's Lidar In-space Technology Experiment (LITE) provided impressive observations of clouds and aerosols from the space shuttle in 1994. By the late 1990s there was more widespread deployment of ground-based cloud radars and lidars, both as part of the US Atmospheric Radiation Measurement programme and the European Cloudnet project. This spurred the development of cloud retrieval algorithms and techniques for evaluating the representation of clouds in models.

In the mid-1990s, the US and European (later European-Japanese) concepts for a joint radar-lidar satellite mission were at a similar stage of development. Originally both envisaged a single satellite, but cost constraints of NASA's Earth System Science Pathfinder programme necessitated separate satellites: the CloudSat radar and the CALIPSO lidar, which were launched on the same rocket in 2006 to join Aqua and form the A-Train constellation. Undoubtedly a factor in the much earlier launch than EarthCARE was the choice to develop a number of small satellites rather than a single larger platform. Important strides have been made in understanding clouds, aerosols and precipitation from CloudSat and the A-Train, but the key new measurements from EarthCARE will take this to the next level. 

The long road to selection

EarthCARE, and its precursor concepts, was proposed to ESA as an Earth Explorer mission at selection meetings four times, led by Anthony Illingworth (University of Reading).

May 1996 (Granada) The initial concept was the "Earth Radiation Mission" (ERM) comprising the GRACE satellite (Global Radiation, Aerosol and Cloud Explorer) and a set of auxiliary measurements from satellites and the ground. At an altitude of 600 km, GRACE would carry four instruments: a nadir-viewing cloud radar at 78 or 94 GHz, a sinusoidally scanning backscatter lidar at 1064 nm, a cloud imager similar to MSI except without the 2.2 micron channel, and a broadband radiometer very similar to the BBR. ERM was one of nine candidate missions for Phase A studies, and was not selected.

July 1999 (Granada) Now consisting of a single satellite "ERM" with the same four instruments, except for a non-scanning lidar to maximize co-location of the radar and lidar. The case was supported by the increased exploitation of ground-based cloud radar observations by that time. There were promising discussions with the Japanese agency NASDA (now JAXA) with similarities noted to the Japanese ATMOS-B1 mission. A possible scenario would be that NASDA would contribute a dual-wavelength lidar, a Fourier Transform Interferometer (FTIR) and a launcher. ERM was one of four candidates but Aeolus was selected instead, and ultimately launched in 2018.

September 2001 (Granada) Now rebranded "EarthCARE" (with thanks to Jacques Testud coming up the name), the mission now had five instruments: a Doppler radar, a 355-nm high spectral resolution lidar, an MSI with an additional 2.2 micron channel, a BBR, and an FTIR to retrieve profiles of water vapour and temperature above cloud. NASDA would provide the CPR and the FTIR. Now one of five candidate missions, the outcome of the meeting was EarthCARE and two of the other candidate missions to go forward for into a Phase A feasibility study, pending a final selection meeting a few years later.

September 2004 (Frascati) The FTIR was dropped... and EarthCARE was selected!

From selection to launch

Scientific oversight of the development of the satellite was provided by the Joint Mission Advisory Group (JMAG), which first met in January 2002 under the joint chairmanship of Anthony Illingworth and Teruyuki Nakajima (pictured above). Hajime Okamoto became Japanese co-chair in 2013 and Robin Hogan became European co-chair in 2023. The 44th and final meeting of the JMAG was in March 2025, at which point scientific oversight passed to the Quality Working Group.

During this time ESA and JAXA had to overcome many engineering challenges, each of which contributed to a delay in the launch from the original target of 2012. For example, the laser used by ATLID is very similar in design to the ALADIN lidar used by Aeolus, so when the problem of laser-induced contamination arose in the development of ALADIN in the early 2000s, it was a blocking issue for ATLID until it was solved. One of the challenges that had to be overcome in the development of the CPR was a sudden shut-off of one of the two High Power Transmitters (HPTs) under vacuum found in 2015. This was found to be due to shorting between components on the printed circuit board due to a failure of the insulating potting material, cured by a revised potting procedure. In 2017, some of the solar bands of the MSI were found to be affected by a spectral misalignment (SMILE) effect whereby the central wavelength varies as a function of across-track viewing angle, which has the potential to degrade retrievals due to the spectral variation of gas absorption and land-surface albedo. Improvements to the instrument enabled this effect to be mitigated for some of the channels, but the 0.67 and 1.65 µm channels are affected by a shift of up to 20 nm, which needs to be treated carefully in retrieval algorithms.

There were other challenges. In 2010 the EarthCARE budget was deemed too high, and an expert committee was set up to provide a report to ESA's Programme Board for Earth Observation (PB-EO) documenting what viable options exist for the continuation of the satellite, to consider viable de-scoping options as well as the run-down cost of a possible cancellation. On 7 February 2011 the committee reported and reaffirmed the high scientific value of the EarthCARE mission. Indeed, the reason why such a complex mission survived to launch with exactly the same instruments proposed 20 years earlier is because the unifying concept of instrument synergy leading to radiative closure meant that if any instrument was de-scoped we would no longer have a mission. A crucial person during this entire period was Tobias Wehr, ESA's EarthCARE Mission Scientist (pictured above), who skilfully steered the mission through all its various challenges as well as doggedly supporting the European scientists developing the complex web of algorithms necessary to achieve the mission goals.

ESA originally selected the Soyuz rocket to launch EarthCARE, but this was not possible after the Russian invasion of Ukraine in 2022 so launch was further delayed while ESA conducted a study into the suitability of the Vega-C and Falcon-9 launchers. It was found that the large size of EarthCARE would require a potentially risky modification to the Vega-C payload faring. A pressing need was to get the satellite in orbit and making measurements by the time of a large international field campaign ORCESTRA planned for August-September 2024, which could not be postponed. After the Vega-C launch failure in December 2022, it became clear that a Vega-C launch could not be carried out in time, and in June 2023, ESA Council made the decision to switch to SpaceX's Falcon-9 rocket. Through a remarkable effort by ESA, JAXA, Airbus and other industry partners, EarthCARE was prepared for launch with no further delays. This included including safely carrying it to Vandenberg Space Force Base in California in March 2024 on a huge Antonov An-124 transport plane.

The big day

EarthCARE was launched at 22.20 UTC on 28 May 2024 from Vandenberg (15.20 local time) with a limited number of team members present in person. ESA held a large launch event at its European Space Operations Centre (ESOC) in Darmstadt, Germany, where the launch occurred shortly after midnight on 29 May: catch-up on the livestream. The launch occurred at 07.20 in Japan, and JAXA hosted a livestream with VTuber Clear Usui (in Japanese). There was widespread media interest in the launch.

Eleven minutes after launch at 22.31 UTC EarthCARE separated from the final stage of the launcher, and at 23.02 its solar array was deployed. At 23.14 the first communication from the satellite was receieved at Hartebeesthoek ground station in South Africa (to much applause at ESOC). A little over a day later at 01.19 UTC on 30 May, the CPR antenna was deployed.

After launch, EarthCARE entered its Commissioning Phase, managed by ESA's Kotska Wallace, in which the platform and each of the four instruments was commissioned. On 23 January 2025 EarthCARE passed its In-Orbit Commissioning Review and entered its Operational Phase managed by Björn Frommknecht.