SWOT: all about the French satellite that will revolutionize hydrology

Tomorrow at 12:46 p.m., the SWOT satellite, for Surface Water Ocean Topography, will take to orbit aboard a SpaceX Falcon 9 launcher. The opportunity to quickly come back to the challenges of this prestigious and unique mission in which France played a central role. Guided tour.

What is SWOT and what will it be used for?

SWOT is first and foremost the name of a major joint mission organized by NASA and its French equivalent, CNES. But it is also the name of the main actor of this mission. In this case, this billion-dollar satellite is a sentinel devoted exclusively to the topological study of surface waters.

And the term is not chosen at random. As such, it is the spiritual successor of TOPEX/Poseidon, an altimetry mission launched in 1992 that completely revolutionized this discipline. For more than 12 years, it has made it possible to measure the relief of the ocean surface with an accuracy of the order of a few centimeters.

And the teams that designed SWOT are hopeful that it will bring just as much progress. Because despite its performance, TOPEX was simply not precise enough to focus on the smallest tanks that litter the surface. This new machine, on the other hand, will be able to work at a much smaller scale.

He will be able to study the topology of lakes over 250 m² and some almost all major rivers more than 50 to 100 meters wide all over the planet. In total, for each period of 21 days (which corresponds to one rotation around the Earth) it will cover a water surface of approximately one million square kilometers to measure its topology!

How it works ?

As a rule, instruments of this type work on the same principle. An antenna begins by emitting a radar signal in the direction of a surface that reflects these waves, such as water.

Part of the signal will thus be sent directly to the instrument where it will be picked up by a receiver. From there, the telescope only has to perform a simple calculation to determine the height of the target water point, since we know the speed of the wave and the time it took to travel. ’round trip.

At SWOT, the basic idea is more or less the same; it’s still a radar altimeter that uses the sleight of hand described above. But the process has an important difference. Indeed, SWOT is equipped with revolutionary equipment: a wide-swath interferometer.

This convoluted term designates an instrument equipped with not one, but two distinct radar antennas. Each will produce a slightly different signal when the wave returns. This will make it possible to obtain an extremely precise and above all very clean signal; enough to considerably facilitate the processing of the mountain of data that it aims to produce.

Pascale Ultré-Guérard, deputy director in charge of CNES strategy, insisted on this last point. Because thanks to this mode of operation, SWOT will be able to harvest data in industrial quantities. If we talk about wide swaths, it is because the telescope will not be content to sample lakes and rivers point by point. Instead, he will pass two in review about 120 km apart.

This will allow him to carry out an interpolation, that is to say to calculate the altitude between these two points on the basis of a mathematical estimate. In other words, it will “fill in the holes” to get a full 120 km measurement strip. And over the rotation of the Earth, it will be able to cover the entire surface of the globe. Enough to produce a very large quantity of interesting statements. “Swot alone will provide as much data as six conventional altimeters“, says Aurélien Ponte, researcher at Ifremer.

What are the scientific issues?

It is an exceedingly important mission in many respects. The first reason is that this scale of 10 to 100 kilometers (we speak of mesoscale) is very poorly observed today. And this is a particularly important gap. Indeed, it is at this mesoscale that a large part of the energy of surface waters is concentrated.

This means that a very important part of the hydrological dynamics takes place at this local scale. By providing information at this level, SWOT will extract crucial information. For example, it will offer an estimate of the flow of almost all the rivers on the planet.

These data will make it possible to make the link between these mechanisms and large-scale ocean circulation. By extension, this will inform researchers about the role of these energy transfers in the biogeochemical cycles that define life on Earth, such as the famous carbon cycle.

It will also provide a better understanding of the dynamics of exchanges between the different Watershed. It will also provide crucial information on the mechanisms at play at the interface of rivers, streams and seas. But above all, it will allowassess available water resources across the entire planet (navigation, irrigation, drinking water, hydroelectric energy, etc.).

In the end, this satellite has a very important potential for lots of applications as important as they are varied; and most of them have yet to be invented.

According to mission officials, this also means informing the researchers. For if oceanologists are used to working with maps, this is not the case for hydrologists; for the latter, SWOT will be a real magic wand. He will allow them to study functional links that they could only dream of until now. For the operators, the challenge will therefore be to make the data as accessible and easy to use as possible, in particular thanks to a great deal of data preprocessing.

And France in all this?

The launch of SWOT is also excellent news for the scientific and academic influence of France. As a reminder, this is a major mission that required years of close collaboration with NASA. And the Hexagon is not content to pose in the photo alongside Uncle Sam, far from it; our country has inherited a structuring role in the program.

Indeed, project leader Thierry Laffont insisted on recalling that CNES has inherited certain important prerogatives. This begins with the management of the satellite itself, which already represents a heavy responsibility. It also ensured the assembly from parts provided by NASA. It is also the French institution that took charge of the oh so important step of certifying the equipment.

France and CNES also inherited a decisive role in the design of the main instrument, the famous interferometer called KarIn (for Ka-band Radar Interferometer). The United States was eager to maintain a monopoly on this key element. But after long and tough negotiations, France obtained the design of the instrument’s radar module; it is Thales Alenia Space who designed the beating heart of this mission. And it is a real gem of technology; he represents a huge step forward in terms of signal and noise management. Each success of the SWOT will therefore also be a great success for French engineering.

What are the other issues?

We sometimes tend to forget it, but water is an extremely important element in the global geopolitical landscape. Some of the most important watersheds are shared by several countries; this can give rise to significant political tensions. And that’s not going to get any better with the rise of global warming.

By allowing all scientific institutions to better control water resources, this satellite could therefore also inherit an almost diplomatic role; as always when we talk about water, with SWOT, the stakes are also eminently geopolitical.

How to follow the launch?

The launch was originally scheduled for today, Thursday December 15, from the Vandenberg base in the United States. But following a moisture problem in the Merlin rocket engines of the Falcon 9 launcher, the launch had to be postponed. Fortunately, another shooting window is available tomorrow; CNES and NASA now plan to fire on December 16 at 12:47 p.m.

Update: SWOT is now targeted for launch no earlier than Dec. 16, at 3:46 am PT 🚀

—NASA Earth (@NASAEarth) December 15, 2022

But to achieve this, it will be necessary to determine the origin of this humidity. If it is simply residual water linked to the passage of a small storm, SWOT should be able to start tomorrow. But that’s not a guarantee. “If it’s anything other than water, we’ll need to suspend launch to replace those engines and make sure they’re reliable.“said Julianna Scheiman, president of the Civil Satellites Division at SpaceX. So we give you an appointment tomorrow around noon to find out what to expect. If the launch goes ahead, it will be streamed on NASA’s YouTube channel (below).