The legacy of TOPEX-Poseidon

Before TOPEX-Poseidon was launched in 1992, little was known about large-scale variation in the global oceans.

What was known came from instruments in two places: the ocean and space. The instruments in the ocean, which include tide gauges on sea-level monitoring stations, provide useful information about the height of water relative to the height of the nearby land, however their scope was – and continues to be – limited. Fixed in place and mainly located along coasts, tide gauges measure only the sea level in their immediate surroundings, leaving the height of the surface over the open ocean, which is dynamic and widely varied, a mystery.

Ocean measurements are also performed by drifting and fixed floats and buoys, voluntary observing ships, and research vessels, all of which are sparse and costly. In addition, they cannot provide a global map of ocean currents or other surface parameters.

This is why the other location, space, is the ideal place from which to measure the height of the ocean surface. A satellite altimeter, an instrument that sends radar pulses down to the surface of the ocean and measures the amount of time it takes for this signal to be reflected back, takes advantage of a satellite’s bird’s eye view to measure a wide swath of the global oceans in just a matter of days.

But before TOPEX-Poseidon, there weren’t any long-lasting satellite missions dedicated to ocean monitoring. Starting in the early 1970s, a handful of satellites proved that such a thing was possible, most notably the very first satellite dedicated to monitoring the ocean, Seasat. Launched in 1978 by the United States, Seasat flew for only three-and-a-half months before a failure in the electric power system led to the mission’s abrupt end.

“Seasat’s design was far from providing the necessary accuracy and its short lifetime did not allow the acquisition of useful scientific data about ocean circulation. But it demonstrated the potential of altimetry and other techniques, triggering the preparation of other higher-precision missions – namely TOPEX-Poseidon,” said Fellous.

But there were still 14 years and a number of significant obstacles to overcome before TOPEX-Poseidon would launch, transforming ocean and climate sciences forever.

“Our altimetry mission was so expensive – probably three quarters of a billion dollars – that we couldn’t afford it on our own,” said Dr Stan Wilson, former Chief of the NASA Oceanic Processes Branch.

Without a clear path ahead, Wilson recalled an impromptu trip he had taken to the CNES Space Centre in Toulouse, France, in 1980 following a conference in Venice. At the conference, he had been invited on the trip by Michel Lefebvre, who would later become known as one of the leading pioneers of satellite geodesy, the science of measuring the shape of the Earth, its orientation in space, and its gravitational field. Geodesy and oceanography are interlinked disciplines, so it made sense that Lefebvre, a geodesist, wanted to introduce Wilson, a new NASA oceanographer, to his team in Toulouse.

“The seed for collaborating with France was planted when I visited the CNES Space Centre and saw that the people there were working with a high level of technical expertise, that their work was scientifically driven, and that they were interested in oceanography,” said Wilson.

“That kind of went in the back of my mind for a couple years. And when we at NASA realised we couldn’t sell the TOPEX programme on our own, I thought, ‘Well, maybe we can do something with the French’,” he recalled. 

Meanwhile, the French space agency had run into an obstacle of its own with its ocean altimetry mission, Poseidon. Although the French government could afford to fund it, they did not see the value in an ocean altimetry mission that would involve only France. As a result, they were not willing to give the mission the green light. To get Poseidon off the ground, CNES would need to find a partner.

In 1983, the first official step toward combining the two missions was taken when a US-French panel convened in Washington, DC, and recommended exploring a potential collaboration. In 1987, both countries approved a joint programme.

But forging the first collaboration between France and the United States on an Earth observation satellite was easier said than done.

“When you are put in a swimming pool as a child, you either swim or you die. So I had to swim,” said Fellous, whose main responsibility as incoming CNES programme manager in late 1982 was to make this collaboration work.

Research and operational communities worldwide use this ocean altimetry data for wide-ranging uses, including climate and ocean forecasting, hurricane tracking and forecasting, fisheries’ management, ship routing, and even the tracking of marine debris. Critically, too, TOPEX-Poseidon kicked off a precise record of sea level rise that continued long past the satellite’s 13½ -year lifetime.

“We now know that although ‘global warming’ is uneven, sea level is steadily evolving. Even though sea level differs by region, it can be observed at any place in the world. Thanks to TOPEX-Poseidon, this understanding of sea level rise has stepped up the global conscience when it comes to climate change,” said Fellous.

Luckily, the collaboration CNES and NASA forged through TOPEX-Poseidon was so strong, that Europe and the United States were able to continue these crucial ocean circulation measurements through four subsequent ocean altimetry missions: Jason-1, Jason-2, Jason-3, and Sentinel-6, with the involvement of the US’s National Oceanic and Atmospheric Administration, EUMETSAT, the European Space Agency, and the European Union. Although these missions continue the collaboration, there are no longer two altimeters on board.

As it turns out, CNES’s experimental altimeter was a success.

“The fact that the French solid state altimeter proved its excellent performance and that CNES exceeded its commitments in its work on TOPEX-Poseidon created a lot of confidence on the NASA side, leading NASA to accept the European altimeter as the only altimeter on the following mission, Jason-1. This also led to a cooperation scheme for follow-on satellites that shifted the responsibility from NASA to CNES,” said Ratier. “It’s absolutely incredible.”

Now, the most recent satellite mission of the series, Sentinel-6, is already taking ocean and climate monitoring into the future through its next-generation, high-precision Poseidon-4 altimeter. The US-European collaboration has also been expanded upon through this mission, a joint effort between EUMETSAT, the European Space Agency, NASA, and NOAA through Europe’s Copernicus Programme, with support from CNES. The two Sentinel-6 satellites, Sentinel-6A Michael Freilich and Sentinel-6B, will continue to operate until at least 2030, ensuring that this crucial data will be collected for years to come.