The legacy of TOPEX-Poseidon
This ocean-observing satellite transformed our understanding of the climate and kicked off a crucial transatlantic collaboration
On the day of the session on European-US collaboration on altimetry at the Ocean Surface Topography Science Team Meeting in Venice, Italy, four key players in facilitating the success of the TOPEX-Poseidon satellite and the approval of Jason-1 reflect on the collaborative missions that changed the way we see climate.
The mysterious ocean
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.
The seed is planted
Originally, TOPEX-Poseidon was conceived of as two separate satellite altimeter missions each with the same goal: to better understand wide scale ocean circulation and tides. The United States’ space agency NASA was planning the TOPEX mission, while across the Atlantic, the French space agency CNES was planning the Poseidon mission. Both intended for their missions to contribute to the World Ocean Circulation Experiment, a major international endeavor of the World Climate Research Programme.
“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.
Two nations join forces
The mission had three main goals: measuring ocean circulation, processing and distributing the measurements to science investigators, and establishing the basis for these observations to continue long term. Each of these goals produced formidable hurdles for the teams on both sides of the Atlantic.
First off, since the joint satellite would carry both countries’ altimeters but only had one antenna, the altimeters would not operate simultaneously. The space agencies had to agree on the best way to alternate the use of their altimeters. After much back and forth, the two countries agreed that the NASA altimeter TOPEX would operate 90% of the time, and the CNES altimeter Poseidon would operate 10% of the time.
This made it possible for NASA’s radar altimeter, based on the technology used in previous satellite altimeters, including the one on the defunct Seasat satellite, to function as the mission’s primary altimeter. CNES’s solid-state altimeter, which used new technology and was only one-fifth the weight of NASA’s instrument, could operate just long enough in order to enable it to be tested out as a lower-power, lighter alternative.
Then came the issue of which agency would provide the satellite bus, the main body of the satellite that carries the instruments, and which would provide the launcher. At a meeting in 1983 between NASA Administrator James Beggs and CNES President Hubert Curien in Washington, DC, Curien offered to provide a SPOT bus for use in the joint mission, leaving NASA to provide a launcher.
Recognising that this arrangement would compromise the scientific goals of the mission, NASA’s Wilson was unable to accept it. The issue stemmed from limitations in the SPOT bus’s orbit. The SPOT bus was only capable of flying in a sun-synchronous orbit, in which the satellite flies over any given point on the Earth at the exact same time every day, with similar illumination by the sun. Although this type of orbit is ideal for optical imaging – such as in missions to monitor ocean colour – it is not ideal for missions to determine changes in global sea level. This is due to the fact that changes in sea level are caused, in part, by tides, and those changes must be taken into account.
The gravitational pull of the sun and moon causes tides, and the solar tidal component has a high and low every 12 hours. Flying an altimeter in a sun-synchronous orbit would mean observing the sea level when the solar component was always at the same phase – high, low, or somewhere in between – every time it flew over a given point on Earth. As such, it would it would not be possible to determine the component of sea level height caused by solar tides. This is why a non-sun-synchronous orbit was essential for TOPEX-Poseidon. This type of orbit would make it possible to observe and determine all phases of the solar component of the tides.
To address this problem, Wilson suggested an alternative arrangement: instead of CNES providing the bus and NASA providing the launcher, why not have NASA provide the bus, which was capable of flying in the required non-sun-synchronous orbit, and have CNES provide the launcher? NASA administrator Beggs proposed this as a counter offer and CNES accepted.
“This decision was extremely unpopular in the political sense. It appeared that the US would be ‘legitimizing’ Ariane, giving it credibility at a time when it was an emerging competitor to the US launcher industry,” said Wilson.
Luckily, political opposition to this decision was short lived.
“In the end, sane heads prevailed, and thanks to the leadership at the most senior levels of NASA, the use of the CNES launcher was fully supported. This was both precedent-setting and groundbreaking,” said Bill Townsend, former TOPEX-Poseidon Programme Manager at NASA.
Another hurdle was the lack of a community that would use the new ocean data TOPEX-Poseidon would provide.
“Convincing the other French agencies to participate in a collective effort to develop the small ocean community into a sizeable group of people able to utilise the new data when they arrived, through increased funding, hiring of young people, and creating new labs, required some sense of diplomacy,” said Fellous.
“In the end, our community was formed from some remote sensing people who turned their interest to ocean and some classic oceanographic people who saw the benefit they could get from satellite remote sensing data,” he added.
Ultimately, it was the ability of CNES and NASA to come together that made it possible to overcome the hurdles posed by the final goal, to establish the basis for these ocean observations to continue long term. Four subsequent ocean altimetry satellites – two of which are currently in orbit – continue to push the boundaries of what is known about the ocean and the climate.
Not long after the launch of TOPEX-Poseidon in August 1992, the data it collected as part of the World Ocean Circulation Experiment began to transform the pre-existing understanding of the ocean.
“One 10-day cycle of TOPEX-Poseidon – a full orbit – showed as good an image of global ocean circulation as the previous 150 years of ocean measurements at sea had shown,” said Fellous.
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.
“TOPEX-Poseidon was a significant pathfinder for Europe and the US using each other’s launch vehicles and, more broadly speaking, for international collaborations in general,” said Townsend.
“This ground-breaking mission made it clear that international collaborations could lead to situations where their significance could be greater than the sum of their parts.”
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