Mission

Scientific Objectives

The new interleaved mode of the Poseidon-4 radar altimeter will allow simultaneous measurements in:

• classical LRM mode providing performance continuity with previous missions,
• new promising SAR mode, which should improve altimetry on coastal and continental areas.

Variations in sea level

Satellite altimetry is used to measure precisely (at centimetre level), globally and almost instantaneously (at the scale of ocean dynamics) sea level variations. Ocean surface topography is variable on several scales of time and space, reflecting a large number of phenomena:

• Constant deviation with regards to the reference ellipsoid (approximately 100 meters) is mainly attributed to the geographical structure of the Earth geoid, i.e. the uneven distribution of mass inside our planet.

• Sea surface deviation with regards to the earth geoid (that can be determined independently using gravimetric satellites such as Champ and Grace), with amplitudes of the order of one meter, is known as "dynamic topography". Such deformations on the sea's surface are related to global oceanic circulation. In a similar manner to atmospheric pressure maps used for meteorology, ocean surface currents follow level curves with a speed proportional to its local slope. We can thus map the main sea currents, such as the Gulf Stream or the Kuroshio.

Mean dynamic topography, i.e. oceanic relief
corresponding to permanent ocean circulation.
Arrows are proportional to current speed.
Credits CLS

• Temporal variations in surface topography are also used to observe and monitor ocean variability (vortex, Rossby waves, etc.), tides, seasonal and/or climatic phenomena, such as El Niño.

Eddies in the Caribbean Sea and  theGulf of Mexico
(Credits Mercator Ocean)

• Lastly, in the long term it is possible to monitor the average sea level. Since the beginning of the Topex/Poseidon mission in 1992, an average global increase in sea levels of around 3 mm has been observed, with strong spatial variability (up to ± 20 mm/yr according to the region). This increase is an indicator of global warming, and in this respect, sustaining the continuity and precision of these measurements is a major challenge for altimetry missions.

Mean sea level since January 1993
(Credits CNES/LEGOS/CLS)

Products derived from altimetry measurements

In addition to surface topography, the signal recorded by altimeters is used to measure two other very useful parameters for marine meteorology: the significant wave height (SWH: average wave height over the footprint of the altimeter) and the surface wind speed. Available in near real-time, these measurements are used for meteorological forecasts and for navigation.

Significant wave height in July 2007
(Credits CNES/CLS)

Altimetry on the continents

Although designed to measure the height of ocean waters (of which the "radar signature" is correctly identified), altimeters can also obtain observations above continents, particularly on any water expanse that is large enough to be detected. This capability has opened new perspectives for continental hydrology. Using altimetry satellites thus makes it possible to monitor seasonal variations in lake levels and certain major rivers. These applications are particularly important in remote and/or poorly instrumented areas, such as the Amazon basin.

Operational oceanography

In addition to science objectives, Sentinel-6 has to secure and enhance new capabilities for operational applications implemented in the OSTM/Jason-2 and soon Jason-3 missions.

Operational oceanography has various applications:

• maritime security
• oil spill prevention
• marine resource management
• climate change
• seasonal forecast
• coastal activities
• ice survey
• water quality and pollution

In Europe, the Copernicus marine environment monitoring service provides regular and systematic reference information on the state of the physical oceans and regional seas. The observations and forecasts produced by the service support all marine applications.

In November 2014, the European Commission signed a Delegation Agreement with Mercator Océan for the implementation of the service.

Mercator Ocean, created in April 2002 and private society since 1 September 2010, is a partner of MyOcean. Mercator Ocean implemented a system used to describe the state of the ocean, an essential component of our environment, at any time and from any corner of our blue planet.

The Mercator system is "fed" through inputs consisting of observations of the ocean measured by satellites (altimetry, and also surface temperature and salinity) as well as in situ measurements (drifting buoys, sensors and temperature, salinity and current profilers). These measurements are "ingested" (assimilated) by the analysis and prediction model. Assimilating observation data in a model is thus used to describe and predict the ocean over periods of up to 14 days. Since October 2005, Mercator has been operating a global oceanographic prediction model with ¼° resolution, i.e. approximately 28 kilometres from the equator.

Mission

Programmatic context

The Sentinel-6 mission follows in the footsteps of the Topex/Poseidon, Jason-1, Jason-2 and Jason-3 missions. It provides continuity between those missions. The planned service life of Sentinel-6 stands at five years and a half after its launch. Consumables are planned for a lifetime of 7.5 years. The mission objective is therefore to provide operational continuity for the collection and distribution of high-precision data for the study of ocean currents and the measurement of sea levels, with a view to improving understanding of these phenomena and their impact on the climate.

The Sentinel-6 mission takes also into account the feedback from the altimetry series. Thus, the Ocean Surface Topography Science Team, in its annual meeting in Seattle in 2009, has emphasized the need of stability across several mission lifetimes for global mean sea level measurement.

The Sentinel-6 mission fulfils the role of reference mission for the ocean surface topography constellation, recognized by the Marine Core Service (MyOcean) of GMES.

Elementary mission requirements

Ocean circulation is studied by measuring the sea level height, derived from two elementary data elements:

• the altimetric distance, between the satellite and the sea level, deduced from altimetry measurements,
• satellite radial height in relation to the reference ellipsoid deduced from measurements taken from different positioning systems (precise orbit determination).

Principle of altimetry measurement (Credits CNES/D. Ducros)

In addition, the altimetric range measurement must be corrected from propagation effects, as the radar signal is delayed in the troposphere and ionosphere. The ionospheric delay is estimated by combining range estimates on two frequencies), and the tropospheric delay is calculated from water content estimates. Therefore, the Poseidon-4 altimeter is dual frequency, and the payload includes a radiometer, which is used to calculate the water content in the troposphere.

The Topex/Poseidon, Jason-1, Jason-2, Jason-3 and Sentinel-6 A and B satellites use the same circular, non-heliosynchronous orbit, inclined at 66° and at an altitude of 1,336 km. The satellite passes over the same points on the ground every 10 days, thus providing homogeneous sampling of the globe's surface over a given period. The orbit altitude is related to the need for precise orbit determination (negligible atmospheric drag and small-scale variations in the Earth's gravitational field have little impact at this altitude). More importantly, thanks to a sun-synchronous orbit, major tidal components (diurnal) can be monitored.

As a secondary objective, the Sentinel 6 mission will support a radio occultation observation mission contributing to climate change monitoring and weather forecasting. From the nonsynchronous orbit, this observation mission will provide unique coverage and sampling in space and time that are not accessible from sun-synchronous orbits providing observations at fixed local solar times.