The OSTM (Ocean Surface Topography Mission) follows in the footsteps of the TOPEX/POSEIDON and JASON-1 missions and provides continuity between them. OSTM plans to run for 20 years and is being conducted by a series of satellites, the first of which is JASON-2. The planned service life of JASON-2 stands at five years (including an extended observation phase) after its launch. 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 JASON-2 project is part of a French development programme for operational oceanography, including the development of in-situ measurements (CORIOLIS project) and of an analysis and prediction centre (MERCATOR project). These two programmes constitute the French contribution to GODAE (Global Ocean Data Assimilation Experiment), the first international operational oceanography experiment.
This mission was determined as part of an agreement between four partners: CNES, NASA, NOAA and EUMETSAT.
Elementary mission requirements
Ocean circulation is studied by measuring sea level height, derived from two elementary data elements:
- the distance or range, between the satellite and the sea surface, deduced from altimetry measurements,
- satellite radial height in relation to the reference ellipsoid deduced from measurements taken from different positioning systems.
In addition, the altimetric range measurement must be corrected for 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. Thhis is achieved with the POSEIDON-3 dual-frequency altimeter and the payload also includes a radiometer, which is used to calculate the water content in the troposphere.
The TOPEX/POSEIDON, JASON-1 and JASON-2 satellites use the same circular, non-sun-synchronous 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 uniform 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, a non sun-synchronous orbit enables major diurnal tidal components to be monitored.
JASON-2 Science Objectives
Sea level variations
Satellite altimetry is used to measure global sea level variations precisely (at centimetre level) and almost instantaneously (at the scale of ocean dynamics). 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 metres) 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 regard to the Earth geoid (that can be determined independently using gravimetric satellites such as CHAMP and GRACE), with amplitudes of the order of one metre, is known as the "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 the local slope. We can thus map the main sea currents, such as the Gulf Stream or the Kuroshio.
- 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.
- Finally, in the long term it is possible to monitor the mean 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 assuring the continuity and precision of these measurements is a major challenge for altimetry missions.
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 realtime, these measurements are used for weather forecasts.
Altimetry on the continents
Although designed to measure the height of ocean waters (of which the "radar signature" is correctly identified), altimeters also have the ability to obtain observations over the continents, particularly any water expanse that is large enough to be detected. This has provided 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.
Combination of altimetry and in-situ measurements: operational oceanography
Data assimilation consists of combining observations and models to make precise predictions about evolutions in complex systems. Traditionally used for weather forecasts, this technique can be transposed to operational oceanography. The international GODAE (Global Ocean Data Assimilation Experiment) experiment, the first "full scale" international operational oceanography experiment of its kind, was initiated in 2003. It aimed to demonstrate that it is possible to observe, model and predict the global ocean in three dimensions, routinely and in real-time.
Pilot systems from the GODAE programme, the Mercator Ocean Public Interest Group, created in April 2002, 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" 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.