The programmatic context
The OSTM (Ocean Surface Topography Mission) follows in the footsteps of the TOPEX/POSEIDON missions and JASON-1 and provides continuity between those missions. OSTM mission aims to run for 20 years and is 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 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 the development 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.
The 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.
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-3 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 and JASON-2 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, an non sun-synchronous orbit permits that major tidal components (diurnal) can therefore be monitored.