Sentinel3 Mission Architecture

In response to the user requirements, the Sentinel-3 system has been defined to support in a long-term sustainable operational fashion four core observing missions: surface topography, ocean colour, ocean and land surface temperature and land surface optical monitoring at medium resolution. Being an operational mission, it is based on the use of demonstrated observing techniques and existing data processing heritage. The Sentinel-3 mission aims at providing remote sensing data in routine, long term (20 years of operations) and continuous fashion with a consistent quality and a very high level of availability for supporting operational oceanography and global land applications.

Figure 8 provides the overall architecture of Sentinel-3 plus all the other elements necessary for the delivery of operational products including the already mentioned Marine Core Services, the value addes the need for Precise Orbit Determination (POD), and the need for the data delivered by the mid inclination radar altimeter satellite Jason.

The surface topography mission to be fulfilled by Sentinel-3 has a primary objective to provide accurate, high density altimetry measurements from a high inclination orbit with long exact repeat cycle, to complement the JASON (CNES, 2008) mid inclination ocean altimeter series. Ocean topography measurements support meso-scale circulation and sea-level monitoring as well as measuring significant wave height which is essential to operational wave forecasting. In addition, sea ice measurements similar to the CryoSat (ESA 2008) mission (though from a slightly different orbit) are supported. The altimeter configuration, a single-antenna radar altimeter with aperture synthesis processing for increased along-track spatial resolution, balances continuity and improved performance needs. Among others, it will extend observations to inland waters and coastal zones. The altimeter will be

Fig. 8 Sentinel-3 Arhitecture and supplementary elements necessary to provide Operational Oceanography products

supported by a Precise Orbit Determination (POD) system and microwave radiometer (MWR) for correcting accompanying water vapor induced propagation delay errors. The altimeter will be able to track over a variety of surfaces: open ocean, coastal sea zones, sea ice and inland waters. The optimal mode of tracking will depend of the surface over-flown, with changes pre-programmed in the satellite to minimize data loss.

The Ocean and Land Color Instrument (OLCI), based on the ENVISAT MERIS instrument, fulfils ocean color and land surface cover mission objectives. The Sea and Land Surface Temperature Radiometer (SLSTR), based on the ENVISAT AATSR instrument, in turn supports the ocean and land surface temperature observation requirements. Unlike AATSR, SLSTR implements a double scanning mechanism for a much larger swath, providing almost horizon-to-horizon coverage and allowing for the synergetic use of both OLCI and SLSTR instruments over the broad region of swath overlap.

The Sentinel-3 satellite is a low Earth orbit satellite that includes a medium-sized spacecraft, large swath/medium spatial resolution optical instruments and a radar altimeter system. The orbit selection, the optimised satellite mechanical configuration and its flight attitude result from intensive mission analysis studies and system trade-offs performed during the definition phase in collaboration with ESA system team, leading to an improved system capacities (with respect to ENVISAT) including features such as the altimeter SAR mode, and additional spectral bands for the optical payload. The satellite is compact and is compatible with small launchers of the type VEGA or Rockot. The satellite accommodates six different payloads with specific sizes, interfaces, severe Earth and calibration field of view constraints, thermal requirements for radiators cold space access. This large number of payloads drives the satellite configuration. The resulting satellite architecture is depicted in Fig. 9 in stowed configuration and in Fig. 10 in deployed configuration.

The sun-synchronous polar orbit chosen is at 814 km altitude (14 + 7/27 revolutions per day) with a local equatorial crossing time of 10:00 a.m., as a compromise between optical instrument and altimetry needs. The present baseline of two simultaneously-orbiting satellites supports full imaging of the oceans in less that 2 days after taking Sun-glint contamination into account (see Table 2), whilst delivering global land coverage in just over 1 day at the equator.

Fig. 9 Sentinel-3 derivation of mission products from the instruments it carries

Fig. 10 Sentinel-3 stowed configuration

Table 2 Satellite features

Launch mass

1240 kg

Main body dimensions

Height (X): 3.854 m

(stowed conf., including

Width (Y): 2.270 m

appendages)

Length (Z): 2.245 m

Pointing type

Geodetic

Absolute pointing error

< 0.1 deg

Pointing knowledge

< 0.05 deg

Observation data (average)

103 Gbit/orbit

PLTM downlink data flow

300 Mbit/s

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