Sentinel-3 is a low Earth-orbit moderate size satellite compatible with small launchers including VEGA and ROCKOT. Sentinel-3 is designed for a 7 year operational lifetime with 120 kg of hydrazine propellant allowing up to 12 years of continuous operations, including de-orbiting at the end of mission. The satellite layout is driven by the need to provide a large face viewing cold-space for thermal control, a modular design for payload accommodation and simplified management of all on-board interfaces. The satellite mechanical configuration and its flight attitude have been optimised through intensive mission analysis studies and system trade-offs performed during the mission definition, resulting in significant improvements in comparison to ENVISAT.

The main subsystems of the satellite are:

• Electrical Power System (EPS), comprising one solar array wing providing power to the spacecraft and payload.
• Attitude and Orbit Control System (AOCS) composed of a coarse sun sensor, magnetometers, coarse rate sensors, star trackers, a GNSS receiver and control actuators including thrusters, magneto-torquers and reaction wheels.
• Satellite management unit for satellite commanding and monitoring.
• Data handling and mass memory unit for payload data handling.
• Mass memory units for the satellite and its payload.
• Satellite telecommunication subsystems including an S-band subsystem for both telecommand (TC) uplink and telemetry (TM) downlink, and a dedicated high volume X-band subsystem for mission data downlink.

The main satellite platform characteristics are:

• Gyroless, three-axis stabilised platform with three star tracker heads, four reaction wheels and magnetic off-loading.
• Geodetic pointing and yaw steering.
• Eight 1 N hydrazine thrusters for in-plane and out-plane manoeuvres.
• Real-time on-board orbit accuracy determination of 3 m based on GPS and Kalman filtering.
• Launch Mass: 1 250 kg (with maturity and system margins, and fuel).
• Stowed dimensions (mm): (H) 3 710, (W) 2 202, (L) 2 207.
• Power: 2.1 kW rotary wing with 10 m² triple junction GaAs European solar cells; Li Ion Battery Capacity: 160 Ah.
• Autonomy: position timeline and on-board sun ephemeris for greater than 2 weeks nominal autonomous operations.

Communication links:

• 64 kbps uplink, 1 Mbps downlink S-band command and control link (with ranging).
• 2 x 280 Mbps X-band science data downlink.
• 384 Gbit solid state mass memory.