This paper describes a new high performance Earth Observation Platform, the SSTL-300, which has been developed to provide customers with a capability that has previously only been available at much higher cost and on larger platforms. This platform offers a 7-year mission lifetime with a very high operational availability. The main payload is a very high-resolution imager (VHRI) with a panchromatic 2.5m ground sampling distance (GSD) channel and four multi-spectral channels offering 5m GSD. The imager swath is 20km in all channels. This imager is an extension of the 4m GSD imager already flying on Beijing-1, which was launched in October 2005. Additional payloads can be accommodated, such as the Medium Resolution Imager (MRI), offering lower resolution of 22m or 32m GSD in four multiple spectral bands with 300km swath width. The 32m MRI has already flown on four previous Disaster Monitoring Constellation (DMC) Missions. High performance geo-location is provided, the performance of which is dependent on the chosen subsystem options. Simultaneous imaging is possible with the VHRI and the MRI and scenes can be as long as 2000km. The image data is compressed on-board, using lossless data compression, for store-and-forward operations. Furthermore, switchable encryption is available, using the Data Encryption Standard (DES), on the TM/TC as well as switchable scrambling on payload data. Near real-time imaging & down-linking is possible for a range of targets close to the ground station. A range of imaging modes are available including: strip mapping, fast response scene capture, stereo imaging, with pitch angles between 10 and 45 degrees to provide digital elevation models, and increased area coverage to provide wide-swath high-resolution imagery of up to 85km. The nominal orbit for the SSTL-300 will be sun-synchronous, with a 10.30am node to provide repeatable global coverage and good lighting conditions. The platform will orbit at approximately 700km, which provides good optimisation for single satellite and constellation revisits. On-board propulsion is included for orbit maintenance.