SES 15 is a geostationary communications satellite operating in Ka- and Ku-bands in North America, which uses electric propulsion for initial orbit raising and all on-orbit manoeuvres.
The 2.1a version includes conversion from analog to digital flight control system and uprated engines on the booster and the first stage with improved injection systems. The new digital flight control and telemetry systems allow the rocket to launch from a fixed rather than angled launch platform and adjust its heading in flight. A digital control system also enables the launch of larger commercial satellites with wider and longer payload fairings such as the ST-type fairing. These fairings introduce too much aerodynamic instability for the old analog system to handle. This stage continues to use the RD-0110 engine. The 2.1a/ST version is sometimes called Soyuz ST-A.
Progress Rocket Space Centre, formerly known as TsSKB-Progress, is a space science and aerospace research company which is known for manufacturing launch vehicles and satellites. Most notably, Progress Rocket Space Centre is the manufacturer of Soyuz launch vehicles.INFO WIKI