1. UCS. Nuclear Weapons & Global Security: UCS Satellite Database [website]. Cambridge MA:Union of Concerned Scientists (updated 13 September 2013). Available: http://goo.gl/wSaVJI [accessed 25 September 2014].
2. ILRS. Current Missions [website]. Greenbelt MD:International Laser Ranging Service Goddard Space Flight Center U.S. Aeronautics and Space Administration (updated 19 May 2014). Available: http://ilrs.gsfc.nasa.gov/missions/satellite_missions/current_missions/ [accessed 25 September 2014].
3. NASA. NASA’s Earth Observing System Project Science Office: Current Missions [website]. Washington DC:National Aeronautics and Space Administration (updated 23 September 2014). Available: http://eospso.gsfc.nasa.gov/current-missions [accessed 25 September 2014].
4. ESA. Observing the Earth [website]. Paris France:European Space Agency. Available: http://www.esa.int/Our_Activities/Observing_the_Earth [accessed 25 September 2014].
5. NASA JPL. Early History > First Satellites [website]. Pasadena CA:National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology (2014). Available: http://www.jpl.nasa.gov/jplhistory/early/firstsatellites.php [accessed 25 September 2014].