1. Planet labs, http://www.planet.com, accessed: 28-07-2017.
2. Astrodigital, http://astrodigital.com, accessed: 28-07-2017.
3. Spire, http://spire.com, accessed: 28-07-2017.
4. Sky and space global, http://www.skyandspace.global, accessed: 28-07-2017.
5. The case for robotic on-orbit servicing of spacecraft: spacecraft reliability is a myth;Ellery;Acta Astronaut.,2008