Secure communications in space

Space systems for earth observation, for navigation and positioning, for telecommunications or for any other application have one essential aspect in common. The systems shall operate in such a safe way as to fullfill the mission they have been designed for. Threat for safe operation can come from failure of technology as well as from self generated interference between components or subsystems. These effects are quite well understood and documented. However, the full reliability against failures of this kind has not been reached and might never be achieved. Operations can also be in danger by natural effects, like solar activity, scintillation and other. A more disturbing effect is the unintentional interference or the detection by authorized or non-authorized individuals or groups. Interference can reach from jamming of the wanted signals up to a timely or even non-repairable interuption of the links to, from or between satellites. A severe situation appears in the case that systems are attacked by intentional interference and jamming. The aim of the attack might reach up to the destruction of the system. An other variant is the undiscovered extraction of information from a transmitted signal by an opponent. It is obvious that military systems must be designed with the presumption that they will operate in a hostile environment. In consequence, the related costs for design, construction and operation are an inherent parameter of such systems. Commercial systems, however, are in general designed, constructed and operated for so called nice weather conditions. If problems of interference appear, they then are handled on an ad hoc basis. The question arises on the degree of hardening against such unwanted effects. More hardening would mean to invest extra money that makes the system more fool proof, but would lower its cost effectiveness. It can be recommended to always consider the balance of risks against costs as a prominent and essential parameter of any space system. Obviously, under estimated or given circumstances, a safer communication would pay to the system provider as well as to users and customers. This paper tries to come to a systematic approach for safeguarding secure satellite communications. Selected examples illustrate the method.