This is the first part of a series dedicated to the importance of securing ADS-B transmissions.  Current air traffic control (ATC) relies on a combination of primary surveillance radar (PSR) based on conventional reflected radio waves and secondary surveillance radar (SSR) that uses an interrogation signal sent to an on-board transponder (Mode-S).  The replies from the aircraft are independent of the primary radar return and provide additional information (i.e., altitude and identity).  Not all aircraft have the Mode-S transponders.  The combined PSR-SSR gives an aircraft location accuracy of 1-2 nmi (nautical miles) with updates every 5-10 seconds, which leads to 3 nmi or greater separation requirement between aircraft.  To achieve the much higher aircraft densities that are forecasted for the years to come and to eliminate the high cost of operation of the PSR-SSR system, the Automatic Dependent Surveillance Broadcast (ADS-B) was introduced as an extension of the Mode-S beaconing system.  Its position accuracy is 0.05 nmi (92.6 m) and the velocity accuracy is 19.4 nmi/h (10 m/s), which means a much smaller separation between aircraft.  Being a satellite-based technology, the benefits of the ADS-B system include increased situational awareness, extended surveillance coverage, enhanced conflict detection, reduced operational costs, and improved routing efficiency.