Abstract
The paper discusses the metrological aspects of building a model of an NTP server intended for use in an autonomous
differential correction system (SDK). It is shown that the deliberate distortion of the data of the GPS / GLONASS signal state vector
and differential corrections of control and correcting stations (CCS) is the main task of a cyberattack on A-GPS or GPS algorithms of
mobile devices (smartphones). This type of hacker attack belongs to modification attacks. To eliminate the threat of violation of the
integrity of differential corrections, the procedure for authenticating the correcting information (CI) transmitted by the CCS is
applied. At the same time, the transmitted KKS data is checked for modification, the order of passage and the timeliness of delivery
of KI messages. The process of authenticating broadcast messages involves attaching a fixed-size data block called a Message
Authentication Code (MAC) to the CI. It is assumed that the two parties involved in the exchange of data, the KKS and the user’s
mobile device, use a common current MAC and NTP server trust timestamp. The main task of the NTP server is to independently
synchronize the transfer of information and the formation of the current MAC. To form the current MAC, the modified code of the
current value of the NTP server time is used in accordance with the X9.95 scheme.
