# SAM-PAY: A Location-Based Authentication Method for Mobile Environments +
## 3 Architecture and Components
For example, the HAUT project [61] defined a Galileo HAS User Terminal, which is a light, small, portable, configurable, and autonomous device powered by a triple-frequency Galileo and GPS receiver. This UT provides free-of-charge, high-accuracy Precise Point Positioning (PPP) corrections (orbits, clocks) and code biases for Galileo and GPS to achieve real-time improved user-positioning performance. From a security point of view, we considered UT to be a trusted tamper-proof device that can store sensitive data (like secret keys), which can be further used in the authentication process. Local Element. The Local Element (LE) is in charge of delivering enhanced performance in terms of accuracy, integrity, availability, and continuity by combining GALILEO/GPSIII satellite-only services with additional information coming from external sources. In particular, the LE provides augmentation and certification features using data from GNSS and GSM cellular networks. The LE accesses the GNSS data via a dedicated connection to the GPS/EGNOS reference station, and it can exploit at the same time all the functions and data available in the mobile operator network. According to the description given in [60], the Local Element provides two important functionalities: Assisted GPS and Certification. Assisted GPS allows the UT to improve its GNSS receiver performance in terms of Time-To-First-Fix (TTFF) and sensitivity. Certification guarantees the reliability of position and time information. This feature is obtained by integrating the GNSS integrity data with the data calculated using cellular-network positioning techniques. Moreover, the cross-check between GNSS and cell-based positions allows the isolating of unexpected high-GNSS errors and acts as an effective anti-fraud mechanism by detecting possible illegal alterations of the GNSS data collected from the user device. Moreover, according to the LE description in [60], an integrity-processing strategy was also defined, which has been optimized for difficult operating conditions. For example, in light-indoor or urban canyons, the GNSS signals may be strongly attenuated and affected by multiple reflections generated by the surrounding environment, such as buildings and trees. In these scenarios, a monitoring technique (which is an extension of the Receiver Autonomous Integrity Monitoring--RAIM--technique) is used for the GNSS signal-integrity check in order to detect and isolate the observations that are degraded (with the Fault Detection Estimation--FDE technique). If a certified position is needed, the system must guarantee that the probability of an outlier being used in the solution is below the so-called \"integrity risk\". In these cases, the Protection Level (PL) may be provided as an upper bound that a position error shall not exceed without being detected. The PL size may be determined as a function of the application requirements (e.g., integrity risk) and other parameters, such as the measurement error model, number of visible satellites, and external aiding. The PL parameter, together with the position estimation, may be used in liability-critical LBS services, such as in commercial applications, in order to guarantee a more reliable certified service and to minimize the probability of performing incorrect actions. e.g., an incorrect service charge.