# A Proposal of a Troposphere Model in a GNSS Simulator for VANET Applications
## 4 GNSS Application in VANET  
GNSS is considered an important component in the automative environment where the driving assistance is gaining always more attention in order to meet the requests of security and safety. The number of applications that use the GNSS technology is always higher and several applications have increasingly stringent requirements. So, in this context, the design of GNSS receivers is an important aspect to be considered. Traditionally, GNSS testing has been subdivided into following three distinct methods: Live sky testing, Record and Replay methods, Simulators. Nowadays, the best practices for testing GNSS receivers concern tests performed in a controlled and repeatable manner in a safe laboratory [24]. In such a way, it is possible to allow tests on the development of receivers currently unavailable or able to operate with a non-complete constellation. The satellite messages contain among other information the so-called ephemeris, in order to compute the satellite position at any time, allowing, with the corresponding distance estimations, the receiver to compute its own position and time. The distance between the receiver and a given satellite, \(d_{i}\), can be so calculated:  
\[d_{i}=c\cdot(t_{i}^{R}-t_{i}^{S}), \tag{1}\]  
where \(c\) is the speed of light, \(t_{i}^{R}\) is the receiving time in the receiver's clock, and \(t_{i}^{S}\) the time of transmission for a given satellite \(i\). Receiver clocks are inexpensive and not perfectly in synchronization with the satellite clock, and thus this time deviation is another variable to be estimated. In this field, the pseudorange term identifies a range affected by a bias. It refers to the clock error between the receiver and satellite. The main error is due to the signal propagation: the atmosphere introduces modification in the time because the difference between light and wave speed that results in an error on the distance estimation. The main task of a simulator is to emulate the different conditions of a GNSS receiver by modeling the satellite and vehicles behavior, the characteristics of the signal, the atmospheric effects and all relevant aspects concerning receiver navigation in the considered simulated environment. Unlike road tests, simulator tests provide full control of simulated satellite signals and simulated environmental conditions. In this way, testers can easily generate and run many different test scenarios for different types of tests, with complete control over all simulation parameters such as date, time position, vehicle movement and environmental conditions. The performance of a receiver varies on the basis of errors and effects applied to the RF signal. The evaluation of the simulation results can be done in real time or by post-test analysis of the recorded data by comparing receiver and simulation data. GPS-SDR-SIM: An Open Source Simulator for GPS Receivers GPS-SDR-SIM [25] is an open source simulator written in \(C\) language that allows to generate baseband GPS data streams, which can be converted into RF signals using Software Defined Radio (SDR) platforms such as for example ADALM-Pluto [26] and HackRF One [27]. The software permits to specify the GPS constellation and, then, to generate the GPS signal based on a file called GPS broadcast ephemeris file. In this work, the file has been downloaded from the site [28] and a daily file containing information about the constellation at a given moment (in RINEX (Receiver Independent Exchange) format [29]) has been obtained. These files are then used to generate the simulated pseudo-range and doppler frequency for the GPS satellites. These simulated data are then used to generate digitized I/Q samples for the GPS signal [30]. The simulation starting can be specified if the corresponding set of ephemeris is available, otherwise it is set on the basis of ephemeris present in the RINEX file.