## 4 Simulation Results  
### Simulation Criteria and Environment Description  
By means of the C-DGNSS positioning framework, the participating vehicles in the neighborhood share PVT-related data with the target vehicles employing low-cost receivers. The NMEA 0183 messaging protocol is an international data format intended for communication between electronic devices similar to the ASCII character standard for computer programming [1; 14]. The NMEA messages are plain sentences with specific serial data fields that can be read by a common digital notepad and are composed of ASCII stringsthat convey useful location-based information. A summary of the basic positioning metrics along with the corresponding NMEA messages is reported below [1; 14]:  
* Geographic latitude, longitude, and altitude ($GNGS);
* Number of GNSS satellites in view ($GNGS);
* Horizontal and vertical dilution of precision ($GNGS);
* Horizontal and vertical accuracy standard deviations ($GNGST);
* Quality of the location (integrity, validity, and integer ambiguity status) ($GNGFA).  
Especially, the $GNGST message is considered a significant requirement for high-precision GNSS positioning because of the role of GNSS metadata in evaluating the quality of GNSS coordinates. The low-cost GNSS receivers may transmit these NMEA messages through various short-range radio-communication interfaces such as the ITS technologies described in Table 1 as well as Wi-Fi, 3G, LTE-4G, 5G, and Bluetooth. The benefits of the NMEA protocol are numerous since it enables the communication between heterogeneous hardware and software while the users are not submitted to develop custom implementations for each GNSS receiver. In Figure 2, the operation of the proposed C-DGNSS framework is depicted. The aiding MADM module assists the target vehicle in selecting the best neighboring vehicle with which to cooperate and hence improves/updates its position in a realistic manner. It parses the incoming NMEA messages that constitute the criteria values of the decision matrix, and then the appropriate MADM algorithm takes over, providing rankings of the neighboring moving vehicles. The MADM module assumes a set of criteria or attributes that collectively define the GNSS position quality of the engaged cars. For the purpose of efficient C-DGNSS and achieving improved position accuracy with the lowest latency and cost, it is advisable that these attributes should be highly uncorrelated and up to 5 or 6. From the reported NMEA messages described earlier, in our contribution, only certain fields are employed as input criteria for the proposed MADM module. Table 4 exhibits the list of criteria investigated per NMEA sentence: (1) The number of GNSS satellites in view (\(NS\)), (2) the root mean square of the double-difference phase residuals in the L1 band (_Range RMS_), (3) the standard deviation of the horizontal coordinates point fix (\(Hz\)_std_), (4) the standard deviation of the vertical coordinates point fix (\(V\)_std_), (5) the ambiguity status of the position solution (_Amb Stat_), and (6) the horizontal dilution of precision (_HDOP_). In greater detail, the _Range RMS_ is measured in meters, the \(Hz\)_std_ and \(V\)_std_ are measured in meters and express the horizontal and vertical accuracy error in the position, and the _Amb Stat_ denotes the GNSS receiver's integer ambiguity status. Thus, _Amb Stat_ may yield an autonomous solution, a differential GNSS, a float solution, and  
Figure 2: Diagram of the Cooperative-Differential GNSS (C-DGNSS) positioning configuration. a high-resolution fixed solution. Finally, the _HDOP_ reveals the effect of the DOP on the horizontal position value. The more satellites that are visible and low in the sky, the better the _HDOP_ and the horizontal position (latitude and longitude). Finally, the criteria are of type \"Expenses\" or \"Benefits\". The first type implies that the lower the criterion value, the better (minimum), while the second type implies that the higher the criterion value, the better (maximum).