# Full Real-Time Positioning and Attitude System Based on GNSS-RTK Technology
## 2 GNSS-RTK Technology
### GNSS Error Correction
* If the position information is not needed in real time, the post-processed kinematic (PPK) technique may be employed. It uses the same method as the RTK technique, but in this case, the receiver records the data and signals received from the satellite. The data of the base or reference stations are subsequently requested in receiver-independent exchange (RINEX) format and post-processed by software, obtaining the same accuracy as the RTK technique. The main drawback of this method is the lack of information in real time, while its main advantage is that real-time corrections are not needed. * The precise point positioning (PPP) technique uses a different concept. This method generates parameters for the models of the error sources so that the receiver can compensate them. Terrestrial reference stations are needed to compare their known positions with the measurements and then generate these parameters. The main advantage of modeling the error sources is that the scope of application can be global and the bandwidth needed to transmit the model parameters is much smaller than the bandwidth needed to transmit the RTCM (Radio Technical Commission for Maritime) corrections used by the RTK technique. The main drawbacks are that the receivers need more computing capacity to process the error models and the precision is lower than the one obtained with RTK. PPP services require payment, and due to the low data consumption, the information is transmitted by Internet and by satellite. Following this trend, there are also many works for the development and modernization of the GNSS. For example, in [19], a review of the satellite navigation status is made, where the problems and challenges are treated, remarking the opportunities, trends, and important values of satellite navigation, and in [20], the current status and planned development of four global navigation satellite systems (the American GPS, Russian GLONASS, European Galileo, and Chinese BeiDou) are reviewed, describing the features of modernization and improvements on positioning, navigation, and timing. For further information about the future and modernization of the different satellite navigation systems, the reader is referred to [21; 22; 23; 24]. It is important to remark at this point that, due to the significant impact of this technology in a wide number of areas, there are multiple research works dealing with techniques and strategies to improve the accuracy and reduce the positioning error. To name but a few, see, for example [25; 26; 27] and references therein. In [25], a new calculation strategy is presented for the computation of the aircraft's resultant position using the GPS (Global Positioning System) SPP (Single Point Positioning) code method in order to improve the quality of determination of the coordinates and accuracy of GPS SPP positioning. In [26], the authors deal with the problem of combining multi-GNSS signals in a single functional model. A methodology and quality assessment of selected methods allowing for multi-GNSS observations is presented, where loose and tight integration strategies applied to the ionosphere and troposphere weighted model are characterized. In [27], precise navigation based on SBAS technology and CORS stations is studied. To estimate reliable and redundant GPS/EGNOS positions, three rovers with GPS receivers are used, and a Kalman filter is employed to compute the final solution. Therefore, from the above, it is clear that GNSS in general, and GNSS-RTK in particular, are important technologies used in multiple applications and areas where maximum precision and reliability are needed, making it a very active research and commercial field.