# Utilization of Quasi-Zenith Satellite System for Navigation of a Robot Combine Harvester
## 2 Materials and Methods
### Positioning Augmentation Systems  
The RTK positioning technique can be used to enhance the precision of the position data from the GNSS to a centimeter-level in real-time. Nowadays, the RTK correction signal can be obtained using a virtual reference station (VRS) via an internet service provider (ISP). The VRS idea is imitative of the multiple reference station (MRS) because it uses the surrounding area of interest's MRS network, but it does not require an actual physical reference station close to the user. The VRS can achieve single-point coordinate accuracies of 2 cm with reference stations separated by 50-70 km under ideal conditions [5]. The QZSS is a satellite navigation system developed by Japan. Users around Japan and Oceania, even in urban canyons and mountainous terrain can receive the positioning, navigation, and timing (PNT) broadcast messages. Further, it is possible to receive augmentation signals by an inclined, elliptical geosynchronous orbit of the Quasi Zenith Navigation Satellites (QZSs); which is the space segment of the QZSS. The shape of QZO coverage area around Japan and Oceania is shown in Figure 3. Another segment of the QZSS is a ground segment consisting of a master control station (MCS), a monitoring station (MS), tracking control stations (TCS), and the system of other national research institutes placed around Japan and overseas to estimate and predict the QZS orbit and clock more precisely. The QZS transmits four GPS interoperable signals and three GPS augmentation signals. The GPS interoperable signals are L1C and L1C/A on L1-band (1575.42 MHz), L2C on the L2-band (1227.60 MHz), and L5 on the L5-band (1176.45 MHz). The GPS augmentation signals are L1-Sub-meter class Augmentation with Integrity Function (SAIF) on the L1-band, L6D, and L6E on the L6-band [12]. The L6D and L6E signals from the QZS are used for the precise point positioning (PPP) service by the CLAS and MADOCA positioning augmentation systems, respectively [13]. Compared to the RTK, both CLAS and MADOCA do not need neither an ISP nor an NTRIP service provider to obtain the correction signal from the reference station. The MADOCA augmentation system was developed by the Japan Aerospace Exploration Agency (JAXA). The measurement principle of MADOCA is shown in Figure 3. MADOCA is based on JAXA's technology for estimating the satellite orbit and clock correction. This technology supports all usable GNSS constellations; GPS, GLONASS, and Galileo, by receiving the L6E signal represented by the dash-dotted line in Figure 3. This signal broadcasts the correction of the positioning error including satellite orbit corrections and satellite clock from the QZSS. Unlike the CLAS, the MADOCA cannot correct the ionosphere and troposphere delays because the global monitoring station of the MADOCA (MGM-net) represented by the pinpoints in Figure 3 is not dense compared to the CORS of the CLAS represented by the flags in Figure 4. However, these delays could be estimated by the receiver [14]. The MADOCA requires a long time to recover to a convergent PPP positioning method [13]. Similar to the CLAS, to make a comparison or to use the positioning results of VRS-RTK and PPP-RTK together requires the coordinate transformations for compensation because the MADOCA also neglects the earth's crustal movement. Figure 2: Combine harvester’s Fulltime Drive System (FDS) with steering wheel for 3 turning modes (a) Gentle turn, (b) Normal turn, (c) Sharp turn. The CLAS was developed from its prototype, the Centimeter-class Augmentation System (CMAS), which does not have a complementary function for GPS. Figure 4 shows the diagram of the CLAS system. The QZS broadcasts the augmentation information via the L-band Experiment (LEX) signal [15]. The LEX signal was renamed L6 in the latest QZSS revision; it is represented by the dash-dotted line in Figure 4. The CMAS uses the  
Figure 4: Diagram of the Centimeter Level Augmentation Service (CLAS) positioning augmentation system.