## 3 Discussion
### Division of Offshore and Deep Sea  
The most obvious difference between ocean tides in the offshore and deep seas is reflected in the tidal height gradient. According to Equations (8)-(15), the tidal correction gradient modulo is affected by both the tidal height and the geoid undulation. The former differs according to time and geographical location like offshore/deep sea. The latter is determined by gravity anomaly at the location of the specular reflection point. Therefore, the tidal height gradient of the offshore and deep sea will have different effects on the tidal correction gradient. The effects will be reflected in the difference of the tidal correction gradient between the two kinds of sea areas on some level. Due to the continuity and equal spacing of the GNSS-R specular reflection points of the tracks on earth surface, the tidal height gradient along the track is also a sequence of equal-spacing samples. The tidal height gradient is calculated as the tidal height difference between the two adjacent points on one track. Figure 5 shows the tidal height (the red curve) and its gradient modulo (the blue curve) of the specular reflection points by the OTCP method on a track containing an offshore part on the right and a deep sea part on the left. The trend of tidal height along track changes gently and nearly linearly in the deep sea (see the 1 st to the 300 th points of the red curve). While in the offshore part, the degree of tidal height change is significantly increased, and the increase and decrease of the tidal height is more random (see the points after the 300 th of the red curve). In addition, due to the periodic black body correction to the noise reference of the GNSS-R receiver on TDS-1 satellite, the position of the specular reflection point is periodically hopped. The influence of the hopping of TDS-1 data on the positioning accuracy has been discussed [13]. Although the hopping has little effect on the positioning accuracy of the correction positioning method, it will cause the tidal height along track to jump (see the jumps of the red curve in Figure 5), so that periodic peaks of the tidal height gradient are produced (see the blue curve). The magnitude of these peaks are generally comparable to those of offshore tidal height gradient mutation peaks; the jumps' effects on the tidal height gradient and the correction gradient in the offshore are negligible. But the jumps have significant effects on the deep sea segments with gentle change of the tidal height gradient, so these jumps are eliminated in the analysis. The sampling time difference between adjacent specular points on one track is only 1 s, the corresponding error of tidal phase and tidal height is insignificant, the influence on the tidal height gradient can be neglected. In this study, the offshore and deep sea parts on each track are divided and compared according to the changes of the tidal height gradient of the track, the specific methods are as follows. 1. Filtering the tracks across both sea and land. There are 28 tracks cross both sea and land among all the 43 tracks, the sea parts of these tracks are extracted. 2. The tidal height gradient and the corresponding tidal elevation position correction gradient of each track is calculated and detrended, the average is then subtracted, and the modulo is taken at last. Division of the offshore and the deep sea segments. Since some of the offshore tracks repeatedly cross land and sea, or cross islands, peninsulas, etc., one track is often divided into multiple sea sub-tracks by land, and the sub-tracks mostly contain offshore segments. Moreover, due to the complexity of the global coastline and tracks characteristics (length, direction, curvature, distribution, etc. [13]), the position of the offshore segment in the track is complicated. It could be divided into four cases: the offshore part is at one end of the track, at both ends of the track, in the middle of the track (the middle of the track is close to the land) or the entire segment is in the offshore (generally short tracks).