## 3 Discussion
### Tidal Height Gradient and the Tidal Positioning Correction Gradient  
The average value and standard deviation of the tidal height gradient modulo and tidal correction gradient modulo of the offshore and deep sea segments are shown in Table 2, and those of each segment are shown in Figures 6-8. The tidal height gradient modulo and its standard deviation in the offshore segments are significantly higher than those in the deep sea, with the gradient of about 2.5 times and the standard deviation of about 3.5 times. The tidal height change in the offshore is more dramatic than that in the deep sea, and the difference between segments (with different tidal amplitudes and phases) is greater. Compared with the large difference of the tidal height gradient modulo in the offshore and deep sea segments, the changes of the positioning correction gradient modulo in the two sea areas are closer. The correction gradient modulo in the offshore and its standard deviation are  
Figure 5: Tidal height (red curve) and its gradient modulo (blue curve) of the specular points by the OTCP method on a deep sea-offshore track.  
respectively 1.2 and 2 times those in the deep sea. The tidal height gradients modulo in the offshore segments have a positive correlation with the positioning correction gradient modulo. They are very close for all the segments, and the difference between them does not increase significantly with the increase of the gradient (see Figures 6 and 8). This indicates that the variation of the tidal height in the offshore increases, and the change of the tidal correction is increased, the tidal positioning correction gradient modulo has a good response to the tidal height gradient modulo in the offshore. However, the difference between the two in the deep sea is larger, and the difference increases significantly with the increase of the gradient. The direct cause is that the gradient of the tidal positioning correction in the deep sea is higher than the tidal height gradient modulo; it does not decrease correspondingly as the tidal height gradient modulo decreases (see Figures 7 and 8). It can be considered that the variation of the tidal positioning correction is more consistent with the tidal height gradient modulo in the offshore where the tidal height changes greatly. While the tidal height change tends to be gentle in the deep sea, the change of the tidal positioning correction does not greatly decrease with it. The sensitivity of the response of tidal positioning correction to the change of the tidal height is reduced in the deep sea compared to offshore. It is speculated that the change in terrain and gravity anomaly in the deep sea is more dramatic than in the offshore with the relatively flat continental shelf underwater, which makes the geoid undulation and the total elevation positioning correction change more in the deep sea ([PERSON] et al., 2018). This may contribute to the decreasing of the response sensitivity and the maintenance of high tidal correction gradient in the deep sea, which is consistent with the expression that the elevation positioning correction is affected by both gravity anomaly and tidal height in Section 2.2.  
Figure 6: Tidal height gradient modulos and the corresponding tidal positioning correction gradient modulos of the offshore segments.  
Figure 7: Tidal height gradient modulos and the corresponding tidal positioning correction gradient modulos of the deep sea segments.