## 4 Results
### Water Bodies Monitored by Different Altimetry Configurations  
Due to the profiled measurement geometry of satellite altimetry, not all detected water bodies can be monitored by this technique. Depending on the orbit configuration (see Section 2.2), multiple lakes will be missed by some missions, especially smaller lakes.  
Since the altimetry mission configuration changes over time, four different scenarios have been defined and investigated regarding their lake coverage:  
1. Jason only;
2. Sentinel-3 only (both satellites);
3. Jason and Envisat (past configuration);
4. Jason and Sentinel-3 and Cryosat-2 and Saral-DP (current configuration).  
Figure 3: Number (left) and total area (right) of detected lakes in the Mississippi River Basin (MRB) depending on the adopted water occurrence probabilities for three different minimum lake sizes (blue: <0.1 km\({}^{2}\); red: <1 km\({}^{2}\); orange: <10 km\({}^{2}\)). The blue dots with a black edge indicates the results for a 50% water occurrence probability and a 0.1 km\({}^{2}\) lake size limit, which are given in the text. Dashed black lines show the number and area from the Global Lakes and Wetlands Database (GLWD) (lower line: GLWD Level 1 only; upper line: GLWD Level 1+2). Solid black lines indicate numbers from the WaterGap Global Hydrology Model (WGHM).  
The statistics presented here are based on a water occurrence of 50% and all 4535 water bodies larger than 0.1 km\({}^{2}\) (blue dot with black edge in Figure 3). In total, 3437 of those targets (75.7%) have not been covered by any of the current and past missions under investigation. Since these are mostly smaller lakes or reservoirs, the percentage of not covered water area is only 17.5% (5108 out of 29,101 km\({}^{2}\)). Depending on the ground tracks of the different missions, they cover a different number of targets. The best scenario is the current configuration of Jason-3, Sentinel-3A/B, Cryostat and Saral-DP, which captures about 19% of the water bodies and 79% of the surface areas. With respect to past altimetry configurations (i.e., Jason-2 and Envisat), this is a significant improvement. All numbers are summarized in Table 2.  
As expected, most of the larger lakes are captured by the altimetry missions. The smaller a lake, the higher the probability that it is missed by the satellites. Figure 4 shows the percentage of missed water bodies for the four different mission scenarios depending on the size of the lakes. With the current configuration, all lakes larger than 50 km\({}^{2}\) and almost 20% of the lakes larger than 10 km\({}^{2}\) are captured. This is a significant improvement with respect to the past configuration of Jason and Envisat, which missed almost 40% of all water bodies larger than 20 km\({}^{2}\). However, even today, about 67% of all water bodies larger than 1 km\({}^{2}\) are missed in the MRB. This number will only be improved significantly when a wide-swath altimetry mission such as SWOT--planned for 2021 (see Section 5.5)--will become active. When only using Jason, more than 60% of lakes larger than 100 km\({}^{2}\) cannot be monitored, whereas the Sentinel-3 configuration alone performs better than the past configuration of Jason and Envisat.