## 2 Study Area and Input Data
### Satellite Altimetry Data  
Satellite altimetry determines the distance between the satellite and the Earth's surface in the nadir direction by measuring the travel time of a microwave signal emitted by an instrument on board of the satellite and reflected by the water surface on the ground. When the satellite's height is known, the surface elevation can be easily derived by subtracting both quantities. More details on the measurement technology can be found in [PERSON] et al. [20], for example. Due to this measurement geometry, satellite altimetry provides measurements along dedicated profiles--i.e., tracks--with a high along-track resolution (depending on sensor and datasets between about 300 m and 7 km). The cross-track resolution strongly depends on the satellite's orbit configuration. Most altimetry missions use repeat orbits and cover the same location on the Earth every 10 to 35 days. The higher the temporal resolution, the lower the spatial cross-track resolution. By combining different missions in a multi-mission approach, the temporal as well as the spatial resolution of the system can be significantly improved. In this study, six different orbits are exploited, which are used by more than 10 different satellites. These are described in more detail in the following. Table 1 summarizes the orbit parameters for all of these altimetry missions.  
Figure 1: The Mississippi River Basin: Lakes and reservoirs that are implemented in WaterGAP are shown in blue, major rivers as grey lines. Orange: River sections that were removed manually (see Section 3.3).  
The lifetime of satellite missions is limited to a few years. In order to ensure the continuous and consistent monitoring of the Earth's surface, successor missions follow the original missions on the same orbit. Important examples of this are the NASA/CNES missions TOPEX/Poseidon, Jason-1, Jason-2 and Jason-3, which will soon be continued by Sentinel-6. All of these missions use the same nominal orbit with a repeat cycle of about 10 days and a track separation at the equator of about 315 km.  
The second long-term orbit is occupied by ERS-1, ERS-2 and Envisat. These ESA missions are followed by the ISRO/CNES mission Saral that uses the same orbit but a different altimeter instrument operating in the Ka-band; in contrast, all other missions emit Ku-band signals. This orbit is defined as a 35-day repeat orbit with an 80 km track separation. Saral was launched in 2013, about three years after Envisat left its nominal orbit. During this data gap between Envisat and Saral, no measurements for this orbit were acquired.  
The Copernicus mission Sentinel-3 consists of two satellites: Sentinel-3A and Sentinel-3B. Both have the same orbit parameters but fly interleaved with each other with a 27-day repeat cycle, meaning that they complement each other in spatial resolution. The Sentinel-3 constellation has a ground track separation at the equator of 52 km.  
In addition to these short-repeat missions, two missions are used within this study that fly on long or non-repeat orbits. Cryosat-2 has a 369-day repeat. This results in a dense ground track pattern but with a sparse temporal resolution at dedicated locations. However, due to its sub-cycle of about 30 days, larger lakes can be monitored with a monthly resolution.  
Saral originally used the Envisat orbit. However, due to problems with the satellite, since 2016, the orbit is no longer maintained on its nominal track but is in a drifting configuration without a regular pattern over the long-term in the ground track. This results in irregular spatial-temporal sampling on the ground but can help with monitoring lakes on a global scale, especially when this mission is combined with other altimetry missions. This part of the missions is named Saral-DP (drifting phase).