Analysis of Sampling Behavior of Candidate SWOT Satellite Orbits
Roohi, Shirzad; Sneeuw, Nico
Institute of Geodesy, University of Stuttgart, GERMANY

A careful repeat orbit design plays an important role in sampling the water bodies from space by satellites. For hydrological application new satellite mission must be able to monitor water bodies such as small lakes and narrow rivers at least at sub-monthly time scale that can not be monitor by current satellite altimetry. Current satellite altimetry can not provide sufficient spatial and temporal coverage due to the orbital geometry and system configuration. Because they are nadir pointing altimetry with limited footprint size. The Surface Water and Ocean Topography (SWOT) mission focused on these issues in monitoring small water bodies. Equipped with swath type SAR measurements, the SWOT satellite provides globally high resolution measurement even in higher latitude area. For example an arbitrary point located at 69° latitude can be monitored 13 and 18 times for orbit inclinations 74° and 78° respectively during a 23 nodal days repeat period. Collecting data from water bodies with such a temporal resolution is very important to study their dynamic behavior. On the other hand radar system mounted on SWOT platform with 4.5° looking angle and 200 MHz bandwidth provides a spatial resolution from 10 m to 60 m for cross track in far and near swath and 2 m for along track direction. In this research we analyze the orbital and radar geometry to find an optimized repeat orbit for SWOT mission. For all possible orbits with orbit height between 750 km and 1000 km and repeat period less than 25 nodal days we compared ground track density, homogeneity and gap evolution as well as spatial-temporal resolution. We identified a number of orbits with repeat period from 20 to 25 day and a few with repeat period between 3 and 5 day that would be good options for the nominal and fast phase of this mission respectively. Also there is not a noticeable difference between candidate orbits with 74° and 78° inclination in terms of sampling other than latitude coverage. Keywords : Repeat orbit, ground track, gap evolution, spatial-temporal resolute