Satellites provide effective and efficient observations of the Earth, especially in hard-to-reach locations, like rivers and lakes in remote areas — but could they offer more comprehensive information? Yes, according to an international team of researchers, who found that satellite altimetry missions making less-frequent observations across broader sample sizes may actually provide more useful information than missions that make more precise and frequent measurements.
They published their results on Aug. 27 in the Journal of Remote Sensing.
The work investigates different types of altimetry data, which involves a satellite sending a signal to a point of interest on Earth and measuring how long it takes for the signal to return. This can reveal water level, surface composition and more — and any detected changes in these measurements over repeated observations may provide insights on the impact of climate change or other external factors.
Altimetric missions are categorized as geodetic or exact repeat missions. Geodetic missions (GMs) typically take a year or longer to take measurements at the same location and usually have a denser ground tracks of a few kilometers that they sample, while exact repeat missions (ERMs) take more frequent measurements every ten days to every month or so and in sparse ground tracks of tens of to a hundred of kilometers.
"This paper specifically highlights the advantages of GM data over ERM data in various applications," said corresponding author Liguang Jiang, assistant professor in the Shenzhen Key Laboratory of Precision Management and Early Warning Technology for Urban Environmental Health Risks, Southern University of Science and Technology's School of Environmental Science and Engineering in China. "In the realm of lake studies, GM data offer the potential to reveal the bigger picture of lake variations across entire regions. This broader perspective becomes invaluable in furthering our understanding of hydrological cycles at continental and even global scales."
The researchers examined data from four different satellites — Jason-1, Jason-2, CryoSat-2 and AltiKa DF — that operated in overlapping periods from October 2010 through November 2023. Each satellite had periods of providing GM or ERM data. When compared, the researchers found satellites in GM cycles sampled several times of the number of lakes as in ERM cycles, providing a wider representation of regional hydrology. They also found that the GM data revealed a clearer understanding of regional and global hydrological cycles and how local influences impacted individual water level observations. Importantly, Jiang said, the team also found that the wider measurements highlighted the broad features of single bodies of water, such as river levels and elevation profiles.
According to Jiang, these cases are neither exclusive nor exhaustive, but rather open the door to enhanced hydrological studies.
"The findings presented in this paper serve as a springboard, demonstrating the exciting possibilities that GM data holds for advancing research on inland water bodies," Jiang said. "By leveraging this rich data source, future studies may unlock the full potential of GM data and make contributions to a deeper understanding of our planet's freshwater systems."
Next, Jiang said, the researchers plan to develop computational tools to more efficiently process GM data for use in hydrology research.
Jiang is also affiliated with the Changjiang River of Scientific Research Institute (CRSRI), Changjiang Water Resources. Karina Nielsen and Ole. B. Andersen, both with the National Space Institute at the Technical University of Denmark, co-authored the paper.
The CRSRI Open Research Program, the Shenzhen Key Laboratory of Precision Management and Early Warning Technology for Urban Environmental Health Risks High-Level University Special Fund and the Southern University of Science and Technology's Research Start-Up Grants supported this research.
