Winner of the Merit Prize, Soon Kit Ying
Reflecting on Our Sea Level – The Use of GNSS-Interferometric Reflectometry to Study Sea-Level Trends
Written by Soon Kit Ying
“It seemed like a magical city, floating on the lagoon as if conjured by an enchanter’s wand.” – Kate Forysth
Being a UNESCO World Heritage Site city, Venice is a popular holiday destination famous for its beautiful churches, intricate architecture and captivating canals. It is made up of more than a hundred canals connecting apartments and museums, people and culture. Yet, being a coastal city, it is extremely vulnerable to rising sea levels. Its tide levels peaked in November 2019, leaving 70% of the city inundated and marking one of the worst floods in the past 50 years.
Also being a coastal city, Singapore is as vulnerable to the consequences of sea-level rise. The threat of rising sea levels has been presented multiple times, most recently being highlighted in Prime Minister Lee Hsien Loong’s speech during National Day Rally 2019, where he said,
“Everything else must bend at the knee to safeguard the existence of our island nation. There is one difference between the two. With the Singapore Armed Forces (SAF), we hope never to go to war. If you have a strong SAF, you may deter threats and avoid having to go to war. But with climate change, we know for sure sea levels will rise.”
Before we dive into the impacts of sea-level rise, how do we even measure sea level in the first place? Could it be as simple as using a ruler? The answer is surprisingly yes, and no.
In the 18th century, tide staffs were used to measure coastal water levels. They are, in fact, tall wooden rulers placed in a metal tube into the water. The water level relative to a fixed point on land was read manually by an observer. As time progressed, new ways of measuring the sea level were invented. A float-activated tide gauge was one of them. Such gauges were used to record the heights of high and low tides. How it works is that the float in the gauge chamber rises and falls with the tide. With the help of a pulley mechanism that is connected to a mechanical recording device, the tidal signal is traced on a paper graph. Driven by a mechanical clock, this paper graph rotates and produces a paper chart recording the tide times and heights.
Figure 1: Tidal leveling: Connecting tidal staff (held by man in picture on the right) with benchmarks. Image from NOAA Photo Library [2].
As technology improves with time, modern inventions such as a radar reflection tide gauge serve as an alternative in monitoring sea level. Each type of tide gauge has its advantages and flaws. The type of instrument used is based on considerations such as the environmental conditions it is placed in, the installation costs and the maintenance costs.
Monitoring sea level using a tide gauge is complicated and definitely not just placing a ruler into the water as the sea level is constantly fluctuating due to factors such as tides. One main disadvantage of using a tide gauge is that it measures relative sea level, which is sea level measured with reference to a fixed point, usually on land. Therefore, it factors in both changes in land height and in sea level. Further processing of data is needed to separate these two contributions to obtain only the sea-level variations. Obtaining land height data can also be difficult at times due to the lack of data or the inaccuracy of it. Moreover, tide gauges are susceptible to damage by heavy winds and extreme weather patterns.
Apart from tide gauges, satellite altimetry can also be used to measure sea level. It measures the distance between the satellite and sea surface height below through radar pulses that are reflected off the sea surface. Unlike tide gauges, satellite altimetry does not measure land height changes. However, its main disadvantage is that it does not perform well in areas near the coasts or land, hence being unsuitable for sea-level monitoring in coastal cities.
Recently, a new method known as Global Navigation Satellite System- Interferometric Reflectometry (GNSS-IR) has been proposed as an alternative method to monitor sea level. GNSS is often a foreign term, so let’s start with GPS. Global Positioning System (GPS) is often used for location purposes such as in applications like Google Maps or other navigation devices. GNSS is a global system of satellites in operation globally, which includes GPS (US) and other systems such as GLONASS (Russia), BeiDou (China) and Galileo (EU).
Figure 2: Schematic diagram illustrating how GNSS-IR works. The signal from the satellite arrives to the GNSS receiver on land both directly and indirectly after reflecting off sea surface. (Modified from Peng et al., 2019 [3])
The main principle of GNSS-IR is that it utilises satellite signals which are reflected off the sea surface. Due to the reflection, these signals are delayed as compared to the direct signals and hence cause an interference that is shown in the form of a signal-to-noise ratio (SNR) oscillation. SNR measurements can be obtained from GNSS receivers on ground, which receive signals from satellites in space. From the frequency of SNR data, we can then estimate the vertical distance (illustrated as H in Figure 2) from the sea surface to the GNSS receiver’s antenna.
The advantage of GNSS-IR is that it can simultaneously measure vertical land motion and sea surface height. The land height data can then be used for processing to separate land and sea contributions to the monitored sea level. Additionally, GNSS-IR can be used in coastal regions which is beneficial for coastal cities such as Singapore. With this potential sea-level monitoring technique, we can look forward to valuable insights on sea-level trends in Singapore and hence better formulate our mitigation strategies.
At the Earth Observatory of Singapore (EOS), a dedicated group of researchers is investigating the applications of GNSS-IR in monitoring sea level. Together with Dr. Emma Hill and Dr. Dongju Peng, I am currently assessing its feasibility in Singapore and hoping to get some insightful results!
References
- Hannah, J. (2011). The difficulties in using tide gauges to monitor long-term sea level change. Survey Quarterly, 300(65), 19.
- National Oceanic and Atmospheric Administration (NOAA) Photo Library https://photolib.noaa.gov/Collections/Coast-Geodetic-Survey/Nautical-Charting/Tides-and-Tidal-Currents/Tides/emodule/990/eitem/46830
- Peng, D., Hill, E. M., Li, L., Switzer, A. D., & Larson, K. M. (2019). Application of GNSS interferometric reflectometry for detecting storm surges. GPS Solutions, 23(2), 47.