How Can a Satellite See Tides?
It might seem strange, but satellites don't 'see' tides in the way we do. Instead, they use a clever technique called radar altimetry. A satellite beams a microwave pulse down to the ocean surface and measures the exact time it takes for the signal to bounce
back. Knowing the satellite's precise position in orbit, scientists can calculate the sea surface height with remarkable accuracy, often within a couple of centimetres. By repeating these measurements over the same spot, they build a detailed picture of how the sea level rises and falls over time. A newer, innovative method even analyses years of satellite photographs of the coastline. By tracking where the water line sits on a beach in thousands of images and combining it with the beach's slope, scientists can deduce tidal movements on a very local scale.
Is This Better Than Traditional Methods?
For centuries, our understanding of tides came from tide gauges—physical measuring stations located along coastlines, often in ports and harbours. While incredibly valuable, these gauges have limitations. They only provide data for a single point, and they are often spaced many kilometres apart, leaving vast stretches of coastline unmeasured. Satellite altimetry, on the other hand, provides near-global coverage, measuring the open ocean where gauges can't go. Missions like the Jason series and the newer Surface Water and Ocean Topography (SWOT) satellite can observe fine details over wide areas, capturing complex tidal patterns that gauges would miss. This allows for a much more complete and detailed map of how tides behave across entire ocean basins and intricate coastlines.
Why Does More Accurate Mapping Matter?
Precise tide mapping has profound real-world implications, especially for a country with a long and densely populated coastline like India. Better tidal data leads to more accurate forecasts, which are critical for safe navigation in and out of busy ports. It also dramatically improves coastal planning and flood risk assessment. By understanding local tidal variations, authorities can better protect coastal communities from storm surges and the compounding effects of sea-level rise. Furthermore, this data is vital for India's scientific community. ISRO utilises satellite data from missions like SARAL and Oceansat for ocean state forecasting, coastal zone studies, and climate research. Accurate tidal models help scientists distinguish between tidal cycles and long-term sea-level changes, leading to a clearer picture of climate change's impact.
What Are the Current Challenges?
Despite its power, satellite tide mapping isn't without its challenges. The biggest issue is the coastal zone—the very area where tidal information is often most needed. Within about 10 to 20 kilometres of the shore, the radar signal can get contaminated by reflections from land, which distorts the measurements. This can lead to a significant loss of valid data close to the coast. Scientists are constantly developing more sophisticated processing techniques, like the ALES retracker, to filter out this 'noise' and retrieve accurate data nearer to shore. Another challenge is separating the tidal signal from other ocean phenomena, such as eddies and currents, which can also cause bumps and dips in the sea surface. This is particularly tricky for observing 'internal tides'—giant underwater waves that propagate within the ocean layers.
