From the Shore to the Stars
For centuries, measuring tides was a local affair. A tide gauge, essentially a specialised ruler, would be installed at a pier or in a harbour to measure the water level at that one specific spot. While incredibly useful for local navigation and coastal
planning, this method left vast stretches of the world's oceans unmeasured. To get a global picture, scientists needed a higher vantage point. Since the 1990s, with missions like TOPEX/Poseidon and the Jason series, we have been measuring the ocean's surface from space, creating a continuous record of sea level that has revolutionised our understanding of the planet.
How Satellites 'See' the Ocean's Height
So how does a satellite hundreds of kilometres up measure the sea level down to a few centimetres? The primary tool is a radar altimeter. Think of it like a bat using echolocation. The satellite sends a microwave pulse down to the ocean surface and precisely times how long it takes for the signal to bounce back. Since the speed of the pulse is known, this travel time gives a very precise distance between the satellite and the water. By combining this with the satellite's exact position in orbit, scientists can calculate the height of the sea surface. This technique has been refined over decades to become astonishingly accurate, allowing us to track global sea-level rise and large ocean features.
The Game Changer: SWOT Mission
While traditional altimeters are excellent, they measure a single point directly beneath the satellite, creating a 1D line of data. The latest generation of technology, however, is a quantum leap forward. The Surface Water and Ocean Topography (SWOT) satellite, a joint mission by NASA and the French space agency CNES, uses a technique called radar interferometry. Instead of one antenna, SWOT has two, mounted at either end of a 10-metre boom. This setup allows it to scan a wide 120-kilometre swath of the ocean, creating a detailed 2D map of the water's surface, rather than just a line. This high-definition view is revealing ocean dynamics at a scale never seen before, capturing smaller eddies and currents that were previously invisible.
More Than Just Tides
This incredibly detailed mapping of the ocean surface has implications far beyond just predicting high and low tide. The data helps scientists create better models for storm surges and tsunami warnings, improving safety for coastal communities. It enhances our understanding of how the ocean absorbs heat and carbon, which is vital for climate change modelling. For India, with its vast coastline and dependence on the monsoon, better ocean models can lead to more accurate weather forecasts. Furthermore, the data has commercial applications, from helping create safer shipping routes to identifying potential sites for offshore wind farms and even helping to track fish populations.
A New Way of Seeing
Recently, scientists have even developed novel methods that use decades of standard satellite images from programs like Landsat to map tides with surprising local detail. By analysing how the coastline's water line moves in thousands of images taken over 40 years, and combining that with the slope of the beach, they can deduce tidal variations. This innovative use of historical data can fill in gaps in coastal areas where tide gauges are sparse. This approach shows that the 'noise' in satellite data—the shifting shoreline—can be turned into a valuable signal, providing a much more granular picture of how tides behave in complex bays and estuaries.
