The background science
The Baltic SEAL team built on over 25 years of significant advances in measuring our oceans from space using altimetry. Altimeters allow us to measure the height of the sea surface. To someone looking at the ocean from shore, it will look flat, except of course for wind-driven surface roughness (i.e. the short wavelets with horizontal scales of a few cm across), and the wave field (i.e. the troughs and crests with horizontal scales of meters to tens of meters). However, if the same observer could look at the ocean over distances of tens or hundreds of km from the vantage point of satellites, they would see variations (‘bumps’ and ‘troughs’) in height of several meters – these being the variations which a satellite altimeter measures. For a brief overview of satellite altimetry, check out a flash intro by Dr. Marcello Passaro, the TUM scientist that led this initiative.
To get sea level we must analyse the sea surface height measurement obtained by the altimeter, and account for all those factors which shape the nature of these “bumps” and “troughs”. These factors include the influence of the Geoid, and the non-spherical shape of the planet. Once we can account for these, we get a value for sea level. Many factors also influence this Sea Level measurement, and we can use existing knowledge and advanced modelling to figure out the contribution (and measurement) of currents, tides, and storm surges. This all starts from extracting the sea level measurement from the altimeter-derived measurement of Sea Surface Height.
However, obtaining Sea Surface Height measurements from altimetry data gets exceptionally difficult near the coast. This is because islands, small rocks, and human infrastructure (and even boats), are captured by the altimeter pulse, and influence the returned signal. Sea-ice also presents a significant challenge, as a hard surface masking the underlying ocean. To account for these, complex masking, and signal analysis procedures developed by coastal altimetry scientists are used to maximise how much usable data we can get near the coast and from ice-prone areas. This is the rational underlying of why the Baltic SEAL project was successful in using the development of a Baltic Sea product to advance global knowledge and innovation in Sea Level products.
The Coastal Altimetry science community has grown significantly over the years. For more information on the community, and their activities check out the ESA-funded workshop and training programme for 2018 and 2020. For a useful introduction to ocean and coastal altimetry, check out the eSurge Coastal Altimetry handbook (downloads as a pdf). It is a really useful gateway to the basics of measuring our oceans using altimeters, and the complexities involved getting Sea-Level estimates from coastal areas. It is also tailored for both expert and novice altimetry data-users.