Satellite Remote Sensing- Measuring Sea Surface Height from Space

How do we measure the Sea Surface Height?

The surface height is measured by a radar altimeter which is attached to a satellite. A radar altimeter measures the distance between a terrain and its own height, the so called range. It emits microwaves and measures the time it takes for the waves to be reflected from the ground to return to the altimeter. For the measurement it is crucial to determine the exact position of the satellite. For this purpose the radar altimeter has three independent tracking systems. The altitude of the satellite is given relative to the reference ellipsoid. The resulting Sea Surface Height (SSH) is the difference between the altitude of the satellite and the range.

How do we correct the data?

There are several factors to be considered that can impair the data.  Firstly it is important to consider the refracting of microwaves by the troposphere. The refraction property is related to pressure, temperature and water vapour partial pressure. It is given by two parts the dry component and the wet component. Furthermore it is important to take the ionosphere into account. It refracts the microwaves because of free electrons. Another point falsifying our data is the sea state bias. The backscatter from wave troughs is greater than from wave crests. At last we have to correct the satellite orbit. For optimal SSH data we have to determine the precise orbit of the satellite.

How is the data processed?

Different Institutes like the NOAA and the CNES process the data they collect from the satellites. There are different levels of data. Level 0 is the raw data and level 2 the Geophysical Data Record (GDR). There are several types of data processing. The real-time processing is available after 3-5 hours. The data it produces is called operational GDR.  It does not contain all environmental/geophysical correction and the orbit is calculated by the DORIS navigator on board. The delayed-mode processing makes its interim GDR available after two days. It uses preliminary corrections. The final GDR is available after 60 days. It uses the precise orbit and the best environmental/geophysical corrections. There are many different data products available like the Sea Surface anomaly (SLA). It subtracts the mean sea surface height from the SSH. It also takes tides into account.

What can we do with the data?

All the data products inferred from the SSH helps us analysing the sea surface topography. The topography helps us in understanding the mean ocean circulation and its variability, the upper ocean heat content, Tides and Tsunamis and many other things. For instance using SLA data we can detect eddies and make estimations over their heat transport.