Ocean Salinity from space It may be more complicated than you think.

The concentration of dissolved salt in our ocean IS known as "salinity".


In the data movie above, red colors show areas of high salinity while low-salinity regions are shown in blue.
NASA has measured sea surface salinity since 2011, thanks to the Aquarius/SAC-D and Soil Moisture Active Passive (SMAP) missions.

Why should YOU care about ocean salinity?

Salinity patterns tell us about changes in Earth's water cycle.

The weight of salt in seawater also helps to drive deep ocean currents. The video below shows the overall path of this type of global circulation, which impacts Earth's climate.

Salt dissolved in water. Sounds simple enough.

How do satellites measure salinity?

To tell this story, it's easiest to start with a flat ocean surface.
The salinity signal is shown as a yellow arrow.

But the path between the ocean surface and the satellite has many factors that disturb the salinity signal or create errors in the measurement.

For example, radiation from the Sun...
... some of which is reflected off the ocean...
... while some radiation directly reaches the satellite sensor.

This type of noise interferes with the salinity measurement.

To a lesser extent, the Moon creates a similar disturbance...
... when "moonbeams" reflect off the ocean surface.

DID YOU THINK WE WERE DONE WITH extraterrestrial bodies? Heavens, No!

To get salinity, you need to correct for the galaxy itself!
The galactic signal and cosmic microwave spectrum create very large signals that must be taken into account.
The galaxy introduces both direct and reflected radiation as error sources.

Now onto more "down to earth" signals.

The salinity signal coming off the ocean must go through Earth's atmosphere to reach the satellite.
Oxygen in the atmosphere attenuates – emits and absorbs – some of the salinity signal.
Radiation from the atmosphere itself also travels up to the sensor.

Salinity signals also pass through THE "ionosphere" to reach satellite sensors. This part of Earth's upper atmosphere is where solar radiation has CREATED ions by removing one or more electrons FROM atoms or molecules.

The ionosphere can mix the "polarization" of signals that pass through it.
In other words, waves that vibrate in one plane can switch to the opposite plane (e.g., go from horizontal to vertical polarization).

The video above represents a linearly polarized wave. Yellow arrows show horizontal polarization and green show vertical polarization.

Well, that was complicated! Are we done?

No. Let's get real: The ocean isn't flat...
... it's roughened by waves.

In fact, ocean roughness is the largest source of error in satellite salinity measurements.

Furthermore, salinity sensors are tuned to measure Earth's oceans, so when they approach land...
... the land signal itself contaminates the salinity signal.

Human "noise" is also a factor in some regions.

The frequency used by NASA Salinity sensors is – by international agreement – supposed to be "listening only."
However, unintended noise from radars and other transmitting devices can penetrate into this quiet band.
This is called "Radio Frequency Interference" or "RFI".

So, To get a "simple" measurement like salinity...

You need to untanglE a lot of COMPLICATED signals:

It's a difficult process to get the accuracy needed for scientific studies...

... equivalent to salinity values of about 2 parts per Ten thousand.

That's a level of saltiness you couldn't taste... but nasa has been able to detect from space!

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