How SWOT Will Work The "Surface Water and Ocean Topography" Mission will be NASA's First Comprehensive View of our Ever-Changing World of Water

Let's begin to explain how NASA's Surface Water and Ocean Topography (SWOT) satellite observatory will work by looking at a water surface being pelted by rain.

The energy from each raindrop disperses as concentric rings, away from its point source. When the rings from different drops overlap, their energy can add together to make a larger wave.

What happens?

High points align with high points

Low points align with low points

The resulting wave has higher peaks and lower troughs... "Constructive" interference!

Raindrops' energy can also interfere in non-constructive ways.

What happens?

High points align with low points

Low points align with high points

The waves cancel out to make a straight line... "Destructive" interference!

Studying how waves interfere – known as "interferometry" – will be used by SWOT to measure water levels with extraordinary accuracy across the globe.

Using Interferometry to Measure Distance

The extent to which one wave is in step with another is known as its phase. Waves 1 & 2 were identical or "in phase." Conversely, Waves 1 & 3 were completely out of phase.

Between these extremes, one wave can be partly in phase with the other. Adding two such waves can create a third wave that has an unusual, rising and falling pattern of peaks and troughs.

Uneven thickness in the exterior film of soap bubbles can produce beautiful colors. These patterns result from interference between waves reflecting off the tops and bottoms of the film. Blue colors show thicker soap film while magenta shows thinner soap film.

A less colorful – but more scientific – way to see wave interference is with an interferometer, an instrument designed to make measurements with great accuracy.

A simple interferometer takes a beam of electromagnetic radiation (like a laser) and splits it into two paths. One of the beams shines onto a mirror and then to a screen. The other beam shines onto a moveable mirror, back through the beam splitter, and onto the same screen. This second beam travels an extra distance, so it gets slightly out of phase.

It introduces "The Experimental Setup," followed by "Using an Interferometer to Measure Wavelength" (0:32 - 2:04) and its math (2:05 - 2:36). The last section is "The Theory Behind the Measurement" (2:37 - 4:06).

When the two beams meet, the phase difference between them creates a pattern of light areas (constructive interference) and dark areas (destructive interference). The pattern is called an "interferogram," which can be used to precisely measure distance.

While my burrito heats up, I'm wondering how SWOT will work...

From Lasers in the Lab to Radars in Space

Lasers are awesome and SWOT will make use of them, but not to produce interferograms. Instead, SWOT will use a microwave radar system. Why? Microwaves can penetrate clouds, so they reach (and bounce off) Earth's surface in any weather conditions, day or night.

Microwaves span many wavelengths. The shortest are 1 millimeter, less than the thickness of a dime. The longest microwaves are 1 meter, about the height of tall traffic cones. Microwave ovens generally use 12-centimeter (4.7-inch) wavelengths to heat your burrito.

Speaking of traffic cones... SWOT will transmit and receive energy in the microwave band known as "Ka" (pronounced "kay-ay"), which is also used by many police radars.

In the Michelson experiment, they used a known distance – how far they moved a mirror – to discover the wavelength of a laser light. SWOT will do the opposite: use its known wavelength (Ka band) to determine the distance from the satellite to Earth's surface.

Don't Worry... It's Just a Phase

Many spaceborne radars can transmit microwaves and receive the signals after they've reflected off a planet's surface. One type, "Synthetic Aperture Radar" (SAR), is used to create two- or three-dimensional images of landscapes.

Position 1 along the satellite's orbital path.

Position 2 along the satellite's orbital path.

Position 3 along the satellite's orbital path.

Radar data from different orbital positions can be merged into a continuous strip. The merged data are shown here as a gray speckled "swath."

One of the most powerful applications of SAR is determining surface elevation. This can be accomplished by collecting data with two antennas simultaneously.

The reflections from one point that reach two antennas are slightly out of phase.

Collecting SAR data at the same time using two antennas can produce "reference" and "secondary" swaths. Merging two swaths is a key step in precisely determining surface elevation.

This principle is being applied by an aircraft-mounted instrument called "AirSWOT," being used to test – and eventually verify – SWOT measurements. AirSWOT uses the same frequency as SWOT: Ka band.

Reference swath from AirSWOT, taken over Lake Tahoe (California/Nevada border).
Secondary swath from AirSWOT, taken over Lake Tahoe.

These SAR images can be merged into interferograms, where phase differences appear as color fringes. Like the Michelson experiment, each cycle of color (purple through green) corresponds to a certain elevation change based on wavelength.

Interferogram from AirSWOT's Lake Tahoe data.
Interference fringes overlain on SAR data, giving a "3D topography" effect.

Science on the Fringe

SWOT will apply this technique by taking two radar swaths simultaneously. These data will be used to create global maps of Earth's water levels, providing essential information on large rivers, lakes, and reservoirs – along with Earth's global ocean – at least twice every 21 days.

In this video, SWOT's interferometric radar is depicted by the green and magenta pulses emanating from a T-shaped antenna reflector. The Ka- band Radar Interferometer ("KaRIn") that generates these pulses will be located in the middle of the observatory.

SWOT's KaRIn instrument will transmit radar signals from one antenna. After reflecting off Earth's surface, each signal will be received by both antennas.

Reflected signals will be received by Antenna 1...

... and also received by Antenna 2.

Return signals to the two antennas...

... will have a phase difference...

... because of the difference in their path lengths.

Knowing the phase difference, distance between antennas, and radar wavelength...

... will allow us to calculate the range difference.

Overall, these measurements will provide the ground location and height for each pixel of the radar swath. However, that's only part of the story...

Let's Get to the Bottom of It

Having a very accurate interferometer is crucial to ensure that SWOT will meet its science goals. However, we also need to precisely determine the observatory's position in orbit and how fast signals will travel between the satellite and Earth's surface using these (and other) instruments:

Altimeter will send and receive signals that travel straight up and down (red dots in previous video). Each pulse's round-trip travel time will be used to determine orbital height.

Microwave Radiometer will measure the amount of water vapor between SWOT and Earth's surface. More water vapor means slower signals.

Laser Retroreflector Array (LRA) is an array of mirrors that will provide a target for laser tracking measurements from the ground.

Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) will pick up signals from 50-60 ground-based radio beacons, equally distributed over Earth to ensure good coverage.

Not shown in this view is the Global Positioning System (GPS) Package, which will communicate with GPS satellites to help determine SWOT's orbital position.

Thanks to this integrated technology system, SWOT will be able to measure water levels over the ocean, large lakes and rivers within about 1 centimeter (cm)!

How Will SWOT Work for You?

SWOT is being jointly developed by NASA and Centre National D'Etudes Spatiales (CNES) with contributions from the Canadian Space Agency (CSA) and United Kingdom Space Agency.


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