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.
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.
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.
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.