SpaceX is scheduled to launch the Jason-3 satellite on Sunday, January 17th at 1:42 PM EST from Vandenberg Air Force Base in California. Jason-3, a mission from NOAA and NASA, will measure sea level heights around the world.

It’s equipped with a radar altimeter which can track sea surface heights by pulsing microwaves down at the surface of the ocean and measuring the time it takes for that pulse to return to the spacecraft.

Using that time, it can calculate the distance between the sea and the spacecraft. Because the scientists know exactly where the spacecraft is relative to the Earth’s core, it can calculate the distance from the Earth’s core to the height of the sea. This is just one important input, however, that’s required to get global sea level.

As it turns out, global sea level is not, in fact, the level of the sea. Or at least, not for most people.

We use “sea level” as a standard reference point for things like aviation, marine navigation, and even determining the height of mountains. For example, the tallest mountain in world, above sea level, is Mount Everest. But the tallest mountain from base to summit is actually the Hawaiian volcano, Mauna Kea, which is partly underwater. And to make matters more interesting, the tallest point on Earth (measured from the center of the planet) is Mount Chimborazo, a volcano in Ecuador.

Defining sea level makes it possible for us to measure the height of everything on land which is important for global maps and those who use them. Measuring sea level over time is important for our understanding of global warming, sea level rise, and how it’s going to affect coastal cities around the world.

So how is this important reference point “sea level” calculated?

Global sea level is the average height from all of the Earth’s oceans from the sea floor. But calculating the average is not as simple as you may think. In order to determine the height of something, you need a reference point to measure from, and the Earth is not a perfect sphere. Also, Earth’s water is always in motion which means their heights are constantly changing.

If the Earth were perfectly spherical, homogeneous in density, and weren’t affected by weather or tides, the level of water above the Earth’s surface would be pretty uniform and the average would be easy to calculate.

But the Earth’s shape isn’t uniform and it’s constantly affected by temperature, pressure, wind and even tidal forces from the moon and the sun.

Imagine you have a ruler and you’re tasked with measuring the height of the ocean at a beach. That height would change by the second due to waves, by the hour because of tides, and even by the week depending on where the Earth was in its orbit around the sun.

Geodesists, mathematicians who measure and monitor the Earth’s size and shape and the exact locations of points on its surface, have tackled the challenge of determining the Earth’s global sea level. In order to do this, they had to first come up with a model for the exact size and shape of the Earth.

Combining this model with measurements from satellites like Jason-3, scientists can calculate both the shape and the height of the global sea level surface. The global sea level is then used as the point of zero elevation and becomes our reference point for determining heights of mountains, the depths of trenches, and even altitudes of objects in space.

Note: there are two types of useful altitudes. Absolute altitude is the height over the local terrain. True altitude is the elevation about global sea level.

Because the Earth isn’t a simple surface like a sphere, the line of global sea level isn’t simple either. It has slight hills and valleys similar to the surface of land.

Jason-3 is part of NOAA and NASA’s effort to continuously monitor sea levels over time. Jason-3 is actually nearly identical to the two Jason satellites, Jason-1 and Jason-2, that came before it, all of which are named after the Greek mythological hero who was an explorer of the seas.

In a recent Reddit AMA, Jason-3 scientists stated that the similarity between the satellites was a good thing because “we’re trying to build a climate record here so the science benefits from the satellites being as similar as possible.”

The main reason the previous satellites needed to retire is because of radiation exposure. They orbit at an altitude of 1,136 km, which is high enough to receive almost no protection from the Earth’s atmosphere. The instruments get bombarded with harmful radiation and need to be replaced after a few years. The Jason-3 scientists noted that the lifetime of the Jason-2 satellite was set to five years for this reason.

Once Jason-3 is launched, NOAA and NASA will move Jason-2 to a new orbit, allowing Jason-3 to take its place.

NASA measurements of sea surface height during El Nino of 1997 and El Nino of 2015 / Image Courtesy of NASA/JPL

Having an accurate, up-to-date measurement of the global sea level is important for maintaining a standard of measurement on the Earth. And because of climate change, the global sea level is changing. Jason-3 will give scientists a constant picture of how the sea level is changing around the world. This in turn can help other scientists forecast things like hurricane intensity, surface waves, tides and currents, and even El Niño and La Niña events.

Jason-3 will be launched by SpaceX from Vandenberg Air Force Base on Sunday at 1:42 PM EST. Live coverage will begin at 11:00 AM EST on NASA TV.

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