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[Upbeat Music]

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Narrator: Where are you right now?

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Or better yet, how do you know where you are?

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Most likely you are using an app that is using a GPS

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to pinpoint where exactly you are on Earth.

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But what you might not know
is that behind your favorite navigation

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app is an invisible

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infrastructure of systems working together
to provide you an accurate location,

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and that the foundation of these systems
is a TRF

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or terrestrial reference frame.

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Dr. Stephen Merkowitz: So reference frame is fundamental

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to any kind of mapping, location,
positioning, navigation application.

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Narrator: That's Dr. Stephen Merkowitz,
 NASA's Space Geodesy Project Manager

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Simply put, geodesy is the science

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of measuring and understanding the shape of the Earth,

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its orientation in space and gravity,
and how they change over time.

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Dr. Stephen Merkowitz: Earth is constantly changing shape,
and the land masses are constantly moving.

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It's at small levels, so it's not something
you would feel, you know, yourself.

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So we're constantly measuring

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what's happening with the Earth as a whole
and the Earth's surface.

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Narrator: To do this, NASA and international partners
monitor measurement

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stations dotted across the globe
that help us determine where,

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how much and in which direction
our planet is changing.

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Dr. Stephen Merkowitz: Since the Earth's surface
is always moving, any measurement

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isn't just in a fixed coordinate system.

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It needs to take into account

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the fact that it's related

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to the landmass and where that landmass
is at any given time.

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Narrator: And just like a coordinate system
plotted on a piece of paper,

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the TRF has an origin or in this case,
the center mass of the

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Earth - called the geocenter.

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Because of this, many Earth observing satellites

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orbit around the center mass of the Earth,
not the Earth's geographic center.

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So it's critical for scientists to pinpoint where that is

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to make sure our Earth observations are accurate.

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Dr. Stephen Merkowitz: One complication is that the center
mass of the Earth

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is constantly changing over time

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with respect to the Earth's surface.

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Narrator: Earthquakes, volcanoes, or even
atmospheric pressure changes all impact

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where exactly the center
mass of the Earth is.

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But by using a network of ground stations
equipped with telescopes and lasers

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that fire pulses at specific satellites,
scientists can calculate

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where the geocenter of the Earth
is at any given time to a few millimeters.

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Dr. Stephen Merkowitz: This is a very important product
that we generate for orbiting satellites,

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particularly for missions

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that are either doing mapping
the gravitational field of the Earth or

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mapping the height of the Earth's surface,
whether it be land, vegetation,

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or sea level.

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We make that available to the public,

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we use the data there
to generate a higher level

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of products, geodetic products,
that can be used by NASA missions

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for many scientific applications.

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Since this is a global measurement, NASA can't do it alone.

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We rely on the global community to

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provide all the data that's needed to provide global coverage.

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Narrator: Having an agreed

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upon international system is
what makes measuring our planet possible,

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whether it be from land,
sea, air, or even space.