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[♩ INTRO ]
If you’re a regular driver or have ever looked at your phone for directions,
you are probably familiar with how useful GPS can be in everyday life.
But GPS can also be really useful for scientists,
and not just in ways you might expect
like by helping them track animals.
Scientists can also use GPS to help predict how bad a tsunami might be
to warn people about flash floods,
or to measure how dry a forest gets during a drought.
All by taking advantage of two very basic ideas.
For context, GPS, or the Global Positioning System, works
thanks to 31 satellites orbiting about 20,000 km above earth.
All of us being served by 31 satellites!
Those satellites transmit signals to the ground,
and then receivers in phones, cars, or other devices can pick them up
and use them to calculate how far away the satellite is.
Then, by triangulating with four or more satellites,
that receiver can figure out its exact position on earth.
Now, there is some nuance.
For instance, scientific receivers use a slightly tweaked version of this process
to pinpoint their location down to a couple of millimeters
—compared to the 1 to 10 meters phones can do.
But that’s basically it! And if you get creative,
this technology can help you measure a lot more than location.
Like, because GPS receivers are so good at calculating where they are,
they’re also good at measuring how the ground moves
—and by extension, how strong earthquakes are.
Traditionally, we use seismometers for this;
these are devices that measure how much a suspended weight bounces around during an earthquake.
But seismometers can get overwhelmed during really strong quakes, above magnitude eight or so.
And that’s especially a problem when it comes to predicting tsunamis.
A tsunami is a giant wave or series of waves
caused by water suddenly being displaced,
like by an undersea earthquake.
So, by measuring how strong a quake is,
scientists can predict when the resulting tsunami will reach land.
Except, if a seismometer gets overwhelmed,
scientists might need to wait for instruments farther from the quake
to give them usable or accurate data.
And the longer the delay, the less time there is to evacuate people.
So, researchers have been investigating using scientific GPS receivers
to predict tsunamis instead, adding that capability to their oldschool seismometers.
And it seems pretty promising!
In one paper, a team found that
the magnitude of the 2004 Sumatra earthquake in Indonesia
could have been calculated accurately in only 15 minutes with GPS.
Hours faster than the system they were using.
This general idea of using GPS to measure ground motion
also allows scientists to measure the amount of water on the ground.
When there’s a lot of water in an area, like because of a high tide or a heavy snowpack,
it can push the earth downwards.
Now it’s too small for us to notice,
but a GPS unit can pick it up, no problem.
And by measuring these tiny depressions,
scientists can estimate things like how much water is in a mountain range,
which could be important for studying climate change
or predicting how bad a drought might be.
That said, there’s another way scientists can study both tsunamis
and climate change with GPS.
And it is completely different.
Instead of watching the ground move, they study interference in GPS signals.
Like, take tsunamis.
As a tsunami travels, it raises the surface of the ocean.
In shallow water, that’s obvious — but at sea, where the ocean is deep,
the waves can be less than a meter high and hard to detect.
Except, the wave’s motion is also transmitted to the air above the water,
generating another set of long, strong waves in the atmosphere.
These waves continue traveling upward
and eventually travel through a region called the ionosphere
that starts about 80 kilometers up.
In this region, radiation from the Sun is so strong,
it strips the electrons off molecules,
creating a region of electrically charged air.
And when the tsunami-generated waves reach that area,
they jostle all those charged particles back and forth.
The key thing is, GPS satellites orbit through the ionosphere,
so they have to transmit their signals through that region.
And all the jostling caused by tsunamis creates interference in those signals.
So, by looking for that interference,
GPS units might be able to detect tsunamis while they are still out at sea,
just by looking at the air.
Again, that means faster, more accurate warnings,
and more time to get people to safety.
Similarly, this general technique also allows GPS
to predict events like flash floods.
In this case, changes in the amount of water vapor in the air
can muck with GPS signals in measurable ways,
since water droplets can absorb and scatter the signal’s energy.
In fact, this principle is already in use in Southern California,
where local weather forecasters use GPS signals
to more accurately predict flash flood warnings.
In one area, this let them update predictions every halfhour,
instead of just twice a day, which had been the norm for weather balloons.
Finally, measuring interference can even tell scientists
最后 通过测量讯号干扰 科学家甚至
how much water is in nearby plants.
Just like water vapor, water in plants can absorb and
scatter the energy of incoming GPS signals.
And scientists have put this to the test.
They were actually able to measure how dry the plants were
at some GPS sites during the 2012 to 2014 drought in California.
This kind of data could be useful for scientists
looking to understand how weather events or climate change
may be affecting what’s growing in a given region
without having to actually drive or hike out there,
helping us map out our changing world more easily.
In each of these examples, scientists have found ways to tweak an existing technology
to learn even more about the world.
And not only that, but they’ve made a lot of progress by looking at GPS interference.
不仅如此 通过关注GPS干扰 他们已经获得了不少成果
They have taken what’s usually considered a problem and turned it to our advantage.
And you thought GPS was just for getting you to the nearest coffee place.
I don’t know about you, but I’m still stuck at the
“tsunamis can be felt from space” thing.
If you would like to learn more about how that’s even possible,
you might like Brilliant’s course Waves and Light,
where you’ll learn about the waves involved in everything
from earthquakes to noisecanceling headphones.
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that will help you learn more about
science, engineering, math, and computer science.
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you’ll still be able to keep learning.
And you can get started at brilliant.org/scishow,
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[ OUTRO ♩ ]
Thanks to Brilliant for supporting this episode of SciShow.