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[ music ]
VO: The ATLAS instrument

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on the ICESat-2 satellite measures elevation by timing the

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flight of laser light pulses that leave the satellite, bounce off 

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the surface and return to ATLAS. The speed of light is very

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fast, and so a regular stopwatch would give us too wide a range

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of time, a big margin of error. The timing has to be ultra-

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precise, less than one billionth of a second, in order the measure the height of the

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surface within just a few centimeters.
Phil: I'm Phil Luers. I'm deputy instrument

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systems engineer on the ICESat-2 mission. Calculating the elevation of the ice

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is all about time of flight. It's time of flight of the photon from the laser

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down to the surface of the ice and back. So what

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we are doing is creating time tags of the

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transmit, when the laser fires, and time tags of the receipt of the photon.

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So it's all starts with the transmitter. When the laser fires, 

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it puts out billions and billions of photons, and we pick off a little bit of that and we send it to a

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detector, and we produce a start pulse, and that is the start of our timing system.

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The spacecraft sends us a position and attitude message every second

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that tells us where we are in the orbit. From that we calculate where we 

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think the spots are on the ground. We have a rough database of the whole 

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Earth with elevation as it travels up and down mountains and down into valleys and over

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ice sheets. For each spot, we calculate where we think the surface is

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and we open up a range window. For every photon that's received in

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that range window, we produce a time tag.

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Now the Sun produces a lot of green photons too,

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many more than our laser does. So our laser produces billions and

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billions of photons with every shot. If you add the noise in there, you won't

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be able to tell the Sun's photons from the laser photons.

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So if you just took one shot, the one laser photon would be lost in the 

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noise, but if you take took two hundred shots, now finally you have enough surface signal

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coming up out of the noise that flight software on board can pick out "this is noise,

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and this is the signal from the surface." Once we downlink the photon time tags from

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the receiver and the transmitter and we have the range windows, we put those all together

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you have the transmit time tags, you have the time until the range window

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opened, you have the received time tags in the range window. You 

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calculate all that put together gives you the time of flight of a photon,

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which ultimately gives you the distance between the spacecraft and the ground.

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So if you know where the spacecraft is, and you know the time of flight, you know the distance

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to the ground, now you have the elevation of the ice.

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VO: Technology that can measure elevation adds a third dimension

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to how we map our Earth, and allows us to study change

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across the poles and beyond.

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[ music, beeping ]

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[ beeping ]

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