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the atlas instrument on the icesat-2

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satellite measures elevation by timing

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the flight of laser light pulses that

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leave the satellite bounce off the

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surface and return to atlas

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the speed of light is very fast and so a

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regular stopwatch would give us too wide

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a range of time a big margin of error

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the timing has to be ultra precise less

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than one billionth of a second in order

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to measure the height of the surface

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within just a few centimeters

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i'm phil lars i'm deputy instrument

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system engineer on the atlas instrument

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on the icesat-2 mission

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calculating the elevation of the ice is

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all about time of flight it's

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time-of-flight of the photon from the

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laser down to the surface of the ice and

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back so what we are doing is creating

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time tags of the transmit when the laser

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fires and time tags of the receipt of

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the photon

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so it all starts with the transmitter

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when the laser fires it puts out

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billions and billions of photons we pick

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off a little bit of that and we send it

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to a detector and we produce a start

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pulse and that is the start of our

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timing system the spacecraft sends us a

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position and attitude message every

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second that tells us where we are in the

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orbit from that we calculate where we

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think the spots are on the ground we

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have a rough database of the whole earth

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with elevation as it travels up and down

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mountains and down into valleys and over

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ice sheets for each spot we calculate

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where we think the surface is and we

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open up a range window for every photon

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that's received in that range window we

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now the sun produces a lot of green

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photons too many more than our laser

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does so our laser produces billions and

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billions of photons with every shot

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if you add the noise in there you won't

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be able to tell the sun's photons from

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the laser photons so if you just took

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one shot the one laser photon would be

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lost in the noise but if you take 200

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shots now finally you have enough

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surface signal coming out by the noise

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that fly software on board can pick out

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this is noise and this is the signal

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from the surface

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once we downlink the photon time tags

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from the receiver and the transmitter

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and we have the range windows you put

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those all together you have the transmit

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time tags you have the time until the

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range window opened you have the receive

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time tags in the range window you

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calculate

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all that put together gives you the time

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of flight of a photon which ultimately

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gives you the distance between the

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spacecraft and the ground so if you know

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where the spacecraft is and you know the

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time of flight you know the distance to

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the ground now you have the elevation of

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the ice

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[Music]

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technology that can measure elevation

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adds a third dimension to how we map our

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earth and allows us to study change