[Music]
GLENN DISKIN: My name is Glenn Diskin,
and I'm the principal investigator for the instrument
called the Diode Laser Hygrometer,
which we often refer to as DLH.
It's flown on a variety of aircraft since the mid 1990s,
making water vapor measurements
from the Earth's surface up to the stratosphere,
from the equator to the poles.
And the objectives we have for the ATTREX campaign
are to measure water vapor concentration
at very high accuracy and precision
in order to help understand the processes
by which water vapor is transported
from the troposphere to the stratosphere,
and also understand its role in cloud formation
at the very high altitudes at which
the Global Hawk will be flying.
[Music]
So the Diode Laser Hygrometer,
as the name suggests, uses diode lasers
which are found in many aspects of people's lives.
But these particular lasers are similar to ones used in
the telecommunications industry,
but they're built specifically for us in order to tune to
portions of the electromagnetic spectrum
where water vapor absorbs.
And we use that absorption of the light by the water vapor
in order to make the measurement
of how much water vapor is there.
And in this instrument,
we do this by sending a beam of light
from the instrument out to a reflector,
which is mounted under the wing of the aircraft.
And that light hits a reflector,
which is very similar to the material used in road signs.
So it sends the light back, not as a mirror,
but as a retro reflector, back into the instrument
where we can collect enough of that return light
to make a measurement of how much of it was absorbed
by the water vapor molecules in the atmosphere.
[Music]
Lets take a look
at where the optics of the instrument are.
This is where the light gets processed, gets sent out
to the reflector and back.
So this is just a cover that we keep on
when it's on the ground.
This is a window that is specially coated
to not to reflect the light
at the wavelength that we are using.
All the optics are inside this box.
So the beam exits this window
comes over to this panel here...
And this is the material that is very similar
to what you find in road signs.
So the light that hits it
preferentially wants to travel back in the same direction
and go back in into the instrument
where we've got a lens that collects the light
and focuses on to a detector where we can analyze it.
So that's in terms of the optics,
very simple compared to some optical instruments.
This is self aligning
because of the retro reflector material.
So it allows us to make very good measurements
without having to have
very precise control over the pointing.
As long as we can hit the target,
which we can easily do in flight.
Then we get signal back
and we can make the measurement.
So the other part of the instrument is the computer,
the specialized computer and other electronics
that are in this housing here.
and this computer is designed to supply
all the information to drive the laser
to collect data from the detectors,
to collect data from other sensors that are inside,
both inside here
and inside the black housing on the other side,
which houses the optics
and sends the light out and collects it back.
This is the fiber optic cable
which we use to transmit the light from this chassis
out to the optical transceiver housing
that is on the other side of the aircraft.
The signals from the detectors that are inside
come back on these black cables here
and other signals go back and forth
on these larger cables.
[Music]
So each molecule
has its own electromagnetic fingerprint.
Water vapor has its own fingerprint.
We use that information of detailed knowledge
of water vapor's absorption characteristics
and we take advantage of that
by moving the laser wavelengths back and forth
across one of the specific fingers
in that in that forest of lines that water vapor has.
And that allows us to determine the concentration
of water in the atmosphere.
[Music]