Posts Tagged ‘LIDAR’

Urban Aerosols: Who CARES?: Starring: King Air

June 11th, 2010 by M. Ottaviani

The NASA Beech B-200 King Air taking flight. It requires the work of many people to have those wheels leaving the tarmac and land with data worth our scientific analyses, and sometimes with data at all.

There’re two instruments on the Beech King Air B200.  One is the HSRL, which stands for High Spectral Resolution LIDAR. The latter word is an acronym born out of the marriage between RADAR and LASER. Even Radiohead use it. The one mounted on the B200 is an advanced device that my colleagues at NASA Langley have been extensively deploying. It pulses a laser beam straight downwards, and records the time it takes for light to come straight back after it bounces off particles. Based on this information, the atmosphere below the airplane is profiled and the height of aerosols layers effectively located.

HSRL "curtain plots' from earlier in the campaign, revealing layers of aerosol of different origins in the Southern California area.

The Research Scanning Polarimeter (RSP) is somewhat weirder, as polarization is a kind of information that is greatly masked to humans’ vision capabilities. Ask bees. If you wear sunglasses though, you should know that they cut the glare because they filter out light waves oscillating in all directions other than that vibrating in a certain direction.

Light is a wave, but most instruments can only measure the amplitude (=intensity) of it. Instead, the RSP adds to it by recording at which angle the light wave vibrates. As you can expect, the change of this angle as a light wave scatters off a a particle is extremely sensitive to the size and shape of the particle.

The RSP scans a line of pixels along the flight track, in the range +/- 60 degrees from the downward direction. Each scan takes about 1 second: as the aircraft proceeds, a point under the aircraft will be seen in subsequent scans at a different angle. The image on top is created by stacking the subsequent scans in columns. Features at different altitudes exhibit different slant angles as an effect of aircraft motion: if you understand what I'm trying to explain, you should be able to tell what causes the dark region on the ground, under the cloud. To be able to collect the information we want, we "straighten up" the data so that each column shows instead the same point seen from different angles (center). Here we straightened up the cloud. Many instruments stop at this point, but the RSP has the capability to measure also polarization. Look at the cloud rainbow emerging in stark contrast when observed in polarized light! Also, the surface loses contrast which is great because we can easily subtract it form the total signal (bottom). What's more important than looks is that the spacing of the rainbow colors drastically depends on the size of the cloud droplet. Similar concepts apply to aerosols.

We hope we can exploit the synergy between these two instruments to nail the vertical distribution and nature of aerosols in the atmosphere. Our studies confirm the potential of integrating the information derived by the complementary capabilities of the two instruments, sort of “Tell me where you are, and I’ll tell you who you are”.

Why do we do this? These instruments are “the little brothers” of the CALIPSO LIDAR (already in space), and the polarimeter that will be launched onboard the Glory mission at the end of this year. These satellites will fly shoulder by shoulder (just a few seconds apart!), therefore observing at any instant the same scene. You can even have your name orbiting with us!

-Matteo