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Summary of this presentation written by John Marshall with all pictures and slides provided by Dr. Weimer

The LIDAR System In his presentation on LIDAR for Remote Sensing and CALIPSO, Dr. Weimer introduced to us LIDAR (light detection and ranging) and its use in Remote Sensing followed by an introduction to CALIPSO a new space based LIDAR to be launched next year (currently undergoing integration in France with Alcatel). Dr. Weimer points out that LIDAR is very similar to radar in principle but with light instead of radar waves. Essentially LIDAR can do two things. First it can be used to gain a picture of topography by measuring the time it takes for light pulses emitted from the sensor to bounce off the target surface and return, and the second which is the case of CALIPSO, is to analyze the return signal to see how the atmosphere affected it which in turn tells us what is in the atmosphere. LIDAR lasers have some unique characteristics such as being very monochromatic, another words the laser is very pure having a very small spectral range which will illuminate an area from orbit about a 70m wide, which is limited to prevent eye injury to animals or humans on the surface. As it shines down three things can happen to the light from a LIDAR. It can be scattered, absorbed, or transmitted straight through the atmosphere. Only a limited number of wavelengths are available that can work from space for laser applications. One problem is passing the laser light through the atmosphere which will have an affect on the laser yet this is also advantageous. By receiving the signal back and analyzing it, a vertical snapshot of the atmosphere directly below the satellite can be obtained. And because the signal is constantly transmitted and received in a line, a vertical curtain can be obtained of the atmosphere. This will lead to a better of understanding of earth’s radiation balance which is critical in understanding global warming.

One of the advantages of a LIDAR sensor Dr. Weimer explains is that we know the exact properties of the laser so when the signal is received back to the sensor we can see accurately how the signal was changed or affected by the atmosphere. The following is a list of the main types of interactions between the laser light and the atmosphere.

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LASER

OPTICS DETECTOR

Laser transmitted

Backscattered Radiation Received

Receiver Subsystem

ATMOSPHERIC SCATTERERS Or ABSORBS

An example of Raman scattering would be the interaction of a laser with carbon dioxide since the light, when it is reemitted, will be shifted. Fluorescence Dr. Weimer explained is caused by molecules “rattling” down through the quantum states, causing different light to come out.

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Rayleigh scattering – laser radiation elastically scattered from atoms and molecules (no change in wavelength)

Mie scattering – laser radiation elastically scattered from particles (aerosols, clouds) of sizes comparable to the wavelength of radiation (no change in wavelength)

Raman scattering – laser radiation inelastically scattered from molecules (wavelength shift that is a characteristic of the molecule)

Resonance scattering – laser radiation matched to specific atomic transition scattered by large cross section (no change in wavelength)

Fluorescence – laser radiation matched in frequency to specific electronic transition of an atom or molecule is absorbed with subsequent emission at longer wavelength

Absorption – attenuation of laser radiation. Can be non-resonant (e.g. black soot) or resonant when wavelength is matched to the absorption band of a given molecule or atom

6

LIDAR for Remote Sensing

CU Seminar3/31/04

Lidar for Remote Sensing& CALIPSO

LIDAR EquationLIDAR Equation

Light scattering particles

R (Range)

∆hAr

For Elastic Scattering, the LIDAR Equation can be written as:

))(2exp(4

)(2

)(0

''2 ∫−=

R

eb

r drrkRkhRCRP

π

Here• Pr is the power returned to the lidar

at the laser wavelength• C = lidar constant (includes Pt laser

power, receiver Area, filter bandwidths and other instrument factors)

• R = range to target• kb/4π [km-1sr-1] is the

backscattering factor(or lidar backscatter coefficient)

• ke [km-1] is the volume extinction coefficient

Scientists Dr. Weimer points out, spend most of there time on the backscatter coefficient.

7

LIDAR for Remote Sensing

CU Seminar3/31/04

Lidar for Remote Sensing& CALIPSO

Ranging EquationRanging Equation

For all lidars, the Range Equation for R is simply:

Where:– c is the Speed of Light– is the round-trip time for the light from lidar to target and back – measured

using precision oscillators

Solutions:

Two measurements (roundtrip time, signal strength)

Three unknowns (Range, backscatter coefficient, extinction coefficient)

Extensive work on finding solutions, see Applied Optics e.g. work by Klett, Fernald, Kovalev

2tcR ∆⋅=

t∆

If you want 1ft resolution your instrument needs to be able to resolve 1 nanosecond. A typical roundtrip for light could be around 4-5 milliseconds.

The CALIPSO Satellite Mission

Dr. Weimer explained that so far there has not been a LIDAR that has completely mapped the earth. With CALIPSO there are 3 instruments on board and it will send down 4 GB of data a day. Because the atmosphere is its own reference, validation is simple he says.

CALIPSO will provide key measurements of aerosol and cloud properties needed to improve climate predictions by reducing uncertainties in :

– radiative forcing from clouds and aerosols

– aerosol-cloud interactions

– cloud-climate feedbacks CALIPSO will fly a 3-channel backscatter

lidar and passive VIS, IR instruments in formation with Aqua, CloudSat, PARASOL,

One of the main needs that LIDAR can help with is providing a better understanding of the affect clouds and aerosols have on the radiation balance. CALIPSO will not be a singular effort; it will be flown in line with other satellites all looking at the same piece of land. This will include Cloudsat, Parasol, Aura, and Aqua. Cloudsat is a microwave radar that sees through clouds, Parasol works to detect aerosols and scattering, and Aura studies atmospheric chemistry. The following slide shows where CALIPSO is needed in studying the radiation balance.

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Above is displayed a color-modulated plot of LITE Level 1 532 nm profile data over the Sahara during orbit 146 at approximately 23 GMT on September 18, 1994. The image displays 5 minutes of LITE data.

The color assigned to each pixel represents the intensity of the return signal in digitizer counts. The count values range from zero for no return to a value less than 4000 for the strongest return.

Prominently featured at the start of this orbit transect is the Atlas Mountain range near 31N, 8W. This mountain range approximately separates a more optically thick aerosol air mass to the southeast from a relatively cleaner air mass to the northwest.

http://www-lite.larc.nasa.gov/n_the_images.html

Strong backscatter

from aerosols

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Aerosol forcing counters the warming caused by greenhouse gases(source: IPCC, 1996)

PICASSO-CENA = CALIPSO For some new data on smoke see Science Magazine, Vol. 303, 27 February, 2004.

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One spacecraft with three science instruments

– Built by Ball Aerospace – Three-channel lidar

• vertical distribution of clouds and aerosols

– Imaging IR radiometer (Sodern in France)

– Wide-field camera • cloud and aerosol

properties

Orbit 705 km, polar 98.08° Launch April, 2005 Currently in Satellite Integration

The CALIPSO Satellite

532nm is in the middle of the green and 1064nm is in the infrared portion of the spectrum. The telescope itself on CALIPSO has a diameter of 1m with a sun shield built around the telescope.

Integrated Lidar Transmitter (ILT)

Imaging InfraredRadiometer (IIR)

Wide Field Camera

X-Band Antenna

Payload Housing Assembly

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Lidar – 532 nm & 1064 nm wavelengths

• Gives particle size estimate

– polarization sensitive at 532 nm • Differentiates ice from water

clouds

– 70-m footprint on ground – sampling 333 m horizontal,

30 m vertical Imaging IR Radiometer

– 10.5 µm & 12 µm wavelengths – 60-km swath

Wide-Field Camera – 620 nm to 670 nm (=MODIS)

wavelengths – 60-km swath

CALIPSO will be launched along with the Cloudsat satellite, in April of next year on a Delta II rocket.

Space Qualification Dr. Weimer points out that space qualification is a huge driving cost for missions. For something to be place into space it has to be built to survive in space and therefore undergo rigorous testing before launch. The following are the points that Dr. Weimer brought up.

Designed, Built, and Tested to withstand the launch and the mission life in-orbit

– High Vibration during launch Since CALIPSO will be placed on top of the Cloudsat satellite on the Delta II it will undergo more vibration during launch than Cloudsat.

– Moderate temperature extremes around an orbit (unless pointed at the sun)

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– Particle Radiation from deep-space and the sun Since CALIPSO will be in LEO some protection will come from the earth’s magnetosphere.

– Mechanical Shock from Pyrotechnics The pyrotechnics are used to deploy the satellite in space.

– Vacuum environment while maintaining low-outgassing

Qualification and Reliability drive costs and schedule

– Low Mass, low power draw, high performance – Extensive design and engineering analysis – Extensive testing at all stages to minimize risk at later stages – High Reliability parts – limits choices, especially electronic – Full traceability of all materials used. Dr. Weimer also noted that for

everything going into space, you have to know the history of every part from large piece to nuts and bolts from who made it to how it was tested

– Lots of Reviews

Conclusion and CALIPSO website Lasers have been used to map both the earth and Mars in fact in some ways we have a better map of mars Dr. Weimer says. The use of LIDARS in space is not new but their capabilities are definitely improving and should help to provide the keys to unlocking the clues to global warming. http://www.ball.com/aerospace/calipso.html http://www-calipso.larc.nasa.gov/ http://earthobservatory.nasa.gov/Library/CALIPSO/CALIPSO.html