ROUGH SENSITIVITY ESTIMATES USING DIRECT SUN

1. Considerations on Validation of Space Based CO2 Measurement Wm. S. Heaps1,E. L. Wilson1, E.M. Georgieva 1,2 1 NASA, Goddard Space flight Center, Greenbelt, MD 220771, phone: 301-286-5106, fax: 301-286-1750, e-mail: William.S.Heaps@nasa.gov 2 Goddard Earth Sciences and Technology Center, Baltimore, MD 21228 13CO2 ABSTRACT The precision requirements for total column CO2 measurement from space are more stringent than any ever attempted. Similarly the requirements on a validation system for a spaceborne CO2 column measurement are very demanding.  We at Goddard have been developing a Fabry-Perot based instrument for remote sensing of CO2 column. This instrument holds high promise for offering a relatively inexpensive method for measuring CO2 column from the ground at very high precision--necessary for validating space borne instruments. We shall present some results obtained using our instrument and discuss some of the factors necessary for making a measurement of total column CO2 with precision less than 1%. • THE SOLAR POINTING PROBLEM • SUN SIZE (~ .5 DEGREE) CAN BE BIG FACTOR IN AIRMASS(SEE FIGURE BELOW) • VIEWING WHOLE SUN RAPIDLY BLOWING THIN CLOUDS SIGNIFICANTLY CHANGE THE EFFECTIVE AIRMASS ON TIMESCALES OF A FEW SECONDS • TO ELIMINATE THIS EFFECT MUST VIEW ONLY A SMALL PART OF THE SUN REDUCING SIGNAL LEVELS AND PUTTING STRICTER REQUIREMENTS ON POINTING SYSTEM ACCURACY • SOLUTIONS TO THE PROBLEM • HIGH PRECISION POINTING IS EXPENSIVE AND DIFFICULT TO IMPLEMENT IN THE FIELD • WE HAVE DEVELOPED A “DITHERING TRACKER” THAT DELIBERATELY POINTS TO SEVERAL POSITIONS ABOVE AND BELOW THE SUN • SIGNAL STRENGTH CAN BE RELATED TO OVERLAP OF SUN WITH INSTRUMENT FOV ALLOWING CALCULATION OF POINTING POSITION • THE MISSING SINK • More CO2 is being removed from the atmosphere than can be explained diagnostically. • Global measurements (ie. satellite) needed to locate sink • Precision requirements for a total column measurement are daunting (~.3%) • Ground based validation methods for satellite must meet or exceed this value The 3 species version of the Fabry-Perot instrument Figure shows 2 minutes of data taken using dithering tracker. Red trace of reference channel power shows tracker central position is “high.” Blue trace of ratio shows that differences in airmass are ~80 ppm between top and bottom of sun. This figure shows the difference in the column average CO2 measured to the bottom of the Sun minus the top of the Sun for various Solar Zenith Angles. Recent Data, June 30, 2006 • DITHERING TRACKER DOESN’T WORK FOR FTIR BECAUSE SCANS TAKE TOO LONG. • FTIR SHOULD USE HIGH PRECSION TRACKER WITH SMALL INSTRUMENT FIELD OF VIEW (FOV) AND STOP TRACKING DURING SCAN • FOR A SYSTEM LOCATED IN WASHINGTON DC THE SZA NEVER GETS <57 DEGREES IN DECEMBER-JANUARY • IMPLIES ELEVATION ACCURACY ~.1 DEGREES FOR 1 PPM PRECISION • SUN APPARENT MOTION PLUS CLOUD VARIABILITY COULD INTRODUCE SIGNIFICANT ERROR DURING 2 MINUTE SCAN OF FTIR The FP to REF channel ratio is plotted as a function of local time for an O2 instrument (yellow), a CO2 instrument (blue), and a water vapor instrument (red).The principal variation is that arising from the change in airmass as the sun rises and sets throughout the day. • CONCLUSIONS • WE PRESENT RESULTS FROM GROUND TESTING OF A NEW INSTRUMENT CAPABLE OF VERY PRECISE MEASUREMENTS OF ATMOSPHERIC CARBON DIOXIDE, OXYGEN, AND WATER VAPOR • THE INSTRUMENT HAS HIGH SENSITIVITY TO SMALL CHANGES IN THESE SPECIES AND IS CAPABLE OF RAPID TEMPORAL RESPONSE. • TO ACHIEVE THE DESIRED DEGREE OF PRECISION FOR CARBON DIOXIDE COLUMN MEASUREMENTS GREAT CARE MUST BE EXCERCISED IN THE TRACKING OF THE SUN. • A LOW COST SOLUTION TO THE PRECISION TRACKING PROBLEM HAS ALSO BEEN DEVELOPED AND PRELIMINARY INDICATIONS SHOW ITS PROMISE AS A SOLUTION • THE OVERALL TECHNIQUE IS APPLICABLE TO GROUND, AIR, AND SPACE BASED PLATFORMS. IT CAN BE EXTENDED TO ADDITIONAL SPECIES. • FUTURE WORK HIGHLY DESIRABLE AIMED AT VERIFYING TECHNIQUES FOR IMPROVING SENSITIVITY, ACHIEVING LONG TERM STABILITY, AND EXTENDING TECHNIQUE TO OTHER ATMOSPHERIC MEASUREMENTS Line strength for CO2 isotopes The ratios from the previous figure are plotted against the calculated airmass. The AM and PM values for O2 are virtually the same and are and cannot be distinguished. The CO2 channel shows some slight variation throughout the day probably arising from consumption by plants. The water vapor channel shows considerable variation consistent with typical changes in humidity throughout a day. ROUGH SENSITIVITY ESTIMATES USING DIRECT SUN • 13CO2 and 12CO2 line strength from HITRAN spectra. • Spectroscopic methods will be used to make the global measurements • OCO (Orbiting Carbon Observatory) uses a large grating spectrometer • Plan is to use a network of FTIR spectrometers for validation. • FTIR spectrometer is commercially available. It can make a high resolution of the solar spectrum in a little less than 2 minutes. • CO2 and O2 column density can be derived from these spectra. • Our group at Goddard has been developing an instrument based upon the Fabry-Perot interferometer to make column measurements BETWEEN 1.5 AND 2 AIRMASSES AIRMASS CHANGE = 0.5 CO2 RATIO CHANGE=.024 SINCE 1 AIRMASS IS ~ 380 PPM WE GET 190/.024=7917. so A RATIO CHANGE OF .001 IS ABOUT 7.9 PPM FOR THE COLUMN AVERAGE Intercomparison of H2O measurement by the Fabry-Perot and by a NOAA GPS based sensor located in Annapolis, Maryland for 5 days in late June early July. The FP only operates during the day. Agreement is consistent with variability expected for sites ~ 30 miles apart. SUN APPARENT SIZE IS ~.5 DEGREE AIRMASS TO TOP AND BOTTOM OF SUN CAN DIFFER SIGNIFICANTLY