Operating wavelengths optimization for a spaceborne lidar measuring atmospheric CO_2

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<ul><li><p>Operating wavelengths optimization for a spacebornelidar measuring atmospheric CO2</p><p>Jrme Caron1,2,* and Yannig Durand1</p><p>1European Space Agency, European Space Research and Technology Centre,Keplerlaan 1, P.O. Box 229, 2200 AG Noordwijk, The Netherlands</p><p>2Consultant from RHEA System SA</p><p>*Corresponding author: jerome.caron@esa.int</p><p>Received 29 June 2009; revised 27 August 2009; accepted 7 September 2009;posted 8 September 2009 (Doc. ID 113050); published 25 September 2009</p><p>The Advanced Space Carbon and Climate Observation of Planet Earth (A-SCOPE) mission, a candidatefor the next generation of European Space Agency Earth Explorer CoreMissions, aims at measuring CO2concentration from space with an integrated path differential absorption (IPDA) lidar. We report theoptimization of the lidar instrument operating wavelengths, building on two performance models devel-oped to assess measurement random errors from the instrument, as well as knowledge errors on geo-physical and spectral parameters required for the measurement processing. A promising approach todecrease sensitivity to water vapor errors by 1 order of magnitude is reported and illustrated. The pre-sented methods are applicable for any airborne or spaceborne IPDA lidar. 2009 Optical Society ofAmerica</p><p>OCIS codes: 010.0010, 010.0280, 010.3640.</p><p>1. Introduction</p><p>The Advanced Space Carbon and Climate Observa-tion of Planet Earth (A-SCOPE) mission, whichhas just completed assessment study, aims to mea-sure atmospheric CO2 concentration from space withan unprecedented coverage and accuracy [1]. Itslaser-based technique is a promising approach to un-ambiguously characterize the CO2 sources and sinks.A-SCOPE belongs to the second generation of spacemissions dedicated to CO2 monitoring, followingthe passive missions Orbiting Carbon Observatory(OCO) [2] and Global Greenhouse Gas Observationby Satellite (GOSAT) [3]. The A-SCOPE instrumentis a quasi-nadir-looking differential absorption lidar(DIAL). Its innovative concept, called integratedpath differential absorption (IPDA), relies on themeasurement of the laser pulse echoes reflected onthe ground surface and provides the total columnconcentration of atmospheric CO2.</p><p>The use of lidar instruments to measure CO2 isexpected to bring several advantages over passivemeasurements: they allow full day/night coverage,coverage of high latitudes during all seasons, andare less impacted by cloud cover thanks to the smallfootprint of the collimated laser beams. Further-more, their time-gated detection technique unam-biguously defines the optical path of the detectedsignal, overcoming some fundamental limitationsof passive instruments that can be impacted by, forinstance, unknown aerosol or cirrus layers [4]. Withan appropriate instrument design, a significant re-duction of the measurement biases is expected forthe IPDA concept [1]. In this paper, the optimizationof the operating wavelengths is investigated with theaim of further reducing measurement and processingerrors. Improvements are not only expected for therandom error budgets, but also for the systematicerrors that are of fundamental importance for thederivation of CO2 fluxes, which is the fundamentalscientific objective of A-SCOPE.Differential absorption lidars emit two laser pulses</p><p>at well-defined, very close wavelengths. The first one,0003-6935/09/285413-10$15.00/0 2009 Optical Society of America</p><p>1 October 2009 / Vol. 48, No. 28 / APPLIED OPTICS 5413</p></li><li><p>called on-line is strongly absorbed by the soundedmolecules, while the second one, called off-line, isnot, and serves as a radiometric reference. Followingrecommendations of preliminary studies [1], a pulseddirect detection lidar was selected for the assessmentstudy of A-SCOPE, as well as two sets of on-/off-linewavelengths, at 1.57 and 2:05 m. The laser wave-length adjustment was made by choosing appropri-ate CO2 absorption lines, by optimizing the valueof the CO2 differential absorption, and byminimizingboth its temperature sensitivity and water vapor in-terference. Starting from these two wavelength sets,the current paper reports on their further optimiza-tion based on substantial improvements: the spectralvariability of the requirement is considered, andsensitivities to errors in water vapor, temperature,ground pressure, scattering surface elevation, andlaser frequency are calculated using a common for-malism based on the inverse approach, where theerrors arising during the measurement processingare quantified.In Section 2, a short overview of the A-SCOPE</p><p>mission is given. A more detailed description canbe found in [1,5]. The model calculating measure-ment relative random errors (RRE) is summarizedin Section 3. The next two sections focus on the opti-mization of the IPDA operating wavelengths, eva-luating the impact of wavelength dependent require-ments (Section 4), and sensitivity to errors in proces-sing parameters (Section 5). Using the formalismintroduced in these two sections, the on-line (Sec-tion 6) and off-line (Section 7) operating wavelengthsare optimized. In Section 8, a method to further re-duce the sensitivity to water vapor errors is pre-sented and illustrated.</p><p>2. A-SCOPE Mission Overview</p><p>A-SCOPE follows a near-polar Sun-synchronizedorbit with 6h local time descending node (LTDN),at a relatively low reference altitude, in the rangeof 325 to 400km. The low altitude improves the lidarradiometric budget, while the dawndusk orbit pro-vides a stable thermal environment. The typicallylow Earth reflected Sun background light is furtherreduced by the measurement geometry, the solarzenith angle being always higher than 60.The lidar instrument operates in direct detection.</p><p>It measures the return from ground backscatter(hard target return), and integrates the CO2 mixingratio information over the full atmospheric verticalcolumn. From the atmospheric CO2 measurement,inversion methods are used to infer the sourcesand sinks that drive the global carbon cycle. The abil-ity of inversion methods to constrain and resolve CO2fluxes strongly improves with increasing measure-ment sensitivity at low altitude, where the fluxesare largest. The fine selection of the wavelengths of-fers the lidar instrument the needed high sensitivityto the planetary boundary layer. The two sets of op-erating wavelengths (denoted by InitialB1 and Initi-alB2) selected in the assessment study are presented</p><p>in Table 1, with the corresponding differential ab-sorption optical depths (DAODs). As required bythe IPDA concept, a small portion of the transmittedbeams is picked up and sent to the receiving channeldetector, which accurately monitors the fluctuationsin the pulse energy ratio on-line/off-line.One observation is defined as the average of all</p><p>measurements over a 50km path. For a low altitudeorbit, where spacecraft velocity is about 7km=s, a la-ser with a repetition rate of 50Hz results in about350 measurements to be averaged in order to obtainthe DAOD at observation level. Distinction is madebetween the measurement products: observation le-vel DAOD, the scattering surface elevation (SSE) re-quired to process it, and the column-averaged CO2dry air mixing ratio that requires further processing.</p><p>3. Measurement Random Errors</p><p>A dedicated performance model, calculating lidarmeasurement RRE has been developed to supportthe A-SCOPE instrument design and sizing. Thismodel first calculates the atmospheric absorptionas a function of wavelength. Then it combines theatmospheric properties with the lidar instrument,simulating the complete measurement optical path,and derives radiometric and noise performances.The following atmospheric effects are calculated: ab-sorption from CO2 and H2O, molecular Rayleighscattering, and extinction due to a median aerosoldistribution [6]. Molecular absorption is evaluatedwith spectroscopic parameters from the HITRAN04 database [7,8], except for the CO2 lines soundedby the on-line laser pulses (R18 and R30), for which[9] is used. Voigt line shapes are considered. Besides,the atmosphere is modeled with a U.S. standard pro-file [6,10], with 380ppm CO2. The DAODs calculatedwith these assumptions are presented in Table 1. Thelidar reflectivities range from 0:035 sr1 (representa-tive of oceans) to 0:3 sr1 (possible value on deserts)[1]. Background light is also calculated, with albedosranging from 0.11 to 0.94 and a worst case Sun ze-nithal angle of 60.The DAOD, denoted by, is obtained from on-line</p><p>and off-line returns and calibration signals with</p><p> 12log</p><p>SONreturn SbgdSOFFcal Sbgd;calSOFFreturn SbgdSONcal Sbgd;cal</p><p>; 1</p><p>where Sbgd and Sbgd;cal are background signals scaledto the laser return and calibration integration times,respectively. The RRE is calculated by differentiatingEq. (1) [11,12]. A sufficiently large detector dynamic</p><p>Table 1. Baseline Operating Wavelengths Sets andCorresponding Differential Absorption Optical Depths for A-SCOPE</p><p>Operating Wavelength Set InitialB1 InitialB2</p><p>On-line wavenumber [cm1] 6361.2246 4875.6487Off-line wavenumber [cm1] 6356.50 4875.22DAOD 0.7679 1.0989</p><p>5414 APPLIED OPTICS / Vol. 48, No. 28 / 1 October 2009</p></li><li><p>range is assumed, so that calibration signals can beset to stronger levels than the laser returns and thushave negligible noises. The following random errorcontributors are considered: shot noise from the lasersignal, shot noise from the background light, and in-trinsic detector noise. All measurements are per-formed with the same detector.</p><p>RRE 1n2</p><p> SONreturnSONreturn Sbgd</p><p>2 SOFFreturnSOFFreturn Sbgd</p><p>2</p><p> 2Sbgdt</p><p>tBG</p><p>1</p><p>SOFFreturn Sbgd</p><p>1</p><p>SONreturn Sbgd</p><p>2</p><p>: 2</p><p>t and tBG are the integration times for the laserreturns and for the background measurement inthe absence of a useful signal. n is the number ofmeasurements used for averaging. S represents thevarious signals and their root-mean-square devia-tions, both expressed in photoelectrons:</p><p>SONreturn QeNONsig Nbgd; 3</p><p>Sbgd QeNbgd; 4</p><p>2SONreturn QeFNONsig Nbgd Ndet; 5</p><p>2Sbgd QeFNbgd Ndet: 6</p><p>N are the incident photons, Qe is the detector quan-tum efficiency, and F is the detector excess noise fac-tor.Ndet is the photon flux, incident onto the detector,that would generate a shot noise having the sameamplitude as the detector noise:</p><p>Ndet QeF</p><p>t2</p><p>NEPh</p><p>2: 7</p><p>NEP stands for noise equivalent power and is thenoise figure of merit of the detector (in W=Hz0:5), h(in joule seconds) is Plancks constant, and (in in-verse seconds) is the frequency of the detected radia-tion. The RRE from Eq. (2) is directly driven by theinstrument sizing parameters: laser transmittedpower, receiver aperture diameter, and detector per-formance. Other measurement random error contri-butors are: (i) noise created by the imperfect overlapbetween on-line and off-line footprints on groundwith a spatially variable lidar reflectivity, (ii) speckle</p><p>noise, and (iii) laser frequency jitter. The main sourceof overlap noise is laser on-line /off-line pointing jit-ter. For a value of 25 rad, considered as achievablethanks to the relaxation from the magnifying trans-mitter telescope, the error is negligible [5,13].Speckle noise can be calculated from the instrumentparameters summarized in Table 2. The number ofspeckle cells is about 9000 at 1:57 m, and 5000 at2:05 m, giving a RRE of about 0.00035 for bothcases. The speckle noise is weaker than shot and de-tector noises for the lowest lidar reflectivity (worstcase), but becomes significant over ground. As it isnearly independent of wavelength on small spectralranges, it has little influence on the operating wave-length optimization, and has not been included in thepresent analysis. It is included in the full instrumenterror budget presented in a separate paper [5]. Final-ly, a laser frequency jitter of a few megahertz, asachieved by candidate laser systems [14], gives anegligible RRE [5]. With a detection bandwidth of2nm, the background signal is weak, so that the ac-tual value of the background integration time tBGdoes not impact the presented analysis, and has beenset tot. The assumed instrument parameters, sum-marized in Table 2, reflect the typical technology per-formance either available or foreseen for the missionimplementation. Detector parameters at 1:6 m havebeen selected according to existing devices [15],while, at 2:0 m, they are target values specifiedby ESA for ongoing technology developments. Moredetails about the A-SCOPE performance modelscan be found in [5]. The calculated random errorsare compared to spectrally variable requirementsin Section 4.</p><p>4. Spectral Variations of the Requirements</p><p>The A-SCOPE IPDA lidar provides a measurementof the total column DAOD. The CO2 total columnDAOD CO2 that is extracted from it contains allthe information about CO2 absorption in a singlenumber. CO2 is linked to the vertical distributionof CO2 concentration by an integral involving theweighting function (WF):</p><p>Table 2. Instrument Parameters used for Calculation of MeasurementRelative Random Errors</p><p>B1 (1:57 m) B2 (2:05 m)</p><p>Laser pulse energy [mJ] 50 50Telescope diameter [m] 1 1Receiver path transmittance 0.65 0.65Integration time t tBG [ns] 200 200Spacecraft altitude [km] 400 400Number of measurementswithin 50km n</p><p>346 346</p><p>Field of view [mrad] 0.200 0.200Background filter bandwidth [nm] 2 2Quantum efficiency Qe 0.75 0.75Excess noise factor F 5 1.5NEP [fW=Hz0:5] 50 100</p><p>1 October 2009 / Vol. 48, No. 28 / APPLIED OPTICS 5415</p></li><li><p>CO2 Z</p><p>CO2pWFpdp</p><p>Z</p><p>CO2pCO2pmdryairg</p><p>1</p><p>1 mH2Omdryair H2Opdp:</p><p>8</p><p>CO2 is a differential absorption cross section permolecule (in square meters),mH2O andmdryair denotethe average masses of one H2O and one dry air mo-lecule (in kilograms), and g 9:8m=s2 is the Earthgravity. CO2 is the CO2 dry air mixing ratio and isdefined by</p><p>CO2 nCO2ndryair</p><p> nCO2nair</p><p>1 H2O; 9</p><p>where nCO2 , ndryair, and nair are numbers of moleculesper unit volume (in inverse cubic meters) and H2O isthe H2O mixing ratio.Due to the integral in Eq. (8), weighting functions</p><p>with a stronger contribution in the lower part of thetroposphere are more favorable to the characteriza-tion of CO2 sources and sinks that are located close tothe ground. Such weighting functions are obtainedwith pressure broadening effects: CO2 absorptionlines are extremely narrow at high elevations, whileclose to ground they are significantly widened. Inconsequence, on-line wavelengths far away fromthe line center have a larger CO2 absorption contri-bution from the lower troposphere and, for the samescientific return, will allow relaxed requirements. Onthe opposite, on-line wavelengths closer to the linecenter constrain the requirement.A goal requirement of 0:5ppm, applying to random</p><p>errors on the column-averaged CO2 dry air mixingratio measurements, was formulated for a constantpressure weighting function [1]. It includes both in-strument random errors (measurement noises, laserfrequency jitter) and random errors in ancillary data(SSE, geophysical parameters) with equal contribu-tions. The ratio Xon; off can be used to rescale thisrequirement to a real set of operating wavelengths:</p><p>Xon; off R pgroundpPB...</p></li></ul>

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