SPACE-BASED ACTIVE OPTICAL REMOTE SENSING OF CARBON DIOXIDE COLUMN USING HIGH-ENERGY TWO-MICRON PULSED IPDA LIDAR

Modeling of a space-based high-energy 2-μm triple-pulse Integrated Path Differential Absorption (IPDA) lidar was conducted to demonstrate carbon dioxide (CO2) measurement capability and to evaluate random and systematic errors. A high pulse energy laser and an advanced MCT e-APD detector were incorporated in this model. Projected performance shows 0.5 ppm precision and 0.3 ppm bias in low-tropospheric column CO2 mixing ratio measurements from space for 10 second signal averaging over Railroad Valley (RRV) reference surface.


INTRODUCTION
Sustained high-quality column CO 2 measurements from space are required to improve estimates of regional and global scale sources and sinks to attribute them to specific biogeochemical processes for improving models of carbon-climate interactions and to reduce uncertainties in projecting future change.Critical regions for CO 2 measurements are: high latitude ecosystems, tropical ecosystems, southern ocean, managed ecosystems, urban and industrial systems and coastal systems.The need for space-based IPDA measurements has been advocated by Active Sensing of CO2 Emission over Nights, Days, and Seasons (ASCENDS) [1] and Advanced Space Carbon and Climate Observation of Planet Earth (A-Scope) [2] studies.Space-based IPDA systems can provide sustained, high precision and low-bias column CO 2 in presence of thin clouds and aerosols.Technology developments are in progress to provide high pulse energy 2-m IPDA that enables optimum, lower troposphere weighted column CO 2 measurements from space.This system provides simultaneous ranging; information on aerosol and cloud distributions, measurements over region of broken clouds, and reduces influences of surface complexities.
Modeling the performance of a direct-detection high pulse energy 2-m IPDA from space is presented in this paper [3].

MODEL BACKGROUND
The IPDA lidar transmitter is based on high-  2, using common off-line wavelength ( off ). Figure 2 indicates that an IPDA operating at the 2-m wavelength region offers optimum CO 2 measurements in the lower troposphere and improves retrieval of the critically important surface flux estimates by a factor of two compared with a similar IPDA operating at 1.6-m [2].

TECHNOLOGY DEVELOPMENTS
Progress in laser and detector technologies and airborne testing resulting in the development of a new aircraft based high-energy triple-pulsed 2-m IPDA lidar.This IPDA promises a new avenue for remote sensing from space [4].The 2-μm triplepulse laser transmitter is the key component for this system.The performance of this transmitter is critical for achieving measurement sensitivity, accuracy, and range.The unique feature of this laser is the production of triple Q-switched pulses, separated by 150-200 μs, using a single pump pulse.The laser was developed at NASA LaRC and is based on Ho:Tm:YLF crystal technology [3].Among other components, LaRC has been developing new technologies for the 2-μm lasers which include timing control, seeding, locking, and narrowing frequencies for both wind and CO 2 measurements [4].Advanced wavelength control of the current triple-pulse laser uses a single seed laser and provides any offset-locked frequency within 32 GHz with respect to the CO 2 R30 line center ( C ) shown in figure 1.The direct-detection IPDA receiver is based on the state-of-the-art, very low noise, 4×4 pixels MCT e-APD array [5].This e-APD detection system has been developed and tested at GSFC for 1.6-m pulsed IPDA lidar for different atmospheric trace gases such as CO 2 and CH 4 .The projected 2-m IPDA lidar parameters for the space-based system, and environmental parameters are listed in Table 1.

IPDA Lidar Transmitter
On-line wavelength   The IPDA performance was estimated using the approach presented by Refaat et al. [3].IPDA return signal power, total noise equivalent power (NEP) and signal-to-noise ratio (SNR) are shown in figure 3   Return power and noise are used to estimate SNR variations with on-line wavelength (bottom).

SENSITIVITY ANALYSIS AND ERROR
Analysis of both random error associated with IPDA, and systematic errors from atmospheric and instrument biases were estimated.Residual systematic errors in CO 2 measurement arise due to uncertainties in the knowledge of atmospheric and the IPDA instrument capabilities are shown in figure 4 with legend list in Table 2.The estimated CO 2 differential optical depth error from atmospheric effects include uncertainties in temperature (0.5°C), pressure (100 Pa), relative humidity (10%) and H 2 O interference.A normally distributed random number generator was used to simulate the variability of these fields to evaluate the systematic error.CO 2 differential optical depth bias errors resulting from the IPDA transmitter uncertainties including on-and off-line laser position jitters (650 kHz) and laser spectral profiles were also estimated.2. Random errors for the combination of 50, 15, and 5 mJ on-and off-line energies and total (random + systematic) errors are shown in figure 5 with legend list in Table 3. Measurements with two weighting functions, at 50 Hz each, with the triple pulse system can be accomplished using two onlines and a common off-line.The near optimum random error for each pair is <0.12% (<0.5 ppm), and the residual systematic error is <0.07%(0.3 ppm).Measurements can be optimized by tuning on-lines based upon ground target scenarios, environment and science objectives.With 10 MHz detection bandwidth, surface ranging with an uncertainty of <3 m can be achieved as demonstrated from earlier airborne flights [6].

CONCLUSIONS
High-energy triple-pulse 2-μm IPDA technology developments at LaRC are in progress to enable CO 2 column measurements from space.Currently  3.
Table 3 Legends of figure 5.
IPDA development is focused on measuring both H 2 O and CO 2 simultaneously and independently.This allows measurement adaptability over a variety of atmospheric and target conditions.An airborne 2-μm lidar has demonstrated CO 2 measurements over land and ocean.Projected capability of high pulse energy 2-μm laser and an MCT e-APD detection system were incorporated for modeling the performance of a space-based IPDA.Random and systematic errors from the instrument and atmospheric effects including influences of H 2 O were evaluated.The projected performance shows that high precision (0.5 ppm) and low bias (0.3 ppm) low tropospheric weighted dry-air column CO 2 mixing ratio measurements can be made from space with total error of <0.8 ppm for standard atmospheric condition and RRV reference with 10 second averaging.
reflectivity including the 24% lidar enhancement factor.# Tunable within 2050.97 nm to 2051.19 nm, as indicated by the shaded area of Figure1. 0 as a function of on-line wavelength.The highest pair of transmitted energies (50 and 15 mJ) are assigned to on-line to account for the absorption loss in the measurement column.Off-line transmitted energies are set to either 15 or 5 mJ, which result in 185.55 or 37.11 nW return power, and 0.81 or 0.37 nW total NEP, respectively.Corresponding off-line SNR are 229 and 101.Online wavelength selection close to R30 line center results in lower return power with total noise dominated by fixed background and electronic noise.Away from line center, on-line return power increases with shot-noise dominating total NEP and resulting in enhanced SNR.High energy and low detection noise combination allow short-time measurement over low reflectivity regions such as snow covered regions and ocean surface.

Figure 3
Figure 3 On-line surface return power detected by the IPDA form space and the corresponding integrated noise equivalent power using 50 and 15 mJ transmitted energies variation with on-line wavelength (top).Return power and noise are used to estimate SNR variations with on-line wavelength (bottom).

Figure 4
Figure 4 Atmospheric and laser transmitter errors and total systematic error estimation versus on-line wavelength for CO 2 measurement using the 2-m IPDA lidar.Figure legend is in Table2.

Figure 5
Figure 5 Random errors variation with on-line wavelength obtained using different on and off-line transmitted energy combinations.Total error for CO 2 measurement using the 2-m IPDA lidar is obtained by adding random and total systematic errors.Figure legend is in Table3.

Table 2
Legends of figure 4.