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Abstract
The spaceborne IPDA LIDAR has the potential to measure the global atmosphere CO2 column concentrations with high accuracy. For this kind of LIDAR, system calibration experiments in the laboratory are of high importance. In this study, a specially-customized CO2 absorption cell is employed to simulate the CO2 column absorption of the spaceborne platform. Then calibration experiments are constructed for the receiving system and the entire LIDAR system. The absorption of several different XCO2 concentrations from 400 to 415 ppm in the atmosphere is equivalent to that of the absorption cell charged with different pressures of pure CO2. Under the zero pressure of the absorption cell, the calculated equivalent column average concentration (XCO2) is 12.53 ppm, which acts as system bias. In the calibration experiments, the absolute errors are all less than 1 ppm. And the standard deviations (STDs) are less than 1.1 ppm (148-shot averaging) and 0.8 ppm (296-shot averaging) for receiving system and less than 1.2 ppm and 0.9 ppm for the IPDA LIDAR system. All the results of different average times are close to each other and less than 1 ppm, which proves the high accuracy of the IPDA LIDAR system. In addition, the XCO2 concentrations Allan deviation of 0.25 ppm and 0.35 ppm at 100 s shows that the receiving system and IPDA LIDAR system function with long-term stability. Using a CO2 absorption cell as a standard calibration device in the laboratory validates the measurement accuracy and stability of the spaceborne IPDA LIDAR prototype. Furthermore, the proposed absorption cell may serve as a standard calibration device for related atmosphere trace gases sounding research.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Global surface temperature in the first two decades of the 21st century (2001-2020) was 0.99 [0.84-1.10]°C higher than that in 1850-1900, and in 2011-2020, especially, was 1.09 [0.95-1.20]°C higher [1]. The increased concentrations of Greenhouse gases (GHGs) in the atmosphere are the primary cause of global warming. Regarding the radiative forcing, CO2 contributes 63.5% of all GHGs [2,3]. To develop appropriate emission reduction programs, it is necessary to accurately obtain the sources and sinks of GHGs (such as CO2) globally. Compared to ground-based and airborne observations, spaceborne observations can cover a global scale. At present, the primary observation method for global CO2 measurements is passive spaceborne remote sensing, such as the Greenhouse gases Observing SATellite (GOSAT) and the Carbon Observatory-2 (OCO-2) [4,5]. However, passive spaceborne spectrometers can only work under sunlight and in cloudless conditions, which means massive useless measurement data and large measurement errors [6–8]. In comparison, the spaceborne integrated path differential absorption (IPDA) light detection and ranging (LIDAR) is a widely recognized instrument currently with high potential in the global and high-precision GHGs column concentrations. Due to its global measurement, all-day operation, high signal-to-noise ratio (SNR), and ability to identify clouds and grounds, CO2 measurement IPDA LIDAR relevant studies have been widely studied, including airborne experiments and feasibility analysis. Several groups had made contributions [9–11].
Since the Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) project was proposed in 2007 [12], NASA Goddard Space Flight Center (GSFC) has proposed several IPDA LIDAR concepts and related validation experiments. As one of the ASCENDS missions, Abshire et al. developed a multiple-wavelength pulsed IPDA LIDAR system based on a multi-wavelength locked laser. The R16 absorption line of CO2 at 1572.23 nm was selected and the first airborne campaign was implemented in 2011 [9]. In the airborne campaigns in 2014 and 2016, the system was improved with a more sensitive HgCdTe APD detector. The XCO2 concentration was consistent with that from in situ sensor. The measurement accuracy averaging on the desert surfaces within 1s was ∼ 0.7 ppm [13]. NASA Langley Research Center (LaRC) and ITT Exelis developed an intensity-modulated continuous-wave (IM-CW) IPDA LIDAR called multifunctional fiber laser lidar (MFLL) [14]. By conducting multiple airborne campaigns under different environmental conditions based on MFLL [15–17], Joel et al. demonstrated that CO2 measurements remained consistent across seasons and had an absolute calibration error (standard deviation) of 0.80 ppm [17]. In addition, NASA LaRC has also developed an airborne double-pulse 2-µm IPDA LIDAR system, and the measured CO2 weighted-average column dry-air volume mixing ratio (XCO2) with an uncertainty of 1.02% and an additional bias of 0.28% was validated [18]. Based on the Advanced Space Carbon and Climate Observation of Planet Earth (A-SCOPE) mission of ESA, Amediek et al. developed a new IPDA LIDAR CHARM-F (CO2 and CH4 Remote Monitoring-Flugzeug) for simultaneous observation of GHGs CO2 and CH4. The results of the first airborne campaign showed an average measurement accuracy of below 0.5% over 20 km [19]. In addition, Fix et al. have studied a high-precision energy monitoring method to reduce the speckle effect of laser, which was important for IPDA LIDAR measurements [20]. The Aerosol and Carbon Detection Lidar (ACDL) developed by Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences was launched in 2022 as the primary payload of the Atmospheric Environment Monitoring Satellite (AEMS). It is the first GHGs spaceborne lidar in space and mainly aims at CO2 column concentrations detection on a global scale with an online wavelength of 1572.024 nm and an offline wavelength of 1572.085 nm [21].
Before the launch of AEMS, the performance of the spaceborne IPDA LIDAR was evaluated and the random errors were less than 0.3% [21]. In addition, Wang et al. demonstrated that the total systematic error was 0.589 ppm and that 61.25% of the global random error was less than 1ppm, which showed its future performance [22]. Zhu et al. accomplished the performance analysis and processing algorithm correction on the airborne flight prototype of the spaceborne IPDA LIDAR system [23]. Then an airborne campaign was carried out in 2019 and 1.02 ppm STD in the open area was validated under atmospheric conditions [24]. Furthermore, the nonlinear correction of the detector module was studied and the detector amplifier circuit board (ACB) was optimized. The simulation results showed that the random error was less than 0.728 ppm in more than 90% of the worldwide regions [25].
However, calibration experiments are significant to verify the measurement accuracy for the spaceborne IPDA LIDAR system. The calibration equipment with an in-door CO2 absorption cell has the potential to simulate the laser absorption of the spaceborne IPDA LIDAR and verify its performance. Sakaizawa et al. tested the performance of the 1.57 µm CO2 measurement LIDAR with a 3-m-long CO2 absorption cell. The result showed good agreement between the CO2 concentration and the measured optical depth (OD), and the correlation between theoretical and experimental OD reached 0.998. These results verified the feasibility of the calibration experiment [26]. However, in the study, the absorption cell cannot simulate the CO2 absorption of the entire atmosphere. Hu et al. designed an absorption cell device that can effectively simulate the CO2 absorption of the atmosphere [27]. Relevant calibration experiments on the receiving system and the IPDA LIDAR system were carried out based on this customized 15.213-m long CO2 absorption cell, respectively. Experimental results confirmed that IPDA LIDAR could achieve 1 ppm measurement accuracy, and the CO2 absorption cell could serve as standard equipment for calibration experiments. As a laboratory calibration test for ACDL, the results of this experiment are highly significant. Meanwhile, the absorption cell calibration equipment can provide a new method for other related researchers to simulate atmosphere GHGs absorption.
In this paper, the basic principle of IPDA LIDAR and the system parameters are introduced briefly in section 2. The experimental setup in detail and measurement results for CO2 absorption cell calibration of the receiving system are shown in section 3. Testing and analysis of the measurement accuracy of the LIDAR system are presented in section 4. In section 5, a summary and outlook of this work are given.
2. Spaceborne IPDA LIDAR system and CO2 absorption cell
2.1 Principle of IPDA LIDAR in the atmosphere
Spaceborne IPDA LIDAR is based on the CO2 differential absorption of the integrated path and selected two wavelengths in the absorption line of CO2 [19]. The online wavelength chooses the wing of the absorption line with strong CO2 absorption. And the offline wavelength locates in the weak absorption area serves as a reference. The IPDA detector receives the reflected echo signals from hard targets such as the ground surface, which is given by the lidar equation as [28]
The vertical-path XCO2 is given as:
2.2 IPDA LIDAR system configuration
The IPDA LIDAR of ACDL is primarily divided into a laser transmitting system, telescope, photoelectric detection system, data acquisition system, and control electronics system. The laser transmitter system consists of a 1572 nm seed laser, frequency stabilization system, pulsed laser, and optical components, resulting in a 1572 nm double-pulse laser at 20 Hz. Using external frequency modulation technique and optical phase-locked loop (OPLL) of a phase-locked system, the seed laser provides continuous wave online and offline laser signals with frequency stability of less than 0.3 MHz (root mean square (RMS)) [29]. A frequency switching output between the online and offline seed lasers is achieved at 200 µs intervals using a magneto-optical switch. Then, pulsed laser is generated based on seed injection OPO (optical parametric oscillator) technology and a frequency stabilization system. The frequency stability is less than 0.6 MHz (RMS) [30]. Finally, OPA (optical parametric amplifier) amplifies the laser energy to above 75 mJ and 35 mJ, for online and offline radiation respectively. The 1572-nm radiation is then divided into two parts by beam splitter mirror 1 (BSM1) after beam pointing control. One part enters the atmosphere through the beam expander, and the other enters the integrating sphere (IS) energy monitoring module. Backscattered echo signals from targets and monitoring signals from IS are ultimately focused on the same InGaAs APD detector in the photoelectric detection system. A filter F2 with a bandwidth of 0.45 nm is inserted in front of the APD detector to block background light. Figure 1. shows the schematic diagram of the IPDA LIDAR system, and Table 1 lists the main parameters of the spaceborne IPDA LIDAR.
Fig. 1. Schematic diagram of the spaceborne IPDA LIDAR system. RM1, 2: reflecting mirrors; BSM1, 2: beam splitter mirrors; IS: integrating sphere; CL1, 2: collimating lens; LPC: laser pointing control; F1, 2: filter; APD: Avalanche photodiode.
Table 1. Main parameters of spaceborne IPDA LIDAR.
2.3 CO2 absorption cell configuration
The free space CO2 absorption cell is specially-designed to simulate the LIDAR beam CO2 absorption of the atmospheric column. When the absorption cell is filled, the absorption cross-section is only related to the pressure and temperature inside. The path integral of the DAOD can convert into the product of the differential absorption cross-section and the effective optical length of the absorption cell (L). DAOD of the absorption cell can be expressed as
The temperature and pressure of the in-door CO2 absorption cell are controllable to simulate the different DAOD of CO2 in the atmosphere. When the absorption cell is at a fixed temperature, the pressure of pure CO2 is the only variable that causes the change of DAOD. The US standard atmosphere model [31] and the spectroscopy database HITRAN 2020 [32] are used to simulate the DAOD and IWF. Several different XCO2 around 400 ppm in the atmosphere can be equivalent to the various pressures charged to the absorption cell. The inversion values of the equivalent XCO2 at different pressures of the absorption cell can verify the accuracy of IPDA LIDAR.
An absorption cell with a 15.213-m length was designed to simulate the DAOD of CO2 from a spaceborne platform according to optimized parameters. The absorption cell device mainly includes a CO2 absorption cell pipe, an optical path turning structure, a temperature control system, a pressure detection system, a vacuuming system, and a CO2 gas charging system [27]. The absorption cell is folded into three parts via two 45° deflection mirrors to reduce the space. The layout of the CO2 absorption cell system is shown in Fig. 2. Table 2 shows the major parameters of the absorption cell, in which several real-time temperature and pressure control points are set.
Table 2. Parameters of the absorption cell.
3. IPDA LIDAR receiver system calibration experiment
3.1 Receiver calibration experimental equipment
An optical double pulse signal simulator based on the acoustic-optical modulator (AOM) modulation is utilized to verify the performance of IPDA LIDAR. The time interval between the double pulses is 200 µs, and the generated double-pulse laser is divided into two parts with a 2 × 2 fiber optical coupler (FOC). One part is delayed by a 3 km long single mode fiber and then enters the CO2 absorption cell to simulate the total atmosphere layer CO2 absorption of the spaceborne platform. The other part is collimated directly and received by the APD detector of the IPDA LIDAR receiving system through the telescope, which acts as an energy monitoring part. The digital adjustable attenuators of AA1 and AA2 are used to control the optical power of the LIDAR system. The timing relationship of the entire experimental equipment is controlled by a multi-channel signal generator, and the schematic diagram of the whole experimental setup is shown in Fig. 3.
Fig. 3. Schematic diagram of absorption cell calibration equipment for IPDA LIDAR based on an optical signal simulator. AOM, acoustic-optic modulator; FOC, fiber optical coupler; AA1, adjustable attenuator 1; AA2, adjustable attenuator 2; FC1, fiber collimator 1; FC2, fiber collimator 2; RM1, reflecting mirror 1; RM2, reflecting mirror 2; SM fiber, 3 km single mode fiber.
When XCO2 changed from 400 to 415 ppm with a step of 5 ppm, DAOD and IWF were calculated according to Eq. (2) and Eq. (5). Table 3 gives the pressure corresponding to different XCO2, and the accuracy of the pressure is 1 hPa. The temperature was fixed at 25°C.
Table 3. XCO2 and corresponding theoretical pressure.
Since the AOM has shifted the frequency online and offline by about 200 MHz, the corresponding DAOD and IWF have changed. During the experiment, CO2 with different pressures was charged into the absorption cell separately. The theoretical results were compared with the experimental results under the online and offline conditions of frequency shift. Table 4 gives the corresponding theoretical equivalent XCO2 (ppm) under several different pressures. According to the simulation, the change in IWF is ignored.
Table 4. The actual pressure and corresponding theoretical XCO2 at shifted online and offline wavelengths.
3.2 Receiver calibration experimental results
When the pressure in the absorption cell is almost zero, the four signals detected by the detector are shown in Fig. 4.
Since the spatial resolutions requirements of land and ocean are 50 km and 100 km, respectively, corresponding to an accumulated pulse number of 148 and 296, which last roughly 7 s and 14 s on the time scale, the analysis in this paper is based on the average of 148 pulses and 296 pulses. The relationship between the SNR of a single-shot and that of a multi-shot is
where Nshots is the number of pulses and SNRs/m represents the SNR of single-shot/multi-shot.A double-path DAOD with more than a 10000-shot (500 s) sample is shown in Fig. 5(a), while Fig. 5(b) shows the results of the 148-shot moving average. Without considering the error of IWF, Fig. 5(c) and Fig. 5(d) show the results of the single-shot and the 148-shot moving average, respectively.
Fig. 5. (a) Single-shot DAOD. (b) DAOD of 148-shot moving average. (c) Single-shot XCO2. (d) XCO2 of 148-shot moving average in case of more than a 10000-shot sample.
The values of XCO2 with zero pressure inside the CO2 absorption cell are 12.53 ppm with STDs of 0.74 ppm for 148-shot moving average and 0.59 ppm for 296-shot moving average, which is a system bias parameter for subsequence measurement correction. The nonzero DAOD bias is mainly induced by the filter transmission difference for the online and offline signals at different wavelengths and different incident angles. The beam pointing is adjusted in advance and monitored in real-time during the experiment to ensure stable beam pointing. In addition, the testing time is determined to be chosen at midnight when the environment disturbance is weak to ensure the relative stability of the beam pointing. Thus, the system bias can maintain a relatively constant value. But the error caused by the spatial optical path of about 20 m outside the absorption cell is negligible.
After the CO2 absorption cell is operated at different pressures to simulate CO2 absorption in the atmosphere, the measurement results are inverted and analyzed. When charging with different pressures, the SNR of the measurement results for the 148-shot and 296-shot moving average lists in Table 5. The results meet the requirements of SNR on the spaceborne platform [10,21,23].
Table 5. SNR for different pressures of CO2.
For the measurement data with the lowest SNR at 466.8 kPa pressure, the results of the “pulse peak method (PPM)” and the “pulse integration method (PIM)” were compared. PPM uses the peak of the pulses to represent the pulse power, while PIM uses the integral values of each point. Besides, the methods of “log before averaging (AVS)” and “log after averaging (AVD)” were also compared [24,33,34]. The methods of AVS and AVD can be expressed as
Fig. 6. (a) DAOD for single-shot calculation with PPM and PIM methods. (b) XCO2 for single-shot calculation. (c) XCO2 for 148-shot moving average with AVS and AVD methods.
Table 6. Statistical results of PPM and PIM.
Table 7. Statistical XCO2 of AVS and AVD for 148-shot moving average.
The summary of the absolute errors and STDs is shown in Fig. 7 and Table 8. The absolute errors of XCO2 meet the accuracy of 1 ppm and remain almost stable for different pulse averaging numbers. It confirms that IPDA LIDAR’s receiving system can achieve a measurement accuracy of 1ppm. The STDs are around 1ppm with 148-shot moving average and almost 0.6 ppm with 296-shot moving average. They show that the system STD can remain stable within the spatial resolution. The accuracy of the IPDA LIDAR receiving system is therefore confirmed and the calibration system performance is validated.
Fig. 7. (a) The inversion results XCO2 of 148-shot moving average after bias correction. (b) The inversion results XCO2 of 296-shot moving average after bias correction.
Table 8. Results of calibration experiments of IPDA receiver system.
3.3 System stability analysis
For the absorption cell with CO2 at a fixed pressure of 439 hPa, the Allan deviation of the inversion results was analyzed. Figure 8(a) shows the XCO2 inversion results for a single-shot calculated for 800 s, and Fig. 8(b) is the corresponding Allan deviation. The corresponding Allan deviation for 7 s, 14 s, and 100 s are approximately 1.07 ppm, 0.78 ppm, and 0.25 ppm, respectively. The Allan deviation decreases with time, and the LIDAR receiving system and calibration system are validated to operate stably over a long period.
Fig. 8. (a) The inversion results XCO2 of single-shot after bias correction. (b) Allan deviation of the inversion results.
The error of the calibration system, regardless of the ACDL IPDA system itself, mainly comes from the optical path. The optical path affects the online and offline beam pointing fluctuations, which results in different transmissions through the filter before the detector. And the error analysis of IPDA LIDAR itself has been analyzed in detail in Ref. [12]. The STD increases from 0.74 ppm to ∼1 ppm (148-shot moving average). Compared to the zero pressure state of the absorption cell and the state with a certain amount of CO2, the airflow in the absorption cell also causes jitter in the beam pointing. In general, the calibration system can meet the measurement accuracy requirements.
4. IPDA LIDAR system calibration experiment
4.1 LIDAR system calibration experimental equipment
After testing the performance of the receiving system, it’s necessary to validate the performance of the entire IPDA LIDAR system using the CO2 absorption cell setup. A relative calibration experiment for the whole IPDA LIDAR system was conducted. Figure 9 shows the schematic diagram of the IPDA LIDAR calibration system. in which reflecting mirror 1 (RM1) is a total-reflection mirror, RM2 is an anti-reflection (AR) mirror, and RM3 is a high-reflection (HR) mirror. The beam splitter (BSM) divides the pulsed laser into two parts. One part enters the absorption cell through RM1, RM2, and RM3 to simulate the absorption of CO2 in the atmosphere, and the other passes through the IS for transmitting energy picking. The output of the IS is delivered via a 1 km multimode fiber (MM fiber), which produced a 5 µs delay for monitoring pulse to separate the echo and monitoring pulse in time. Both optical signals are received by the same APD detector of the LIDAR system. In the LIDAR system, the IS for energy monitoring is devoted to reducing the impact of pointing jitter and speckle, and an error of less than 0.2 ppm is validated [20,35].
Fig. 9. Schematic diagram of IPDA LIDAR calibration system. LA, light absorber; RM1, 2, 3, 4, 5, reflecting mirror; MM fiber, 1 km multimode fiber; IS, integrating sphere; BSM, beam splitter mirror.
4.2 LIDAR system calibration experiment results
First, the theoretical XCO2 based on the actual pressure was calculated. Table 9 shows the relationship between the charged pressure and the theoretically corresponding equivalent XCO2. The original four pulse signals are shown in Fig. 10.
Table 9. The actual pressure and corresponding theoretical XCO2 at 1572.024 nm and 1572.085 nm.
Fig. 11. (a) The inversion results XCO2 of 148-shot moving average after bias correction. (b) The inversion results XCO2 of 296-shot moving average after bias correction.
PPM and AVS methods are adopted to invert the equivalent XCO2. The beam pointing is adjusted to be constant in the experiment and monitored in real-time by other optical systems, so the bias under zero pressure acts as a fixed bias. The statistics of the final inversion results of 148-shot moving average and 296-shot moving average are shown in Fig. 11. The inversed XCO2 increases with the increasing CO2 pressure in the absorption cell equipment. The bias and STDs are counted in the XCO2 inversion results and shown in Table 10. For the CO2 absorption cell charged with different pressures, the differences between the inversion results and the simulated theoretical values are less than 1 ppm. Under the accuracy requirements of 50 km resolution (148 shots average) on the land and 100 km resolution (296 shots average) on the ocean, it can be satisfied.
Table 10. Results of calibration experiments of IPDA LIDAR system.
Comparing the calibration results of the receiving system and the LIDAR system, they have in common the absolute error of less than 1ppm, and the STD of 148-shot moving average is almost close to 1 ppm. The absolute bias verifies the absorption cell’s ability as a standard calibration device and the IPDA LIDAR measurement accuracy. STDs are less than 1.2 ppm with 148-pulse averaged. The main reason for the different STDs is the effect of the spatial optical path. The mirrors used in the calibration system cause inevitably stray light interference, which affects the stability of the beam pointing. In calibration experiments, it is important to reduce the parasitic laser pulse interference.
4.3 System stability analysis
For the absorption cell with CO2 at a fixed pressure of 467.2 hPa, the Allan deviation of the XCO2 inversion result was analyzed. As the results in Fig. 12(b) show, the Allan deviation decreases monotonically, indicating that the calibration system can maintain stability over time. The corresponding Allan deviations for 7 s, 14 s, and 100 s are 1.27 ppm, 0.91 ppm, and 0.35 ppm, respectively.
Fig. 12. (a) The inversion results XCO2 of single-shot after bias correction. (b) Allan variance of the inversion results XCO2.
The absolute errors of both calibration systems are less than 1ppm. It proved that the calibration systems based on a CO2 absorption cell with changed pressure functioned with high accuracy. In addition, it can be an effective setup for IPDA LIDAR calibration on the ground. The spatial optical path of the calibration system needs to be further optimized to reduce the jitter of the beam pointing.
5. Conclusions
In this study, a specially-customized CO2 absorption cell in the laboratory was utilized to calibrate and validate the performance of the spaceborne IPDA LIDAR. It can simulate the atmospheric CO2 absorption of the spaceborne platform. Using the CO2 absorption cell, the calibration experiments for the receiving system and the entire IPDA LIDAR system were constructed. The measurement concentrations set in the experiment ranged from 400 to 415 ppm with a step size of 5 ppm. By establishing the relationship between XCO2 concentrations and the pressure inside the CO2 absorption cell, the performance of IPDA LIDAR was analyzed.
In the calibration experiment of the receiving system, an optical double-pulse signal simulator is used to simulate a 20 Hz double-pulse laser. The equivalent XCO2 concentration that acted as system bias is 12.53 ppm. The STDs are 0.74 ppm and 0.59 ppm with 148-shot and 296-shot moving averages for the experiments, respectively. The PPM and AVS methods were utilized in inversions. The inversion results show that the absolute bias between the inversion results and the theoretical values is less than 1 ppm for all measurements. The STDs are less than 1.1 ppm and 0.8 ppm using 148-shot and 296-shot moving averages, respectively. The XCO2 Allan deviation calculated for 100 s is 0.25 ppm, which proves the long-term stability of the IPDA LIDAR receiving system.
In the calibration experiment of the entire IPDA LIDAR system, the experimental results of different CO2 concentrations were validated. The absolute errors are less than 1 ppm for all measurements, and the STDs are less than 1.2 ppm and 0.9 ppm using 148-shot and 296-shot moving averages, respectively. The XCO2 Allan deviation for an accumulation time of 100 s is validated to be 0.35 ppm for the ACDL IPDA system.
The accuracy and long-term stability of the IPDA LIDAR system used for XCO2 measurements are validated. In addition, a CO2 absorption cell was creatively designed and customized to simulate the CO2 absorption of the spaceborne platform, which can provide a new method IPDA LIDAR of calibration for relevant researchers. In future studies, we plan to reduce the influence of beam pointing in the spatial optical path of the experimental system.
Funding
ACDL LIDAR project; China National Space Administration.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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