CO2, CH4, and N2O are recognised as the most important greenhouse gases, the concentrations of which increase rapidly through human activities. Space-borne integrated path differential absorption lidar allows global observations at day and night over land and water surfaces in all climates. In this study we investigate potential sources of measurement errors and compare them with the scientific requirements. Our simulations reveal that moderate-size instruments in terms of telescope aperture (0.5–1.5 m) and laser average power (0.4–4 W) potentially have a low random error of the greenhouse gas column which is 0.2% for CO2 and 0.4% for CH4 for soundings at 1.6 μm, 0.4% for CO2 at 2.1 μm, 0.6% for CH4 at 2.3 μm, and 0.3% for N2O at 3.9 μm. Coherent detection instruments are generally limited by speckle noise, while direct detection instruments suffer from high detector noise using current technology. The wavelength selection in the vicinity of the absorption line is critical as it controls the height region of highest sensitivity, the temperature cross-sensitivity, and the demands on frequency stability. For CO2, an error budget of 0.08% is derived from our analysis of the sources of systematic errors. Among them, the frequency stability of ± 0.3 MHz for the laser transmitter and spectral purity of 99.9% in conjunction with a narrow-band spectral filter of 1 GHz (FWHM) are identified to be challenging instrument requirements for a direct detection CO2 system operating at 1.6 μm.

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Space-borne remote sensing of CO2, CH4, and N2O by integrated path differential absorption lidar: a sensitivity analysis

A high-performance airborne water vapor differential absorption lidar has been developed during the past years. This system uses a four-wavelength/three-absorption line measurement scheme in the 935 nm H2O absorption band to cover the whole troposphere and lower stratosphere simultaneously. Additional high spectral resolution aerosol and depolarization channels allow precise aerosol characterization. This system is intended to demonstrate a future space-borne instrument. For the first time, it realizes an output power of up to 12 W at a high wall-plug efficiency using diode-pumped solid-state lasers and nonlinear conversion techniques. Special attention was given to a rugged optical layout. This paper describes the system layout and technical realization. Key performance parameters are given for the different subsystems.

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The airborne multi-wavelength water vapor differential absorption lidar WALES: system design and performance

The global observation of profiles of the atmospheric wind speed is the highest-priority unmet need for global numerical weather prediction. Satellite Doppler lidar is the most promising candidate to meet the requirements on global wind profile observations with high vertical resolution, precision, and accuracy. The European Space Agency (ESA) decided to implement a Doppler wind lidar mission called the Atmospheric Dynamics Mission Aeolus (ADM-Aeolus) to demonstrate the potential of the Doppler lidar technology and the expected impact on numerical weather forecasting. An airborne prototype of the instrument on ADM-Aeolus was developed to validate the instrument concept and retrieval algorithms with realistic atmospheric observations before the satellite launch. It is the first airborne direct-detection Doppler lidar for atmospheric observations, and it is operating at an ultraviolet wavelength of 355 nm. The optical design is described in detail, including the single-frequency pulsed laser and th...

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The Airborne Demonstrator for the Direct-Detection Doppler Wind Lidar ALADIN on ADM-Aeolus. Part I: Instrument Design and Comparison to Satellite Instrument

Active remote sensing is a promising technique to close the gaps that exist in global measurement of atmospheric carbon dioxide sources, sinks and fluxes. Several approaches are currently under development. Here, an experimental setup of an integrated path differential absorption lidar (IPDA) is presented, operating at 1.57 μm using direct detection. An injection seeded KTP-OPO system pumped by a Nd:YAG laser serves as the transmitter. The seed laser is actively stabilized by means of a CO2 reference cell. The line-narrowed OPO radiation yields a high spectral purity, which is measured by means of a long path absorption cell. First measurements of diurnal variations of the atmospheric CO2 mixing ratio using a topographic target were performed and show good agreement compared to simultaneously taken measurements of an in situ device. A further result is that the required power reference measurement of each laser pulse in combination with the spatial beam quality is a critical point of this method. The system described can serve as a testbed for further investigations of special features of the IPDA technique.

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Development of an OPO system at 1.57 μm for integrated path DIAL measurement of atmospheric carbon dioxide

Measurements of winds throughout the atmosphere are crucial for both numerical weather prediction and for climate studies. Nevertheless, observations of height profiles of the global wind field are lacking. Thus the European Space Agency decided to establish a Doppler wind lidar mission named Atmospheric Dynamics Mission ADM-Aeolus. The mission will provide vertical profiles from ground up to the lower stratosphere of one component of the horizontal wind vector, which is perpendicular to the flight track. Pre-launch validation of the instrument with an airborne prototype was performed by DLR.

The Spaceborne Wind Lidar Mission ADM-Aeolus

The design of a 50 Hz single longitudinal mode, diode-pumped and frequency-tripled Nd:YAG master oscillator power amplifier is described, and the first measurements of output parameters are presented. The laser oscillator is injection-seeded by a tuneable monolithic Nd:YAG ring laser and frequency stabilized by minimising the Q-switch build-up time. The laser system will be an integral part of an airborne instrument demonstrator for a first satellite based Doppler wind lidar to measure vertical profiles of one component of the atmospheric wind vector. This paper focuses on the investigation of the frequency jitter and the linewidth of the laser, which are measured on a pulse-to-pulse basis. For this purpose a compact, high accuracy beat frequency monitoring system has been developed at DLR. By operating the amplifier stage at half the repetition rate (50 Hz) of the oscillator, we could reduce the frequency stability from 10 MHz (rms) to 1.3 MHz (rms) (over a 14 s period). We have determined a mean linewidth of 15 MHz (FWHM) at 1064 nm. These measured laser parameters enable wind velocity measurements in the atmosphere (0–15 km) at an accuracy of 1 to 2 m/s.

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Torsten Schröder

Papers

Frequency jitter and spectral width of an injection-seeded Q-switched Nd:YAG laser for a Doppler wind lidar