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

A review of remote sensing technology for lower-tropospheric thermodynamic (TD) profiling is presented with focus on high accuracy and high temporal-vertical resolution. The contributions of these instruments to the understanding of the Earth system are assessed with respect to radiative transfer, land-surface-atmosphere feedback, convection initiation, and data assimilation. We demonstrate that for progress in weather and climate research, TD profilers are essential. These observational systems must resolve gradients of humidity and temperature in the stable or unstable atmospheric surface layer close to the ground, in the mixed layer, in the interfacial layer – usually characterized by an inversion – and the lower troposphere. A thorough analysis of the current observing systems is performed revealing significant gaps that must be addressed to fulfill existing needs. We analyze whether current and future passive and active remote sensing systems can close these gaps. A methodological analysis and demonstration of measurement capabilities with respect to bias and precision is executed both for passive and active remote sensing including passive infrared and microwave spectroscopy, the global positioning system as well as water-vapor and temperature Raman lidar and water-vapor differential absorption lidar. Whereas passive remote sensing systems are already mature with respect to operational applications, active more » remote sensing systems require further engineering to become operational in networks. However, active remote sensing systems provide a smaller bias as well as higher temporal and vertical resolutions. For a suitable mesoscale network design, TD profiler system developments should be intensified and dedicated observing system simulation experiments should be performed. « less

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014RG000476

A review of the remote sensing of lower-tropospheric thermodynamic profiles and its indispensable role for the understanding and the simulation of water and energy cycles

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...

/pdf/the-airborne-demonstrator-for-the-direct-detection-doppler-1ke8occ5y4.pdf

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

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.

/pdf/frequency-jitter-and-spectral-width-of-an-injection-seeded-q-7btspko6re.pdf

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

We report on the design and performance of the laser deployed in the airborne demonstrator Doppler wind lidar for the Aeolus mission of the European Space Agency (ESA). The all-solid-state, diode-pumped and frequency-tripled Nd:YAG laser is realized as a master oscillator power amplifier (MOPA) system, generating 60 mJ of single-frequency pulses at 355 nm wavelength, 50 Hz repetition rate and 20 ns pulse duration. For the measurement of the Doppler frequency shift over several accumulated laser shots, the frequency stability of the laser is of crucial importance. Injection-seeding, in combination with an active cavity control based on the Ramp-Delay-Fire technique, provides a pulse-to-pulse frequency stability of 0.25 MHz measured at 1064 nm under laboratory conditions. This value increases to 0.31 MHz for airborne operation in a vibration environment that has been characterized by multiple acceleration sensors during different flight conditions. In addition, a pure Ramp-Fire setting was tested for comparison leading to a frequency stability of 0.16 MHz both in airborne operation and on ground. The laser cavity control electronics also have to provide a trigger signal for the lidar detection electronics, about 60 μs prior to the expected laser pulse emission and with high timing stability. An in-flight timing stability of below 100 ns was measured decreasing to 20 ns for a shorter pre-trigger time of 10 μs.

Frequency and timing stability of an airborne injection-seeded Nd:YAG laser system for direct-detection wind lidar.

The Methane Remote Sensing LIDAR Mission (MERLIN) is a joint French-German cooperation on the development, launch and operation of a climate monitoring satellite, executed by the French Space Agency CNES and the German DLR Space Administration. It is focused on global measurements of the spatial and temporal gradients of atmospheric Methane (CH4) with a precision and accuracy sufficient to constrain Methane fluxes significantly better than with the current observation network. Merlin is a LIDAR Instrument using the IPDA principle. This instrument principle relies on the different absorption of the laser signal by atmospheric Methane at two laser wavelengths – on-line and off-line – both around 1.645 μm, reflected by the Earth surface or by cloud tops. The attenuation is strong at the on-line wavelength; the off-line “reference” wavelength is selected to be only marginally affected by Methane absorption. Being an active instrument with its own light source, the MERLIN LIDAR Instrument does not have to rely on sun illumination of the observed areas and can therefore continuously operate over the orbit. Airbus DS GmbH was selected by the German DLR Space Administration as the industrial Prime Contractor for the Mission Phase C/D, to build the MERLIN Payload, which is the first realization of such an instrument for space in Europe. This presentation will concentrate on the Architecture and the Design of the MERLIN Payload developed during the ongoing Mission Phase C. Further details of the instrument development status will be shown by an overview of the current hardware and design status of the major subsystems.

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11180/1118028/MERLIN--overview-of-the-design-status-of-the-lidar/10.1117/12.2535999.pdf

MERLIN: overview of the design status of the lidar Instrument

This paper reports on a workshop which took place on April 08/09, 2002 at the University of Hohenheim, Stuttgart, Germany. During this workshop, laser and lidar experts as well as experts in atmospheric sciences came together. The goals of this workshop were to establish an interdisciplinary network on lidar research, to develop operational reference systems for water vapor and wind lidar measurements and to demonstrate the significant impact of water vapor and wind lidar systems on climate and weather research. The main result was the setup of an interdisciplinary expert network Lidar research water vapor and wind. This network is responding to large gaps in our knowledge of the global water vapor and wind distribution and shall improve weather and climate research. The network has mainly the following objectives: 1) The development of high-performance, high-power laser transmitters for wind and water vapor measurements based on novel pump laser designs, 2) research on efficient frequency conversion techniques of the high-power pump light into wavelengths required for water vapor and wind lidar systems, 3) the development of an operational reference water vapor differential absorption lidar system, also as a demonstrator for space borne missions like the Water Vapor Lidar Experiment in Space (WALES), 4) the performance of observation system simulation experiments (OSSEs) in order to demonstrate the impact of lidar data on nowcasting and short-range weather forecast, 5) the extension of the application of current and future lidar data in combination with other instruments for process studies, 6) the design of future field studies including a new generation of remote sensing systems. The network shall investigate key questions in atmospheric sciences, particularly, the role of water vapor in the Earth's weather and climate system by the application of lidar systems with unprecedented accuracy, resolution and range.

Lidar research network Water Vapor and wind

The Methane Remote Sensing LIDAR Mission (MERLIN) is a joint French–German cooperation on the development, launch and operation of a climate monitoring satellite, executed by the French Space Agency CNES and the German DLR Space Administration. It is focused on global measurements of the spatial and temporal gradients of atmospheric methane (CH4) with a precision and accuracy sufficient to constrain methane fluxes significantly better than with the current observation network. Airbus Defence and Space GmbH was selected by the German DLR Space Administration as the industrial prime contractor for the mission phase C/D, to build the MERLIN Payload, which is the first realization of such an instrument for space. This presentation will concentrate on the architecture and the optical design of the MERLIN Payload, which ensures reliable, high-performance operation of the bi-static DIAL, consisting of separate transmitter (Tx) and receiver paths (Rx). The MERLIN satellite is a secondary passenger payload on the launcher. As such, the satellite places many constraints on the instrument, pertaining to the power, mass and volume allowable. The available resources force the MERLIN instrument to have passive thermal control while necessitating a very compact design due to the demanding envelope constraints. This creates a large operational temperature range with thermal gradients on the structure, requiring an extremely robust optical design in a compact envelope. The robust optical design, for the Rx and Tx paths, employs several passive measures and an active pointing control for Rx and Tx co-alignment. Further details of the instrument development status during the ongoing phase C will be shown by an overview of the current hardware and design status of the major subsystems.

Christian Wührer

Papers

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

Frequency and timing stability of an airborne injection-seeded Nd:YAG laser system for direct-detection wind lidar.

MERLIN: overview of the design status of the lidar Instrument

Lidar research network Water Vapor and wind

MERLIN: design of an IPDA LIDAR instrument