63 research outputs found
Messung von CO2-Säulengehalten in der Atmosphäre mit Lidar-Methoden
Eine der fundamentalen Fragen der aktuellen Wissenschaft ist die Entschlüsselung des globalen Kohlenstoff-Kreislaufs insbesondere vor dem Hintergrund des anthropogenen Einflusses durch die Emission großer Mengen von CO2 in die Atmosphäre. Aktuell gibt es keine Messsysteme, die die nötige Messgenauigkeit zur Bestimmung von Quellen, Senken und Flüssen atmosphärischen Kohlendioxids erreichen.
Im Rahmen der vorliegenden Arbeit wurde die Methode der aktiven Fernerkundung atmosphärischen Kohlendioxids mittels Lidar theoretisch und experimentell untersucht. Im Vordergrund stand dabei die Entwicklung eines Differential-Absorptions-Lidars (DIAL). Der Aufbau und die Charakterisierung einer geeigneten Laserlichtquelle sowie einer präzisen Wellenlängen-Stabilisierung stellten zentrale Aspekte dar. Für ein einsetzbares System mussten darüber hinaus ein Empfangssystem, eine Steuerung und Datenerfassung sowie Systeme zur Analyse der Strahlqualität aufgebaut werden. Da die atmosphärische Rückstreuung im verwendeten Wellenlängenbereich bei 1,57 µm gering ist, wurde das System in der DIAL-Variante IPDA (Integrated Path Differential Absorption) verwirklicht, das statt der atmosphärischen Rückstreuung den Laser-Reflex einer festen Oberfläche verwendet, womit der CO2-Säulengehalt entlang der Messstrecke bestimmt wird. Als Laserlichtquelle wurde ein Nd:YAG-gepumptes KTP-OPO-System mit alternierendem Injection Seeding durch zwei DFB-Laserdioden verwirklicht.
Sensitivitätsstudien anhand von numerischen Simulationen lieferten Informationen über geeignete Messwellenlängen, über die maximal erreichbare Messgenauigkeit und über Anforderungen an das Messsystem. Es ergab sich, dass sich im Hinblick auf die untersuchten Aspekte die geforderte Genauigkeit im Subprozent-Bereich erreichen lässt, wobei höchste Anforderungen an die spektralen Eigenschaften der Laserlichtquelle gestellt werden.
In Testmessungen wurde die erreichbare Messgenauigkeit des entwickelten Labor-Prototypen untersucht. Es wurden Stärken und Schwächen des aufgebauten Systems und der Messmethode herausgearbeitet. So zeigte sich insbesondere, dass die beim IPDA-Messprinzip notwendige Leistungsreferenz-Messung sowie die räumliche Laserstrahl-Qualität als kritisch zu betrachten sind. Bei direkten Vergleichsmessungen mit einem In-situ-Gerät erreichte der Prototyp auf Anhieb eine Standardabweichung von 2 % bei einem Bias von unter 10 %. Die erreichte spektrale Qualität der Laserstrahlung übertraf die aufgestellten Anforderungen.
Diese Arbeit liefert einen Beitrag zum Fortschritt auf dem Gebiet der Entwicklung aktiver Systeme zur Fernerkundung atmosphärischen Kohlendioxids
Determination of the emission rates of CO2 point sources with airborne lidar
We report on CO2 emissions of a coal-fired power plant derived from flight measurements performed with the IPDA lidar CHARM-F during the CoMet campaign in spring 2018. Despite the results being in broad agreement with reported emissions, we observe strong variations between successive flyovers. Using a high-resolution large eddy simulation, we identify strong atmospheric turbulence as the cause for the variations and recommend more favorable measurement conditions for future campaign planning
Analysis of Range Measurements From a Pulsed Airborne CO2 Integrated Path Differential Absorption Lidar
Determining the CO2 column abundance from an integrated path differential absorption (IPDA) lidar requires accurate
knowledge of the range to the scattering surface, i.e., the
column height. We have adapted and tested a ranging algorithm for the airborne IPDA CO2 lidar designed at the NASA Goddard Space Flight Center, and have evaluated its accuracy and precision. We applied a quasi-maximum-likelihood method, using cross correlation, to estimate the range from the lidar’s 1-μs-wide echo pulses. The objective was to show that the use of these temporally long laser pulses allows the determination of the optical path length with required precision. We analyzed airborne measurements made in August 2009 over the Chesapeake Bay near the Eastern Shore of Virginia. The results indicate a ranging recision of better than 3 m, which is sufficient for airborne and space-based retrievals of CO2 column concentration
Development of an OPO system at 1.57 µm for integrated path DIAL measurement of atmospheric carbon dioxide
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 beamquality 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
Pointing Verification Method for Spaceborne Lidars
High precision acquisition of atmospheric parameters from the air or space by means of lidar requires accurate knowledge of laser pointing. Discrepancies between the assumed and actual pointing can introduce large errors due to the Doppler effect or a wrongly assumed air pressure at ground level. In this paper, a method for precisely quantifying these discrepancies for airborne and spaceborne lidar systems is presented. The method is based on the comparison of ground elevations derived from the lidar ranging data with high-resolution topography data obtained from a digital elevation model and allows for the derivation of the lateral and longitudinal deviation of the laser beam propagation direction. The applicability of the technique is demonstrated by using experimental data from an airborne lidar system, confirming that geo-referencing of the lidar ground spot trace with an uncertainty of less than 10 m with respect to the used digital elevation model (DEM) can be obtained
Airborne lidar reflectance measurements at 1.57 �¼m in support of the A-SCOPE mission for atmospheric CO2
The characteristics of the lidar reflectance of the
Earth�s surface is an important issue for the IPDA lidar technique (integrated path differential absorption lidar) which is the proposed method for the spaceborne measurement of atmospheric carbon dioxide within the framework of ESA�s ASCOPE project. Both, the absolute reflectance of the ground
and its variations have an impact on the measurement sensitivity. The first aspect influences the instrument�s signal
to noise ratio, the second one can lead to retrieval errors, if
the ground reflectance changes are strong on small scales.
The investigation of the latter is the main purpose of this
study. Airborne measurements of the lidar ground reflectance
at 1.57�¼m wavelength were performed in Central and Western
Europe, including many typical land surface coverages
as well as the open sea. The analyses of the data show,
that the lidar ground reflectance is highly variable on a wide
range of spatial scales. However, by means of the assumption
of laser footprints in the order of several tens of meters, as
planned for spaceborne systems, and by means of an averaging of the data it was shown, that this specific retrieval error is well below 1 ppm (CO2 column mixing ratio), and so compatible with the sensitivity requirements of spaceborne CO2 measurements. Several approaches for upscaling the data in terms of the consideration of larger laser footprints, compared to the one used here, are shown and discussed. Furthermore, the collected data are compared to MODIS ground reflectance data
Towards a greenhouse gas lidar in space
Highly accurate measurements of atmospheric carbon dioxide (CO2) and methane (CH4) by a space-borne lidar will help to substantially improve knowledge of greenhouse gas fluxes. The method of integrated-path differential-absorption lidar for total column measurements has proven to be a suitable means for CH4 detection in natural gas leak surveillance and active remote sensing of CO2. This pioneering work facilitated the instrument development of an advanced greenhouse gas lidar on HALO and set the stage for the development of a CH4-lidar in space instrument foreseen in the Franco-German climate mission MERLIN
- …