Airborne Measurements for Environmental Research E-Book
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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Serie: Wiley Series in Atmospheric Physics and Remote Sensing
- Sprache: Englisch
This first comprehensive review of airborne measurement principles covers all atmospheric components and surface parameters. It describes the common techniques to characterize aerosol particles and cloud/precipitation elements, while also explaining radiation quantities and pertinent hyperspectral and active remote sensing measurement techniques along the way. As a result, the major principles of operation are introduced and exemplified using specific instruments, treating both classic and emerging measurement techniques.
The two editors head an international community of eminent scientists, all of them accepted and experienced specialists in their field, who help readers to understand specific problems related to airborne research, such as immanent uncertainties and limitations. They also provide guidance on the suitability of instruments to measure certain parameters and to select the correct type of device.
While primarily intended for climate, geophysical and atmospheric researchers, its relevance to solar system objects makes this work equally appealing to astronomers studying atmospheres of solar system bodies with telescopes and space probes.
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Veröffentlichungsjahr: 2013
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Table of Contents
Related Titles
Title Page
Copyright
Preface
A Tribute to Dr. Robert Knollenberg
List of Contributors
Chapter 1: Introduction to Airborne Measurements of the Earth Atmosphere and Surface
Chapter 2: Measurement of Aircraft State and Thermodynamic and Dynamic Variables
2.1 Introduction
2.2 Historical
2.3 Aircraft State Variables
2.4 Static Air Pressure
2.5 Static Air Temperature
2.6 Water Vapor Measurements
2.7 Three-Dimensional Wind Vector
2.8 Small-Scale Turbulence
2.9 Flux Measurements
Chapter 3: In Situ Trace Gas Measurements
3.1 Introduction
3.2 Historical and Rationale
3.3 Aircraft Inlets for Trace Gases
3.4 Examples of Recent Airborne Missions
3.5 Optical In Situ Techniques
3.6 Chemical Ionization Mass Spectrometry
3.7 Chemical Conversion Techniques
3.8 Whole Air Sampler and Chromatographic Techniques
Chapter 4: In Situ Measurements of Aerosol Particles
4.1 Introduction
4.2 Aerosol Particle Number Concentration
4.3 Aerosol Particle Size Distribution
4.4 Chemical Composition of Aerosol Particles
4.5 Aerosol Optical Properties
4.6 CCN and IN
4.7 Challenges and Emerging Techniques
Chapter 5: In Situ Measurements of Cloud and Precipitation Particles
5.1 Introduction
5.2 Impaction and Replication
5.3 Single-Particle Size and Morphology Measurements
5.4 Integral Properties of an Ensemble of Particles
5.5 Data Analysis
5.6 Emerging Technologies
Acknowledgments
Chapter 6: Aerosol and Cloud Particle Sampling
6.1 Introduction
6.2 Aircraft Influence
6.3 Aerosol Particle Sampling
6.4 Cloud Particle Sampling
6.5 Summary and Guidelines
Chapter 7: Atmospheric Radiation Measurements
7.1 Motivation
7.2 Fundamentals
7.3 Airborne Instruments for Solar Radiation
7.4 Terrestrial Radiation Measurements from Aircraft
Chapter 8: Hyperspectral Remote Sensing
8.1 Introduction
8.2 Definition
8.3 History
8.4 Sensor Principles
8.5 HRS Sensors
8.6 Potential and Applications
8.7 Planning of an HRS Mission
8.8 Spectrally Based Information
8.9 Data Analysis
8.10 Sensor Calibration
8.11 Summary and Conclusion
Chapter 9: LIDAR and RADAR Observations
9.1 Historical
9.2 Introduction
9.3 Principles of LIDAR and RADAR Remote Sensing
9.4 LIDAR Atmospheric Observations and Related Systems
9.5 Cloud and Precipitation Observations with RADAR
9.6 Results of Airborne RADAR Observations–Some Examples
9.7 Parameters Derived from Combined Use of LIDAR and RADAR
9.8 Conclusion and Perspectives
Acknowledgments
Appendix A: Supplementary Online Material
A.1 Measuring the Three-Dimensional Wind Vector Using a Five-Hole Probe
A.2 Small-Scale Turbulence
A.3 Laser Doppler Velocimetry: Double Doppler Shift and Beats
A.4 Scattering and Extinction of Electromagnetic Radiation by Particles
A.5 LIDAR and RADAR Observations
A.6 Processing Toolbox
List of Abbreviations
Constants
Greek
Latin
References
Index
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The Editors
Dr. Manfred Wendisch
Leipzig Institute for Meteorology (LIM)
Leipzig, Germany
Dr. Jean-Louis Brenguier
Météo-France, CNRM, GMEI
EUFAR
Toulouse, France
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Cover Picture
Photo taken near the Caribbean island of Antigua from the University of Wyoming King Air research aircraft during the Rain in Cumulus over the Ocean (RICO) project (funded by the US National Science Foundation). Courtesy of Gabor Vali.
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany
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Preface
This book summarizes the knowledge of international experts in airborne measurements from 13 countries, which they have developed over many years of field experiments and application to environmental research. The book is produced within the framework of the European Facility for Airborne Research (EUFAR, http://www.eufar.net/). EUFAR is a research infrastructure network supported since 2000 by the European Commission, as part of the research infrastructures integration program; see the respective web page at http://cordis.europa.eu/fp7/capacities/home_en.html.
One of the EUFAR Networking Activities is dedicated to Expert Working Groups (EWGs), which facilitate cross-disciplinary fertilizations and a wider sharing of knowledge and technologies between academia and industry in the field of airborne research. Over the past 10 years, numerous workshops have been organized by the EWGs addressing technical, logistic, and scientific issues specific to airborne research for the environment. From the beginning, these workshops involved international experts; however, an ever increasing number of scientists from outside the European airborne science community became involved and played an active role. Thus, the EWGs within EUFAR have become a truly international collaborative effort and, as a consequence, the workshops had a continuously increasing impact on defining research foci of future international airborne research.
The EUFAR EWGs currently publish workshop reports and recommendations (i) to aircraft operators on best practice and common protocols for operation of airborne instruments, (ii) to scientific users on best usage and interpretation of the collected data, and (iii) to the research institutions on future challenges in airborne measurements. To ensure legacy of this accumulated knowledge, this book summarizes the major outcome of the EWG discussions on the current status of airborne instrumentation. The book has been designed to provide an extensive overview of existing and emerging airborne measurement principles and techniques. Furthermore, the book analyzes problems, limitations, and mitigation approaches specific to airborne research to explore the environment.
The target audience of the book is not only experienced researchers but also graduate students, the book intends to attract to this exciting scientific field. Also university teachers, scientists experienced in related fields and looking for additional airborne data, for example, for validation or analysis of their own measurements, modelers, and project managers will find a concise overview of airborne scientific instrumentation to explore atmospheric and Earth's surface properties in this book.
Chapter 1 examines the strengths and weaknesses of airborne measurements. The subsequent Chapter 2 deals with the description of instruments to measure aircraft state parameters and basic thermodynamic and dynamic variables of the atmosphere, such as static air pressure, temperature, water vapor, wind vector, turbulence, and fluxes. The next three chapters consider in situ measurements of gaseous and particulate atmospheric constituents (Chapters 3–5). Chemical instruments to measure gaseous atmospheric components are introduced in Chapter 3, whereas the instrumentation for particulate atmospheric constituents is described in Chapters 4 (aerosol particles) and 5 (cloud and precipitation particles). Special problems associated with airborne particle sampling (aerosol and cloud/precipitation particles) are discussed in Chapter 6. The following two chapters deal with airborne radiation measurements (Chapter 7) and with techniques for passive remote sensing of the Earth's surface (Chapter 8). The most commonly applied airborne active remote sensing techniques are introduced in Chapter 9. An extensive, albeit not complete, list of references the reader may consult for airborne instrumentation is given at the end of the book. Furthermore, some supplementary material has been compiled, which is not printed but available from the publisher's Web site.
We are very grateful to the European Commission, Research Infrastructure Unit, for its financial support to EUFAR, more specifically for the organization of expert workshops and the preparation of this book. We also acknowledge the support of the national research organizations from Europe and the United States, which are supporting the 91 scientific experts contributing to the book.
We particularly appreciate the considerable efforts of Ulrich Schumann and David W. Fahey to organize and steer the review process for the book; we also acknowledge their useful comments and suggestions. Before publication, the book was peer-reviewed by external experts, which has contributed to improve the quality of the book significantly. We explicitly thank the external reviewers of the book listed in alphabetic order: Charles Brock, Peter Gege, Jim Haywood, Dwayne E. Heard, Jost Heintzenberg, Robert L. Herman, Lutz Hirsch, Andreas Hofzumahaus, Peter Hoor, Ruprecht Jaenicke, Greg McFarquhar, Matthew McGill, Ottmar Mühler, Daniel Lack, George Leblanc, Hanna Pawlowska, Tom Ryerson, Johannes Schneider, Patrick J. Sheridan, Geraint Vaughan, Peter Vörsmann, and Elliot Weinstock. Technical editor Dagmar Rosenow led many of the thankless but necessary tasks to pull this book together. We are grateful for her talents and dedication, without which the book could not have been completed. We also thank Matt Freer and Frank Werner for their help with editing the text and figures; the students Kathrin Gatzsche and Marcus Kundisch from the Leipzig Institute for Meteorology (LIM) of the University of Leipzig were of great help in compiling the extensive bibliography. Furthermore, we would like to list the leading authors of the chapters emphasizing their active role in writing this book.
Chapter 1: Ulrich Schumann, David W. Fahey, Manfred Wendisch, and
Jean-Louis Brenguier
Chapter 2: Jens Bange, Marco Esposito, and Donald H. Lenschow
Chapter 3: Jim McQuaid and Hans Schlager
Chapter 4: Andreas Petzold and Paola Formenti
Chapter 5: Jean-Louis Brenguier
Chapter 6: Martina Krämer, Cynthia Twohy, and Markus Hermann
Chapter 7: Manfred Wendisch and Peter Pilewskie
Chapter 8: Eyal Ben-Dor
Chapter 9: Jacques Pelon and Gabor Vali
Leipzig and Toulouse
Dr. Manfred Wendisch
2012
Leader of EWGs
within EUFAR (Universität Leipzig)
and
Dr. Jean-Louis Brenguier
EUFAR coordinator (Météo-France)
List of Contributors
Chapter 1
Introduction to Airborne Measurements of the Earth Atmosphere and Surface
Ulrich Schumann, David W. Fahey, Manfred Wendisch, and Jean-Louis Brenguier
Aircraft have been applied very effectively in many aspects of environmental research. They are widely used to investigate the atmosphere and observe the ground visually and by measurements with instruments on board the aircraft. They allow for
The high maneuverability of aircraft allows researchers to chase atmospheric phenomena, follow their evolution, and explore their chemistry and physics from small spatial scales up to thousands of kilometers and over time scales of fractions of seconds to many hours or even days. Aircraft instruments uniquely complement remote sensing instruments by measuring many parameters that are currently not available from space- or ground-based sensors (e.g., turbulence, nanometer-sized particles, and gases without or with only low radiation absorption efficiencies, such as nitrogen monoxide). Aircraft can reach remote locations and can carry in situ as well as active and passive remote sensing instruments. Observations may be performed along streamlines or in a fully Lagrangian manner with repeated sampling of the same air mass over extended periods. Instrumented aircraft enable remote observations of the Earth surface with very high resolution and with minimum disturbances by the atmosphere between sensor and object.
Worldwide, an impressive fleet of research aircraft is available with airborne instruments designed for many applications, although further demand still exists. Traditionally, research has been performed with manned aircraft, but increasingly, unmanned aircraft are also being used. Research aircraft include stratospheric aircraft (e.g., the Russian Geophysica and NASA ER-2 and NASA Global Hawk Unmanned Aerial System), high-level jets (e.g., Gulfstream-505 aircraft at National Center for Atmospheric Research (NCAR), the High-Altitude and Long-Range Research Aircraft in Germany, and two Falcon 20 in France and Germany), large and mid-sized aircraft operating between near ground and in the lower stratosphere (e.g., the NCAR C130 in the United States, a BAe-146 in the United Kingdom, and an ATR-42 in France), and several smaller (e.g., CASA-212 in Spain and Do-228 in the United Kingdom) and low-level aircraft (e.g., Sky-Arrow in Italy and Ultra-light at the University of Karlsruhe). In addition to specialized research aircraft, commercial airliners have been equipped with inlets and instruments and have participated in measurement programs (e.g., Swiss Nitrogen Oxides and Ozone along Air Routes Project, Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft, Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container, and the forthcoming In-Service Aircraft for a Global Observing System) that have produced large amounts of climatologically relevant data on atmospheric composition and properties during many long-distance flights.
Improving access to these powerful research platforms and providing professional support and user training are objectives of leading international research institutions and agencies and they have been supported by European Facility For Airborne Research (EUFAR), in particular.
The design, integration, and operation of in situ and remote instrumentation on aircraft platforms require a number of special considerations to achieve desired performance. The first is the rapid motion of an aircraft through the atmosphere, which in contrast to other airborne platforms, such as balloons, is required for continuous aerodynamic lift. The motion changes the pressure, temperature, and flow fields near the aircraft surfaces that generally contain air sampling inlets and other openings. In situ instruments, in particular, require inlets and suitable sampling strategies. An important example is the measurement of ambient air temperature using a probe that involves contact of a temperature sensor with ambient air. In this case, the deceleration (relative to the aircraft reference frame) of gas molecules from the speed of the aircraft to stagnation conditions in the probe induces a compression heating of the order of 4 K (at 100 m/s) to 25 K (at 230 m/s). Accurate temperature measurements require careful probe calibrations and the availability of accurate airspeed and ambient pressure measurements. In clouds, phase changes reduce the temperature changes and cause strong humidity changes. Short-range, remote (e.g., infrared temperature) measurements offer an option that avoids airspeed effects.
Many aerosol instruments have been successfully designed to mount inside or outside the aircraft fuselages or on the underside of the wings. In aerosol sampling, particles below a certain size follow the flow lines around the curved fuselage and wing surfaces and make sampling inlets straightforward. Larger particles cross the flow lines due to their greater inertia, thereby complicating or simplifying sampling strategies depending on the objective. Particles approach sampling inlets with the airspeed of the aircraft. In clouds, droplets and ice particles affect inlet probe surfaces, and larger particles may shatter into many smaller ones. This effect may invalidate the intended measurements if the additional particles cannot be properly accounted for. New inlet designs have minimized these shattering effects.
Many gas and aerosol instruments are too large to be installed outside the aircraft. Therefore, an atmospheric sample must be continuously transported into the pressurized cabin or other payload area. As some gases react with or are absorbed on the walls of the sample inlet lines, special inlet materials or fast sample flow rates must be used to acquire representative samples. Similarly, aerosol particles can be lost by turbulent and inertial deposition in the inlet lines. Specialized inlet systems have been designed and implemented to provide ambient air, from outside the possibly polluted aircraft boundary layer, to a suite of sampling instruments inside the aircraft, while minimizing the loss of particles on the inlet walls.
Another consideration in the design of airborne instruments is accommodation of environmental conditions that sometimes rapidly change. For instance, some instruments are located in unpressurized payload areas and exposed to ambient pressure and temperature conditions during flight. In some cases, in descent from high altitudes and low temperatures, instruments experience rapid changes in temperature or humidity, which may lead to condensation of water vapor on optical, electronic, and other components. Pressure and temperature problems are typically avoided by the use of pressurized enclosures for critical components and heaters that control temperatures throughout a flight. In many in situ sampling instruments, special provisions are required to maintain ambient sample flows or other parameters constant in response to changes in ambient pressure between the ground and cruise altitudes. Inlet systems and instrument sampling volumes are typically sealed to avoid contamination from cabin air.
Another important consideration is aircraft turbulence encountered both in clear air and in convective cloud systems and lightning strikes. In turbulence, aircraft instruments (and crew members) are exposed to rapid and often large accelerations in all three dimensions. Vibration from turbulence or engine operation is also a concern. With the application of good materials and structural engineering principles in the design and construction phases, instruments are generally able to maintain high measurement quality under these conditions. Lightning strikes are a physical threat when sampling near convective cloud systems. These systems are of significant scientific interest because of their chemical and dynamical properties. Although lightning strikes often cause minor structural damage and can be unnerving to the crew and passengers, the aircraft systems and instrument payload are generally unaffected. After a strike, flight directors often end the scientific portion of a flight in the interests of safety and direct the aircraft to return to base for inspection.
For many measurements, precise geographical positions and three-axis orientations are needed along the flight track. Examples are measurements of the wind vector, of upward and downward irradiances, and all types of active or passive remote sensing. Accurate information on aircraft position, velocity, and translational and rotational accelerations can be provided accurately by advanced inertial systems at high frequencies (up to 100 Hz) and by global positioning systems at frequencies up to 1 Hz.
Airborne measurements require careful consideration of aviation safety. In recent years, the effort required for aircraft and instrument safety certification has increased. Aviation safety and airworthiness certification regulations have a significant impact on the development of airborne instrumentation and the planning and execution of field experiments. Like other structural components of the aircraft, airborne instruments must withstand extreme accelerations as in the case of severe turbulence or unexpected airframe loads. This requires special attention to allow installation of a comprehensive measurement system, especially in small aircraft. Research instruments may contain radioactive, explosive, flammable, toxic, or chemically active constituents that carry additional safety and regulatory requirements. Moreover, instruments mounted outside the aircraft must be able to withstand bird strikes, icing, and lightning. Less constraining, but still crucial for aircraft integration, are instrument weight, volume, and electrical power consumption. Limits for total payload weight and payload center of gravity are necessary considerations, which are often a challenge when the payload contains a number of large and heavy instruments.
An important part of carrying out airborne measurements is campaign planning. Planning includes identification of scientific objectives and key scientific questions, site selection, aircraft preparation, and preparation of instrumentation, flight templates, time lines, and the on-site decision process. Planning activities and strategies, which are usually not well represented in the scientific literature, are generally undertaken by the scientific leaders of a campaign. It is important to recognize that planning may extend many months before a campaign and that a high-quality planning effort greatly increases the likelihood of a successful campaign.
During or shortly after campaign science flights, preliminary “quick look” results and analyses often become available on board or on the ground. These analyses can be used for “in flight” modification of flight objectives or planning for subsequent flights. Quality data processing often requires postflight instrument calibration or corrections that depend on aircraft state parameters or other variables and may take months of work effort. Intercomparison flights of two or more instrumented aircraft operating in the same or equivalent air masses, sometimes wing by wing, have been found to be important for data quality checks and instrument improvements in many campaigns. Finally, instrument data sets generally must be made available to other investigators and the public following a campaign and archived in data banks. These data sets, alone or in combination with model and other observational results, provide the basis for subsequently addressing the scientific objectives and key scientific questions of a campaign in the scientific literature.
The preface introduces the objectives and chapters of this book, which address issues specific to airborne measurements. The book serves, in part, as a handbook to guide engineers and researchers involved in airborne research in the integration of airborne instrumentation, its operation in flight, and processing of acquired data. It also provides recommendations for the development of novel instrumentation and examples of successful projects to help researchers in the design of future flight campaigns. The substantial success of instrumentation on board aircraft platforms in the past decades suggests that instrumented aircraft will continue to play an important role in meeting the ongoing challenge of understanding the processes in our complex Earth system.
Chapter 2
Measurement of Aircraft State and Thermodynamic and Dynamic Variables
Jens Bange, Marco Esposito, Donald H. Lenschow, Philip R. A. Brown, Volker Dreiling, Andreas Giez, Larry Mahrt, Szymon P. Malinowski, Alfred R. Rodi, Raymond A. Shaw, Holger Siebert, Herman Smit, and Martin Zöger
2.1 Introduction
Insofar as the atmosphere is part of a giant heat engine, the most fundamental variables that must be quantified are those describing its thermodynamic state and the air motions (wind). Therefore, this chapter focuses on describing methods for measuring basic thermodynamic and dynamic variables of the atmosphere, including aspects and calibration strategies that are unique to performing such measurements from airborne platforms. However, in order to be able to analyze airborne thermodynamic and dynamic measurements, aircraft motion and attitude have to be measured as well, both for the purpose of placing measurements in an Earth coordinate system and for making corrections that depend on those factors. Therefore, this chapter starts by describing techniques to measure these aircraft state parameters.
The chapter begins with some historical context (Section 2.2), immediately followed by a description of methods for measuring the motion, position, and attitude of the airborne measurement platform itself (Section 2.3). The structure of the remainder of the chapter is organized with the following train of logic: scalar properties of the atmosphere are dealt with first, followed by vector properties, and finally, the two properties are combined in the discussion of flux measurements. The scalar properties that are of primary relevance to the thermodynamic state of the atmosphere are static air pressure (Section 2.4), atmospheric temperature (Section 2.5), and water vapor (Section 2.6). Water vapor is one of several trace gases of atmospheric relevance, but it is particularly highlighted here because of its profoundly important coupling to the atmospheric thermodynamic state (e.g., through latent heating/cooling and through infrared (IR) absorption) and the fact that water is common in the atmosphere in all three phases (gaseous, liquid, and solid, i.e., ice), with respective phase transitions. Water vapor by itself could be the subject of its own chapter, but we have chosen to keep it in the context of the other thermodynamic and dynamic variables, for example, temperature, that combine to give critical thermodynamic variables such as relative humidity and supersaturation. The treatment of the dynamic motions of the atmosphere is divided into measurement of the large-scale, three-dimensional wind vector (Section 2.7) and the measurement of smaller-scale turbulent motions (Section 2.8). The chapter culminates with a treatment of flux measurements (Section 2.9), which ultimately are responsible for the changing state of the atmosphere itself.
2.2 Historical
The history of airborne measurements for atmospheric research can be traced back to free air balloon sounding of the atmosphere. The first meteorological ascent was reported by the French physicist, Jacques Charles, on 1 December 1783 in a hydrogen balloon equipped with a barometer and a thermometer. He recorded a decrease in temperature with height and estimated the atmospheric lapse rate. Joseph Louis Gay-Lussac and Jean-Baptiste Biot made a hot-air balloon ascent in 1804 to a height of 6.4 km in an early investigation of the Earth's atmosphere and measured temperature and moisture at different heights. They reported that the composition of the atmosphere does not change with decreasing pressure (increasing altitude). Manned balloons continued to be used throughout the next couple of centuries with the obvious advantage of being able to follow an air mass and thus allowing very detailed measurements in a small volume of air, but with the disadvantage of limited sampling statistics. In the early 1930s, Heinz Lettau and Werner Schwerdtfeger made direct measurements of vertical wind velocity in the lowest 4 km of the troposphere from a balloon using a combination of a rate-of-climb meter to keep the balloon height constant and a sensitive anemometer to measure the vertical air velocity relative to the balloon. They estimated that the accuracy of their technique was better than 0.2 m s−1 (Lewis, 1997).
The use of powered aircraft for airborne measurements of atmospheric parameters goes back to at least 1911 when in Germany, Richard Assmann, the inventor of the aspirated psychrometer, motivated the aircraft designer, August Euler, to modify one of his aircraft to make upper-air soundings. The following year a meteorograph was installed in an Euler monoplane and it recorded pressure and temperature up to 1100 m altitude. Aircraft continued to be used for temperature soundings, in some cases on a daily basis, from the 1920s through the World War II. These measurements played a role in the major advances that occurred in synoptic meteorology during these years. Eventually, their routine sounding role diminished as pilot balloons and radiosondes became the standard tools for atmospheric sounding.
Thermodynamic and turbulence measurements were performed in 1936 with a Potez 540 aircraft from the French Air Force in the Puy de Sancy Mountain area (Dupont, 1938). The aircraft was equipped with an “anémoclinomètre” for the airspeed and attack and drift angles measurements, an accelerometer with three piezoelectrical channels for the vertical acceleration component, and a “météograph” for the pressure, temperature, and hygrometry measurements. Several flights were performed over the National Glider School Center to characterize turbulence and dynamic properties over the mountain site.
The use of aircraft for intensive research programs continued to expand. For example, a series of temperature and humidity soundings from aircraft in the lowest 300 m over the ocean in the fall of 1944 was used to study modification of stably stratified air along its trajectory as it passed from land to a relatively cold ocean offshore of Massachusetts, USA (Craig, 1949).
Turbulence measurements from aircraft date back to at least the early 1950s when a US Navy PBY-6A instrumented with a vertical accelerometer was used by Joanne Malkus and Andrew Bunker to estimate a “turbulence index” for cloud dynamics observations (Malkus, 1954). Later, an anemometer was combined with the vertical acceleration measurements to estimate vertical and longitudinal air velocity fluctuations, and thus to calculate vertical momentum flux (Bunker, 1955).
In the mid-1950s, a more complete turbulence measuring system was used on a McDonnell FH-1 (the first all-jet aircraft) to measure vertical velocity spectra in the planetary boundary layer. This system used either a rotating vane or a differential pressure probe mounted on a nose boom to measure the aircraft attack angle, an integrating accelerometer to measure aircraft velocity fluctuations relative to the Earth, and an integrating rate gyroscope to measure pitch angle fluctuations. By combining these measurements, fluctuations of vertical wind velocity were estimated (Lappe and Davidson, 1963).
A different approach to measuring turbulence intensity was used by MacCready (1964) starting in the early 1960s, who disregarded the long wavelength contributions to the longitudinal air velocity fluctuations by band-pass filtering the output of an airspeed sensor to estimate the turbulence dissipation from the Kolmogorov hypothesis. This provided a simple easily implemented system to provide a standardized measure of turbulence, albeit over a limited wavelength region, as well as a measure of the total turbulence energy production by equating it to the turbulence dissipation.
The next step in improving the complexity and accuracy for vertical wind velocity measurements was taken in the early 1960s in Australia, with the development of a system on a Douglas DC-3 by Telford and Warner (1962). They combined a nose-boom-mounted vane with a free gyroscope and a vertically stabilized (using signals from the free gyroscope) accelerometer. This reduced errors present in previous systems due to the varying contribution of gravity to the measured acceleration resulting from attitude angle variations. They also incorporated a fast temperature sensor and wet-bulb thermometer to measure heat and water vapor fluxes.
Afterward, an inertial navigation system (INS) was integrated, with improved accuracy and reduced drift rates, to measure the translational and rotational aircraft motions, as well as the absolute location of the aircraft. Today, GPS-based instruments are also utilized in combination with Inertial Measuring Units (IMUs) to provide a lighter and less expensive alternative to INS. In contrast, the air motion sensing systems have changed little in the past few decades and are now the limiting factor in measuring air motion.
At present, there is a remarkable variety of instrumented airborne platforms for atmospheric and environmental measurements, including high-performance jet aircraft for high-altitude and long-range measurements, smaller turboprop aircraft for intensive boundary layer measurements, armored aircraft for thunderstorm penetration, slow-moving helicopter-towed platforms for high-resolution measurements, and an emerging fleet of relatively small, remotely piloted vehicles carrying miniaturized but still highly capable instrument packages. Indeed, the airborne platforms are as varied and innovative as the instruments they carry, all matched to the specialized research objectives that drive the continuing innovation.
2.3 Aircraft State Variables
In order to place measurements into a proper geographical reference frame it is necessary to precisely measure the position and attitude of the aircraft from which measurements are made. These variables, including aircraft height or altitude, attitude (e.g., yaw, pitch, roll angles), position, and velocity, are collectively defined as the aircraft state.
2.3.1 Barometric Measurement of Aircraft Height
Hypsometric (or pressure) altitude can be estimated by an integration of the hydrostatic equation using measurements of virtual temperature Tvir and static air pressure p, assuming that the sounding is invariant as follows:
2.1
Alternatively, standard atmosphere models can be used to estimate the temperature from the pressure, which can then be integrated to obtain pressure altitude. The International Standard Atmosphere (ISA) sets the international standard (ISO, 1975). Below 30 km altitude, the ISA model is identical to that of the International Civil Aviation Organization (ICAO) and the US Standard Atmosphere, with variables as shown in Table 2.1. These standard atmospheres assume dry atmospheric conditions.
Table 2.1 ISA Standard Atmosphere Properties (base values) in the Troposphere and Stratosphere
Variation in the value of gravitational acceleration g is small. To account for this, instead of the geometric altitude, atmospheric models use geopotential height measured in geopotential meters (gpm), defined as
2.2
2.3
and
2.4
see Iribarne and Godson (1981). For the dry, tropospheric layer, using the ISA constants, we obtain
2.5
where h is obtained in gpm. In Eq. (2.5), p0 is 1013.25 hPa, corresponding to the lowest atmospheric layer in the ISA (Table 2.1). An aircraft pressure altimeter in this lowest atmospheric layer indicates the ISA altitude when the altimeter setting is 1013.25 hPa. Typically, the altimeter setting is adjusted so that the altimeter reads exactly the airport altitude on landing. The details of how altimeter setting is mechanized in an aircraft pressure altimeter can be found in Iribarne and Godson (1981).
Both the hypsometric altitude from Eq. (2.1) and the pressure altitude from Eq. (2.5) assume that there are no horizontal pressure gradients. Height measurements based on RADAR are not covered here. The sum of RADAR altitude plus the height of the terrain above sea level approximates hypsometric or pressure altitude measurements, but accurate terrain data is not available at very fine scale, and surface artifacts such as buildings can complicate that determination except, of course, over the sea. Neither of these altitude estimates is as inherently accurate as those from the Global Navigation Satellite System, as described in Section 2.3.3. For use in comparing airborne measurements with atmospheric model output, pressure or potential temperature could be less ambiguous measures of height.
2.3.2 Inertial Attitude, Velocity, and Position
2.3.2.1 System Concepts
IMUs using Newton's laws, applied to motion on a rotating planet, integrate a triad of linear accelerations to determine aircraft velocity and position. Detailed theory of operation and design criteria for navigation units based on IMUs are presented in the studies by Broxmeyer (1964) and O'Donnell (1964). The accelerometer orientation must be known to accommodate accelerations due to gravity, and this is accomplished by mounting on a stable platform.
Two main approaches are in general use: gimballed and strapdown systems. The gimballed system is typically mechanized to keep the stabilized platform containing the accelerometers level with respect to the Earths gravity, rotating as necessary to maintain verticality as the aircraft moves, incorporating the effect of changes in the gravity vector as the aircraft changes latitude and altitude. The gimbals are a set of three rings that let the platform keep the same orientation while the vehicle rotates around it. Attitude angles can then be measured directly from the gimbal orientation. The big disadvantage of this approach is the relatively high cost and mechanical complexity causing reliability challenges related to the many precision mechanical parts. The coordinate transformation between Earth-fixed and aircraft body axis systems is described by Axford (1968) and Lenschow (1972). Figure 2.1 (Axford, 1968) shows the arrangement of a gimballed system and defines the coordinate transformation variables for its use.
Figure 2.1 Sketch of gimbal system. (Source: Redrawn from Axford (1968). Copyright 1968 American Meteorological Society. Reprinted with permission.)
In strapdown systems, which comprise most of the IMUs used in atmospheric research at present, accelerometers are fixed to the aircraft, and linear and angular acceleration measurements are integrated using a model to continually compute the orientation of gravity to the vehicle axis, creating a virtual stabilized platform. Compared to the gimballed systems, strapdown systems offer lower cost and higher reliability but require higher maximum angular rate capability and higher sampling rate capability to sufficiently capture aircraft motion on a maneuvering aircraft (Barbour, 2010). IMUs integrated into an INS with a gyroscope error of 0.01° h−1 will result in a navigation error of ∼2 km h−1 of operation.
2.3.2.2 Attitude Angle Definitions
Standard definitions for the attitude angles and other motion variables can be found in ISO 1151-1 (ISO, 1985) and ISO 1151-2 (ISO, 1988). A conventional INS defines the Earth-based coordinate system to be north-east-down (NED) and the aircraft body axis system to be forward-right-down (XYZ). The transformation matrix from the body axis XYZ to the Earth-based NED system has three successive rotations that are prescribed by the order of the gimbals (roll innermost). For strapdown systems, the equations are written to emulate this gimbal order that defines the attitude angles using the Tait-Bryan sequence of rotations: (i) rotate to wings horizontal around body X (forward)-axis by roll angle (ϕ, right wing down positive); (ii) rotate to X-axis horizontal about body Y (right)-axis by pitch angle (θ, nose up positive); and (iii) rotate about Z (down)-axis to north by heading (ψ, true heading, positive from north toward east). Transforming a vector in the XYZ body axis to Earth-based NED coordinates requires the roll R, pitch P, and heading H rotation matrices
2.6
to be applied to the vector in the following order:
2.7
2.3.2.3 Gyroscopes and Accelerometers
Doebelin (1990) presents an overview of linear accelerometers and 3D gyroscopic angular displacement and angular velocity (rate) sensors. Barbour (2010) and Schmidt and Phillips (2010); Schmidt (2010) survey current inertial sensor issues and trends. Spinning electrically suspended gyroscopes (ESGs) offer the highest accuracy and stability, with the rotor supported in vacuum by an electric field, thus nearly eliminating errors caused by friction. Currently, there is an upsurge in solid-state sensors that include microelectromechanical systems (MEMS) devices, ring laser gyros (RLGs), fiber-optic gyros (FOGs), and interferometric gyros (IFOGs), which have significant cost, size, and weight advantages over spinning devices. Accelerometers are pendulous servo accelerometers, resonant vibrating beam accelerometers (VBAs), or MEMS implementations of either of these.
Table 2.2 indicates the gyro bias and accelerometer bias requirements of each class of application. In an unaided INS, initial alignment must be accomplished carefully so that the initial tilt of the system does not put a component of gravity into the horizontal accelerometers. Alignment is accomplished by tilting to zero the horizontal acceleration to establish level, and the initial heading is accomplished by establishing north by alignment with the Earth's rotation rate (0.002 ° s−1).
Table 2.2 Performance of Classes of Unaided INS
Table 2.3 shows the expected uncertainties from unaided navigation-grade INS.
Table 2.3 Accuracy of Unaided Navigation-Grade INS (Honeywell LaserRef2 SM after 6 h).
Variable
Accuracy
Position
1.5 km h
−1
Ground velocity
4.10 m s
−1
Vertical velocity
0.15 m s
−1
(baro-damped)
Pitch and roll angles
0.05°
True heading
0.2°
Source: From Honeywell (1988).
2.3.2.4 Inertial-Barometric Corrections
Unaided INS does not have sufficient information available to damp errors in the Earth-vertical coordinate. Barometric pressure can be used to limit errors in the vertical acceleration that cause unbounded drift. A third-order baro-inertial loop described by Blanchard (1971) can be used for this. Lenschow (1986) discusses the considerations for choosing the time constant for the mechanization of the loop, being a trade-off among minimizing the effect of high-frequency noise in the pressure measurement, minimizing the recovery time from errors, and improving long-term stability. A time constant of 60 s has been used for the National Center for Atmospheric Research (NCAR) aircraft.
2.3.3 Satellite Navigation by Global Navigation Satellite Systems
Global Navigation Satellite Systems (GNSS) are constellations of satellites in medium Earth orbit at heights of about 2.5 × 107 m, corresponding to an orbital period of roughly 12 s. Gleason and Gebre-Egziabher (2009) provide detailed information about GNSS methodology and expected errors. Table 2.4 lists the status of GNSS as of 2010. Receivers compatible with multiple constellations benefit from the larger number of satellites in view.
Table 2.4 Overview of Operational and Planned Global Navigation Satellite Systems
Each satellite vehicle (SV) broadcasts a precise time measurement along with its ephemeris. GPS receivers use this to determine the transit time of the signal, which is then converted to distance (called pseudorange). Satellite positions can be obtained from either the broadcast ephemeris or the more accurate ephemeris published within hours or days, which can be incorporated into postprocessing of the GNSS signals. Four (or more) pseudorange measurements are used to unambiguously compute the receiver position using triangulation. Adding more SV signals increases the accuracy of the position estimate.
2.3.3.1 GNSS Signals
The details of the coding and decoding of GNSS signals are discussed in the texts of Gleason and Gebre-Egziabher (2009); Bevly and Cobb (2010); Hofmann-Wellenhof, Lichtenegger, and Collins (2001). The US GPS provides two precision positioning signals (military-accessible P-code) on frequencies L1 (1575.42 MHz) and L2 (1227.6 MHz) and a clear acquisition signal (C/A code) on L1. A third frequency L5 (1176.5 MHz) was added in 2009. Selective availability (SA) – the intentional addition of time varying errors of up to 100 m (328 ft) to the publicly available navigation signals – was discontinued in 2001.
