In-flight Validation Research Articles

IN-FLIGHT AND ON-ORBIT CALIBRATION OF LUNAR IMAGING SPECTROMETERS – A REVIEW

Lunar imaging spectrometers play a leading role in the study of the Moon’s mineral composition. The accuracy and reliability of the acquired data depend on the calibration process. Key stages of it include laboratory calibration, in-flight validation, on-orbit calibration, and cross-calibration. During these stages, a variety of techniques and methods are used for calibration with the aim of achieving higher radiometric accuracy when recording the spectral reflective characteristics of materials in scenes from the lunar surface. These methods include capturing well-known calibration astronomical targets and calibration sites, comparing data from previous lunar surface studies obtained from orbital devices or ground-based telescopes. On-board sources for calibration, such as lamps with a standardized emission spectrum, are also used. Capturing the Earth’s atmosphere for validation and calibration of data recorded by optical spectrometers is another method. The review presents an overview of techniques and methods used to calibrate optical spectrometers in flight to the Moon and in lunar orbit. This paper provides a review of the techniques and methods utilized for in-flight and on-orbit calibration of lunar imaging spectrometers, drawing from an extensive overview of referenced science papers.

In-flight validation of the Metis visible-light polarimeter coronagraph on board Solar Orbiter

Context. The Metis coronagraph is one of the remote sensing instruments of the ESA-NASA Solar Orbiter mission. The goal for the instrument is to enable the study of the solar atmosphere and solar wind by simultaneously acquiring images of the solar corona at two different wavelengths: visible light (VL), within a band ranging from 580 nm to 640 nm, and ultraviolet light, in the HI Lyα 121.6 ± 10 nm. The visible-light channel of the coronagraph includes a polarimeter with electro-optically modulating liquid crystal variable retarders to measure the linearly polarized brightness of the K-corona and derive the electron density. Aims. In this paper, we present the first in-flight validation results of the Metis polarimetric channel together with a comparison to the on-ground calibrations. This paper seeks to validate the first use of an electro-optical device, the liquid crystal-based polarimeter, in deep space and within a hard radiation environment. Methods. We used the orientation of the K-corona’s linear polarization vector during the roll maneuvers of the Space Orbiter spacecraft for the in-flight calibration. Results. The Metis coronagraph on board the Solar Orbiter shows good agreement with the on-ground measurements. The in-flight validation confirms the expected performance of the visible-light channel polarimeter. Furthermore, a comparison between the first polarized brightness value obtained by Metis and the polarized brightness values obtained by the space-based coronagraph LASCO and the ground-based coronagraph K-Cor shows the consistency of the Metis calibrated results.

AstroBio-CubeSat: A lab-in-space for chemiluminescence-based astrobiology experiments

Space exploration is facing a new era in view of the planned missions to the Moon and Mars. The development and the in-flight validation of new technologies, including analytical and diagnostic platforms, is pivotal for exploring and inhabiting these extreme environments. In this context, biosensors and lab-on-chip devices can play an important role in many situations, such as the analysis of biological samples for assessing the impact of deep space conditions on man and other biological systems, environmental and food safety monitoring, and the search of molecular indicators of past or present life in extra-terrestrial environments. Small satellites such as CubeSats are nowadays increasingly exploited as fast and low-cost platforms for conducting in-flight technology validation. Herein, we report the development of a fully autonomous lab-on-chip platform for performing chemiluminescence-based bioassays in space. The device was designed to be hosted onboard the AstroBio CubeSat nanosatellite, with the aim of conducting its in-flight validation and evaluating the stability of (bio)molecules required for bioassays in a challenging radiation environment. An origami-like microfluidic paper-based analytical format allowed preloading all the reagents in the dried form on the paper substrate, thus simplifying device design and analytical protocols, facilitating autonomous assay execution, and enhancing the stability of reagents. The chosen approach should constitute the first step to implement a mature technology with the aim to conduct life science research in space (e.g., for evaluation the effect of deep space conditions on living organisms or searching molecular evidence of life) more easily and at lower cost than previously possible.

Spaceflight validation of technology for point-of-care monitoring of peripheral blood WBC and differential in astronauts during space missions

During long duration orbital space missions, astronauts experience immune system dysregulation, the persistent reactivation of latent herpesviruses, and some degree of clinical incidence. During planned NASA ‘Artemis’ deep space missions the stressors that cause this phenomenon will increase, while clinical care capability will likely be reduced. There is currently minimal clinical laboratory capability aboard the International Space Station (ISS). The ability to monitor the white blood cell count (WBC) and differential during spaceflight has been an unmet NASA medical requirement, primarily due to a lack of capable hardware. We performed ground and flight validation of a device designed to monitor WBC and differential within minutes from a fingerstick blood sample. This device is miniaturized, robust, and generally compatible with microgravity operations. Ground testing for spaceflight consisted of vibration tolerance, power/battery and interface requirements, electromagnetic interference (EMI), and basic evaluation of sample preparation and operations in the context of spaceflight constraints. The in-flight validation performed aboard the ISS by two astronauts included assessment of three levels of control solution (blood) samples as well as a real time analysis of a fingerstick blood sample by one of the crewmembers. Flight and ground testing of the same lot of control solutions yielded similar total WBC values. There was some select discrepancy between flight and ground data for the differential analysis. However, the data suggest that this issue is due to compromise of the control solutions as a result of storage length before flight operations, and not due to a microgravity-associated issue with instrument performance. This evaluation also yielded lessons learned regarding crewmember training for technique-sensitive small-volume biosample collection and handling in microgravity. The fingerstick analysis was successful and was the first real-time hematology assessment performed during spaceflight. This device may provide an in-mission monitoring capability for astronauts thereby assisting Flight Surgeons and the crew medical officer during both orbital and deep space missions.

Development of a small unmanned aircraft system to derive CO<sub>2</sub> emissions of anthropogenic point sources

Abstract. A reduction of the anthropogenic emissions of CO2 (carbon dioxide) is necessary to stop or slow down man-made climate change. To verify mitigation strategies, a global monitoring system such as the envisaged European Copernicus anthropogenic CO2 monitoring mission (CO2M) is required. Those satellite data are going to be complemented and validated with airborne measurements. Unmanned aerial vehicle (UAV)-based measurements can provide a cost-effective way to contribute to these activities. Here, we present the development of an sUAS (small unmanned aircraft system) to quantify the CO2 emissions of a nearby point source from its downwind mass flux without the need for any ancillary data. Specifically, CO2 is measured by an NDIR (non-dispersive infrared) detector, and the wind speed and direction are measured with a 2-D ultrasonic acoustic resonance anemometer. By means of laboratory measurements and an in-flight validation at the ICOS (Integrated Carbon Observation System) atmospheric station Steinkimmen (STE) near Bremen, Germany, we estimate that the individual CO2 measurements have a precision of 3 ppm and that CO2 enhancements can be determined with an accuracy of 1.3 % or 0.9 ppm, whichever is larger. We introduce an anemometer calibration method to minimize the effect of rotor downwash on the wind measurements. This method derives the fit parameters of a linear calibration model accounting for scaling, rotation, and a potential constant bias. For this purpose, it analyzes wind measurements taken while following a suitable flight pattern and assuming stationary wind conditions. From the calibration and validation experiments, we estimate the single measurement precision of the horizontal wind speed to be 0.40 m s−1 and the accuracy to be 0.33 m s−1. By means of two flights downwind of the ExxonMobil natural gas processing facility in Großenkneten about 40 km west of Bremen, Germany, we demonstrate how the measurements of elevated CO2 concentrations can be used to infer mass fluxes of atmospheric CO2 related to the emissions of the facility.

Design and flight test of a linear parameter varying flight controller

Future aircraft generations require improved performance and efficiency to enable a reduced environmental footprint. To acquire this goal, for example new material and wing concepts are perused at the moment by the aircraft industry. These developments, which include aspects such as over-actuation and lowly damped flexible modes, give rise to more complex, multi-objective control problems. One candidate method, which delivers a solution to these problems for the whole flight envelope, is linear parameter varying (LPV) control. It naturally incorporates the controller scheduling in the synthesis process, guarantees stability and robustness over the entire parameter envelope, and enables intuitive multi-objective, multiple-input multiple-output (MIMO) controller designs. This paper proves the concept of LPV control in practice: The paper presents and discusses the LPV controller design process, simulation results, motion simulator test and finally, the in-flight validation of the control system on a Cessna Citation II aircraft. The developed inner loop controller structures are inspired by classical flight controllers used on state-of-the-art fly-by-wire airliners. The longitudinal aircraft motion is augmented with load-factor command and the lateral motion controller features a roll rate command with attitude hold behavior. The control laws are validated in flight by automated and actual pilot inputs with respect to functionality, flying and handling qualities. Test results are encouraging with the provided key findings and lessons learned aiming to provide a simplification for future LPV flight controller development and testing campaigns.

The role of laboratory activities in aerospace control education: two case studies

Aerospace control education can significantly benefit from actual hands-on experience. In most cases, however, such experience can only be provided to students in small-scale project activities. In this paper the experience gathered in integrating laboratory activities in aerospace control education in the UAV Lab and in the Advanced Aerospace Control courses is presented and discussed. UAV Lab is an extra-curricular course aimed at an interdisciplinary group of students covering the whole design cycle for a multirotor UAV, from conceptual design to in-flight validation, with specific emphasis on hands-on experience in hardware/software integration, data collection and analysis and flight testing. Advanced Aerospace Control, on the other hand, is a curricular Master course in robust and nonlinear control, in the framework of which students are requested to solve a control design problem formulated over the dynamics of a multirotor UAV. The paper presents the course syllabi, discusses the role of laboratory activities and provides an overview of the obtained results.

Lever-arm and boresight correction, and field of view determination of a spectroradiometer mounted on an unmanned aircraft system

This study addresses the correction of lever-arm offset and boresight angle, and field of view (FOV) determination to enable accurate footprint determination of a spectroradiometer mounted on an unmanned aircraft system (UAS). To characterise the footprint, an accurate determination of the spectroradiometer position and orientation (pose) must be acquired with a global navigation satellite system (GNSS) and an inertial measurement unit (IMU). Accurate pose estimation requires an accurate lever-arm and boresight correction between the pose measuring sensors and the spectroradiometer. Similarly, the spectroradiometer FOV is required to determine the footprint size as a function of above ground level (AGL) flying height. The system used in this study consists of an IMU with dual-frequency and dual-antenna GNSS receiver, a machine vision camera, and a point-measuring spectroradiometer (Ocean Optics QE Pro). The lever-arm offset was determined from a scaled 3D point cloud of the system, created using photos of the airframe and processed with the structure-from-motion (SfM) algorithm. The boresight angles were estimated with stationary experiments by computing the difference between the orientations of the IMU, the spectroradiometer, and the camera. The orientation of the spectroradiometer was determined by moving a spectrally distinct target into the FOV. The orientation of IMU was measured by averaging its readings during the stationary epoch, while SfM was employed as an independent technique to estimate the orientation of the camera. The footprint of the spectroradiometer for a combination of AGL height and Gershun tube aperture ring was determined experimentally, enabling computation of the effective FOV. In-flight validation of the lever-arm and boresight correction was performed by comparing the corrected pose of the co-mounted camera with the pose derived from SfM as the reference. Our experimental results demonstrate that controlled determination and correction of lever-arm and boresight increases the pose estimation accuracy and thereby supports the direct georeferencing of a UAS-mounted spectroradiometer point observation.

UAV Lab: a multidisciplinary UAV design course

Abstract Aerospace control education can significantly benefit from actual hands-on experience. In most cases, however, such experience can only be provided to students in small-scale project activities. In this paper the experience of the UAV Lab course, held at Politecnico for the first time in the fall of 2018, is presented and discussed. The course, aimed at an interdisciplinary group of students, covers the whole design cycle for a multirotor UAV, from conceptual design to in-flight validation, with specific reference to modelling, simulation, identification and control. The course has been conceived as an extra-curricular activity, so the emphasis is not on conventional lectures but rather on hands-on experience in hardware/software integration, data collection and analysis and flight testing. The paper presents the course syllabus and organisation and provides an overview of the obtained results and the feedback provided by the students.

Possible Technologies of Progress Transport Cargo Vehicle Control during Experiments in Free Flight

The International Space Station (ISS) project involves Russian Progress transport cargo vehicles (TCV) which often retain residual resources of their basic systems after they have completed their main tasks in the ISS program. Utilization of these resources for research purposes during free flight of the TCV after undocking from the station increases the efficiency of both the TCV operation and the ISS research program as a whole. Transport cargo vehicles can be used for research in various fields: in-flight tests, trials, validation and certification of various equipment, materials, and systems to the benefit of other spacecraft programs; conducting experiments within the framework of the Earth study using additional equipment; microgravity experiments taking into account specific capabilities of TCV; launch of small satellites and explorers after TCV undocking from the station and settling into specified orbit, etc. To perform research using the Progress transport cargo vehicles, new efficient technologies are proposed. These technologies required some specific methods of control to be developed. This paper addresses these technologies, as well as some of the developed control methods.

General Aviation Weather Alerting: The Effectiveness of Different Visual and Tactile Display Characteristics in Supporting Weather-Related Decision Making

ABSTRACTObjective: This research analyzed the effects of weather technology interface characteristics on the quality and timeliness of weather-related decision making via mental workload and situational awareness (SA) metrics using flight training devices (FTDs) and follow-up in-flight validation testing.Background: A significant number of fatal general aviation (GA) accidents are attributable to pilots flying into instrument meteorological conditions without the necessary certification for safe operation. Even when takeoff conditions are clear, they can degrade in flight. Pilots might consult in-flight weather technologies to inform flight-plan-related decisions.Method: GA pilots flew 2 scenarios in FTDs while receiving weather notifications (referred to here as alerts) via complex graphical or smart-watch-based visual displays and under different vibrotactile cuing conditions. In-flight testing examined if vibrotactile cuing effects found in simulation were present in real flight.Results: Results suggest the quality and timeliness of weather-related flight decisions and flight-related SA are better supported when displays use vibrotactile cues and embed alert text within maps rather than present it separately on a smart watch. In-flight data confirmed that pilots can reliably perceive vibrotactile cues in real aircraft.Conclusion: Vibrotactile cues improve the reception of notifications and can reduce cognitive demands while flying. Visual weather information should be displayed on larger screens that are positioned to reduce pilots’ postural reorientation. These findings can inform the design of weather technologies for supporting pilot in-flight decision making.

HAI, a new airborne, absolute, twin dual-channel, multi-phase TDLAS-hygrometer: background, design, setup, and first flight data

Abstract. The novel Hygrometer for Atmospheric Investigation (HAI) realizes a unique concept for simultaneous gas-phase and total (gas-phase + evaporated cloud particles) water measurements. It has been developed and successfully deployed for the first time on the German HALO research aircraft. This new instrument combines direct tunable diode laser absorption spectroscopy (dTDLAS) with a first-principle evaluation method to allow absolute water vapor measurements without any initial or repetitive sensor calibration using a reference gas or a reference humidity generator. HAI contains two completely independent dual-channel (closed-path, open-path) spectrometers, one at 1.4 and one at 2.6 µm, which together allow us to cover the entire atmospheric H2O range from 1 to 40 000 ppmv with a single instrument. Both spectrometers each comprise a separate, wavelength-individual extractive, closed-path cell for total water (ice and gas-phase) measurements. Additionally, both spectrometers couple light into a common open-path cell outside of the aircraft fuselage for a direct, sampling-free, and contactless determination of the gas-phase water content. This novel twin dual-channel setup allows for the first time multiple self-validation functions, in particular a reliable, direct, in-flight validation of the open-path channels. During the first field campaigns, the in-flight deviations between the independent and calibration-free channels (i.e., closed-path to closed-path and open-path to closed-path) were on average in the 2 % range. Further, the fully autonomous HAI hygrometer allows measurements up to 240 Hz with a minimal integration time of 1.4 ms. The best precision is achieved by the 1.4 µm closed-path cell at 3.8 Hz (0.18 ppmv) and by the 2.6 µm closed-path cell at 13 Hz (0.055 ppmv). The requirements, design, operation principle, and first in-flight performance of the hygrometer are described and discussed in this work.

In-flight performance validation of the TanDEM-X autonomous formation flying system

The in-flight performance validation of the experimental autonomous formation keeping system embarked by the German TanDEM-X formation has been performed during a 12-day-long closed-loop campaign conducted in June 2012. Relative control performance better than 10 m was achieved, demonstrating that a significant gain of performance can be achieved when the control of the formation is done autonomously on-board instead of on-ground. Furthermore, the formation keeping system was shown to be operationally robust, easy to operate and fully predictable, i.e., fully suited for routine mission operations. This campaign concludes successfully a series of validation activities, opening new doors to future innovative scientific TanDEM-X experiments for which enhanced formation control is required.

Enabling technologies for space exploration systems: The STEPS project results and perspectives

The project STEPS (Sistemi e Tecnologie per l'EsPlorazione Spaziale) is a joint development of technologies and systems for Space Exploration supported by Regione Piemonte, the European Regional Development Fund (E.R.D.F.) 2007-2013, Thales Alenia Space Italia (TAS-I), SMEs, Universities and public Research Centres belonging to the network “Comitato Distretto Aerospaziale del Piemonte” the Piedmont Aerospace District (PAD) in Italy. The project first part terminated in May 2012 with a final demonstration event that summarizes the technological results of research activities carried-out during a period the three years and half. The project developed virtual and hardware demonstrators for a range of technologies for the descent, soft landing and surface mobility of robotic and manned equipment for Moon and Mars exploration. The two key hardware demonstrators—a Mars Lander and a Lunar Rover—fit in a context of international cooperation for the exploration of Moon and Mars, as envisaged by Space Agencies worldwide. The STEPS project included also the development and utilization of a system of laboratories equipped for technology validation, teleoperations, concurrent design environments, and virtual reality simulation of the Exploration Systems in typical Moon and Mars environments. This paper presents the reached results in several technology domains like: vision-based GNC for the last portion of Mars Entry, Descent and Landing sequence, Hazard avoidance and complete spacecraft autonomy; Autonomous Rover Navigation, based on the determination of the terrain morphology by a stereo camera; Mobility and Mechanisms providing an Integrated Ground Mobility System, Rendezvous and Docking equipment, and protection from Environment effects; innovative Structures such as Inflatable, Smart and Multifunction Structures, an Active Shock Absorber for safe landing, balance restoring and walking; Composite materials Modelling and Monitoring; Human-machine interface features of a predictive Command and Control System; Energy Management systems based on Regenerative Fuel Cells; aerothermodynamic solutions for Atmospheric Re-entry of Commercial Transportation Systems; novel Design and Development Tools, such as a Rover S/W simulator and prototypes of the DEM viewer and of a S/W Rock Creator/visualizator.The paper also provides perspectives on the proposed STEPS 2 project that will likely continue the development of a subset of the above technologies in view of their possible in-flight validation within next five years.

Experimental Validation of L1 Adaptive Control: The Rohrs Counterexample in Flight

The paper presents new results on the verification and in-flight validation of an L1 adaptive flight control system, and proposes a general methodology for verification and validation of adaptive flight control algorithms. The proposed framework is based on Rohrs counterexample, a benchmark problem presented in the early 80s to show the limitations of adaptive controllers developed at that time. In this paper, the framework is used to evaluate the performance and robustness characteristics of an L1 adaptive control augmentation loop implemented onboard a small unmanned aerial vehicle. Hardware-in-the-loop simulations and flight test results confirm the ability of the L1 adaptive controller to maintain stability and predictable performance of the closed loop adaptive system in the presence of general (artificially injected) unmodeled dynamics. The results demonstrate the advantages of L1 adaptive control as a verifiable robust adaptive control architecture with the potential of reducing flight control design costs and facilitating the transition of adaptive control into advanced flight control systems.

Numerical Rebuilding of SMART-1 Hall Effect Thruster Plasma Plume

The SMART-1 flight data provided an excellent opportunity for extending validation of plume prediction codes to actual flight conditions. This paper will present the results obtained for the numerical simulation of the SMART-1 plasma plume using the PICPluS particle in cell code, which had been previously validated using ground experiment data. This activity can be considered as the first important step in the direction of a full in-flight validation for the code models and algorithms. A description of the implemented physical models is provided, followed by simulation results of experimental data from Alta's tests and from the literature. Finally, the results of the code application to the SMART-1 mission are presented, focusing on the simulation of retarding potential analyzer measurement, spacecraft floating potential, and interaction of the plasma field with the satellite (in particular the solar array). The results show that, although the possibility of dangerous interaction between the electric propulsion system and the spacecraft is very limited, a number of complex phenomena arise due to the use of the electric propulsion system in orbit. For example, the interaction between the plume and the solar array orientation leads to complex patterns for the spacecraft floating potential. Eventually, full three-dimensional simulation is therefore needed to accurately model this kind of phenomena with a realistic description of the involved geometries.

In-Flight Attitude Perturbation Estimation for Earth-Orbiting Spacecraft

This paper presents four innovative techniques, three Kalman-based and one observer-based, to estimate environmental perturbation torques acting on an Earth-orbiting spacecraft. The Kalman-based techniques all use simple generic models for the state dependence of the perturbation, and for estimating the unknown coefficients included in these generic mathematical formulations. The observer-based technique is developed with the nonlinear disturbance observer theory. The proposed strategies are validated in numerical simulations and are traded-off in terms of estimation accuracy and computational load requirement. Then, the most suitable estimation technique is combined with a batch least square filter algorithm to yield a perturbation estimation system with low computational load, which can be implemented onboard a spacecraft. Finally, the proposed estimation strategy is applied to a realistic gyroless Earth-orbiting spacecraft mission: the European Space Agency’s Project for Onboard Autonomy (PROBA)-2 mission. Ultimately, the selected estimation strategy will be implemented onboard the PROBA-2 spacecraft for in-flight validation. All strategies proposed in this article are general and are applicable to any Earth-orbiting spacecraft.

Absolute Radiometric In-Flight Validation of Mid Infrared and Thermal Infrared Data From ASTER and MODIS on the Terra Spacecraft Using the Lake Tahoe, CA/NV, USA, Automated Validation Site

In December 1999, the first Moderate Resolution Imaging Spectroradiometer (MODIS) instrument and an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument were launched into polar orbit on the Terra spacecraft. Both instruments measure surface radiance, which requires that they are calibrated and validated in flight. In-flight validation is essential to independently verify that instrument calibration correctly compensates for any changes in instrument response over time. In order to meet this requirement, an automated validation site was established at Lake Tahoe on the California/Nevada border in 1999 to validate the ASTER and MODIS thermal infrared (TIR, 7-13 mum) and MODIS mid infrared (MIR, 3-5 mum) land-monitoring channels. Daytime and nighttime data were used to validate the TIR channels, and only nighttime data were used to validate the MIR channels to avoid any reflected solar contribution. Sixty-nine ASTER scenes and 155 MODIS-Terra scenes acquired between years 2000 and 2005 with near-nadir views were validated. The percent differences between the predicted and instrument at-sensor radiances for ASTER channels 10-14 were 0.165plusmn0.776, 0.103plusmn0.613, -0.305plusmn0.613, -0.252plusmn0.464, and -0.118plusmn0.489, respectively. The percent differences for MODIS-Terra channels 20, 22, 23, 29, 31, and 32 were -1.375plusmn0.973, -1.743plusmn1.027, -0.898plusmn0.970, 0.082plusmn0.631, 0.044plusmn0.541, and 0.151plusmn0.563, respectively. The results indicate that the TIR at-sensor radiances from ASTER and MODIS-Terra have met the preflight radiometric calibration accuracy specification and provide well-calibrated data sets that are suitable for measuring absolute change. The results also show that the at-sensor radiances from the MODIS-Terra MIR channels have greater bias than expected based on the preflight radiometric calibration accuracy specification

Measurements of the Total Water Content of Cirrus Clouds. Part II: Instrument Performance and Validation

Abstract This paper describes the performance and in-flight validation of an instrument mounted in a pallet on the NASA WB-57 research aircraft that measures the sum of gas phase and solid phase water, or total water, in cirrus clouds. Using a heated isokinetic inlet and a Lyman-α photofragment fluorescence technique for detection, measurements of total water have been made over three orders of magnitude. During the Cirrus Regional Study of Tropical Anvils and Cirrus Layers Florida Area Cirrus Experiment (CRYSTAL FACE), the instrument operated at duct temperatures sufficiently warms to completely evaporate particles up to 150-μm diameter. Laboratory calibrations, in-flight diagnostics, intercomparison with water vapor measured by absorption in flight, and intercomparisons in clear air with the Harvard water vapor instrument validate the detection sensitivity of the instrument and illustrate the minimal hysteresis from instrument surface contamination. The Harvard total water and water vapor instruments together provide measurements of the ice water content of cirrus clouds in the mid- and upper troposphere with an uncertainty of 18%.

Measurements of the Total Water Content of Cirrus Clouds. Part I: Instrument Details and Calibration

Abstract This paper describes an instrument designed to measure the sum of gas phase and solid phase water, or total water, in cirrus clouds, and to be mounted in a pallet in the underbelly of the NASA WB-57 research aircraft. The ice water content of cirrus is determined by subtracting water vapor measured simultaneously by the Harvard water vapor instrument on the aircraft. The total water instrument uses an isokinetic inlet to maintain ambient particle concentrations as air enters the instrument duct, a 600-W heater mounted directly in the flow to evaporate the ice particles, and a Lyman-α photofragment fluorescence technique for detection of the total water content of the ambient air. Isokinetic flow is achieved with an actively controlled roots pump by referencing aircraft pressure, temperature, and true airspeed, together with instrument flow velocity, temperature, and pressure. Laboratory calibrations that utilize a water vapor addition system that adds air with a specific humidity tied to the vapor pressure of water at room temperature and crosschecked by axial and radial absorption of Lyman-α radiation at the detection axis are described in detail. The design provides for in-flight validation of the laboratory calibration by intercomparison with total water measured by radial absorption at the detection axis. Additionally, intercomparisons in clear air with the Harvard water vapor instrument are carried out. Based on performance of the Harvard water vapor instrument, this instrument has the detection capability of making accurate measurements of total water with mixing ratios in the mid- to upper troposphere of up to 2500 ppmv and mixing ratios in the lower stratosphere of about 5 ppmv, corresponding to almost three orders of magnitude in measurement capability.