To prepare the mission of the Infrared Atmospheric Sounding Interferometer (IASI), which will be launched in 2005 aboard the European METOP satellite, the measurement capability to retrieve trace gases concentrations from nadir radiances was investigated. This included sensitivity studies, development of an innovating inversion algorithm, cloud-filtering, and use of data assimilation techniques for validation purposes. The availability of nadir radiances measured by the IMG instrument allowed us to test the tools under development on real data. This paper summarizes the current status of the work.

The paper describes the activities of the European Centre for Medium Range Weather Forecasts focused on the preparation for the assimilation of advanced high spectral resolution sounders that will become available for operational use within the next decade. A particular emphasis is put on a robust and comprehensive monitoring facility as well as on an accurate cloud detection scheme based on pattern recognition.

The interest in aerosol observations from satellite passive instrument is steadily increasing since satellite instruments supply unique global observations for establishing an aerosol climatology. A correct characterization of single aerosol events from satellite requires adequate temporal and spatial resolution. Most state-of-the-art algorithms are based on a single sensor, so that they often suffer from specific limitations: poor spatial or spectral resolution, large re-visitation time, poor de-clouding,... A method to exploit the synergy between the polar orbiting instrument GOME (Global Ozone Monitoring Experiment) and the METEOSAT geostationary system was proposed, aiming at increasing the accuracy of the aerosol characterization over the ocean by determining with GOME the actual aerosol model to be adopted for aerosol optical thickness determination with METEOSAT. Applications of the algorithm to relevant aerosol events are presented characterizing aerosol optical properties and thickness. The comparison with results obtained via independent space-time co-located ground-based measurements and retrievals from other algorithms that make use of satellite measurements such as POLDER, allows for a first validation of the algorithm. Comparisons also address limitations of the retrieved aerosol model in terms of time-space coverage.

The Mesosphere Lower Thermosphere (MLT) is the most inaccessible and least understood region of the Earth's atmosphere. AtmophericGravity waves play a substantial role in the dominant processes of energy and momentum transport in this region. The WAVES mission (proposed as a NASA Midex) will be the first mission dedicated to studying atmospheric gravity waves, their sources and how they affect the MLT. An instrument, the Multi-spectral Limb Photometer (MLP) will make limb viewing observations to support the WAVES mission. This instrument will observe airglow variations caused by longer wavelength waves passing through the region and will make measurements of temperature and composition in various regions in the MLT. The MLP instrument images the earth atmosphere in limb view in the orbital plane. The resolution in the vertical dimension (altitude) is about 2 km. In the horizontal dimension the MLP collects an averaged intensity over a region of 250 km in width. Vertical imaging vs. horizontal non imaging is realized by cylindrical lenses. The stray light baffling design is specially adapted to allow for day and night observation. The MLP is a single optical channel instrument using a CCD sensor. We propose to use a grille filter spectrometer consisting of a telecentric imager in which a set of narrow vertical strip interference filters are included. The image of the limb is projected onto these strip filters preserving the imaging qualities (vertical dimension). With the interference filter it is possible to realize a spectral function fitting with the multiple spectral bandpass of the emitting species. The full wavelength range is 555-892 nm where about 10 emission lines are to be resolved. The instrument sensitivity is adapted to the intensity and spectral spacing (with respect to neighboring emission lines) of each line: spatial and spectral width of each interference filter strip are independently optimized. This is unique compared to spectrograph using grating technology where spectral resolution and sensitivity are undesirably coupled through the slit width. Finally, the CCD sensor captures a composite image where columns depict the vertical limb imaging and rows indicate the spectral signature.

This paper describes the dual-mission Mars 2003 Miniature Thermal Emission Spectrometer (Mini-TES) being built by Raytheon Santa Barbara Remote Sensing (SBRS) under contract to Arizona State University (ASU). Mini-TES is a single detector Fourier Transform Spectrometer (FTS), covering the spectral range 5-29 microns (micrometers ) at 10 cm^-1 spectral resolution. Scheduled for launch in 2003, one Mini-TES instrument will fly to Mars aboard each of the two missions of NASA's Mars Exploration Rover Project (MER). Mini-TES is designed to provide a key minerological remote sensing component of the MER mission, which includes several other science instruments. Originally intended for the Athena Precursor Experiment (APEX) slated for a 2001 launch, the first Mini-TES unit was required to meet a two-year development schedule with proven, flight-tested instrumentation. Therefore, SBRS designed Mini-TES based on proven heritage from the successful Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES), which was launched in 1996 and successfully completed its intended mission of one Martian year (two earth years) with over 100 million spectra collected to date, and counting. Relevance of the Mini-TES to MER science, overall design, performance, assembly and test flow as well as details of the hardware fabricated at SBRS, are discussed.

The Stellar Absorption and Refraction Sensor (STARS) is a compact, large-aperture instrument that combines a UV-IR imaging spectrograph with a co-aligned visible-light imager to make simultaneous absorptive and refractive stellar occultation measurements. The absorption measurements provided by the spectrograph allow the determination of vertical profiles of atmospheric constituents. The coincident refraction observations made by the image yield high-precision measurements of atmospheric density, pressure, and temperature and provide independent knowledge of both the refracted light path and Rayleigh extinction, which are critical in reducing the uncertainty in the retrieved constituent profiles in the lower atmosphere. STARS employs a two-axis gimbaled telescope to acquire and track the star and a two-axis, high-precision, fast-steering mirror to correct for spacecraft jitter and maintain the star within the spectrograph field of view. The relative star position measured by the imager provides position feedback to the active tracking loop of the fast-steering mirror. With funding from NASA's Instrument Incubator Program, a laboratory facility has been developed to demonstrate the overall instrument performance and, in particular, its capability to acquire and track a setting, refracting, and scintillating star, to compensate for various degrees of platform jitter, and to provide the pointing knowledge required for accurate determination of the atmospheric quantities. The combination of built-in image tracking and motion compensation capabilities, small size, and limited spacecraft resource requirements makes STARS and its tracking mechanism suitable for deployment on existing and future commercial spacecraft platforms for applications that require high-precision pointing. In this paper, we present details of the instrument design and its expected performance based on our laboratory tests.

The Spaceborne Infrared Atmospheric Sounder (SIRAS) represents a new approach to imaging spectrometry in the infrared by combining next generation WFOV refractive optics with high-dispersion gratings to minimize size and mass. Prototype hardware was developed and tested on this program to demonstrate that high spectral resolution could be achieved in a package of small size and mass. The performance, development and testing of the prototype spectrometer are discussed as well as potential applications for future missions. This effort was sponsored under the NASA Instrument Incubator Program (IIP).

Considering global increase in greenhouse-gases, observation and monitoring of the earth's atmosphere with space-borne instruments are essential. Satellite measurement offers the advantage of global and long-term monitoring. In the short wave infrared (SWIR) region of 1.5-1.9 micrometers , major greenhouse gases (carbon dioxide (CO₂), water vapor (H₂O), and methane (CH₄)) have absorption spectra of moderate strength without interference by other molecule absorption. In addition, we can use the un-cooled detector for this wavelength region. Two different types of observation geometry will be discussed; one is nadir-looking with sun glint light source for the column amount retrieval and the other is limb-looking with scattered light source for the vertical profile retrieval. We propose the four- ports Fourier transform spectrometer (FTS) for this application. One input port is for nadir-looking measurements and the other input port is for limb-looking measurements. One output port is used for greenhouse gases measurements and the other port is used for the oxygen (O₂) absorption spectra measurement for the optical path length calibration. The instrumentation of the FTS, retrieval algorithm and expected performance are discussed, and ground test results are also presented.

The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounding Testbed-Interferometer (NAST-I), is the infrared component of a suite of airborne infrared and microwave spectrometers, developed for the Integrated Program Office (IPO), that has been flying on high-altitude aircraft as part of the risk reduction effort for NPOESS. It is a high spectral resolution (0.25 cm^-1, unapodized) and high spatial resolution (~2 km, nadir) cross-track scanning (~ 45 km swath width, at 20 km aircraft altitude) Fourier Transform Spectrometer (FTS) observing within the 3.7 - 16.1 micron spectral range. In addition to characterizing the atmospheric thermal and moisture structure and providing information on radiatively active trace gases (e.g. O₃ & CO) during field experiments, NAST-I measurements greatly contribute toward instrument and forward model pre-launch specification optimization (i.e., for the Cross-track Infrared Sounder, CrIS, to fly on NPOESS) and will enhance post-launch calibration/validation activities for CrIS as well as for other future advanced atmospheric spaceborne sensors (e.g., the EOS AIRS, CERES, MODIS, MOPITT, & TES instruments). In this paper, we investigate some of the challenges associated with validating infrared spectral radiances obtained from remote sensing measurements and forward model simulations. Specifically, measured infrared spectral radiances are compared with radiance calculations performed using a Line by Line forward radiative transfer model based on nearly-coincident temperature and water vapor profiles observed with several independent in-situ, passive, and active measurement systems.

Airborne profiles of spectral radiances between 3.5 and 15 microns are used to derive profiles of atmospheric temperature and the concentrations of radiatively active absorbing gases. These measurements are intended for the validation of atmospheric radiative transfer physics and associated forward models as well satellite remote sensing techniques and products. Vertical profiles of upwelling spectral radiance are obtained during ascents and descents of the Proteus aircraft carrying the NPOESS Airborne Sounding Test-bed - Interferometer (NAST-I). Opaque region radiance measurements provide the vertical temperature profile while the vertical derivative of radiance with respect to Planck radiance provides the vertical distribution of spectral emissivity. Spectral regions where only a single absorbing gas is active can be used to derive the mixing ratio profile for that gas, assuming the spectroscopy (i.e., line strengths, shapes, and pressure and temperature dependencies) is accurately known for the spectral region used. For the case where the gaseous mixing ratio is either known to be well mixed (e.g., CO₂) or is measured in-situ from aircraft or balloon profile observations, the accuracy of the spectroscopic parameters used for radiative transfer calculations can be validated. In this paper, results obtained from vertical profile measurements associated with four different air mass conditions are used to depict the errors associated with radiative transfer calculations within the R-branch of the 15micrometers CO₂ band. The airborne spectrometer observation technique illustrated here is proving to be a useful spectroscopic validation tool.

Concern about the climatic effects of anthropogenic emissions of carbon dioxide has resulted in a growing need, both scientifically and politically, to monitor atmospheric carbon dioxide. The development of a satellite instrument which could measure the global distribution of atmospheric carbon dioxide would greatly improve our understanding of the global carbon cycle and provide a means of monitoring regional sources and sinks. In this paper, we propose and analyze the potential of a nadir-viewing, satellite-based remote sounding instrument consisting of a simple filter radiometer tuned to the 6300 cm^-1 (1.6 micrometers ) region to globally measure the atmospheric carbon dioxide column. Such an instrument would be among the simplest of all potential remote sounding instruments to make this measurement. Retrievals by a radiometer instrument are modeled using high-resolution FTS spectra and compared with SFIT2 retrievals. We find that the proposed instrument has potential, and that the sensitivity is likely to be limited by our knowledge of the atmospheric temperature and uncertainty line strengths and widths.

The measurements of HCl and CH4 atmospheric total abundance is very important, because these minor gases play a fundamental role in the stratospheric ozone cycle and in the climatic change. In fact, the first is considered source and sink for chlorine compounds; the latter is a greenhouse gas (26%) and can contribute to the hydrochloric acid formation. HCl and CH₄ present a vibrorotational absorption spectrum in the near infrared (3-4 micron). For this reason it is possible to use a Fabry-Perot interferometer (FPI) as a multiple narrow band filter with an appropriate free spectral range (FSR), so its transmission bands overlap the absorption lines of gas under observation. A remote sensor called NISES (Near Infrared Single-Etalon Sensor) and based on a plane FPI with a dynamic control of the etalon gap, is developing. It utilizes the direct sun radiation collected by a solar tracker to detect atmospheric HCl and CH4 slant columns and its suitable for both ground based and airborne applications. The model MAES (Mathematical Algorithm for Etalon Sensor) has been used to study the main optical characteristics of the FPI (free spectral range, finesse, transmission lines number) and to optimize the instrumental response over a wide range of atmospheric conditions. Moreover, line by line computations of atmospheric layer optical depths and radiances are performed, using HARTCODE (R. Rizzi et al. submitted to Applied optics), so a sequence of different Free spectral Range (FSRs), during the measurements itself, is proposed to minimize the water vapor and gases interfering contribution. The main optical characteristics of an FPI and its deployment for atmospheric sensing are discussed; the result of model simulation and the layout of the HCl sensor are presented as well as some preliminary tests.

The future of remote sensing includes a significant role for imaging spectrometers in operational environmental satellite systems. Optimizing an imaging spectrometer design for geostationary orbit requires detailed study and evaluation of available technology and design architectures versus performance requirements and possible resource limitations that are unique to geostationary orbit operation. This paper summarizes a trade study of two candidate geostationary imaging spectrometer architectures that is constrained by performance requirements, cost and technical risk factors.

The Stratospheric Aerosol and Gas Experiment III (SAGE III) Meteor mission was originally scheduled for launch in the early summer of 2001. This paper will discuss the overall SAGE III/Meteor mission and provide a description of the instrument performance based on different pre-launch tests that have been performed over the last two years. Pre- launch tests include instrument radiometric throughput and calibration; wavelength calibration; polarization response; and in-atmospheric testing including sun and moon viewing. The resulting data demonstrate the capability of the instrument to provide high spectral resolution atmospheric spectral measurement in the visible to the near IR wavelength region with a high SNR. The instrument has been integrated onto the Meteor spacecraft at the NIIEM facility in Russia. Since the launch of the Meteor SAGE III has been delayed until the end of 2001, this paper will only provide a description of the planned initial operation of the SAGE III instrument after launch.

Five functional UV-VIS-NIR spectrometer/telescopes were built for the Stratospheric Aerosol and Gas Experiment III (SAGE III) satellite instrument project. Three will be on satellite platforms in the early part of the decade, and a fourth, the SAGE III Test Model (TM) is functioning as a ground-based instrument. The fifth is used as a software test-bed to support Mission Operations for the space instruments. The SAGE III instrument is spatially scanning UV-VIS-NIR holographic grating spectrometer using a cooled CCD detector. This paper presents performance results from the TM instrument. The TM instrument has been used in cross calibration studies during which one of the SAGE III flight instruments directly sampled the Sun, Moon, and twilight sky from the ground. Comparisons of SNRs, and relative responsivities are presented. TM zenith twilight spectra and measurement sensitivities are presented including a comparison of its twilight radiance measurements with the MODTRAN 3.7 atmospheric radiance/transmittance model. In addition the TM results of sampling planetary and stellar spectra are presented.

This paper presents an overview of the SAGE III water vapor retrieval algorithm. Results of simulated retrieval are shown to demonstrate the advantages of the non-linear optimization algorithm in reducing the influence of the contributions due to interfering species. Diagnostic analyses of the retrieval are conducted to examine the characteristics of the matrices of contribution functions and the averaging kernels. The results indicate that high vertical resolution profiles of water vapor can be retrieved form the SAGE III measurements and the total random errors are less than 10% for altitudes below 42 km.

The radiative balance of the troposphere, and hence climate, is influenced strongly by radiative cooling associated with emission of infrared radiation by water vapor, particularly at far-infrared (far-IR) wavelengths greater than 15 micrometers and extending out beyond 50micrometers . Water vapor absorption and emission is principally due to the pure rotation band, which includes both line and continuum absorption. The distribution of water vapor and associated far-IR radiative forcings and feedbacks are well-recognized as major uncertainties in understanding and predicting future climate. Up to half of the outgoing longwave radiation (OLR) from the Earth occurs beyond 15.4 micrometers (650 cm^-1_ depending on atmospheric and surface conditions. Cirrus clouds also modulate the outgoing longwave radiation in the far-IR. However, despite this fundamental importance, far-IR emission (spectra of band- integrated) has rarely been directly measured from space, airborne, or ground-based platforms. Current and planned operational and research satellites typically observe the mid-infrared only to about 15.4 micrometers . In this talk we will review the role of the far-IR radiation in climate and will discuss the scientific and technical requirements for far-IR measurements of the Earth's atmosphere.

The clear sky radiative energy balance in the infrared spectrum is investigated with particular attention to the role of water vapor pure rotational band in the spectral range 10-600 cm-1. Main results for Tropical and Sub Arctic Winter atmospheres are in good agreement with those previously reported by other authors. Multiple scattering layers, simulating the presence of cirrus clouds, are used in Tropical atmosphere. The cloudy tropospheric heat balance is studied by the introduction of spectral power densities instead of spectral heating rates. Distribution of radiant energy between air molecules and cloud ice crystals is taken into account and the effects of cirrus absorption and scattering are distinguished. Finally a comparison between Mie and Fu parameterization is performed to understand the effects of more realistically shaped cirrus particles.

Monochromatic calculations have been used to ascertain the far-infrared (wave numbers less than 650 cm^-1) contribution to the thermal energy budget of the Earth's atmosphere-surface system. The results of the model calculations for clear-sky conditions have demonstrated that nearly half of the outgoing thermal energy emanates from the far-infrared. Despite the critical importance of the far- infrared, however, very few direct measurements of this spectral region have been made by satellite, aircraft, and surface instruments. Thus the present study has used the monochromatic calculations both to quantify the magnitude of the radiative impact which the infrared-active molecules have upon the absorption and emission of far-infrared energy within the atmosphere, and to focus our attention on the subintervals within the far-infrared that may provide the most useful measurements for climate studies. The results of the monochromatic calculations have illustrated the importance of the radiative effects attributed to the line and continuum features associated with the pure rotation band of water vapor. Subdividing the far-infrared into moderately narrow band (~100 to 200 cm^-1) spectral regions has facilitated an analysis of the relative contributions of those spectral regions for a variety of atmospheric conditions. The results from the contribution function calculations have demonstrated that a careful selection of the far-infrared narrow band subintervals can prove very useful in determining both upper and lower tropospheric humidity.

Broad band radiance measurements at top of the atmosphere are simulated using two diverse line-by-line codes for a number of standard atmospheric conditions to evaluate the sensitivity of broad band spectral and spectrally integrated radiometry to realistic changes of water vapor content in the troposphere and of other parameters of climatic significance. . It is shown that the rotational band is very sensitive to realistic changes in upper tropospheric water vapor. Moreover our ability to retrieve middle and upper troposphere H₂O is enhanced if spectral measurements in the water vapor rotational band are compared to measurements in the strong 6.3 micrometers vibrational band, which has been used exclusively since the early 1970's.

Since the early 1970's, infrared remote sensing of water vapor has been based exclusively on observations of mid-infrared (mid-ir) emission from the strong 6.3 micrometers vibration-rotation band. However, the outgoing longwave radiation from Earth is dominated by emission in the far-infrared (far-ir) at wavelengths greater than 15 micrometers . The source of this far-ir radiation is upper tropospheric water vapor. Furthermore, there are a growing number of novel instrument concepts that propose measuring the thermal far-ir spectrum for the purpose of radiation budget sensing and climate fingerprinting. As a result, we investigate the feasibility of retrieving upper tropospheric water vapor from nadir-viewing observations of far-ir spectral emission. We find that it is possible to retrieve upper tropospheric water vapor from measurements of the far-ir spectral radiation, and that the vertical resolution can be significantly improved by combining measurements from the far-ir and mid-ir spectrum.

The REFIR (Radiation explorer in the far infrared) project is a study, funded by European Union, of feasibility of a novel space-borne instrument that will measure the atmospheric spectral radiance of the Earth in the broad spectral range 100-1100 cm^-1 from space with sufficient spectral resolution (0.5 cm^-1) and signal-to-noise ratio (SNR > 100). The main scientific objectives of the REFIR experiment are the measurement of the outgoing FIR radiation at the top of the atmosphere and the improvement of our knowledge of the principal drivers of this flux, e.g. temperature structure, water vapor, and clouds throughout the troposphere-surface system. The REFIR concept consists of a far infrared Fourier transform spectrometer (FTS) as the core instrument, of an embedded imager operating in an infrared 'window', sharing the same bore-sight as FTS, for scene/cloud signature identification in the FIR, of an add-on imager to provide multi-channel imagery, and of an absolute single-pixel radiometer with a single broad-band channel, used to measure the emitted radiation contextually with the spectral measurements. The integration of all the systems leads to a very compact satellite instrumentation, working at room temperature, with an estimated overall mass of 70 kg and a power consumption of 80 W, including electronics. The overall data rate toward the ground station is foreseen to be of 170 kbps before on-board data compression. This work highlights the main technical results at the end of phase-B0 study. The technical solutions adopted for the instrument are outlined and an accurate analysis of performances is shown.

An analysis of spectral errors induced by the sampling method used for the interferogram acquisition in Fourier-transform spectroscopy is given. A model is presented that can be used for a rigorous determination of the expected spectral noise in the case of different sampling methods as a function of the quality of the implemented instrumental devices. Using this model numerical evaluations are made in the case of the REFIR (radiation explorer in the far infrared) instrument developed for the measurement of the long wavelength Earth's emission from satellite platforms. The different sampling techniques are considered and evaluated for the selection of the most suitable one for this application. The equal time sampling followed by a numerical filter and a re-sampling has shown the capability of best correcting for sampling errors in the case of this instrument. Furthermore, from the general model an approximate expression is derived that can be used for a preliminary estimate with simple analytical calculations of the spectral noise for different sampling methods. In the specific case of REFIR, a comparison between the numerical modeling and the approximate expression has confirmed that the latter gives a good estimation and can be very useful for the assessment of the relative relevance of the different components of the spectral noise induced by sampling errors.

Global measurements of far infrared emission from the upper troposphere are required to test models of cloud radiative forcing, water vapor continuum emission, and cooling rates. Spectra with adequate resolution could also be used for retrieving atmospheric temperature and humidity profiles,and yet there are few spectrally resolved measurements of outgoing longwave flux at wavelengths longer than 16 micrometers . It has been difficult to make measurements in the far infrared due to the need for cooled detectors and large optics to achieve adequate sensitivity. We review design considerations for infrared Fourier transform spectrometers, including the dependence of system performance on basic system parameters, and discuss the prospects for achieving useful sensitivity from a satellite platform with a lightweight spectrometer using uncooled detectors.

GASCODs are UV-Visible ground-based spectrometers developed at the ISAO Institute and used to detect stratospheric trace gases involved in the ozone cycle such as NO₂, OClO, BrO, by application of Differential Optical Absorption Spectroscopy (DOAS) methodology to the zenith scattered light collected data. After several tests both in laboratory and in Antarctic region, one of the spectrometers was modified for unattended and continuous measurement in extreme high-latitude environment. The instrument was installed in December 1995 in the Italian Station at Terra Nova Bay (74 degree(s)26'S, 164 degree(s)03E', Ross Sea). The GASCOD is still working and causing very interesting data for the study of the denitrification processes during the formation of the so-called ozone hole over the Antarctic region. When the station is unmanned, to allow for the continuous NO₂ monitoring for whole the year without mechanical problems, the fixed [407 - 460] nm spectral region is investigated. The results for Nitrogen Dioxide, obtained by application of DOAS algorithms to the data recorded during the year 2000, are presented. During a leg (December 2000 - January 2001) of the 16th Italian Antarctic Expedition, after the usual instrument check, many measurements were carried out in other spectral regions, with the aim to obtain information about the stratospheric tracers contents. The results obtained for Ozone, Nitrogen dioxide and Formaldehyde at different Solar Zenith Angle are presented.

We recently demonstrated trace detection using Cavity Ring Down Spectroscopy (CRDS) coupled with telecom DFB diode lasers. Our scheme exploits optical feedback from a V-shaped cavity back to the laser. We built trace-gas detectors for CH4 and HF, characterized by a low cost, simplicity, compactness and sensitivity. Operating wavelength are 1.312 micrometers for HF and 1.65 micrometers for methane. The optical setup includes a distributed feed-back (DFB) diode laser, temperature stabilized by a Peltier, a collimating lens, 2 steering mirrors, a V-shaped optical resonator and a photodiode. The V-cavity is made of three low-cost super mirrors R 99.995%) and contains the air sample to be analyzed (20cm³). In standard atmospheric conditions the detection limits for 1 second integration time are of 50 ppbv for HF and 200 ppbv for methane. We present an analysis of the mechanisms of cavity injection and laser feedback, allowing to estimate the influence of various parameters on the performances of this type of apparatus. Calculations and results are given, with particular emphasis on the detection limit and the dynamic range.

We have developed a field portable optical gas sensor for the ultra sensitive detection of ethane. The system is based on an adaptation of a commercially available system, which uses a cryogenically cooled, lead-salt, laser diode at 3.34 microns and a multi-pass astigmatic Herriott sample cell. We have adapted this system to a second derivative wavelength modulation scheme giving a lower detection limit of less than 100 parts per trillion for a one second measurement time. Our custom designed software controls every aspect of the instrument operation from spectral scanning of the laser diode, to automatic calibration, optical alignment, spectral analysis and complete data logging.

The Atmospheric Neutral Density Experiment (ANDE) is a mission proposed by the Naval Research laboratory to monitor the thermospheric neutral density at an altitude of 400km. The mission objectives are to provide total neutral density along the orbit for improved orbit determination of resident space objects. The measurements also provide a critical validation point for the upcoming Special Sensor Ultraviolet Limb Imagers (SSULI) to be launched on each of the five Defense Meteorological Satellite Program (DMSP) spacecraft in block 5D3 starting in 2001. In addition ANDE provides two calibration objects for use in the Air Force High Accuracy Satellite Drag Model (HASDM) initiative to help maintain and improve accuracy of the space object catalog. The mission consists of two spherical satellites fitted with retro-reflectors for satellite laser ranging (SLR). One satellite is completely passive, the other carries active instrumentation to measure the partial pressure of atmospheric constituents, GPS positioning, acceleration in all 3 axes, and surface temperature. The active satellite will be fitted with modulating retro-reflectors. The spacecraft telemetry will be transferred to the ground by modulating and reflecting the SLR laser interrogation beam.

The Unconventional Stellar Aspect (USA) experiment is a multi-purpose experiment built around an X-ray sensor viewing celestial sources. The objectives include both basic research in X-ray astronomy and exploration of applied uses of X-ray sensors in space. The applied uses depend in large part upon exploiting understanding of celestial X-ray point sources. The experiment was launched on February 23, 1999 from Vandenberg AFB, CA aboard the Advanced Research and Global Observation Satellite (ARGOS). USA operated from April 1999 to November 2000. It consists of two proportional counters mounted in a two-axis gimbal for offset pointing from the nadir-pointed ARGOS. We present an overview of the experiment and then describe how it is used to provide a new atmospheric diagnostic that takes the form of redundant sets of atmospheric column density determinations. The data analyzed are energy-resolved photon extinction curves of X-ray celestial sources occulted by the Earth's atmosphere. As each X-ray source is occulted by the mesosphere and lower thermosphere (80-160 km), the density profile is derived from the extinction curve and temperature is derived from the scale height; limited composition information may be derived from the energy- dependence of extinction. These data are compared to standard atmospheric models. This research is the first to study the neutral atmosphere using X-ray source occultations, and complements UV airglow remote sensing techniques used aboard ARGOS that are insensitive to nighttime neutral density.

We present the concept for an instrument designed to map and monitor the ionosphere from geostationary orbit. This instrument will be used to study the spatial and temporal behavior of mesoscale (> 10 km) ionospheric structures. The instrument is designed to primarily operate at night as irregularities are generally more prevalent during the evening. The instrument will be sensitive enough to gather a high signal-to-noise image viewing the nadir in approximately 100 seconds. The instrument can also be operated as a limb imager. The instrument will image a 1.6 degree(s) field-of-view with 10 km spatial resolution. The experiment will use a two-axis gimbal to point to various points on the limb and disk of the Earth. The instrument consists of an extreme ultraviolet (EUV) imager and a far-ultraviolet (FUV) imager. The EUV imager will operate at 83.4 nm using a low resolution imaging spectrograph to set the passband. The O II 83.4 nm emission is produced by photoionization of O during the daytime. Above the limb, this emission can be used to determine altitude distribution of the O⁺ density. The FUV imager will operate at 130.4 , 135.6, and 143.0 nm. At night, The O I 130.4 and O I 135.6 nm emissions are produced by primarily by radiative recombination, and therefore these emissions provide useful ionospheric diagnostics at night. During the daytime, the 130.4 and 135.6 nm lines are primarily produced by photoelectron impact excitation of O; however the 130.4 nm line is also excited by resonant scattering of sunlight. The O I 130.4 and 135.6 nm lines provide information on the O density during the daytime. The instrument will map the daytime N₂ column density using the Lyman-Birge-Hopfield bands near 143.0 nm.

The Volumetric Imaging System for the Ionosphere (VISION) is designed to use limb and nadir images to reconstruct the three-dimensional distribution of electrons over a 1000 km wide by 500 km high slab beneath the satellite with 10 km x 10 km x 10 km voxels. The primary goal of the VISION is to map and monitor global and mesoscale (> 10 km) electron density structures, such as the Appleton anomalies and field-aligned irregularity structures. The VISION consists of three UV limb imagers, two UV nadir imagers, a dual frequency Global Positioning System (GPS) receiver, and a coherently emitting three frequency radio beacon. The limb imagers will observe the O II 83.4 nm line (daytime electron density), O I 135.6 nm line (nighttime electron density and daytime O density), and the N₂ Lyman-Birge-Hopfield (LBH) bands near 143.0 nm (daytime N₂ density). The nadir imagers will observe the O I 135.6 nm line (nighttime electron density and daytime O density) and the N₂ LBH bands near 143.0 nm (daytime N₂ density). The GPS receiver will monitor the total electron content between the satellite containing the VISION and the GPS constellation. The three frequency radio beacon will be used with ground-based receiver chains to perform computerized radio tomography below the satellite containing the VISION. The measurements made using the two radio frequency instruments will be used to validate the VISION UV measurements.

Two-dimensional images of Sporadic-E layers have been produced using a new technique called radio induced fluorescence (RIF). This technique makes the ion layers glow when being stimulated by high power radio waves. Normally the ion-layers do not radiate visible emissions. Experiments on January 1998 at Arecibo Observatory in Puerto Rico have shown that the layers can be made to glow at 557.7 nm and other wavelengths by illuminating them with radio waves at 3.175 MHz with effective radiated powers of 80 megawatts. The regions of the sporadic-E layers that have electron densities greater than the critical density for reflection of the radio waves emit electrons that collide with and excite atmospheric atomic oxygen and molecular nitrogen. A charge-coupled-device (CCD) imager located on the ground is used to capture images of the glowing E-region structures. The camera exposure times were in the range of 15 to 45 seconds. The images obtained using this technique show a wide variety of structures in the sporadic-E layers. Some layers cover the 15 x 30 km region illuminated by the radio wave beam. Other layers show strong modulation of the E-region by neutral wind instabilities. Two-dimensional computer simulations of the coupling between neutral wind turbulence and the ion layers simulate the structure in the images.

A bistatic lidar has been assembled at the HIPAS Observatory in Alaska (64.9 degree(s) N latitude and 146.8 degree(s) W) around a 2.7 m diameter rotating Liquid Mirror Telescope (LMT) with parabolic mercury reflecting surface. The LMT is isolated in a tower under a float glass skylight for operation when outside temperature can be at -40 degree(s)C. The lidar operates in conjunction with a 70MW (Effective Radiated Power) Radio Frequency array and ionospheric heater, which has been shown to perturb the arctic ionosphere and the electrojet. Bistatic laser illuminators include a Doubled YAG pumped dye laser (presently tuned to the 590 nm sodium D₂ resonance), an Excimer pumped dye laser (also tuned to the D₂ line), and a Doubled Alexandrite laser for future N₂ and Ca⁺ detection. Observations include sporadic Na formation due to the aurora, detection of Leonid's meteor trails (starting at 180 km) and changes in the sodium layer due to the HIPAS-RF heater. The 590 nm lidar is now being modified to detect polar stratospheric clouds during the summer. Ozone and OH can be detected in the future with the 308 nm wavelength of the excimer laser. A more recent application will use the LMT to focus a several hundred Joule - nanosecond duration laser pulse to 100 km altitudes, for the purpose of creating multi kilometer long plasma columns in the sky for direct electroject modification experiments. The 1057 nm laser pulse will be generated by surplus Nova components; namely, 9.2 cm and 15 cm disk amplifiers in a double pass SBS configuration.

Within the Space Sciences Division at the Naval Research Laboratory (NRL), new facilities have been built and old facilities have been upgraded to provide greater accuracy and precision during the calibration of instruments used for observation of Extreme and Far ultraviolet airglow emission in the upper atmosphere. In addition to the calibration of whole instruments, the facilities necessary for the construction and characterization of the detectors used in the aforementioned spectral region now either exist or are in their final stages of construction. Also in existence at the Naval Research Laboratory are the facilities for the environmental testing of instruments and components for the space environment. The heart of the NRL ultraviolet calibration facilities is an oil-free vacuum chamber, 2-meters in length, with a diameter of 1.67 meters containing an optical test bench 1.2 m wide by 1.5 m long. Some of the various instruments that have already been calibrated in the chamber are the High Resolution Atmosphere and Auroral Spectrographs flown aboard the Air Force Advance Research and Global Observing Satellite and the five Special Sensor Ultraviolet Limb Imagers for the Air Force Defense Meteorological Satellite Program. The chamber's hardware and control software have been upgraded. The software upgrades to the vacuum calibration chamber will allow for autonomous operation with failure and emergency handling procedures to protect the instruments under test from a loss of vacuum environment. The hardware upgrades allow for chamber pressures in the low 10^-6 torr range during the operation of windowless gas discharge lamps. In addition, the upgrades provide the capability to stimulate an instrument using two sources of light simultaneously, one through a monochromater the other by direct illumination.

The Global Imaging Monitor of the Ionosphere (GIMI) is one of the scientific instruments aboard the Department of Defense's Advanced Research and Global Observation Satellite (ARGOS). GIMI contains two far-ultraviolet electron- bombarded CCD (EBCCD) cameras, with 9 degree(s)-square fields of view. GIMI is intended for far-UV imagery of upper atmospheric and ionospheric airglow and auroras, occultations of UV-bright stars by the neutral upper atmosphere, and deep-space celestial objects and diffuse far-UV background. To obtain quantitative information from the images, an extensive program of calibrations, prior to launch and in-orbit, was required. Laboratory calibrations before launch consisted of imaging monochromatic laboratory UV light sources whose integrated intensities were monitored by separately calibrated photon-counting detectors. The in- flight calibrations involved observations of UV-bright starts, for which previously obtained and calibrated UV- bright stars, for which previously obtained and calibrated UV spectra exist from previous UV astronomy satellites (such as OAO-2 and IUE). We describe the procedures used in obtaining data for calibration purposes, and for reducing and analyzing the calibration data. We also discuss procedures and results for determining the absolute sensitivities, and their variations with wavelength, position in the field of view, and on-orbit time during the ARGOS mission.

The Special Sensor Ultraviolet Spectrographic Imager (SSUSI) is currently slated for launch on the Defense Meteorological Satellite Program (DMSP) F-16 in November 2001. This instrument consists of a scanning imaging spectrograph (SIS) whose field-of-view is scanned from horizon-to-horizon and a nadir-looking photometer system (NPS). It will provide operational information about the state of the atmosphere above 100 km. The unique problems incurred by the observational requirements (e.g. that we be able to make daytime and nighttime observations) and the design trade-offs needed to meet those requirements were strong drivers on calibration requirements. Those design trade-offs and the expectation that the instrument calibration will change appreciably in-flight have led to the requirement to perform a large instrument characterization in-flight using only natural sources. We focus, in this paper, on the flight characterization of the SSUSI instrument. This includes discussions of the stellar calibration approach for radiometric calibration, measurements of internally scattered light, sensitivity to the South Atlantic Anomaly, measurements of changing pulse height distributions, and measuring changing reflectivity of a nadir viewing scan mirror. In addition, the calibration of the NPS system using natural sources is addressed.

Operational sensors are designed and intended to reliably produce the measurements needed to develop high-value key environmental parameters. The Special Sensor Ultraviolet Spectrographic Imager (SSUSI) is slated to fly on the next five Defense Meteorological Satellite Program launches (beginning with the launch of F16 in Fall 2001). SSUSI will routinely produce maps of ionospheric and upper atmospheric composition and image the aurora. In this paper we describe these products and our validation plans and the process through which we can assure our sponsors and data products users of the reliability and accuracy of these products.

The Advanced Research and Global Observation Satellite (ARGOS) has been operating since February 1999 and includes three spectrographs comprising the High Resolution Airglow and Auroral Spectroscopy (HIRAAS) experiment. The HIRAAS instruments remotely sense the Earth's mid-, far- and extreme-ultraviolet airglow to study the density, composition, and temperature of the thermosphere and ionosphere. The Low Resolution Airglow and Aurora Spectrograph (LORAAS) is a limb scanner covering the 80-170 passband nm with 1.8 nm spectral resolution. Repeated serendipitous observations of hot O- and B-type stars have been used to improve the aspect solution, characterize the instrument field-of-view, and monitor relative sensitivity degradation of the instrument during the mission. We present the methodology of performance characterization and report the observed performance degradation of the LORAAS wedge-and-strip microchannel plate detector. The methods and results herein can be utilized directly in on-orbit characterization of the SSULI operational sensors to fly aboard the DMSP Block 5D3 satellites.

The Cosmic Origins Spectrograph (COS) will be the most sensitive UV spectrograph to be flown aboard the Hubble Space Telescope. The COS FUV and NUV channels will provide high sensitivity at resolution greater than 20000 over wavelengths ranging from 115nm to 320nm. We present a brief review of the instrument design, results from the optical testing of FUV gratings and predicted on orbit performance.

Today's large aperture telescopes have created a demand for diffraction gratings that are larger than the capabilities of the largest ruling engines. To overcome this obstacle, a method to produce a stable and passive assembly of two or more gratings is required. Replicated monolithic mosaics are produced by the simultaneous optical replication of two submaster diffraction gratings that are aligned relative to one another using a massive kinematic support structure. Relative surface alignment of the individual grating segments of better than 1 arc second is reported.

The fabrication of large high-quality diffraction gratings remains one of the most challenging tasks in optical fabrication. Traditional direct-write methods, such as diamond ruling or electron-beam lithography, can be extremely slow and result in gratings with undesired phase errors. Holographic methods, while generally resulting in gratings with smoother phase, frequently require large aspheres and lengthy optical setup in order to achieve desired period chirps. In this paper we describe a novel interference lithography method called scanning-beam interference lithography (SBIL) that utilizes small phase-locked scanning beams to write general periodic patterns onto large substrates. Small mutually coherent beams are phase controlled by high-bandwidth electro-optic components and caused to overlap and interfere, generating a small grating image. The image is raster-scanned over the substrate by use of a high-precision interferometer-controlled air bearing stage, resulting in large grating patterns. We will describe a prototype system in our laboratory designed to write gratings with extremely low phase distortion. The system is being generalized to pattern gratings with arbitrary period progressions (chirps). This technology, with extensions, will allow the rapid, low cost patterning of high-fidelity periodic patterns of arbitrary geometry on large substrates that could be of great interest to astronomers.

We present design examples for instruments making use of micromachined silicon grisms and immersion gratings. The capabilities of high index grisms, transmission grating-prism hybrids, open up new possibilities in compact IR spectrograph design with spectral resolving power, R~500-5000. Coarsely grooved immersion gratings will provide for unique high resolution spectrograph designs in the near and mid-infrared (resolving power, R~10⁴-10⁵). The high refractive index of silicon shortens the required grating depth, to produce a given resolving power, by up to a factor of 3.4. Alternatively, at a given resolving power, an immersion grating can allow a spectrograph slit to be widened by this factor relative to an instrument using a grating illuminated in air or vacuum; this increases the instrument sensitivity without degrading the spectral resolution. Our analysis here illustrates the potential of these devices to improve spectrograph throughput, spectral resolution, and wavelength coverage while reducing the required instrument volume relative to similar instruments using non-immersed diffraction gratings and low index prisms and grisms.

We report new results on silicon grism and immersion grating development using photolithography and anisotropic chemical etching techniques, which include process recipe finding, prototype grism fabrication, lab performance evaluation and initial scientific observations. The very high refractive index of silicon (n=3.4) enables much higher dispersion power for silicon-based gratings than conventional gratings, e.g. a silicon immersion grating can offer a factor of 3.4 times the dispersion of a conventional immersion grating. Good transmission in the infrared (IR) allows silicon-based gratings to operate in the broad IR wavelength regions (~1- 10 micrometers and far-IR), which make them attractive for both ground and space-based spectroscopic observations. Coarser gratings can be fabricated with these new techniques rather than conventional techniques, allowing observations at very high dispersion orders for larger simultaneous wavelength coverage. We have found new etching techniques for fabricating high quality silicon grisms with low wavefront distortion, low scattered light and high efficiency. Particularly, a new etching process using tetramethyl ammonium hydroxide (TMAH) is significantly simplifying the fabrication process on large, thick silicon substrates, while providing comparable grating quality to our traditional potassium hydroxide (KOH) process. This technique is being used for fabricating inch size silicon grisms for several IR instruments and is planned to be used for fabricating ~ 4 inch size silicon immersion gratings later. We have obtained complete K band spectra of a total of 6 T Tauri and Ae/Be stars and their close companions at a spectral resolution of R ~ 5000 using a silicon echelle grism with a 5 mm pupil diameter at the Lick 3m telescope. These results represent the first scientific observations conducted by the high-resolution silicon grisms, and demonstrate the extremely high dispersing power of silicon- based gratings. The future of silicon-based grating applications in ground and space-based IR instruments is promising. Silicon immersion gratings will make very high-resolution spectroscopy (R>100,000) feasible with compact instruments for implementation on large telescopes. Silicon grisms will offer an efficient way to implement low-cost medium to high resolution IR spectroscopy (R~ 1000-50000) through the conversion of existing cameras into spectrometers by locating a grism in the instrument's pupil location.

Methods for surface metrology have advanced significantly in the last few years, driven largely by the metrology needs for advanced lithographic processes. This paper applies recently developed metrology techniques to the specific problem of determining the groove structure of diffraction gratings well enough to reliably predict performance. Metrology devices used include an atomic force microscope, a contact profilometer, and a late-model optical microinterferometer. Examples of shallow (far-UV, high dispersion) and deep (IR echelle) gratings are presented, along with some conclusions of which metrology techniques are applicable for which types of diffraction grating. Also required along with the metrology is the use of fast, full electromagnetic model efficiency calculation codes which calculate the efficiency to be expected from a given mount, materials set, and grating profile. We present results qualifying codes we use against known and published results.

Design and performance of Fully Automated Ultraviolet Spectrographic Tester (FAUST) providing Bidirectional Scatter Distribution Function (BSDF) measurements at wavelengths ranging from the vacuum ultraviolet to the infrared has been described in details. The instrument is capable of measuring both very near (3 arcseconds) and very wide angle (over 120 degrees) scatter off both highly specular (mirrors and gratings) and highly diffuse surfaces for +/- 90 degrees incidence angle variations. Scatterometer dynamic range of over 11 orders of magnitude has been demonstrated. Stray light reduction techniques practicing for instrument signature improvement are discussed. Principles of light sources and detectors choice, instrument automation and calibration are explained. Instrument signature data along with some examples of a list of scatter measurements and gratings efficiency measurements performed by the use of FAUST are also presented here.

An overview of the measured performance for a variety of volume-phase holographic (VPH) gratings is presented. Many of the gratings analyzed were developed as part of a National Science Foundation funded effort to explore the viability of this technology for astronomical applications. Additional emphasis is given to some remaining issues with this technology and the likely means to resolve them.

Superb quality small Volume Phase Holographic Gratings are available and in operation at ESO. Compared to Surface Relief Transmission Gratings, they have better efficiencies at high dispersion. Their role at ESO would/will expand with larger sizes, high index modulation (larger bandwidth; access to the near-IR domain), cooled operation (also for the near-IR) and the articulated spectrograph approach (higher dispersion & efficiency than with Grisms).

The upgrade of already existing instrumentation keeps always the flavor of a second-hand choice when an insufficient amount of financial and material resources can be allocated to a project. The advent of VPH gratings into the astronomical instrumentation scenario offers a new possibility to reach spectral resolutions and efficiencies that could not be dreamt of in existing low dispersion spectrograph. d.o.lo.res. is a spectrograph of the FOSC series, which couples imaging to spectroscopy by means of a grism that can be put in the parallel beam between collimator and camera. The recent upgrade with a 1435 gr/mm VPH of d.o.lo.res. is the final point of a study carried out in the gOlem labs at Brera Observatory. The lab is equipped with an modified spectrophotometer (mod*) which is able to measure transmission of optical elements which will be equipped for cryogenic environment. The working study lead to the design of a high resolution VPHG in the Ha spectral region for the low resolution spectrograph. An increase of the hitherto limiting resolution of 2250 up to 5150 together with an increase of global efficiency of a factor of 1.4 give the added value of a 5000 USD investment for an 'old' instrument.

We present the design for an optical spectrograph for the 6.5-meter Magellan II Telescope. The spectrograph covers the full visible spectrum in a single exposure at very high efficiency through a dual-channel design and the use of volume phase holographic (VPH) gratings in lieu of traditional surface gratings. A pair of symmetric fold mirrors about the grating keep the spectrograph in Littrow configuration, eliminating the need for an articulated camera. Efficient VPH prescriptions have been developed for all resolution modes up to R=11,000. The resulting design is, mechanically and optically, relatively simple, compact, and inexpensive.

The recent interest of the astronomer community for volume phase holographic gratings is directly related to the enhancement of spectrograph throughput since the grating can rise higher diffraction efficiency. Indeed, dichromated gelatin technology has demonstrated capability for 70-90% efficiency. From the heritage of several diffractive and holographic projects and applications, the Centre Spatial de Liege has recently decided to invest in the large-scale DCG grating technology. This paper will present the new facility presently under construction. The goal is to be ready to respond to the market demand in 2002 with a capacity for producing 30 cm dia. holographic gratings. The challenge is not the size itself but the quality control in each process step. Thanks to the heritage of space instrumentation, CSL is trained to fulfill requirements on product and quality control. Large clean rooms are equipped with DCG coating machine, optical bench, development lab, and conditioning processes. The grating period may range from 325 to 3000 lp/mm. Low frequencies are especially hard to holographically record because it induces a cumbersome set-up. The working wavelength of DCG gratings is limited by the gelatin transmissivity (from 350 nm to 2 micrometers ). But the actual limitation factor in the IR is the refractive index modulation, equivalent to etching depth on ruled gratings: working wavelength of 1.5 micrometers means a need for 3 times the modulation of a visible grating. Large efforts are needed to insure that IR volume-phase gratings can reach efficiency higher than alternative grating technologies. In that field, this paper presents experimental results on small grating samples. A realistic performance goal is discussed to advise the astronomer community of our near-future products.

Thermally irreversible photochromic materials, mainly belonging to the diarylethene class, are potentially useful as optoelectronic devices. Poly-1,2-bis-(2-methyl thiophen-3-yl)perfluorocyclopentene has been studied in order to verify its possible application in astronomy for the production of re-writable focal plane masks and volume phase holographic gratings. Films of photochromic materials embedded in an amorphous polymer matrix are obtained by casting from chloroform solution. The films show a good transparency in the visible region when the photochromic molecule is in the open-ring form, while they totally absorb visible wavelength radiations when the photochrome is in the other isomeric form. The variation in refractive index between the two photochromic isomers, evaluated by UV-vis-NIR absorption spectra, is large enough to make this photochromic apt for the production of volume phase holographic gratings.

This research employs UV-DOAS to log ambient atmospheric measurements of criteria pollutants and to gather data specifically for determining transformation rates. Species of concern include oxides of nitrogen (NO + NO₂ = NO_x) and sulfur dioxide (SO₂). Combustion sources such as coal fired power plants continuously emit NO_x and SO₂, which enter the atmosphere and become dispersed and transformed, resulting in the formation of (formula available in paper). The goal of this research is to determine the transformation rates from power plant plumes for (formula available in paper) in a coastal environment. Continuous emission rates are known and ambient primary pollutant concentrations are continuously monitored with UV-DOAS. Secondary pollutant concentration levels are estimated from integrated annular denuder system measurements. The results of this research will be used to estimate the impacts of the nitrogen emissions from local power plants on the atmospheric nutrient loading to Tampa Bay and to assess the validity of current SO₂ to H₂SO₄/SO₄^2- modeled transformation rates in the Tampa environment.

The UV-Vis DOAS spectrometer GASCOD/A4p (Gas Analyzer Spectrometer Correlating Optical Differences, Airborne version) was installed on board the stratospheric Geophysica aircraft during the APE-THESEO and APE-GAIA campaign in February-March and September-October 1999 respectively. The instrument is provided by five input windows, three of which measure scattered solar radiation from the zenith and from two horizontal windows, 90 degree(s) away from the zenith to perform limb-absorption measurements. Spectra from 290 to 700 nm were processed through DOAS technique to obtain trace gases column amounts. Data from horizontal windows, which are performed for the first time from an airborne spectrometer, are used to retrieve an average concentration of the gases along a characteristic length of the line of sight. An atmospheric Air Mass Factor model (AMEFCO) is used to calculate the probability density function and the characteristic length used to reduce the slant column amounts to in-situ concentration values. The validation of the method is performed through a comparison of the values obtained, with a in-situ chemiluminescent ozone analyzer (FOZAN) which performed synchronous measurements on board Geophysica aircraft. Data from the APE-GAIA campaign was presented and discussed.