The Airborne Visible/Infrared Imaging Spectrometer-4 (AVIRIS-4) represents the next generation in the series of airborne imaging spectrometers built by NASA JPL. Commissioned by the Swiss ARES research consortium, AVIRIS-4 is geared towards delivering cutting-edge imaging spectroscopy data for scientific and practical applications as a replacement for its predecessor APEX. AVIRIS-4 is based on a Dyson-type imaging spectrometer design, also employed by NASA-operated AVIRIS-3 and EMIT, and integrates a scaled two-mirror telescope housed in a compact vacuum vessel. This enables airborne measurements in unpressurized aircraft at altitudes ranging from 500 m to 7620 m, achieving image resolutions between 0.3 and 4.5 m with a field of view of 40.2° in 1241 spatial pixels. AVIRIS-4 surpasses previous state-of-the-art sensor heads in signal-to-noise ratio performance and features a spectral range of 375 to 2504 nm and 7.4 nm spectral sampling. The operation, data capture and mission control hardware as well as the calibration and data processing software is developed by UZH, EPFL, and ZHAW. This paper outlines the design and calibration strategies implemented in AVIRIS-4’s development and highlights its performance during its first year of operation in 2024.

Reference data sets are crucial for quality assessments of airborne and space-based spectral data. This study estimates the uncertainty on a data set consisting of more than 8400 in-situ collected white reference spectra stored in the spectral data base SPECCHIO. The given measurements are evaluated quantitatively using LibRadtran simulations.

The accurate characterisation of the instrumental spectral performance of field spectroradiometers acquires particular importance in view of the instruments' widespread use for calibrating and validating airborne- and satellite-based optical sensor systems. Typically, spectral calibration relies on the basic assumption of a Gaussian instrumental spectral response function (ISRF). We test alternative ISRF parameterisations and examine the resulting changes in spectral performance of four ASD field spectroradiometers. We then use a MODTRAN-simulated spectrum to compare an ideal Bottom Of Atmosphere (BOA) reflectance with the reflectances derived from ASD-convolved radiances using nominal (per ASD specifications), Gaussian and Symmetric Super Gaussian parameterisations of the spectroradiometers. We highlight the impact of the small but significant differences by retrieving the Photochemical Reflectance Index (PRI) values, which change by a few percentage points compared to the reference PRI.

Imaging spectroscopy gains more importance in various and diverse applications. Using the appropriate spectral resolution of a spectral sensor for the targeted study area is important to obtain meaningful data. In this study, a Principal Component Analysis (PCA) was performed on spectrally convolved spectral ground control points of different land cover types obtained using field spectroradiometers to estimate the information content of the data in a wavelength range from ultraviolet to short-wave infrared (350 - 2500nm). Based on the results, an optimal spectral resolution (Full Width at Half Maximum (FWHM)) for the modelled Spectral Response Function (SRF) can be established and discussed.

Remote sensing has demonstrated its potential for wide-scale biodiversity monitoring. It overcomes the spatial limitations of ground-based measurements and allows acquiring high-temporal, -spatial and -spectral resolution data. The temporal trajectories of vegetation spectral responses hold information about time-specific physiological and morphological turnover that may express genetic constraints. We investigated the spectral response of individual trees derived from multi-temporal airborne imaging spectroscopy data along with genetic information and climatic variables. By deriving similarities of spectral and genetic distances, we present a multitemporal approach using remote sensing data to improve biodiversity monitoring at the level of genetic diversity within species.

This work tested the practical mapping of floating plastic waste at surface area abundances between 1% - 5% using airborne imaging spectroscopy. APEX and AVIRIS-ng sensors were flown over deployed targets of known abundance during the ESA HyperSense campaign in 2018. Results show that such low abundances can be detected and mapped, while actual material identifications start to fail for abundances of 2.5% and lower.

Airborne hyperspectral imaging data recently gains more attention due to its potential in a variety of fields. Ensuring good data quality is a crucial step in order to produce valuable results from hyperspectral imagery. We therefore present a MATLAB based tool to perform an automated basic data quality analysis. The program compares airborne hyperspectral data cubes with its spectral ground control points and extracts basic statistical measures. The user selects one or several flight lines of either the same or different sensors and coordinate systems. The result is a single PDF file per target enabling a convenient and straightforward way to assess the spectroradiometric data quality. Up until now, the tool can be applied to APEX and AVIRIS-NG reflectance data cubes.

Hyperspectral target detection experiments under nonideal conditions are scarce. An extensive multi-scale and multi-temporal field experiment was designed towards the goal of knowledge expansion under such circumstances. A range of camouflage materials and specific targets of interest were placed in a realistic natural environment with vegetation cover and varying illumination. In several experiments, aspects like changes in the sun position, variable moisture, and relocations of targets were analysed. Using an aircraft-based and a drone-based imaging spectrometer, the target scenarios were mapped at different daytimes. The data were radiometrically, atmospherically and geometrically processed to allow subsequent data analysis. First insights deliver promising results.

Imaging spectroscopy is frequently used to assess traits and functioning of vegetated ecosystems. Applied reflectance- and radiance-based approaches critically rely on accurate estimates of surface irradiance. Accurate retrievals of surface irradiance are, however, nontrivial and often error-prone, thus causing inaccurate estimates of vegetation information. We analyze the irradiance field surrounding an isolated tree using the 3-D radiative transfer model DART in high spatial (25 cm) and spectral (1 nm, 350-2500 nm) resolution. We validate modeled irradiance with in situ measurements and quantify the impact of erroneous surface irradiance estimates on the retrieval of vegetation indices. We observe the irradiance gradients in the cast shadows of <;560% in the blue spectral range, while this gradient decreases with increasing wavelength and becomes negligible in the near infrared (NIR). Furthermore, we quantify a vegetation-induced decrease in the irradiance of <;6% in the visible spectral region and an increase of <;7% in the NIR outside the cast shadow. Commonly employed vegetation indices are also affected by such brightening or darkening effects. Outside the cast shadow, indices sensitive to the relative content of chlorophyll (CHL) and carotenoids (CAR) show an overestimation of <;14%. The photochemical reflectance index shows an underestimation of <;5%. This paper provides first quantitative insight in high spatial and spectral resolution, on the impact of vegetation on its surrounding irradiance field. Findings highlight important implications for vegetation assessments and provide the fundamental base to advance retrievals of vegetation traits and functioning from imaging spectroscopy data.

Many field spectroradiometer user interfaces have been designed by industry without aligning them with typical scientific end-user requirements. We use two case studies to demonstrate the potential of streamlined graphical user interfaces, adhering to a number of generic software requirements for such applications. The software developed includes an Apple iPad based application to control an ASD Fieldspec spectroradiometer and a Java application to interface with the new autonomous dual field of view FloX/RoX fieldspectroradiometers and manage spectral data imported from their SD storage cards. Both applications are available for testing and usage. It is hoped that industry will be inspired by these case studies, improving the design of future implementations and making their instrument interface documentation freely available to the scientific community, allowing the implementation of specialized instrument control software.

Shifts of center wavelengths (CWLs) and related changes of full-widths at half-maximums (FWHMs) occur during in-flight data acquisitions of push-broom imaging spectrometers such as the airborne prism experiment (APEX). Combined with the spectrally changing properties of the dichroic coating that acts as a beam splitter between the visible and near infrared (VNIR) as well as the short-wave infrared (SWIR) channels, these shifts affect both the spectral and radiometric accuracies of the spectrometer data, and hence the accuracy of higher level products. In this paper, two independent standards, i.e., atmospheric absorption features as well as features of the standard reference material filter built in the APEX in-flight characterization facility, are used in a complementary way to improve in-flight spectral calibration. The CWL shift and FWHM change for each detector element are simultaneously detected by using spectrum-matching and surface fitting techniques under constraints from pregenerated shift realizations. Subsequently, the APEX spectroradiometric response model is improved in the aspect of spectral resolution by using performance parameters of optics and detector modules. The radiometric gain and offset for each detector element are corrected according to the detected CWLs and FWHMs, as well as the improved APEX response model. Compared with the spectral and radiometric parameters acquired during laboratory calibration, the detected CWLs and FWHMs promote the accuracy of the atmospheric feature positions in the SWIR channel by 10 nm, whereas the corrected gains and offsets reduce the radiance deviation in the spectral regions 375-550 nm and 950-1080 nm both by 70% on average.

The airborne prism experiment (APEX) is an imaging spectrometer developed by a joint Swiss-Belgian consortium composed of institutes (University of Zurich, Flemish Institute for Technological Research) and industries (RUAG, OIP, Netcetera), supported by the European Space Agency's PRODEX programme. APEX is designed to support the development of future space-borne Earth observation systems by simulating, calibrating or validating existing or planned optical satellite missions. Therefore, periodic extensive calibration of APEX is one major objective within the project. APEX calibration under laboratory conditions is done at its dedicated calibration and characterization facility at the German Aerospace Center (DLR) in Oberpfaffenhofen, Germany. While environmental influences under laboratory conditions are reduced to a minimum, the effects of atmospheric absorption and the properties of the underlying calibration infrastructure may still influence the measurements and subsequently the accuracy of the sensor spectral response estimations. It is demonstrated that even a lightpath of ~2 m through the atmosphere or the monochromator grating can have significant impact on the spectral response estimation of the sensor. A normalization approach described in this letter is able to compensate for these effects. The correction algorithm is exemplarily demonstrated on actual measurements for the short wavelength-IR range channel.

The APEX airborne imaging spectrometer has been shown to exhibit spectral shifts during in-flight conditions, linked to changes in the nitrogen gas density within the APEX optical subunit. These shifts lead to features in the recorded spectra caused by the dichroic coating used as a beam splitter between VNIR and SWIR channels. Consequently dichroic features are no longer compensated for by the radiometric calibration coefficients obtained under laboratory conditions. This paper presents results of a numerical simulation that can model the impact of spectral shifts on radiometry. As a consequence the APEX sensor model has been upgraded and according correction functions have been implemented in the APEX level 1 processor to compensate for shift dependent changes in radiometry due to the dichroic coating.

APEX system calibration has reached on operational level with calibration procedures at the CHB being well established and the APEX CAL IS enabling an efficient and transparent data storage and processing for the timely generation of calibration cubes required for the radiometric calibration of APEX imagery. Overall, these achievements result in shorter times from laboratory calibration to processed end user products.

There is an urgent need within the international remote sensing community to establish a metadata standard for field spectroscopy that ensures high quality, interoperable metadata sets that can be archived and shared efficiently within Earth observation data sharing systems. Careful examination of all stages of metadata collection and analysis can inform a robust standard that is applicable to a range of field campaigns. This paper presents approaches towards a standard that encompasses in situ metadata collection and initiatives towards sharing metadata within intelligent archiving systems.

The calibration of remote sensing instruments is a crucial step in the generation of products tied to international reference standards. Calibrating imaging spectrometers is particularly demanding due to the high number of spatiospectral pixels and, consequently, the large amount of data acquired during calibration sequences. Storage of these data and associated meta-data in an organized manner, as well as the provision of efficient tools for the data analysis and fast and repeatable calibration coefficient generation with provenance information, is key to the provision of traceable measurements. The airborne prism experiment (APEX) calibration information system is a multilayered information technology solution comprising a database based on the entity-attribute-value (EAV) paradigm and software written in Java and Matlab, providing data access, visualization and processing, and handling the data volumes over the expected lifetime of the system. Although developed in the context of APEX, the system is rather generic and may be adapted to other pushbroom-based imagers with little effort.

The APEX system is fully operational since 2011 and ready to provide well-calibrated data as standard processing output of the APEX PAF and the VITO CDPC to the user community. We sincerely hope that the freely available APEX Open Science Data Set will assist the community in gaining insights into the huge potential of APEX and help in assessing its applicability to specific domains of spectroscopy research.

Spectral ground data are one important asset in the provision of reliable imaging spectroscopy products by the application of Cal/Val procedures. These data should firstly be stored in an organised manner, documented by rich metadata, and secondly be exchangeable between database systems to enable data sharing between research groups. In this paper we present a solution of the exchange of spectral data collections including their full metadata context in normalised form. The presented methods were implemented in the SPECCHIO spectral database system and allow the operational data exchange between distributed SPECCHIO instances.

The In-Flight Characterization (IFC) facility of the Airborne Prism Experiment (APEX) is presumably the first onboard characterization unit implemented in an airborne imaging spectrometer. This study is meant to test methodologies for the retrieval of temporal relative center wavelength drifts based on IFC data. A rare Earth material filter with a set of well-known absorption features is imaged through the IFC on APEX and recorded at several time positions. The shift of the center wavelengths covered by a spectral feature is estimated by means of curve matching algorithms. Two algorithms are evaluated: in the former the shift is determined by using the correlation coefficient as merit function to determine changes of the feature shape and position, while the latter evaluates the distance between centers of gravity. These methods have demonstrated an uncertainty in the order of 6-9 % of a pixel. A test case has been designed in which the APEX system was exposed to a temperature profile with a thermal excursion of 26°C, reproducing flight conditions. Results show the spectral stability of the APEX imaging spectrometer.

The correction of BRDF (bidirectional reflectance distribution function) effects in imaging spectrometer data requires object specific spectrodirectional information. Such data may be acquired using goniometer systems such as the dual-view Field Goniometer System (FIGOS). Their use in an operational processing environment requires optimized solutions for their storage, assessment, pre-processing and application in the actual correction procedure. The final goal is the provision of a spectral albedo product to the end user.

The product generation from hyperspectral sensor data has high requirements on the processing infrastructure, both hardware and software. The Airborne Prism Experiment (APEX) processing and archiving facility has been set up to provide for the automated generation of level-1 calibrated data and user-configurable on-demand product generation for higher processing levels. The system offers full reproducibility of user orders and processing parameters by employing a relational database. The flexible workflow software allows for the quick integration of novel algorithms or the definition of new processing sequences. Reprocessing of data is supported by the archiving approach. Configuration management based on the database enables the control over different versions of processing modules to be applied. The system is described with a focus on the APEX instrument; however, its generic design allows adaptation to other sensor systems.

The facilities to support the ESA's airborne APEX hyperspectral mission simulator are described. These facilities include calibration tools, such as specific processing in a dedicated Processing and Archiving Facility (PAF), operational calibration and characterization using the Calibration Home Base (CHB), the In-Flight Characterization facility (IFC) and the Calibration Test Master (CTM). Further on, a preview on major applications and the corresponding development efforts to provide scientific data products up to level 2/3 to the user are outlined. Products dedicated for the retrieval of limnology, vegetation, atmospheric parameters, as well as general classification routines and rapid mapping tasks are currently under development and prepared for dissemination by the APEX Science Center (ASC) and the APEX Operations Center (AOC).