This conference presentation was prepared for Remote Sensing, 2024.

Current critical needs of NASA, NOAA, and other agencies carrying out Earth environmental monitoring require higher-performance observing systems that offer lower noise, finer resolution, broader coverage, etc., but that are also lower-cost, can be accommodated on a wide range of launch vehicles and hosted payload platforms, as well as provide flexibility in how they are deployed and used. To achieve these ambitions, it is necessary to consider the observing system as comprising not only the sensor but also the concept of operations, processing, and potential for collaborative and synergistic observations. Here we present a new approach that enables dynamic, data-driven sensing and provides a way to test and evaluate the overall end-to-end system performance in the laboratory prior to launch with realistic Earth scenes. Recent technology advances now enable the utilization of new sensing concepts that reconfigure the sensor in real time to adjust where they are looking, their dwell time, their spatial resolution, and depending on the platform, their geometrical vantage point. For example, at frequencies spanning approximately 10-100GHz, phased array and reflectarray observations of sea surface wind (speed and direction), wide-swath polarimetric imagery, soil moisture and sea surface temperature, and atmospheric thermodynamic state that are deemed critical by NASA’s Earth Science strategic goals are now possible. These measurements would all be improved by this work, since the sensor would be configured for maximum resolution, coverage, and dwell time for regions in the scene that exhibit the highest variability and would therefore benefit the most from high-fidelity sensing. This approach also efficiently optimizes the use of a fixed set of resources. Here we describe two new systems recently funded by the NASA Earth Science Technology Office (ESTO) to improve present capabilities for high-resolution atmospheric sensing from small satellite platforms: the Configurable Reflectarray Wideband Scanning Radiometer (CREWSR) and a Versatile, Intelligent, and Dynamic Earth Observation (VIDEO) testing and development platform for data driven and configurable sensors.

The Electrojet Zeeman Imaging Explorer (EZIE) is a three-satellite mission that images the magnetic fingerprint of intense electrical currents flowing in the Earth’s ionosphere. The multi-point measurements of these electrojets will provide closure to decades-old, and much debated, mysteries of the interaction between the Earth and the surrounding space. Each one of the three 6U CubeSats carries a microwave electrojet magnetogram (MEM) instrument which consists of four identical 118-GHz heterodyne spectropolarimeters. These MEM instruments were designed to meet the stringent mass, power and volume requirements of the CubeSat spacecraft. MEM instruments use the Zeeman splitting effect on the 118.75GHz oxygen absorption line to infer magnetic fields at ~80km altitude. These magnetic fields are used to calculate the local ionospheric electrical currents flowing at 100-130km altitude. The flight models of the instruments were built and tested in-house at JPL within 12 months. Each of the final instruments weighs 4.1kg, and operates with 20W of power. Despite the compact size and low power consumption the noise temperatures of the instruments are at 450K level and the measurement noise is less than 1.5K on the 49kHz polarimetric spectrometer system. The planned launch of the EZIE mission is in March 2025.

The NOAA-21 satellite successfully completed its post-launch checkout, verification, and validation of all the sensor data and key performance parameters. It is now the primary satellite in NOAA’s Joint Polar Satellite System with redundant back up provided by NOAA/NASA Suomi National Polar-orbiting Partnership (SNPP) and NOAA-20 satellites. The performance of all NOAA-21 data meets specifications and are within family compared to SNPP and NOAA-20. The global data provided by the mission is critical for numerical weather prediction models for making timely and accurate weather forecasts, as well as for detecting and monitoring environmental events such as floods, fires, and changes in atmospheric chemistry such as ozone concentration.

Monitoring aerosols in the stratosphere requires measurements with good vertical resolution and comprehensive spatial sampling. We have developed a small satellite instrument that utilizes the limb scattering observation technique to meet these requirements. The Aerosol Radiometer for Global Observation of the Stratosphere (ARGOS) instrument measures radiance profiles in eight equally spaced directions simultaneously, using two near-infrared wavelengths (870nm and 1550nm) to improve penetration into the upper troposphere and lower stratosphere (UT/LS). The combination of multiple viewing directions and multiple wavelengths provides improved spatial sampling and statistical leverage to validate the particle size distribution used to retrieve aerosol extinction. ARGOS is scheduled for a technology demonstration flight in February 2025.

The OreSat 0.5 is a novel small satellite developed in collaboration between Portland State University in Portland, Oregon, USA, the University of Maryland, Baltimore County, in Baltimore, MD, USA, and the Mullard Space Science Laboratory at University College London, Surrey, UK. OreSat 0.5 will demonstrate global cirrus cloud detection and mapping from a compact, low-cost platform. In this work, we preview the OreSat 0.5 mission and demonstrate the calibration and science behind its primary payload, the Cirrus Flux Camera (CFC). The CFC is a three-channel shortwave infrared radiometer (870, 1390, 1590 nm bands). Flux ratios between its three bands will be used to differentiate ice versus water and noncloud signals. Along-track and Across-track pointing up to ±45° will allow retrievals of heights and winds of the cirrus cloud tops. We discuss a preliminary pre-launch calibration of CFC and plans to expand upon and maintain this calibration vicariously on-orbit and through proxy sources. OreSat 0.5 launched to space on August 16 2024 and first light data is expected by Q4 2024.

The Ocean Color Instrument (OCI) on NASA’s Plankton, Aerosol, Cloud, and ocean Ecosystem (PACE) mission has been providing data to the science community since April 2024. OCI is a hyperspectral imager, providing almost daily global coverage, at a spatial resolution of 1.2km. Its design specifications were optimized for ocean color and atmospheric applications, but terrestrial studies could benefit from its hyperspectral coverage as well. The ocean color requirements called for very high radiometric accuracy, which could benefit a wide variety of applications. This paper presents results from the first 6 months of on-orbit calibration and characterization measurements, including absolute calibration, spectral registration, temporal trending of radiometric sensitivity, signal to noise ratio, and linearity, with a focus on the commissioning results obtained in the first 2 months after launch.

The Hyper-Angular Rainbow Polarimeter-2 (HARP2) was launched on board the Plankton, Aerosol, Cloud and ocean Ecosystem (PACE) mission, in February 2024, for the global measurement of aerosol and cloud properties as well as to provide atmospheric correction over the footprint of the Ocean Color Instrument (OCI). HARP2 is designed to collect data over a wide field of view in the cross-track direction (+/-47deg) allowing for global coverage in about two days, as well as an even wider field of view in the along-track direction (+/-54deg) providing measurements over a wide range of scattering angles. HARP2 samples 10 angles at 440, 550, and 870nm focusing on aerosol and surface retrievals, and up to 60 angles at 670nm for the hyper-angular retrieval of cloud microphysical properties. The HARP2 instrument collects three nearly identical images with linear polarizers aligned at 0°, 45°, and 90° that can be converted to push-broom images of the I, Q, and U Stokes parameters for each angle, and each wavelength. The HARP2 technology was first demonstrated with the HARP CubeSat satellite which collected a limited dataset for 2 years from 2020 to 2022. HARP2 extends these measurements to a full global coverage in two days, seven days a week.

Launched in February 2024, the PACE mission represents NASA’s next investment in ocean biology, clouds, and aerosol data records. A key feature of PACE is the inclusion of an advanced satellite radiometer known as the Ocean Color Instrument (OCI), a global mapping radiometer that combines multispectral and hyperspectral remote sensing.

The OCI flight-unit was built at NASA’s Goddard Space Flight Center. At the time of this writing, PACE/OCI has launched and completed on-orbit commissioning activities and four months of normal science operations. A key aspect of the OCI architecture is the capability to trend absolute and relative calibration changes over the course of mission life with solar calibration. Every 24 hours a quartz Quasi-Volume Diffuser (QVD), mounted at 90deg from the nadir position of the OCI spinning aperture, is oriented towards the sun via a mechanism as the spacecraft ground-track nears the North Pole. By knowing the irradiance of the sun and the reflectivity of the target, the absolute radiance at the input to the OCI aperture can be determined. The allowable absolute uncertainty budget for each solar calibration measurement is 1.6% 1-σ below 900nm at beginning of life (BOL) and the allowable relative uncertainty budget is ~0.26% 1-σ. The Solar Calibration Assembly (SCA) consists of two quartz QVD targets, one Acktar Fractal Black target, a baffle, and a mechanism which selects targets and opens a door.

This paper provides an overview of driving solar calibration requirements, SCA design and orbital maneuver, pre-launch tests and preliminary on-orbit results. Certain pre-launch measurements and analyses are covered in detail including diffuser Bidirectional Reflectance Distribution Function (BRDF) measurements, metrology measurements, and optical measurements of the SCA including stray light evaluation.

The NASA Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission launched from Kennedy Space Center in the early morning of February 8, 2024. Just 63 days later, data from NASA’s newest Earth-observing satellite became available to the public. These data will extend and improve upon NASA’s 20+ years of global satellite observation of our living oceans, atmospheric aerosols, and cloud and initiate an advanced set of climate-relevant data records. Ultimately, PACE is the first mission to provide daily, global measurements that will enable prediction of the “boom-bust” cycle of fisheries, the appearance of harmful algae, and other factors that affect commercial and recreational industries. PACE also observes clouds and tiny airborne particles known as aerosols that influence air quality and absorb and reflect sunlight, thus warming and cooling the atmosphere. In the months since launch and initial data release, the PACE Project pursued instrument temporal and system vicarious calibrations, executed cross-instrument comparisons, conducted performance assessments, explored synergies with other missions, and released advanced science data products. In parallel, the PACE Validation Science Team left for the field and the Post-launch Airborne eXperiment (PACE-PAX) prepared for its mission. And, most importantly, preliminary science results were realized. Here, we present a snapshot of these activities and their impacts and outcomes, encompassing the first half year of the PACE mission.

Launched in February 2024, the Plankton, Aerosol, Cloud, and ocean Ecosystem (PACE) mission represents NASA’s next investment in ocean biology, clouds, and aerosol data records. The Ocean Color Instrument (OCI) is the primary instrument supporting PACE by collecting accurate radiometric data of the Earth by observing the top of atmosphere reflectance and is a combination hyperspectral (ultra-violet to near infrared) / multiband imager (shortwave infrared). An important aspect of the radiometry is the ability of OCI to resolve contrast from scene to scene to achieve data accuracy requirements. This paper reviews the analysis that was performed to verify the modulation transfer function (MTF) requirement levied against the instrument and presents the supportive test data collected during pre-launch instrument testing to supplement the analysis. The requirements levied against OCI drove to a 1.2km² ground resolution leading to a Nyquist period of 2.4km (0.417cycles/km). The analysis is comprehensive with incorporation of as many contributors as practical to provide the clearest possible picture of the margin against requirements. The analysis provided input for additional design trade studies and insight into the aspects of the design that required the most attention during the implementation phase. We describe four separate MTF contributor networks based on the instrument design and mutual exclusivity of contributing factors. The various networks represent the hyperspectral and multiband detection systems separately as well as the along track and cross track imaging dimensions. Ground-test was performed against the MTF primary contributors and the results were used to validate the analysis as will be presented here.

The Ocean Color Instrument on NASA’s PACE mission is a 322-887nm hyperspectral imager with 1km x 1km nadir spatial resolution and 5nm spectral resolution utilizing charge-coupled devices (CCDs) operating in Time Delay Integration (TDI) mode where each TDI column represents a different wavelength in 0.625nm increments. After TDI, the charge is moved into serial output pixels and read out. The spatial resolution requires an 8.5MHz readout rate. This only allows 59ns for the CCD reset and video to be asserted and settled before sampling. The response exhibits serial pixel-to-pixel readout interference due to the lack of full settling. Each serial pixel value has a dependence on the value of the preceding pixel value. This leads to a spectrally dependent radiometric measurement error of up to 0.3%. We explain the operation of the detection system, the behavior of the interference, and show the resulting measurement error based on data from both ground testing and on-orbit characterization.

The Hyper-Angular Rainbow Polarimeter (HARP2) is a novel wide-field of view imaging polarimeter instrument on the recently-launched NASA Plankton Aerosol Cloud ocean Ecosystem (PACE) mission. Since launch on February 8 2024, HARP2 has taken over 6 months of global Earth data. In order for this data to meet scientific quality standards, we must ensure that it is as accurate as possible and over long periods of time. We use well-characterized Earth targets, such as Saharan deserts, as well as regular views of the Sun and dark frames to trend our on-orbit calibration. In this work, we discuss the preliminary performance trends derived from these activities and how well they compare with the HARP2 prelaunch calibration.

In February the PACE observatory was launched. Among the instruments on board is the multi viewport spectropolatimeter SPEXone. The instrument will be used for the characterisation and quantification of aerosols in the atmosphere. For the retrieval of aerosol properties it is important to observe the degree of linear polarisation and the intensity of reflected sun light from different viewing angles. Hence the need to ensure viewports are well aligned and ground pixels observed with different viewports are correctly matched. Once the aerosol properties are determined the results need to be interpreted. For the interpretation of the results knowledge of the location on Earth is required. Therefore we match the obtained images as well with other instruments (e.g. Sentinel 5). In this presentation we explain the algorithm used to register the various images, demonstrate sub-pixel accuracy on simulated data, and finally apply the method to in flight data.

Currently, Japan Aerospace Exploration Agency (JAXA), Japan Meteorological Agency (JMA) and Japan Space Systems (JSS) are operating major Earth Observation Satellites. Ibuki (GOSAT) carrying TANSO-CAI and -FTS, GOSAT-2 carrying TANSO-CAI2 and -FTS2, Shizuku (GCOM-W) carrying AMSR2, Daichi-2 (ALOS-2) carrying PALSAR-2 , DPR on GPM-core satellite of NASA, and Shikisai (GCOM-C) carrying SGLI, are being operated by JAXA under cooperation with some domestic agencies, such as Ministry of Environment (MoE), National Institute of Information and Communications Technology (NICT). JMA is operating meteorological satellite Himawari-8 and -9 on geostationary orbit. Next generation of meteorological satellite is about to develop by JMA. JSS is operating ASTER on EOS-Terra satellite of NASA and HISUI on ISS. For coming satellites or instruments, JAXA is preparing CPR on EarthCARE satellite of ESA, ALOS-4 carrying PALSAR-3 and GOSAT-GW carrying TANSO-3 + AMSR-3 as follow-on mission for GOSAT-2. And the first Japanese Lidar mission MOLI on ISS is expected to entering development phase

The Earth Cloud Aerosol and Radiation Explorer (EarthCARE) is a Japanese-European collaborative earth observation satellite mission. The Cloud Profiling Radar (CPR) is one of the EarthCARE instruments and it is the world's first onboard millimeter-wave Doppler radar in space. The Japan Aerospace exploration agency (JAXA) is responsible for development of the CPR in this mission in cooperation with the National Institute of Information and Communications Technology (NICT). The EarthCARE satellite was successfully launched at 07:20 JST on 29th May 2024 on the Space X Falcon-9 rocket from Vandenberg Space Force Base in California, USA. The first observations of the CPR were conducted on 12th and 13th June. The CPR observed the internal structure of cloud and succeeded in the world's first measurement of vertical cloud motion from space.

This work is an extensive study of the efficiency of the image regularization (Alternating Directions Method of Multipliers i.e., ADMM) procedure implemented in tandem with one of the popular denoising techniques (Total Variation or Block Matching and 3D Filtering or Convolutional Neural Networks) for reconstruction of remotely captured images in the electro-optical domain using telescopes with optically-sparse aperture (OSA) mirrors. A suite of thirty-four images taken by the Sentinel-2 telescope with a ground resolved distance (GRD) of 1m and circumscribing a multitude of features and scenes is considered as the ground truth images. These range from houses and roads in urban settlements to pastoral lands with houses and rivers in the same Field-of-View (FoV) to landscapes of rivers and their adjoining areas - thereby offering an ideal testbed to infer on the edge detection as well as texture, contrast and smoothness retention capabilities of the post-processing routines when observed by OSA mirrors. Such mirrors are already known to have constraints on their performance, albeit the compromised collecting area and Modulation Transfer Function (MTF). This situation is, further, complicated by incorporation of noise of various origins, for e.g., gaussian, impulse and shot noise. In this work, the authors present a comparative analysis on the imaging performance of a Standard OSA primary mirror with sub-apertures having equal sizes relative to two Non-Uniform Sized (NUS)-OSA mirror configurations - the Taylor-ln and One-by-Three configurations. The latter designs promise a significant improvement in the overall mass budget of the imaging system, capable of good imaging performance, attributed to their significant sidelobe suppression. However, the quality of the reconstructed images, including identification of small features, edge detection and preserving the contrast and texture depend significantly on the choice of the regularization parameters (λ and ρ), based on the features of the ground truth images and the type of mirror under consideration. Hence, to design an efficient imaging system where a series of observed images, corrupted with noise, is fed spontaneously to the post-processing pipeline via the downlink channel, the pipeline should be automated with an optimized pair of values for λ and ρ which could result in high quality reconstructed images, irrespective of the features in the input images. This is to enable near-real time observations for remote-sensing purposes and security and surveillance purposes, by avoiding any human intervention for determining these values on a case-to-case basis. It is concluded that assigning very low values to the penalty parameter ρ is of consequence for good quality reconstructed images, irrespective of the features in the input images. Furthermore, the One-by-Three mirror has a better imaging quality, nearly similar to that of the more conventional Standard mirror. It is also concluded that CNN is the least preferred denoiser when trying to process a series of images captured by the imaging system with the least human intervention for adjusting the regularization parameters. Therefore, with proper post-processing pipeline in place for such ultra-lightweight imaging systems, NUS-OSA mirrors could find an efficient application where very large monolithic mirrors (diameter sim few tens of m) would have been required, for example, in space-based remote-sensing and security and surveillance or even astronomical systems in the Longwave Infrared-Thermal Infrared (LWIR-TIR) range to achieve GRDs of a few tens of cm.

This conference presentation was prepared for Remote Sensing, 2024.

The EarthDaily Constellation (EDC), planned to be operational in early 2025, is a revolutionary Earth observation system designed to provide daily global coverage of the Earth's landmass with scientific-grade data. The mission's origins can be traced back to the agriculture sector, where there was a pressing need for high-quality, frequent Earth observation data to support critical decisions. Over time, the mission evolved to address a wide range of environmental applications including water management, forestry, disaster response, wildfire risk and wildfire propagation, greenhouse gas monitoring, and more. EDC addresses the need for more frequent, higher-resolution scientific-quality monitoring to understand and mitigate the impacts of climate change.The ten-satellite constellation, equipped with 22 spectral bands ranging from visible to long-wave thermal infrared, will collect an unprecedented 100 TB of data per day with a 10-year design life.The spectral bands have been carefully modeled after Landsat-8/9 and Sentinel-2 to ensure compatibility with historical archives, supporting long-term studies of Earth's evolution, and maximizing the value for environmental monitoring

As global climate change severely impacts our world, there is an increasing demand to monitor trace gases with a high spatial resolution and accuracy. At the same time, these instruments need to be compact in order have constellations for short revisit times. Here we present a new spectrometer instrument concept for trace gas detection, where photonic crystals filters replace traditional diffraction based optical elements. In this concept, 2D photonic crystal slabs with unique transmission profiles are bonded on a detector inside a regular telescope. As the instrument flies over the earth, different integrated intensities for each filter are measured for a single ground resolution element with a regular telescope. From this detector data, trace gas concentrations are retrieved. As an initial test case we focused on methane and carbon dioxide retrieval and estimated the performance of such an instrument. We derive the Cramér-Rao lower bound for trace-gas retrieval for such a spectrometer using Fisher information and compare this with the achieved performance. We furthermore set up a framework how to select photonic crystal filters based on maximizing the Fisher information carried by the filters and how to use the Cramér-Rao lower bound to find good filter sets. The retrieval performance of such an instrument is found to be between 0.4% to 0.9% for methane and 0.2% to 0.5% for carbon dioxide detection for a 300×300m² ground resolution element and realistic instrument parameters.

SPEXone is a compact multi-angle spectropolarimeter that measures both spectral intensity and the state of linear polarization of light scattered by aerosols in the Earth’s atmosphere at five different viewing angles simultaneously. This enables a very accurate quantification and characterization of atmospheric aerosols, helping us to better understand their effects on global climate and air quality. Building upon the success of its predecessor SPEXone, which has been launched in 2024 as part of the NASA PACE observatory, a second and improved instrument, SPEXone Second Generation, has been built within the ESA PRODEX program. Most recently, the integrated instrument underwent full on-ground characterization and calibration in ambient conditions at SRON. This contribution gives an overview of the measurements and presents preliminary results from the characterization and calibration campaign, focusing on the instrument performance. A few key performance aspects such as straylight, spatial and spectral resolution are discussed, with data from SPEXone for PACE serving as a comparison. The result of the analysis shows excellent image quality and indicates an improvement in the amount of diffuse straylight.

The Geostationary Synthetic Aperture Radar (GEOSAR) concept, first proposed in 1978, aims at defining an Earth Observation system able to provide regional coverage with large swaths, subcontinental access with very short revisit time, and quasi-persistent monitoring capabilities, by exploiting the unique characteristics of the GEO orbit. These peculiar characteristics make GEOSAR suitable to perform imaging and interferometry intended for the observation of fastevolving large-scale phenomena, such as ground motion in natural and urban environments. However, stable acquisition configurations that such applications require, are affected, even on the short-time scale, by the GEO perturbing forces. Therefore, a specific control strategy must be implemented to ensure small cross-track baselines and maximum Doppler bandwidth overlap between subsequent acquisitions. The paper proposes a novel orbit maintenance strategy tailored for GEOSAR. It assesses the near-zero inclination GEOSAR feasibility considering interferometric requirements and International Telecommunication Union (ITU) regulations compliance, demonstrating that the overall delta-V budget of the mission remains similar to that of standard GEO satellites.

The European Space Agency (ESA), in collaboration with the European Commission (EC) and EUMETSAT, is developing as part of the EC’s Copernicus program, a space-borne observing system for quantification of anthropogenic carbon dioxide (CO2) emissions. The anthropogenic CO2 monitoring (CO2M) mission will be implemented as a constellation of identical Low Earth Orbit satellites, to be operated over a nominal period of more than 7 years. Each satellite will continuously measure CO2 concentration in terms of column-averaged dry air mole fraction (denoted XCO2) along the satellite track on the sun-illuminated part of the orbit, with a swath width of 250km. Observations will be provided at a spatial resolution < 2 × 2km², with high precision (< 0.7ppm) and accuracy (bias < 0.5ppm), which are required to resolve the small atmospheric gradients in XCO2 originating from anthropogenic activities. The demanding requirements led to a payload composed of three instruments, which simultaneously perform co-located measurements: a push-broom imaging spectrometer in the Near Infrared (NIR) and Short-Wave Infrared (SWIR) for retrieving XCO2 and in the Visible spectral range (VIS) for nitrogen dioxide (NO2), NO2 serving as a tracer to high temperature combustion of fossil-fuel and related emission plumes. High quality retrievals of XCO2 will be ensured even in presence of aerosol loading, thanks to co-located measurements of aerosol properties resulting from a second instrument called Multiple Angle Polarimeter (MAP). The third instrument is a three-band Cloud Imager (CLIM) that will provide the capacity to detect small tropospheric clouds and cirrus cover.

Starting with a summary of the main scientific drivers, this paper will provide an overview of the progress of the space segment development: platform, payload as well as the end-to-end simulator. The consolidated design of the CO2M instruments which have passed their Critical Design Review, the results of the critical development models as well as the first delivery of the flight hardware are included in this paper.

As alternative to a CCD-based, charge domain, Time Delayed Integration (TDI) operation, we developed a analog domain TDI image sensor. The 143μm pixel pitch, 4-stage TDI operation is emulated by analog summation of 4 time-delayed samples of 4 rows of pixels. The frame period is 142μs. The device is intended for “push-broom” operation in a remote sensing mission. These pixels are pretty large; the individual pixel has a sun-shape (star-shape) geometry that speeds up the photo charge diffusion to the central transfer gate with an estimated factor of at least four.

We present a 96x96 InGaAs/InP single photon avalanche diode (SPAD) array for detection at 1550nm wavelength. The pixels have a diameter of 15μm and a 25μm pitch, resulting in a fill factor of 28.3%. The dark count rates (DCR) of the array were measured for a subset of 6x24 pixels. The DCR vs. photon detection efficiency of a representative SPAD and the breakdown voltage statistics for a subset of 6x96 pixels were recorded at room temperature. The DCR of the measured subset has a median value of very low 87kcps. At corresponding excess bias, we measured a photon detection efficiency (PDE) of 15%. The breakdown voltage of the 6x96 subset has a median value of 62.2V with a standard deviation of only 72mV. The results indicate a strong candidate for a SWIR imaging sensor in low-level-light applications.

As the imaging devices increase in complexity so the development of electronic to interface those detectors due to their high level of flexibility and programmability. Referred to as Front End Electronics (FEE) it is often the bottleneck in new developments. It became clear that developing the FEE platform solution along aside the detector gave a critical advantage to the mission development time and therefore cost. Also providing a solution including FEE that can interface with the rest of the payload/instrument without further integration work reduces the integration time and cost. This is an area where Teledyne-e2v has worked and invested to develop an effective FEE platform solution for a range of imaging detectors from VUV to IR. In order to supply effective imaging solutions, it is clear it is critical to keep investing in developing state-of-the-art detectors but also to combine those detectors with FEE to interface those complex detectors and ease further their integration at instrument level. The presentation will cover innovations at Teledyne-e2v in both detector and FEE areas.

The NOAA-21 VIIRS has successfully operated since its launch in November 2022 in an orbital constellation that includes the S-NPP and NOAA-20 launched in 2011 and 2017, respectively. Each VIIRS instrument makes daily global observations with 22 spectral bands, covering wavelengths from 0.41 to 12μm. It includes a day-night band (DNB) with a nominal spectral bandwidth of 0.5-0.9μm. The VIIRS instrument is regularly calibrated on orbit by a set of on-board calibrators (OBC), which include a solar diffuser (SD), a solar diffuser stability monitor (SDSM), a blackbody (BB), and a space view (SV) port. On-orbit calibration activities and strategies also include near-monthly lunar observations performed via spacecraft roll maneuvers. The lunar observations are often used along with SD measurements to help improve on-orbit calibration of the reflective solar bands. In this paper, we provide an update of NOAA-21 VIIRS on-orbit calibration and performance since launch, and a comprehensive assessment of on-orbit changes of its spectral band responses before and after the second mid-mission outgas (MMOG) performed in February 2024. In addition to detector noise characterization, the OBC performance of NOAA-21 VIIRS is illustrated and compared with that of S-NPP and NOAA-20 over the same operating period. Results show that the NOAA-21 VIIRS overall performance is comparable to or better than its predecessors.

The Visible Infrared Imaging Radiometer Suite (VIIRS), a key instrument in the Joint Polar Satellite System constellation, generates high fidelity land, ocean, and atmospheric data for a suite of science products that include monitoring of vegetation, algal blooms, wildfires, drought, flooding etc. There are three VIIRS instruments on orbit, one on each of the following: Suomi National Polar-orbiting partnership, NOAA-20 (formerly JPSS-1), and NOAA- 21 (formerly JPSS-2) spacecraft. Starting March 2024, NOAA-21 and NOAA-20 are the primary and secondary satellites of the constellation and providing meteorologists detailed information on severe weather events. In addition to the 21 spectral bands, covering the wavelength range from 0.41 to 12.2 μm, VIIRS has a day-night band (DNB) that collects valuable daytime and nighttime measurements at three different gain stages. The VIIRS instrument on the JPSS-4 spacecraft, scheduled to launch in 2027, recently (in Fall 2023) underwent sensor thermal vacuum (TVAC) environmental testing. Several key performance parameters such as the instrument gains, detector SNR or NEdT, dynamic range, relative spectral response, response versus scan angle, polarization sensitivity, stray light, and nearfield response were characterized during the various testing phases. In this paper, we provide an overview of the JPSS- 4 VIIRS prelaunch calibration activities, with a focus on radiometric performance assessments and their comparisons with previous VIIRS instruments.

The MODIS instruments on NASA’s Terra and Aqua satellites each use an on-board solar diffuser (SD) as the primary calibration source for the reflective solar bands (RSB). The reflectance properties of the MODIS solar diffusers were extensively characterized prior to launch and those measurements form the basis for the absolute reflectance and radiance calibration of the MODIS Level-1B data products. On orbit, additional characterization was done during early mission yaw maneuvers to verify the angular dependence of the SD reflectance and measure the transmission of an optional attenuation screen that can be placed in front of the SD. To account for degradation of the SD reflectance from cumulative solar radiation exposure, MODIS is also equipped with an on-board solar diffuser stability monitor (SDSM). Regular SD and SDSM calibrations have been made throughout the duration of the Terra and Aqua missions, for more than 20 years now. In the past few years, both Terra and Aqua have been drifting away from their historically maintained orbital planes. Observations of the SD continue to be made, but with incident solar angles that are outside the limits of the pre-launch and early mission characterizations, which presents a problem for accurate calibration. Recently, in August 2024, an additional set of yaw maneuvers was performed for Aqua to help characterize the upcoming drift-induced changes in SD measurements. We discuss the current state of Terra and Aqua MODIS SD calibration, the results of the Aqua yaw maneuvers, and the various options available to extend the SD measurements and RSB calibration through the end of the Terra and Aqua missions.

Dome Concordia (Dome C) in Antarctica is an excellent calibration site for polar-orbiting Earth observation instruments due to its spectral, spatial, and temporal uniformity. These instruments also observe Dome C multiple times a day and at a variety of geometries. The MODIS Characterization Support Team uses regular observations of Dome C by Aqua and Terra MODIS to help validate and improve the calibration of the detector gain and response versus scan angle of the reflective solar bands used to generate NASA’s Level 1B reflectance products. The reflectance trends at Dome C are typically assessed on a yearly basis, due to a six-month sunlit observation period. In this work, we increase the temporal resolution of the trends from yearly to bi-monthly and reduce measurement noise using a reflectance-based snow BRDF model. We show results for Terra and Aqua MODIS BRDF-normalized reflectance using the Collection 7 calibration for bands 1-4, 8-9, and 17. The BRDF model significantly reduces the variations in the bi-monthly reflectance trends with the best results observed near nadir and for the blue bands 3, 8, and 9. The higher temporal sampling allows for better real-time identification of any calibration errors during the sunlit season. In addition, due to its polar location, Dome C is largely insensitive to the recent orbit drift of the Terra and Aqua satellites which has created challenges for MODIS calibration based on other on-board and Earth targets. Combined, these advantages will make Dome C a particularly important calibration reference target during the final years of the Terra and Aqua missions.

The Plankton, Aerosol, Cloud, and ocean Ecosystem (PACE) mission is NASA’s next investment in ocean biology, clouds, and aerosol data records. A key feature of PACE is the inclusion of an advanced satellite radiometer, the Ocean Color Instrument (OCI), a global mapping radiometer that combines multispectral and hyperspectral remote sensing. Geolocation processing is performed for OCI using spacecraft navigation data and an instrument geometry model. To evaluate geolocation accuracy for OCI and develop refinements to processing methods, control point matching using Landsat data has been implemented as a step in operational processing of OCI data at the Science Data Segment. This processing provides between 200 and 300 high-quality matchups per day with good global and geometric distribution, allowing rapid evaluation of OCI geolocation accuracy. Initial results provided an early indication of the overall quality of geolocation processing and of specific aspects needing improvement. A standard set of granules was identified to support rapid implementation and testing of geolocation refinements, and this approach has been highly successful in improving geolocation processing accuracy to meet science requirements. This evaluation will continue throughout the mission to ensure the ongoing accuracy of geolocation. This paper describes the control point matching methodology, the approach to development of geolocation processing refinements, and recent results.

The Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) mission was selected by NASA as part of the Earth Venture-Instrument (EVI-3) program. Post-launch Cal/Val indicated a significant benefit to using a new calibration approach relative to the original, more traditional calibration scheme. The new approach uses early-orbit observations minus background (O-B) to optimize the calibration parameters by minimizing the difference between the O-B under clear-sky conditions. A multilayer feed-forward neural network was used in some channels to infer and correct complicated calibration artifacts. The calibration parameterization follows a generalized version of the periodic absolute calibration equation typically used for cross-track passive microwave radiometers. Calibration parameter regression and machine learning use various telemetry as predictors. Finally, the scan bias correction of the sidelobe contamination and a causal calibration sustainment system is applied. Absolute calibration performance is on par with present operational sounders.

Hydrosat’s dual band Longwave Infrared Imager (LIRI) is an uncooled thermal pushbroom imager that leverages a microbolometer, an on-board blackbody calibration source, and implements Time Delay Integration (TDI) via software. LIRI’s mission is to measure Land Surface Temperature (LST) from Low Earth Orbit (LEO). An overview of the instrument is presented, and the process to convert raw LIRI imagery into Top of Atmosphere (TOA) radiance is described. Due to data rate constraints, the application of a two-point Non-Uniformity Correction (NUC) and TDI occurs on orbit, prior to radiometric calibration. The processing to radiometrically calibrate the LIRI data then occurs on the ground. It is shown that the method to compute the NUC and TDI prior to radiometric calibration yields TOA radiance identical to applying the radiometric calibration to the raw imagery directly and then applying TDI.

HEO is a commercial company that performs Non-Earth Imaging (NEI), also referred to as Satellite-to-Satellite imaging or Space-to-Space imaging (S2S). This involves using space-based sensors to capture resolved imagery of Resident Space Objects (RSOs) in orbit. This approach has several challenges: objects in space are far apart from one another, and are moving with very high relative velocity. To perform NEI successfully and within a sensible time-scale, multiple imaging platforms are required with high resolution telescopes and a suitable automated infrastructure. HEO achieves this by leveraging existing Earth Observation platforms to perform high-cadence, resolved images of objects in space. Through use of HEO's Inspect platform which monitors imaging opportunities and controls sensor tasking, this capability is able to provide several key applications to space operations. This including defence applications (such as space object identification and capability assessment) as well as civil applications, such as mitigating the risk of space debris.

In the frame of the Copernicus program, ESA launched the Copernicus Sentinel- 2 optical imaging satellites, which are fully operational since June 2017. Sentinel-2C- and -2D satellites will be launched following Sentinel-2A and -2B units with identical sensors.

This paper reports on a sensitivity analysis of Sentinel-2 atmospheric correction / cloud masking vs Signal-to Noise Ratio (SNR) in specific spectral bands. Some Sentinel-2 L1C products are selected to study this effect. Noisy products are simulated adding noise to original L1C-data applying different Gaussian noise models. Finally, both original and noisy L1C-products are processed with Level-2A processor Sen2Cor and resulting L2A-products are compared. Results showed, that added noise to B10 is most critical due to performance reduction of cloud masking. Added noise to B01 is less critical because it does not lead to systematic changes of average surface reflectance. It results in increased scatter of surface reflectance. Added noise to B09 is found to be uncritical because the impact on water vapor retrieval is within uncertainty of validation method.

Climate Absolute Radiance and Refractivity Observatory (CLARREO) Pathfinder (CPF) mission’s Hyperspectral Imager for Climate Science (HySICS) instrument’s transmissive flight diffuser calibration is presented. The absolute Bidirectional Transmittance Distribution Function (BTDF) measurement of the transmissive diffuser is needed to calculate the instrument’s absolute efficiency. Along with a known solar irradiance source such as Total Solar Irradiance Sensor (TSIS), it can provide an absolute irradiance measurement path on orbit, with NIST traceability. This provides an additional path for CPF to cross compare with other on orbit sensors’ measurement such as Visible-Infrared Imaging Radiometer Suite (VIIRS), Clouds and the Earth’s Radiant Energy System (CERES). The flight diffuser was calibrated at NASA’s Goddard Space Flight Center (GSFC) using the Facility’s Optical Scatterometer.

In satellite-based hyperspectral Earth observation, spatial scanning of a spectrally dispersed one-dimensional field of view is a common approach. Here, the spatial and spectral resolutions of the image acquisition are directly coupled via a slit aperture and cannot be adjusted independently. Spatio-spectral scanning systems, on the other hand, acquire two-dimensional, spectrally coded images with decoupled spatial and spectral resolutions. As an alternative to the use of variable filters in such systems, we investigated an approach based on two dispersion stages and a variable slit. We give a short theoretical overview and present a first experimental validation with a physically doubled Czerny-Turner laboratory setup. We also consider a virtually doubled setup based on a mirrored slit as a compact alternative. Lastly, we discuss benefits, challenges, and possible applications, focusing on SmallSat-based observation.