Ground-based Calibration Research Articles

Radiometric Calibration of Spaceborne Optical Sensors, Lecture Series 2:Pre-launch Ground-based Calibration, Part 2

Radiometric Calibration of Spaceborne Optical Sensors, Lecture Series 2:Pre-launch Ground-based Calibration, Part 2

ABSOLUTE RADIOMETRIC CALIBRATION OF THE GÖKTÜRK-2 SATELLITE SENSOR USING TUZ GÖLÜ (LANDNET SITE) FROM NDVI PERSPECTIVE

TÜBİTAK UZAY has conducted a research study on the use of space-based satellite resources for several aspects of agriculture. Especially, there are two precision agriculture related projects: HASSAS (Widespread application of sustainable precision agriculture practices in Southeastern Anatolia Project Region (GAP) Project) and AKTAR (Smart Agriculture Feasibility Project). The HASSAS project aims to study development of precision agriculture practice in GAP region. Multi-spectral satellite imagery and aerial hyperspectral data along with ground measurements was collected to analyze data in an information system. AKTAR aims to develop models for irrigation, fertilization and spectral signatures of crops in Inner Anatolia. By the end of the project precision agriculture practices to control irrigation, fertilization, pesticide and estimation of crop yield will be developed. Analyzing the phenology of crops using NDVI is critical for the projects. For this reason, absolute radiometric calibration of the Red and NIR bands in space-based satellite sensors is an important issue. The Göktürk-2 satellite is an earth observation satellite which was designed and built in Turkey and was launched in 2012. The Göktürk-2 satellite sensor has a resolution 2.5 meters in panchromatic and 5 meters in R/G/B/NIR bands. The absolute radiometric calibration of the Göktürk-2 satellite sensor was performed via the ground-based measurements - spectra-radiometer, sun photometer, and meteorological station- in Tuz Gölü cal/val site in 2015. In this paper, the first ground-based absolute radiometric calibration results of the Göktürk-2 satellite sensor using Tuz Gölü is demonstrated. The absolute radiometric calibration results of this paper are compared with the published cross-calibration results of the Göktürk-2 satellite sensor utilizing Landsat 8 imagery. According to the experimental comparison results, the Göktürk-2 satellite sensor coefficients for red and NIR bands estimated in this work sustained to agree within 2% of calibration coefficients estimated in the cross-calibration results.

Toward High Altitude Airship Ground-Based Boresight Calibration of Hyperspectral Pushbroom Imaging Sensors

The complexity of the single linear hyperspectral pushbroom imaging based on a high altitude airship (HAA) without a three-axis stabilized platform is much more than that based on the spaceborne and airborne. Due to the effects of air pressure, temperature and airflow, the large pitch and roll angles tend to appear frequently that create pushbroom images highly characterized with severe geometric distortions. Thus, the in-flight calibration procedure is not appropriate to apply to the single linear pushbroom sensors on HAA having no three-axis stabilized platform. In order to address this problem, a new ground-based boresight calibration method is proposed. Firstly, a coordinate’s transformation model is developed for direct georeferencing (DG) of the linear imaging sensor, and then the linear error equation is derived from it by using the Taylor expansion formula. Secondly, the boresight misalignments are worked out by using iterative least squares method with few ground control points (GCPs) and ground-based side-scanning experiments. The proposed method is demonstrated by three sets of experiments: (i) the stability and reliability of the method is verified through simulation-based experiments; (ii) the boresight calibration is performed using ground-based experiments; and (iii) the validation is done by applying on the orthorectification of the real hyperspectral pushbroom images from a HAA Earth observation payload system developed by our research team—“LanTianHao”. The test results show that the proposed boresight calibration approach significantly improves the quality of georeferencing by reducing the geometric distortions caused by boresight misalignments to the minimum level.

The RUSALKA device for measuring the carbon dioxide and methane concentration in the atmosphere from on board the International Space Station

The high-resolution near-IR RUSALKA spectrometer is intended for developing a technique for measuring the carbon dioxide and methane concentrations in the atmosphere from on board the International Space Station. It consists of two main elements: an echelle spectrometer and an acoustooptic tunable filter used to select the diffraction orders of the grating. The device provides high resolving power (at least 20 000) in the 0.73–1.68 −µ m region, is compact, has low weight, and contains no moving parts. The concentrations of the gases are determined from the unsaturated lines of the CO2 band (1.58 µm) and the CH4 band (1.65 µm). This paper describes the technical characteristics of the device as well as the results of its ground-based calibrations.

“The Times They Are a-Changin” - Factors affecting observed spatio-temporal patterns in ground conductivity along the Murray River Floodplains, in south eastern Australia.

The recent advent of calibrated airborne EM systems, coupled with effective ground-based calibration procedures and more robust inversion tools that can account for system geometry, have given added impetus to their deployment as tools to aid the quantitative monitoring of variations associated with floodplain ecosystems, and in particular processes connected with surface water and groundwater interactions. This also has implications for understanding the consequences of floodplain management.In a pilot study in the Riverland of South Australia, we report on spatio-temporal variations observed in near surface determined from multi-temporal monitoring of a floodplain with AEM data. Significant increases and decreases in ground conductivity have been observed in particular locations adjacent to the river and on the floodplain, indicating areas where water levels associated with the underlying saline groundwater system have dropped, and others where it may have risen. These variations are attributed in part, to floodplain management strategies, including an operational salt interception scheme (SIS) and to changes in the regulated height of the river. The results also suggest that some parts of the river have changed from gaining to losing sections, indicating that the SIS may be achieving it’s desired objective, namely of extract saline groundwater before it discharges into the Murray River.Overall, these results suggest that airborne EM systems should now be considered as one means for quantifying spatial changes in the amount of salt stored on the floodplain, for monitoring the effects of particular management strategies, and for understanding floodplain processes.

Polarization Calibration of the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO)

As part of the overall ground-based calibration of the Helioseismic and Magnetic Imager (HMI) instrument an extensive set of polarimetric calibrations were performed. This paper describes the polarimetric design of the instrument, the test setup, the polarimetric model, the tests performed, and some results. It is demonstrated that HMI achieves an accuracy of 1% or better on the crosstalks between Q, U, and V and that our model can reproduce the intensities in our calibration sequences to about 0.4%. The amount of depolarization is negligible when the instrument is operated as intended which, combined with the flexibility of the polarimeter design, means that the polarimetric efficiency is excellent.

Planckpre-launch status: The HFI instrument, from specification to actual performance

Context. The High Frequency Instrument (HFI) is one of the two focal instruments of the Planck mission. It will observe the whole sky in six bands in the 100 GHz-1 THz range. Aims: The HFI instrument is designed to measure the cosmic microwave background (CMB) with a sensitivity limited only by fundamental sources: the photon noise of the CMB itself and the residuals left after the removal of foregrounds. The two high frequency bands will provide full maps of the submillimetre sky, featuring mainly extended and point source foregrounds. Systematic effects must be kept at negligible levels or accurately monitored so that the signal can be corrected. This paper describes the HFI design and its characteristics deduced from ground tests and calibration. Methods: The HFI instrumental concept and architecture are feasible only by pushing new techniques to their extreme capabilities, mainly: (i) bolometers working at 100 mK and absorbing the radiation in grids; (ii) a dilution cooler providing 100 mK in microgravity conditions; (iii) a new type of AC biased readout electronics and (iv) optical channels using devices inspired from radio and infrared techniques. Results: The Planck-HFI instrument performance exceeds requirements for sensitivity and control of systematic effects. During ground-based calibration and tests, it was measured at instrument and system levels to be close to or better than the goal specification.

Using airborne geophysics to define the 3D distribution and landscape evolution of Quaternary valley-fill deposits around the Jamestown area, South Australia

The combined analysis of airborne electromagnetics, airborne gamma-ray spectrometry, magnetics and a digital elevation model with ground-based calibration has enabled construction of a 3D architectural and landscape evolution model of valley-fill deposits around the township of Jamestown in South Australia. The valley-fill sediments consist of traction, suspension and debris-flow deposits that range in age (optically stimulated luminescence dating) from 102 ± 12 ka to the present day. A sediment isopach map generated from the airborne electromagnetics dataset reveals the 3D structure of the valley-fill deposits. The sediments are up to 40 m thick in asymmetrical valleys and are the result of colluvial fan, floodplain and sheet-wash processes. The sediments fine upwards with a higher proportion of coarser bedload deposits toward the base and fine sand, silt and clay towards the top of the sequence. A strong linear correlation between airborne K response and soil texture allowed the percentage of surface silt to be modelled over the depositional landforms. The sediments are thought to have been derived by a combination of eolian dust accessions, and weathering and erosion of bedrock materials within the catchment. Older drainage lines reflected in the distribution of relatively closely spaced and well-connected ‘magnetic channels’ differ markedly from present-day streams that are largely ephemeral and interrupted. This is thought to reflect a change in local hydrology and associated geomorphic processes from relatively high to lower energy conditions as the valley alluviated. These hydrological changes are likely to be associated with a drying climate, lower recharge and runoff.

Calibration and characterization of a digital camera for bidirectional reflectance distribution function retrieval of vicarious calibration sites

This work describes the calibration and characterization of a commercial digital camera that is being evaluated as a potential inexpensive imaging system to measure the bidirectional reflectance factor of the test site surfaces that are used by the Remote Sensing Group for the ground-based vicarious calibration of airborne and spaceborne systems. The main advantage to using a lightweight digital camera in combination with a fisheye lens is that a test site can be simultaneously spatially and spectrally sampled with greater ease. The motivation for using an inexpensive, lightweight digital camera to measure surface bidirectional distribution factor is the ability to spatially sample many points on the ground. An additional benefit to such cameras is their ability to operate without the need for cooling, which was required on a previous camera system developed at the Remote Sensing Group. Laboratory measurements include the gain and offset, spectral responsivity, linearity, cosine response, and point spread function. The results show that the uniformity of the Nikon detector array ranges from 0.7%-1.2%. The spectral bandwidth is approximately 90 nm for the blue channel, and 110 nm for the green, and 75 nm for the red channel. Linearity measurements show that the camera responsivity is nonlinear due to the internal image-processing algorithms, and a logarithmic fitting function is suggested. Measurements of the camera response versus the angle of incidence reveal that the camera does not exhibit a cos4 falloff on detector irradiance as one would expect, which is another consequence of the internal algorithms.

Ground-based calibration of the TAUVEX flight model

We describe in detail the procedures used for the ground testing and calibration of the flight model of the TAUVEX space telescope. We present the measurements done for this purpose, the various assumptions adopted during the ground calibration process, the results obtained, and provide a first approximation to the real performance of the experiment.

Ground-based calibration and characterization of the Fermi gamma-ray burst monitor detectors

One of the scientific objectives of NASA’s Fermi Gamma-ray Space Telescope is the study of Gamma-Ray Bursts (GRBs). The Fermi Gamma-Ray Burst Monitor (GBM) was designed to detect and localize bursts for the Fermi mission. By means of an array of 12 NaI(Tl) (8 keV to 1 MeV) and two BGO (0.2 to 40 MeV) scintillation detectors, GBM extends the energy range (20 MeV to > 300 GeV) of Fermi’s main instrument, the Large Area Telescope, into the traditional range of current GRB databases. The physical detector response of the GBM instrument to GRBs is determined with the help of Monte Carlo simulations, which are supported and verified by on-ground individual detector calibration measurements. We present the principal instrument properties, which have been determined as a function of energy and angle, including the channel-energy relation, the energy resolution, the effective area and the spatial homogeneity.

Vega is a rapidly rotating star

Vega, the second brightest star in the northern hemisphere, serves as a primary spectral type standard. Although its spectrum is dominated by broad hydrogen lines, the narrower lines of the heavy elements suggested slow to moderate rotation, giving confidence that the ground-based calibration of its visible spectrum could be safely extrapolated into the ultraviolet and near-infrared (through atmosphere models), where it also serves as the primary photometric calibrator. But there have been problems: the star is too bright compared to its peers and it has unusually shaped absorption line profiles, leading some to suggest that it is a distorted, rapidly rotating star seen pole-on. Here we report optical interferometric observations that show that Vega has the asymmetric brightness distribution of the bright, slightly offset polar axis of a star rotating at 93 per cent of its breakup speed. In addition to explaining the unusual brightness and line shape peculiarities, this result leads to the prediction of an excess of near-infrared emission compared to the visible, in agreement with observations. The large temperature differences predicted across its surface call into question composition determinations, adding uncertainty to Vega's age and opening the possibility that its debris disk could be substantially older than previously thought.

The Spectral Irradiance Monitor: Measurement Equations and Calibration

The Spectral Irradiance Monitor (SIM) is a satellite-borne spectrometer aboard the Solar Radiation and Climate Experiment (SORCE) that measures solar irradiance between 200 and 2700 nm. This instrument employs a Fery prism as a dispersing element, an electrical substitution radiometer (ESR) as the primary detector, and four additional photodiode detectors for spectral scanning. Assembling unit level calibrations of critical components and expressing the sensitivity in terms of interrelated measurement equations supplies the instrument's radiant response. The calibration and analysis of the spectrometer's dispersive and transmissive properties, light aperture metrology, and detector characteristics provide the basis for these measurement equations. The values of critical calibration parameters, such as prism and detector response degradation, are re-measured throughout the mission to correct the ground-based calibration.

X-ray telescope onboard Astro-E III Guidelines to performance improvements and optimization of the ray-tracing simulator

We present a detailed study of the performance of the Astro-E x-ray telescope (XRT) onboard the Astro-E satellite. As described in preceding papers the ground-based calibrations of the Astro-E XRT revealed that its image quality and effective area are somewhat worse than that expected from the original design. Conceivable causes for such performance degradation are examined by x-ray and optical microscopic measurements at various levels, such as individual reflectors, sectors, and quadrants of the XRT and their alignments. We can attribute, based on detailed measurements, the degradation of the image quality to a slope error in the individual reflectors and the positioning error of reflectors. As for the deficit of the effective area, the shadowing of x rays within the XRT body is the dominant factor. Error budgets for the performance degradation of the Astro-E XRT are summarized. The ray-tracing simulator, which is needed to construct the response function for arbitrary off-axis angles and spatial distributions of any celestial x-ray sources, has been developed and tuned based on the results of detailed measurements. The ray-tracing simulation provides results that are consistent within 3% with the real measurement except for large off-axis angles and higher energies. We propose, based on knowledge obtained from all the measurements and simulations, several plans for future developments to improve the performance of the nested thin-foil mirrors.

Calibration of the AIRS microwave instruments

Aqua carries three microwave radiometers that form an integral part of the Atmospheric Infrared Sounder (AIRS) sounding suite. Two Advanced Microwave Sounding Unit-A modules, one operating with two channels in the 23-31-GHz range and one operating with 12 channels in the 50-60-GHz range and one channel at 89 GHz, provide all-weather temperature soundings and cloud information. The Humidity Sounder for Brazil operates with four channels in the 150-190-GHz range and provides all-weather humidity and cloud soundings. All are cross-track scanners, as is AIRS. While there are significant differences between these three instruments, they are sufficiently alike that a common approach can be used to calibrate them. We describe the instruments and their heritage, the onboard calibration system, and the ground-based calibration processing.

On the Absolute Calibration of the Cepheid Distance Scale Using [ITAL]Hipparcos[/ITAL] Parallaxes

The fundamental Hipparcos parallaxes (HIPP) of 219 Cepheids are used for the absolute calibration of the Galactic distance scale sampled by a modern Baade-Wesselink (BW) distance indicator, which reliably accounts for pulsation and thermal properties of Cepheid variable stars. Notably we map thermal properties into the Johnson-Cousins color (V-I). The BW realization is found to be much less affected than previously adopted optical luminosity laws by intrinsic scatter and systematic errors in representing individual Cepheid distances and thus is best suited for a calibration of the galactic distance scale using the fundamental Hipparcos parallaxes (HIPP). Comparisons between the actual Hipparcos calibration and three independent ground-based calibrations of the same BW distance scale show very close agreement at the 0.04 mag level, i.e., at the 1 σ level of the absolute accuracy claimed for BW realizations, although the Hipparcos calibration is affected by an uncertainty of ±0.10 mag due to propagation of parallax errors alone. Comparisons include the zero-age main-sequence calibration by Cepheids in clusters (Pleiades distance modulus at 5.57 mag), the calibration by pulsation parallaxes of Cepheids, and the calibration by updated model calculations of synthetic stellar spectra of Cepheids. Notably, the resulting galactic distance scale is found to be ∼0.1 mag shorter than the value obtained in the original calibration of Feast & Catchpole. The implications of the actual calibration on the Cepheid-based distance to the Large Magellanic Cloud (LMC) and the extragalactic distance scale are briefly discussed. A true distance modulus of 18.59 ± 0.04 mag is presently achieved. Evidence from Hipparcos-based calibrations by different methods strongly supports the actual upward revision of 0.09 ± 0.04 mag for the LMC distance of 18.50 mag adopted in the Hubble Space Telescope Key Project program, corresponding to no more than a 5% decrease in the value of the Hubble constant.

An Analysis of [CLC][ITAL]BeppoSAX[/ITAL][/CLC] LECS Observations of Extreme-Ultraviolet Emission in Clusters of Galaxies

Kaastra et al. have used the BeppoSAX LECS instrument to search for excess extreme-ultraviolet (EUV) emission in A2199. They claim that the results obtained confirm an independent report of an excess EUV emission in this cluster by Lieu et al. Using an in-flight-derived procedure that is better suited to the analysis of extended sources and that avoids uncertainties related to ground-based calibrations for the overall detector sensitivity profile, we find no excess EUV emission in A2199. We also used these procedures to search for an EUV excess in A1795, but no excess was found.

Radiometric characterization of field radiometers in support of the 1997 Lunar Lake, Nevada, experiment to determine surface reflectance and top-of-atmosphere radiance

A continuing series of field campaigns to Lunar Lake, Nevada, has been established to develop measurement protocols and assess the uncertainties of ground-based calibrations of on-orbit satellite sensors. In June 1997, an ensemble of field radiometers was deployed to validate the fundamental reflectance and radiance measurements of the Land Satellite (Landsat-5) Thematic Mapper (TM) and Satellite Pour l'Observation de la Terre (SPOT-2) Haute Resolution Visible (HRV) satellite instruments. Prior their deployment to Lunar Lake, many of the field instruments measured a common sphere source at the University of Arizona (UA). The results, presented in this work, showed variations in the relative stabilities of the field instruments, and demonstrate the need for in-depth characterization of field instruments for an accurate assessment of instrument performance and measurement uncertainty.

Confirmation of previous ground-based Cepheid P-L zero-points using Hipparcos trigonometric parallaxes

Comparisons show agreement at the 0.1-mag level between the calibration of the Cepheid period–luminosity (P–L) relation by Feast & Catchpole (FC) using the early release of Hipparcos data and four previous ground-based calibrations, three of which are either largely or totally independent of the distance to the Large Magellanic Cloud (LMC). Each of the comparisons has the sense that the FC calibration is brighter, but only at the level of ≲0.1 mag. In contrast, FC argue that their Hipparcos recalibration leads to a 0.2-mag revision in the distance to the LMC, and thereby to a 10 per cent decrease in the Hubble constant. We argue differently. The comparison of the Hipparcos recalibration with others should be made using only local Galactic Cepheids, not based on Cepheids in the LMC that require a set of precepts that are not germane to the direct Hipparcos recalibration. The comparison made here, using only Galactic Cepheids, gives a correction of ∼4 per cent or less to our value of H0 based on Type Ia supernovae, keeping all other factors and precepts the same. A second success of the Hipparcos mission is the calibration of the position of the main sequence in the Hertzsprung–Russell diagram as a function of metallicity using local subdwarfs. These data have been used by Reid and by Gratton et al. to obtain, similarly to FC, a brighter absolute magnitude of RR Lyrae stars by ∼0.3 mag from that often currently adopted. These new calibrations confirm the earlier brighter calibrations by Walker, by Sandage, and by Mazzitelli, D'Antona & Caloi, thereby reducing the ages of globular clusters by ∼30 per cent. This removes most of the cosmological time-scale problem if H0∼55 km s−1 Mpc−1. A similar conclusion, based on pulsation theory and MACHO data, has been reached by Alcock et al.

Ground-based calibration facility for the Scanner for radiation Budget instrument in the solar spectral domain

The Ground Calibration Unit (GCU) is described. The GCU uses a solar-illuminated diffuser as calibration source. The absolute radiance of the diffuser is calculated from its absolute reflectivity and the incoming solar irradiance. The GCU was used to calibrate the on-board lamp sources of the flight model 1 of the Scanner for Radiation Budget (ScaRaB) instrument in February 1993.