Directional Reflectance Studies in Support of the Radiometric Calibration Test Site (RadCaTS) at Railroad Valley

The Radiometric Calibration Test Site (RadCaTS) is a suite of commercial and custom instruments used to make measurements of the surface reflectance and atmosphere throughout the day at Railroad Valley, Nevada. It was developed in response to the need for daily radiometric calibration data for the vast array of Earth-observing sensors on orbit, which is continuously increasing as more nations and private companies launch individual environmental satellites as well as large constellations. The current suite of instruments at RadCaTS includes five ground-viewing radiometers (GVRs), four of which view the surface in a nadir-viewing configuration. Many sensors such as those on Landsat-7 and Landsat-8 view Railroad Valley within 3 of nadir, while others such as those on Sentinel-2A and -2B, RapidEye, Aqua, Suomi NPP, and Terra can view Railroad Valley at off-nadir angles. Past efforts have shown that the surface bidirectional reflectance distribution function (BRDF) has minimal impact on vicarious calibration uncertainties for views <10, but the desire to use larger view angles has prompted the effort to develop a BRDF correction for data from RadCaTS. The current work investigates the application of Railroad Valley BRDF data derived from a BRF camera developed at the University of Arizona in the 1990s (but is no longer in use) to the current RadCaTS surface reflectance measurements. Also investigated are early results from directional reflectance studies using a mobile spectro-goniometer system during a round-robin field campaign in 2018. This work describes the preliminary results, the effects on current measurements, and the approach for future measurements

Ultra-Portable Field Transfer Radiometer for Vicarious Calibration of Earth Imaging Sensors

A small portable transfer radiometer has been developed as part of an effort to ensure the quality of upwelling radiance from test sites used for vicarious calibration in the solar reflective. The test sites are used to predict top-of-atmosphere reflectance relying on ground-based measurements of the atmosphere and surface. The portable transfer radiometer is designed for one-person operation for on-site field calibration of instrumentation used to determine ground-leaving radiance. The current work describes the detector-and source-based radiometric calibration of the transfer radiometer highlighting the expected accuracy and SI-traceability. The results indicate differences between the detector-based and source-based results greater than the combined uncertainties of the approaches. Results from recent field deployments of the transfer radiometer using a solar radiation based calibration agree with the source-based laboratory calibration within the combined uncertainties of the methods. The detector-based results show a significant difference to the solar-based calibration. The source-based calibration is used as the basis for a radiance-based calibration of the Landsat-8 Operational Land Imager that agrees with the OLI calibration to within the uncertainties of the methods

Monitoring the On-Orbit Calibration of Terra MODIS Reflective Solar Bands Using Simultaneous Terra MISR Observations

On December 18, 2015, the Terra spacecraft completed 16 years of successful operation in space. Terra has five instruments designed to facilitate scientific measurements of the earths land, ocean, and atmosphere. The Moderate Resolution Imaging Spectroradiometer (MODIS) and the Multiangle Imaging Spectroradiometer (MISR) instruments provide information for the temporal studies of the globe. After providing over 16 years of complementary measurements, a synergistic use of the measurements obtained from these sensors is beneficial for various science products. The 20 reflective solar bands (RSBs) of MODIS are calibrated using a combination of solar diffuser and lunar measurements, supplemented by measurements from pseudoinvariant desert sites. MODIS views the on-board calibrators and the earth via a two-sided scan mirror at three spatial resolutions: 250 m using 40 detectors in bands 1 and 2, 500 m using 20 detectors in bands 3 and 4, and 1000 m using 10 detectors in bands 819 and 26. Simultaneous measurements of the earths surface are acquired in a push-broom fashion by MISR at nine view angles spreading out in the forward and backward directions along the flight path. While the swath width for MISR acquisitions is 360 km, MODIS scans a wider swath of 2330 km via its two-sided scan mirror. The reflectance of the MODIS scan mirror has an angle dependence characterized by the response versus scan angle (RVS). Its on-orbit change is derived using the gain from a combination of on-board and earth-view measurements. The on-orbit RVS for MODIS has experienced a significant change, especially for the short-wavelength bands. The on-orbit RVS change for the short-wavelength bands (bands 3, 8, and 9) at nadir is observed to be greater than 10 over the mission lifetime. Due to absence of a scanning mechanism, MISR can serve as an effective tool to evaluate and monitor the on-orbit performance of the MODIS RVS. Furthermore, it can also monitor the detector and scan-mirror differences for the MODIS bands using simultaneous measurements from earth-scene targets, e.g., North Atlantic Ocean and North African desert. Simultaneous measurements provide the benefit of minimizing the impact of earth-scene features while comparing the radiometric performance using vicarious techniques. Long-term observations of both instruments using select ground targets also provide an evaluation of the long-term calibration stability. The goal of this paper is to demonstrate the use of MISR to monitor and enhance the on-orbit calibration of the MODIS RSB. The radiometric calibration requirements for the MODIS RSB are +/- 2% in reflectance and +/- 5% in radiance at typical radiance levels within +/- 45 deg. of nadir. The results show that the long-term changes in the MODIS reflectance at nadir frames are generally within 1. The MODIS level 1B calibrated products, generated after correcting for the on-orbit changes in the gain and RVS, do not have any correction for changes in the instruments polarization sensitivity. The mirror-side-dependent polarization sensitivity exhibits an on-orbit change, primarily in the blue bands, that manifests in noticeable mirror side differences in the MODIS calibrated products. The mirror side differences for other RSB are observed to be less than 1%, therefore demonstrating an excellent on-orbit performance. The detector differences in the blue bands of MODIS exhibit divergence in recent years beyond 1%, and a calibration algorithm improvement has been identified to mitigate this effect. Short-term variations in the recent year caused by the forward updates were identified in bands 1 and 2 and are planned to be corrected in the next reprocess

Observations and Recommendations for the Calibration of Landsat 8 OLI and Sentinel 2 MSI for Improved Data Interoperability

Combining data from multiple sensors into a single seamless time series, also known as data interoperability, has the potential for unlocking new understanding of how the Earth functions as a system. However, our ability to produce these advanced data sets is hampered by the differences in design and function of the various optical remote-sensing satellite systems. A key factor is the impact that calibration of these instruments has on data interoperability. To address this issue, a workshop with a panel of experts was convened in conjunction with the Pecora 20 conference to focus on data interoperability between Landsat and the Sentinel 2 sensors. Four major areas of recommendation were the outcome of the workshop. The first was to improve communications between satellite agencies and the remote-sensing community. The second was to adopt a collections-based approach to processing the data. As expected, a third recommendation was to improve calibration methodologies in several specific areas. Lastly, and the most ambitious of the four, was to develop a comprehensive process for validating surface reflectance products produced from the data sets. Collectively, these recommendations have significant potential for improving satellite sensor calibration in a focused manner that can directly catalyze efforts to develop data that are closer to being seamlessly interoperable

Radiometric calibration, validation and correction of multispectral photogrammetric imagery

Osajulkaisut: Publication 1: Lauri Markelin, Eija Honkavaara, Jouni Peltoniemi, Eero Ahokas, Risto Kuittinen, Juha Hyyppä, Juha Suomalainen, and Antero Kukko. 2008. Radiometric calibration and characterization of large-format digital photogrammetric sensors in a test field. Photogrammetric Engineering and Remote Sensing, volume 74, number 12, pages 1487-1500. © 2008 American Society for Photogrammetry and Remote Sensing (ASPRS). Publication 2: Lauri Markelin, Eija Honkavaara, Teemu Hakala, Juha Suomalainen, and Jouni Peltoniemi. 2010. Radiometric stability assessment of an airborne photogrammetric sensor in a test field. ISPRS Journal of Photogrammetry and Remote Sensing, volume 65, number 4, pages 409-421. doi:10.1016/j.isprsjprs.2010.05.003. © 2010 International Society for Photogrammetry and Remote Sensing (ISPRS). Publication 3: Eija Honkavaara, Lauri Markelin, Tomi Rosnell, and Kimmo Nurminen. 2012. Influence of solar elevation in radiometric and geometric performance of multispectral photogrammetry. ISPRS Journal of Photogrammetry and Remote Sensing, volume 67, pages 13-26. doi:10.1016/j.isprsjprs.2011.10.001. © 2011 International Society for Photogrammetry and Remote Sensing (ISPRS). Publication 4: L. Markelin, E. Honkavaara, U. Beisl, and I. Korpela. 2010. Validation of the radiometric processing chain of the Leica ADS40 airborne photogrammetric sensor. In: Wolfgang Wagner and Balázs Székely (editors). 100 Years ISPRS, Advancing Remote Sensing Science. ISPRS Technical Commission VII Symposium. Vienna, Austria. 5-7 July 2010. International Society for Photogrammetry and Remote Sensing. International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences, volume 38, part 7A, pages 145-150. ISSN 1682-1777. © 2010 by authors. Publication 5: Lauri Markelin, Eija Honkavaara, Daniel Schläpfer, Stéphane Bovet, and Ilkka Korpela. 2012. Assessment of radiometric correction methods for ADS40 imagery. Photogrammetrie - Fernerkundung - Geoinformation (PFG), volume 2012, number 3, pages 251-266. doi:10.1127/1432-8364/2012/0115Vast amounts of remote sensing data are acquire daily all over the globe from satellites, from manned or unmanned airborne platforms, and from the ground. Airborne photogrammetry provides a flexible method for acquiring high-resolution imagery in a timely manner over large areas. Aerial images are increasingly being used in a more automatic and quantitative way for applications such as land cover classification and environmental monitoring. Apart from the high geometric quality of photogrammetric sensors, also their radiometric properties are important. Different objects reflect solar irradiance according to their individual spectral and directional properties, and radiometric analysis can be used to identify such objects and changes in them. The perquisite for quantitative radiometry is the absolute radiometric calibration of the sensor, which links the recorded digital numbers to physical units. The major benefit of a radiometrically calibrated sensor is the possibility to radiometrically correct images form atmospheric effects to surface reflectance. Radiometric correction becomes a necessity, when imagery from different dates and sensors are used for quantitative image analysis. The objectives of this study were, first, to develop a vicarious method for the radiometric calibration and validation (Cal/Val) of a photogrammetric sensor in a test field. Second, three radiometric correction methods suitable for reflectance image product generation from photogrammetric images were evaluated. Finally, the influence of the solar elevation angle in the radiometric performance of multispectral photogrammetry was evaluated. The Cal/Val method developed in this study utilizes field measured nadir reflectance factors of the reference targets to match the reflectance factors measured at a laboratory in an exact imaging geometry to the current weather conditions. When evaluating the radiometric correction methods, a reflectance accuracy level of 5 % was achievable with all of the evaluated methods when using well-defined isotropic reference targets. For other targets, reflectance accuracies of between 5 and 20 % were possible. The results showed that a low solar elevation of 25° did not cause the general performance of the photogrammetric processes and 3D point cloud generation to deteriorate. The radiometric Cal/Val method presented in this study presents a step towards developing traceable processes for photogrammetric sensors. The results also confirmed the high radiometric quality of photogrammetric sensors and proved the suitability of the photogrammetric imagery for radiometric correction. This makes possible the rigorous radiometric processing of photogrammetric images and improves the quality and accuracy of automatic image interpretation and classification tasks.Kaukokartoitusdataa kerätään päivittäin suuria määriä ympäri maailmaa satelliiteista, miehitetyistä ja miehittämättömistä lentokoneista sekä maasta käsin. Fotogrammetrinen ilmakuvaus on erinomainen tapa kerätä tarkkoja kuvia haluttuna ajankohtana suuriltakin alueilta. Ilmakuvia käytetään yhä enemmän automaattisissa ja kvantitatiivisissa sovelluksissa kuten maan pinnan luokittelussa ja ympäristön seurannassa. Laadukkaiden geometristen ominaisuuksien lisäksi olennaista fotogrammetrisissa sensoreissa on niiden radiometriset ominaisuudet. Koska kohteet heijastavat auringon säteilyä yksilöllisesti aallonpituudesta ja havaintogeometriasta riippuen, voidaan radiometrisiä ominaisuuksia hyödyntää kohteiden tunnistamisessa ja muutosten seurannassa. Kvantitatiivisen radiometrian perusvaatimus on radiometrialtaan absoluuttisesti kalibroitu sensori. Radiometrisen kalibroinnin avulla sensorin tallentamat sävyarvot voidaan muuntaa fysikaalisiksi suureiksi. Kalibroidun sensorin kuvilla näkyvät ilmakehän aiheuttamat häiriöt voidaan korjata ja kuvat muuntaa vastaamaan maanpinnan heijastusta radiometrisillä korjausmenetelmillä. Radiometrinen korjaus on välttämätöntä, kun halutaan käyttää eri ajankohtina ja eri sensoreilla kerättyjä kuva-aineistoja kvantitatiivisessa analyysissä. Tämän työn tarkoituksena oli ensinnäkin kehittää menetelmä fotogrammetristen sensorien epäsuoraan radiometriseen kalibrointiin ja arviointiin (Cal/Val) testikentällä. Toiseksi tutkittiin kolmen eri radiometrisen korjausmenetelmän soveltuvuutta fotogrammetrisille ilmakuville. Kolmanneksi tutkittiin auringon korkeuskulman vaikutusta ilmakuvien radiometriaan ja siten fotogrammetristen prosessien suorituskykyyn. Kehitetty radiometrinen Cal/Val menetelmä hyödyntää laboratoriossa tarkassa havaintogeometriassa tehtyjä heijastusmittauksia, jotka muunnetaan vastaamaan kuvausaikaisia sääolosuhteita maastossa tehtyjen referenssikohteiden nadiiriheijastusmittauksilla. Työssä tutkituilla radiometrisen korjauksen menetelmillä pystyttiin saavuttamaan 5 % heijastustarkkuus, kun käytettiin tarkkoja referenssikohteita. Muita kohteita käyttäen oli mahdollista saavuttaa 5-20 % heijastustarkkuus. Tulokset osoittivat myös, että 25° auringonkulma ei vaikuttanut fotogrammetristen prosessien suorituskykyyn eikä kolmiulotteisten pistepilvien luomiseen. Tässä työssä esitetty radiometrinen Cal/Val menetelmä on askel kohti fotogrammetrisen sensorien jäljitettävää kuvienkäsittelyketjua. Tulokset vahvistivat sensorien hyvät radiometriset ominaisuudet sekä todistivat niiden kuvien soveltuvan radiometriseen korjaukseen. Tämä mahdollistaa ilmakuvien radiometrian kvantitatiivisen käsittelyn sekä lisää automaattisten kuvantulkintamenetelmien tarkkuutta