Polarised Multiangular Reflectance Measurements Using the Finnish Geodetic Institute Field Goniospectrometer

The design, operation, and properties of the Finnish Geodetic Institute Field Goniospectrometer (FIGIFIGO) are presented. FIGIFIGO is a portable instrument for the measurement of surface Bidirectional Reflectance Factor (BRF) for samples with diameters of 10 – 50 cm. A set of polarising optics enable the measurement of linearly polarised BRF over the full solar spectrum (350 – 2,500 nm). FIGIFIGO is designed mainly for field operation using sunlight, but operation in a laboratory environment is also possible. The acquired BRF have an accuracy of 1 – 5% depending on wavelength, sample properties, and measurement conditions. The angles are registered at accuracies better than 2°. During 2004 – 2008, FIGIFIGO has been used in the measurement of over 150 samples, all around northern Europe. The samples concentrate mostly on boreal forest understorey, snow, urban surfaces, and reflectance calibration surfaces

Sea Ice Albedo from MISR and MODIS: Production, Validation, and Trend Analysis

he Multi-angle Imaging SpectroRadiometer (MISR) sensor onboard the Terra satellite provides high accuracy albedo products. MISR deploys nine cameras each at different view angles, which allow a near-simultaneous angular sampling of the surface anisotropy. This is particularly important to measure the near-instantaneous albedo of dynamic surface features such as clouds or sea ice. However, MISR’s cloud mask over snow or sea ice is not yet sufficiently robust because MISR’s spectral bands are only located in the visible and the near infrared. To overcome this obstacle, we performed data fusion using a specially processed MISR sea ice albedo product (that was generated at Langley Research Center using Rayleigh correction) combining this with a cloud mask of a sea ice mask product, MOD29, which is derived from the MODerate Resolution Imaging Spectroradiometer (MODIS), which is also, like MISR, onboard the Terra satellite. The accuracy of the MOD29 cloud mask has been assessed as >90% due to the fact that MODIS has a much larger number of spectral bands and covers a much wider range of the solar spectrum. Four daily sea ice products have been created, each with a different averaging time window (24 h, 7 days, 15 days, 31 days). For each time window, the number of samples, mean and standard deviation of MISR cloud-free sea ice albedo is calculated. These products are publicly available on a predefined polar stereographic grid at three spatial resolutions (1 km, 5 km, 25 km). The time span of the generated sea ice albedo covers the months between March and September of each year from 2000 to 2016 inclusive. In addition to data production, an evaluation of the accuracy of sea ice albedo was performed through a comparison with a dataset generated from a tower based albedometer from NOAA/ESRL/GMD/GRAD. This comparison confirms the high accuracy and stability of MISR’s sea ice albedo since its launch in February 2000. We also performed an evaluation of the day-of-year trend of sea ice albedo between 2000 and 2016, which confirm the reduction of sea ice shortwave albedo with an order of 0.4–1%, depending on the day of year and the length of observed time window

Multiwavelength studies of regolith effects in planetary remote sensing

A large proportion of our knowledge about the surfaces of atmosphereless solar-system bodies is obtained through remote-sensing measurements. The measurements can be carried out either as ground-based telescopic observations or space-based observations from orbiting spacecraft. In both cases, the measurement geometry normally varies during the observations due to the orbital motion of the target body, the spacecraft, etc.. As a result, the data are acquired over a variety of viewing and illumination angles. Surfaces of planetary bodies are usually covered with a layer of loose, broken-up rock material called the regolith whose physical properties affect the directional dependence of remote-sensed measurements. It is of utmost importance for correct interpretation of the remote-sensed data to understand the processes behind this alteration. In the thesis, the multi-angular effects that the physical properties of the regolith have on remote-sensing measurements are studied in two regimes of electromagnetic radiation, visible to near infrared and soft X-rays. These effects are here termed generally the regolith effects in remote sensing. Although the physical mechanisms that are important in these regions are largely different, notable similarities arise in the methodology that is used in the study of the regolith effects, including the characterization of the regolith both in experimental studies and in numerical simulations. Several novel experimental setups have been constructed for the thesis. Alongside the experimental work, theoretical modelling has been carried out, and results from both approaches are presented. Modelling of the directional behaviour of light scattered from a regolith is utilized to obtain shape and spin-state information of several asteroids from telescopic observations and to assess the surface roughness and single-scattering properties of lunar maria from spacecraft observations. One of the main conclusions is that the azimuthal direction is an important factor in detailed studies of planetary surfaces. In addition, even a single parameter, such as porosity, can alter the light scattering properties of a regolith significantly. Surface roughness of the regolith is found to alter the elemental fluorescence line ratios of a surface obtained through planetary soft X-ray spectrometry. The results presented in the thesis are among the first to report this phenomenon. Regolith effects need to be taken into account in the analysis of remote-sensed data, providing opportunities for retrieving physical parameters of the surface through inverse methods.Helsingin yliopiston tähtitieteen laitokselta 6.3.2009 väittelevä Jyri Näränen on tutkinut väitöstyössään Aurinkokunnan ilmakehättömien kappaleiden valonheijastusominaisuuksia. Hän on myös kartoittanut laajalti Kuun pinnan fysikaalisia ominaisuuksia sekä saanut uraauurtavia tuloksia planeettojen pintojen ns. röntgenfluoresenssista. Aurinkokunnan kappaleista heijastuneen valon määrä riippuu siitä, missä kulmassa niitä havaitaan suhteessa Aurinkoon. Valtaosa Aurinkokunnan kappaleiden tutkimuksesta tehdään Maasta tai luotaimista käsin. Tällöin sekä havainto- että valaistuskulmat muuttuvat usein havaintojen aikana paljonkin, ja heijastuskulman vaikutukset havaintoihin on tunnettava, jotta tulokset voidaan tulkita oikein. Kiviplaneettoja ja Aurinkokunnan pienkappaleita peittää tavallisesti kerros väljästi pakkautunutta kivimurskaa ja pölyä, jota kutsutaan regoliitiksi. Mm. regoliitin huokoisuudesta ja regoliittihiukkasten muotojen ja kokojen jakaumasta riippuu, miten valo heijastuu siitä takaisin. Jos Aurinko valaisee kappaleen suoraan Maan takaa, kappale on täysin valaistu ja puhutaan nollaheijastuskulmasta. Tällöin kappale on myös kirkkaimmillaan. Kun valonlähteen ja havaitsijan välinen kulma kasvaa, kappale näyttää himmenevän. Himmenemisen määrä riippuu yllämainituista regoliitin ominaisuuksista. Regoliitin heijastuskulmaefektiä voidaan käyttää hyväksi kun halutaan selvittää heijastavan pinnan ominaisuuksia. Tässä nk. inversiomenetelmässä muodostetaan tietokonemalli heijastavasta pinnasta. Mallin fysikaalisia ominaisuuksia muutetaan, kunnes sillä voidaan hyvin mallintaa havaittu heijastuskulmaefekti. Näränen on ollut mukana tutkimassa Kuun tummien merialueiden regoliitin ominaisuuksia ESA:n SMART-1 kuuluotaimen vuosina 2004-2006 keräämän havaintoaineiston perusteella. Tulokset vastasivat melko hyvin Apollo -lennoilta saatuja tuloksia, mutta kattoivat huomattavasti suuremman osan Kuun pintaa kuin mitä miehitetyillä lennoilla pystyttiin tutkimaan. Tutkimuksessa käytetty havaintoaineisto on lajissaan yksi laajimmista maailmassa. Näränen on myös kehittänyt ja käyttänyt tutkimuksessaan uudenlaisia koejärjestelyitä, joilla on tutkittu regoliitin pintakarkeuden vaikutusta mitattuun röntgensäteilyyn mittauskulman muuttuessa. Karkean pinnan huomattiin muuttavan havaittua röntgensäteilyä energisemmäksi kuin sileän pinnan. Tulos on merkittävä, sillä regoliitista lähtevän röntgensäteilyn energiaa mitataan usein, kun tutkitaan sisemmän Aurinkokunnan kappaleiden pinnan alkuainekoostumusta. Koostumuksen avulla pystytään ymmärtämään paremmin pinnan kemiaa ja mineraaleja. Tulokset ovat hyödyksi ensi vuosikymmenellä, kun NASA:n MESSENGER ja ESA:n BepiColombo -luotaimet saapuvat tutkimaan Merkuriusta

Application of Sentinel-2 MSI in Arctic Research: Evaluating the Performance of Atmospheric Correction Approaches Over Arctic Sea Ice

Multispectral remote sensing may be a powerful tool for areal retrieval of biogeophysical parameters in the Arctic sea ice. The MultiSpectral Instrument on board the Sentinel-2 (S-2) satellites of the European Space Agency offers new possibilities for Arctic research; S-2A and S-2B provide 13 spectral bands between 443 and 2,202 nm and spatial resolutions between 10 and 60 m, which may enable the monitoring of large areas of Arctic sea ice. For an accurate retrieval of parameters such as surface albedo, the elimination of atmospheric influences in the data is essential. We therefore provide an evaluation of five currently available atmospheric correction processors for S-2 (ACOLITE, ATCOR, iCOR, Polymer, and Sen2Cor). We evaluate the results of the different processors using in situ spectral measurements of ice and snow and open water gathered north of Svalbard during RV Polarstern cruise PS106.1 in summer 2017. We used spectral shapes to assess performance for ice and snow surfaces. For open water, we additionally evaluated intensities. ACOLITE, ATCOR, and iCOR performed well over sea ice and Polymer generated the best results over open water. ATCOR, iCOR and Sen2Cor failed in the image-based retrieval of atmospheric parameters (aerosol optical thickness, water vapor). ACOLITE estimated AOT within the uncertainty range of AERONET measurements. Parameterization based on external data, therefore, was necessary to obtain reliable results. To illustrate consequences of processor selection on secondary products we computed average surface reflectance of six bands and normalized difference melt index (NDMI) on an image subset. Medians of average reflectance and NDMI range from 0.80–0.97 to 0.12–0.18 while medians for TOA are 0.75 and 0.06, respectively

Improvements, calibration and accuracy of the Finnish Geodetic Institute Field Goniospectrometer

Tässä diplomityössä esitellään Geodeettisen laitoksen kenttägoniospektrometri, FIGIFIGO, siihen tehdyt kalibraatiot, tarkkuusmittaukset ja parannukset. Työssä esitellään myös muita samankaltaisia laitteita, selitetään mittauksen teoria ja esitellään joitakin saatuja tuloksia. FIGIFIGO on kenttäkäyttöön suunniteltu mittalaite, jolla voidaan määrittää erilaisista pinnoista heijastuneen valon spektri useista eri suunnista. Tätä tietoa voidaan käyttää pintojen heijastavuuden mallintamiseen, satelliiteista saadun datan varmentamiseen ja ilmakuviin tehtäviin korjauksiin. FIGIFIGO on suunniteltu mahdollisimman varmatoimiseksi, nopeaksi ja kevyeksi. Tämän työn kuluessa laitteistoon on lisätty muun muassa kalansilmälinssillä varustettu kamera, jolla määritetään laitteen asento suhteessa aurinkoon, sekä uudet optiikat, joilla voi mitata valon polarisaatiota. Nämä lisäykset ovat mahdollistaneet entistä monipuolisemmat mittaukset ja siten uusien tieteellisten tulosten hankkimisen. Laitteiston kalibraatio on antanut lisää varmuutta saatujen tulosten oikeellisuudesta ja tarkkuustestien perusteella tehdyt muutokset ja parannukset ovat entisestään lisänneet laitteiston mittatarkkuutta ja luotettavuuttaThis work presents the features of Finnish Geodetic Institute Field Goniospectrometer (FIGIFIGO), concentrating on recent improvements, calibrations, and accuracy of the device. Several similar instruments by international research colleagues are introduced and the theories of bidirectional reflectance factor and bidirectional reflectance distribution function are briefly explained. Some data are presented to give an idea of the measurement results. FIGIFIGO is a device for multiangular reflectance factor measurements under field conditions. Some most noteworthy features of the instrument, compared to similar devices, include: fast operation and assembly, high portability, possibility for both laboratory and field measurements, and high degree of automation. The most important improvements to the system include: a sky camera for device orientation, optics with rotatable linear polarizer, and a fine tune mirror for spatial correction of optics footprint. With the addition of these features, FIGIFIGO has became more reliable and accurate, and scientifically more useful. Calibration of the system has increased the reliability of the data, and has provided the operators with information on how to operate the instrument most efficiently and accurately. The utilization of this information translates directly to more reliable and scientifically more important data

Multiangular Spectrometry and Optical Properties of Debris Covered Surfaces

Due to the recent development in CCD technology aerial photography is now slowly changing from film to digital cameras. This new aspect in remote sensing allows and requires also new automated analysis methods. Basic research on reflectance properties of natural targets is needed so that computerized processes could be fully utilized. For this reason an instrument was developed at Finnish Geodetic Institute for measurement of multiangular reflectance of small remote sensing targets e.g. forest understorey or asphalt. Finnish Geodetic Institute Field Goniospectrometer (FiGIFiGo) is a portable device that is operated by 1 or 2 persons. It can be reassembled to a new location in 15 minutes and after that a target's multiangular reflectance can be measured in 10 - 30 minutes (with one illumination angle). FiGIFiGo has effective spectral range approximately from 400 nm to 2000 nm. The measurements can be made either outside with sunlight or in laboratory with 1000 W QTH light source. In this thesis FiGIFiGo is introduced and the theoretical basis of such reflectance measurements are discussed. A new method is introduced for extraction of subcomponent proportions from reflectance of a mixture sample, e.g. for retrieving proportion of lingonberry's reflectance in observation of lingonberry-lichen sample. This method was tested by conducting a series of measurements on reflectance properties of artificial samples. The component separation method yielded sound results and brought up interesting aspects in targets' reflectances. The method and the results still need to be verified with further studies, but the preliminary results imply that this method could be a valuable tool in analysis of such mixture samples.Viimevuosina tapahtunut kehitys CCD teknologiassa on vihdoin mahdollistanut myös digitaalisten ilmakuvakameroiden valmistamisen. Digitaalisen kuvauksen käsittelyssä ei ole enää järkevää käyttää manuaalisia filmeille kehitettyjä menetelmiä vaan uusi tekniikka mahdollistaa myös automaattisen tietokoneprosessoinnin. Uusien tehokkaiden datan käsittelytapojen kehittäminen kuitenkin vaatii huomattavaa panostusta kaukokartoituskohteiden valonsironnan perustutkimukseen. Tämän vuoksi Geodeettisella Laitoksella on kehitetty goniospektrometri FiGIFiGo (Finnish Geodetic Institute Field Goniospectrometer), jolla voidaan mitata pienten kaukokartoituskohteiden reflektanssi monisuuntaisesti. Mitattaviksi kohteiksi käyvät esimerkiksi metsän aluskasvillisuus tai asvaltti. FiGIFiGo on helppokäyttöinen ja kannettava laite jonka operoimiseen tarvitaan kaksi ihmistä. Se voidaan koota 15 minuutissa käyttövalmiiksi, jonka jälkeen yhden kohteen sirontaominaisuuksien mittaamiseen kuluu 10 - 30 minuuttia käytetystä tarkkuudesta riippuen. FiGIFiGo:lla voidaan tehdä mittauksia tehokkaasti välillä 400 - 2000 nm sekä ulkona auringonvalolla että sisällä 1000 W laboratorio lampun kanssa. Tässä 'Pro Gradu'-tutkielmassa käsitellään FiGIFiGo ja sen mittausten teoreettinen pohja. Tutkielmassa esitellään myös uusi metodi monesta eri komponentista koostuvan kohteen tutkimiseen. Metodilla voidaan jakaa kohteesta mitattu monisuuntainen reflektanssi sen osien välille, eli esimerkiksi puolukka-jäkälä-kohteesta sironneen valon spektristä voidaan erottaa puolukan osuus. Työssä mitattiin useiden eri keinotekoisten kohteiden reflektanssit ja metodia testattiin soveltamalla sitä näihin tuloksiin. Saadut tulokset vaikuttivat järkeviltä ja ne toivat esille mielenkiintoisia huomioita kohteiden sironnasta. Pienen näytemäärän vuoksi metodin ja tulosten vahvistamiseen tarvitaan vielä lisänäyttöä. Alustavat tulokset vihjaavat kuitenkin että metodi voisi olla hyödyllinen työkalu vastaavien mittausten analysoinniss

A fractional snow cover mapping method for optical remote sensing data, applicable to continental scale

This thesis focuses on the determination of fractional snow cover (FSC) from optical data provided by satellite instruments. It describes the method development, starting from a simple regionally applicable linear interpolation method and ending at a globally applicable, semi-empirical modeling approach. The development work was motivated by the need for an easily implementable and feasible snow mapping method that could provide reliable information particularly for forested areas. The contribution of the work to the optical remote sensing of snow is mainly associated with accounting for boreal forest canopy effect to the observed reflectance, thus facilitating accurate fractional snow retrievals also for ground beneath the tree canopies. The first proposed approach was based on a linear interpolation technique, which relies on a priori known reference reflectances at a) full snow cover and b) snow-free conditions for each calculation unit-area. An important novelty in the methodology was the utilization of a forest sparseness index determined from AVHRR reflectance data acquired at full dry snow cover conditions. This index was employed to describe the similarity between different unit-areas. In practice, the index was used to determine the reference reflectances for such unit-areas for which the reflectance level could not be determined otherwise, e.g. due to frequent cloud cover. This approach was found to be feasible for Finnish drainage basins characterized by fragmented landscape with moderate canopies. Using a more physical approach instead of linear interpolation would allow the model parameterization using physical quantities (reflectances), and would therefore leave space for further model developments based on measuring and/or modeling of these quantities. The semi-empirical reflectance model-based method SCAmod originates from radiative transfer theory and describes the scene-level reflectance as a mixture of three major constituents: opaque forest canopy, snow and snow-free ground, which are interconnected through transmissivity and snow fraction. Transmissivity, in turn, can be derived from reflectance observations under conditions that highlight the presence of forest canopy, namely the presence of full snow cover on the ground. Thus, SCAmod requires a priori information on transmissivity, but given that it can be determined with the appropriate accuracy, it enables consideration of the obstructing effects of forests in fractional snow estimation. In continental-scale snow mapping, determination of the transmissivity map becomes a key issue. The preliminary demonstration of transmissivity generation using global land cover data was a part of this study. The first implementations and validations for SCAmod were presented for AVHRR data at Finnish drainage basin scale. In subsequent work, determination of the feasible reflectance constituents was addressed, followed by a sensitivity analysis targeting at selection of optimal spectral bands to be applied with SCAmod. Feasibility of the NDSI-based approach in FSC-retrievals over boreal forests is also discussed. Finally, the implementations and validations for MODIS and AATSR data are presented. The results from relative (using high-resolution Earth Observation data to represent the truth) and absolute validation (using in situ observations) indicate a good performance for both forested and non-forested regions in northern Eurasia. Accounting for the effect of forest canopy in the FSC-retrievals is the key issue in snow remote sensing over boreal regions; this study provides a new contribution to this research field and provides one solution for continental scale snow mapping

Boreal forest albedo and its spatial and temporal variation

Surface albedo refers to the fraction of solar irradiance that is reflected by a surface. Accurate characterisation of the albedo of various land cover types is required for evaluating the energy exchange between the Earth s surface and the atmosphere. The optical and structural properties of a surface determine its albedo. Boreal forest albedo can vary due to factors such as tree species composition, forest structure, understorey vegetation composition, and seasonal changes in vegetation and snow cover. The aim of this study was to characterise typical albedos of Finnish forests dominated by different tree species, evaluate the seasonal variation in forest albedo, and to estimate the effects of structural forest variables and understorey composition on forest albedo or spectral reflectance. To achieve these aims, forest albedo was measured in-situ using pyranometers, estimated from satellite data and calculated using a forest albedo model. Unmixing methods were used to estimate forest albedo from coarse spatial resolution MODIS albedo retrievals and understorey spectral reflectance from Landsat observations. Mature or middle aged pine, spruce and broadleaved deciduous (mainly birch) forests had distinctly different albedos in both summer and winter. Coniferous forest albedo was lower and showed less seasonal variation than albedo in open areas or broadleaved deciduous forests. Albedo of pine was somewhat higher than that of spruce. Snow cover on the ground and canopy increased forest albedo. Young stands with an assumedly high proportion of deciduous species in the under- and overstorey were characterised by a higher albedo than the mature coniferous forests. The high albedo at early succession rapidly decreased as the forest matured. The forest floor was typically covered by green understorey vegetation with rather low albedo, which decreased the influence of a changing canopy cover or leaf area index (LAI) on forest albedo. The spectral reflectances of the understorey varied with site fertility and forest age.Metsän albedoksi kutsutaan sitä osuutta metsään saapuvasta auringonsäteilyn energiasta, joka ei sitoudu metsään vaan heijastuu takaisin taivaalle. Metsän albedon tiedetään vaihtelevan muun muassa puulajin, metsän rakenteen ja lumipeitteen mukaan, mutta näiden vaihtelujen suuruutta ei ole Suomessa tarkemmin arvioitu. Tietoa albedosta tarvitaan kun arvioidaan metsien energiatasetta sekä metsien vaikutusta ilmastoon. Tämän tutkimuksen tavoitteena oli arvioida kuinka suomalaisten metsien albedo vaihtelee puulajin, metsän rakenteen, aluskasvillisuuden ja vuodenaikojen vaihtelun mukaan. Aineistona käytettiin maastossa tehtyjä albedomittauksia, satelliittimittauksista laskettuja arvoja sekä mallinnusta. Maastossa tehdyistä mittauksista saadut tulokset ovat vain suuntaa-antavia pienen koealamäärän takia. Satelliittikuva-aineistojen tulkinnassa käytettiin apuna malleja, joilla voitiin arvioida heijastusarvoja kuvanalkioita suuremmassa mittakaavassa. Kasvukauden aikainen vaihtelu havumetsien albedossa oli melko pientä, mutta lehtimetsissä albedo oli keväällä ja syksyllä lumettomana aikana hieman matalampi kuin keskikesällä. Lumipeite kasvatti albedoa sekä havu- että lehtimetsissä. Albedo oli kaikkina vuodenaikoina matalin kuusimetsissä, hieman korkeampi mäntymetsissä ja korkein lehtimetsissä. Poikkeuksen muodostivat jaksot, jolloin havumetsien latvus oli lumen peitossa keskitalvella. Saman puulajin keski-ikäisissä tai varttuneissa metsissä lehtiala tai latvuspeittävyys vaikutti lumettoman ajan albedoon vain vähän, mikä saattoi osittain johtua melko matalasta aluskasvillisuuden albedosta. Nuorissa havumetsissä albedo oli suurempi kuin varttuneissa, mikä todennäköisesti johtui nuorten metsien pienemmästä lehtialasta sekä aluskasvillisuuden suuremmasta näkyvyydestä. Aluskasvillisuuden aallonpituudesta riippuva heijastus muuttui metsikön varttuessa ja riippui metsätyypistä