Nadir Direction Research Articles

Calculated sensitivities of several optical radiometric indices for vegetation canopies

A radiative transfer model for a vegetation canopy is used to compute several quantities used in remote sensing applications and to determine the sensitivity of these radiometric quantities to several of the important problem parameters. In particular, the following quantities are considered: 1) the ratio of near IR (700–1100 nm) to visible (600–690 nm) reflected intensities in the nadir direction, 2) the ratio of the fraction of incident energy reflected in the near IR to that in the visible (ratio of canopy hemispheric reflectances), 3)the normalized difference between the near IR and visible nadir reflected intensities, 4) the visible intensity transmitted in a given downward direction, and 5) the visible energy incident on the soil surface. A realistic radiative transfer model is proposed for calculating these radiometric quantities. Also simple approximate models are derived for the radiation transmitted through the canopy. With these models, the sensitivity of the reflected and transmitted quantities to various canopy and illumination conditions are then determined. Specifically, the effects of 1) sun angle, 2) the ratio of the diffuse-to-direct incident radiation, 3) the leaf-normal distribution for the canopy, 4) the scattering properties of the leaves, and 5) the soil albedo are considered. The sensitivity of the calculated quantities are presented as a function of the leaf area index of the canopy, and, for the reflected quantities, also as a function of the fraction of visible light absorbed in the canopy.

A multiangle imaging spectroradiometer for terrestrial remote sensing from the earth observing system

AbstractThe Multiangle Imaging SpectroRadiometer (MISR) instrument for the Earth Observing System (EOS) will provide a unique opportunity for studying the ecology and climate of the Earth through the acquisition of systematic, global multiangle imagery in reflected sunlight. MISR employs nine discrete cameras pointed at fixed angles, viewing the nadir direction and forward and aftward along the spacecraft ground track. Each camera is a charge‐coupled‐device –based pushbroom imager. Within a 7‐minute period, every point in a 204‐km‐wide swath is imaged at the nine viewing angles, ensuring observations acquired under virtually identical illumination and atmospheric conditions. The cameras will image the Earth in the nadir direction and at 30.7°, 45.6°, 60.0°, and 72.5° forward and aftward of the local vertical at the Earth's surface. Images at each angle will be obtained in four spectral bands centered at 440, 550, 670, and 860 nm. MISR is capable of taking image data in two different spatial resolution modes: Local Mode, in which selected targets are observed with 240‐m spatial sampling, and Global Mode, where the entire sunlit Earth is observed continuously with 1.92‐km sampling. Absolute radiometric calibration of the MISR instrument will be performed in‐flight using special on‐board hardware. The data produced by MISR will be valuable in a number of scientific discipline areas, and MISR images and geophysical products will be archived at the EOS Data and Information System to make them available to the broad scientific community.

Diurnal trends in reflectance of bare soil surfaces in Southern Tunisia

Abstract In an area in southern Tunisia diurnal trends of bare soil surfaces have been investigated. The study area comprises two main parts: the footslopes and the playas. The diurnal variation of the bidirectional reflectance factor (BRF) in nadir direction on the footslopes is dominated by the effect of roughness. Maximum BRF is found with small solar zenith angles due to decrease in shadow related to surface roughness. For Landsat overpass it implies that the normal ground reflectance for a bare surface on the footslopes at identical surface conditions is up to 10% lower in December (solar zenith angle 63 degrees) than in June (28 degrees). Band ratios on the footslopes hardly change with variation of zenith angle. The diurnal variation in the playas is dominated by moisture. Asymmetric daily curves, with the lowest reflectance in the morning have been found. Four phenomena are reported which can be held responsible for this effect. This daily effect of moisture is weather dependent and may obscure lo...

A technique for interpretation of auroral bremsstrahlung X-ray spectra

The precipitating kiloelectronvolt electrons in the auroral zone that produce enhanced E-region ionization also produce bremsstrahlung X-rays. The spectral distribution of the emitted X-rays contains information from which the energy spectrum of the precipitating electrons may be inferred. Modeling of the interaction of auroral electrons with the atmosphere shows that the flux F of vertically-emitted X-rays as a function of energy K is given approximately by F(K) = g 1(K) ∫ K ∞ (E − K)J(E) dE where J( E) is the differential flux of electrons in the energy range E − E + dE and g 1( K) is a modified X-ray production function. With this functional form for the production function the quantity F(K) g 1(K) is an exponential if the electron distribution is exponential. Thus, by dividing the observed X-ray fluxes at energy K by g 1( K) and fitting the result to an exponential function, the average energy and energy flux of the electron distribution can be determined directly. The technique is especially useful for determining ionospheric properties that depend on the integral of the electron fluxes, for example, the total energy flux, the E-region peak density and the height-integrated electrical conductances. The best-fit exponential electron spectrum yields values for these parameters that are very nearly the same as the values computed from the actual spectrum, even though the actual spectrum may deviate substantially from an exponential. Accurate knowledge of the function g 1 ( K) is important in applying the technique to observations. For X-ray fluxes emitted at an escape angle of 180° from the nadir direction and assuming an isotroplc distribution of incident electrons g 1( K) is approximately equal to 1.96 × 10 −5 K −1.9 when energies are expressed in kiloelectronvolts. The values derived here for g 1( K) agree well with those derived from simultaneous measurements of electron fluxes and resulting X-ray bremsstrahlung emission.

Equations for Estimating the Similarity Parameter from Radiation Measurements within Weakly Absorbing Optically Thick Clouds

Abstract Series expansions are derived for estimating the similarity parameter, which is a combination of the single-scattering albedo and asymmetry factor, from radiance measurement deep in the interior of a weary absorbing cloud. One expansion requires the measurements of the radiance in only the zenith and nadir directions, while a second uses the downward and upward irradiances. Numerical calculations show the estimates for the similarity parameter are very insensitive to different values of the asymmetry factor.

A test of the Suits vegetative-canopy reflectance model with LARS soybean-canopy reflectance data

The Suits vegetative-canopy reflectance model is tested with an extensive set of field reflectance measurements made by the Laboratory for Application of Remote Sensing (LARS) for soybean canopies. The model is tested for the full hemisphere of observer directions as well as the nadir direction. The results show moderate agreement for the visible channels of the Landsat MSS and poor agreement in the near-infrared channel of Landsat MSS. An analysis of errors is given.

Comparative study of suits and sail canopy reflectance models

In order to fully exploit the potential of remote sensing from aircraft and spacecraft altitude to map vegetation and estimate key agronomic parameters such as leaf area index (LAI) and biomass, the relationships between the canopy reflectance and properties of canopy elements must be well understood. A number of canopy reflectance models exist in the literature. Much of the model(s) verifications have been done primarily with observations from nadir direction. This was partly due to near nadir view observations of the Landsat series of satellites and partly due to the convenience of such measurements. The most significant development in these models, their ability to predict angular reflectance properties could not, however, be evaluated. The Laboratory for Applications of Remote Sensing, Purdue University, with strong support from modeling community, collected two excellent data sets, one on corn and one on soybeans, that contain the full hemisphere of off-nadir reflectance measurements. These data sets have been used to verify the multilayer one-component (leaves) Suits and SAIL canopy reflectance models. Software to evaluate multilayer multicomponent canopies was developed, but could not be evaluated because of the lack of information on stems. This evaluation suggests that these and similar other models have generic deficiency. The SAIL model, because of a more realistic inclusion of the leaf angle distribution, is in better agreement with observations than the Suits model. Further improvement and additional evaluation of these models is needed.

Effect of heliotropism on the bidirectional reflectance of irrigated cotton

The dynamic movement of cotton leaves in the presence of sunlight has been described using gyroscopic coordinates. These coordinates were used to calculate the position of leaf normals with respect to the sun on an individual basis. A sensitivity analysis established that the angle between a leaf normal and the sun was most affected by changes in pitch and roll. Averaging leaf normals by quadrant (as established by the direction of the compass) showed the pronounced heliotropic behavior of cotton leaves and the differential behavior of cotton leaves relative to the quadrant of the sun. These results were subsequently applied to the estimation of the phase angle γ between the canopy normals and sun from cross sections of bidirectional reflectance curves obtained for a band spanning 0.57–0.69 μm. From the cross section determined by the principal plane at 6:07 MST, which turned out to be concave and nearly symmetrical, the canopy averaged value for γ was estimated to be about 90°, while the calculated value from the leaf angle data was 85°. At 12:08 MST the same cross section was convex and asymmetrical with respect to rotation about the nadir direction, indicating distinguishable forescattered and backscattered components. For the forescattered component the estimated value for γ was 15°, and the calculated value 20°. For the backscattered component the estimated value for γ was 12°, and the calculated value 21°. These results suggested that bidirectional reflectance information analyzed sequentially using a visible band for which the optical transmission is nearly zero can be used to monitor leaf movement and hence moisture availability.

Methods for estimating the similarity parameter of clouds from internal measurements of the scattered radiation field

The similarity parameter is a combination of the single scattering albedo and the scattering asymmetry factor and can be used in conjunction with particle-size information to estimate the fractional absorption of a cloud layer. Methods are presented for estimating the parameter without estimating the optical thickness of the cloud or the ground albedo, but radiance measurements must be made at two elevations deep within the cloud layer. If the radiance data is for a sufficient number of directions, then one method can be used to give the parameter without using any correlation of the parameter with properties of the cloud. With the appropriate correlations, the parameter can be obtained using either the mean radiance and net flux or the radiance in only the zenith and nadir directions. The methods are based on an asymptotic transport theory and are shown to be a special case of more general transport results.

On the brightness coefficient of the Baltic Sea

Measurement series of the upwelling solar radiation above the Baltic Sea and the corresponding actinometric and meteorological observations were carried out during the scientific cruises of the RV “Ayu-Dag” in the summer of 1981 and 1982. The values and variability of the sea brightness coefficient at serveral mooring stations of the Baltic were determined. Two components of the sea brightness coefficient can be distinguished: the first component which is due to the light, directly reflected from the sea surface and the second one which is due to the light, diffusely backscattered from the water. It is expedient to separate these components within the experimental values of the sea brightness coefficient: the data analysis becomes much easier when the two distinguished spectra are considered. The spectral distribution of the sea brightness coefficient and its components within the range of wavelengths from 392 to 700 nm and their variability in different regions of the Baltic Sea are discussed. For this separation the directly reflected component is determined by model calculations. The spectral brightness coefficient as well as the colour index of the Baltic waters vary on a large scale. This is caused mainly by the water purity variation, but also by a dependence of the brightness coefficient on the illumination conditions: a) for a cloudy sky its values are always higher than those for a clear sky; b) the sea brightness coefficient decreases with the increase of the solar zenith angle. In most cases the obtained spectra of the sea brightness coefficient (and, even more, the spectra of its backscattered component) show the characteristic minimum caused by chlorophyll, at 440 nm; however, there is practically no correlation between the spectral values of the brightness coefficient at 440 nm and chlorophyll concentration in the Baltic waters. The validity of the assumption, that the sea radiance value in the nadir direction is approximately equal to 2% of the sky radiance value in the zenith direction, was estimated by model calculations. It was shown that this assumption may be valid only for wind speeds up to ≈3 m/s.

Atmospheric Spectral Imaging

An array of imaging spectrometers flown on the Spacelab 1 mission was capable of providing spectra of the atmospheric emissions over a broad wavelength range from 300 to 12,700 angstroms and acquiring each complete spectrum nearly simultaneously. The instrument was used to make observations on the day side and night side of the earth, looking down in the nadir direction, radially away from the earth, and in various limb-scanning modes. Observations were made looking at various angles to the vehicle velocity vector and during thruster firings and water dumps as well as at times when such events were inhibited. As a result of the mission a data base has been acquired that is valuable for studies of both the upper atmosphere and the shuttle-Spacelab vehicle environment.

Radiative transfer in an atmosphere–ocean system: an azimuthally dependent matrix-operator approach

Azimuthally dependent shortwave radiance in an atmosphere–ocean system is calculated for different types and concentrations of suspended matter in ocean and atmosphere. The transfer code, the matrix-operator method, is also applied to a rough ocean surface. With emphasis on remote sensing of oceanic constituents conditions for measurements are simulated to estimate the contribution of phytoplankton, sediment, and yellow substances to the ocean-leaving radiance within the 0.415–0.740-μm wavelength interval. The masking of these upward radiances by surface reflection and atmospheric extinction is discussed. In most conditions upward spectral radiance in the nadir direction usually contains the highest proportion of the oceanic underlight, even for an ocean surface roughened by 7-m/sec wind speed at all sun elevations below the mid-latitude noon condition.

The O+ 834‐Å dayglow: Satellite observations and interpretation with a radiation transfer model

The O+ 834‐Å dayglow observations made with the EUV spectrometer on the Air Force satellite STP‐78‐1 are interpreted with the use of a radiation transfer model. At 600‐km altitude the dayglow intensity was observed to vary with magnetic latitude from 30 R to 300 R in the near zenith direction (θ = 40°) and from 400 R to 500 R in the near nadir direction (θ = 140°); the correspondingly large near zenith to near nadir intensity ratio of 0.3–0.65 over much of the dayside indicates that the O+ ions in the topside ionosphere constitute an optically thick medium for resonance scattering of 834‐Å airglow emission even at this high altitude. Simultaneous measurements of the O+ density from the AE‐E spacecraft at 460‐km altitude near the equator are used to normalize the STP 78‐1 airglow data. A g value of 1.1×10−8 s−1 at zero optical depth is required for the ionization excitation of atomic oxygen leading to the production of O+ atoms in 4P state. The latitudinal distribution of O+ density derived from the 834‐Å airglow data shows a double peak (at ±13° latitude) surrounding an equatorial trough, characteristic of the equatorial anomaly, at altitudes below 1000 km and a single peak at the magnetic equator above 1000 km. At high latitudes (>20°) the O+ densities in the summer hemisphere are substantially lower than those in the winter hemisphere.

Effect of terrain orientation and solar position on satellite-level luminance observations

Changes in the luminance of a terrain along the local nadir direction are studied as a function of terrain slope, and the solar azimuth and elevation angles. The digital terrain data for the Santa Clara valley-Mt. Hamilton region of California, available from the U.S. National Cartographic Information Center, are used for this purpose. The elevation above mean sea-level of this selected region varies between 24 and 1,282 m. Luminance computations are performed by assuming the Lommel-Seeliger law of reflection, and the terrain reflectance to be a constant and independent of its geographic location. The atmospheric effects and contributions due to multiple reflections are neglected. It is shown that the ratio of the actual terrain luminance to that for the corresponding flat surface for 64 equal-size (∼ 4 km 2) finite subregions, changes by a very significant amount over the selected region, and also with changes in the solar position. Selected Landsat MSS band 7 data of the same region are presented for validation purposes.

Combined eye–atmosphere visibility model

The effect of the earth's atmosphere on the visibility in the nadir direction, as determined by the human eye, is presented. The visibility characteristics can be obtained from the combined eye-atmosphere modulation transfer function. This function is applied to the calculation of visibility thresholds of an area of an albedo different from its surroundings. It is shown that the atmospheric effect of diffusing sharp boundaries between adjacent areas has a significant influence on the visibility threshold. A sensitivity study is carried out to investigate the effects of different atmospheric optical conditions on the visibility thresholds. This model emphasizes the use of a combined eye-atmosphere theory to predict visibility conditions from airplanes.

Simulation colorimetry of the earth-atmosphere system

Luminance, dominant wavelength, and spectral purity of the sky radiation as a function of the zenith angle, and of the radiation reflected by different types of Lambert surfaces along the nadir direction, but received by observers at ground level as well as at the top, are presented for several plane-parallel models of the terrestrial atmosphere. These colorimetric parameters of the diffuse radiation are determined from the computed radiances at 23 unequally spaced wavelengths in the spectral region 0.365–0.784 μ. The diffuse radiation is evaluated after taking into account all orders of scattering. The atmospheric models used are for average cloudfree midlatitude summer situations, and vary from an aerosol-free absorption-free model at one end to a model with a large amount of aerosol and absorption by ozone and water vapor at the other. This study illustrates the manner in which the aforementioned colorimetric parameters are affected by the atmospheric conditions, solar position, direction of observation, position of observer, and the wavelength dependence of the reflectivity of the surface underlying the atmospheric model.

Attitude Determination System for a Nadir-Pointing Satellite

An attitude determination system for a nadir-pointing satellite with a high inclination orbit is investigated. The major engineering difficulty of such a system lies in estimating the nadir direction due primarily to large uncertainties of Earth atmospheric conditions caused by seasonal and diurnal variations over a wide range of latitude. Presented in this paper are investigation results of an attitude determination system approach which automatically adapts itself to those variations and augments the end-to-end attitude determination accuracy of science-sensor boresights through in-flight calibration. The engineering approaches discussed in this paper will be applied to the SEASAT-A mission.

Earth-atmosphere system and surface reflectivities in arid regions from LANDSAT MSS data

Previously developed programs for computing atmospheric transmission and scattering of the solar radiation are used to compute the ratios of the Earth-atmosphere system (space) directional reflectivities in the nadir direction to the surface Lambertian reflectivity, for the four bands of the Landsat multispectral scanner (MSS). These ratios are presented as graphs for two water vapor levels, as a function of the surface reflectivity, for various sun elevation angles. Space directional reflectivities in the vertical direction are reported for selected arid regions in Asia, Africa and Central America from the spectral radiance levels measured by the Landsat MSS. From these space reflectivities, surface reflectivities are computed applying the pertinent graphs. These surface reflectivities are used to estimate the surface albedo for the entire solar spectrum. The estimated albedos are in the range 0.34–0.52, higher than the values reported by most previous researchers from space measurements, but are consistent with laboratory and in situ measurements.

An atlas of low-latitude 6300-A [O I] night airglow from Ogo 4 observations

The atomic oxygen emission line at 6300 A, measured in the nadir direction by a photometer on the polar orbiting satellite OGO-4, was plotted between 40 deg N and 40 deg S latitude on a series of maps for the moon-free periods between 30 August 1967 and 10 January 1968 The longitudinal and local time variations which occur during the northern fall-winter season are indicated. The northern tropical arc is more widespread while the southern arc is not present at all longitudes. The conditions under which the observations were made are described, and four airglow maps were selected to show the local time variations.

Measurements of Microwave Emission from a Foam-Covered, Wind-Driven Sea

Abstract Measurements were made from aircraft of the 1.55-cm microwave emission from the North Sea and North Atlantic at surface wind speeds ranging from less than 5 to 25 m sec−1. Brightness temperatures in the nadir direction increased almost linearly with wind speed from 7 to 25 m sec−1 at a rate of about 1.2C (m sec−1)−1. At 70° from nadir the rate was 1.8C (m sec−1)−1. This increase was directly proportional to the occurrence of white water on the sea surface. At wind speeds <7 m sec−1, essentially no white water was observed and brightness temperatures in the nadir direction were ∼120K; at wind speeds of 25 m sec−1 white water cover was on the order of 30% and average brightness temperatures at nadir were −142K. Maximum brightness temperatures for foam patches large enough to fill the entire radiometer beam were 220K.