Philip D. Hammer

LEAFMOD: A New Within-Leaf Radiative Transfer Model

Abstract We describe the construction and verification of a within-leaf radiative transfer model called LEAFMOD (Leaf Experimental Absorptivity Feasibility MODel). In the model, the one-dimensional radiative transfer equation in a slab of leaf material with homogeneous optical properties is solved. When run in the forward mode, LEAFMOD generates an estimate of leaf reflectance and transmittance given the leaf thickness and optical characteristics of the leaf material (i.e., the absorption and scattering coefficients). In the inverse mode, LEAFMOD computes the total within-leaf absorption and scattering coefficient profiles from measured reflectance, transmittance, and leaf thickness. Inversions with simulated data demonstrate that the model appropriately decouples scattering and absorption within the leaf, producing fresh leaf absorption profiles with peaks at locations corresponding to the major absorption features for water and chlorophyll. Experiments with empirical input data demonstrate that the amplitude of the fresh leaf absorption coefficient profile in the visible wavebands is correlated with pigment concentrations as determined by wet chemical analyses, and that absorption features in the near-infrared wavebands related to various other biochemical constituents can be identified in a dry-leaf absorption profile.

Digital array scanned interferometer: sensors and results

Digital array scanned interferometers (DASI) blend characteristics of a grating spectrometer and a two-beam interferometer for acquisition of hyperspectra. DASIs posses field-widened capabilities that permit very high throughput. Aspects of DASI design, hyperspectra, and data processing methods are presented. In particular, we provide data showing that photon-noise-limited hyperspectra are achievable for DASI data.

Imaging interferometer for terrestrial remote sensing

A prototype imaging interferometer called DASI (digital array scanned interferometer) is under development at our laboratories. Our objective is to design an instrument for remote sensing of Earths atmosphere and surface. This paper describes the unusual characteristics of DASIs which make them promising candidates for ground and aircraft-based terrestrial measurements. These characteristics include superior signal-to-noise, design simplicity and compactness, relative to dispersion based imaging spectrometers. Perhaps one of the most notable features of DASIs is their ability to acquire an entire interferogram simultaneously without any moving optical elements. We also describe selected laboratory and ground based field measurements using the prototype DASI. A CCD detector array was placed at the DASI detector plane for wavelength coverage from 0.4 to 1.0 micrometers . A NICMOS MCT detector was used for coverage from 1.1 to 2.2 micrometers . The DASI was configured to have a spectral resolution of about 300 cm-1, a spatial field of view of 5 degrees, and a constant number of transverse spatial elements (detector dependent) for each exposure frame. Frame exposure rates were up to 0.6 Hz with the potential to achieve 5 Hz. Image cube measurements of laboratory targets and terrestrial scenes were obtained by multiple frame scanning over the field of view. These data sets reveal the potential science yields from obtaining simultaneous high resolution spatial and spectral information.

Platform options of free-flying satellites, UAVs or the International Space Station for remote sensing assessment of the littoral zone

Over the years, making or creating a choice for a specific platform from which to conduct remote sensing observations of specific targets brings in many factors related to the target characteristics and how the data are going to be used. Attempts to measure Earths diverse objects have generated a wide range of platform alternatives, from geostationary satellites to low-flying aircraft. Now several additional options possessing unique attributes are available: the International Space Station (ISS) and Un-inhabited Aerial Vehicles (UAVs). This paper explores some of the tradeoffs among these alternatives for the special problem of remotely sensing the littoral zone, but especially the shallow ecosystems. Though the surface area of the littoral zone is relatively large, it is geographically disbursed and somewhat linear. Also, the spatial, spectral and temporal variability of ecosystems in this zone is very high, and signals are masked by the overlying water column. Ideally, a frequent revisit time would be desirable to monitor their health and changing condition. These characteristics place important constraints on platform choice as one tries to design a system to monitor these critical ecosystems and provide useful information for managing them. This paper discusses these tradeoff issues as offered mainly by three platform choices: free-flying satellites, ISS, UAVs and other aircraft.

Imaging interferometry for terrestrial remote sensing: digital array scanned interferometer instrument developments

An imaging spectrometer concept called digital array scanned interferometry (DASI) is being explored in our laboratories for terrestrial remote sensing applications. The essence of DASI operation is that interferograms are resolved spatially over one coordinate at the detector plane, and spatial information is obtained over the orthogonal coordinate. In this paper we focus on recent developments for approaching the fundamental capabilities of the DASIs performance, specifically the signal-to-noise ratio. We also describe selected land observations acquired from an airborne DASI operating in the 4550-9090 cm-1 (1.1-2.2 micrometers ) spectral region with a spectral resolution of 266 cm-1.

Interferometric imagery from a solar powered RPV

Describes the design, integration, and deployment of a compact, high-spectral-resolution imaging interferometer built specifically for use on the Pathfinder solar powered remotely piloted aircraft (RPA) that has been developed under NASAs Environmental Research Aircraft and Sensor Technology (ERAST) program. Emphasis is placed on the engineering challenges associated with RPA instruments, including low power and weight constraints, extreme thermal and pressure environment, and remote operation. Sample data are presented that illustrate the strengths and weaknesses of the composite RPA/scanned interferometer remote sensing system.

Spectral imaging of clouds using a digital array scanned interferometer

Abstract Spectral images of various cloud fields have been made from the ground using a prototype imaging interferometer. The instrument, called DASI (digital array scanned interferometer), is under development as an alternative technique for terrestrial remote sensing. Our paper describes the unusual characteristics of DASIs that make them promising candidates for both ground and aircraft based measurements of clouds. These characteristics include superior signal-to-noise potential, design simplicity and compactness. The prototype DASI had a wavelength sensitivity range of 4550–9090 cm −1 (1.1–2.2 μm), a spectral resolution of 300 cm −1 , a spatial field of view of 5 degrees, and 256 transverse spatial elements for each exposure frame with exposure rates up to 1 frame/s. Spectrally resolved images of cirrus and cumulus clouds over the San Francisco Bay area were obtained by multiple frame scanning over the field of view. The DASI measurements are compared with radiative transfer calculations using LOWTRAN-7. Modification of the LOWTRAN code was necessary so that high spectral resolution data for ice optical properties could be used. With these modifications, the agreement between measured and model results is good in some cases. However, quantitative interpretation of these preliminary measurements was not generally possible because of various sources of uncertainty. Finally, a discussion is given on the potential applications and science yields of DASI instruments in the framework of coordinated field missions such as project FIRE. Such applications include the use of ground and airborne based DASIs to retrieve microphysical properties of clouds and to investigate current problems in atmospheric radiation.

Surface reflectance mapping using interferometric spectral imagery from a remotely piloted aircraft

During October 1997, NASAs Environmental Research Aircraft and Sensor Technology (ERAST) Program conducted flight tests of the Pathfinder Remotely Piloted Aircraft (RPA) out of the Pacific Missile Range Facility (PMRF), Kauai (Hawaii). The Pathfinder was a light-weight (=500 lb), solar-powered RPA with an extremely limited (=35 lb) sensor payload capacity. One of the ERAST objectives was to evaluate the feasibility of using such RPA platforms to collect remotely sensed data in support of Earth systems science. A key aspect of this objective was to make simultaneous spectral measurements of a common target using an airborne spectral imager and a ground-based spectrometer. Intercomparison of these results, together with modeling of atmospheric radiative effects, enable the establishment of ground-truth and the verification of the spectral imagers radiometric calibration. The outcome of this calibration exercise is a procedure for deriving the surface albedo for a variety of different regions of the overall scene containing the ground-truth target. A Digital Array Scanned Interferometer (DASI) hyperspectral imager jointly developed by NASA Ames Research Center and and Washington University was selected as the imaging spectrometer payload. Compactness, light weight, low power draw, rugged design and simplicity of operation made the DASI an ideal candidate for this mission.

A new within-leaf radiative transfer model: preliminary results

As part of the NASA Ecological Processes and Modeling Program, researchers at NASA/Ames and INRA in the LEAFMOD group are developing a new within-leaf radiative transfer model. The model is based on a radiative transfer formulation and on the assumption of the separation of the origins of the scattering and absorption transport effects. Preliminary results are presented.

Calibration of infrared satellite images using high altitude aircraft measurements

The use of infrared radiance measurements made from high altitude aircraft for satellite image validation is discussed. Selected examples are presented to illustrate the techniques and the potentials of such validation studies.