David B. Pollock

Achieving satellite instrument calibration for climate change

For the most part, satellite observations of climate are not presently sufficiently accurate to establish a climate record that is indisputable and hence capable of determining whether and at what rate the climate is changing. Furthermore, they are insufficient for establishing a baseline for testing long-term trend predictions of climate models. Satellite observations do provide a clear picture of the relatively large signals associated with interannual climate variations such as El Nino-Southern Oscillation (ENSO), and they have also been used to diagnose gross inadequacies of climate models, such as their cloud generation schemes. However, satellite contributions to measuring long-term change have been limited and, at times, controversial, as in the case of differing atmospheric temperature trends derived from the U.S. National Oceanic and Atmospheric Administrations (NOAA) microwave radiometers.

On diode thermometers

Abstract Forward voltage vs. temperature characteristics are presented for 3 groups of silicon planar p-n junction diodes for the 4–300°K temperature range. Group ‘intrinsic’ characteristics are obtained at temperatures above about 40°K and existing diode theory appropriate for high level injection is successfully applied. Comparison of experimental results with theory permits the determinations of the zero temperature energy gaps of silicon (as well as germanium and gallium arsenide). The values thus obtained are in good agreement with values obtained by other methods. At temperatures below about 40°K, the forward voltage increases more rapidly with decreasing temperature. Deviations are noted among diodes within the same group as well as between groups. This low temperature ‘extrinsic’ characteristic is attributed to carrier freezeout effects at impurities which by increasing the junction width, convert the p-n junction to a p-i-n junction. Confirmation of this hypothesis is found by the current dependence as a function of temperature as well as by the decrease of capacitance with temperature. The role of the impurities in the semiconductor in controlling the slope in the intrinsic range and the sensitivity and deviation from linearity in the extrinsic range is discussed.

Review Article: Uncertainty analysis of remote sensing optical sensor data: guiding principles to achieve metrological consistency

Climate change monitoring requires decades-long time-series radiometric measurements using multiple optical sensors in multiple platforms covering the globe. The problem of achieving traceability to SI units for these measurements is discussed. A major challenge is to determine the result of a measurement and its associated uncertainty using various calibration and validation processes. These processes are plagued by systematic (non-statistical) uncertainties that are not well understood. In particular, different, but in principle equivalent, SI traceable measurements may differ by more than would be expected from the uncertainties associated with the individual measurements. We propose a methodology based on the International Organization for Standardization (ISO) Guide to the Expression of Uncertainty in Measurement (GUM) for the analysis of uncertainties in such measurements along with consistency checking. This allows the measurement result and its associated uncertainty to evolve as new knowledge is gained from additional experiments, and it promotes greater caution in drawing conclusions in view of the sparse measurements. We use data from ongoing total solar irradiance measurements from various instruments in orbit to illustrate the principles.

INTERFEROMETRIC OPTICAL MODULATOR

A modulator for collimated monochromatic light is described. The arrangement of components is similar to that of a Twyman-Green interferometer. The two mirrors, however, are replaced by piezoelectric disks with a reflective coating. Thus the optical path length of each leg of the interferometer can be adjusted electrically to achieve modulation. Theoretical analysis is presented concerning bandwidth, required voltage, and power dissipation. It is shown that modulation bandwidths up to 20 Mc/sec are possible with nearly 100% depth of modulation, that the required driving voltages are less than 100-V rms, and that power dissipation in the modulator is a fraction of a watt. Experimental results are presented.

Radiometric standards in space: the next step

The lack of accurate calibration sources is preventing current sensor technology from achieving the minimum possible uncertainty levels in remote-sensing radiometric measurements. This has been established in a recent study undertaken at the National Institute of Standards and Technology (NIST) following the identification of the problem at the Calibration Source Requirements Workshop at the NIST in 1997. This paper presents the concept of establishing high-accuracy calibration sources in space, utilizing the International Space Station (ISS) as a platform. For example, a retrievable total solar irradiance radiometer standard could be deployed on the ISS in order to establish the Sun as a calibration source of the required accuracy. Periodic measurements of solar irradiance with this radiometer standard would help to establish the Sun as a radiometric standard with the accuracy needed for various radiometric sensors on other platforms in space. As the radiometer on the ISS would be retrievable, its traceability to the International System of Units (SI) could be maintained by periodically bringing it down for calibration with SI-traceable standards maintained in ground laboratories. Other suitable radiometric standards on the ISS could also establish the Moon, the stars and various ground sites as calibration sources.

Aerial video reconnaissance using large sensor arrays (updated June 11, 2008)

The goal of an imaging sensor with nearly constant response, constant image quality, with a focal plane array of pixels whose overlap can be scaled, that can still provide a nearly hemispherical field-of-view has been demonstrated. The topic of this paper is the optical design of just such a sensor. A flow down of these performance constraints to hardware specifications is bounded by information theory, diffraction theory, plus practical matters that constrain the overlap of focal plane arrays. A set of performance goals for a sensor are the ability to observe features ≤25 μr features in size within a 45° scene using >1 G pixels.

Applying the Buchdahl dispersion model to infrared hybrid refractive-diffractive achromats

The Buchdahl dispersion model provides a rapidly converging polynomial form for describing the dispersion of refractive materials. Via this model, the dispersion of a material over the waveband of concern can be accurately characterized by a simple polynomial form, often out to only the second order. In this paper, the Buchdahl model is applied to hybrid refractive-diffractive achromats for both 3-5μm (MWIR) band and 8-12μm (LWIR) band. For each waveband, Buchdahl dispersion coefficients of IR materials and the diffractive optical element (DOE) are defined by optimally choosing the Buchdahl chromatic coordinate and best-fitting the Buchdahl model to the dispersion of materials and the DOE. The principles for selecting 1 to 2 IR materials combined with a DOE to produce hybrids achromatized at 3 and 4 wavelengths are discussed. A series of thin lens predesign examples are presented.

Cohering of multiple polariton lasers for sensing applications

Active modelocking of multiple polariton lasers mediated by real time sensing offers novel capabilities for optically based sensing. We outline a strategy based in part on short range polariton-polariton interactions and in part on an actively managed external optical field coherent with each of the individual polariton lasers. This actively managed coherent optical field is required to establish long range coherence between multiple spatially distinct polariton lasers. Polariton lasers offer nonlinear behavior at excitation levels of a few quanta of the optical field, time constants of picoseconds or less, and optical wavelength dimensions of individual lasers. Achievement of useful long range, hundreds of meters, polariton based optical sensing appears useful, but to require active cohering of arrays of polariton lasers. Continuous metrology and active control of the system coherence offer unique opportunities for sensing approaching quantum limited operation. We consider strategies and capabilities of sensing systems based on such arrays of spatially distinct, but collectively coherent, polariton lasers. Significant advances in a number of technical areas over decades appear needed to achieve such systems.

The Space Metrology Program

The full potential of current remote sensor technology is limited by the inability to correct biases once an exo-atmospheric remote sensor becomes operational. Even when the calibration is traced to the International System of Units, SI, and the instrument is performing within the operational envelope wherein it is calibrated, the problem exists and a Space Metrology Program is a potential solution to the problem. This paper discusses such a program, suggests a feasibility study to address the issues and recommends a plan of action. Any operational instrument has a bias and reducing the magnitude of the bias can only be accomplished when an adequately accurate standard is accessible by the instrument while the instrument is in its operational environment. Currently the radiometric flux from the sun, the moon and the stars is inadequately accurate SI to provide a standard that is consistent with the remote sensor state-of-the-art technology. The result is data that is less accurate than it could be often leading to confusing and conflicting conclusions drawn from that data. Planned remote sensors such as those required to meet future program needs (e.g. the United States National Polar-Orbiting Operational Environmental Satellite System (NPOESS) and the proposed international Global Earth Observation Program) are going to need the higher accuracy radiometric standards to maintain their accuracy once they become operational. To resolve the problem, a set of standard radiometers on the International Space Station is suggested against which other exo-atmospheric radiometric instruments can be calibrated. A feasibility study for this program is planned.

An All-Reflective, Wide Field Of View Collimator

The test and evaluation of infrared optical systems is requiring collimators with rather stringent performance characteristics which must be simultaneously realized. Fields of view as large as 15 degrees, flat focal surfaces, clear apertures with uniform intensity across the focal plane and minimal spectral variations over passbands as wide as 7 pms are within the realm of common place. A centered, rotationally symmetric optical system with a tilted field of view and a decentered stop has been used to design just such an optical system. Evaluation shows that it meets performance requirements and that it is nearly diffraction limited over the central 5 of its 15-degree field of view.