Danette P. Ryan-Howard

Laser radar reflective tomography utilizing a streak camera for precise range resolution

Tomography is used to reconstruct 2-D images from 1-D range-resolved laser radar data. A doubled mode-locked Nd:YAG pulsed laser illuminates a conical object, and a receiver utilizing a streak camera resolves the reflected light in time.

Three-Dimensional Imaging Using A Single Laser Pulse

We describe results from an optical detector capable of producing three-dimensional images using single laser pulses. The method consists of detecting reflected light from an object illuminated by a short pulse from a laser with a detector that resolves many pixels in the objects image into fine time bins equivalent to range resolution of 4 cm. The detector utilizes a fiber optic image converter to transform a square focal plane into a line array that is input to a streak camera to obtain high time resolution in all the pixels. We show data from simple objects, like posts and cones, as well as more complicated objects. This work builds upon our results reported in September 1988 at the Laser Radar III SPIE conference. In that work we imaged the entire object to a point that was a single input to the streak camera. In addition we viewed the object from many aspect angles and used the range measurements to produce a two-dimensional projection image of the object. That method of reflective tomography requires gathering data from many aspect angles while the method reported here, which we call angle-angle-range, provides a three-dimensional image with data from a single aspect angle using a single laser pulse. Here we will compare the methods in detail.

Two-Dimensional Tomographs Using Range Measurements

We demonstrate a method of constructing two-dimensional projection images of objects using reflective tomography. The method consists of detecting reflected light from an object illuminated by short pulses (FWHM ≈ 100 ps) from a laser with high time resolution (FWHM ≈ 250 ps) to produce one-dimensional range-resolved data. Repetition at many aspect angles provides input to a filtered back projection algorithm that produces a 2D projection image of the object. The receiver consists of 1) a lens to image the object to a point, 2) a streak camera to provide time resolution, 3) a tv detector to record the streaked light, and 4) electronics to control the system and to store data. This paper describes the concept on which the receiver is based, the details of the prototype receiver, and the characteristics of images of many objects.

Architectural trades for an advanced geostationary atmospheric sounding instrument

The process of formulating a remote sensing instrument design from a set of observational requirements involves a series of trade studies during which judgments are made between available design options. The outcome of this process is a system architecture which drives the size, weight, power consumption, cost, and technological risk of the instrument. In this paper, a set of trade studies are described which guided the development of a baseline sensor design to provide vertical profiles (soundings) of atmospheric temperature and humidity from future Geostationary Operational Environmental Satellite (GOES) platforms. Detailed trade studies presented include the choice between an interferometric versus a dispersive spectrometer, the optical design of the IR interferometer and visible imaging channel, the optimization of the instrument spatial response, the selection of detector array materials, operating temperatures, and array size, the thermal design for detector and optics cooling, and the electronics required to process detected interferograms into spectral radiance. The trade study process was validated through simulations of the radiometric performance of the instrument, and through simulated retrievals of vertical profiles of atmospheric temperature and humidity. The flexibility of these system trades is emphasized, highlighting the differing outcomes that occur from this process as system requirements evolve. Observations are made with respect to the reliability and readiness of key technologies. The results of this study were disseminated to industry to assist their interpretation of, and responses to, system requirements provided by the U.S. Government.

Pupil apodization as a means of mitigating diffraction effects in remote sensing instruments

The Geostationary Operational Environmental Satellite (GOES) platform carries an infrared atmospheric sounding instrument which is used to obtain vertical profiles of atmospheric temperature and humidity throughout much of the western hemisphere. These profiles are numerically retrieved from measured nadir-viewing spectral radiances. The opacity of clouds to IR radiance makes such instruments functional only in clear-air regions. Because severe weather is associated with clouded regions, it is highly desirable to obtain soundings through holes in the cloud cover and up to the edge of frontal boundaries. There is much difficulty in performing this task with the existing GOES sounder because cloud cover gives rise to radiance errors in adjacent, and more distant, clear-air fields-of-view. A primary cause for this problem is diffraction, which introduces optical crosstalk between fields-of-view, and which is exacerbated by the large radiance contrast between clouds and clear air. This paper describes a novel application of tapered, or apodized, aperture illumination which may be employed in future GOES sounding instruments to mitigate the effects of diffraction. Tapering the aperture illumination at the edges (or applying this taper at accessible pupils, which are images of the aperture stop) reduces the subsidiary rings of the point-spread function. The benefits of pupil apodization are quantified, as are the penalties incurred by effectively making the aperture smaller. The construction of a graded-transmission spatial filter is described, and its optimal location in a sounding instrument based on a Michelson spectrometer is defined. Finally, the results of measurements taken on a fabricated filter are presented.

An advanced wide area chemical sensor testbed

In order to meet current and emerging needs for remote passive standoff detection of chemical agent threats, MIT Lincoln Laboratory has developed a Wide Area Chemical Sensor (WACS) testbed. A design study helped define the initial concept, guided by current standoff sensor mission requirements. Several variants of this initial design have since been proposed to target other applications within the defense community. The design relies on several enabling technologies required for successful implementation. The primary spectral component is a Wedged Interferometric Spectrometer (WIS) capable of imaging in the LWIR with spectral resolutions as narrow as 4 cm-1. A novel scanning optic will enhance the ability of this sensor to scan over large areas of concern with a compact, rugged design. In this paper, we shall discuss our design, development, and calibration process for this system as well as recent testbed measurements that validate the sensor concept.

GOES advanced sounder design study

Interim results of a current study on upgrading the GOES infrared Sounder are presented. Considered are a 15 cm diameter telescope to reduce instrument size and weight, use of a Fourier transform infrared (FTIR) interferometer for high spectral wavelength resolution, a small detector focal plane array operating at 65K, and combining the instrument with a microwave sounder. Retrieval performance improvement from the FTIR sounder is estimated.

Hyperspectral environmental suite for the Geostationary Operational Environmental Satellite (GOES)

The GOES satellites will fly a Hyperspectral Environmental Suite (HES) on GOES-R in the 2012 timeframe. The approximately 1500 spectral channels (technically ultraspectral), leading to improved vertical resolution, and approximately five times faster coverage rate planned for the sounder in this suite will greatly exceed the capabilities of the current GOES series instrument with its 18 spectral channels. In the GOES-R timeframe, frequent measurements afforded by geostationary orbits will be critical for numerical weather prediction models. Since the current GOES soundings are assimilated into numerical weather prediction models to improve the validity of model outputs, particularly in areas with little radiosonde coverage, this hyperspectral capability in the thermal infrared will significantly improve sounding performance for weather prediction in the western hemisphere, while providing and enhancing other products. Finer spatial resolution is planned for mesoscale observation of water vapor variations. The improvements over the previous GOES sounders and a primary difference from other planned instruments stem from two-dimensional focal plane array availability. These carry an additional set of challenges in terms of instrument specifications, which will be discussed. As a suite, HES is planned with new capabilities for coastal ocean coverage with the goal of including open ocean coverage. These new planned imaging applications, which will be either multispectral or hyperspectral, will also be discussed.

Optical performance measurements of large IR focal plane arrays for GOES

Remote sensing of the atmosphere and the surface of the earth is performed by the Imager and Sounder instruments onboard the GOES (Geostationary Operational Environmental Satellite) Satellites. By employing large PV Hg1CdTe focal plane array (FPA) detectors, instruments like the Advanced Baseline Imager (ABI) and Advanced Baseline Sounder (ABS) will provide improved update times, resolution, and sensitivity. However, uniformity in the pixel geometry across the array must first be demonstrated in order to maintain the accuracy of weather products at each spot on the ground. This uniformity is particularly important in weather products involving radiance subtractions and ratios from multiple spectral bands employing different detectors. Measurement ofthe spatial response associated with a pixel is important in determining both ground resolution and the effect ofradiance from outside the pixel field-of-view. Therefore, a high precision test set-up has been developed at Lincoln Laboratory to measure both the modulation transfer function (MTF) associated with each pixel in the array and the cross-talk from pixel to pixel. Details of the test set up and initial results of the testing will be discussed.

Emergency GOES Imager (EGI)

The Emergency GOES Imager study responds to the potential need for a small, back-up imager for weather observations in the event of failure of one or more of the current GOES satellites. The Emergency GOES Imager (EGI) is designed to be compact and lightweight. Minimal spatial resolution is required in the visible and IR band for the purpose of synoptic forecasts. The ground resolution requirement is 16 km for the 10.2 to 11.2 micrometers IR band and 4 km for the 0.5 to 0.7 micrometers visible band. Due to the small size of the instrument, the EGI has the potential to be deployed either alone on a small launcher or as an auxiliary payload on a larger satellite. The overall size of the EGI is dependent on the orientation of the satellite because of the dependence on amount of solar shielding required for the cooler, and the choice of coolers for specific satellite orientations. Although the EGI design is for an emergency system, the design utilizes recent technology in the form of both a linear IR focal plane array, in front of its constant-motion mirror, and a visible CCD array with a staring-format. The IR array has the potential to present a technical challenge to array manufacturers in the area of calibration, assuming a 0.1 K NEDT. We discuss the means by which the emergency requirements are met with this small and simple system, define the limiting technologies in the design, and explore modifications necessary to expand these requirements.