Leslie L. Thompson

Multispectral Resource Sampler - An experimental satellite sensor for the mid-1980s

An experimental pushbroom scan sensor, called the Multispectral Resource Sampler (MRS), is being developed by NASA for an earth orbiting spacecraft flight in the mid-1980s. This sensor will provide new and unique earth survey research capabilities beyond those possible with current sensor systems, and is designed with flexibility to provide a research facility for a number of preselected experiments. The sensor will have a ground resolution (IFOV) of 15 meters over a swath width of 15 kilometers, in four bands, or 30 kilometers in two bands. A data rate limitation of 15 megabits/second controls the permitted swath width. Each of the four arrays will have five separate spectral filters that will be selectable by command while in orbit. The basic sensor uses four 2000 element detector arrays in the focal plane of a 70 cm focal length (F/3.5) telescope. The four arrays are aligned on a common focal surface; thus no beamsplitters are required. This causes a spatial separation on the ground which requires computer processing to register the bands. A 2.2 ms dwell time of the pushbroom array allows bandwidths as narrow as 20 nanometers over the spectral range from 0.35 to 1.0 micrometers. Response in each band will be quantized into eight bits. The MRS can be pointed at ± 40° in the across track direction and ± 55° in the along track direction. Along track pointing permits stereo coverage at variable base/height ratios and atmospheric correction experiments, while across track pointing will provide repeat coverage, from a Landsat-type orbit, of every 1 to 3 days. A number of significant experiments which could be performed with the MRS include experiments in crop discrimination and status, rock discrimination, geobotanical mineral exploration, land use classification and forestry.

Time-delay-and-integration charge-coupled devices using tin oxide gate technology

A time-delay-and-integration (TDI) CCD area array organized into two sections of 10 by 9 integration stages has been designed, built, and tested. The device uses four-phase buried channel construction and provides 65-percent quantum efficiency with smooth spectral response by front surface imaging through transparent doped tin oxide gates. TDI operation with forward and reverse scan clocking for bi-directional image motion has been demonstrated, with the 10 input parallel-in/serial-out output shift register operating at 1.25-MHz video rate.

A design study for an advanced ocean color scanner system

The spaceborne ocean color scanners currently being planned for flights on Nimbus-G satellite or space shuttle craft are, in every aspect, only a modest beginning towards what is to be expected of ocean color scanners in the eighties. Improvements are necessary in the following areas: present systems provide a spatial resolution on the order of 1 km at nadir, which would not satisfy most of the coastal zone study requirements. Also the present design of radiomers is less than optimum for the removal of the atmospheric effects on ocean colorimetry.Along with a colorimetric data analysis scheme, the instrumental parameters which need to be optimized in future systems are outlined. One technique for meeting these requirements entails use of large linear array detector technology.

Time-Delay-And-Integration Charge Coupled Devices (CCDs) Applied To The Thematic Mapper

The Thematic Mapper (with its 30 m ground footprint in five-spectral bands from 0.4 μm to 2.0 μm and 120 m GFT in a 10.5 μm to 12.5 μm spectral band) represents the next generation sensor system for application to earth resource survey. The Thematic Mapper (TM) uses a 56 cm x 42 cm oscillating mirror to scan the scene, and is designed to provide accurate radiometric measurements. Currently, in the visible focal plane the discrete detectors must provide 80% quantum efficiency, nominally 100 electrons noise, less than 5% ripple in the spectral response, and 0.5% signal crosstalk to meet system requirements. This focal plane uses complex hybrid assembly techniques to interface silicon photodiodes to JFET preamplifiers. This complexity can be ameliorated by using a new approach in signal pro-cessing which takes advantage of the properties of charge-coupled devices (CCD). A 20-channel time-delay-and-integration (TDI) CCD with nine stages of integration per channel has been built for a TM application. By going to a CCD array which operates in a TDI mode, over 700 individual Op Amps can be replaced with only 48 Op Amps. Smooth spectral response and 70% quantum efficiency have been provided by using doped tin oxide gates over the imaging region. The 20 x 9 TDI device when operated with all 9 integration stages, ,exceeded SNR requirements by a factor of 2. The crosstalk performance is controlled by providing high charge transfer efficiency using a buried channel CCD. The TDI device has demonstrated charge integration for both forward and reverse scans of the oscillating mirror, and a form of exposure control is provided by integrating over 3, 6, or all 9 stages in either direction of scan.

Moderate resolution imaging spectrometer for the NASA earth observing system

MODIS is intended to provide daily global surveys for the oceans, the atmosphere, and land. To achieve this capability, this instrument requires at least a 1700 km swath width and provides geometric-instantaneous-fields-of-view that are either 214 m, 428 m, or 856 m in size with reference to a 705 km satellite altitude. The range of products to be supported by MODIS includes ocean chlorophyll concentration, ocean primary productivity, dissolved organic matter in the oceans, global SST, total suspended solids, land cover type, land surface temperatures, vegetation indices, and volcanic and fire events. Details of the MODIS-N and MODIS-T instrument concepts are discussed.

Overview of the potential instrument payload for the EOS system

The Earth Observing System (EOS) is a key element of the U.S. Program in Global Change. It is NASAs initiative to provide the scientific means to observe the earth as a system. The EOS satellite system and accompanying Data and Information System (DIS) provide spacebased platforms and groundbased elements for scientific research and information extraction. The program recognizes the need for global coverage, long term observations, and multidisciplinary analyses in order to detect subtle changes in the environments state. A multidisciplinary payload of active and passive sensors is proposed to accomplish the objectives of the program. International partners are an important element of the program. This paper focuses on the instrument complement for the EOS-A spacecraft since those decisions will occur this year. Significant programmatic constraints and program science decisions will impact the selection of the EOS-A payload.

MODIS-N - Moderate Resolution Imaging Spectrometer-Nadir

The Moderate Resolution Imaging Spectrometer-Nadir (MODIS-N) for the Earth Observing System (EOS) is intended to provide daily global surveys for the atmosphere, the oceans, and the land. To achieve this capability, MODIS-N requires an at-least 2300-km swath width, and provides geometric-instantaneous-fields-of-view (GIFOVs) that are either 856 m, 428 m, or 214 m in size with reference to a 705 km satellite altitude. The 214 m GIFOV may or may not be used depending on total data rate impact assessments traded with science needs. To achieve the data for the multiplicity of science investigations MODIS-N provides nominally 36 spectral bands that are selected for specific locations and bandpasses in the spectral range from the visible to the long wave infrared. Another driver of this instrument combination is the need for long term spectral and radiometric calibration stability. Specific calibration capabilities are to be built into MODIS-N to achieve calibration knowledge over a 5 year operational life.

Technical Issues In Focal Plane Development For Terrestrial Resource Observations

The use of solid-state detector arrays which operate in a pushbroom scan mode for remote sensing of the Earths resources and environment has received increased attention in the last several years. The potential for improved radiometric sensitivity, geometrical accuracy and signal processing permit consideration of new levels of sensor performance in the areas of spatial resolution, spectral resolution, mapping accuracy, and improved system throughput of data products. These benefits depend on the ability to manufacture and accurately align thousands of detectors into a multispectral focal plane. Two key performance goals are to achieve: (1) radiometric calibration to 0.5% precision detector-to-detector over a dynamic range of 1000:1; and (2) geometric alignment to place the detector elements to within 0.1 resolution element of their desired perfect positions. System cost (including the ground segment) and complexity should be traded against these goals. Science experiments continue to be needed to establish the tolerances on these goals.

Silicon Solid/State Linear Arrays For Multisfectral High Resolution Imaging Systems

As will be pointed out many times in the next few months, the Earth Resources Technology Satellite (ERTS-1) has now completed two years of successful operation in orbit as of July 23, 1974. This satel-lite provides two sensor systems capable of imaging the Earths surface with nominally 100m ground resolution across a 185 Km swathwidth. In addition, accurate relative radiometry is provided in four spectral bands by one of these ERTS systems known as the Multispectral Scanner (MSS). The four spectral bands cover the visible to near-IR (.5 -.6; .6 -.7; .7 -.8; and .8 -1.1 micrometers). The high quality of the ERTS-1 MSS imagery provides a known standard to which future systems can be compared.