MOMS is a new optoelectronic scanning system designed for the acquisition of geoscientific remote sensing data. The mission of MOMS-01 aboard STS-7 and STS-11/41-B flights served the technological space verification of the sensor and the demonstration of the instuments's capability for thematic mapping up to scale 1:50,000 in globally distributed test sites. The future development of the MOMS instrument family aims at the realization of a highly advanced modular remote sensing system, featuring 3-4 band multispectral and in-track stereoscopic capabilities.
The concept of the Imaging Spectrometer is becoming established as a major new thrust in remote sensing of the Earth. JPL is currently operating the Airborne Imaging Spectrometer on a NASA C-130 which has demonstrated the direct identification of surface materials using Imaging Spectrometry. An advanced aircraft instrument, the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS), is currently under development, and will begin operation on a NASA U-2 in 1986. The next step will be the Shuttle Imaging Spectrometer (SISEX) currently planned for a 1990 flight. This paper will describe the current state of development of SISEX, including the development of a modular concept which will allow major elements of SISEX to be used on NASA's Space platform, the Earth Observing System. This modular approach is expected to result in a substantial overall cost saving.
Three imaging spectrometers and two camera systems for remote sensing are described. Two of the imaging spectrometers are versions of the Visible and Infrared Mapping Spectrometer (VIMS) for Mars Observer and the Comet Rendezvous Asteroid Flyby (CRAF) mission. The other spectrometer is the Airborne Visible/Infrared Imaging Spectrometer (AVIRTS) which is currently under construction. The optical imaging systems are the wide angle and narrow angle cameras for the CRAF mission.
The paper presents the main results of a feasibility study (ESA Contract n° 5234/82 with AEROSPATIALE and Marconi Space Systems) of an 8-channel multispectral push-broom instrument, operating in the earth-reflected solar spectrum, for oceans applications. The objective for this so-called "Advanced OCM" (Ocean Colour Monitoring) is the mapping of sea-surface chlorophyll conteit with an accuracy of 20 %, which calls for exceptional radio-metric accuracy of NEΔ<5.10-4 at sea level and very good relative and absolute calibrations.
A design is presented for an imaging spectropolarimeter/photometer based on the LANDSAT multispectral scanner (MSS) for the wavelength range 0.5 to l.lμm. The proposed scanner has essentially the same wavelength sensitivity, field of view and scanning parameters of the original MSS, except for the duplication of sensing and data systems for the determination of the amount of plane polarization. A unique non-polarizing scanning system is described as well as a beamsplitter that does not introduce extraneous polarization.
To perform in the laboratory the final calibration of a pushbroom scanner a flexible test arrangement has been developed. Herewith radiometric and geometric calibration can be performed to a CCD based multispectral scanner. With the same arrangement spectral calibration and a simple check of instrumental sensitivity to polarisation and straylight are possible. For proper understanding of the calibration procedure both the scanner and the test arrangement are described.
In remote sensing, increasing demand for accuracy has appeared since the first satellite has been launched. SPOT is a new generation of remote sensing satellites caracterized by push broom technique and high resolution. SPOT system calibrating procedures are presented and on board calibration devices are described and discussed.
A NASA laboratory for evaluating linear detector arrays for remote sensing applications from the visible to shortwave infrared is de-scribed. Nominal requirements are presented on the critical parameters of signal-to-noise ratio, modulation transfer function, response linearity, dynamic range, and detector-to-detector response uniformity. Results of tests on a custom developed multi-spectral linear array from RCA and two commercial arrays from Texas Instruments and Reticon for the visible and near-infrared are presented. The custom array was clearly superior in signal-to-noise ratio and dynamic range to its off-the-shelf counterparts. All the arrays were found to have somewhat less than desirable modulation transfer functions, but excellent response linearity.
A line of 3000 detectors for the 1.55 µm - 1.7 µm band is being developed by Thomson-CSF for the SPOT III program. It consists of 10 elementary modules of 300 InGaAs detectors, multiplexed in the focal plane by a silicon CCD.
A scheme for simplifying the design of CCD-based multispectral pushbroom scanners is presented. A new type of real-time signal processor permits registration of the images in the various spectral bands to ±0.1 pixel even though the system is mechanically aligned to only ±8 pixels. This permits design flexibility and cost reduction in the sensor optical system. The same signal processing can also correct for earth rotation skew and platform attitude errors at the same time, thereby simplifying the archiving and use of the data. An operational land observing system design that uses these ideas is described.
ISAMS is a temperature and composition sounder due to fly on the NASA UARS in 1989. The mission objectives, design rationale and expected performance are described. Features of this limb viewing infrared radiometer are its use of the pressure modulation technique of spectral selection and closed cycle cooling of the detectors to achieve background limited performance.
The optical arrangement of the ISAMS telescope and a typical channel are shown and some theoretical aspects of the optical design and the constraints on it are described. Data is then given on the predicted image resolution allowing for residual geometrical aberrations, diffraction and a manufacturing/assemblv error budget.
Earlier pressure modulator designs have been developed to produce a new mechanism having opposed and balanced pistons. The mechanism operates as a coupled resonant system, with new control electronics providing better amplitude and frequency stability. Careful choice of materials used in construction of the modulators, together with the use of a silicalite molecular sieve, is expected to lead to improved stored gas stability. Stirling cycle coolers developed for ISAMS are to be installed as servo controlled balanced pairs, the effectiveness of this compensating technique has been demonstrated during testing. Connection of the coolers to the widely spaced detectors is effected by a co-axial thermal connection system. Detector mounts which minimise heat leaks whilst providing mechanical rigidity and adjustment of the optics within them have been designed employing tubular metal legs in a stool arrangement.
The design and in-orbit performance data are presented for the Stratospheric Aerosol and Gas Experiment II (SAGE II) instrument which was launched by Shuttle on the Earth Radiation Budget Satellite. SAGE II is a Sun photometer that measures the extinction of Solar radiation caused by the Earth's atmosphere in seven spectral channels ranging in center wavelength from 0.385 to 1.02 micrometers. These measurements, which occur twice each orbit during satellite sunrise and sunset, are inverted to yield vertical distributions of stratospheric aerosols, ozone, water vapor, and nitrogen dioxide. The SAGE II instrument consists of a Cassegrain telescope with a two axis gimbal mounting, a grating spectrometer, and a 12 bit data system. The instrument tracks the Solar centroid in the aximuth plane and vertically scans the instrument's instantaneous field of view across the Sun for tangent altitudes ranging from the Earth's horizon to 150 km. SAGE II is a third generation instrument following the highly successful Stratospheric Aerosol Measurement II (SAM II) and SAGE I programs.
The ATSR is an infrared imaging radiometer which has been selected to fly aboard the ESA Remote Sensing Satellite No. 1 (ERS1) with the specific objective of accurately determining global Sea Surface Temperature (SST). Novel features, including the technique of "along track" scanning, a closed Stirling cycle cooler, and the precision on-board blackbodies are described. Instrument sub-systems are identified and their design trade-offs discussed.
The optical arrangement of ATSR (for sea-surface temperature measurement) is described. Radiometric constraints upon the sizes, positions and temperatures of stops in the system are discussed and their impact on the design summarized. Data on the predicted image resolution are given, allowing for residual geometrical aberrations, diffraction, signal integration time and a manufacturing/assembly error budget.
The Along Track Scanning Radiometer is a radiometer designed to measure sea surface temperature to be flown aboard ERS1. This paper shows how the focal plane assembly was designed to meet the high performance required for each channel. It also presents solutions to additional thermal and mechanical problems presented by the design.
The successfully flown Shuttle Imaging Radar-B (SIR-B) instrument is described, giving details of the hardware, the subsystem functions, and the Shuttle interfaces. The preliminary design of the SIR-C instrument is described, giving an overview of the preliminary hardware design, the subsystem functional design, and the Shuttle interfaces.
The surface of Venus has remained a relative mystery because of the very dense atmosphere that is opaque to visible radiation and, thus, normal photographic techniques used to explore the other terrestrial objects in the solar system are useless. The atmosphere is, however, almost transparent to radar waves and images of the surface have been produced via earth based and orbital radars. The technique of obtaining radar images of a surface is variously called side looking radar, imaging radar, or synthetic aperture radar (SAR). The radar requires a moving platform in which the antenna is side looking. High resolution is obtained in the cross-track or range direction by conventional radar pulse encoding. In the along-track or azimuth direction, the resolution would normally be the real antenna beam width, but for the SAR case, a much longer antenna (or much sharper beam) is obtained by moving past a surface target as shown in Figure 1, and then combining the echoes from many pulses, by using the doppler data, to obtain the images.
The Dutch Additional Experiment (DAX) was a focal-plane instrument package flown successfully on the IRAS satellite. The package contained a low-resolution spectrometer, operating between 8 and 23 μm, and a two-channel photometer with bands at 60 and at 100 μm. The design and the in-flight performance of both instru-ments are discussed.
This paper presents the project ISOCAM for an infrared camera which will be one of the four focal plane instruments on ISO. The camera contains two optical channels, one with an InSb CID array (3 to 5 microns), the other with a Si:Ga DVR array (5 to 17 microns). Interference filters and CVF's provide spectral resolutions between 2 and 50. The optics sets the pixel field of view at 3, 6 or 12 arc second.
The Short Wavelength Spectrometer (SWS) being developed for the Infrared Space Observatory (ISO) is described. The instrument is a pair of grating spectrometers, used in low orders, that will provide resolving powers of about 1000 from 3 μm to 45 μm. In combination with Fabry Perot etalons, the resolving power will be as high as 30,000 over the 15 to 30 μm range.
The photometer for the wavelength range 3 ... 200 µm also has modes for polarimetric and 3 ... 18 µm spectrophotometric observations. An imaging capability is available over the full range. The design goal is towards highest sensitivity and flexibility as a multi-user instrument on ISO.
This paper presents the main results of a feasibility study (ESTEC contract n° 6122/84/NL/GM with AEROSPATIALE and REOSC) of a cold lightweight mirror. This is the pri-mary mirror of a RITCHEY-CHRETIEN telescope for the ISO satellite, diffraction limited at 5 μm ( λ/14 RMS). The following points are the main difficulties which must be taken into account to define the mirror : - Choice of mirror material, based on the best optical suitability, homogeneity of the thermal expansion coefficient and straylight characteristics. The available materials are ZERODUR optical grade and fused silica HERASIL 1 grade. Fused silica seems to be the best material because there is no deformation of a sample under radiations and no hysterisis during temperature cycles, opposite to ZERODUR. - Lightweighting, REOSC know-how with numerical machine tools allows to reach a weight of 20 kg (instead of 57 kg) and in terms of aspherical coefficient, the wave front er-ror is about λ/86 in RMS value at 5 μm. - Mirror fixation device, based on a patented fixation for ambient temperature, does not introduce torque in the mirror and take up the differential thermal expansion between the mirror and the optical support structure. A calculation gives a wave front error of λ/70 RMS at 5 μm. - Mirror cooling, temperature of the mirror in space has to be less than 10° K. It is necessary to bond thermal straps at the rear of the optical surface of the mirror in lightweighting holes. The number of straps necessary to cool down to 10° K the mirror in 48 hours must be over 40. The number of holes in the mirror is 57.
A spectroscopic instrument for the ESA Infrared Space Observatory is described, covering the wavelength range 45 to 180 microns and with operating modes offering a choice of two resolving powers. The lower resolving power of around 230 is given by a diffraction grating system, with the spectrum recorded by a line of 10 detectors. The high resolving power of around 1.5E4 is achieved by switching a Fabry-Perot (F-P) interferometer into the parallel part of the instrument beam. Either of two F-Ps can be selected, one optimised for the 45 to 90 micron wavelength range and the other for 90 to 180 microns. The operating modes offered include differential measurement, using wavelength switching and synchronous detection, and absolute measurement with an internal chopper/reference. Development aspects of the instrument are also discussed.
The Infrared Space Observatory, an approved and funded project of the European Space Agency is an astronomical satellite operating at wavelengths from 3 to 200 microns. The launch date is 1992. It will obtain spectroscopic, imaging, photometric and polarimetric measurements of selected celestial objects using a cryogenically-cooled 60cm telescope and a complement of four nationally-funded scientific instruments.
This paper describes the scientific and technical background and prospects for the Space Infrared Telescope Facility (SIRTF). SIRTF is a cryogenically-cooled, one meter-class space telescope which will be operated by NASA as an observatory for infrared astronomy, in the mid-1990's. SIRTF will build on the scientific and technical success of IRAS, but will have greater capabilities in all important scientific areas - wavelength coverage, spectral and spatial resolution, and sensitivity. SIRTF will provide detailed studies of even the faintest IRAS sources, important new capabilities for the study of known astrophysical phenomena, and the potential to make new and unexpected discoveries about the nature of the Universe. SIRTF will be a long-life observatory and will support a vigorous general investigator program accessible to the international scientific community. The long-life SIRTF mission has undergone intensive review by the SIRTF Science Working Group, which was selected in mid-1984. In this paper, we present the outcome of that review process and describe the SIRTF program as it is now envisioned. Particular emphasis will be placed on the choice of orbit for SIRTF, the SIRTF scientific performance requirements and the baseline design concept for the SIRTF facility and mission.
The temperature of the SIRTF optical system has two significant effects: it determines the contribution of the telescope thermal emission to the photon shot noise seen by the detectors, and it determines the magnitude of the telescope emission that constitutes the radiometric baseline against which astronomical measurements are made. Changes in the system temperature therefore produce shifts in the radiometric baseline which can set the limit on the usable sensitivity of the system. We compare these two effects and explore some of their implications. We find that temperature gradients on the primary mirror are not likely to pose a serious problem if spatial chopping is employed.
The Infrared Array Camera for the Space Infrared Telescope Facility (SIRTF/IRAC) is capable of two-dimensional photometry in either a wide field or diffraction-limited mode over the wavelength interval from 2 to 30 microns. Three different two-dimensional direct readout (DRO) array detectors will be used: Band 1 - InSb or Si:In (2 - 5 microns) 128 x 128 pixels, Band 2 - Si:Ga (5 - 18 microns) 64 x 64 pixels, and Band 3 - Si:Sb (18 - 30 microns) 64 x 64 pixels. The hybrid DRO readout architecture has the advantages of low read noise, random pixel access with individual readout rates, and non-destructive readout. The scientific goals of IRAC are discussed, which are the basis for several important requirements and capabilities of the array camera: 1) diffraction-limited resolution from 2 - 30 microns, 2) use of the maximum unvignetted field of view of SIRTF, 3) simultaneous observations within the three infrared spectral bands, 4) the capability for broad and narrow bandwith spectral resolution. A strategy has been developed to minimize the total electronic and environmental noise sources to satisfy the scientific requirements.
A conceptual design for an infrared spectrometer capable of both low resolution (λ/Δ-λ = 50; 2.5-200 microns) and moderate resolution (1000; 4-200 microns) and moderate resolution (1000; 4-200 microns) has been developed. This facility instrument will permit the spectroscopic study in the infrared of objects ranging from within the solar system to distant galaxies. The spectroscopic capability provided by this instrument for SIRTF will give astronomers orders of magnitude greater sensitivity for the study of faint objects than had been previously available. The low resolution mode will enable detailed studies of the continuum radiation. The moderate resolution mode of the instrument will permit studies of a wide range of problems, from the infrared spectral signatures of small outer solar system bodies such as Pluto and the satellites of the giant planets, to investigations of more luminous active galaxies and QS0s at substantially greater distances. A simple design concept has been developed for the spectrometer which supports the science investigation with practical cryogenic engineering. Operational flexibility is preserved with a minimum number of mechanisms. The five modules share a common aperture, and all gratings share a single scan mechanism. High reliability is achieved through use of flight-proven hardware concepts and redundancy. The design controls the heat load into the SIRTF cryogen, with all heat sources other than the detectors operating at 7K and isolated from the 4K cold station. Two-dimensional area detector arrays are used in the 2.5-120μm bands to simultaneously monitor adjacent regions in extended objects and to measure the background near point sources.
The Multiband Imaging Photometer for SIRTF (MIPS) is to be designed to reach as closely as possible the fundamental sensitivity and angular resolution limits for SIRTF over the 3 to 700μm spectral region. It will use high performance photoconductive detectors from 3 to 200μm with integrating JFET amplifiers. From 200 to 700μm, the MIPS will use a bolometer cooled by an adiabatic demagnetization refrigerator. Over much of its operating range, the MIPS will make possible observations at and beyond the conventional Rayleigh diffraction limit of angular resolution.