As part of the geolocation accuracy assessment of lightning flashes detected by the Geostationary Lightning Mapper (GLM) on the GOES-16 and GOES-17 satellites (Geostationary Operational Environmental Satellite), two satellite laser ranging stations employed laser beacon systems to generate transient light pulses that simulate natural lightning around 777.4 nm to validate the pre-launch spec of 5 km. The pulse width, repetition rate, wavelength, and power of the laser-pulses were configured to produce sufficient instrument response to be detected as synthetic lightning events by the GLM instrument. During the testing period from April 2017 to January 2018, the laser systems illuminated the GOES-16 satellite to observe diurnal variation of the GLM system response, with particular emphasis on geolocation accuracy. The final GOES-16 laser beacon tests, which used the latest updates of the geolocation algorithms implemented by the GOES-R Ground Segment, showed the offsets between the GLM geolocated location and the known laser locations were within 5 km.
The Advanced Baseline Imager (ABI) will image Earth in 16 spectral channels, including 10 thermal IR (TIR) channels.
The instantaneous field of view (IFOV) of each TIR detector element is (56 μrad)2. The ABI has an onboard fullaperture
blackbody, the Internal Calibration Target (ICT), used in conjunction with deep space looks to calibrate the
ABI's TIR channels. The ICT is only observed over a small range of temperatures and at one specific pair of reflection
angles from the ABI's two scan mirrors. The sunlit area on Mercury's surface underfills the IFOV's of the ABI's TIR
channels, but has a much higher range of characteristic temperatures than the ICT, so its radiation is weighted more
strongly toward shorter wavelengths. Comparison of a TIR channel's responses to the ICT and to Mercury provides a
sensitive means to evaluate variations in spectral response functions among detector elements, across the ABI's field of
regard, and among instruments on different satellites. Observations of Mercury can also verify co-registration among
the ABI's atmospheric absorption channels that do not observe features on Earth's surface. The optimal conditions for
viewing Mercury typically occur during one or two intervals of a few weeks each year when it traverses the ABI's FOR
(-10.5o < declination < +10.5o) with an elongation angle from the Sun of at least 20.5o.
A 4 mm diameter zone plate was calibrated in the 7 nm to 18.5 nm wavelength range using synchrotron radiation. The efficiency in the focused 1st order was measured using the scanning monochromator at the Naval Research Laboratory beamline X24C at the National Synchrotron Light Source. The measured efficiencies were compared to efficiencies calculated by accounting for the partial transmittance through the molybdenum zone plate rings and the resulting phase enhancement of the efficiencies. Accurate absolute efficiency calibrations enable the use of zone plates in EUV solar
irradiance monitors having excellent stability against contamination and oxidation.
Measuring the solar extreme ultraviolet (EUV) irradiance is a high priority for space weather and upper atmospheric
research. We have designed and fabricated a sensor to measure the total solar irradiance in the extremely variable 17.1-
19.5 nm spectral band that contains numerous Fe emission lines and we are developing another similar sensor for the
30.4 nm He II spectral line. Each of these sensors uses a 4 mm diameter zone plate (ZP) to focus the in-band solar
radiation onto a small pinhole in front of a detector. The focal length of a ZP is inversely proportional to wavelength,
and the pinhole's diameter is sufficient to admit the in-band radiation from the full solar disk and inner corona. A 2 mm
diameter central occulting disk minimizes the undiffracted (0-order) and out-of-band radiation that reaches the pinhole.
Two thin Al film filters, one in front of the ZP and one deposited on the detector, are virtually opaque to wavelengths
longer than EUV and prevent the detector from responding to most of the solar spectrum. A ZP has a number of
advantages over a diffraction grating. Because it focuses in-band radiation, a ZP allows the detector to be smaller than
the aperture, reducing both the dark current and the out-of-band response. The circular symmetry of the ZP eliminates
both the polarization sensitivity and the shift in the spectral band with field angle that are intrinsic to a linear grating.
The optical assembly is contained in a very small volume. We are planning to fly the Fe-band sensor as a secondary
payload on NRL's VERIS sounding rocket solar measurement mission in the near future and are investigating the
feasibility of flying both sensors on a future CubeSat mission.
The Geostationary Operational Environmental Satellite-R (GOES-R), scheduled for launch in 2014, will be the first in a series of next generation weather satellites. It will be 3-axis stabilized in geostationary orbit, and will have an Advanced Baseline Imager (ABI) that can make full-Earth disk images, covering most of the Western Hemisphere, in spectral bands ranging from 0.47-13.3 μm. We are now designing a Full-Disk Ratioing Radiometer (FDRR) to determine the ratio of the full-disk irradiance to the solar irradiance in spectral bands that match the four shortest wavelengths of the ABI's Visible and Near IR (VNIR) spectral bands. When hard-mounted to the nadir face of a GOES-R satellite, this FDRR can determine the full-disk albedo in each band, with the added benefit that the ABI's corresponding channels can be calibrated by comparison of these measurements to the Earth's irradiance measurements derived from simultaneous full disk images made by the ABI. The FDRR uses an integrating sphere with two baffled pinholes. One pinhole has a baffle that restricts its field-of-view (FOV) to a circle 20.1° in diameter, centered at nadir, viewing the Earth's full disk continuously throughout its daily cycle. This baffle has a shutter that allows it to be closed for dark current measurements during the day and to prevent solar intrusion at night. The second pinhole, with a much smaller diameter, has a baffle that restricts its FOV to about 1° in the East-West direction and +/-25° in the North/South direction, allowing the direct solar irradiance to enter the sphere for a brief interval once each night. A radiationhardened fiber optic light pipe couples the output of the sphere to filters and detectors in an electronics box. These filters and detectors have spectral bands matched to those in the ABI. This technique measures the ratio of the full-disk irradiance to the direct solar irradiance, determining the Earth's albedo independent of the detector's response, the transmission of the filters and the fibers, and the sphere's reflectivity.
The diffraction efficiencies of a Fresnel zone plate (ZP), fabricated by Xradia Inc. using the electron-beam writing technique, were measured using polarized, monochromatic synchrotron radiation in the extreme ultraviolet wavelength range 3.4-22 nm. The ZP had 2 mm diameter, 3330 zones, 150 nm outer zone width, and a 1 mm central occulter. The ZP was supported by a 100 nm thick Si3N4 membrane. The diffraction patterns were recorded by CMOS imagers with phosphor coatings and with 5.2 μm or 48 μm pixels. The focused +n orders (n=1-4), the diverging -1 order, and the undiffracted 0 order were observed as functions of wavelength and off-axis tilt angle. Sub-pixel focusing of the +n orders was achieved. The measured efficiency in the +1 order was in the 5% to 30% range with the phase-shift enhanced efficiency occurring at 8.3 nm where the gold bars are partially transmitting. The +2 and higher order efficiencies were much lower than the +1 order efficiency. The efficiencies were constant when the zone plate was tilted by angles up to ±1° from the incident radiation beam. This work indicates the feasibility and benefits of using zone plates to measure the absolute EUV spectral emissions from solar and laboratory sources: relatively high EUV efficiency in the focused +1 order, good out-of-band rejection resulting from the low higher-order efficiencies and the ZP focusing properties, insensitivity to (unfocused) visible light scattered by the ZP, flat response with off-axis angle, and insensitivity to the polarization of the radiation based on the ZP circular symmetry. EUV sensors with Fresnel zone plates potentially have many advantages over existing sensors intended to accurately measure absolute EUV emission levels, such as those implemented on the GOES N-P satellites that use transmission gratings which have off-axis sensitivity variations and poor out-of-band EUV and visible light rejection, and other solar and laboratory sensors using reflection gratings which are subject to response variations caused by surface contamination and oxidation.
Recent work by some of the authors presented a novel construction of a multiresolution analysis on manifolds and graphs, acted upon by a given symmetric Markov semigroup {Tt}t≥0, for which Tt has low rank for large t. This includes important classes of diffusion-like operators, in any dimension, on manifolds, graphs, and in nonhomogeneous media. The dyadic powers of an operator are used to induce a multiresolution analysis, analogous to classical Littlewood-Paley and wavelet theory, while associated wavelet packets can also be constructed. This extends multiscale function and operator analysis and signal processing to a large class of spaces, such as manifolds and graphs, with efficient algorithms. Powers and functions of T (notably its Green's function) are efficiently computed, represented and compressed. This construction is related and generalizes certain Fast Multipole Methods, the wavelet representation of Calderon-Zygmund and pseudo-differential operators, and also relates to algebraic multigrid techniques. The original diffusion wavelet construction yields orthonormal bases for multiresolution spaces {Vj}. The orthogonality requirement has some advantages from the numerical perspective, but several drawbacks in terms of the space and frequency localization of the basis functions. Here we show how to relax this requirement in order to construct biorthogonal bases of diffusion scaling functions and wavelets. This yields more compact representations of the powers of the operator, better localized basis functions. This new construction also applies to non self-adjoint semigroups, arising in many applications.
Classically, analysis on manifolds and graphs has been based on the study of the eigenfunctions of the Laplacian and its generalizations. These objects from differential geometry and analysis on manifolds have proven useful in applications to partial differential equations, and their discrete counterparts have been applied to optimization problems, learning, clustering, routing and many other algorithms.1−7 The eigenfunctions of the Laplacian are in general global: their support often coincides with the whole manifold, and they are affected by global properties of the manifold (for example certain global topological invariants). Recently a framework for building natural multiresolution structures on manifolds and graphs was introduced, that greatly generalizes, among other things, the construction of wavelets and wavelet packets in Euclidean spaces.8,9 This allows the study of the manifold and of functions on it at different scales, which are naturally induced by the geometry of the manifold. This construction proceeds bottom-up, from the finest scale to the coarsest scale, using powers of a diffusion operator as dilations and a numerical rank constraint to critically sample the multiresolution subspaces. In this paper we introduce a novel multiscale construction, based on a top-down recursive partitioning induced by the eigenfunctions of the Laplacian. This yields associated local cosine packets on manifolds, generalizing local cosines in Euclidean spaces.10 We discuss some of the connections with the construction of diffusion wavelets. These constructions have direct applications to the approximation, denoising, compression and learning of functions on a manifold and are promising in view of applications to problems in manifold approximation, learning, dimensionality reduction.
A small, but highly variable fraction of the total solar irradiance lies in the extreme ultraviolet (EUV) spectrum. EUV radiation heats Earth's ionosphere, sometimes disrupting microwave communication and navigation and increasing the drag on satellites in low-Earth orbits. Each of the next series of Geostationary Operational Environmental Satellites (GOES), scheduled to operate from 2012 until at least 2029, will fly an EUV Sensor (EUVS) to measure the solar irradiance operationally in several EUV spectral bands. We propose a novel approach using zone plates (ZPs) instead of the transmission gratings that are now in use. A ZP can be used to form a solar image on a small detector array at a selected EUV wavelength. Since the focal length of a ZP is inversely proportional to wavelength, other wavelengths within the passband of the sensor will be blurred at the focal plane. The ZP can be mounted on a thin-film metallic substrate that can act as a filter, transmitting EUV radiation while blocking light at longer wavelengths. Another thin-film spectral filter on the front surface of the detector can further increase the spectral selectivity of the EUVS and make it less sensitive to defects in either thin film. The circular symmetry of the ZP minimizes the variation in detected signal with field angle and polarization, and its focusing capability allows the detectors to be small, making them easier to fabricate and improving their radiometric performance. ZPs are now used routinely at soft X-ray and short EUV wavelengths.
Large 2-D focal plane arrays (FPAs) can be used for multispectral imaging by sequentially viewing a scene through multiple discrete narrow-band filters on a continuously rotating filter wheel. The constant angular velocity of the wheel minimizes momentum disturbances, jitter and power consumption and maximizes reliability, but the optical beam straddles two adjacent filters during each transition between them. The duty cycle of the imager can be maximized by separating these narrow band filters into two equal groups: long-wavelength and short-wavelength. The narrow-band filters can then be mounted contiguous to one another on a ring of the filter wheel, alternating between long and short wavelength filters. The optical beam transmitted by the filter wheel can then be divided into a long-wavelength beam and a short-wavelength beam by a dichroic beamsplitter that transmits the output of one group of filters and reflects the output of the other group. Each beam can then be transmitted through an additional broadband spectral filter and can be imaged on a separate FPA. When the optical beam straddles two narrow-band filters on the wheel, the dichroic beamsplitter and the broadband filters prevent out-of-band radiation from reaching either FPA, allowing FPA to integrate a spectrally-pure 2D image (although the flux will decrease and the diffraction will increase as the beam is vignetted.) When the incident beam passes through a long-wavelength filter on the wheel, the short wavelength FPA can be read our without requiring a shutter, and vice versa.
The Visible/Infrared Imager/Radiometer Suite (VIIRS) will be the operational imaging instrument on three NPOESS satellites, in Sun-synchronous orbits at altitudes of 833 km. The VIIRS is presently planned to have a total of 14 solar reflective spectral bands, with central wavelengths ranging from 412nm to 2250nm. The Advanced Baseline Imager (ABI) will be the operational imaging instrument on two GOES-R satellites in geostationary orbits. The ABI is presently planned to have a total of 6 solar reflective spectral bands, with central wavelengths ranging from 470nm to 2260nm. Some of the ABI’s spectral bands are similar, but not identical to, those of the VIIRS. Each VIIRS instrument and each ABI instrument will be equipped with a solar diffuser for on-board, end-to-end calibration of its reflective channels.
Due to the high scan rates of both instruments and the flexible scheduling of the ABI, there will be several opportunities each day for the two instruments to simultaneously view the same area on Earth's surface along nearly identical lines of sight. It should be possible to cross-calibrate the ABI and the VIIRS instruments to far greater precision than has been achieved before, and to merge data from multiple platforms into fused data products. The utility of the combined VIIRS/ABI weather imagery can be improved still more if the ABI's reflective spectral bands are changed to match corresponding bands of the VIIRS.
A small integrating sphere with two pinhole apertures can be hard-mounted to the nadir-facing surface of a 3-axis stabilized GOES satellite in geostationary orbit. One pinhole can be baffled to produce a circular field-of-fiew (FOV) 18° in diameter, centered at nadir, allowing it to view the Earth's full disk continuously. A second, smaller pinhole can be baffled to produce a rectangular FOV that subtends 1° in the East/West direction and +/- 25° in the North/South direction, centered 22.5° west of nadir. The solar irradiance transmitted through the smaller pinhole will be added to the Earth's irradiance for a brief interval at 2230 hrs, local time, once each night. A detector in the integrating sphere can measure the ratio of the full-disk irradiance to the direct solar irradiance in any desired solar-reflective spectral band, independent of the detector's gain and the sphere's reflectivity. These stable, long-term measurements of the daily and seasonal albedo variations are valuable for climatic studies. This full-disk ratioing radiometer (FDRR) can be placed on a GOES-R satellite and equipped with a six spectral channels matched to the six solar-reflective channels of the Advanced Baseline Imager (ABI). Each ABI channel can then be calibrated by comparing the full-disk albedo derived from every one of its full disk images to that measured simultaneously by the FDRR. The FDRR is small and light, has no moving parts, requires minimal electrical power, has a low data rate, and calibrates the ABI continuously without interrupting its Earth observations or blocking its aperture.
Two of the GOES instruments, the Imager and the Sounder, perform scans of the Earth to provide a full disc picture of the Earth. To verify the entire scan process, an image of a target that covers an 18o circular field-of-view is collimated and projected into the field of regard of each instrument. The Wide Field Collimator 2 (WFC2) 1 has many advantages over its predecessor, WFC1, including lower thermal dissipation, higher far field MTF, smaller package, and a more intuitive (faster) focusing process. The illumination source is an LED array that emits in a narrow spectral band centered at 689 nm, within the visible spectral bands of the Imager and Sounder. The illumination level can be continuously adjusted electronically. Lower thermal dissipation eliminates the need for forced convection cooling and minimizes time to reach thermal stability. The lens system has been optimized for the illumination source spectral output and athermalized to remain in focus during bulk temperature changes within the laboratory environment. The MTF of the lens is higher than that of the WFC1 at the edge of FOV. The target is focused in three orthogonal motions, controlled by an ergonomic system that saves substantial time and produces a sharper focus.
Due to optical misalignment, visible and infrared channels of the Geostationary Operational Environmental Satellite (GOES) I-M Imager may not be properly registered. This “co-registration” error is currently estimated by comparing groups of visible and infrared observation residuals from the GOES Orbit and Attitude Tracking System (OATS). To make the channel-to-channel comparison more direct, it was proposed to compare individual observations rather than groups of observations. This has already been done for landmarks but not for stars. Stars would help determine nighttime co-registration when visible landmarks are not available. Although most stars in the GOES catalog are not detectable in the shortwave infrared channel, many are. Because stars drift west-to-east across the detectors and because of their high observation frequency, stars provide good east-west co-registration information. Due to the large detector fields-of-view, stars do not provide much information about north-south co-registration.
Observations of coastal waters require high spectral and radiometric resolution, as compared to land, and high spatial resolution, as compared to the open ocean. An imaging instrument in geostationary orbit with a nominal aperture diameter of one meter in the spectral region from 400 - 1000 nm, ould meet these requirements on demand, over a large area of the Earth's surface. Observations made during daylight hours using filter wheel technology and large 2-D silicon focal plane arrays can achieve these objectives at reasonable coverage rates. Polarization-sensitive measurements would allow this instrument to optimize its observations of water-leaving radiance and to better compensate for atmospheric background. This instrument can be fabricated with existing technology.
The GOES Imager and Sounder instruments each observe the full Earth disk, 17.4° in diameter, from geostationary orbit. Pre-launch, each instrument's dynamic scanning performance is tested using the projection of a test pattern from a wide-field collimator. We are fabricating a second wide-field collimator (WFC2) to augment this test program. The WFC2 has several significant advantages over the existing WFC1. The WFC2 target illumination system uses an array of light-emitting diodes (LEDs) radiating at 680nm, which is within the visible bands of both the Imager and Sounder. The light from the LEDs is projected through a non-Lambertian diffuser plate and the target plate to the pupil of the projection lens. The WFC2's power dissipation is much lower than that of WFC1, decreasing stabilization time and eliminating the need for cooling fans. The WFC2's custom-designed 5-element projection lens has the same effective focal length (EFL) as the WFC1 projection lens. The WFC2 lens is optimized for the LED's narrow spectral band simplifying the design and improving image quality. The target plate is mounted in a frame with a mechanized micro-positioner system that controls three degrees of freedom: tip, tilt, and focus. The tip and tilt axes intersect in the WFC's image plane, and all adjustments are controlled remotely by the operator observing the target plate through an auto-collimating telescope.
KEYWORDS: Black bodies, Mirrors, Calibration, Space telescopes, Telescopes, Sensors, Imaging systems, Temperature metrology, Space mirrors, Aerospace engineering
The Imagers and the Sounders on the present generation of GOES spacecraft each have a full-aperture blackbody for on-board infrared (IR) calibration. Each instrument interrupts its Earth-viewing operations at regular intervals and views this blackbody by rotating its scan mirror. IR Radiation from this blackbody follows the same path through the entire optical train as radiation from the Earth. The instrument also observes space to measure background generated by the instrument. The difference between signals from the blackbody and from space is used in conjunction with the measured temperature of the blackbody to compute the gain for each IR channel of the instrument. The GIFTS is an experimental, next-generation IR sounder with a Michelson interferometer. It performs on-board gain calibration by sequentially viewing two internal blackbodies that operate at different temperatures, measuring the difference between the resultant signals. A flip mirror is used to insert the blackbodies into the optical beam after the beam has been converged by the telescope. The apertures of the GIFTS blackbodies are much smaller than that of the telescope. This approach allows the blackbodies to have deep cavities and minimizes the time lag and the momentum disturbance in the calibration process. On the other hand, GIFTS is unable to directly measure long-term changes in the throughput of the optical elements that are not included in the calibration path: the scan mirror and two telescope mirrors. The GIFTS approach is capable of high radiometric precision, but requires augmentation by occasional end-to-end IR calibration measurements to achieve the required absolute accuracy over the lifetime of an operational GOES mission.
Attenuated sunlight is a valuable reference for on-board calibration of spacebome instruments that observe reflected sunlight from the earth. Direct viewing of the sun through a perforated plate can provide full aperture, end-to-end calibration. Since the transmissivity of the perforated plate depends only upon its geometry, it is potentially more stable than the diffuse reflectivity of a diffuser plate, particularly when exposed to the space environment. We have observed the sun through a sheet metal plate with a hexagonal array of small holes placed in front of a telescope. A pinhole in the telescope’s focal plane, followed by a spectral filter and a silicon photo-diode, were selected to approximate the IFOV and spectral bands proposed for imagers on future GOES missions. In each observation, the center of the solar image was found to have a smooth, symmetrical maximum, with no significant angular structure due to interference. These observations demonstrated that the perforated plate technique is a promising method for stable, long-term, on-orbit calibration of visible and near IR channels on spacebome optical instruments.
The AGSI instrument contains four focal plane arrays (FPAs) including the long wave IR (LWIR), medium wave IR (MWIR), short wave IR (SWIR), and the visible (VIS) FPA. The LWIR and MWIR FPAs are housed in a dewar. The SWIR FPA is housed in a separate dewar. The visible FPA will be located outside of the dewar. The AGSI instrument thermal subsystem is required to maintain the LWIR and MWIR FPAs at 65K, the SWIR FPA at 210 K, and the visible FPA between 210 K and 300 K. The stability of all four FPAs must be +/- 0.1 degrees C. The remainder of the optical system, telescope structure and instrument electronics must be maintained between 0 degrees C and 40 degrees C. Telescope alignment must also be maintained through the cooldown of the instrument optical bench, and during the diurnal and seasonal variations in orbital environment. The thermal design will accommodate such directly in the field-of-view for short periods of time without damage to the instrument, and with the ability to recover science operation quickly. This paper will discuss the thermal design needed to achieve these goals as well as detail the cryocooler and solar intrusion issues.
The Advanced Geosynchronous Studies Imager (AGSI) system design combines the latest available technologies into an instrument design concept which could deliver the improved performance sought by the National Weather Service at NOAA and meet NASA earth system science goals in a joint program. The instrument could cover the Earth disk every 15 minutes with subsatellite point resolution form 1/2 kilometer in the visible to 2 kilometers in the long wave IR. Simultaneously, it could provide coverage of a 3000 by 5000 kilometer region in 5 minute intervals and 30 second updates of a 1000 kilometer square region containing a weather system of interest. We found that performance margins could be improved even as we drove the design interactions with emphasis on reducing the mass. Scan speed was chosen by maximizing performance while trading off the acceptable impact on the total systems. The resulting 18-channel design could deliver vastly improved performance over the present GOES without great increases in mass or volume, while still paying close attention to control of development cost sand impact on the host spacecraft. The design could be adapted to changed requirements or descoped to have lower data rates and fewer channels.
The AGSI is a visible and IR instrument being proposed to satisfy both NOAA's operational weather and NASA's geostationary science requirements. It scans the full earth disk with a plane scan mirror in object space, mounted on a two-axis gimbal system. Image rotation is an intrinsic problem: scanning about one gimbal axis rotates the projection of the focal plane array (FPA) onto the earth's surface. The AGSI's needs for both higher angular resolution and higher radiometric resolution are satisfied by time delay and integration (TDI) in several FPA's. The electronic and opto-mechanical scan vectors must match to maintain image quality: the projection of the TDI axis of each FPA onto the earth's surface must always coincide with the scan direction and the scan rates must be equal. A new gimbal geometry, focal plane layout, and associated scanning techniques have been developed to scan the earth's surface in a series of conical arcs that satisfy these conditions. This technique has the additional advantages that the outer gimbal axis remains stationary during the data-taking portion of the scan pattern and that the magnitude of the angle of reflection remains relatively constant during a single scanning arc.
The AGSI design permits scan rates slow enough to detect stars as dim as visual magnitude eight in the coarse of normal imaging. This gives many times the number of stars seen with the current Geosynchronous Operational Environmental Satellite (GOES) Imager and can eliminate the need to schedule special star looks. Besides improving image navigation and registration accuracy, the frequency observations enable the Imager to fly aboard a spacecraft with loose attitude control. The slow scan rate is thanks to the long CCD detector arrays and to the time delay integration made possible by the unique windshield wiper scan pattern. The Bremer star detection algorithm describe can be implemented onboard to reduce downlink requirements and so permit star detection across a dedicated full silicon passband. The wide passband increases the number of detectable stars, and cross checking with narrower science passbands eliminates false alarms from high energy particles while preserving low detection thresholds and sensitivity.
Each Imager and Sounder on the present generation of GOES satellites has an eight-channel visible detector array. These visible arrays perform star-sensing measurements to establish inertial attitude references so that earth observations can be registered to the earth's latitude and longitude coordinates. The Imager's visible array also performs earth observations. We have used the archived signal levels of star measurements to estimate the long-term throughput loss in these channels of the Imager and Sounder on GOES 8 and GOES 9. An exponential decay rate was determined for each sensor by averaging the values derived from each of approximately 30 stars over a time interval of at least 500 days. Large degradations in image quality occur during local night, when direct sunlight enters the optical ports of the two sensor. Therefore, we have deleted observations made during the 10 hour interval around midnight, satellite time, from our analysis. Variable stars and stars with low signal-to-noise ratios were also excluded. The annual throughput losses for the four sensors, derived from measured star signal levels, range from 3.8 percent to 9.6 percent.
The far ultraviolet spectroscopic explorer (FUSE) satellite will make high spectral resolution ((lambda) /(Delta) (lambda) equal 30,000) measurements in the 905 - 1195 angstrom bandpass from low-earth orbit. The optical system of the instrument consists of four coaligned telescopes and gratings, which disperse their spectra onto two detectors; both the mirrors and slit assemblies will be adjustable in flight. Because of this complicated, coupled optical system, it is important to understand all of the effects which may affect performance. The ability of the FUSE instrument to maintain its high resolving power and effective area is dependent on many factors, including the optical design, manufacturing errors, the ability to coalign the system on orbit, and the stability of the structure holding the optical elements. In order to predict the on-orbit performance, a detailed optical performance budget has been developed. This budget includes all effects which affect the resolution and throughput. Included are short term effects (such as the stability of the metering structure due to thermal variations during a single orbit); long term effects (such as moisture desorption from the graphite/cyanate ester structure and gravity release); and installation tolerances. We present the results of this exercise, and describe the dependence of the instrument performance on the expected errors.
Future GOES missions, augmented by the addition of very stable visible calibration, could measure long-term changes in the earth's albedo. Direct sunlight can be viewed through a plate perforated by small holes, attenuating the radiance at the center of the solar image by both blockage and diffraction. Attenuation by a factor of 50,000 produces a full-aperture, end-to-end calibration source with radiance comparable to a high-albedo target. Since its radiance depends only upon geometry, the perforated plate technique avoids the materials degradation problems inherent to calibration techniques based on reflection or refraction. Our analysis indicates that a plate with 50 micrometers holes spaced 4.9 mm apart in a hexagonal array has the potential to satisfy the requirement for a source stable to < 1 percent over a 5-year GOES mission. Some randomization of the hole positions may be desirable to suppress interference effects. We have made preliminary measurements of the large- scale angular structure of a simulated solar image viewed through a perforated screen, and have demonstrated their agreement with out theoretical predictions.
KEYWORDS: Imaging systems, Calibration, Black bodies, Monte Carlo methods, Interference (communication), Sensors, Signal to noise ratio, Computer simulations, Analog electronics, Signal detection
The raw output of many scanning radiometers is a small, rapidly varying signal superimposed on a large background that varies more slowly, due to thermal drifts and 1/f noise. To isolate the signal, it is necessary to perform a differential measurement: measure a known reference and subtract it from each of the raw outputs, cancelling the common-mode background. Calibration is also a differential measurement: the difference between two outputs is divided by the difference between the two known references that produced them to determine the gain. The GOES-I Imager views space as its background subtraction reference and a full-aperture blackbody as its second reference for calibration. The background suppression efficiency of a differential measurement algorithm depends on its timing. The Imager measures space references before and after each scan line and performs interpolated background subtraction: a unique, linearly weighted average of the two references is subtracted from each scene sample in that line, cancelling both constant bias and linear drift. Our model quantifies the Gaussian noise and 1/f noise terms in the noise equivalent bandwidth, which is minimized to optimize the algorithm. We have obtained excellent agreement between our analytical predictions and Monte Carlo computer simulations.
Standard diffractive estimates based on the system f/# were at odds with geometric predictions concerning beam footprint size at a relayed pupil image plane due to an object point at infinity. The reason for this was that the relay optic was well within the defractive depth of focus of an intermediate star image. This invalidated geometric predictions for the system f/# and the location and size of the system impulse response. Accurate predictions emerged when Fourier transforms of the highly apodized planar wavefront incident on the relay optic were calculated.
KEYWORDS: Black bodies, Calibration, Imaging systems, Sensors, Signal to noise ratio, Interference (communication), Computer simulations, Analog electronics, Monte Carlo methods, Radiometry
Many scanning sensors produce raw outputs in which their response to the signal is superimposed on a large background. The signal changes rapidly, due to the scanned input, while the background varies more slowly, due to thermal drifts and 1/f noise. Whenever such a sensor is used as a radiometer, it is necessary to perform a differential measurement: to measure a known reference and subtract it from the raw signals, cancelling the common-mode background. Calibration is also a differential measurement: the difference between each channel's response to two known inputs is divided by the difference between these two inputs to determine the linear gain of the channel. The GOES-I Imager obtains its background subtraction references by viewing space, with radiance virtually equal to zero, during the turn- around intervals at the ends of scan lines. It used a temperature-monitored blackbody as a second reference to measure the gain. We have verified our analytical predictions by computer simulations. Gaussian and 1/f noise were generated and combined, filtered, and processed using the differential measurement algorithms. Excellent agreement was demonstrated between these simulations and the analytical model.
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