David R. Hearn

Design of an Optical Photon Counting Array Receiver System for Deep-Space Communications

Demand for increased capacity in deep-space to Earth communications systems continues to rise as sensor data rates climb and mission requirements expand. Optical free-space laser communications systems offer the potential for operating at data rates 10 to 1000 times that of current radio-frequency systems. A key element in an optical communications system is the Earth receiver. This paper reviews the design of a distributed photon-counting receiver array composed of four meter-class telescopes, developed as a part of the mars laser communications demonstration (MLCD) project. This design offers a cost-effective and adaptable alternative approach to traditional large, single-aperture receive elements while preserving the expected improvement in data rates enabled by free-space laser communications systems. Key challenges in developing distributed receivers and details of the MLCD design are discussed.

Design and Performance of the EO-1 Advanced Land Imager

An Advanced Land Imager (ALI) will be flown on the first Earth Observing mission (EO-1) under NASAs New Millennium Program (NMP). The ALI contains a number of key NMP technologies. These include a 15 degree wide field-of-view, push-broom instrument architecture with a 12.5 cm aperture diameter, compact multispectral detector arrays, non-cryogenic HgCdTe for the short wave infrared bands, silicon carbide optics, and a multi-level solar calibration technique. The focal plane contains multispectral and panchromatic (MS/Pan) detector arrays with a total of 10 spectral bands spanning the 0.4 to 2.5 micrometer wavelength region. Seven of these correspond to the heritage Landsat bands. The instantaneous fields of view of the detectors are 14.2 (mu) rad for the Pan band and 42.6 (mu) rad for the MS bands. The partially populated focal plane provides a 3 degree cross-track coverage corresponding to 37 km on the ground. The focal plane temperature is maintained at 220 K by means of a passive radiator. The instrument environmental and performance testing has been completed. Preliminary data analysis indicates excellent performance. This paper presents an overview of the instrument design, the calibration strategy, and results of the pre-flight performance measurements. It also discusses the potential impact of ALI technologies to future Landsat-like instruments.

EO-1 Advanced Land Imager overview and spatial performance

The Advanced Land Imager (ALI) is the primary instrument flown on the first Earth Observing mission (EO-1), which was developed under NASAs New Millennium Program (NMP). The ALI contains a number of innovative features. These include the basic instrument architecture which employs a push-broom data collection mode, a wide field of view optical design, compact multi-spectral detector arrays, non-cryogenic HgCdTe for the short wave infrared bands, silicon carbide optics, and a multi-level solar calibration technique. The sensor includes detector arrays that operate in ten bands, one panchromatic, six VNIR and three SWIR, spanning the range from 0.433 to 2.35 /spl mu/m. This paper describes the instrument design, provides an overview of the ground testing and calibration of the instrument and a summary of the sensor performance in space. In particular, the spatial imaging performance of ALI is discussed. Sample images are shown that demonstrate the improved capability of the sensor in terms of Pan band resolution and signal-to-noise ratio in all bands. On-orbit images have been analyzed and the results are compared with pre-launch calibrations. The instrument performance appears to meet all expectations.

Spectral calibration of the EO-1 Advanced Land Imager

Spatial calibrations have been performed on the Advanced Land Imager (ALI) of the EO-1 satellite. Topics discussed in this paper include end-to-end imaging test, measurements of system modulation transfer function (MTF), and pixel lines of sight. The MTF measurements were made by recording scans of a knife-edge past the pixels. The techniques used to place the focal plane at the correct focal position are described, since they make use of MTF measurements. Line-of- sight measurements combine theodolite measurements of the telescope distortions and the photolithographic patterns of the detector arrays with images of a stationary Ronchi ruling recorded with the instrument at its normal operating conditions in a thermal vacuum chamber.

Vacuum window optical power induced by temperature gradients

The Advanced Land Imager (ALI) of the EO-1 satellite has been calibrated under its normal operating conditions in a thermal vacuum chamber. The optical equipment for the calibrations had to be placed outside the chamber. The effects on the calibrating beam of the vacuum chamber window are described here. Since the instrument senses the reflected solar spectrum, the existing fused-silica window was found to have adequate spectral transmission properties. It also was measured to cause little wavefront error by itself. When the chamber is under vacuum, and the interior cold shroud is cooled to approximately 100 K, the window develops significant optical power. This is a result of radiational cooling of the window, coupled with change of its index of refraction with temperature. Such an effect seriously compromises the spatial calibrations of the instrument, particularly MTF measurements. This effect was overcome in two ways: A heating plate placed outside the window was used to alter the temperature distribution until its effect on the wavefront was negligible. The more practical solution was to measure the window power with an interferometer, and compensate for it by shifting the target reticle of the collimator.

EO-1 Advanced Land Imager in-flight calibration

The EO-1 Advanced Land Imager (ALI) is the first earth- orbiting instrument to be flown under NASAs New Millennium program. The ALI employs novel wide-angle optics and a multispectral and panchromatic spectrometer. EO-1 is a technology verification project designed to demonstrate comparable or improved Landsat spatial and spectral resolution with substantial mass, volume, and cost savings. This paper provides as overview of in-flight calibration and performance assessment of the Advanced Land Imager. Include dare techniques for calibrating and assessing focus and MTF using long, straight man-made objects and monitoring of radiometric linearity and offsets using an internal calibration source, standard Earth references scenes, and solar and lunar observations.

Initial flight test results from the EO-1 Advanced Land Imager: radiometric performance

The Advanced Land Imager (ALI) is one of three instruments flown on the first Earth Observing mission (EO-1) under NASAs New Millennium Program (NMP). The primary NMP mission objective is to flight-validate advanced technologies that will enable dramatic improvements in performance, cost, mass and schedule for future, Landsat-like, Earth remote sensing instruments. ALI contains a number of innovative features, including all the Category 1 technology demonstrations of the EO-1 mission. These include the basic instrument architecture which employs a push-broom data collection mode, a wide field of view optical design, compact multispectral detector arrays, non-cryogenic HgCdTe for the short wave infrared bands, silicon carbide optics and a multi-level solar calibration technique. The Earth Observing-1 spacecraft was successfully launched on November 21, 2000. During the first sixty days on orbit, several Earth scenes were collected and on-orbit calibration techniques were exercised by the Advanced Land Imager. This paper presents the status of ALI radiometric performance characterization obtained from the data collected during that period.

EO-1 Advanced Land Imager calibration and performance overview

The pre-launch measurements and test required for calibration and characterization of the advanced land imager (ALI), which will be flown on NASAs EO-1 mission, have been completed. The instrument level performance testing was conducted at MIT Lincoln Laboratory with the ALI in an operational environment. The overall calibration strategy, which includes both pre-launch and post-launch components, will be described in this paper. The fundamental sensor calibration data comprise five measurement categories: angular position in object space for each pixel; normalized spectral response functions; response coefficients; zero signal offsets; and modulation transfer functions. Performance and characterization test include measurements of noise, SNR, linearity, repeatability, image artifacts, stray light rejection, and cross-talk. An overview of the facilities, equipment, tests and results is presented here.

Characterization of instrument spectral resolution by the spectral modulation transfer function

Comparison of the spectral resolving power of different types of spectrometer can be facilitated by the use of the spectral modulation transfer function (SMTF). It is the Fourier transform of the instrument spectral response function. The SMTFs of the two imaging spectrometers included in the Advanced Land Imager of the EO-1 satellite are shown as examples. The SMTF offers a way to specify the spectral resolution required for the detection of known spectral features. It can also be used to determine the ultimate resolution that can be achieved through processing signals form any given instrument and observation.

Summary of the EO-1 ALI performance during the first 2.5 years on-orbit

The Advanced Land Imager (ALI) is a VNIR/SWIR, pushbroom instrument that is flying aboard the Earth Observing-1 (EO-1) spacecraft. Launched on November 21, 2000, the objective of the ALI is to flight validate emerging technologies that can be infused into future land imaging sensors. During the first two and one-half years on-orbit, the performance of the ALI has been evaluated using on-board calibrators and vicarious observations. The results of this evaluation are presented here. The spatial performance of the instrument, derived using stellar, lunar, and bridge observations, is summarized. The radiometric stability of the focal plane and telescope, established using solar, lunar, ground truth, and on-board sources, is also provided.