Contemporary and emerging sensor systems typically require in-flight calibration reference sources. These are required to satisfy increasingly stringent specifications for stability, repeatability, dynamic range, absolute irradiance accuracy, and irradiance distribution uniformity, while meeting stray light, weight, and power constraints. While SSG has successfully designed and flight-qualified an internal calibration source for a telescope in a Schmidt configuration, future remote sensing programs are more likely to require telescopes in a 3-mirror off-axis re-imaging configuration. A major advantage to developing an internal calibration reference source for a re-imaging telescope is the availability of an intermediate field stop where the illumination from the calibration source can be inserted into the optical train. SSG's internal source design offers important advantages over existing approaches using in-flight blackbodies, including reduced volume, weight, and power requirements and the ability to generate multiple irradiance levels over a short period of time. The GIFTS (Geosynchronous Imaging Fourier Transform Spectrometer) telescope has been used as a representative platform to demonstrate this new internal calibration source, as it is representative of a design that may be used for future programs including the HES (Hyperspectral Environmental Suite) telescopes.
An Internal Calibration Unit (ICU) has been designed, built, and tested to meet a stringent set of quantitative performance requirements for an infrared sensor in a Schmidt telescope configuration. The design concept is readily applicable to re-imaging telescopes. The design optimizes optics, source design, and power supply electronics for irradiance accuracy, repeatability, and uniformity, as well as power and weight.
The Engineering Model (EM) and Flight Model (FM) Focal Plane Assembly (FPA)/dewar assemblies have been fabricated, tested, and delivered for system integration, and the EM instrument has been assembled and tested. The key design features of the FPA and dewar have been presented in previous SPIE symposia and will only be briefly reviewed. The primary emphasis in this paper is on the performance results such as sensitivity, linearity and spectral response as well as environmental test results and a review of the assembly of the flight hardware.
The HIRDLS instrument is an infrared limb-scanning radiometer designed to sound the upper troposphere, stratosphere, and the mesosphere as part of the Chemistry Platform for NASA's Earth Observing System (EOS) Program. The instrument performs limb scans at multiple azimuth angles, measuring CO2, O3, H2O, aerosols and other significant greenhouse trace gases in 21 channels ranging from 6.12 to 17.76 micrometer. The Detector Subsystem (DSS) focal plane assembly (FPA) contains the 21 detectors for the science measurements and a set of alignment detectors to be used for instrument integration. All detector elements are Photoconductive HgCdTe operating in the 60 - 65 K range and each channel has a separate cold filter. The FPA is mounted in a customized vacuum dewar which couples to a Stirling-cycle mechanical cryocooler via a sapphire rod. Lockheed Martin has designed, fabricated and tested detectors covering the entire HIRDLS spectral range. All the n-type HgCdTe starting material was grown at Lockheed Martin. The vacuum dewar and the preamplifier designs were done at Lockheed Martin. In this paper, we will discuss the key features of and design drivers for the DSS design and the design validation activities. The details of the DSS to instrument interfaces will be discussed. We will consider the solutions found for design and packaging issues with the DSS, and the design trades made at the subsystem level to optimize the instrument performance and increase the ease of assembly and instrument integration.
There has been significant progress during the past several years in photovoltaic (PV) HgCdTe technology for advanced long wavelength remote sensing applications. Useful cutoff wavelengths have been extended to beyond 17.0 micrometer. Junction quality has been improved to the point that D* greater than 3 multiplied by 1011 cm-(root)Hz/W can be achieved in arrays at temperatures of 60 - 65 K. The atmospheric infrared sounder (AIRS) instrument, scheduled for launch in the year 2000 as part of the NASA EOS Program, uses over 4000 PV HgCdTe detector elements organized into ten linear multiplexed arrays, with cutoff wavelengths extending as far as 15.0 micrometer at 60 K. The AIRS instrument also uses two long linear arrays of photoconductive (PC) HgCdTe detectors for the 13.7 - 15.4 micrometer band. These PC detector arrays have cutoff wavelengths of 16.0 - 17.0 micrometer and achieve D* values of 3 - 5 multiplied by 1011 cm-(root)Hz/W at 60 K. PV HgCdTe offers many advantages over PC HgCdTe for advanced remote sensing instruments: negligible 1/f noise, much higher impedance so that cold preamps or multiplexers are possible, configurational versatility with backside- illuminated two-dimensional arrays of closely spaced elements, 10X - 100X better linearity, dc coupling for measuring the total incident photon flux, and a (root)2 higher BLIP D* limit. In this paper we compare the relative merits of PV and PC HgCdTe for advanced remote sensing instruments, and we review recent data for both PV and PC HgCdTe arrays with cutoff wavelengths as long as 17.5 micrometer.
The GOES Imager and Sounder instruments each utilize several HgCdTe photoconductive (PC) detectors and detector arrays for detection over the 6.5 to 14.7 micrometers region. These high performance detectors are integrated with germanium aplanat lenses and mounted in miniature hermetically sealed housings. There are demanding requirements on the radiometric performance of these detector assemblies. For LW Sounder detectors, the highest possible sensitivity achievable by a practical HgCdTe photoconductor at the operating temperatures of 100 to 105 K was required. Lockheed Martin designed, fabricated, tested, packaged, qualified, and delivered 7 of the 11 HgCdTe PC detector assemblies for GOES-8, and 9 of the 11 assemblies for GOES- 9. All the n-type HgCdTe starting material was grown at Lockheed Martin.
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