James A. Stobie

Update on the imaging sensor for GIFTS

Remote temperature sounding from the vantage point of Earth Orbit improves our weather forecasting, monitoring and analysis capability. Recent advances in the infrared hyperspectral sensor technology promise to improve the spatial and temperature resolution, while offering relatively quick re-look times to witness atmospheric dynamics. One approach takes advantage of a two-dimensional, imaging Fourier transform spectrometer to obtain a data cube with the field of view along one plane and multiple IR spectra (one for every FPA pixel) along the orthogonal axis. Only the pixel pitch in the imaging focal plane and the optics used to collect the data limit the spatial resolution. The maximum optical path difference in the Michelson FTS defines the spectral resolution and dictates the number of path-length interferogram samples (FPA frames required per cube). This paper discusses the unique challenges placed on the focal plane by the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) approach and how advanced focal plane technology is applied to satisfy these challenges. The instrument requires a midwave spectral band from 4.4 to 6.1m to capture the C02 and H20 absorption bands, and an optional VLWIR spectral band to cover from 8.85-14.6m. The paper presents performance data of Liquid Phase Epitaxy (LPE) fabricated HgCdTe detectors and design details of the advanced readout integrated circuit necessary to meet the demanding requirements of the imaging sensor for the GIFTS instrument. Point defects are removed by using a unique super-pixel approach to improve operability for the VLWIR focal plane. Finally, early focal plane performance measurements are reported, including Noise Equivalent Input, responsivity uniformity, output offset stability and 1/f noise knee.

A multispectral hybrid HgCdTe FPA/dewar assembly for remote sensing in the Atmospheric Infrared Sounder (AIRS) instrument

AIRS is a key instrument in NASAs Earth Observing System (EOS) Program. Passive IR remote sensing is performed using a high resolution grating spectrometer design with a wide spectral coverage focal plane assembly (FPA). The hybrid HgCdTe focal plane consists of twelve modules, ten photovoltaic (PV) and two photoconductive (PC), providing spectral response from 3.7 to 15.4 micrometers. The PV modules use silicon readout integrated circuits (ROICs) joined to the detector arrays as either direct or indirect hybrids. The PC modules are optically chopped and led out to warm electronics. Operating at 58 K, the sensitivity requirements approach BLIP in the critical 4.2 and 15.0 micrometer bands. The optical footprint coupled with the support and interface components of the focal plane make it a very large assembly, 53 mm multiplied by 66 mm. Dispersed energy from the grating is presented to the modules through 17 narrowband filters mounted 0.2 mm above the focal plane in a single, removable precision assembly. With PV and PC devices on the same focal plane operating simultaneously, shielding and lead routing as well as ROIC design have been optimized to minimize any interactions between them. Multilayer carriers have been designed to lead out the closely spaced PC arrays and the entire focal plane itself. Multilayer shielded flex cables are used to interconnect the focal plane to a very unique dewar. The tightly spaced optical pattern, along with more than 50 components in the focal plane, make this a highly complex assembly. The vacuum dewar, while providing approximately 600 leadouts, is directly coupled to the cold spectrometer and operates at 155 K while cooling the focal plane to 58 K via a sapphire rod interfaced to a pulse tube cooler. This paper discusses the key features of the FPA/dewar assembly, modeling/analyses done in support of the design, and results of design validation activities to date.

VLIWR HgCdTe staring focal plane array development

Atmospheric remote-sensing have been one of the primary drivers toward longer wavelength infrared sensors beyond the 8 to 12 um atmospheric window typically used for terrestrial imaging systems. This paper presents the recent performance improvement attained with very long wavelength infrared (VLWIR) focal plane arrays, by the stringent control of the small bandgap HgCdTe material quality. Array operability is further enhanced by design using a 2:1 super-pixel detector format scheme with programmable bad element de-select and our new detector input offset optimization circuitry within each unit cell. Focal plane arrays with peak quantum efficiencies in excess of 80 percent, and cutoff wavelengths out to 15 μm have NEI operabilities around 95 percent for mid 1014 ph/s-cm2 fluxes operating near 50 K. Average NEI of 3.5 x 1010 ph/s-cm2 was demonstrated for a 14 μm cutoff wavelength focal plane array, consisting of over 55,000 elements, operating with an effective sample time of 87.5 ms.

Advanced readout integrated circuit signal processing

Readout integrated circuits (ROICs) for focal plane arrays (FPAs) have become increasingly complex to meet the needs of modern infrared systems. BAE Systems has pioneered a number of advanced signal processing architectures for FPA ROICs. Demonstrated signal processing capabilities of BAE Systems FPAs include analog-to-digital conversion, offset subtraction, individual pixel automatic gain compensation, transient noise suppression, on-FPA defect deselection, reconfigurable pixels, spatial neural network processing and subframe noise averaging. BAE Systems FPA advanced signal processing is not just for demonstrations, but is used in many of their deliverable FPAs, improving real system performance.

Performance of the atmospheric infrared sounder (AIRS) in the radiation environment of low-earth orbit

The Atmospheric Infrared Sounder (AIRS), a hyperspectral infrared sounder, was launched onboard NASAs Aqua spacecraft on May 4, 2002 into sun-synchronous polar Earth orbit for a mission expected to last 7 years. By monitoring calibration data from views of deep space and two on-board calibrators, we have identified a number of effects attributed to in-orbit radiation. Transient effects include 1. steps in the output level of individual channels, attributed to injection of charge into a large capacitor in the read-out electronics integrated circuit (ROIC); and 2. spikes in the calibration data and, by inference, in the scene data, attributed to the passage of ionizing radiation through the active region of the HgCdTe detectors. On-board signal processing corrects for most of the spike effects, and ground processing smoothes the hot and cold calibration data and provides a system of flags to alert the user in cases where the calculated radiances are still suspect. Persistent effects include 1. extremely rare degradations of channels due to large charge injection events; and 2. slow increases in noise levels for a small number of channels, attributed to bias shifts due to the slow accumulation of radiation dose in the ROIC input cells for some channels. In addition to these detector effects, two operational anomalies have been attributed to the high radiation levels in the South Atlantic Anomaly (SAA), one an unplanned cooler shut-down, the second an unplanned stopping of the scan mirror. This paper presents statistics on the frequency and location of these radiation events, and provides a description of the mechanisms by which such events are identified and accounted for. It should be emphasized that the vast majority of the 2378 AIRS infrared channels, and the instrument as a whole, have shown excellent stability and operability throughout the mission.

Imaging sensor for the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS)

Accurate high resolution temperature sounding through our atmosphere is paramount to improving our weather forecasting, monitoring, and analysis capability. From the vantagepoint of earth Orbit, remote temperature sounding is becoming a reality and its accuracy is bolstered by recent advances in infrared hyper-spectral sensor capability. One promising approach takes advantage of a two-dimensional, imaging Fourier transform spectrometer to obtain a data cube with the field of view along one plane and multiple IR spectra (one for every FPA pixel) along the orthogonal axis. The spatial resolution is limited only by the pixel pitch in the imaging focal plane and the optics used to collect the data. The maximum optical path difference in the Michelson FTS defines the spectral resolution and dictates the number of path-length interferogram samples (FPA frames required per cube. This paper discusses the unique challenges placed on the focal plane by the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) approach and how advanced focal plane technology is applied to satisfy these challenges. Two focal planes are required to provide spectral coverage from 4.4 to 6.1um and 8.85-14.6um. Currently, the GIFT’s LWIR focal plane is the longest wavelength two-dimensional PV HgCdTe array of this size (128 square on 60 um centers) planned for space deployment. The paper presents performance data of Liquid Phase Epitaxy (LPE) fabricated HgCdTe detectors and design details of the advanced readout integrated circuit necessary to meet the demanding requirements of the imaging sensor for the GIFTS instrument.

Low-power highly linear output buffer

A class AB CMOS output buffer has been designed for use on an IR focal plane array. Given the requirements for power dissipation and load capacitance a class A output, such as a source follower, would be unsuitable. The approach taken uses a class AB amplifier configured as a charge integrator. Thus it converts a charge packet in the focal plane multiplexer to a voltage which is then the output of the focal plane. With a quiescent current of 18 micro-a and a load capacitance of 100 pf, the amplifier has an open loop unity gain bandwidth of 900 khz. Integral nonlinearity is better than .03 percent over 5.5 volts when run with VDD-VSS = 6v.

Performance of the PV/PC HgCdTe focal plane/dewar assembly for the Atmospheric Infrared Sounder instrument (AIRS)

AIRS is a key facility instrument in the NASA Earth Observing System (EOS) program, a spaceborne, global observation system being implemented to obtain comprehensive long-term measurements of earth processes affecting global change. Designed to provide new and more accurate data about the atmosphere, land, and oceans for application in climate studies and weather prediction, AIRS performs passive IR remote sensing using a high resolution grating spectrometer with a wide spectral coverage focal plane assembly (FPA) operating at 58 K in a unique vacuum dewar package cooled to 155 K. The hybrid HgCdTe focal plane consists of 12 modules, 10 photovoltaic (PV) utilizing silicon readout integrated circuits (ROICs) in both direct and indirect hybrid configurations, and 2 photoconductive (PC) led out to warm electronics. This complex focal plane has a large optical footprint, 53 mm X 66 mm, and receives energy dispersed from the grating through a precision filter assembly containing 17 narrowband filters. Designed to prevent any interaction between the PV and PC devices, the FPA incorporates extensive shielding and lead routing in the multilayer carriers and flex cables, as well as features in the ROIC design. The 526 lines necessary to operate the FPA are led out of the vacuum dewar, which is cooled via the spectrometer. The focal plane is cooled to 58 K through a sapphire rod interfaced to a pulse tube cooler. The Engineering Model (EM) and Protoflight Model (PFM) detector/dewar assemblies have been fabricated, assembled, tested, and delivered for system integration, and the EM instrument has been assembled and tested. The key design features of the FPA and dewar assembly have been presented in previous SPIE symposiums and will be briefly reviewed. In this paper the emphasis will be on performance results such as sensitivity, linearity, assembly tolerances, environmental test results, and other parameters of interest, as well as a detailed review of the actual flight hardware assembly.

Focal plane performance for the AIRS instrument

Higher resolution and wider IR spectral coverage is needed to improved infrared sounding instruments. The Atmospheric Infrared Sounder (AIRS), chosen by NASA to fly on the Earth Observing System, addresses these needs with advanced PV HgCdTe detector arrays designed to cover the spectral range from 3.7 micrometers to 13.6 micrometers with an average resolution of (lambda) /(Delta) (lambda) equals 1200. High performance detectors and advanced readout integrated circuit electronics make it possible to meet mission requirements. For convenience, the AIRS focal plane has been partitioned into four MWIR modules spanning the spectral range from 3.7 micrometers to 8.22 micrometers , and six LWIR modules for wavelengths above 8.8 micrometers . This paper focuses on the AIRS readout device and recent developments in p-on-n heterojunction detector technology at Loral. The detector arrays, operating at 60 K, readily satisfies the requirements of the AIRS instrument. Detector arrays with 4.7 micrometers cutoff wavelength at 60 K and 20 mV reverse bias have RdAs typically greater than 1010 (Omega) (DOT) cm2, with dark signals less than 0.6 fA and detector capacitances less than 0.6 pf for a 50 micrometers by 10 micrometers detector. AR coated MW arrays exhibit quantum efficiencies of greater than 80 percent. Reverse breakdowns are more than -150 mV. Module data for 15.1 micrometers detectors with anti-reflection coating exhibit quantum efficiencies greater than 70 percent and dark currents less than 8 nanoamps at 20 mV reverse bias. Also, excellent module linearity meeting the AIRS stringent requirements is achieved. Of course, measurements of MW detectors require extremely high gain transimpedance amplifiers. The AIRS MWIR readout structures prove to be exceptional in their ability to characterize these high impedance detectors. The charge sensitive input amplifiers on these readout devices utilize an equivalent input integration capacitor of less than 10 fFd to achieve ultrahigh transimpedance gain, and reset noise is suppressed with on focal plane correlated double sampling. LWIR readouts use ultralow noise buffered direct injection preamplifiers. The readouts have a robust architectures with differential input and outputs to minimize EMI and built in redundancy for survivability. Description of the readout device is presented, as well as linearity measurements of both the readout and complete modules.