John G. Whitney

Remote sensing of atmospheric structure and composition by pressure modulator radiometry from space : the ISAMS experiment on UARS

The scientific objectives of the improved stratospheric and mesospheric sounder (ISAMS) experiment involve the measurement of global temperature and composition profiles from an instrument on the Upper Atmosphere Research Satellite (UARS). This paper describes the instrument concept, its design, and its performance as calculated and as measured in the laboratory. The data retrieval technique, operating modes, observing strategy, and the error budget are briefly discussed.

The High-Resolution Dynamics Limb Sounder (HIRDLS) experiment on AURA

Space-based experiments have contributed much to our knowledge of the stratosphere in recent years. These observations have been characterized by large horizontal or vertical scales, leaving a range of unobserved phenomena at smaller scales. This is especially true at the tropopause, the boundary between the troposphere and stratosphere, where rapid changes in the vertical in temperature and composition have been unobserved on a global basis. The HIRDLS instrument has been designed to address these issues. HIRDLS is a 21 channel limb scanning infrared radiometer designed to make global measurements at smaller vertical and horizontal scales than have been previously observed, from pole to pole, at altitudes of 8-80 km. This paper will present an overview of the HIRDLS science and instrument, as well as the data retrieval process. It will serve as an introduction to the series of subsequent papers dealing with the calibration and other aspects of the experiment.

Description of the High Resolution Dynamics Limb Sounder (HIRDLS) instrument

We describe the top level design of the High Resolution Dynamics Limb Sounder (HIRDLS) instrument including the optical and scanning subsystems which have been developed to meet 0.7 arcsec pointing and the 1% radiometric accuracy requirements. The HIRDLS instrument is an infrared limb- sounding radiometer designed to sound the upper troposphere, stratosphere, and mesosphere. The instrument performs high resolution limb scans at multiple azimuth angles, measuring infrared emissions in 21 spectral channels ranging from 6 to 18 microns. The instrument design includes an off-axis Gregorian telescope with high resolution optical shaft encoders, a silicon carbide scanning mirror, and a vibration isolation system incorporating accelerometers in a feed- forward scanning control system. The detector subsystem includes 21 HgCdTe detector elements cooled by a mechanical Stirling cycle cooler.

Spectral design and verification of HIRDLS filters and antireflection coatings using an integrated system performance approach

The HIRDLS instrument contains 21 spectral channels spanning a wavelength range from 6 to 18 micrometer. For each of these channels the spectral bandwidth and position are isolated by an interference bandpass filter at 301 K placed at an intermediate focal plane of the instrument. A second filter cooled to 65 K positioned at the same wavelength but designed with a wider bandwidth is placed directly in front of each cooled detector element to reduce stray radiation from internally reflected in-band signals, and to improve the out- of band blocking. This paper describes the process of determining the spectral requirements for the two bandpass filters and the antireflection coatings used on the lenses and dewar window of the instrument. This process uses a system throughput performance approach taking the instrument spectral specification as a target. It takes into account the spectral characteristics of the transmissive optical materials, the relative spectral response of the detectors, thermal emission from the instrument, and the predicted atmospheric signal to determine the radiance profile for each channel. Using this design approach an optimal design for the filters can be achieved, minimizing the number of layers to improve the in- and transmission and to aid manufacture. The use of this design method also permits the instrument spectral performance to be verified using the measured response from manufactured components. The spectral calculations for an example channel are discussed, together with the spreadsheet calculation method. All the contributions made by the spectrally active components to the resulting instrument channel throughput are identified and presented.

High-Resolution Dynamics Limb Sounder (HIRDLS) for the Earth Observing System

The HIRDLS instrument is being designed to obtain data to address critical questions related to the middle atmosphere and its role in global change. We briefly state the scientific objectives of the experiment, and then describe the requirements placed on the instrument. These include the ability to obtain measurements with 4 degree(s)latitudinal and longitudinal resolution, and 1 km vertical resolution, the ability to sound down into the upper troposphere when clouds are absent, and the ability to measure radiance profiles in order to infer temperature and the concentrations of a number of trace species of different chemical lifetimes, along with the gradients of the geopotential height fields, for 5 or more years. The HIRDLS instrument is a multichannel infrared limb scanner that significantly extends the measurement capabilities of earlier instruments such as LIMS and ISAMS. Advances include the use of a two-axis scanner to allow limb scans at multiple azimuths, narrow fields of view coupled with over-sampling, digital filtering and low noise to enhance vertical resolution, the use of larger numbers of channels to acquire data over a larger range of altitudes and the use of a gyroscope to determine motions of the optical bench. The ways in which this is done are described. The most demanding requirements are for radiometric accuracy and precision, and for precise pointing knowledge (in the presence of vibration). The results of trade-off studies are presented, and the current conceptual design is described.

HIRDLS proto-flight model radiometric calibration from pre-launch calibration data

The High Resolution Dynamics Limb Sounder (HIRDLS) flight instrument, which is currently in orbit on the NASA Aura Satellite, went through a pre-launch calibration at Oxford University during Autumn 2002. One of the calibration exercises was to characterize the radiometric signals of the HIRDLS proto-flight model (PFM). It was discovered during the data-analysis phase, that the radiometric data required special treatment. Because of the stringent radiometric requirements imposed on HIRDLS, these additional analyses were necessary. This manuscript will detail these specific analysis techniques that were used on the data and present results based on a full analysis of the data, including a complete accounting of the statistical error analysis.

Spectral Characterization of the HIRDLS Flight Instrument From Prelaunch Calibration Data

Results from an instrument-level spectral characterization of the 21-channel High Resolution Dynamics Limb Sounder (HIRDLS) flight instrument will be presented. These data were obtained during the prelaunch calibration of HIRDLS at the University of Oxford (fall 2002). A monochromator, equipped with a controllable diffraction grating, was used to produce monochromatic light for these tests. The monochromator was housed, along with HIRDLS, in a large vacuum chamber. The monochromator was also equipped with a polarizer, which allowed data to be acquired at known polarizations for each channel. A calibration detector, with a flat spectral response, was used to measure the output from the monochromator. This paper will document the analysis procedures used to obtain a measured instrument spectral response for each channel, along with the associated error analyses for these measurements.

HIRDLS field-of-view calibration techniques and results

The techniques used to calibrate the field of view of the High Resolution Dynamics Limb Sounder (HIRDLS) instrument and the results of the calibration are presented. HIRDLS will be flown on the NASA EOS Aura platform. Both in-field and out-of-field calibrations were performed. The calibration results are compared to the requirements and, in the case of out-of-field, mechanisms explaining the results are discussed.

HIRDLS monochromator calibration equipment

A specially designed and built monochromator was developed for the spectral calibration of the HIRDLS instrument. The High Resolution Dynamics Limb Sounder (HIRDLS) is a precision infra-red remote sensing instrument with very tight requirements on the knowledge of the response to received radiation. A high performance, vacuum compatible monochromator, was developed with a wavelength range from 4 to 20 microns to encompass that of the HIRDLS instrument. The monochromator is integrated into a collimating system which is shared with a set of tiny broad band sources used for independent spatial response measurements (reported elsewhere). This paper describes the design and implementation of the monochromator and the performance obtained during the period of calibration of the HIRDLS instrument at Oxford University in 2002.

Prelaunch calibration of the NASA AURA HIRDLS instrument

The High Resolution Dynamics Limb Sounder (HIRDLS) instrument is scheduled for launch on the NASA AURA satellite in January 2004; it is a joint project between the UK and USA. HIRDLS is a mid-infrared limb emission sounder which will measure the concentration of trace species and aerosol, and temperature and pressure variations in the Earths atmosphere between about 8 and 100 km altitude on a finer spatial scale than has been achieved before. This will depend upon both a high quality of instrument build, and very precise pre-launch calibration. Proto Flight Model calibration was performed in a purpose-built laboratory at Oxford University during an 13-week period in 2002. The tests were made in vacuum under cryogenic conditions close to the space environment. The measurements were divided into spectral, spatial and radiometric, with the HIRDLS pointing capability being used to control which item of test equipment was viewed. A large degree of automation was achieved, and this combined with 24-hour/7-day working enabled a large quantity of information to be obtained.