The High Resolution Dynamics Limb Sounder (HIRDLS) instrument was launched on the NASA Aura satellite in July
2004. HIRDLS is a joint project between the UK and USA, and is a mid-infrared limb emission sounder designed to
measure the concentrations of trace species, cloud and aerosol, and temperature and pressure variations in the Earth's
atmosphere from the upper troposphere to the mesosphere. The instrument is intended to make measurements at both
high vertical and horizontal spatial resolutions, but validating those measurements is difficult because few other
measurements provide that vertical resolution sufficiently closely in time. However, the FORMOSAT-3/COSMIC suite
of radio occultation satellites that exploit the U.S. GPS transmitters to obtain high resolution (~1 km) temperature
profiles in the stratosphere does provide sufficient profiles nearly coincident with those from HIRDLS. Comparisons
show a good degree intercorrelation between COSMIC and HIRDLS down to about 2 km resolution, with similar
amplitudes for each, implying that HIRDLS and COSMIC are able to measure the same small scale features. The
optical blockage that occurred within HIRDLS during launch does not seem to have affected this capability.
The HIRDLS instrument is a limb viewing infra-red radiometer on the NASA Aura spacecraft in a sun synchronous low
earth orbit and obtains measurements of the composition of the atmosphere covering the whole Earth each day. The
MIPAS instrument is a limb viewing infra-red interferometer on board the European Envisat satellite in a very similar
orbit to Aura except that the local solar time is different. The complement of geophysical data products of both
instruments is very similar, and because of similar observation strategies their two data sets can be usefully compared.
The comparison provides the means to support validation in order to obtain statistics such as systematic differences and
variance. This is performed over the full latitude range of HIRDLS and height range of MIPAS and thereby helps to
identify sources of errors. The identification of known atmospheric features is a useful diagnostic, and includes such
things as regions of upwelling of tracer gases, or the propagation of coherent structures as with mid-latitude waves and
we can test whether these structures are consistently represented in both data sets. HIRDLS version 2.04.19 (v004)
temperature, ozone and nitric acid show very low systematic 'errors' compared to MIPAS over most of the spatial range.
Currently pre-released water vapour, nitrous oxide and F-11 are reasonably similar, CH4 somewhat more restricted, and
nitrogen dioxide, N2O5, chlorine nitrate and F-12 as yet susceptible to complications from the obstructed telescope.
Further details are discussed in the paper.
The HIRDLS instrument, like any other remote sensor must be able to maintain a high degree of measurement accuracy
through its mission life. There are many factors that influence radiometric stability including direct and indirect thermal
effects and other aging processes. Ideally the sensor should be capable of 'self-calibrating' and there must be
independent methods to track its long term accuracy. For the HIRDLS instrument, being handicapped with regard to
'self-calibration', the high fidelity data available to the ground data processors provide substantive evidence that it has
retained good long term 'accuracy'. Details of the long term performance are presented and discussed, together with
reference to some problems and their solutions.
The High Resolution Dynamics Limb Sounder (HIRDLS) instrument was launched on NASA's Aura spacecraft on 15 July 2004. When activation was completed 25 days later, it was discovered that the measured radiances were very different from those that were expected. After a long series of analyses and diagnostic tests, the cause was confirmed to be a blockage that covers much of the front aperture, preventing even one completely clear view of the atmosphere. In this paper the steps required to correct the radiances for the effects of the blockage are noted. These are calibrating the radiances, removing the effects of the blockage oscillating, and the radiance coming from the blockage, correcting for the effects of the partial aperture, and filtering the noise. The paper describes the algorithms needed, and presents the results of their application. The success of the procedures will be demonstrated by the quality of the resulting radiances and retrieved profiles of temperature and trace species. The difficulties that have been eliminated, and that still remain are noted, along with plans for further improvement. Finally, the scientific implications are briefly discussed.
The High Resolution Dynamics Limb Sounder (HIRDLS) instrument was launched on the NASA Aura satellite in July 2004. HIRDLS is a joint project between the UK and USA, and is a mid-infrared limb emission sounder designed to measure the concentrations of trace species and aerosol, and temperature and pressure variations in the Earth's atmosphere between about 8 and 100 km. The instrument is performing correctly except for a problem with radiometric views out from the main aperture. A series of tests has led to the conclusion that optical beam is obstructed between the scan mirror and the aperture by what is believed to be a piece of Kapton film that became detached during the ascent to orbit. The paper describes measurements aimed at mapping the geometric and radiometric properties of the obstruction using different positions of the aperture door, including in some cases where the sun was made to illuminate the aperture. The aim of the work is to facilitate atmospheric observations through a small part of the aperture which remains clear.
The functional performance of the NASA Aura HIRDLS instrument since launch on the 15th July 2004 is presented and discussed. The HIRDLS (High Resolution Infra-red Limb Sounder) is a 21-channel infra-red radiometer, using actively cooled MCT detectors on a common focal plane. It has many features that provide considerable flexibility of the commanding, control and the format and content of the telemetry. HIRDLS also features a precision 2-axis scan mirror
and gyroscopes that are attached to the optical bench and together they provide additional data on the line of sight on small time scales. The stability of the temperature control of the focal plane and critical optical components is also presented and discussed. To-date the instrument has performed functionally without fault and in many aspects well within specifications. The only problem (and a serious one) so far encountered has been the optical blockage of the main aperture, which is discussed in other papers. Some aspects of the instrument that have been utilised to help characterise the blockage are outlined.
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.
A pre-launch calibration of the High Resolution Dynamics Limb Sounder (HIRDLS) flight instrument was performed at Oxford University in Fall 2002. The in-band spectral characterization was performed was performed as part of this exercise. Spectral response data for all 21 channels were obtained for three different experimental conditions (nominal and two off-nominal operating conditions). Results from these data sets will be presented, as well as the analysis procedures used, along with a discussion on error analysis.
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.
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.
Christopher Hepplewhite, John Barnett, Robert Watkins, Frederick Row, Roger Wolfenden, Karim Djotni, Olusoji Oduleye, John Whitney, Trevor Walton, Philip Arter
A state-of-the-art calibration facility was designed and built for the calibration of the HIRDLS instrument at the University of Oxford, England. This paper describes the main features of the facility, the driving requirements and a summary of the performance that was achieved during the calibration. Specific technical requirements and a summary of the performance that was achieved during the calibration. Specific technical requirements and other constaints determined the design solutions that were adopted and the implementation methodology. The main features of the facility included a high performance clean room, vacuum chamber with thermal environmental control as well as the calibration sources. Particular attention was paid to maintenance of cleanliness (molecular and particulate), ESD control, mechanical isolation and high reliability. Schedule constraints required that all the calibration sources were integrated into the facility so that the number of re-press and warm up cycles was minimized and so that all the equipment could be operated at the same time.
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 Earth's 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.
Results from a pre-launch radiometric calibration of the 21-channel HIRDLS instrument will be presented. These data were obtained during a pre-launch calibration of HIRDLS at Oxford University (Fall 2002). Two external blackbody cavities were used to generate temperatures between ~90 K to ~320 K. These blackbodies were located, along with HIRDLS, inside a large vacuum chamber. Data were taken at three different focal-plane temperatures (61 K, 66 K, and 71 K). This paper will cover a variety of details; such as, data--taking procedures, analysis methodology, and the resulting linearity analyses.
The pre-launch calibration of the HIRDLS instrument took place in a dedicated facility at the University of Oxford. One aspect of this calibration was the determination of the response of the instrument to black body radiation. This was achieved with the use of purpose built full aperture black body targets which were mounted in the vacuum chamber together with all of the calibration equipment. Special attention was placed on the absolute knowledge of the emission from these targets. This was done through a combination of thermometric sensor calibration traceable to the International Temperature Standard (ITS-90), surface emission measurements, cavity design and modeling and controlling the stray light sources in the vacuum chamber. This paper describes the design requirements, implementation and performance achieved.
Christopher Hepplewhite, John Barnett, Karim Djotni, John Whitney, Justain Bracken, Roger Wolfenden, Frederick Row, Christopher Palmer, Robert Watkins, Rodney Knight, Peter Gray, Geoffory Hammond
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.
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 Earth's atmosphere between about 8 and 100 km altitude on a finer spatial scale than been achieved before. HIRDLS has particularly stringent radiometric calibration accuracy requirements. A warm (280-300K) 'In-Flight Calibrator' (IFC) black cavity within the instrument plus a view to cold space are used to perform radiometric calibration. The cavity has an entrance aperture which is much smaller than the full beam size, and it is viewed through a focusing mirror. The cavity and focusing mirror are ideally maintained at the same temperature but differences of up to 1 C may exist, in which case a correction utilising the mirror emissivity can usefully be made. That emissivity has been measured at instrument level during pre-launch calibration by viewing an external target at the same temperature as the IFC while varying the calibration mirror temperature.
Results from a pre-launch in-band spectral characterization of the 21-channel HIRDLS flight instrument will be presented. These data were obtained during a pre-launch calibration of HIRDLS at Oxford University (Fall 2002). A monochromator, equipped with a controllable diffraction grating, was used to produce monochromatic light for these tests. The monochromator was enclosed, with HIRDLS, in a large vacuum chamber. The monochromator was also equipped with a polarizer, which allowed for data to be procured at known orthogonal polarizations for each channel. A calibration detector, with a flat spectral response, was used to monitor the output from the monochromaotr. This report will consist of a description of the analyiss methodlogy, leading to an unpolarized instrument spectral response function for each channel.
The Traceable Radiometry Underpinning Terrestrial- and Helio-Studies (TRUTHS) mission offers a novel approach to the provision of key scientific data wtih unprecedented radiometric accuracy for Earth Observation (EO) and solar studies, which will also establish well-calibrated reference targets/standards to support other SI missions. This paper will present the TRUTHS mission and its objectives. TRUTHS will be the first satellite mission to calibrate its instrumentation directly to SI in orbit, overcoming the usual uncertainties associated with drifts of sensor gain and spectral shape by using an electrical rather than an optical standard as the basis of its calibration. The range of instruments flown as part of the payload will also proivde accurate input data to improve atmospheric radiative transfer codes by anchoring boundary conditions, through simultaneous measurements of aerosols, particulates and radiances at various heights. Therefore, TRUTHS will significantly improve the performance and accuracy of Earth observation misison with broad global or operational aims, as well as more dedicated missions. The providision of reference standards will also improve synergy between missions by reducing errors due to different calibration biases and offer cost reductions for future missions by reducing the demands for on-board calibration systems. Such improvements are important for the future success of strategies such as Global Monitoring for Environment and Security and the implementation and monitoring of international treaties such as the Kyoto Protocol. TRUTHS will achieve these aims by measuring the geophysical variables of solar and lunar irradiance, together with both polarized and un-polarized spectral radiance of the Moon, and the Earth and its atmosphere.
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.
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.
The High Resolution Dynamics Limb Sounder (HIRDLS) instrument is being built jointly by the UK and USA, and is scheduled for launch on the NASA EOS Chem satellite in 2002. HIRDLS will measure the concentration of trace species and aerosol, and temperature and pressure variations in the Earth's atmosphere between about 8 and 100 km altitude. It is an infrared limb emission sounder, and a primary aim is that it should measure to much finder spatial resolution than has previously been achieved, with simultaneous 1 km vertical and 500 km horizontal resolutions, globally, every 12 hours. Achieving these objectives will depend upon very precise pre-launch calibration. This will be undertaken at Oxford University in a test laboratory that is currently being constructed specifically for the task. The instrument will be surrounded by cryogenically cooled walls, and mounted together with the test equipment on an optical table contained in a vacuum chamber. The table will be mounted independently of the chamber, on an inertial mass supported on pneumatic isolators. Test equipment is being manufactured to measure (1) the radiometric response (with an absolute accuracy equivalent to 70 mK) using full aperture black body targets, (2) the spectral response of each of the filter channels using a grating monochromator, (3) the spatial response of the instrument field of view, including low level out-of-field contributions, to 10 (mu) rad accuracy using a monochromator. The methods and equipment used are described together with the principal requirements.
The scientific objectives and requirements for HIRDLS are presented, and the ways in which these flow down to some of the most important instrument requirements are shown. An overview of the conceptual design of the HIRDLS instrument, a 21-channel infrared limb scanner is presented, followed by a brief summary of the key requirements on the 9 subsystems, and an outline of some of their noteworthy design features.
In this paper we describe the scientific design work behind the selection of the IR spectral passbands for the 21 sounding channels of the high resolution dynamics limb sounder (HIRDLS) which is scheduled to fly aboard the Earth Observing System (EOS) chemistry platform at the beginning of the next century. At least one radiometer channel must be used for each gas that is being measured. Preferably the interfering contributions to the radiance by other gases in a channel should be small, but the principle requirements are that the desired emission be measured with high signal-to-noise ratio, and that there be separate channels for the measurement of interfering species. However, more than one channel is required to provide full altitude coverage of those target gases such as CO2, H2O, and O3, which have emission bands whose centers become optically thick in the middle atmosphere. Further channels, in which gaseous absorption is low, are required for the characterization of aerosol effects. We describe the HIRDLS channels selected for each gas, with emphasis on signal-to-noise considerations and altitude coverage, the elimination of contaminating signal between channels, and non-LTE processes for high altitude sounding and space view definition.
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.
The High Resolution Dynamics Limb Sounder is described, with particular reference to the atmospheric measurements to be made and the rationale behind the measurement strategy. The demands this strategy places on the filters to be used in the instrument and the designs to which this leads to are detailed. A second set of filters at an intermediate image plane to reduce 'Ghost Imaging' is discussed together with their required spectral properties. The spectral characteristics of the primary and secondary filters in each channel are combined together with the spectral response of the detectors and other optical elements to obtain the system response weighted appropriately for the Planck function and atmospheric limb absorption. This method is used to demonstrate whether the out-of-band spectral blocking requirement for a channel is being met, and an example calculation is demonstrated showing how the blocking is built up for a representative channel. Finally, the techniques used to produce filters of the necessary submillimeter sizes together with the testing methods and procedures used to assess the environmental durability and establish space flight quality are discussed.
John Barnett, Paul Morris, T. Nightingale, Christopher Palmer, Guy Peskett, Clive Rodgers, Fredric Taylor, P. Venters, Robert Wells, John Whitney, John Ballard, R. Knight
The improved stratospheric and mesospheric sounder (ISAMS) is one of the instruments on the NASA Upper Atmosphere Research Satellite which was launched in September 1991. ISAMS is a limb-viewing infrared radiometer which measures thermal emission in 24 pass bands (some of which are obtained by gas correlation). This enables the daily mapping over much of the Earth of temperature, the concentrations of 8 chemical species (water vapor, methane, ozone, nitric acid, nitrogen dioxide, nitric oxide, dinitrogen pentoxide, carbon monoxide), and aerosol opacity in the stratosphere and mesosphere. The instrument has eight separate focal planes, each consisting of a 4-element detector array, which are cooled by two mechanical coolers developed specifically for the instrument. The instrument uses a moveable mirror to scan the limb in elevation and to view at a variable azimuth angle to avoid Doppler shifts; the view may be to either side of the spacecraft in order to improve the geographical coverage.
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