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, 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 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 functional performance of the NASA Aura HIRDLS instrument since launch on the 15<sup>th</sup> 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) 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 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.
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 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.
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.
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
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.
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.
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 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.
The High Resolution Dynamics Limb Sounder (HIRDLS) mission has particularly demanding scientific goals which represent especially challenging engineering technologies, assembly and calibration strategies. These goals include the measurement of various atmospheric species and associated geophysical parameters that have spatial resolutions and absolute accuracies that are at the same time commensurate with the resolution of current global general circulation models and with the observed fine structure of processes important in the upper troposphere and lower stratosphere (UTLS) and polar vortex mixing. The performance of the HIRDLS instrument and the pre-launch calibration equipment and procedures are essential to the achievement of these goals. The results of tests of the equipment to-date are described. These results and the procedures and equipment that are in place for the pre-launch calibration suggest that HIRDLS will be well capable of meeting the science requirements.
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.