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C. Donlon, I. S. Robinson, W. Wimmer, G. Fisher, M. Reynolds, R. Edwards, and T. J. Nightingale

Abstract

The infrared SST autonomous radiometer (ISAR) is a self-calibrating instrument capable of measuring in situ sea surface skin temperature (SSTskin) to an accuracy of 0.1 K. Extensive field deployments alongside two independent research radiometers measuring SSTskin using different spectral and geometric configurations show that, relatively, ISAR SSTskin has a zero bias ±0.14 K rms. The ISAR instrument has been developed for satellite SST validation and other scientific programs. The ISAR can be deployed continuously on voluntary observing ships (VOS) without any service requirement or operator intervention for periods of up to 3 months. Five ISAR instruments have been built and are in sustained use in the United States, China, and Europe. This paper describes the ISAR instrument including the special design features that enabled a single channel radiometer with a spectral bandpass of 9.6–11.5 μm to be adapted for autonomous use. The entire instrument infrared optical path is calibrated by viewing two blackbody reference cavities at different temperatures to maintain high accuracy while tolerating moderate contamination of optical components by salt deposition. During bad weather, an innovative storm shutter, triggered by a sensitive optical rain gauge, automatically seals the instrument from the external environment. Data are presented that verify the instrument calibration and functionality in such situations. A watchdog timer and auto-reboot function support automatic data logging recovery in case of power outages typically encountered on ships. An RS485 external port allows supporting instruments that are not part of the core ISAR package (e.g., a solarimeter) to be logged using the ISAR system. All data are processed by the ISAR instrument and are relayed to a host computer via the RS232 serial link as (National Electronics Manufacturers Association) NEMA-style strings allowing easy integration into many commercial onboard scientific data logging systems. In case of a communications failure, data are stored on board using a CompactFlash card that can be retrieved when the instrument is serviced. The success of the design is demonstrated using results obtained over 21 months in the English Channel and Bay of Biscay as part of a campaign to validate SSTskin observations derived from the Environmental Satellite (Envisat) Advanced Along-Track Scanning Radiometer (AATSR).

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C. W. P. Palmer, P. Venters, R. J. Knight, J. Ballard, T. J. Nightingale, and P. E. Morris

Abstract

In Part I the Improved Stratospheric and Mesospheric Sounder instrument and its calibration facility were described, and the results of the radiometric and spectral calibrations were presented. In Part II the remaining prelaunch calibrations are discussed.

The optical calibrations include the procedure used to coalign the different spectral channels and the mapping of the resulting instantaneous fields of view. These maps revealed detailed stray light processes within the instrument. The remaining optical calibration is of angular motion of the instrument scan mirror. The pressure modulator calibrations consist of the filling procedures and algorithm validation, which allow the gas conditions within the modulator to be determined from the instrument telemetry. Some overall conclusions to the calibration process are drawn.

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J. Ballard, C. W. P. Palmer, P. E. Morris, T. J. Nightingale, J. Firth, R. J. Knight, P. Venters, R. J. Wells, and J. G. Whitney

Abstract

The Improved Stratospheric and Mesospheric Sounder (ISAMS) is an infrared spectroradiometer that formed part of the science instrument payload of the Upper Atmosphere Research Satellite. An essential part of the success of ISAMS in orbit was a program of prelaunch calibration and characterization of many aspects of the instrument's performance. A brief description of ISAMS is followed by a detailed discussion of the calibration and characterization methodology, the facilities used in this program, and the results from the spectral and radiometric measurements. The results are discussed in terms of factors affecting the in-flight performance of ISAMS, particularly the spectral response of the measurement channels, the radiometric linearity, stray radiations and their dependence on the line of sight view, signal-to-noise ratios, and the sensitivity of the in-flight radiometric calibration to anticipated changes to the thermal environment within ISAMS. Some of the “lessons learned” are discussed with reference to the ISAMS design and the design of future instruments and test facilities.

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A. E. Roche, J. B. Kumer, J. L. Mergenthaler, R. W. Nightingale, W. G. Uplinger, G. A. Ely, J. F. Potter, D. J. Wuebbles, P. S. Connell, and D. E. Kinnison

Abstract

This paper discusses simultaneous measurements of stratospheric CIONO2, HNO3, temperature, and aerosol extinction coefficient by the Cryogenic Limb Array Etalon Spectrometer (CLAES) on the NASA Upper Atmosphere Research Satellite (UARS), obtained over the period 9 January 1992 through 23 April 1993. The discussion concentrates on the stratosphere region near 21 km of particular interest to heterogeneously driven ozone depletion. For periods between 12 June and 1 September 1992 at latitudes poleward of about 60°S, when temperatures were below type I polar stratospheric cloud (PSC) formation thresholds throughout the lower stratosphere, CLAES observed high levels of PSCs coincident with highly depleted fields of both HNO3 and CIONO2. By 17 September, the incidence of PSCs had greatly diminished in the lower stratosphere, but both CIONO2 and HNO3 remained highly depleted. These observations are consistent with the removal of gaseous HNO3 through the formation of nitric acid trihydrate (NAT) particles and the removal of CIONO2 through heterogeneous reactions on the particle surfaces. They also suggest substantial denitrification of the lower Antarctic vortex through sedimentation of PSC particles. In the Northern Hemisphere winter of 1992/93 far fewer PSCs were observed in the Arctic lower-stratosphere vortex, which had shorter periods and more localized regions of cold temperatures. Both HNO3 and CIONO2 maintained much higher levels inside the Arctic vortex than those seen in the Antarctic throughout the winter/spring period. Following 28 February 1993 when Arctic vortex temperatures rose above 195 K, CIONO2 was observed in large quantities [>2.1 ppbv near 21 km] inside the vortex. The persistence of relatively high levels of HNO3 inside the Arctic spring vortex compared with the low levels seen in the Antarctic spring vortex suggest a much lower level of denitrification in the Arctic.

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E. Theocharous, N. P. Fox, I. Barker-Snook, R. Niclòs, V. Garcia Santos, P. J. Minnett, F. M. Göttsche, L. Poutier, N. Morgan, T. Nightingale, W. Wimmer, J. Høyer, K. Zhang, M. Yang, L. Guan, M. Arbelo, and C. J. Donlon

Abstract

To ensure confidence, measurements carried out by imaging radiometers mounted on satellites require robust validation using “fiducial quality” measurements of the same in situ parameter. For surface temperature measurements this is optimally carried out by radiometers measuring radiation emitted in the infrared region of the spectrum, collocated to that of a satellite overpass. For ocean surface temperatures the radiometers are usually on board ships to sample large areas but for land and ice they are typically deployed at defined geographical sites. It is of course critical that the validation measurements and associated instrumentation are internationally consistent and traceable to international standards. The Committee on Earth Observation Satellites (CEOS) facilitates this process and over the last two decades has organized a series of comparisons, initially to develop and share best practice, but now to assess metrological uncertainties and degree of consistency of all the participants. The fourth CEOS comparison of validation instrumentation: blackbodies and infrared radiometers, was held at the National Physical Laboratory (NPL) during June and July 2016, sponsored by the European Space Agency (ESA). The 2016 campaign was completed over a period of three weeks and included not only laboratory-based measurements but also representative measurements carried out in field conditions, over land and water. This paper is one of a series and reports the results obtained when radiometers participating in this comparison were used to measure the radiance temperature of the NPL ammonia heat-pipe blackbody during the 2016 comparison activities (i.e., an assessment of radiometer performance compared to international standards). This comparison showed that the differences between the participating radiometer readings and the corresponding temperature of the reference blackbody were within the uncertainty of the measurements, but there were a few exceptions, particularly for a reference blackbody temperature of −30°C. Reasons that give rise to the discrepancies observed at the low blackbody temperatures were identified.

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