Remote Sensing of Environment 116 (2012) 4–16

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Remote Sensing of Environment

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ATSR infrared radiometric calibration and in-orbit performance

Dave Smith a,⁎, Chris Mutlow a, John Delderfield a, Bob Watkins b, Graeme Mason c

a STFC, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdomb Department of Atmospheric Oceanic and Planetary Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdomc ESA, ESRIN, Via Galileo Galilei, Casella Postale 64, 00044 Frascati, Roma, Italy

⁎ Corresponding author. Tel.: +44 1235 445996.E-mail address: dave.smith@stfc.ac.uk (D. Smith).

0034-4257/$ – see front matter. Crown Copyright © 20doi:10.1016/j.rse.2011.01.027

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 July 2010Received in revised form 25 January 2011Accepted 29 January 2011Available online 30 June 2011

Keywords:ATSRATSR-2AATSRAlong-Track Scanning RadiometerCalibrationRadiometerSea Surface TemperatureBlackbodyThermalInfrared

Three Along Track Scanning Radiometers (ATSRs) form a series of space-borne instruments specificallyoptimised to provide accurate remotely sensed measurements of Sea Surface Temperature (SST), which is akey geophysical parameter required to inform the debate on climate change and global warming. Thesesensors' well-calibrated, high quality data have wide applicability and are being used in amuchwider range ofearth observation studies and applications, in addition to the planned SSTmission. Each successive instrumenthas been an incremental improvement over its predecessor. Since early 1991 the ATSR sensors have providedglobal observations from the European Space Agency's Earth Observation satellites; namely ATSR-1 on ERS-1(European Remote-sensing Satellite), then ATSR-2 on ERS-2 and then AATSR (Advanced ATSR) on ENVISAT.The missions have been operated with good overlaps between successive sensors; AATSR is currently theoperational instrument. The fundamental requirement for each ATSR instrument is a design that is capable ofdelivering absolutely calibrated infrared data; therefore this paper concentrates on how the sensors providethe calibrated radiometric observations required for the SST retrieval algorithms to work. It does not discussthe validation of the algorithms to produce SST derived from these basic observations. Described are therigorous pre-launch measurements over a range of simulated flight environments which verify that this aimhas been achieved, a calibration which exercises the same brightness temperature algorithm that is then usedwhen calibrating in-flight measurements. Crucially, the ATSRs measure calibration source radiances in-flightwithout interrupting Earth-viewing which permits continuous gain and offset monitoring and calibration. Forclarity where it is thought necessary for a data user's better understanding of the calibration for flight data,some detail is provided concerning the differences between the three different ATSRs.

Crown Copyright © 2011 Published by Elsevier Inc. All rights reserved.

1. Introduction

Each ATSR senses a ±250 km ground swath symmetricallyarranged across the sub-satellite track by scanning four, single pixel,spatially co-registered channels (centred at 1.6, 3.7, 10.8 and 12 μm)that are recorded by cryogenic detectors within its Focal PlaneAssembly (FPA). For ATSR-2 and AATSR an additional three shorterwavelength channels (at 0.55, 0.65, 0.85 μm) are included, these arerecorded with ambient temperature silicon detectors. They arehoused in their own compartment, as part of an enhanced FPA, andoptically configured to use the full aperture of the other four channelsand share the same common, single pixel field-stop. This paperconcentrates on the thermal infrared channels, which are selected tomatch atmosphere transmission “windows” at centre wavelengths of3.7, 10.8, and 12 μm.

All three spacecraft are in sun-synchronous orbits with a near 3-day repeat cycle and approximately 14 orbits per day. Each ATSR, with

11 Published by Elsevier Inc. All rig

its combined day and night coverage and 500 km swath width,provides virtually complete global coverage every three days; exceptfor the two polar regions excluded by the orbit's inclination and theimpact of cloud coverage. Note that coverage refers here to datacollection rather than retrieved SST, since clouds are opaque in theinfrared and SST can only be sensed through cloud-free atmosphericcolumns.

At the sub-satellite point, each sensor's Instantaneous Field ofView (IFOV) is 1 km square and data are sampled on nominal 1 kmsquare grid. ATSRs produce observations of the surface at two-angles over their ground swath, one viewing nearly verticallythrough the atmosphere and the other viewed obliquely. (BecauseATSRs are conical scanning radiometers, the term “swath” is alsoused to describe telescope viewing vectors that are swept out by thescan mirror as opposed to a temporally combined ground coveragestrip.)

To meet scientific goals over the dynamic range used for SST,radiances reaching the satellite must be measured to an absoluteaccuracy equivalent to better than 0.1 K in brightness temperature fora stated spectral response. Also to avoid noise multiplication in theretrieval algorithm calibrated, single pixel, random radiometric noise

hts reserved.

5D. Smith et al. / Remote Sensing of Environment 116 (2012) 4–16

must be less than 0.08 K at 3.7 μmand 0.05 K at 11 μmand 12 μm, for ascene temperature of 265 K (relaxed to 270 K for AATSR).

2. Instrument outline and scanning geometry

The reader can find detailed descriptions of the ATSR-1 and -2instruments in Edwards et al. (1990) and Stricker et al. (1995). Theinstruments have a small IFOV so they can achieve good image qualitywith a single f5 off-axis parabolicmirror feeding to a single on-axisfield-stop at prime focus. An inclined scan mirror covers the telescope'sslightly diverging beam, and it is rotated at 400 rpm with a constantangular velocity vector parallel to this beam's primary ray to generate ascanned cone. The angle of incidence at the scan mirror is ~11.7° andfour times this, the full scan cone angle, is ~47°. With its minimisedoptical surfaces this configuration is well suited to the IR, and the scanplus paraboloid mirrors only need to be cooled to ~−10 °C for theiremitted photon noise to fit within allocation in the noise budget. Lowangles of incidence and high mirror reflectivity mean that these Earthimaging fore-optics are essentially non-polarising.

A conical scanner offers a number of advantageous features:

• Automatic generation of two Earth-viewing swaths.• Space between these two swaths to accommodate full aperture in-flight calibration sources, that are viewed completely each scanwithout discarding any useful Earth views or requiring anyadditional optical components (Fig. 1).

• Chopped operation compatible with maintaining IR offset calibration.• Both the Earth views are calibrated with reference to the same pairof blackbodies using the same electro-optical system; removing thepossibility of relative calibration errors when combined in the SSTretrieval.

It is of the essence in ATSRs' design that all the optical elements areused with constant areas and at the same angles for collecting both IRcalibration data and each of the dual swath Earth signals. Clearly as noadditional optical elements are required for calibration, no artefactsare introduced by extra optic, by area/angle variations, or operatingcycle timing changes — the same is not true of all radiometerconfigurations (e.g. MODIS Barnes et al., 1998).

Nadir View

Along-Track View

Hot Blackbody Cold Blackbody

VISCAL

Scan Direction

Fig. 1. The ATSR scan pattern showing the positions of the on-board calibration targetsand earth views.

3. Calibration principles

The IR radiometric calibration concept inherent in the ATSR designis that two on-board blackbody sources, at differing temperaturesoptimised to cover the important range of Earth scene temperatures,is used to establish two reference radiances. These reference signalsare combining with other characterisation data determined by pre-launch calibration, to derive the IR radiometric response of the sensorcontinuously in time for scene pixels around the scan.

Built into the instrument design are several features that make itpossible to simplify the calibration schema and reduce the number ofassumptions that need to be made in the process. Of these the keyfeatures are:

• Cooled fore-optics to minimise any background signals and ensureany small residual signals remain stable over each “scan cycle” (i.e.during sampling of the two Earth view swaths and two calibrationsources).

• Same optical path used to view the Earth and the on-board sources.• Stray-light baffles and other stray-light control features.• Use of a blackbody calibration system employing two blackbodies attemperatures covering the cold and hot extremes of SST. (Thissignificantly reduces the effects of system non-linearity in the rangeof expected SST temperatures compared to that those obtained byinstruments which calibrate using a “zero” point (offset) derivedfrom viewing cold space.)

Hence the measured signal Cscene for a scene of radiance Lscene canbe expressed as:

Cscene = gainLscene + offset: ð1Þ

This is known as the “ATSR Radiometric Model”. Using this model(excluding for the moment any detector non-linearity) we can thendetermine the gain and offset for each spectral channel by using twosources of known radiance carried into orbit. The ATSRs have a coldand a hot blackbody, at temperatures Tcold and Thot respectively, thatprovide the known calibration radiances Lcold,λ and Lhot,λ. Thesetemperatures have been chosen to span the range of expected SST,and minimise the need for extrapolation and therefore the effects ofnon-linearity over the range from 260 K to 300 K.

To convert from temperature to radiance, and vice versa, weintegrate the Planck function with a spectral response as a function ofwavelength, Rλ(λ) to give the integrated in-band radiance as:

Lλ Tð Þ = ∫Rλ λð ÞB λ; Tð Þdλ; ð2Þ

where for a perfect blackbody with emissivity ε=1.0, the spectralradiance B(λ,T) at wavelength λ emitted by a blackbody attemperature T, per unit area, per steradian, per second, per unitwavelength is given by:

B λ; Tð Þ = 2hc2 = λ5 exp hc = λkbTð Þ−1ð Þ� �

; ð3Þ

where h is Planck's constant, c is the velocity of light and kb isBoltzmann's constant.

The radiometric calibration is dependent only on the radiancesdetermined from the on-board calibration sources and does notrequire an absolute knowledge of the AΩ. The ATSRs work because thecalibration sources are located ahead of all the optical componentsand are sampled as part of the normal measurement cycle so that thecomplete electro-optical chain is calibrated. However, note that it isessential that there be no observable contribution to the measuredradiance from other sources, and so stray light control and entrancebaffle clearances on optical beams are vital factors in the instrumentdesign and performance verification.

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Non-linearity is an expected characteristic of the conductiveHgCdTe detectors for the 10.8 μm and 12 μm channels, Baker et al.(1978). Essentially, the electron–hole recombination rate increases asthe number of carriers (electrons and holes); the result is a fall-off inthe detector's response as the photon-flux increases.

The approach for determining and correcting for the effects of non-linearity was established for ATSR (Mason, 1991) and has been usedfor the other two sensors. Here the nonlinearity is expressed as thefractional fall-off in response, fλ(T), compared to the linear responsefunction, such that

fλ Tð Þ = z0;λ + z1;λLλ Tð Þ

Lλ 320Kð Þ + z2;λLλ Tð Þ

Lλ 320Kð Þ� �2

: ð4Þ

This fall-off shape is applied within the calibration algorithm byconsidering a corrected radiance, L ′λ (T), where:

L′λ Tð Þ = fλ Tð ÞLλ Tð Þ: ð5Þ

4. On-board blackbody sources

The on-board blackbody sources are described in detail in Masonet al. (1996); two of these highly accurate blackbody sources are builtinto each ATSR, and provide the basis for in-flight absolute calibrationof the thermal infrared channels. During every scan, each of the twoblackbodies is viewed in turn by all of the spectral channels, to providethe cold and hot reference radiances. One blackbody temperature‘floats’ cold at the optimised temperature of the instrument fore-optics enclosure, whilst the other is heated at a constant power toprovide a hot reference. The blackbodies and optical system aredesigned such that the instrument's full optical beam has anunobstructed view of the base-plate; is not clipped by any apertureor the blackbody cavity walls. Each base-plate has been designed tomaintain a uniform temperature. The cavities provide a very higheffective emissivity (εN0.999) by a combination of Martin Mariettablack coating and a re-entrant cone base geometry.

The temperatures of the blackbody bases are measured with highaccuracy precision platinum resistance thermometers (PRTs). At unitlevel, these are calibrated with their flight electronics against atransfer standard PRT traceable to ITS-90.

Even the high emissivity of ε=0.999 is not unity, so the blackbodyradiances will have a small reflected component originating from theinstrument optical enclosure that itself behaves as a blackbody source.The calibrated blackbody radiance from a blackbody targets is:

Lλ Tbbð Þ = ελ∫Rλ λð ÞB λ; Tbbð Þdλ + 1−ελð Þ∫Rλ λð ÞB λ; Tinstð Þdλ: ð6Þ

Eq. (6) can be used in conjunction with Eqs. (1), (2) and (3) toprovide all the information needed to convert a measured signalcounts to a linearly calibrated radiance.

5. Pre-launch radiometric calibration

The principle steps in the pre-launch calibration applied to each ofthe thermal infra-red channels instruments in the three ATSRinstruments are:

• Measurement of the spectral response for each channel at FPA sub-system level.

• Operation of the instrument in simulated flight environment withits hot blackbody power set to give the required temperaturedifferential with respect to the ambient (cold) one.

• Verification of the “on-board” radiometric calibration over the fullrange of scene temperatures to be measured in flight (i.e. between210 K and 315 K).

• Measurement of system non-linearity and formulation of a param-eterisation to correct it.

• Assessment of the measurement noise and derivation of theequivalent brightness temperature difference.

• Confirmation of radiometry around the scanned swaths to establishthat it is free from around-scan effects.

• Inversion of the derived calibration algorithms to show they fit backto the raw results.

• Investigation of radiometric performance under different generalthermal conditions.

• Verification that the algorithms return accurate radiometry duringsimulated orbital transient thermal conditions.

The dates of the calibration campaigns for the three instrumentswere as follows:

ATSR

Oxford University April-1989 to July-1989 ATSR-2 Oxford University November-1992 to March-1993 AATSR RAL October-1997 to December-1997

October-1998 to December-19981

1 TheAATSRcalibration campaignwas repeatedafter some reworkon the instrument torectify a technical problem that had been identified during the first test campaign.

5.1. Test facilities' description

For infrared radiometric calibration it is essential that the thermalenvironment be well controlled, and accurately monitored. This wasachieved by performing tests in a vacuum chamber with theinstrument under test surrounded by temperature controlled panelsto allow it to operate at temperatures close to those expected from thethermal model at flight conditions. To accomplish this goal, four mainthermal zones in the test facility were controlled. These were an“Earth-shine” plate (ESP) which was used to simulate radiation fromthe Earth, a “Payload Electronics Module (PEM) simulator” whichmimicked the interfacing panel of the spacecraft, and a “cold box” toprovide a uniform space temperature environment around theinstrument and a “drum baffle” to shield the instrument to straysfrom the chamber walls. The ESP was used to support the externalcalibration targets which could be rotated about the instrument's scancone, nominally set to be the tank's axis.

ATSR and ATSR-2 were calibrated in a purpose built facility at theDepartment of Atmospheric Oceanic and Planetary Physics (DAOPP)at Oxford University (Mason, 1991). Although of the same focal lengthand aperture, AATSR was a larger instrument than ATSR and ATSR-2,and could not be fitted within the Oxford test facility. The AATSRcalibration activities were therefore performed in the larger SpaceTest Chamber (STC) at the Rutherford Appleton Laboratory retainingthe original design concepts and philosophy developed for the Oxfordtesting, and a subset of the test equipment. This is described in Smith(1999), and Smith et al. (2001).

5.2. External blackbody sources

The external blackbody sources used for the pre-launch calibrationof all three ATSR instruments were designed and built by the UKMeteorological Office to provide a scene radiance corresponding to anaccurately known temperature. Using the same sources and pro-cedures in ground testing has provided continuity of calibrationbetween the different sensors.

The requirement of these blackbodies is to provide a referencesource for the scene radiance known to an uncertainty equivalentbetter than 0.1 K, an error that is apportioned as follows:

• The errors caused by the emissivity not being unity must be lessthan 0.035 K.

• The temperature of the target must be known to better than 0.02 K.

Table 1Calculated and measured emissivities of external blackbodies.

Calculated Measured Difference

3.7 μm 0.99899±0.00035 0.99911±0.00055 0.0001211 μm 0.99847±0.00036 0.99870±0.00040 0.0002312 μm 0.99871±0.00037 0.99871±0.00032 0.00000

Table 2Percentage fall-off at 310 K for all ATSR instruments.

ATSR ATSR-2 AATSR

10.8 μm 3.0% 4.2% 3.5%12.0 μm 4.2% 3.9% 7.2%

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• The temperature differences across the target are less than 0.02 K.• The temperature fluctuations over 5 min must be less than 0.01 K.

For ATSR testing, the operating temperatures of each target weremeasured using six Rosemount E109 100 Ω platinum resistancethermometers (PRTs). Four sensors were mounted in the baseplate in

Fig. 2. Results from the radiometric calibration at the centre of nadir-view for BOL thermal baas a function of target temperature, the differences between the brightness temperature as mthe resistance thermometers.

probes that ensured good thermal contact with the blackbody, theother two sensors were mounted one halfway up the baffle and theother positioned near the aperture plate. However, as there werequestions about the self-heating in these PRTs, for ATSR-2 they werereplaced by 27Ω rhodium–iron resistance thermometers (RIRTs)supplied by Oxford Instruments; these were demonstrated to beaccurate to within ±0.01 K with self-heating of less than 0.001 Kwhen measured by an AC resistance bridge.

As is the case for the instrument's internal blackbodies, theradiance emanating from the external calibration units is derivedusing the formulation in Eq. (4). The emissivity of each target wascalculated using a geometric model and the spectral emissivity of thepaint, Table 1, Mason (1991). These values were validated bycomparison against the ATSR-2 and AATSR on-board blackbodies(see Section 5.4).

5.3. Test procedure

Radiometric calibration and instrument performance verificationwas performed by measuring the signal channel responses over arange of preset stable external blackbody target temperature from

lance conditions for ATSR (top), ATSR-2 (middle) and AATSR (bottom). Each plot shows,easured by the instrument and the actual target brightness temperature as measured by

8 D. Smith et al. / Remote Sensing of Environment 116 (2012) 4–16

210 K to 310 K. Tests were performed under different thermalconditions for the instrument, to establish the stability and robustnessof its outputs. In practice, only one of the targets was changed during acalibration run (usually nadir) whilst the other (along-track target)was maintained at a fixed temperature for reference (Table 2).

The initial detailed calibration runs were undertaken at the centreof nadir view, with the nadir target temperature being moved at 5 Kintervals from 210 K to 315 K. Once the initial capability of theinstrument was established the remaining calibration tests covering afull range of viewed scan swath angles were performed using areduced set of target temperatures at 10 K intervals between 240 Kand 310 K. The purpose of these tests was to measure any scan-dependent variations; the calibration was verified at differentpositions around the scan to cover all nadir and along-track pixels.A subset of these tests was carried out holding both external targets ata constant temperature of 240 K and cycling the Earth Shine Platetemperature to provide a more stringent test that the baffles werecontributing negligible stray radiation. Measurements were also takenwith an external target at approximately the same temperature as

Fig. 3. Brightness temperature errrors with the external blackbodies set at the same temperData for the +XBB are shown as red diamonds and the –XBB are shown as blue triangles.

each of the on-board blackbodies, to verify calibration at points whereno correction for non-linearity was needed. For these tests the fixedtarget was usually set to 280 K, being approximately mid-waybetween the temperatures of the two on-board blackbodies.

To ensure that the overall radiometric uncertainty from thesources was below 0.04 K, measurements were collected only whenboth target base-plates were drifting at rates below 0.02 K over 5 min,and the temperature differences across the targets were below 0.02 K.

To verify the stability of the sensor over its lifetime, as well as innormal daily operation, each radiometric test was executedwith ATSRand the test environment under thermal equilibrium at differingthermal conditions: these included the conditions expected atbeginning of life (BOL) and end-of-life (EOL), as well as simulatedorbital cycles. The orbital simulations were performed with the nadirtarget at 240 K, 275 K and 310 K for two orbits each, and with along-track target maintained at 280 K.

Low radiance measurements were performed by cooling thealong-track target with liquid nitrogen. When the target temperaturehad reached 96 K, the LN2 supply was disconnected and the blackbody

atures as the on-board blackbodies for ATSR (left), ATSR-2 (centre) and AATSR (right).

Fig. 4. NEΔT as a function of target temperature for ATSR (top), ATSR-2 (middle) and AATSR (bottom) at BOL thermal conditions.

Table 3NEΔT of IR channels at target temperature of 270 K.

Requirement ATSR ATSR-2 AATSR

3.7 μm 0.080 0.046 0.051 0.03710.8 μm 0.050 0.023 0.021 0.02512.0 μm 0.050 0.031 0.024 0.023

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was allowed to warm up slowly, and measurements taken at 1 Kintervals. These measurements were used to determine non-linearity,Table 2, and provide the coefficients for the corrections.

5.4. Summary of results

Themain pre-launch calibration results and analyses are describedin Mason (1991), Smith et al. (1993) and Smith (1999).

A summary of the results at BOL conditions for eachATSR instrumentis shown in Fig. 2. The calibration error is defined as the difference in thebrightness temperature of the external targets asmeasured byATSR andthe actual target brightness temperature as determined from thetemperatures as measured by the thermometers. Where the externalblackbody temperature is close to that of an on-board blackbody, errorsdue to non-linearity become insignificant and only residual errorsremain. So for each of the calibration runs, measurements were takenwith the external blackbody temperature matched to the on-boardtarget temperatures to investigate these residuals, without the need toallow for detector non-linearity. The differences between the brightnesstemperatures and actual target temperatures, for all tests atBOL thermalenvironment, are shown in Fig. 3.

As well as establishing the radiometric bias errors, the radiometricnoise was measured as a function of scene temperature. The

instrument noise is expressed as the noise equivalent brightnesstemperature difference, NEΔT, and is given by:

NEΔTλ = A1;λ ΔCλ∂Lλ∂T

���T

� �−1

ð7Þ

where ΔCλ is the signal channel noise taken to be the standarddeviation of the blackbody pixel counts.

Fig. 4 shows the NEΔTs for each instrument plotted as a function oftarget temperature for the BOL calibrations at the centre-of-nadirview. Formally, the requirement for NEΔT is set for a scenetemperature of 270 K and the measurements compared against therequirement at this temperature are given in Table 3. In all cases, themeasured noise was within the requirement and there is goodagreement across instruments. It should be noted that the apparent

10 D. Smith et al. / Remote Sensing of Environment 116 (2012) 4–16

rise in NEΔT as the target temperature decreases is purely related to∂L/∂T as a function of scene temperature. For the 11 μm and 12 μmchannels the dominant noise source is detector noise, which remainsalmost constant for all scene temperatures. At 3.7 μm, pre-amplifiernoise dominates for low photon fluxes (Tsceneb250 K) whilst at higherphoton fluxes the noise becomes dominated by statistical photonsignal noise.

6. In orbit performance and monitoring

To state the obvious, once in orbit there is no longer access to anindependent absolute calibration facility external to the instruments.Nevertheless, careful monitoring of the in-orbit trends described inthis section contributes towards confidence in the radiometricmeasurements.

6.1. Cooler performance

During in-flight commissioning, the ATSR-1 Stirling cycle coolerwasseennot to cool the FPA ashad been achievedduring ground calibration,

Fig. 5.Mission trends of the IR detector temperature (top), commanded gains (middle) and oare shown for 12 μm (RED), 11 μm (green) and 3.7 μm (blue).

i.e. to run at a stable FPA temperature of ~80 K it needed to be run atmuch higher amplitudes. For ATSR on ERS-1 the Experiment InterfaceDocument with the satellite specified that ATSR had a good view-factorto space from its anti-earth side. In addition, spacecraft panels had beenbuilt with extensions so as to prevent direct sunlight reaching this sideof the instrument on which the cooler and FPA are situated. The UKcalibration facility was implemented on such a basis and the StirlingCycle cooler easily cooled the FPA down to 80 K with mechanismamplitude to spare; ATSR was then calibrated. However neither of thespacecraft interface provisions was met. At spacecraft level testing thecooler bodywas getting toohot and aheatpipe solutionwas successfullyretro-fitted. However, in flight it was soon apparent that the cooler wasrunning at much higher amplitudes than during instrument calibrationand struggling to reach 80 K. To prevent premature ageing of themechanism and maximise the lifetime of the mission, the decision wastaken to modify the operations, and the cooler was operated at fixedamplitude levels rather than with active control. Thus the orbitaltemperature cycling of the cryogenic elements was not constrained,although the average temperature was maintained by occasionalmanual updates to the commanded cooler amplitudes and frequency.

ffsets (bottom) for ATSR (left), ATSR-2 (centre) and AATSR (right). The gain and offsets

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Initially, at the start of the mission, the aim was to keep orbitaltemperature peaks below 90 K, but after a couple of years this wasrelaxed to 100 K. Care was taken to minimise excursions above thistemperature in order to keep the 12 μmdetector in the operating regionwhere its HgCdTe long-wavelength responsivity cut-off did notsignificantly alter SST retrievals. Later in the mission, even 100 K wasnot always achievable on a continuous basis and retrievals had to takespectral response changes into account. Optimising cooler operationwas the most challenging part of running ATSR in flight, andunavoidably the dataset has variable noise performance, and later inthe mission some variation in spectral response.

One factor that helped late in the ATSR mission was that theinstrument was de-hibernated at roughly 3 month intervals for justover a day, giving data overlap periods with ATSR-2, with the resultthat the FPA was always clean and needed minimum cooling.

It is known that the response non-linearity is dependent on thedetector temperatures. This was characterised pre-launch at thenominal operating temperature of 80 K and at an elevated temper-ature of 90 K then fitting the results to theory. Although there was asmall change in non-linearity, with the calibration algorithm appliedthe differences in the measured brightness temperatures between260 K and 300 K were within the ±0.1 K requirement and it wasconcluded that the same temperature to radiance look-up-tables werevalid for all anticipated detector temperatures.

Fig. 6. Mission trends of the NEΔTs for ATSR (left), ATSR-2 (centre) and AATSR (right). Hot

The detectors for the 12 μm channel were specified to have a longwavelength cut-off so that, over the normal operating range up to90 K, their change in temperature would not affect spectral response.The pre-launch measurements of noise performance showed theexpected worsening between 80 K and 90 K over which range thespectral response was taken as fixed.

However, eventually an increase in detector temperature willaffect the 12 μm spectral response, and in orbit it became apparentthat the detectors would have operate at up to 100 K. So additionalpost launch measurements were performed on detectors from thesame batch to characterise the spectral response changes that cameinto play at these elevated temperatures. These measurements wereused in the adjustment of instrument products when running athigher temperatures.

To counter these problems ATSR-2 on ERS-2 had many detailsaltered in its thermal blanket and finish. Crucially, RAL designed a lowemissivity radiation shield around the inside cryogenic part of the FPAassembly such that even with the now correctly understoodspacecraft interfaces (essentially unchanged from ERS-1) the loadon its cooler was below the specified 400 mWatts. Then the cooler,unchanged from ERS-1, successfully maintained the IR FPA at 80 K forthe whole mission, conferring stable noise and spectral performance.Other design changes were made for ATSR-2 to minimise cooler noisepick-up that was a problem on ATSR-1; the detector signal

BB signals are shown in red diamonds and cold BB signals are shown in blue triangles.

12 D. Smith et al. / Remote Sensing of Environment 116 (2012) 4–16

preamplifier was relocated away from the cooler and plugged directlyinto the FPA, which also removed the need for a heavily screened lead.In-flight the ATSR-2 cooler behaved extremely well with nodiscernable degradation in its behaviour or performance throughoutthe instrument's 13-year operating lifetime — the ATSR-2 coolerremained perfectly functional even though instrument's mission wasterminated because the scan mirror mechanism failed!

The AATSR cooler, a much bigger mechanism provided by BritishAerospace Bristol (now EADS ATSRIUM-UK) and integrated into asub-system with its drive electronics by RAL. It has performed in anexemplary fashion throughout the mission lifetime to date and ismaintaining the infrared focal plane assembly at its target temper-ature 80±0.5 K during nominal operations. A cool-down fromambient to 80 K typically takes approximately 6 h and there are nosigns of any degradation in performance.

A significant influence on the AATSR cooler performance has beenthe build up of water ice contamination on the outer surfaces of the FPAcryogenic components. Some level of water ice contamination wasexpected because of the FPA is vented and carbon fibre structures areused for the instrument and spacecraft. These are known to absorbwater vapour whilst on ground which then desorbs on-orbit and cancondense on cold surfaces below 150 K. The effect of the water icecondensation is to increase emissivity and hence increase the thermalloading on the FPA. To maintain the FPA at 80 K, the cooler controlsoftware compensates by increasing the cooler drive levels. The coolingperformance is recovered after the FPA has been decontaminated byallowing the FPA to warm to ambient for 48 h. As the water vapoursurrounding the spacecraft slowlydisperses, the rate atwhich the coolerdrive levels and temperatures rise has gradually decreased after eachsuccessive cool-down. For ATSR-2 the contamination rate was estimat-ed to be 0.1 μm per day during commissioning which decreased to0.01 μmper day after thefirst year of operation,whichmeant that itwas

Fig. 7. Fourier Transform spectrum of ATSR blackbody noise signals. This analysis was perfvariations of the blackbodies. It should be noted that because the cooler frequency is 43 Hz

only necessary to decontaminate ~6 month intervals. However, thedeposition rate for AATSR was an order of magnitude higher thanobserved at the start of the ATSR-2 mission, which resulted in a morerapid build up of water ice contamination. This means that regularoutgassing at ~4 month intervals are necessary to prevent the layerthickness from reaching excessive levels and having a detrimental effecton the optical and thermal performance.

Although contamination does affect AATSR optics, the overallinstrument radiometric performance was not affected since the on-board sources provide continuous calibration. Also, despite theincreased water ice contamination, the AATSR Stirling cycle coolerhas been operating well within its operational limits with no sign ofdegradation and has not needed any adjustment to its softwareconfiguration since launch.

6.2. Throughput gain

In ATSRs, with their d.c. coupled signal channels, an IR Auto-Gain-Offset (AGO) loop optimally maintains each detector signal within its12 bit digitisation ranges. The loop compares the measured signalchannel counts for the hot and cold blackbodies against a nominalcharacteristic of counts versus temperature. From this it steps towardsimproved commanded pairs of gain and offset values and maintainsthe required overall brightness temperature range. Therefore thetrends of the gain provide an inverse indication of the variation oftotal ATSR sensitivity over time, Fig. 5.

Somemonths into operations, the ATSR 3.7 μm channel failed. Thishas been a one-off failure in the series for which no proven cause hasbeen established. The effect was temporarily to remove a valuablesource for night-time SST information (daytime SSTs were unaffectedbecause the 3.7um channel is not used due to the presence of reflectedsolar energy in the measured signals). For ATSR, the main factor

ormed over 512 scans and after removal of background drifts due to the temperatureand the scan period is 6.7 Hz, the noise signal is aliased strongly.

Table 4Typical blackbody thermometer readings for AATSR taken on 3rd of June 2002. The top row shows the average of the 5 baseplate sensors (PRT1-PRT5). Note that PRT6 is the baffletemperature and is not used in the average. The difference column shows the differences between the individual sensor readings and the average temperature. The final column is atypical reading from the pre launch calibration in December 1998.

+XBB temperatures (K) −XBB temperatures (K)

Reading Difference Pre-launch Reading Difference Pre-launch

Baseplate average 301.522 – 293.527 Baseplate Average 262.897 – 252.773PRT1 301.513 −0.009 −0.009 PRT1 262.898 0.001 0.001PRT2 301.518 −0.004 −0.002 PRT2 262.899 0.002 0.000PRT3 301.526 0.004 0.002 PRT3 262.897 0.000 0.000PRT4 301.525 0.003 0.001 PRT4 262.892 −0.005 −0.001PRT5 301.530 0.008 0.006 PRT5 262.897 0.000 −0.002PRT6 (Baffle Sensor) 301.905 0.383 0.391 PRT6 (Baffle Sensor) 262.882 −0.015 −0.017

Fig. 8. Mission trends for ATSR (left), ATSR-2 (centre) and AATSR (right) of the daily averages of the +XBB and –XBB baseplate mean temperatures, and the differences from themean of the individual sensor readings.

13D. Smith et al. / Remote Sensing of Environment 116 (2012) 4–16

14 D. Smith et al. / Remote Sensing of Environment 116 (2012) 4–16

affecting the long term gain stability was the FPA temperature. As thistended to increase over time, the commanded gains were graduallyincreased by the AGO loop to compensate for the decrease in theradiometric response.

With the FPA/IR detector temperatures having remained stablethroughout the ATSR-2 and AATSR missions, we note the AGOcommanded gains after each outgassing have remained stable, whichwould also indicate that there has been negligible degradation of anyinstrument component including the optics.

6.3. Radiometric noise

In-flight, the radiometric noise is monitored using the two on-board blackbodies signals. The mission trends of the NEΔTs for eachinstrument are presented in Fig. 6.

For ATSR, the NEΔT trend of the 11 and 12 μm channels increasesover the mission lifetime. This is due to the noise signal beingamplified as the electronic gain was increased to compensate for theloss of radiometric response. When considering the raw detectorsignals in Volts the noise signals actually remained stable suggestingthat the detectors and preamplifiers have not degraded inperformance.

Again for ATSR, there is an additional significant noise contributionfrom magnetic cooler drive pick-up. This signal can be observed bycareful inspection of ATSR images over uniform scenes and is clear inFourier analysis of the blackbody signals (see Fig. 7). Due to cooleroperating frequency the signal is almost an odd number of half cyclesper scan and drifts slowly in relation to the scan cycle. This signal wasnot present (or at least insignificant) on ATSR-2 or AATSR.

Fig. 9. Blackbody temperatures and IR channel blackbody signals for A

For ATSR-2, we note that the noise performance derived from theon-board blackbodies appears variable over the mission, as it can besubtly affected by scan mirror rotation instability. All ATSRs use twointegrators for alternate pixels so as to maximise noise performance.Because the integrators have marginally different characteristics forthe same digitally commanded settings, the data from odd and evenpixel integrators are calibrated separately. Normally there are exactly2000 pixels during the 150 ms scan period and so odd/even pixelsoccur keep fixed locations in a scan. However, when a scan jitteroccurs it is possible to get 2001 or 1999 pixels per scan, so thelocations of odd/even are swapped. As the software used to producethis figure does not track the details of this effect, such a scan mirrorjitter gives an apparent increase in the radiometric noise. However, asthere is no long term drift it can be inferred that the detector andoptics performance has remained stable throughout the mission.

The AATSR noise performance has remained very stable through-out the mission. Occasional outliers can be observed in the trends,Fig. 6, but these can be traced to the start and finish of outgassingcycles when the detectors are not at their nominal operatingtemperature.

6.4. Blackbody radiances

As described, each blackbody has multiple temperature sensors,each of which has its own precision amplifier before their signals aremultiplexed. Therefore consistency within a blackbody's tempera-tures builds confidence. Table 4 shows such a set of in-orbit readingscompared against measurements taken during the pre-launchcalibration. Although the in-flight readings are warmer by about10 K, the differences between the individual sensors readings and the

ATSR blackbody crossover test performed on 21st of April 2009.

Table 5Results from ATSR-2 blackbody cross-over test performed on 21st and of 22nd April2004.

1995 results 2004 results

+XBB −XBB +XBB −XBB

3.7 μm −0.0047 0.0024 0.0115 −0.013811 μm −0.0027 0.0010 0.0049 −0.006012 μm −0.0015 0.0009 0.0058 −0.0069

15D. Smith et al. / Remote Sensing of Environment 116 (2012) 4–16

base plate averages are well maintained. Fig. 8 suggests both that thetemperature differences across the blackbodies and also that therelative calibrations of the PRTs have not changed, linking well to pre-launch temperature baselines.

It is also possible in flight to compare signals as measuredradiometrically from both blackbodies in what has come to beknown as a “blackbody cross-over test” by switching the heaters overbetween the +XBB and –XBB blackbodies (and allowing thetemperatures to “cross-over” and stabilise, and then to reverse thesettings back to normal). The basic idea is to compare the radiometricsignals in the thermal channels when the two blackbodies are atidentical indicated temperatures. Any significant difference wouldimply a drift in the blackbody thermometer calibration, an aroundscan stray, or a change in one emissivity caused by a deterioration ofthe black surface finish. To draw conclusions from these data one hasto take account of some esoteric blackbody data collection timings andrates of change of temperature. The blackbody temperatures andradiometric signals during a typical blackbody cross-over test forAATSR are shown in Fig. 9.

For ATSR-2, the test was performed during the commissioningphase and repeated on 14 and 15 July 2004. For the second test the on-board tape drives of ERS-2 had not been functioning since July 2003,and hence science data were only available when the spacecraft wasin range of the Kiruna ground station. To ensure that the cross-overoccurred during the 10 min of Kiruna visibility it was thereforenecessary to schedule the commands using timings calculated fromthe 1995 test results, a challenge that was successfully achieved.

Results for ATSR-2 commissioning showed that there were nosignificant differences between the two on-board blackbodies. The2004 results show that although there has been some drift, therelative radiometric differences between the ATSR-2 blackbodieswere less than 0.05 K, Table 5.

For AATSR, the test was performed during commissioning and isnow performed roughly annually. The results for AATSR indicate that,relative to each other, the brightness temperature errors from theblackbodies are less than 10 mK at 11 and 12 μm, and below 20 mK at3.7 μm. Comparing with earlier measurements, Fig. 10, it can be seenthat the 11 and 12 μm channels are stable over time, whilst thereappears to be a very slow increase in the 3.7 μm channel of

+XBB Temperat

0

2

4

6

8

10

12

14

16

18

20

Jan-02 Jan-03 Jan-04 Jan-05 Jan

Tem

per

atu

re (

mK

)

3.7µm

11µm

12µm

Trend (3.7µm)

Trend (11µm)

Trend (12µm)

Fig. 10. Blackbody temperature uncertain

approximately 6 mK over 7 years. Even for the trend in this channel,the onewith the lowest blackbody emissivity, the apparent brightnesstemperature difference is still much smaller than the radiometricnoise.

It should be noted that the test is a comparison of one-blackbodyagainst the other on the assumption that the reference is stable, andtherefore does not provide an absolute calibration of the blackbodies.Although this not a direct verification of the overall radiometric error,the results combined with the verified consistency of the blackbodythermometry and signal channel gains and offsets, Fig. 5, suggest thatthe radiometric performance is being maintained.

The blackbodies were developed for ATSR (Mason et al., 1996). Aspart of this process extremely demanding experiments wereperformed directly measuring emissivity. Also thermometry waschecked for stability over temperature cycling and for extendedtimelines. With this bedrock it is hardly surprising that ATSRradiometry is good.

However, although in-flight the cross-over tests exercise much ofthe overall radiometry function of the instruments; even they areclearly not an independent absolute radiance re-calibration. There areat least two further techniques that have been employed on ATSRs' IRchannels. First, radiances as received at the satellite can be correlatedbetween satellites, especially when they orbit in tandem. Alterna-tively the SST/surface radiance product itself can be validated incampaigns involving satellite overpasses. Many campaigns have beenperformed to support the validation of satellite SSTs that demonstratethat the ATSRs radiometric performance is achieved. Examples ofATSR validation results can be found in Corlett et al. (2006) andO'Carroll et al. (2006).

7. Conclusions

By viewing external blackbody sources and operating the in-struments in vacuo under flight conditions, included simulated orbits,the pre-launch calibration activities of the ATSR series have clearlydemonstrated that the infrared radiometric accuracies to be betterthan 0.05 K traceable to ITS-90. The accuracy is maintained around thefull instrument swath and under different thermal conditions. Alsosuch calibration tests demonstrate that the radiometric noiseperformance for each instrument was below 0.05 K in all channelsat a reference temperature of 270 K.

In-orbit monitoring has verified that the on-board radiometryincluding calibration sources have remained stable throughoutmission lifetimes. This is confirmed by blackbody cross-over testsperformed for ATSR-2 and AATSR, which imply that any drift of thethermometry and/or emissivity of the two on-board sources wasbelow 0.02 K.

For the early ATSR, the main factor affecting the scientificperformance has been the gradual increase in detector temperature

ure Uncertainty

-06 Jan-07 Jan-08 Jan-09 Jan-10

ties from the AATSR cross-over tests.

16 D. Smith et al. / Remote Sensing of Environment 116 (2012) 4–16

during the mission due to factors discussed. This aside, all analyses ofthe data are consistent with the ATSR series having provided state-of-the-art stable precision radiometry throughout their missions.

Acknowledgements

The authors would particularly like to thank Jack Abolins, JohnWright and BrianMaddison of RAL, Gary Corlettt and David Llewellyn-Jones from Leicester University, Francois Bousquillon de Freschville,Serge Paturaud, Hugues Dufort and Jean Noel Berger of ESOC, PhillipeGoryl, Wolfgang Lengert and Pascal Lecomte of ESRIN and MiguelCanella from ESTEC and many others for their inputs over the yearswithout whom this work would not have been possible.

ATSR and ATSR-2 were originally funded by the Science andEngineering Research Council (SERC) with responsibility beingtransferred to the Natural Environment Research Council (NERC).AATSR was funded by the UK Department of Environment Food andRural Affairs (DEFRA), and now the UK Department for Energy andClimate Change (DECC).

ATSR and ATSR-2 data are provided courtesy of the EuropeanSpace Agency (ESA), NERC, the British National Space Centre (BNSC)and RAL.

AATSR data are provided courtesy of the NERC Earth ObservationData Centre (NEODC), the UK Department for Energy and ClimateChange (DECC) and ESA.

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