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3.2.1 Pre-flight characteristics and expected performance

This section describes the pre-flight characteristics and expected performance of AATSR in the following areas:

These subsections rely on the results of the pre-launch testing and analysis of AATSR, as well as AATSR's specifications, to describe the expected performance.

3.2.1.1 Spectral

3.2.1.1.1 Specifications

The choice of wavebands for AATSR was derived from the scientific requirements for sea surface temperature and land surface mapping (for more detail, see section 1.1.2. ). For continuity of the SST and land surface reflectance data sets, which commenced with ATSR-1 and ATSR-2, the channel responses of the AATSR 0.55 µm, 0.66 µm, 0.87 µm, 1.6 µm, 3.7 µm, 11 µm and 12 µm channels were specified to have similar spectral shapes to those on ATSR-1 and ATSR-2.

The AATSR signal channel end-to-end response characteristics as specified for AATSR are shown in table 3.2 .

Table 3.2 AATSR Specified Spectral Responses
Channel / µm Centre Wavelength / µm 50% of Peak / µm Error on 50% Points / µm Slope 5% - 80% / µm <1 % of Peak / µm
0.55 0.555 0.545 - 0.565 ± 0.003 < 0.008 0.530 - 0.580
0.66 0.659 0.649 - 0.669 ± 0.003 < 0.008 0.634 - 0.684
0.87 0.865 0.855 - 0.875 ± 0.003 < 0.008 0.840 - 0.890
1.6 1.61 1.58 - 1.64 + 0.01/- 0.04 < 0.30 1.52 - 1.70
3.7 3.70 3.55 - 3.93 ± 0.06 < 0.12 3.40 - 4.12
11 10.85 10.40 - 11.30 ± 0.09 < 0.34 9.80 - 11.90
12 12.00 11.50 - 12.50 ± 0.09 < 0.37 10.90 - 13.10

In addition to the specifications in table 3.2 , requirements were set for the out of band response for each AATSR channel. The peak out of band response was specified as less than 10-4 of the peak in-band response and the out of band response for each channel was specified as contributing less than 1 part in 4095 of the total signal integrated over wavelength.

From ATSR-2 experience, it was understood that for the 0.55 µm, 0.66 µm and 0.87 µm channels, the tolerances of the half power points given in table 3.2 are not independent for each edge. In order to meet the spectral width specification, a further requirement was necessary, to specify that the difference between the half power points (i.e. the width) for the 0.55 µm, 0.66 µm and 0.87 µm channels needs to be 20 nm, with a tolerance of -0/+2 nm.

3.2.1.1.2 Measurements

Measurements of the spectral responses were made for the Focal Plane Assembly and generally the above specifications were met. The full set of spectral response measurements can be downloaded here. The measured response curves for the AATSR channels can also be seen in the plots below. Click on each plot to view a larger version.

Figure 3.9
Figure 3.10
Figure 3.11
Figure 3.12
Figure 3.13
Figure 3.14
Figure 3.15

The out of band response proved difficult to measure and quantify accurately, but the results showed that generally the out of band responses were less than 10-3 of peak response.

Further details of the AATSR spectral characterisation are available on request from the Rutherford Appleton Laboratory (initial contact to be made via EOHelp@esa.int).

3.2.1.2 Spatial

3.2.1.2.1 Description and Specifications

AATSR has two ~500 km wide curved swaths, with 555 pixels across the nadir swath and 371 pixels across the forward swath (see figure1.1 in the User Guide). Each pixel (or sample) is defined by integrating the detector signals over 75 ±0.75 µs as the scan mirror rotates, thus giving 2000 samples per 150 ms scan (only the useful samples of the earth views and calibration sources are transmitted to the ground, and are selected using a pixel map).

The nominal Instantaneous Field of View (IFOV) is specified to be 1 x 1 km at the centre of the nadir swath. With ENVISAT orbiting at 800 km, the angular IFOV set at the field stop in the Focal Plane Assembly (FPA) is specified to be a square 1/777 rad (265 arc seconds) ±1.44% by 1/777 rad (265 arc seconds) ±1.44%, which gives a surface projected IFOV at nadir of 1.03 km by 1.03 km.

The varying viewing angle to the Earth's surface which results from AATSR's conical scan 1.1.3. affects the projection of the IFOV on the ground. As already described, at nadir, the projection of the IFOV is 1 x 1 km, but at the centre of the forward view, it is 1.5 x 2 km.

The Field of View (FOV) along-track has a response shape like the IFOV, varying from a 1 km length at nadir to 2 km length at the centre of the forward view due to the angle it is projected onto the Earth (see figure1.1 in the User Guide). Around the scan (which is across-track at nadir and at the centre of the along-track view - see figure1.3 in the User Guide), the FOV is the IFOV convolved with itself for its displacement during one 75 µs integration. Thus, assuming that the response across the IFOV is uniform, the FOV in the around-scan direction has an approximately triangular shape. As the nominal sampling distance is approximately 1 km, at nadir the FOV is a 1 km wide half-height triangle, with a base width of 2 km. Figure3.16 illustrates the calculation and shape of the FOV around the scan.

Calculation of the FOV from the IFOV
Figure 3.16 Calculation of the FOV from the IFOV around the scan

Co-registration of the different spectral channels is important, and the specification is for errors in optical and electrical alignment between channels not to exceed 0.1 sample distance at nadir. This is helped by having a single field stop, but the integration periods for each channel must also be accurately synchronised.

One specific difference in AATSR's FOV from ATSR-1 and ATSR-2 results from a change to the orientation of the 11 µm and 12µm detectors. It is known from previous ATSR missions that the detectors used for these channels have a response variation across their sensitive areas, which is related to electrical bias. This causes a variation in the response over the IFOV in one dimension. In previous ATSRs, this response variation was imaged along-track. For AATSR, the construction of the detectors has been altered to minimise the inherent variation and the detectors are oriented so that the variation now appears in the across-track direction.

3.2.1.2.2 Measurements

The instrument level measurements of the IFOV (which were made at RAL) are described here.

The objectives of the tests were:

  • To map out the IFOV of each signal channel to a resolution of better than 10% of a pixel.
  • To measure the relative alignment of each channel.
  • To determine and quantify the effect on the IFOV due to significant distortion of the instrument's fore optics as a result of changes in the thermal environment between BOL and EOL thermal conditions.
  • To determine whether the instrument optical alignment is affected by outgassing of the carbon fibre structure.

The IFOV of each channel was mapped out by scanning the image of a point source across the instrument's field stop and recording the detector's response as a function of position.

Two methods for mapping the IFOV were used:

  • A 'static' method where the instrument's scan mirror is kept stationary and the image of the point source is steered across the field stop in both azimuth and elevation.
  • A 'dynamic' method, where the instrument's scan mirror is rotating, using a gimbal mounted mirror to move the image in the along-track direction and the scan mirror to move the image in the across-track direction.

The results of the measurements are described in detail in RAL's FOV Test Report Ref. [1.2 ] and are summarised below. The static IFOV measurements are shown in figure3.17 and figure3.18 and summarised in table 3.3 , with the dynamic measurements shown in figure3.19 .

VFPA static IFOV results
Figure 3.17 IFOV maps for 0.87 µm, 0.66 µm and 0.56 µm channels in air (the dashed lines show the nominal IFOV as defined by the FPA's field stop)

IRFPA static IFOV results
Figure 3.18 IFOV maps for 12 µm, 11 µm, 3.7 µm and 1.6 µm channels in air (the dashed lines show the nominal IFOV as defined by the FPA's field stop)

Table 3.3 Widths and centre (relative to 0.87µm co-alignment scans) of AATSR static IFOV maps
Channel Width of IFOV (arc seconds) Centre of IFOV (arc seconds)
Along-Track Across-Track Along-Track Across-Track
12µm 235 245 6 -5
11µm 223 235 -1 -3
3.7µm 246 253 -6 6
1.6µm 259 259 1 -3
0.87µm 261 258 0 0
0.66µm 262 255 -2 5
0.56µm 263 256 -1 8

Dynamic response measurements
Figure 3.19 Dynamic response measurement for the IR channels (solid lines). These are compared against the integrated, static IFOVs in the across-track direction (dashed lines)

In conclusion, the results of the measurements at RAL (see RAL's FOV Test Report Ref. [1.2 ] ) showed that the centres of the IFOVs for all AATSR channels agree to within 5% of a sample period. The widths of the 3.7 µm, 1.6 µm, 0.87 µm, 0.66 µm and 0.55 µm channels are within 10% of the expected IFOV defined by the field stop. The 12 µm and 11 µm channels are also well centred but were smaller in area. This is due to the smaller active areas of the detectors.

The IFOV map of the 0.87 µm channel did not show any significant differences between ambient, BOL or EOL conditions.

The drift in alignment between ambient and BOL conditions is less than 0.1 pixel in the along-track direction and 1 pixel in the across-track direction. This will have no effect on the beam clearance. Between BOL and EOL, there was a 60% shift in the across-track direction. However, the drifts induced under normal orbital variations will be much less and should have a minimal effect on geolocation.

The dynamic response tests demonstrated that there were no observable effects on the IFOVs of the 12 µm, 11 µm and 1.6 µm channels. The 3.7 µm channel does show some slight difference just as for ATSR-2, which may be the result of a longer time constant in the detector/preamplifier combination.

RAL and the Principal Investigator concluded that the IFOV results were acceptable for flight.

Further comments on the implication of the true FOV on the exploitation of AATSR data, particularly over land, are given in Section 2.12.1. .

3.2.1.3 Pointing

3.2.1.3.1 Alignment and platform pointing

AATSR's pointing has to be known and stable. Thus, the angle of the scan cone for AATSR is specified to be 46.90° (tolerance ±1.0 arc minutes), with one cone side including the nadir, and the cone axis pointing 23.45° (tolerance ±0.5 arc minutes) forward from nadir. The Scan Mirror normal is, therefore, inclined at an angle of 11.725° (tolerance ±0.25 arc minutes). These angles are shown in figure3.20 below. AATSR also has an alignment cube, so that AATSR's alignment can be referenced to the spacecraft alignment and pointing.

Scan cone characteristics (4K)
Figure 3.20 Scan Cone Characteristics

Measurements have shown that the scan cone axis deviates slightly outside specification as it lies within 2.5 arc minutes of the ideal position at 23.478°. The measurements of the scan cone characteristics and the alignment between AATSR and the platform are used in AATSR data processing (see section).

ENVISAT's pointing accuracy and stability (add link to satellite-level information) do not present any constraints on AATSR's performance (e.g. for geolocation).

3.2.1.3.2 Pointing performance of the scan mechanism

AATSR uses the same technique as ATSR-1 and ATSR-2 of using a scan reference pulse, provided once per scan, in conjunction with the difference from the time of the pulse to work out where the scan mirror is pointing on the ground at any time around the scan. Therefore, the scan mechanism must rotate uniformly, so as not to introduce errors in the pointing for an individual sample.

To give an example, imagine that the scan mechanism runs very slightly slow. Thus, the instrument is actually viewing a point on the scan slightly behind where it would be expected to be if the mechanism was running at the correct speed. Using the time since the scan reference pulse to locate the sample would thus introduce a pointing error. Unfortunately, ATSR-2's scan mechanism has suffered problems with minor rotation speed variations or "jitter", which can affect the products. Extensive effort has gone into AATSR to reduce the possibilities of similar problems to those experienced at times with ATSR-2.

To minimise the pointing errors introduced by non-constant scan mechanism rotation, AATSR is specified such that the scan mechanism shall rotate at a sufficiently uniform angular velocity that its maximum positional error in angle of rotation, from that implied by assuming a uniform rate, shall be less than 5 arc minutes at any position around the scan, when measured in air. This performance in air is expected to be equivalent to an error of 1 arc min (i.e. 0.09 sample distance) (1 sigma RMS) in vacuum. Test results have shown that the worst case normal mode error budget is ±2.4 arc minutes.

The AATSR scan mechanism had backup systems introduced after the problems experienced in-flight with ATSR-2. The pointing performance of the backup Phase Locked Loop mode is not as good as that of the main system, with the worst case backup mode error budget being ±4.8 arc minutes.

The stability of the sample spacing around the scan could also be affected by mechanical disturbances from AATSR mechanisms (other than the scan mechanism) and any satellite mechanisms. Thus, AATSR is specified such that mechanisms shall affect the pointing of the scan vector by less than 0.1 sample distance.

3.2.1.4 Radiometric

3.2.1.4.1 Specifications

From the scientific requirements for SST and land surface mapping, the dynamic range and noise performance required for the AATSR channels have been derived and are shown in table 3.4 .

Table 3.4 Dynamic Range and Noise Performance for a Single Calibrated AATSR Sample
Channel/µm Nominal Working Range NEDelta (1K)T at 270K/K S/N at 0.5% Albedo
0.55 0 - 50 mW cm-2 µm-1 sr-1 N/A 20:1
0.66 0 - 45 mW cm-2 µm-1 sr-1 N/A 20:1
0.87 0 - 30 mW cm-2 µm-1 sr-1 N/A 20:1
1.6 0 - 7 mW cm-2 µm-1 sr-1 N/A 20:1
3.7 0 - 311 K 0.08 N/A
11 200 - 321 K * 0.05 N/A
12 200 - 318 K * 0.05 N/A

* - Extended range or low gain mode (previously known as forest fire mode on ATSR-2) can extend these to higher values (see section of the User Guide), although it is not part of baseline planning to use this mode.

AATSR has 12 bit digitisation, with the full range downlinked for all channels all of the time.

So that AATSR can extract the maximum performance from each channel's detector/preamplifier combination, the instrument includes a system to automatically optimise the signal channel offsets for all channels, with the gain of the thermal channels also automatically optimised. Visible channel gains are commanded manually. The gain and offset for all channels is set up using the onboard calibration systems.

Responsivity can be affected by the polarisation of the radiance, and AATSR was specified so that the difference in response between any two orthogonal polarisations was to be not more than 4% for the thermal channels and 6% for the reflection channels. In addition, for all channels, the response variation with plane of polarisation was specified to be known to better than 0.5%. Unfortunately, the specification was not met, and the actual performance is described below.

3.2.1.4.2 Measurements

3.2.1.4.2.1 Polarisation

Polarisation measurements were made at FPA level and instrument level. Concerning the FPA level measurements, some non-compliances with the polarisation specification were found and the results are summarised in table 3.5 .

Table 3.5 FPA-level Measurements of Variation in Response with Polarisation
Channel/µm Variation in Response with Polarisation
0.55 1-2%
0.66 14-15%
0.87 3.5-4.5%
1.6 33-35%
3.7 3-5%
11 3.5-7%
12 0-6%

Ref. [1.10 ] Polarisation measurements were also performed at instrument level for the visible and 1.6 µm channels (see RAL's Visible Calibration Report) and the results are summarised in table 3.6 .

Table 3.6 Instrument-level Measurements of Variation in Response with Polarisation
Channel/µm Variation in Response with Polarisation
0.55 2%
0.66 15%
0.87 10%
1.6 40%

The magnitude of the polarisation variation is in good agreement with the FPA-level measurements, with the main difference being that the 0.87 µm channel appears to be slightly more sensitive.

3.2.1.4.2.2 Instrument-level visible channel results

The objectives of the Visible Calibration were:

  • Measure the normalised detector response as a function of input intensity over the expected operational range to better than 5%.
  • Determine and measure any scan dependent effects by measuring the response at all points around the nadir and along track views and VISCAL opal.
  • Characterise the variation of the normalised radiometric response due to polarisation to better than 0.5% (the results are discussed above).
  • Measure the radiometric noise.

The test results are discussed below (details are provided in RAL's Visible Calibration Report Ref. [1.10 ] ).

The tests showed that the 0.87 µm, 0.66 µm and 0.55 µm channel responses were linear with input radiance, whereas the 1.6 µm channel was found to have a significantly non-linear response (13.4% for radiances corresponding to full albedo). The non-linearity of the 1.6 µm channel has been quantified and a calibration algorithm derived for use within the Level 1B processing (see section). The radiometric responses of the other VNIR channels are assumed to be linear in the data processing. Measurements of the radiometric responses with the FPA evacuated and with the IRFPA at ambient temperatures have been selected for use in the data processing as representing the conditions seen at the start of the mission. The errors in the radiometric response measurements are summarised in table 3.7 .

Table 3.7 Errors in Radiometric Response Results
Channel/µm Error in Radiometric Response Measurements
0.55 3.9%
0.66 3.4%
0.87 3.6%
1.6 9.0%

The measured signal-to-noise ratios are summarised in table 3.8 .

Table 3.8 Signal-to-Noise Ratio Results
Channel/µm S/N Ratio at 0.5% albedo
0.55 25:1
0.66 28:1
0.87 25:1
1.6 31:1

Variations in the radiometric calibration around the scan were <1%, although it is likely that the observed variations were caused by inaccuracy in positioning the source.

The reflectance factor of the VISCAL was measured for all channels and was found to be in good agreement with prediction. An interesting result was the 10% variation of signal across the VISCAL in the 1.6µm channel, which was very similar to in-flight data from ATSR-2. This does not arise from features of the VISCAL, but is caused by non-uniform response across the main aperture, and will not affect the in-flight calibration of the channel. The predictions and measurements are summarised in table 3.9 .

Table 3.9 Measured VISCAL Reflectance Factors compared with the expected values calculated from the geometry and optical properties
Channel/µm VISCAL Reflectance Factor
Measured Expected
0.55 0.163±0.004 0.165±0.003
0.66 0.163±0.005 0.163±0.003
0.87 0.154±0.003 0.156±0.004
1.6 0.191±0.006 0.192±0.005

In conclusion, the measurements showed that the visible channels meet the performance requirements.

3.2.1.4.2.3 Instrument-level infra-red results

The principles behind the calibration of the thermal infrared channels are the same as those for the calibration tests performed on ATSR-1 and 2. These are to:

  • Verify the 'on-board' radiometric calibration for a range of target temperatures between 210K and 310K.
  • Measure any detector non-linearities and make appropriate corrections.
  • Verify that the different channels produce self consistent results.
  • Measure the radiometric noise.
  • Measure the effect on the radiometric calibration with the detectors operating at an increased temperature.
  • Verify the calibration with the on-board black bodies set at different power levels.
  • Investigate the radiometric performance under different thermal conditions.
  • Determine and measure any scan dependent variation in the radiometric performance.
  • Determine and measure radiometric leaks.
  • Verify the calibration under simulated orbital transient thermal conditions.

The test results are discussed below (details are provided in RAL's Infra-red Radiometric Calibration Report Ref. [1.5 ] ).

The calibration of the 3.7µm, 11µm and 12µm channels was verified against precision black body targets having an accuracy of ±0.037K. The thermometer, resistance and emissivity calibrations of these targets can be traced to international standards. In particular, the rhodium-iron resistance thermometers have been calibrated to the International Temperature Scale of 1990 (ITS90).

The main radiometric calibration was performed with the instrument operating under Beginning-of-Life thermal balance conditions, for target temperatures between 210K and 315K. The initial results showed that the detectors had non-linear responses similar to those observed on ATSR-2. These data were used to derive non-linearity corrections for the Level 1B processing (see section).

Measurements taken with the external black bodies matched to the on-board target temperatures revealed an emissivity error at 3.7 µm. It was concluded that the true emissivities of the on-board black bodies were best represented by calculated values based on reflectance measurements of witness samples.

After applying the corrections, the brightness temperatures at 270K were within 30 mK of the target temperatures measured by the RIRTs. These results and the NEDelta (1K)T measurements are summarised in table.

T for a target temperature of 270K with the FPA at 80K" rel="eph_tab">
Channel/µm TAATSR-TRIRT / K NEDelta (1K)T / K (FPA at 80K)
3.7 -0.014 0.037
11 -0.030 0.025
12 -0.020 0.025

The calibration was also performed at all points around the nadir and along-track swaths (see figure1.3 in the User Guide for the swath pattern), under different thermal environments and different on-board black-body power settings. The results showed that there were no significant scan dependent effects or radiometric leaks.

Although the orbital simulations did not return data under the planned conditions, the full sequence of orbits was carried out and the results show that the calibration was not affected by the transient thermal environment of the tests.

In conclusion, the measurements showed that the 3.7 µm, 11 µm and 12 µm channels meet the performance requirements.

3.2.1.5 Stability

AATSR's responses need to be stable on a number of timescales, from around the scan right up to the mission duration.

To minimise the possibilities of stray signals reaching the detectors around the scan, view baffles are necessary to keep out all direct sunlight during normal instrument operation. Similarly, the instrument enclosure must also be light-tight (other than for necessary apertures) and internally must be generally of low reflectivity to minimise the possibility of stray signals reaching the detectors at any point around the scan. The scan mirror is also sufficiently specular so that no significant signals from outside the telescope beam are passed back to the detectors. The mirror's scattering caused by surface roughness is less than 0.05% for the shortest wavelength AATSR channel. Verification that AATSR's measurements did not vary around the scan took place during radiometric calibration 3.2.1.4.2. , with no problems being observed.

The response of the thermal channels has to be stable over the period between forward and nadir views of the Earth's surface, so as not to introduce large errors into forward/nadir differencing algorithms. Items of the instrument which are "visible" to the detectors must remain stable in temperature over the time period between forward and nadir views. Each of the items was assigned an error budget of 0.02K over the two minutes between views, assuming that the 12µm channel is viewing a 265K scene and the instrument is at 264K. From the ATSR-1/2 design, this corresponded to stability requirements on the paraboloid mirror aperture stop of 0.2K/120s and on the focal plane assembly baffle of 0.8K/120s. Modelling, correlated with test results, has shown that the temperature stability requirements should all be comfortably met.

The onboard calibration sources are also designed to be stable. Uncertainties in the black body calibration source radiance must not exceed the equivalent of 100mK temperature error so that the overall SST accuracy requirements to be met. The SST retrieval algorithms also require that the radiances of the BBCs are stable over the period between nadir and forward views (approximately two minutes). Therefore, the BBCs are designed so that their temperatures are stable to better than 0.03K over a two minute period and have been shown to meet this requirement.

For land reflectance measurements, the stability of the reflection channel gains has to be to 0.1% over the time between forward and nadir views and 1% around the orbit.

AATSR is also designed so that the linearity of the electronics for each signal channel is not expected to change by >0.5LSB between pre-launch beginning of life (BOL) non-linearity assessment and end of life (EOL) flight performance.