SPEXone is a compact multi-angle channeled spectropolarimeter with five viewing angles and hyperspectral imaging of a ~100 km swath with a spatial resolution of 4.6 x 5.4 km in the along track and across-track directions and a spatial oversampling ratio of 2. SPEXone has a spectral range from 385 to 770 nm and a spectral resolution slightly below 2 nm. High accuracy polarimetry is achieved by implementation of the dual-beam snapshot spectral polarization modulation concept, yielding two complementary spectrally modulated hyperspectral images of the same scene from which both radiance and state of linear polarization can be extracted.

The automated in-flight monitoring of the SPEXone instrument include in-orbit stability of the thermal subsystem, the optics, and the detector. The results of the in-flight monitoring are presented in separate tabs. For more data, you can consult Richard van Hees.

 

SPEXone radiometric inter-comparison

 

The radiance measured by the SPEXone instrument is compared to the radiance measured by the OCI instrument. This is done for specific regions, listed in the table below. Only pixels with a solar zenith angle below 60 degrees and a homogeneous cloud-region of 50×50 km2 are chosen. For the pixels passing these requirements and the region-specific requirements the radiance is taken from both instruments and the SPEXone radiance is divided by the OCI instrument. A monthly mean is
taken of this fraction and is shown in the plots. A separate plot is made for each region and each plot has a separate line for each chosen wavelength.

Region id General location Latitude range Longitude range Requirements
0 Pacific Ocean TBD TBD • Clear sky
1 Pacific Ocean TBD TBD • Fully cloudy
• Cloud optical thickness > 10
2 Sahara Desert TBD TBD • Clear sky
3 Antarctica TBD TBD • Clear sky
4 Railroad Valley TBD TBD • Clear sky
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SPEXone dark stability

Dark measurements are performed during eclipse, mostly outside the South Atlantic Anomaly (SAA), with the detector in Diagnostic mode (full-frame) and Science mode (binned). For the stability analysis, all measurements performed during one day are averaged and the unbiased standard deviation (uncertainty) is determined. In the trend plots, the detector average are shown as a function of time. The relevant instrument settings are listed below. The results of the dark algorithm are also used in pixel-quality analysis.

MPS ID Mode Texp [ms] Co-additions Repetitions
46 Science 60 5 28
47 Science 64 5 28
132 Diagnostic 0.4 32 4
162 Diagnostic 60 32 4
163 Diagnostic 64 32 4
187 Diagnostic 6000 16 2
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SPEXone housekeeping (trends)

The relevant housekeeping parameters for stability of the SPEXone instrument are listed in the Table below. The reports of these parameters are generated from PACE HKT products. These products contain the instrument telemetry sampled at 1 Hz, which are averaged over 30 seconds for the daily plots and averaged over a full orbit in the trend plots. The mean value is shown as a purple line and the range between the minimum and maximum as a grey band.

Description Units Nominal Lower limit Upper limit
TS1_DEM_N_T C 18.33 18.25 18.41
TS2_HOUSING_N_T C 19.61 19.55 19.67
TS3_RADIATOR_N_T C 0.5 -1.9 1.6
TS4_DEM_R_T C 17.9 17.65 18.0
TS5_HOUSING_R_T C 19.3 19.05 19.55
TS6_RADIATOR_R_T C 0.6 -1.9 1.6
Heater 2 power W 3.5 3 4.5
Heater 3 power W 4.25 3.5 4.5 (7)
Detector temperature C 21 19 22
ICU DIGV C 32 31 33
ICU heater-group 1 C 46 45 47
ICU heater-group 2 C 43 42 44
ICU MCU-RAM C 34 33 35
ICU midboard C 33.5 32.5 34
DEM supply mA 495 430 510
ICU 1.2V bus mA 167.5 165 170
ICU 3.3V bus mA 175 165 190
ICU 4V bus mA 210 200 235
ICU 5V bus mA 485 435 500
DEM supply V 4.77 4.76 4.81
ICU 1.2V bus V 1.245 – –
ICU 3.3V bus V 3.269 3.268 3.270
ICU 4V bus V 3.905 3.89 3.915
ICU 5V bus V 4.855 4.845 4.88
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SPEXone polarimetric zero-point

 

A selection of the smallest DoLP (degree of linear polarization) measurements is collected for the verification over bright clouds (cloud optical thickness > 30) and a scattering angle between 160 and 180 degrees. In these cases, it is expected that the DoLP is near-zero (within 0.001). From the pixels that pass the requirements the mean and median value of the smallest 1% of DoLP values is taken. The mean and median are shown as a monthly average. A separate plot is made for each viewport and each plot has a separate line for each chosen wavelength.

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SPEXone radiometric inter-angle

 

The non-nadir radiance measured by SPEXone over deep convective clouds (DCCs) is collected and compared to the nadir radiance. The definition of DCCs is given in the table below. As such the Iviewport/Inadir is calculated where Iviewport is the radiance of a non-nadir viewport (-50, -20, 20, 50 degrees) and Inadir is the radiance of the nadir viewport (0 degrees). The monthly mean Iviewport/Inadir value is plotted. A separate plot is made for each viewport and each plot has a separate line for each chosen wavelength. Besides the measured Iviewport/Inadir, these two values are also theoretically calculated using a radiative transfer (RT) calculation. These values are also averages over a month and thus allow a comparison of the measured Iviewport/Inadir mean and the RT-based calculated
Iviewport/Inadir mean.

 

Criteria for DCC Characteristics Motivation
Reflectivity at 750 nm > 0.7 Top-of-atmosphere (TOA) signal is
dominated by cloud		 TOA signal can be well predicted
Inter-tropical locations Favorable areas for convection Convective cloud
Measurements over
ocean		 Negligible surface contribution
(outside glint)		 Cloud signal dominates TOA signal
Cloud top height > 10 km Negligible contribution from
aerosols		 Cloud signal dominates TOA signal
RMS of radiance at 770
nm < 3% (from OCI)		 Homogeneous area 1D RT calculation is sufficient
Solar zenith angle < 30
degrees		 Minimize shadow effects 1D RT calculation is sufficient
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