Intercomparison of airborne multi-angle polarimeter observations from the Polarimeter Definition Experiment
Kirk Knobelspiesse, Qian Tan, Carol Bruegge, Brian Cairns, Jacek Chowdhary, Bastiaan van Diedenhoven, David Diner, Richard Ferrare, Gerard van Harten, Veljko Jovanovic, Matteo Ottaviani, Jens Redemann, Felix Seidel, and Kenneth Sinclair
Kirk Knobelspiesse,1,*
Qian Tan,2,3
Carol Bruegge,4
Brian Cairns,5
Jacek Chowdhary,5,6
Bastiaan van Diedenhoven,5,6
David Diner,4
Richard Ferrare,7
Gerard van Harten,4
Veljko Jovanovic,4
Matteo Ottaviani,5,8
Jens Redemann,9
Felix Seidel,4
and Kenneth Sinclair5,6
1NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
2NASA Ames Research Center, Moffett Field, California 94035, USA
3Bay Area Environmental Research Institute, Petaluma, California 94952, USA
4Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
5NASA Goddard Institute for Space Studies, New York, New York 10025, USA
6Columbia University, New York, New York 10027, USA
7NASA Langley Research Center, Hampton, Virginia 23681, USA
8SciSpaceLLC, Bethesda, Maryland 20817, USA
9University of Oklahoma, Norman, Oklahoma 73072, USA
Kirk Knobelspiesse, Qian Tan, Carol Bruegge, Brian Cairns, Jacek Chowdhary, Bastiaan van Diedenhoven, David Diner, Richard Ferrare, Gerard van Harten, Veljko Jovanovic, Matteo Ottaviani, Jens Redemann, Felix Seidel, and Kenneth Sinclair, "Intercomparison of airborne multi-angle polarimeter observations from the Polarimeter Definition Experiment," Appl. Opt. 58, 650-669 (2019)
In early 2013, three airborne polarimeters were flown on the high altitude NASA ER-2 aircraft in California for the Polarimeter Definition Experiment (PODEX). PODEX supported the pre-formulation NASA Aerosol–Cloud–Ecosystem (ACE) mission, which calls for an imaging polarimeter in polar orbit (among other instruments) for the remote sensing of aerosols, oceans, and clouds. Several polarimeter concepts exist as airborne prototypes, some of which were deployed during PODEX as a capabilities test. Two of those instruments to date have successfully produced Level 1 (georegistered, calibrated radiance and polarization) data from that campaign: the Airborne Multiangle Spectropolarimetric Imager (AirMSPI) and the Research Scanning Polarimeter (RSP). We compared georegistered observations of a variety of scene types by these instruments to test whether Level 1 products agreed within stated uncertainties. Initial comparisons found radiometric agreement, but polarimetric biases beyond measurement uncertainties. After subsequent updates to calibration, georegistration, and the measurement uncertainty models, observations from the instruments now largely agree within stated uncertainties. However, the 470 nm reflectance channels have a roughly bias of AirMSPI relative to RSP, beyond expected measurement uncertainties. We also find that observations of dark (ocean) scenes, where polarimetric uncertainty is expected to be largest, do not agree within stated polarimetric uncertainties. Otherwise, AirMSPI and RSP observations are consistent within measurement uncertainty expectations, providing credibility for the subsequent creation of Level 2 (geophysical product) data from these instruments, and comparison thereof. The techniques used in this work can also form a methodological basis for other intercomparisons, for example, of the data gathered during the recent Aerosol Characterization from Polarimeter and Lidar (ACEPOL) field campaign, carried out in October and November of 2017 with four polarimeters (including AirMSPI and RSP).
J. Martijn Smit, Jeroen H. H. Rietjens, Gerard van Harten, Antonio Di Noia, Wouter Laauwen, Brian E. Rheingans, David J. Diner, Brian Cairns, Andrzej Wasilewski, Kirk D. Knobelspiesse, Richard Ferrare, and Otto P. Hasekamp Appl. Opt. 58(21) 5695-5719 (2019)
Gerard van Harten, David J. Diner, Brian J. S. Daugherty, Brian E. Rheingans, Michael A. Bull, Felix C. Seidel, Russell A. Chipman, Brian Cairns, Andrzej P. Wasilewski, and Kirk D. Knobelspiesse Appl. Opt. 57(16) 4499-4513 (2018)
S. Stamnes, C. Hostetler, R. Ferrare, S. Burton, X. Liu, J. Hair, Y. Hu, A. Wasilewski, W. Martin, B. van Diedenhoven, J. Chowdhary, I. Cetinić, L. K. Berg, K. Stamnes, and B. Cairns Appl. Opt. 57(10) 2394-2413 (2018)
Neranga K. Hannadige, Peng-Wang Zhai, Meng Gao, Bryan A. Franz, Yongxiang Hu, Kirk Knobelspiesse, P. Jeremy Werdell, Amir Ibrahim, Brian Cairns, and Otto P. Hasekamp Opt. Express 29(3) 4504-4522 (2021)
Kirk Knobelspiesse, Brian Cairns, Michael Mishchenko, Jacek Chowdhary, Kostas Tsigaridis, Bastiaan van Diedenhoven, William Martin, Matteo Ottaviani, and Mikhail Alexandrov Opt. Express 20(19) 21457-21484 (2012)
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Instrument Characteristics for the Polarimeters Used During the PODEX Field Campaigna
Instrument
Polarimetric Analysis Method
Imager?
Polarimetric Uncertainty
Number of View Angles Per Pixel
Nadir Ground Resolution
Band Center Wavelengths, nm
Observations per Pixel
AirMSPI
Photoelastic modulation
yes
Sw: 0.3%
Sw: 1
Sw: 25 m
355, 380, 445, 470,
up to 210
SaS: 0.5%
SaS: 9-15
SaS: 10 m
555, 660, 865, 935
PACS
Philips prism, linear polarizers
yes
unknown
65
37 m
470, 550, 670, 766, 870
975
RSP
Wollaston prisms
no
0.15%
155
277 m
410, 470, 555, 670, 865, 960, 1590, 1880, 2250
4,185
Note that AirMSPI has two targeting modes. The “Sweep” (abbreviated Sw) mode scans a wide along-track field of view, and provides multi-angular views of extended scenes in which the different view angles are not spatially co-registered. The “Step and Stare” (abbreviated SaS) mode observes a shorter along-track target at multiple spatially co-registered viewing angles, typically 9 or 15 during PODEX. Both modes are mapped to different ground spatial resolutions, and have subsequently different measurement uncertainties. Spatial resolutions are reported for nadir views from a typical ER-2 aircraft altitude (19 km). Boldface for band centers indicates polarization sensitivity.
Table 2.
RSP Relative Gain Coefficients of the Detectors Measuring Orthogonal Polarizations in Telescopes Measuring and (K1) and in Telescopes Measuring and (K2)
Date
K1 (470 nm)
K1 (670 nm)
K1 (865 nm)
K2 (470 nm)
K2 (670 nm)
K2 (865 nm)
5/20/12
0.97704
0.98149
1.00105
0.97742
1.01994
0.98632
5/8/13
0.97599
0.98108
1.00157
0.97551
1.02034
0.98500
5/2/16
0.97511
0.98116
1.00117
0.97724
1.02141
0.98498
12/15/16
0.97516
0.98115
1.00094
0.97772
1.02126
0.98499
Mean
0.97583
0.98122
1.00118
0.97697
1.02074
0.98532
Standard Deviation
0.09%
0.02%
0.03%
0.10%
0.07%
0.07%
Table 3.
RSP Radiometric Calibration Coefficients in Digital Numbers Pre- and Post-PODEX and Since that Field Campaign for Telescopes Measuring and (Cal1) and Telescopes Measuring and (Cal2)
Date
Cal1 (470 nm)
Cal1 (670 nm)
Cal1 (865 nm)
Cal2 (470 nm)
Cal2 (670 nm)
Cal2 (865 nm)
5/20/12
17254.9
18106.5
20287.6
17426.2
19328.2
19100.6
5/8/13
17138.5
18013.8
20133.1
17303.6
19226.2
18971.3
5/2/16
17189.9
18122.5
20347.0
17340.8
19307.6
19164.9
12/15/16
16909.7
18124.2
20431.7
17064.8
19318.3
19250.6
Mean
17123.2
18091.8
20299.8
17283.9
19295.1
19121.8
Standard deviation
150.1
52.5
125.9
154.8
46.7
117.7
Relative standard deviation
0.9%
0.3%
0.6%
0.9%
0.2%
0.6%
Table 4.
Comparison Scene Characteristics, Including Mean and DoLP Values
Differences are calculated as RSP–AirMSPI. The critical value for 4., the correlation of normalized difference () with respect to , is calculated to be 0.077 based upon the sample size.
Table 7.
Statistics for Comparison Dataset Excluding Coastal Ocean Scenes, a
(470 nm)
(660/670 nm)
(865 nm)
DoLP (470 nm)
DoLP (660/670 nm)
DoLP (865 nm)
1. Pearson correlation
0.990
0.985
0.979
0.998
0.992
0.978
2. Linear regression slope
0.922
0.963
0.964
1.014
0.959
0.989
3. Linear regression Y-intercept
0.008
0.017
0.030
−0.001
0.001
−0.001
4 Difference correlation
−0.001
−0.131
−0.176
0.201
−0.235
0.063
5. Bias (norm. difference, )
−1.53
0.03
0.68
0.41
−0.67
−0.75
6. Upper limit of agreement
0.47
2.32
3.04
3.68
1.15
1.24
7. Lower limit of agreement
−3.53
−2.26
−1.69
−2.87
−2.49
−2.75
8. Percentage of
35.6%
8.4%
13.0%
15.6%
7.5%
8.4%
Differences are calculated as RSP–AirMSPI. The critical value for 4., the correlation of normalized difference () with respect to is 0.092. Confidence intervals for (5.) are roughly for all bands, and for limits of agreement (LOA) (6., 7.) roughly .
Table 8.
Statistics for Comparison Dataset with Only Coastal Ocean Scenes, a
(470 nm)
(660/670 nm)
(865 nm)
DoLP (470 nm)
DoLP (660/670 nm)
DoLP (865 nm)
1. Pearson correlation
0.999
0.999
0.999
0.999
0.999
0.996
2. Linear regression slope
0.953
1.004
1.062
1.060
1.043
1.067
3. Linear regression Y-intercept
−0.001
−0.001
−0.001
−0.010
−0.007
−0.002
4. Difference correlation
0.745
0.820
0.558
0.877
0.706
0.113
5. Bias (norm. difference, )
3.19
6. Upper limit of agreement
4.90
7. Lower limit of agreement
1.47
8. Percentage of
0.0%
0.0%
0.6%
87.8%
64.1%
91.2%
Differences are calculated as RSP–AirMSPI. The critical value for 4., the correlation of normalized difference () with respect to is 0.144. Only for DoLP(865 nm) can be considered independent of , so and limits of agreement (LOA) were calculated for this band alone. The confidence intervals for (5.) is and for LOA (6., 7.) roughly .
Table 9.
AirMSPI and RSP Data Availability
AirMSPI, ellipsoid projected (cloud and ocean scenes):
DOI 10.5067/AIRCRAFT/AIRMSPI/PODEX/RADIANCE/ELLIPSOID_V005
AirMSPI, terrain projected (land scenes):
DOI 10.5067/AIRCRAFT/AIRMSPI/PODEX/RADIANCE/TERRAIN_V005
RSP, all data
https://data.giss.nasa.gov/pub/rsp/PODEX
Table 10.
Spectrally Dependent AirMSPI Uncertainty Model Parameters
Detector noise floor, with scaling for normalized radiance
Shot noise parameter, with scaling for normalized radiance
0.005
0.002
Relative gain coefficient characterization uncertainty
0.015
0.03
Absolute radiometric characterization uncertainty
0.002
0.002
Polarimetric characterization uncertainty
Cosine of the solar zenith angle
Intensity reflectance
Polarized reflectance
DoLP
Degree of linear polarization,
Some coefficient values are different for the pair of instruments (RSP1 and RSP2). RSP2 was used during PODEX, so the uncertainty model for those coefficients was used in this study.
Note the “CloudC” scene was used for validation of cloud retrievals (case 1) in [11].
Tables (12)
Table 1.
Instrument Characteristics for the Polarimeters Used During the PODEX Field Campaigna
Instrument
Polarimetric Analysis Method
Imager?
Polarimetric Uncertainty
Number of View Angles Per Pixel
Nadir Ground Resolution
Band Center Wavelengths, nm
Observations per Pixel
AirMSPI
Photoelastic modulation
yes
Sw: 0.3%
Sw: 1
Sw: 25 m
355, 380, 445, 470,
up to 210
SaS: 0.5%
SaS: 9-15
SaS: 10 m
555, 660, 865, 935
PACS
Philips prism, linear polarizers
yes
unknown
65
37 m
470, 550, 670, 766, 870
975
RSP
Wollaston prisms
no
0.15%
155
277 m
410, 470, 555, 670, 865, 960, 1590, 1880, 2250
4,185
Note that AirMSPI has two targeting modes. The “Sweep” (abbreviated Sw) mode scans a wide along-track field of view, and provides multi-angular views of extended scenes in which the different view angles are not spatially co-registered. The “Step and Stare” (abbreviated SaS) mode observes a shorter along-track target at multiple spatially co-registered viewing angles, typically 9 or 15 during PODEX. Both modes are mapped to different ground spatial resolutions, and have subsequently different measurement uncertainties. Spatial resolutions are reported for nadir views from a typical ER-2 aircraft altitude (19 km). Boldface for band centers indicates polarization sensitivity.
Table 2.
RSP Relative Gain Coefficients of the Detectors Measuring Orthogonal Polarizations in Telescopes Measuring and (K1) and in Telescopes Measuring and (K2)
Date
K1 (470 nm)
K1 (670 nm)
K1 (865 nm)
K2 (470 nm)
K2 (670 nm)
K2 (865 nm)
5/20/12
0.97704
0.98149
1.00105
0.97742
1.01994
0.98632
5/8/13
0.97599
0.98108
1.00157
0.97551
1.02034
0.98500
5/2/16
0.97511
0.98116
1.00117
0.97724
1.02141
0.98498
12/15/16
0.97516
0.98115
1.00094
0.97772
1.02126
0.98499
Mean
0.97583
0.98122
1.00118
0.97697
1.02074
0.98532
Standard Deviation
0.09%
0.02%
0.03%
0.10%
0.07%
0.07%
Table 3.
RSP Radiometric Calibration Coefficients in Digital Numbers Pre- and Post-PODEX and Since that Field Campaign for Telescopes Measuring and (Cal1) and Telescopes Measuring and (Cal2)
Date
Cal1 (470 nm)
Cal1 (670 nm)
Cal1 (865 nm)
Cal2 (470 nm)
Cal2 (670 nm)
Cal2 (865 nm)
5/20/12
17254.9
18106.5
20287.6
17426.2
19328.2
19100.6
5/8/13
17138.5
18013.8
20133.1
17303.6
19226.2
18971.3
5/2/16
17189.9
18122.5
20347.0
17340.8
19307.6
19164.9
12/15/16
16909.7
18124.2
20431.7
17064.8
19318.3
19250.6
Mean
17123.2
18091.8
20299.8
17283.9
19295.1
19121.8
Standard deviation
150.1
52.5
125.9
154.8
46.7
117.7
Relative standard deviation
0.9%
0.3%
0.6%
0.9%
0.2%
0.6%
Table 4.
Comparison Scene Characteristics, Including Mean and DoLP Values
Differences are calculated as RSP–AirMSPI. The critical value for 4., the correlation of normalized difference () with respect to , is calculated to be 0.077 based upon the sample size.
Table 7.
Statistics for Comparison Dataset Excluding Coastal Ocean Scenes, a
(470 nm)
(660/670 nm)
(865 nm)
DoLP (470 nm)
DoLP (660/670 nm)
DoLP (865 nm)
1. Pearson correlation
0.990
0.985
0.979
0.998
0.992
0.978
2. Linear regression slope
0.922
0.963
0.964
1.014
0.959
0.989
3. Linear regression Y-intercept
0.008
0.017
0.030
−0.001
0.001
−0.001
4 Difference correlation
−0.001
−0.131
−0.176
0.201
−0.235
0.063
5. Bias (norm. difference, )
−1.53
0.03
0.68
0.41
−0.67
−0.75
6. Upper limit of agreement
0.47
2.32
3.04
3.68
1.15
1.24
7. Lower limit of agreement
−3.53
−2.26
−1.69
−2.87
−2.49
−2.75
8. Percentage of
35.6%
8.4%
13.0%
15.6%
7.5%
8.4%
Differences are calculated as RSP–AirMSPI. The critical value for 4., the correlation of normalized difference () with respect to is 0.092. Confidence intervals for (5.) are roughly for all bands, and for limits of agreement (LOA) (6., 7.) roughly .
Table 8.
Statistics for Comparison Dataset with Only Coastal Ocean Scenes, a
(470 nm)
(660/670 nm)
(865 nm)
DoLP (470 nm)
DoLP (660/670 nm)
DoLP (865 nm)
1. Pearson correlation
0.999
0.999
0.999
0.999
0.999
0.996
2. Linear regression slope
0.953
1.004
1.062
1.060
1.043
1.067
3. Linear regression Y-intercept
−0.001
−0.001
−0.001
−0.010
−0.007
−0.002
4. Difference correlation
0.745
0.820
0.558
0.877
0.706
0.113
5. Bias (norm. difference, )
3.19
6. Upper limit of agreement
4.90
7. Lower limit of agreement
1.47
8. Percentage of
0.0%
0.0%
0.6%
87.8%
64.1%
91.2%
Differences are calculated as RSP–AirMSPI. The critical value for 4., the correlation of normalized difference () with respect to is 0.144. Only for DoLP(865 nm) can be considered independent of , so and limits of agreement (LOA) were calculated for this band alone. The confidence intervals for (5.) is and for LOA (6., 7.) roughly .
Table 9.
AirMSPI and RSP Data Availability
AirMSPI, ellipsoid projected (cloud and ocean scenes):
DOI 10.5067/AIRCRAFT/AIRMSPI/PODEX/RADIANCE/ELLIPSOID_V005
AirMSPI, terrain projected (land scenes):
DOI 10.5067/AIRCRAFT/AIRMSPI/PODEX/RADIANCE/TERRAIN_V005
RSP, all data
https://data.giss.nasa.gov/pub/rsp/PODEX
Table 10.
Spectrally Dependent AirMSPI Uncertainty Model Parameters
Detector noise floor, with scaling for normalized radiance
Shot noise parameter, with scaling for normalized radiance
0.005
0.002
Relative gain coefficient characterization uncertainty
0.015
0.03
Absolute radiometric characterization uncertainty
0.002
0.002
Polarimetric characterization uncertainty
Cosine of the solar zenith angle
Intensity reflectance
Polarized reflectance
DoLP
Degree of linear polarization,
Some coefficient values are different for the pair of instruments (RSP1 and RSP2). RSP2 was used during PODEX, so the uncertainty model for those coefficients was used in this study.