Pre-Launch Polarization Assessment of JPSS-3 and -4 VIIRS VNIR Bands and Comparison with Previous Builds

Abstract

The Visible Infrared Imaging Radiometer Suite (VIIRS) instrument, deployed on multiple satellites including the Suomi National Polar-orbiting Partnership (S-NPP), National Oceanic and Atmospheric Administration 20 (NOAA-20), NOAA-21, Joint Polar Satellite System (JPSS-3), and JPSS-4 spacecraft, with launches in 2011, 2017, 2022, 2032, and 2027, respectively, has polarization sensitivity that affects the at-aperture radiometric Sensor Data Record (SDR) calibration in the Visible Near InfraRed (VNIR) spectral region. These SDRs are key inputs into the VIIRS atmospheric, land, and water Environmental Data Records (EDRs) that are integral to weather and climate applications. If the polarization sensitivity of the VIIRS instrument is left uncorrected, EDR quality will degrade, causing diminished quality of weather and climate data. Pre-launch characterization of the instrument’s polarization sensitivity was performed to mitigate this on-orbit calibration effect and improve the quality of the EDRs. Specialized ground test equipment, built specifically for the VIIRS instrument, enabled high-fidelity characterization of the instrument’s polarization performance. This paper will discuss the polarization sensitivity characterization test approach, methodology, and results for the JPSS-3 and -4 builds. This includes a description of the ground test equipment, instrument requirements, and how the testing was executed and analyzed. A comparison of the polarization sensitivity results of the on-orbit S-NPP, NOAA-20, and -21 instruments with the JPSS-3 and -4 VIIRS instruments will be discussed as well.

1. Introduction

The Visible Infrared Imaging Radiometer Suite (VIIRS) instrument is in a low Earth orbit aboard the Suomi National Polar-orbiting Partnership (S-NPP), National Oceanic and Atmospheric Administration 20 (NOAA-20), and NOAA-21 spacecraft with launches in 2011, 2017, and 2022, respectively [1]. Two more VIIRS instruments, Joint Polar Satellite System 3 (JPSS-3) and JPSS-4, will be launched in 2032 and 2027, respectively. Its mission is to provide observations of different geophysical phenomena such as land, sea, and ice surface temperatures, fires, Earth albedo, aerosols, and cloud properties [2]. This information is an important input into weather forecasting as well as global climate modeling [3,4]. VIIRS is also part of the historical climate dataset that includes observations from Advanced Very High Resolution Radiometer (AVHRR) [5] and Moderate Resolution Imaging Spectroradiometer (MODIS) [6], which spans more than 40 years. This dataset is vital for modeling changes in climate that predict future global impacts and requires radiometric and calibration consistency between these instruments over their missions. The improved radiometric calibration of VIIRS with respect to AVHRR provides an opportunity to perform vicarious calibrations to transfer the VIIRS calibration back through the historical climate dataset. This improves the fidelity of the climate dataset by removing calibration biases that may be present from heritage instrument calibration bias errors. Therefore, the performance of the VIIRS products is important in the present for weather forecasting and geophysical dynamics, as well as for understanding the future implications of climate change.

VIIRS has 22 bands at two spatial resolutions spread across four focal planes. There are 5 imaging (I-) bands with 32 detectors in track, with each having a nadir dynamic field of view (DFOV) of 375 m and 16 moderate resolution (M-) bands and a Day Night Band (DNB) with a nadir DFOV of 750 m and 16 detectors in track. The Visible Near Infra-Red (VNIR) focal plane has nine bands, M1–M7, I1 and I2, covering a spectral range of 0.402–0.900 µm, eight Shortwave Midwave Infra-Red (SMIR) bands, M8–M13, I3 and I4, with a spectral range of 1.230–4.130 µm and four Longwave Infra-Red (LWIR) bands, I5 and M14–M16, with a spectral coverage of 8.400–12.490 µm. The DNB has its own Focal Plane Assembly (FPA) and covers a spectral range of 0.500–0.900 µm.

Table 1. VIIRS band information that includes typical radiance or temperature for each gain stage (high, low, and variable gain), maximum radiance or temperature, and signal-to-noise ratio or noise-equivalent-delta temperature.

Band	Ground FOV (m)	Spectral Range (µm)	Band Gain	Ltyp or Ttyp	Lmax or Tmax	SNR or NEdT
VNIR
DNB	750	0.500–0.900	VG	0.00003	200	6
M1	750	0.402–0.422	High	44.9	135	352
			Low	155	615	316
M2	750	0.436–0.454	High	40	127	380
			Low	146	687	409
M3	750	0.478–0.498	High	32	107	416
			Low	123	702	414
M4	750	0.545–0.565	High	21	78	362
			Low	90	667	315
I1	375	0.600–0.680	Single	22	718	119
M5	750	0.662–0.682	High	10	59	242
			Low	68	651	360
M6	750	0.739–0.754	Single	9.6	41	199
I2	375	0.846–0.885	Single	25	349	150
M7	750	0.846–0.885	High	6.4	29	215
			Low	33.4	349	340
SWIR
M8	750	1.230–1.250	Single	5.4	165	74
M9	750	1.371–1.386	Single	6	77.1	83
I3	375	1.580–1.640	Single	7.3	72.5	6
M10	750	1.580–1.640	Single	7.3	71.2	342
M11	750	2.225–2.275	Single	0.12	31.8	10
MWIR
M12	750	3.660–3.840	Single	270	353	0.396
M13	750	3.973–4.128	High	300	343	0.107
			Low	380	634	0.423
I4	375	3.550–3.930	Single	270	353	2.5
LWIR
M14	750	8.400–8.700	Single	270	336	0.091
M15	750	10.263–11.263	Single	300	343	0.07
I5	375	10.500–12.400	Single	210	340	1.5
M16	750	11.538–12.488	Single	300	340	0.072

lists the 22 VIIRS bands’ spectral, spatial, radiometric, and noise requirements. The nominal orbital altitude of its spacecraft is ~824 km with an equatorial crossing of 13:30 Universal Coordinated Time and a swath width of 3000 km. Each VIIRS has a temporal frequency of two visits per day, but with two VIIRS 180° out of phase, their combined temporal frequency improves to ~6 h.

VIIRS has two main product types: the Sensor Data Record (SDR) and Environmental Data Record (EDR) [7,8,9]. The SDRs contain a geolocated and radiometrically calibrated product with top-of-atmosphere (TOA) radiance, reflectance, or brightness temperatures as its main deliverable. EDRs use ancillary data combined with the SDRs to create geophysical products such as sea surface temperature (SST), cloud cover/layers, active fires, and ocean color/chlorophyll. There are a total of 26 EDRs produced by VIIRS with SDR as their key input [10]. The VIIRS EDRs are banded together with EDRs from other instruments onboard the S-NPP or NOAA-20/21 spacecraft to facilitate the weather model’s forecasts and monitor the global properties and dynamics of the different geophysical phenomena [11].

The VIIRS SDR radiometric calibration uses a combination of pre-launch and on-orbit measurements. The pre-launch measurements characterize the instrument’s response to known ground test equipment (GSE) to enable a National Institute of Standards and Technology (NIST) traceable calibrated scene that is uniform in both the scan and track directions. This includes characterization of the internal on-orbit sources such as the Solar Diffuser (SD), SD stability monitor (SDSM), and on-board blackbody calibrator (OBC). On-orbit, the internal calibration sources are used to monitor response changes in the instrument and remove them from the SDR product to maintain a consistent calibrated performance over the mission. Both the pre-launch and on-orbit calibration inputs provide an SDR radiometric product for an unpolarized scene. However, TOA Earth scenes are not always unpolarized. If not accounted for, the partial polarization of the TOA scene can cause biases in the SDR calibration and impact the performance of EDR. This can lead to striping and bias in the ocean color/chlorophyll products that can significantly degrade their ability to assess the required geophysical parameters of interest [12,13]. Therefore, the instrument’s sensitivity to polarization is characterized pre-launch to help correct for this effect. The pre-launch polarization characterization data is used as a Look-Up Table (LUT) input into EDR algorithms to remove this effect in their product. This paper will discuss the JPSS-3 and -4 polarization sensitivity measurements and compare them to the VIIRS builds currently on-orbit.

2. Materials and Methods

2.1. VIIRS Optical Design

The VIIRS instrument’s optical design sensitivity to polarization needs to be understood because the observed TOA radiance can be partially polarized. Rayleigh scattering in the atmosphere in the blue spectrum, glint off the ocean surface, and atmospheric aerosols are examples of Earth scenes that can create significant polarization effects in the TOA radiance. EDRs, such as ocean color/chlorophyll, that view these scenes need to consider polarization of the TOA radiance since the SDR radiance being reported is for an unpolarized scene. Figure 1 shows an example of the ocean color/chlorophyll EDR product water leaving radiance for S-NPP VIIRS. The top row shows band M1 with (a) and without (b) a polarization correction. The bottom row is band M2 with (c) and without (d) a polarization correction [13]. The striping observed in the first column is due to a mixture of polarization effects and residual HAM side calibration errors for band M1. The right-hand column shows how the striping in the water leaving radiance is reduced by implementing the VIIRS polarization sensitivity characterization correction into the EDR product.

There is a VIIRS requirement, see

Table 2. VIIRS polarization sensitivity requirements for each VNIR band with the center wavelength, as well as maximum allowable polarization amplitude and characterization uncertainty, are listed.

Band	Wavelength (nm)	Maximum Amplitude for ±45° Scan Angle	Characterization Uncertainty Requirement in Percent
M1	412	3.0	0.5
M2	445	2.5	0.5
M3	488	2.5	0.5
M4	555	2.5	0.5
M5	672	2.5	0.5
M6	746	2.5	0.5
M7	865	3.0	0.5
I1	640	2.5	0.5
I2	865	3.0	0.5

, to limit the polarization sensitivity of the instrument so that the required EDR TOA radiance correction is relatively small. This reduces the residual EDR uncertainty due to SDR polarization adjustments.

The VIIRS polarization requirement was a driver in the development of the instrument’s optical design. The VIIRS optical design, shown in Figure 2, consists of three main assemblies: the Rotating Telescope Assembly (RTA), the Half-Angle Mirror (HAM), and the aft-optics. The RTA is an afocal three-mirror anastigmat telescope and fold mirror that consists of all reflective optics that rotate 360° to capture light from the ±56.03° Earth View and three internal calibrators. The HAM is a flat mirror that rotates at half the speed of the RTA and directs the output energy from the RTA to the non-rotating aft-optics. The aft-optics has a reflective telescope to focus the RTA output onto 4 different Focal Plane Assemblies (FPAs). This is accomplished using two dichroic beam splitters in combination with steering mirrors to guide the spectral energy to the proper FPA. The VNIR and DNB FPAs receive reflected light off dichroic 1. The transmitted energy from dichroic 1 is further split by dichroic 2, with the reflected energy reaching the SMIR FPA and the transmitted light off both dichroics reaching the LWIR FPA. The VNIR FPA is not temperature-controlled; the DNB is temperature-controlled close to the nominal ambient temperature of the instrument. A dewar with a window for each of the SMIR and LWIR FPAs is used to cool the FPAs to anywhere from 80 to 82.5 K, depending on the specific VIIRS build.

The same silver coating design is used for each mirror in the optical system, with an s and p reflectance that is dependent on the orientation of the incident polarization. The mirror’s s and p reflectance values are spectrally and angle of incidence (AOI) dependent. The optical AOIs within the RTA and aft-optics are fixed for each detector as a function of the HAM scan’s AOI but do vary detector-to-detector and band-to-band. The HAMs AOIs vary from 29.60° to 56.47° across the Earth view scan, resulting in a cross-track dependence on the instrument’s polarization sensitivity. The sophisticated nature of the dichroic’s dielectric layers makes these optical elements prone to having divergent s and p polarization reflectance and transmission values that are spectral and angular in dependence. Another key element to the instrument’s polarization performance is the bandpass filters. They have numerous dielectric layers that can create spectrally dependent s and p polarization transmission values. The initial VIIRS design selected a silver mirror coating, dichroic, and bandpass filters to meet all the instrument requirements, including polarization sensitivity. As the instrument’s optical elements evolved from build-to-build due to modifications for specification, non-compliance, or workmanship differences, the polarization sensitivity has changed between the VIIRS instruments [14]. The impacts of these optical changes will be discussed in Section 3.3.

2.2. VIIRS Polarization Ground Test Equipment

The pre-launch testing of the VIIRS polarization sensitivity and characterization methodology is covered here briefly. A more detailed description of how the testing is performed can be found in the documentation of the NOAA-20 VIIRS polarization results [15]. The pre-launch JPSS-3 and -4 VIIRS polarization testing data were collected at Raytheon Intelligence and Space in El Segundo, California, in 2020 and 2021, respectively. The Ground Support Equipment (GSE) used to characterize the VIIRS polarization sensitivity is the Polarization Test Source Assembly (PTSA). The PTSA, shown in Figure 3, has a 100 cm Spherical Integration Source (SIS) with a wide FOV of unpolarized illumination. The FOV fully illuminates the VIIRS entrance aperture and can adjust its intensity to maximize the instrument response across all bands. A polarization sheet is between the SIS and the VIIRS sensor to polarize the energy from the source. A spectral blocking filter and stray light baffling are also part of the PTSA to limit unwanted energy from reaching the detectors. The blocking filter removes spectral energy greater than 0.625 µm so that bands M1–M3 do not include the out-of-band spectral response in the polarization sensitivity characterization. Since the on-orbit solar irradiance is much higher in the blue spectral region than in the red and near infra-red (NIR) regions and the SIS is brightest in the red/NIR, the out-of-band would be unequally weighted in bands M1–M3 polarization sensitivity characterization. Hence, the blocking filter is needed to reduce the uncertainty in the characterization of these bands. Historically, there have been two different types of sheet polarizers (BVONIR and BVO777) that are used during the VIIRS polarization testing and can easily be interchanged onto the rotation stage with excellent repeatability. The sheets are mounted on a rotation stage that rotates 360° in 15° increments, resulting in 25 VIIRS measurements per rotation. The orientation of the polarization sheet with respect to the VIIRS coordinate frame is shown in Figure 4. A second polarization sheet can be mounted between VIIRS and the rotary stage. This fixed polarization sheet, of the same material as the mounted sheet, can be used to determine the polarization efficiency of the two sheets. When the polarization electric fields of the two sheets are parallel, the transmission through the sheets is at its maximum, and when they are orthogonal, full extinction of the light to VIIRS should occur. Since the polarizer sheets are not perfectly efficient, some unpolarized light reaches the VIIRS aperture when the sheets’ transmission axis orientations are orthogonal. The signal modulation with the cross-polarization configuration is observed by VIIRS to determine the polarization efficiency of the sheets and remove its effects from the polarization sensitivity characterization. The PTSA performance is characterized before the test to understand the uncertainties of the source and is used as inputs into the characterization requirement in Table 2. This includes knowledge of the rotation stage accuracy, straylight influences, and polarization sheet uniformity.

2.3. VIIRS Polarization Analysis Method

The VIIRS polarization analysis methodology is illustrated in Figure 5. The polarization sensitivity test (PST) is performed in ambient conditions with the instrument mounted on a rotation stage and scanning horizontally. The PTSA is positioned so that the full VIIRS aperture is illuminated and the instrument scanning so that both HAM sides, A and B, view the source.

VIIRS requires a dark source to remove the digital number offset, and this is achieved by viewing the internal On-Board Calibrator Blackbody (OBCBB) in each scan. Equation (1) shows the offset corrected digital number (dn).

$$dn\left(b,d,ms,\theta ,\phi \right)=\frac{\sum _{scan}\left(\overline{{DN}_{EV}^{sample}}-\overline{{DN}_{OBCBB}^{sample}}\right)}{N_{scan}}$$

(1)

The lower case dn is background corrected while the capital DN is not. The dns are averaged over ~30 samples, and all scans of the PTSA Earth View (DN_EV) values are subtracted by the dark digital numbers of the OBCBB (DN_OBCBB). A set of dns is measured across scan angle θ, polarization angle φ, band b, detector d, and HAM side ms. All three polarization sensitivity measurement configurations (straylight, cross-polarizer, and VIIRS polarization) use the same dn technique from Equation (1). A 2-phi Fourier filtering fit is performed for each band, detector, and scan angle measurement to characterize the polarization modulation across polarization angles. This is shown in Equation (2),

$$dn\left(b,d,ms,\theta \right)=\frac{1}{2}{a}_0+a_2\cos 2\phi +b_2\sin 2\phi$$

(2)

where the dn and the polarization angle φ are the dependent and independent variables, respectively. The ½ a₀ term is the dc offset of the measurement that corresponds to the average dn signal over the 360° polarization angle rotation. The a₂ and b₂ capture the 2φ modulation of the dns over the 360° polarization angle rotation and are used to model the instrument’s polarization sensitivity. The fit residuals of Equation (2) are used to estimate the measurement uncertainty quality. The non-2φ modulations in the dns are most likely a result of detector noise or PTSA artifacts contaminating the measurements. These residuals provide information about the quality of the measurements and are included in the uncertainty analysis. They also inform the test execution and can be used to determine if a re-measurement is necessary. The a₀, a₂, and b₂ are inputs into the polarization amplitude (PA) and phase (β), as shown in Equations (3) and (4), respectively.

$$PA=\frac{2\sqrt{a_2^2+b_2^2}}{a_0}*\frac{1}{F_{cross}}$$

(3)

$$\beta =\frac{\mathit{atan}2\frac{b_2}{a_2}}{2}$$

(4)

$$F_{cross}=\sqrt{\frac{2\sqrt{a_{2\_cross}^2+b_{2\_cross}^2}}{a_{0\_cross}}}$$

(5)

The sheet polarizer efficiency is measured using the cross-polarization of two sheets and is shown in Equation (5). It uses the Fourier fit-in Equation (2) to get the $a_{2\_cross}$ and $b_{2\_cross}$ values. A square root of the cross-polarization amplitude using $a_{2\_cross}$ and $b_{2\_cross}$ is required because the PTSA source is traveling through two sheets. The F_cross term removes the polarization sheet inefficiency in Equation (3) by scaling the PA value determined by the single sheet measurement. The F_cross term assumes the polarization efficiency of both sheets (the fixed and rotation stage ones) are the same, so the square root can be used to determine a single sheet’s efficiency. Ideally, the F_cross value will be 1, but it falls somewhere between 0.5 and 0.99, depending on the band and sheet type. Equation (3) uses the band average F_cross with the assumption that the polarizer efficiency is not detector and HAM side dependent. The uncertainty analysis captures any additional errors resulting from the band and HAM averaging assumptions.

The data analysis results in a PA and β for each band, detector, HAM side, and scan angle that are plotted in polar plots with the radius term representing the PA and polar angle corresponding to twice the β angle (this is due to the 2φ covering 0° to 180° while the polar plot is a full 360° angular extent). These polar plots allow a visual representation of the PA and β behavior across the detector, scan angle, and HAM side for each band. Instrument design characteristics, like element surface AOI changes across detector or scan, can be correlated with the polar plots to investigate the root causes of the observed polarization sensitivities.

3. Results

3.1. JPSS-3 and -4 VIIRS Polarization Straylight Results

PTSA straylight testing used two different configurations to identify light contamination. In the first, VIIRS views the PTSA with the SIS off, and the polarization sheet is rotated to look for laboratory light contamination. In the second configuration, the PTSA SIS is on, but a lollipop is placed between VIIRS and the polarization sheet. The sheet is rotated, and out-of-field straylight from the SIS is identified. Equations (1)–(3) are used to process the straylight measurements, with the a₀, PA, and β angles evaluated. A non-zero a₀ term would bias the polarization sensitivity characterization and would affect the PA value in the denominator of Equation (3). Any modulation in the straylight signal would affect the polarization sensitivity measurement’s PA and β angle by mixing two 2φ signals. Figure 6 (JPSS-3) and Figure 7 (JPSS-4) show an example of dark signal dns as a function of polarization angle and detectors (colors) for band M4 HAM B and M2 HAM B, respectively. A Fourier fit, with cycles up to 6φ, is overlayed for each detector to look for period behaviors in the dns versus polarization angle. The dn response is less than ~4 dn for all detectors, with no significant impact on the polarization signal response. There is also no significant 2φ component in the dn variation over the polarization angle that would affect the PA or β angle.

Table 3. JPSS-3 and -4 VIIRS signal for the dark and lollipop normalized by the SIS signal for each VNIR band, showing how the dark signal relates to the polarization sensitivity measurement signal.

	JPSS-3		JPSS-4
Band	Median of Dark dn/SIS dn	Median of Lollipop dn/SIS dn	Median of Dark dn/SIS dn	Median of Lollipop dn/SIS dn
M1	0.00074	0.00069	0.00063	0.00058
M2	0.00008	0.00013	0.00007	0.00011
M3	0.00004	0.00010	0.00004	0.00009
M4	0.00015	0.00037	0.00014	0.00034
M5	0.00016	0.00021	0.00015	0.00020
M6	0.00006	0.00009	0.00005	0.00008
M7	0.00013	0.00013	0.00012	0.00013
I1	0.00013	0.00019	0.00012	0.00016
I2	0.00006	0.00010	0.00005	0.00009

lists the median of the ratio of the dn from the dark measurements to the SIS signal during the test for both JPSS-3 (left) and -4 (right). The dn response levels are within the noise of the detectors and not large enough to influence the polarization sensitivity characterization. A straylight contamination PA using these a₂ and b₂ coefficients with the a₀ from a nominal SIS illumination level is used as an input into the uncertainty analysis.

3.2. JPSS-3 and -4 VIIRS Cross-Polarization Results

The PTSA sheet efficiency was determined using the cross-polarizer configuration in conjunction with VIIRS. The methodology discussed in the previous section was used to evaluate the VIIRS response and determine each band’s sheet polarization efficiency. Figure 8 (JPSS-3) and Figure 9 (JPSS-4) show examples of the band M1 response during the cross-polarization test for all detectors and HAM side A as a function of the polarizer rotation angle. A 2φ frequency signal modulation can be observed with the square root of the amplitude corresponding to the PTSA efficiency and the phase providing the transmission axis orientation of the polarizer sheet.

Table 4. JPSS-3 VIIRS cross-polarizer results showing the resulting PTSA efficiencies for each VNIR band.

Band	Band Average Amplitude	Band Sigma Amplitude	Band Average Phase	Band Sigma Phase	NIR Correction Factors
M1	0.9673	0.0017	173.8812	0.0145	0.9835
M2	0.9722	0.0008	173.8497	0.0090	0.9860
M3	0.9745	0.0003	173.8338	0.0055	0.9872
M4	0.9736	0.0013	173.8203	0.0090	0.9867
M5	0.9736	0.0005	173.7641	0.0056	0.9867
M6	0.9743	0.0003	173.7731	0.0053	0.9870
M7	0.9664	0.0002	173.7209	0.0059	0.9831
I1	0.9762	0.0006	173.7727	0.0080	0.9880
I2	0.9706	0.0003	173.7100	0.0067	0.9852

(JPSS-3) and

Table 5. JPSS-4 VIIRS cross-polarizer results showing the resulting PTSA efficiencies for each VNIR band.

Band	Band Average Amplitude	Band Sigma Amplitude	Band Average Phase	Band Sigma Phase	NIR Correction Factors
M1	0.9664	0.0017	177.7562	0.0098	0.9831
M2	0.9719	0.0006	177.7434	0.0065	0.9859
M3	0.9745	0.0003	177.7300	0.0000	0.9871
M4	0.9686	0.0010	177.6884	0.0085	0.9842
M5	0.9688	0.0007	177.6072	0.0046	0.9843
M6	0.9691	0.0003	177.6316	0.0037	0.9844
M7	0.9605	0.0003	177.5853	0.0051	0.9800
I1	0.9710	0.0007	177.6305	0.0072	0.9854
I2	0.9645	0.0003	177.5897	0.0035	0.9821

(JPSS-4) list the cross-polarization results for each VNIR band using the BVONIR sheet. Bands M1–M3 were measured with the PTSA configured with the Sonoma filter, while the remaining bands were measured without spectral attenuation. The polarizer sheet efficiency is above 98% for all bands for both JPSS-3 and JPSS-4 measurements. This is consistent with previous VIIRS measurements and shows that the polarization sheets have not changed significantly during the years between tests. The correction applied to the measured PAs in Equation (3) is less than 2%. The phase is ~6° and ~2° rotated away from zero for JPSS-3 and -4, respectively. This is most likely due to the fixed polarization sheet’s transmission axis not being perfectly aligned with the rotation stage. Since the phase is not used in the polarization correction in Equation (3), it does not impact the VIIRS polarization sensitivity analysis. A band-averaged polarization sheet efficiency value was used in the PST analysis, and the impact of averaging across bands was included in the uncertainty analysis discussed later.

3.3. JPSS-3 and -4 VIIRS Polarization Results

The PST results are shown in this section using polar plots of the polarization sensitivity. The x- and y-axis in the polar plots correspond to the a₂ and b₂ terms from Equation (2), respectively. From the origin of the polar plot, a vector can be used to evaluate the polarization amplitude (magnitude of this vector) and phase (half of the angle of the vector with respect to the positive x-axis). There are multiple colors on the polar plot that correspond to each unique scan angle. There are 16- or 32-line connected symbols within each color corresponding to each detector of the band of interest. Detector 1 (in instrument order) has a larger symbol to identify the detector order of the points on a connected line. Visualizing the polarization sensitivity in polar plot fashion allows for amplitude, phase, and scan angle variations to be more easily compared and patterns in their behavior to be examined.

The JPSS-3 and JPSS-4 polarization sensitivity for band M1 HAM A, all detectors, and scan angles are shown in Figure 10 and Figure 11, respectively. The band M1 amplitude meets the 3% requirement across the ±45° scan angles measured for both VIIRS builds. The scan variation is similar between builds, but the detector-to-detector shape varies. This is most likely due to workmanship differences in the dichroic beam splitter 1.

Table 6. JPSS-3 maximum detector PA (in percentage) for all VNIR bands, scan angles, and HAM sides from PST.

		Band
	Scan Angle	M1	M2	M3	M4	M5	M6	M7	I1	I2
HAM A	−55.5	1.992	1.413	1.015	0.920	1.494	0.713	1.307	0.788	1.419
	−45	1.763	1.303	0.950	1.090	1.419	0.554	1.253	0.868	1.464
	−37	1.580	1.177	0.845	1.167	1.382	0.473	1.284	0.870	1.503
	−30	1.616	1.204	0.855	1.273	1.416	0.397	1.345	0.941	1.572
	−20	1.627	1.112	0.797	1.361	1.368	0.338	1.385	0.931	1.618
	−15	1.708	1.142	0.820	1.393	1.324	0.327	1.405	0.928	1.643
	−8	1.862	1.176	0.846	1.434	1.308	0.347	1.437	0.933	1.673
	4	2.032	1.222	0.863	1.489	1.262	0.373	1.492	0.915	1.728
	22	2.292	1.375	0.968	1.533	1.181	0.390	1.560	0.892	1.797
	45	2.551	1.528	1.070	1.543	1.050	0.374	1.615	0.795	1.851
	55.5	2.628	1.589	1.105	1.532	1.009	0.365	1.645	0.792	1.873
HAM B	Scan Angle	M1	M2	M3	M4	M5	M6	M7	I1	I2
	−55.5	2.233	1.545	1.101	0.964	1.478	0.727	1.350	0.807	1.462
	−45	1.958	1.414	1.019	1.111	1.440	0.557	1.263	0.890	1.475
	−37	1.733	1.259	0.897	1.201	1.410	0.466	1.287	0.893	1.505
	−30	1.754	1.278	0.908	1.288	1.435	0.393	1.341	0.946	1.569
	−20	1.698	1.171	0.833	1.396	1.382	0.327	1.384	0.952	1.615
	−15	1.811	1.199	0.862	1.410	1.349	0.339	1.409	0.954	1.647
	−8	1.938	1.242	0.887	1.466	1.326	0.356	1.434	0.954	1.669
	4	2.118	1.270	0.893	1.506	1.267	0.383	1.488	0.938	1.726
	22	2.410	1.425	1.002	1.560	1.193	0.398	1.562	0.910	1.798
	45	2.674	1.603	1.116	1.570	1.045	0.367	1.608	0.803	1.846
	55.5	2.759	1.662	1.163	1.544	1.021	0.355	1.637	0.814	1.868

and

Table 7. JPSS-4 maximum detector PA (in percentage) for all VNIR bands, scan angles, and HAM sides from PST.

		Band
	Scan Angle	M1	M2	M3	M4	M5	M6	M7	I1	I2
HAM A	−55.5	2.592	1.626	1.097	0.939	1.512	1.061	1.242	0.745	1.415
	−45	2.203	1.471	1.005	1.028	1.416	0.835	1.204	0.739	1.296
	−37	1.991	1.403	0.942	1.134	1.321	0.663	1.160	0.777	1.353
	−30	1.866	1.358	0.919	1.203	1.325	0.553	1.221	0.791	1.428
	−20	1.754	1.288	0.876	1.309	1.307	0.419	1.307	0.822	1.526
	−15	1.722	1.265	0.857	1.345	1.304	0.384	1.345	0.836	1.565
	−8	1.719	1.225	0.839	1.395	1.276	0.340	1.389	0.840	1.616
	4	1.758	1.222	0.828	1.440	1.245	0.303	1.458	0.824	1.685
	22	1.997	1.340	0.937	1.460	1.180	0.310	1.551	0.803	1.783
	45	2.224	1.477	1.016	1.490	1.102	0.346	1.646	0.741	1.889
	55.5	2.261	1.528	1.057	1.499	1.075	0.355	1.681	0.721	1.926
HAM B	Scan Angle	M1	M2	M3	M4	M5	M6	M7	I1	I2
	−55.5	2.740	1.704	1.147	0.957	1.511	1.087	1.262	0.754	1.437
	−45	2.306	1.524	1.040	1.026	1.416	0.851	1.215	0.740	1.307
	−37	2.078	1.437	0.972	1.141	1.326	0.675	1.152	0.778	1.345
	−30	1.951	1.385	0.939	1.216	1.326	0.559	1.219	0.795	1.422
	−20	1.808	1.321	0.894	1.311	1.308	0.426	1.309	0.823	1.527
	−15	1.753	1.296	0.874	1.356	1.305	0.384	1.351	0.837	1.571
	−8	1.748	1.244	0.853	1.417	1.276	0.343	1.395	0.838	1.621
	4	1.802	1.251	0.841	1.446	1.247	0.303	1.462	0.825	1.691
	22	2.043	1.355	0.947	1.494	1.186	0.308	1.554	0.803	1.788
	45	2.262	1.494	1.030	1.493	1.102	0.344	1.646	0.742	1.891
	55.5	2.307	1.550	1.070	1.488	1.076	0.355	1.681	0.723	1.927

list the maximum PA for each band and HAM side at all the measured PST scan angles for JPSS-3 and -4, respectively. This gives a quantitative look into any HAM side or build-to-build differences between these two VIIRS versions.

Figure 12 illustrates the polarization sensitivities for bands M2–M4 HAM side A for JPSS-3 (left column) and JPSS-4 (right column). All the bands meet their sensor requirement of 2.5% between ±45° scan angles. The bands are also consistent in their detector-to-detector sensitivity in both amplitude and phase between JPSS-3 and -4 builds. Unlike band M1, where the dichroic had build-to-build performance differences, bands M2–M4 have a consistent mirror and dichroic polarization performance. The scan variation decreases as the band’s wavelength increases. This is due to the properties of the silver mirror coatings, where the s and p reflectance difference converges and reverses sign near the band M4 spectral band region. The polarization sensitivity variation across HAM sides for these bands is very similar and should limit HAM side striping effects.

Figure 13 and Figure 14 show the JPSS-3 (left column) and JPSS-4 (right column) polarization sensitivities for bands M5–M7 and I1–I2, respectively. The polarization sensitivities fall within the sensor requirements. The detector-to-detector amplitude and phase behavior are larger for bands M5 and M6 for JPSS-4 when compared to JPSS-3. Bands M7, I1, and I2 are consistent between the two builds. These bands have a different detector-to-detector and scan variation when compared to bands M1–M4. Bands M1–M4 have a mostly linear detector-to-detector pattern that moves across the polar plots with scan angle, but the shapes remain relatively constant. Bands M5–M7 and I1–I2 have large parabolic-like detector-to-detector shapes where the curvature changes based on scan angle position. The scan angle pattern also changes between bands M1–M3 with bands M4–M7 and I1–I2. The left-most polarization sensitivity on the polar plots is a scan angle of +55° or +45° for bands M1–M3, and the right-most scan angle values corresponding to −55°. For bands M4–M7 and I1–I2, the left-most points on the polar plots correspond to a scan angle of −55° and the right-most are for +55°. This is caused by the inversion of the s and p polarization reflectance for the VIIRS mirrors near the spectral region of band M4. This s and p reflectance inversion is a characteristic of the VIIRS mirror coating design and is similar to S-NPP and, NOAA-20 and NOAA-21 VIIRS. The scan angle and detector-to-detector phase variation make the s and p orientation important because they affect each band differently. Errors in the polarization orientation will cause striping in the EDRs because each band is affected differently, and the EDR algorithms rely on band combinations to extract geophysical parameters. While bands I2 and M7 share the same spectral bandpass, the AOIs on their optics are different, and therefore, the polarization sensitives are slightly different between the two bands.

The results shown in Figure 10, Figure 11 and Figure 12 are used to create LUTs as inputs into VIIRS EDR algorithms. The amplitude (PA) and phase (β), determined during pre-launch testing, are converted into a Mueller matrix that is used in combination with a Stokes vector to create the on-orbit polarization correction [13]. The uncertainty in the amplitude and phase characterization is important because they are both used in the creation of the on-orbit Mueller matrix LUT. Errors in the pre-launch characterization could lead to stripping or other artifacts in the EDR products.

The estimate of uncertainty for the JPSS-3 and -4 polarization sensitivity test uses a combination of sensor-level measurements combined with PTSA characterization information. The error tree in Figure 15 illustrates the inputs into the estimate of uncertainty and factors included in the budget. The measurement total box in Figure 15 includes sheet efficiency, source stability, polarization sheet angle, dn noise, and straylight errors. The source stability was estimated by splitting the 360° rotation measurements into two 180° segments and comparing the 2φ results of each segment. The dn noise was computed using the standard deviation of the dns over samples and scans for a single polarization angle. This varies by band, detector, scan angle, and polarization angle, with the maximum value for each band included in the uncertainty tree. The cross-polarization measurement a₂ and b₂ fit uncertainties are propagated into the PA and β as F_cross uncertainties and correspond to the polarization efficiency correction uncertainty in Figure 15. The estimated uncertainty in the polarization angle was determined during the cross-polarization measurement. The maximum response in the modulation during the cross-polarization test was expected to be aligned with the rotation axis of the RTA, and any deviations from this are included in the phase uncertainty. The lollipop and dark measurements were used to estimate the straylight contribution of the polarization testing. Any straylight will affect the a₀ term in Equation (2) and bias the PA. Since the straylight was not significant during the JPSS-3 and -4 tests, it was not mitigated in the PA data processing and was carried in the estimated uncertainty instead.

Multiple collections were obtained at the scan angle of −8° to determine the repeatability portion in Figure 15. These collections were taken at the beginning, middle, and end of the test to evaluate the repeatability of the test and the PTSA. The maximum minus minimum of the −8° measurements of the PA was used to determine the test repeatability. A polynomial fit was used to model the scan angle dependence of the polarization sensitivity. The fit residuals of the PA polynomial fit were used to estimate the uncertainty from scan angle interpolation. The SIS-TOA term accounts for the spectral differences between the PTSA and on-orbit Solar sources. A correlated polarization model that accounted for the spectral weighting differences between the two source profiles was used to determine the SIS-TOA impact [16]. While the absolute value in the modeled polarization sensitivities has errors, the relative values, when comparing different spectral source profiles, provide an accurate estimate of its uncertainty. The out-of-band box in Figure 15 was estimated for bands M1–M3 by comparing the difference in PAs between the Sonoma blocking filter in and out. Uncertainty from the PTSA setup was allocated in the final box in Figure 15. These values were determined during the PTSA performance characterization before the execution of the PST.

Each error component discussed above and illustrated in Figure 15 is considered a random error and not a bias. This allows them to be root-sum-squared (RSSed) into the total measurement polarization characterization uncertainty. This combines both the PTSA and test measurement uncertainties into a final rolled-up value and is used consistently across all the VIIRS instrument builds. The polarization sensitivity characterization uncertainty values are shown in

Table 8. Polarization sensitivity characterization uncertainty for each VNIR band and instrument build.

Band	Center Wavelength (nm)	Req (%)	JPSS-4 (%)	JPSS-3 (%)	NOAA-21 (%)	NOAA-20 (%)	S-NPP (%)
M1	412	0.5	0.27	0.3	0.38	0.35	0.14
M2	445	0.5	0.25	0.27	0.3	0.27	0.28
M3	488	0.5	0.17	0.21	0.16	0.14	0.14
M4	555	0.5	0.22	0.24	0.22	0.19	0.24
I1	640	0.5	0.23	0.23	0.24	0.14	0.14
M5	672	0.5	0.11	0.11	0.15	0.1	0.13
M6	746	0.5	0.09	0.08	0.1	0.1	0.13
M7	865	0.5	0.15	0.14	0.09	0.21	0.14
I2	865	0.5	0.37	0.36	0.35	0.35	0.36

for all instrument builds and are compared with the instrument requirement listed in Table 2. JPSS-3 and -4 are well within the requirements for all bands.

3.4. VIIRS Polarization Performance Comparison with Previous Builds

The maximum PA values across the band, scan angle, and HAM side are shown in

Table 9. S-NPP (top), NOAA-20 (middle), and NOAA-21 (bottom) maximum detector PA (in percentage) for all VNIR bands, scan angles, and HAM sides [15,17].

			Band
		Scan Angle	M1	M2	M3	M4	M5	M6	M7	I1	I2
S-NPP	HAM A	−55	2.119	2.055	1.033	0.644	1.504	1.063	0.306	1.664	0.258
		−45	1.950	1.927	0.981	0.848	1.166	1.008	0.268	1.415	0.215
		−20	1.912	2.069	1.248	1.018	0.754	0.935	0.230	0.914	0.226
		−8	1.836	1.999	1.217	0.932	0.652	0.849	0.242	0.856	0.270
		22	2.248	2.146	1.415	0.835	0.500	0.713	0.268	0.610	0.316
		45	2.441	2.125	1.348	0.752	0.426	0.651	0.287	0.588	0.369
		55	2.387	1.894	1.142	0.788	0.368	0.592	0.483	0.614	0.466
	HAM B	−55	1.962	2.047	1.116	1.244	0.992	1.389	0.645	0.962	0.626
		−45	2.080	1.940	1.171	1.212	0.752	1.184	0.558	0.823	0.555
		−20	2.187	1.967	1.229	1.193	0.517	0.919	0.425	0.622	0.444
		−8	2.021	1.977	1.198	1.087	0.450	0.828	0.389	0.629	0.431
		22	2.331	2.158	1.377	0.947	0.414	0.653	0.337	0.694	0.420
		45	2.286	2.048	1.448	0.749	0.414	0.612	0.325	0.661	0.420
		55	2.394	1.884	1.142	0.646	0.389	0.527	0.496	0.724	0.490
			Band
		Scan Angle	M1	M2	M3	M4	M5	M6	M7	I1	I2
NOAA-20	HAM A	−55	5.125	3.719	2.885	3.613	1.905	1.621	0.732	0.814	0.736
		−45	5.275	3.804	2.863	3.909	1.864	1.321	0.624	0.741	0.626
		−37	5.351	3.847	2.825	4.073	1.894	1.130	0.542	0.758	0.542
		−30	5.486	3.944	2.840	4.162	1.863	0.998	0.466	0.740	0.472
		−20	5.548	3.906	2.729	4.170	1.821	0.867	0.367	0.733	0.367
		−15	5.570	3.914	2.706	4.229	1.850	0.855	0.369	0.799	0.375
		−8	5.633	3.924	2.672	4.180	1.795	0.790	0.320	0.755	0.366
		4	5.715	3.954	2.631	4.179	1.836	0.754	0.387	0.810	0.432
		22	5.654	3.888	2.620	4.042	1.816	0.737	0.442	0.826	0.503
		45	5.512	3.981	2.806	3.889	1.799	0.749	0.549	0.851	0.613
		55	5.369	4.047	2.842	3.799	1.801	0.761	0.601	0.857	0.658
	HAM B	Scan Angle	M1	M2	M3	M4	M5	M6	M7	I1	I2
		−55	5.559	4.065	2.914	4.029	2.099	1.037	1.185	0.854	1.193
		−45	5.742	4.097	2.866	4.210	2.171	0.922	0.917	0.899	0.921
		−37	5.868	4.123	2.826	4.325	2.223	0.900	0.739	0.954	0.749
		−30	6.022	4.221	2.846	4.349	2.190	0.873	0.614	0.951	0.625
		−20	6.172	4.184	2.764	4.321	2.134	0.869	0.478	0.936	0.500
		−15	6.205	4.202	2.750	4.361	2.144	0.917	0.474	0.996	0.507
		−8	6.321	4.216	2.742	4.302	2.067	0.908	0.431	0.946	0.478
		4	6.426	4.256	2.740	4.285	2.035	0.951	0.456	0.986	0.503
		22	6.414	4.186	2.851	4.147	2.017	0.958	0.469	1.001	0.525
		45	6.168	4.359	3.077	3.991	1.992	0.951	0.524	1.033	0.581
		55	5.955	4.468	3.115	3.907	1.978	0.946	0.557	1.040	0.609
			Band
		Scan Angle	M1	M2	M3	M4	M5	M6	M7	I1	I2
NOAA-21	HAM A	−55	4.224	1.730	1.138	0.886	1.601	1.447	1.077	0.801	1.140
		−45	4.252	1.518	1.008	0.943	1.596	1.201	0.964	0.798	1.032
		−37	4.325	1.440	0.968	0.950	1.578	1.053	0.882	0.813	0.967
		−30	4.419	1.383	0.933	1.030	1.598	0.961	0.850	0.835	1.022
		−20	4.591	1.307	0.883	1.083	1.565	0.898	0.900	0.842	1.101
		−15	4.640	1.333	0.900	1.114	1.576	0.879	0.936	0.858	1.140
		−8	4.728	1.304	0.911	1.134	1.529	0.847	0.974	0.855	1.178
		4	4.815	1.329	1.026	1.149	1.525	0.845	1.036	0.862	1.255
		22	4.845	1.503	1.144	1.149	1.542	0.855	1.112	0.868	1.332
		45	4.723	1.701	1.274	1.123	1.520	0.860	1.196	0.875	1.408
		55	4.530	1.774	1.326	1.068	1.493	0.856	1.233	0.866	1.445
	HAM B	Scan Angle	M1	M2	M3	M4	M5	M6	M7	I1	I2
		−55	4.152	1.635	1.079	0.848	1.588	1.492	0.998	0.811	1.052
		−45	4.204	1.455	0.971	0.922	1.573	1.239	0.911	0.794	0.978
		−37	4.301	1.387	0.942	0.937	1.544	1.080	0.848	0.801	0.928
		−30	4.394	1.347	0.912	1.019	1.573	0.985	0.833	0.822	1.004
		−20	4.521	1.286	0.871	1.067	1.546	0.908	0.900	0.830	1.098
		−15	4.590	1.305	0.889	1.118	1.550	0.885	0.938	0.846	1.141
		−8	4.645	1.286	0.902	1.124	1.509	0.850	0.982	0.841	1.185
		4	4.727	1.305	1.012	1.144	1.511	0.838	1.050	0.850	1.269
		22	4.758	1.461	1.122	1.150	1.521	0.838	1.128	0.855	1.350
		45	4.649	1.649	1.247	1.114	1.508	0.849	1.210	0.859	1.427
		55	4.481	1.712	1.296	1.064	1.481	0.841	1.248	0.853	1.459

for S-NPP, NOAA-20, and NOAA-21 VIIRS [15,17]. The polarization sensitivity requirements were not met for the NOAA-20 and -21 VIIRS builds but do meet requirements for S-NPP, JPSS-3, and JPSS-4. Requirements exceedances for NOAA-20 bands M1–M4 and NOAA-21 band M1 were due to component-level requirements gaps or unexpected behavior from modifications to the VIIRS optical system. The S-NPP polarization sensitivity met all its requirements, with I1, I2, M4, M5, and M7 showing lower PA values than JPSS-3 or -4. The NOAA-20 VNIR bandpass filters were modified due to out-of-band spectral response non-compliances in S-NPP VIIRS. The new bandpass filters had large polarization sensitivities that resulted in bands M1–M4 failing the polarization requirement. NOAA-21, JPSS-3, and -4 had updates to the bandpass filter design in bands M1–M4 to reduce its contribution to the polarization sensitivity of the instrument. On NOAA-21, the polarization sensitivity for band M1 failed requirements due to large s and p reflectance differences in dichroic 1. The dichroic 1 vendor did not have s and p reflectance requirements for the band M1 spectral region, and the NOAA-21 dichroic had high polarization in that region. For JPSS-3 and -4, s and p reflectance requirements for dichroic 1 were flowed down to the vendor. With the bandpass, silver mirror coatings, and bandpass filter behaviors well understood, JPSS-3 and -4 met the polarization sensitivity requirements with margin.

4. Discussion

The VIIRS SDR calibrated unpolarized radiance and brightness temperature product is a key input into the EDR algorithms. For most EDRs, the unpolarized radiance scene is sufficient to ascertain the global physical parameters of interest. However, some EDRs, such as ocean color/chlorophyll, observe highly polarized scenes that impact their performance without mitigation. The SDR ocean scenes for NOAA-20 band M1 observed striping on the order of ~11% [13]. Without the pre-launch polarization sensitivity characterization, these EDRs would have significant degradation in their product. Application of the NOAA-20 VIIRS polarization correction removed much of the effect with a residual of ~1% remaining. The JPSS-3 and -4 maximum polarization sensitivity of ~2.5% is less than half of the NOAA-20’s ~5.7% and should not impact the EDR performance as much as its predecessor. This should improve the fidelity of the EDR products on JPSS-3 and -4 due to polarized scene influences.

5. Conclusions

Understanding the VIIRS instrument’s sensitivity to polarization is important for optimum EDR performance. Pre-launch, the instrument’s response variation as a function of the at-aperture polarization orientation was characterized. This required special ground test equipment that minimized the characterization uncertainty to below 0.5% and provided the necessary information to create a polarization sensitivity correction for on-orbit EDR algorithms. The JPSS-3 and -4 VIIRS polarization sensitivity characterization was discussed here and showed the polarization amplitude was less than the 2.5–3% requirement, and the uncertainty in the characterization was below 0.5%. A comparison with the previous S-NPP, NOAA-20, and -21 VIIRS builds showed that the JPSS-3 and -4 VIIRS polarization sensitivity was similar to or outperformed its predecessors. This indicates that JPSS-3 and -4 should provide high-quality polarization corrections on-orbit and not limit the capabilities of the EDR products that require this information. The lessons learned throughout the VIIRS polarization characterization on the S-NPP and JPSS missions are used in the design, pre-launch testing, and on-orbit application trade studies for the NASA and NOAA next-generation efforts. This is important for providing consistent EDR datasets across future missions for long-term climate dataset continuity.