Eric C. Fest

VIIRS/J1 polarization narrative

The polarization sensitivity of the Visible/NearIR (VISNIR) bands in the Joint Polar Satellite Sensor 1 (J1) Visible Infrared Imaging Radiometer Suite (VIIRS) instrument was measured using a broadband source. While polarization sensitivity for bands M5-M7, I1, and I2 was less than 2.5 %, the maximum polarization sensitivity for bands M1, M2, M3, and M4 was measured to be 6.4 %, 4.4 %, 3.1 %, and 4.3 %, respectively with a polarization characterization uncertainty of less than 0.38%. A detailed polarization model indicated that the large polarization sensitivity observed in the M1 to M4 bands is mainly due to the large polarization sensitivity introduced at the leading and trailing edges of the newly manufactured VISNIR bandpass focal plane filters installed in front of the VISNIR detectors. This was confirmed by polarization measurements of bands M1 and M4 bands using monochromatic light. Discussed are the activities leading up to and including the two polarization tests, some discussion of the polarization model and the model results, the role of the focal plane filters, the polarization testing of the Aft-Optics-Assembly, the testing of the polarizers at the National Aeronautics and Space Administration’s (NASA) Goddard center and at the National Institute of Science and Technology (NIST) facility and the use of NIST’s Traveling Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (T-SIRCUS) for polarization testing and associated analyses and results.

Staring MWIR, LWIR and 2-color and scanning LWIR polarimetry technology

Polarimetry sensor development has been in work for some time to determine the best use of polarimetry to differentiate between manmade objects and objects made by nature. Both MWIR and LWIR and 2-color staring Focal Plane Arrays (FPAs) and LWIR scanning FPAs have been built at Raytheon Vision Systems each with exceedingly higher performance. This paper presents polarimetric performance comparisons between staring 2562 MWIR, 2562 LWIR, 5122 LWIR/LWIR staring FPAs and scanning LWIR FPAs. LWIR polarimetry has the largest polarimetric signal level and a larger emissive polarimetric signature than MWIR which makes LWIR less dependent on sun angles. Polished angled glass and metal objects are easily detected using LWIR polarimetry. While single band 9-11 um LWIR polarimetry has advantages adding another band between 3 and 7 um improves the capability of the sensor for polarization and spectral phenomenology. In addition the 3-7 um band has improved NEDT over the 9-11 um band due to the shorter detector cutoff reducing the Noise Equivalent Degree of Linear Polarization. (NEDOLP). To gain acceptance polarimetric sensors must provide intelligence signatures that are better than existing nonpolarimetric Infrared sensors. This paper shows analysis indicating the importance of NEDOLP and Extinction ratios.

VIIRS polarization sensitivity testing and analysis

A requirement for the Visible/Infrared Imager Radiometer Suite (VIIRS) is that its polarization sensitivity be 3% or less for all VISNIR bands (412-865 nm). A test using a rotating polarizer sheet was performed on the sensor to validate this requirement, and though the test results show that the requirement is met, they also show a large variation in this polarization sensitivity (as much as 2%) across the field of view (FOV) in track. Though this result is unexpected, it may be the result of natural variations in the diattenuation and retardance of the VIIRS optics as a function of field angle. To test this theory, a raytracing model of the system was constructed using measured ellipsometric data from the VIIRS optics, and the polarization sensitivity of the model was computed. Using the nominal ellipsometric data, good correlation between the predicted and measured polarization sensitivity was not achieved. However, by applying small variations to the ellipsometric data as a function of position on the optics, it was possible to achieve good correlation. This paper gives the details of the sensor polarization sensitivity measurements, ellipsometric measurements, and raytracing analysis.