Eugene Waluschka

VIIRS on-orbit optical anomaly: investigation, analysis, root cause determination and lessons learned

A gradual, but persistent, decrease in the optical throughput was detected during the early commissioning phase for the Suomi National Polar-Orbiting Partnership (SNPP) Visible Infrared Imager Radiometer Suite (VIIRS) Near Infrared (NIR) bands. Its initial rate and unknown cause were coincidently coupled with a decrease in sensitivity in the same spectral wavelength of the Solar Diffuser Stability Monitor (SDSM) raising concerns about contamination or the possibility of a system-level satellite problem. An anomaly team was formed to investigate and provide recommendations before commissioning could resume. With few hard facts in hand, there was much speculation about possible causes and consequences of the degradation. Two different causes were determined as will be explained in this paper. This paper will describe the build and test history of VIIRS, why there were no indicators, even with hindsight, of an on-orbit problem, the appearance of the on-orbit anomaly, the initial work attempting to understand and determine the cause, the discovery of the root cause and what Test-As-You-Fly (TAYF) activities, can be done in the future to greatly reduce the likelihood of similar optical anomalies. These TAYF activities are captured in the “lessons learned” section of this paper.

Performance of cat's eye modulating retro-reflectors for free-space optical communications

Modulating retro-reflectors (MRR) couple passive optical retro-reflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing/acquisition/tracking system required on only one end of the link. In operation a conventional free space optical communications terminal, the interrogator, is used on one end of the link to illuminate the MRR on the other end of the link with a cw beam. The MRR imposes a modulation on the interrogating beam and passively retro-reflects it back to the interrogator. These types of systems are attractive for a asymmetric communication links for which one end of the link cannot afford the weight, power or expense of a conventional free-space optical communication terminal. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large area MQW placed in front of the aperture of a corner-cube. For the MQW MRR, the maximum modulation can range into the gigahertz, limited only by the RC time constant of the device. This limitation, however, is a serious one. The optical aperture of an MRR cannot be too small or the amount of light retro-reflected will be insufficient to close the link. For typical corner-cube MQW MRR devices the modulator has a diameter between 0.5-1 cm and maximum modulation rates less than 10 Mbps. In this paper we describe a new kind of MQW MRR that uses a cat’s eye retro-reflector with the MQW in the focal plane of the cat’s eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 10 Mbps free space link over a range of 1 km is demonstrated. In addition a laboratory of a 70 Mbps MQW focal plane is described.

MODIS solar diffuser: modeled and actual performance

The MODIS instruments solar diffuser is used in its radiometric calibration for the reflective solar bands (VIS, NIR, and SWIR) ranging from 0.41 to 2.1 micron. The sun illuminates the solar diffuser either directly or through an attenuation screen. The attenuation screen consists of a regular array of pin holes. The attenuated illumination pattern on the solar diffuser is not uniform, but consists of a multitude of pin-hole images of the sun. This non-uniform illumination produces small, but noticeable radiometric effects. A description of the computer model used to simulate the effects of the attenuation screen is given and the predictions of the model are compared with actual, on-orbit, calibration measurements.

Cat's eye quantum well modulating retroreflectors for free-space optical communications

Modulating retro-reflectors (MRR) couple passive optical retro-reflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing/acquisition/tracking system required on only one end of the link. In operation a conventional free space optical communications terminal, the interrogator, is used on one end of the link to illuminate the MRR on the other end of the link with a cw beam. The MRR imposes a modulation on the interrogating beam and passively retro-reflects it back to the interrogator. These types of systems are attractive for asymmetric communication links for which one end of the link cannot afford the weight, power and expense of a conventional free-space optical communication terminal. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large area MQW placed in front of the aperture of a corner-cube. For the MQW MRR, the maximum modulation can range into the gigahertz, limited only by the RC time constant of the device. This limitation, however, is a serious one. The optical aperture of an MRR cannot be too small or the amount of light retro-reflected will be insufficient to close the link. For typical corner-cube MQW MRR devices the modulator has a diameter between 0.5-1 cm and maximum modulation rates less than 10 Mbps. In this paper we describe a new kind of MQW MRR that uses a cats eye retro-reflector with the MQW in the focal plane of the cats eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 50 Mbps device has been demonstrated.

Modeling studies of the MODIS solar diffuser attenuation screen and comparison with on-orbit measurements

The MODIS instrument relies on solar calibration to achieve the required radiometric accuracy. This solar calibration occurs as the TERRA spacecraft comes up over the North Pole. The earth underneath the spacecraft is still dark for approximately one minute and the sun is just rising over the earths north polar regions. During this time the sun moves through about 3.3 degrees, the scan mirror rotates about 19 times and about 50 exposures (frames) are taken each time the field of view is directed to the approximate center (sweet spot) of the solar diffuser. For some of MODISs bands the brightness of the diffuser is reduced, to prevent detector saturation, by means of a retractable pinhole screen, which produces approximately 600 pinhole images of the sun, within the field of view of any one detector. Previous attempts at creating a radiometric model of this, reduced intensity, calibration scenario produced intensity variations on the focal planes with insufficient detail to be useful. The current computational approach, gets around these limitations and is fast enough to permit simulation of the motion of the sun and the scan mirror. The results resemble the observed focal plane temporal and spatial intensity variations well enough to be useful. The computational approach is described and a comparison with observational data is presented.

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.

Analysis of JPSS J1 VIIRS polarization sensitivity using the NIST T-SIRCUS

The polarization sensitivity of the Joint Polar Satellite System (JPSS) J1 Visible Infrared Imaging Radiometer Suite (VIIRS) measured pre-launch using a broadband source was observed to be larger than expected for many reflective bands. Ray trace modeling predicted that the observed polarization sensitivity was the result of larger diattenuation at the edges of the focal plane filter spectral bandpass. Additional ground measurements were performed using a monochromatic source (the NIST T-SIRCUS) to input linearly polarized light at a number of wavelengths across the bandpass of two VIIRS spectral bands and two scan angles. This work describes the data processing, analysis, and results derived from the T-SIRCUS measurements, comparing them with broadband measurements. Results have shown that the observed degree of linear polarization, when weighted by the sensor’s spectral response function, is generally larger on the edges and smaller in the center of the spectral bandpass, as predicted. However, phase angle changes in the center of the bandpass differ between model and measurement. Integration of the monochromatic polarization sensitivity over wavelength produced results consistent with the broadband source measurements, for all cases considered.

Measured polarized spectral responsivity of JPSS J1 VIIRS using the NIST T-SIRCUS

Recent pre-launch measurements performed on the Joint Polar Satellite System (JPSS) J1 Visible Infrared Imaging Radiometer Suite (VIIRS) using the NIST T-SIRCUS monochromatic source have provided wavelength dependent polarization sensitivity for select spectral bands and viewing conditions. Measurements were made at a number of input linear polarization states (twelve in total) and initially at thirteen wavelengths across the bandpass (later expanded to seventeen for some cases). Using the source radiance information collected by an external monitor, a spectral responsivity function was constructed for each input linear polarization state. Additionally, an unpolarized spectral responsivity function was derived from these polarized measurements. An investigation of how the centroid, bandwidth, and detector responsivity vary with polarization state was weighted by two model input spectra to simulate both ground measurements as well as expected on-orbit conditions. These measurements will enhance our understanding of VIIRS polarization sensitivity, improve the design for future flight models, and provide valuable data to enhance product quality in the post-launch phase.

Evaluation of the polarization properties of a Philips-type prism for the construction of imaging polarimeters

The design and construction of wide FOV imaging polarimeters for use in atmospheric remote sensing requires significant attention to the prevention of artificial polarization induced by the optical elements. Surface, coatings, and angles of incidence throughout the system must be carefully designed in order to minimize these artifacts because the remaining instrumental bias polarization is the main factor which drives the final polarimetric accuracy of the system. In this work, we present a detailed evaluation and analysis to explore the possibility of retrieving the initial polarization state of the light traveling through a generic system that has inherent instrumental polarization. Our case is a wide FOV lens and a splitter device. In particular, we chose as splitter device a Philips-type prism, because it is able to divide the signal in 3 independent channels that could be simultaneously analyze to retrieve the three first elements of the Stoke vector (in atmospheric applications the elliptical polarization can be neglected [1]). The Philips-type configuration is a versatile, compact and robust prism device that is typically used in three color camera systems. It has been used in some commercial polarimetric cameras which do not claim high accuracy polarization measurements [2]. With this work, we address the accuracy of our polarization inversion and measurements made with the Philips-type beam divider.

MODIS polarization ray tracing analysis

On-orbit optical sensors are the primary data source for the remote sensing community. A rigorous pre-flight characterization and calibration is a key to the success of their mission. Indeed, preliminary calibration and correction factors are determined during this process. As part of this process, prior to the launch of NASAs Moderate Resolution Imaging Spectroradiometer (MODIS) its polarization sensitivity was measured. In this work, our goal was to simulate these measurements using computer ray tracing software. Based on that, we could evaluate the evolution of the different coatings (Mirror, Beam splitters, Anti-reflection and Band pass filters) due to degradation over time. We were able to simulate the measurements and obtained what the theoretical polarization sensitivity should be. The results were compared to the pre-launch measurements and an analysis of the whole MODIS optical system was performed in order to explain these differences. A full description of the MODIS polarization ray tracing procedure along with a discussion on the results and their implications on past, present and future work will be given.