Rudolph Nolasco

Passive SWIR airglow illuminated imaging compared with NIR-visible for low-light nighttime observations

It is well known that luminance from photo-chemical reactions of hydroxyl ions in the upper atmosphere (~85 km altitude) produces a significant amount of night time radiation in the short wave infra-red (SWIR) band with wavelength between 0.9 and 1.7 μm. By examining images in an urban and a rural setting, we investigate the correlation between the appearances of passive dark of night images in the SWIR with NIR- visible. The experimental setup consists of two sensors, a NIR-visible CCD and an InGaAs array sensitive in the SWIR, both colocated on an AZ-EL mount, and both co-boresighted so that different viewing angles of the sky and terrestrial scenes are possible. By making corrections for focal length and pixel size, the visible and SWIR data can be compared. After taking several nights of data in the urban environment of Albuquerque, NM, the entire system was then re-located to a rural location on the island of Kauai in a rural setting with very low ambient light. It is shown that under most conditions the SWIR sensor produces significantly better imagery using the airglow illumination source.

SWIR air glow mapping of the night sky

It is well known that luminance from photo-chemical reactions of hydroxyl ions in the upper atmosphere (~85 km altitude) produces a significant amount of night time radiation in the short wave infra-red (SWIR) band of wave length 0.9 to 1.7 μm. Numerous studies of these phenomena have demonstrated that the irradiance shows significant temporal and spatial variations in the night sky. Changes in weather patterns, seasons, sun angle, moonlight, etc have the propensity to alter the SWIR air glow irradiance pattern. By performing multiple SWIR measurements a mosaic representation of the celestial hemisphere was constructed and used to investigate these variations over time and space. The experimental setup consisted of two sensors, an InGaAs SWIR detector and a visible astronomical camera, co-located and bore sighted on an AZ-EL gimbal. This gimbal was programmed to view most of the sky using forty five discrete azimuth and elevation locations. The dwell time at each location was 30 seconds with a total cycle time of less than 30 minutes. The visible astronomical camera collected image data simultaneous with the SWIR camera in order to distinguish SWIR patterns from clouds. Data was reduced through batch processing producing polar representations of the sky irradiance as a function of azimuth, elevation, and time. These spatiotemporal variations in the irradiance, both short and long term, can be used to validate and calibrate physical models of atmospheric chemistry and turbulence. In this paper we describe our experimental setup and present some results of our measurements made over several months in a rural marine environment on the Islands of Kauai and Maui Hawaii.

Comparison of fast correlation algorithms for target tracking

Laser Aided Detection and Ranging (LADAR) imaging systems can be used to provide high resolution imaging and tracking of moving targets at night. Central to the tracking system is a high speed correlation algorithm to determine target motion between sensor image frames. Several issues complicate the correlation calculations. These include coherent speckle noise and atmosphere induced scintillation of the illuminator beam. The Fitts correlation algorithm is commonly used because of its simplicity and speed. However it is only optimal when the shift between sensor frames is less than a pixel. In addition it can be sensitive to certain types of noise. Projection based phase only (PBPO) is another type of correlation algorithm that is also high speed and in many cases less sensitive to noise. In this paper we compare the Fitts algorithm with PBPO in terms of number of computations and noise immunity when used in a LADAR tracker.

Spatial and temporal variability of SWIR airglow measurements

It is well known that luminance from photo-chemical reactions of hydroxyl ions in the upper atmosphere (~85 km altitude) produces a significant amount of night time radiation in the short wave infra-red (SWIR) band between 0.9 and 1.7 μm wave length. This phenomenon, often referred to as airglow, has been demonstrated as an effective illumination source for passive low light level night time imaging applications. It addition it has been shown that observation of the spatial and temporal variations of the illumination can be used to characterize atmospheric tidal wave actions in the airglow region. These spatio-temporal variations manifest themselves as traveling wave patterns whose period and velocity are related to the wind velocity at 85 km as well as the turbulence induced by atmospheric vertical instabilities. In this paper we present nearly a year of airglow observations over the whole sky, showing long term and short term fluctuations to characterize SWIR night time image system performance.

Drift in correlation algorithms for high speed video trackers

The FITTS correlation algorithm has been widely used for over forty years in high speed video trackers. It has the advantage that it is very simple implement in a digital processor with a small number of calculations. At each step the algorithm attempts to estimate the shift between an image of a moving target and a proto-type image. There are several well-known short comings of the FITTS algorithm. First the error in the shift estimate increases if the shift is greater than one pixel of the digital image. Secondly all correlation based tracking algorithms are susceptible to low frequency drift. These errors can force a lower tracker closed loop bandwidth to maintain track loop stability. In this paper we examine in an intuitive manner the underlying cause of the drift and approaches to its mitigation.

Comparison of sensor noise effects on FITTS and projection based phase only correlation algorithms for high speed video trackers

The FITTS correlation algorithm has been widely used for over forty years in high speed video trackers. It has the advantage that it is very simply implemented in a digital computer with a small number of calculations. At each step the algorithm attempts to estimate the shift between an image of a moving target and a proto-type image. There are several well-known short comings of the FITTS algorithm. First the error in the shift estimate increases if the shift is greater than one pixel of the digital image. Second the FITTS algorithm is susceptible to errors from sensor noise if the video images have low signal to noise ratio. These errors can force a lower tracker closed loop bandwidth to maintain track loop stability. An alternative correlation tracker algorithm is known as Projection Based Phase Only Correlation. In this paper we compare the two algorithms with respect to the effect of sensor noise.

Retractable imaging system on a Cessna T-206H

The Standoff Intelligence Detection (SID) program is an Air Force Research Laboratory (AFRL) quick reaction program, tasked with providing the warfighter ready-now technologies related to directed energy, optics, and photonics. The first variant of these aircraft was started in 2008 utilizing two Cessna 182Q aircraft retrofitted with a wing mounted imaging systems and mission equipment. The 3rd generation of these imaging aircraft is equipped on a Cessna T-206H, Turbo Stationair. The aircraft is a 6-seat, single-engine aircraft, retrofitted with an MX-15HDi sensor system and supporting equipment. The SID Program has produced the aircraft to provide an ISR platform to support tests, exercises, search and rescue, and real-world needs.

Hyper-spectral measurements using a compact SWIR camera

Recent advances in InGaAs camera technology has stimulated interest in the short wave infra-red (SWIR) band in the spectral region 0.84 – 1.7 μm. Located between the visible and thermal infra-red, the SWIR band shows interesting properties of both. Images tends to have the look of the visible and are less affected by scattering from aerosol haze, however the solar irradiance is dropping rapidly with wavelength in the SWIR. Spectral signatures, particularly of paints and dyes, may be different in the SWIR band compared to the visible. For these reasons we have chosen to investigate hyper-spectral measurements in this band using the NovaSol μHSI SWIR hyper-spectral imager system.

Maximum a-posteriori LADAR Image estimation in thepresence of scintillation and speckle noise

Laser aided detection and ranging (LADAR) imaging systems are usually corrupted by several pathologic noises. Speckle noise is due to the coherent nature of the laser illuminator. Scintillation noise is introduced by atmospheric turbulence over the outgoing illumination path and manifests itself as a multiplicative noise in the imagery. These noises can be mitigated by a simple recursive averaging algorithm when looking at fixed targets in staring mode. However if the target under observation is moving with respect to the imaging platform, the averaging will cause the target image to smear. In such a case, a maximum a-posteriori (MAP) approach can be used to estimate localized statistics of the scene under observation as well as the scintillation. The parameter estimates can then be incorporated into a spatially and temporally adaptive averaging approach which mitigates the noise while at the same time preserving motion in the scene.

Seasonal hemispherical SWIR airglow imaging

Airglow luminescence in the SWIR region due to upper atmospheric recombination of solar excited molecules is a well accepted phenomenon. While the intensity appears broadly uniform over the whole sky hemisphere, we are interested in variations in four areas: 1) fine periodic features known as gravity waves, 2) broad patterns across the whole sky, 3) temporal variations in the hemispheric mean irradiance over the course of the night, and 4) long term seasonal variations in the mean irradiance. An experiment is described and results presented covering a full year of high resolution hemispheric SWIR irradiance images. An automated gimbal views 45 hemispheric positions, using 30 second durations, and repeats approximately every half hour through out the night. The gimbal holds co-mounted and bore-sighted visible and SWIR cameras. Measuring airglow with respect to spatial, temporal, and seasonal variations will facilitate understanding its behavior and possible benefits, such as night vision and predicting upper atmosphere turbulence. The measurements were performed in a tropical marine location on the island of Kauai Hi.