In a stand-off imaging scenario from a high-altitude airborne platform, the camera line-of-sight (LOS) passes through atmosphere layers of variable density at an oblique angle. Dispersion blur arises because of wavelength-dependent refraction in the atmosphere. In visible-and-near-infrared (VNIR) spectral band the atmospheric dispersion is significant and has some effect onto resolution performance in LOS elevation dimension. Historically the atmospheric refraction and dispersion issues were studied and analyzed in astronomy, considering an infinitely distant celestial body imaged by a ground-based telescope. The calculation involves an atmosphere properties model as well as an expression for air refractive index as function of pressure, temperature and humidity. However, a straightforward application of the star–to-ground telescope refraction formulae to the ground target–toairborne camera case leads to incorrect results. In the latter case boundary conditions for light ray refraction are different, since the ground target-to-camera distance is finite rather than infinite. There are dispersion-related ray angle differences both at departure of light from the target and on arrival to the camera. Only the latter part of the overall atmospheric dispersion is perceived at the camera and causes image blur. An iterative method of the camera-perceived dispersion calculation is presented. Parametric study results illustrate the dispersion dependence on camera altitude and LOS zenith angle. The model results are validated by comparison to vertical-to-horizontal sharpness ratio statistics calculated from images taken by Condor2 cameras at various ranges. The dispersion-related NIIRS grade loss is relatively small, and in most practical cases no optical compensation of the atmospheric dispersion is necessary.
The Spectro XRTM is an advanced color/NIR/SWIR/MWIR 16’’ payload recently developed by Elbit Systems / ELOP.
The payload’s primary sensor is a spotter camera with common 7’’ aperture. The sensor suite includes also MWIR
zoom, EO zoom, laser designator or rangefinder, laser pointer / illuminator and laser spot tracker. Rigid structure,
vibration damping and 4-axes gimbals enable high level of line-of-sight stabilization. The payload’s list of features
include multi-target video tracker, precise boresight, strap-on IMU, embedded moving map, geodetic calculations suite,
and image fusion.
The paper describes main technical characteristics of the spotter camera. Visible-quality, all-metal front catadioptric
telescope maintains optical performance in wide range of environmental conditions. High-efficiency coatings separate
the incoming light into EO, SWIR and MWIR band channels. Both EO and SWIR bands have dual FOV and 3 spectral
filters each. Several variants of focal plane array formats are supported.
The common aperture design facilitates superior DRI performance in EO and SWIR, in comparison to the
conventionally configured payloads. Special spectral calibration and color correction extend the effective range of color
imaging. An advanced CMOS FPA and low F-number of the optics facilitate low light performance. SWIR band
provides further atmospheric penetration, as well as see-spot capability at especially long ranges, due to asynchronous
pulse detection. MWIR band has good sharpness in the entire field-of-view and (with full HD FPA) delivers amount of
detail far exceeding one of VGA-equipped FLIRs.
The Spectro XR offers level of performance typically associated with larger and heavier payloads.
The Condor2 long-range oblique photography (LOROP) camera is mounted in an aerodynamically shaped pod carried by a fast jet aircraft. Large aperture, dual-band (EO/MWIR) camera is equipped with TDI focal plane arrays and provides high-resolution imagery of extended areas at long stand-off ranges, at day and night. Front Ritchey-Chretien optics is made of highly stable materials. However, the camera temperature varies considerably in flight conditions. Moreover, a composite-material structure of the reflective objective undergoes gradual dehumidification in dry nitrogen atmosphere inside the pod, causing some small decrease of the structure length. The temperature and humidity effects change a distance between the mirrors by just a few microns. The distance change is small but nevertheless it alters the camera's infinity focus setpoint significantly, especially in the EO band. To realize the optics' resolution potential, the optimal focus shall be constantly maintained. In-flight best focus calibration and temperature-based open-loop focus control give mostly satisfactory performance. To get even better focusing precision, a closed-loop phase-matching autofocus method was developed for the camera. The method makes use of an existing beamsharer prism FPA arrangement where aperture partition exists inherently in an area of overlap between the adjacent detectors. The defocus is proportional to an image phase shift in the area of overlap. Low-pass filtering of raw defocus estimate reduces random errors related to variable scene content. Closed-loop control converges robustly to precise focus position. The algorithm uses the temperature- and range-based focus prediction as an initial guess for the closed-loop phase-matching control. The autofocus algorithm achieves excellent results and works robustly in various conditions of scene illumination and contrast.
Based on the experience gained with the Condor2 long-range oblique photography (LOROP) camera, ELOP is
expanding its airborne reconnaissance product line with the Condor TAC tactical photography system. The latter was
designed for overflight imaging of extended areas from a fighter or special mission aircraft, at day and night. The
Condor TAC is mounted in an aerodynamically shaped pod and can operate in wide envelope of flight altitude and
speed. Besides the camera, the pod contains mission management and video processing unit (MVU), solid state recorder
(SSR), wide-band data link (DL) for real-time imagery transmission, and two environmental control units (ECU).
Complex multi-segment optical windows were successfully developed for the system.
The camera system design is modular and highly flexible. Two independent imaging payload modules are mounted
inside a gimbal system. Each of the modules is equipped with a strap-down IMU, and may carry a cluster of cameras or
a single large camera with gross weight up to 35 kg. The payload modules are interchangeable, with an identical
interface to the gimbal. The modularity and open architecture of the system facilitate its adaptation to various
operational requirements, as well as allow easy and relatively non-expensive upgrades and configuration changes.
In the current configuration, both EO and IR payload modules are equipped with a combination of longer focal length
cameras for bi-directional panoramic scan at medium and high flight altitudes, and shorter focal length cameras for
fixed wide angle coverage at low altitudes. All the camera types are equipped with standard format, off-the-shelf area
detector arrays. Precise motion compensation is achieved by calibrated back-scan mirrors.
Large-aperture, high-resolution medium-wave IR sensors are applied for oblique imaging from a variety of airborne platforms. Applicable optical configurations include both reflective (aplanatic Cassegrain) and refractive devices. In the former case the MWIR typically complements a primary EO band, with a common front objective and a dichroic beam splitter. ELOP's successful Condor2 LOROP camera is an example of such configuration. The camera provides consistent EO and IR performance at long stand-off ranges from a high-altitude fast jet platform.
The long range oblique IR imaging represents a particularly complex case for system analysis and performance prediction. Atmospheric attenuation changes significantly as a function of altitude and line-of-sight depression angle. Temperature of the camera optics may vary in wide bounds and could differ substantially from the target temperature. Pixel pitch, optical transmittance and (in case of Cassegrain optics) a ratio of central obscuration all have a strong effect onto the resolution performance.
The paper presents a normalized multi-parametric performance study where the resolved spatial frequency is expressed as a fraction of the optical cutoff frequency. The major dependencies of the resolution on the key parameters are illustrated. The analysis includes a relatively wide range of variation of the design parameters, covering most of the feasible reflective and refractive optical configurations.
A dual band camera for long-range airborne reconnaissance has a large common Cassegrain objective, a beam splitter, VNIR and MWIR channels. Non-uniformity correction (NUC) must be especially accurate in this type of application, due to the low dynamic range of a raw image acquired through tens of kilometers of atmosphere.
Accurate calibration of non-uniformity in the MWIR band represents a challenge, because of considerable emissivity of the optics, variable optics temperature, high cos4 effect, vignetting, complex focal plane geometry, residual misalignment between the exit pupil and the dewar's cold stop, and insertion of a blackbody temperature reference source (TRS) directly in front of the dewar window. The paper describes a special calibration method which overcomes the complexities and achieves high NUC accuracy. The method combines in-laboratory transmissibility measurement with two-stage in-flight periodic calibration. The detector non-uniformity is calibrated in wide signal range. The TRS temperature follows a curve giving linear rise of radiance in time. Inner surface of the pod between the optical windows is used as a uniform source for evaluation of a pattern caused by the optics radiation. This method was successfully implemented in the ElOP long-range oblique photography (LOROP) camera.
The ELOP dual band LOROP camera was designed as a payload of a 300 gal reconnaissance pod capable of being carried by a single-engineerd fighter aircrat like F-16. The optical arrangement provides coincidence of the IR and EO optical axes, as well as equality of the fields-of-view. These features allow the same sacn coverage to be achieved, and the same gimbals control software to be used for the visible-light-only, IR-only and simultaneous dual band photography. Because of intensive, broadband vibration existing in teh pod environment, special attention was given to image stabilization system. Nevertheless, residual vibraiton still exists in a wide frequency range spreading from zero frequency to the detector integration rate and beyond it. Hence, evaluation of the camera performance could not rely on the well-known analytical solutions for MTFMOTION. The image motion is deinfed in terms of the Power Spectral Density throughout the whole frequency range of interest. The expected MTFMOTION is calculated numerically using a statistical approach. Aspects of the staggered-structure IR detecotr application in oblique photography are discussed. Particuarly, the ground footprint of the IR detector is much wider along-scan than one of the EO detector, requiring considerations to be implemented in order to prevent IR image deformation.