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Patent Issued for Method for Ground-To-Space Laser Calibration System

July 16, 2014



By a News Reporter-Staff News Editor at Defense & Aerospace Week -- A patent by the inventors Lukashin, Constantine (Newport News, VA); Wielicki, Bruce A. (Yorktown, VA), filed on December 13, 2012, was published online on July 1, 2014, according to news reporting originating from Alexandria, Virginia, by VerticalNews correspondents.

Patent number 8767210 is assigned to The United States of America as represented by the Administrator of the National Aeronautics and Spa.

The following quote was obtained by the news editors from the background information supplied by the inventors: "Quality of Earth Science data products based on observations from spaceborne radiometric sensors depends on their performance and accuracy on orbit. The accuracy of measuring reflected solar radiance can be affected by multiple factors. First, instruments with complex optics are sensitive to polarization. The response of such instruments is characterized before the launch, however, sensitivity to polarization can change on orbit significantly (e.g. Moderate Resolution Imaging Spectroradiometer (MODIS) launched on board the Terra satellite). Other factors are the degradation of optics, particularly in blue wavelength range below 500 nm (e.g. Clouds and Earth's Radiant Energy System (CERES) launched on board the Terra & Aqua satellites), and on-orbit changes in the instrument response to stray light. None of the existing sensors has the ability to monitor all these changes in calibration on orbit.

"Accurate verification of space born sensors calibration on orbit plays a crucial role in meeting mission accuracy requirements. Onboard verification systems significantly impact mission costs by increasing the mass of instrumentation and required power. Also, onboard verification systems are not accessible for adjustment, maintenance, improvement, or repairs in the case of failure. Accordingly, a need exists for an improved calibration concept that does not suffer from the drawbacks of known calibration systems and methods."

In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "The present invention is a method of calibrating an optical sensor, and more specifically, a method of calibrating an optical sensor onboard a satellite orbiting the Earth. The calibration is achieved by transmitting expanded and uniform laser beam to the instrument in low Earth or geo-stationary orbit, and varying beam polarization and its wavelength within short time intervals. The method is applicable to the instruments observing the reflected solar radiance. The method includes utilization of a ground-based laser, ground-to-space laser calibration (GSLC) system, with Continuous Wave (CW) laser, to generate a light signal on orbit (radiance) with controlled wavelength and polarization on the ground. The expanded beam with uniform top-hat profile, generated by ground-based laser system is aimed at and transmitted to the satellite, whereby entire aperture of optical sensor on the satellite is exposed to transmitted light. The optical sensor measures the intensity of incident signal on orbit while operations with beam polarization and wavelength are performed using optics on the ground.

"One aspect of the method is to determine sensor sensitivity to polarization on orbit, which can be defined as instrument response to the same light intensity with different polarization. The concept of calibrating sensitivity to polarization of spaceborne sensor is illustrated in FIG. 1. The physical principle is to safely expose a radiometer on orbit to 100% linearly polarized light generated by the GSLC, and to map sensor response to polarization at different polarization angles within a short time period (seconds). The operations are considered when spacecraft overpasses the ground site during nighttime with clear atmospheric conditions. While the spaceborne sensor observes the laser signal, with its line-of-sight aligned with the laser beam vector, the direction of linear polarization of laser light is rotated 360 degrees using beam optics on the ground. There are three key advantageous points used in this approach: (a) polarization parameters of laser light are not affected by a clear atmosphere even if its intensity changes between the surface and TOA; (b) it is a relative measurement: the response of the spaceborne sensor should be the same at polarization angles 0.degree., 180.degree. and 360.degree., and this feature provides a normalization reference for the calibration cycle; and the measurements should be performed during a short time interval--within seconds. Although the Earth's atmosphere attenuates the intensity of the laser beam, during short time intervals (e.g. 5-10 seconds) the atmospheric conditions do not substantially change, and beam intensity should be as stable on orbit as it is on the ground. Since the calibration is relative, only stability of the beam is required, and therefore, application of a Continuous Wave (CW) laser is preferable choice.

"Another aspect of the present invention is verification of instrument spectral response. The GSLC can be used to verify sensor spectral response on orbit by using lasers with different wavelengths or a tunable laser. In this case, the beam polarization is not required. The spectral verification measurements are also relative--ratio of instrument response in blue to its response in red and near-infrared (NIR) wavelengths as function of time from mission start date (generally, degradation in NIR is negligible). Atmospheric correction must be applied depending on the wavelength to improve accuracy of sensor calibration. Additionally, it is possible to map Relative Spectral Response (RSR) of narrowband and hyperspectral instruments by changing laser wavelength fast (e.g. 5-10 seconds) by tuning the laser around a central wavelength. The measurement is also relative, being normalized to the sensor response at the central wavelength.

"The present invention can also be used to verify instrument response to both kinds of stray light on orbit, geometric and out-of-band, by choosing appropriate geometry of observations and laser wavelength. During nighttime the laser beam tracks the satellite as it comes up over the horizon until it goes down. Meanwhile, the on-orbit sensor acquires data in its nominal mode. All measurements are normalized to the signal from direct view of the laser site by the sensor to provide a relative measurement. The same operation is performed for many orbit tracks in order to develop an integrated stray light model. Verification of out-of-band stray light, or spectral cross-talk, can achieved by operating the laser at a selected wavelength and mapping instrument response in near-by bands.

"The present invention includes several unique aspects or features, including radiometric measurement of ground-based laser signal for long time periods (e.g. 2-5 minutes) by an on orbit sensor. This requires satellite tracking by a laser system and also requires that the sensor is pointing at the laser site on Earth surface. All calibration measurements are relative, and the measurements are performed during short time intervals (i.e. the duration of the signal is about 5-10 seconds). Atmospheric effects cancel out due to the short time interval. The polarization parameter of laser light is not affected by clear atmosphere. Aerosol can de-polarize laser beam for 0.1-0.2% at most (forward scattering), but at a high altitude site this effects are negligible. Operations including radiometric calibration using polarization and wavelength operations have not previously been performed utilizing a ground-to-space laser.

"These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings."

URL and more information on this patent, see: Lukashin, Constantine; Wielicki, Bruce A.. Method for Ground-To-Space Laser Calibration System. U.S. Patent Number 8767210, filed December 13, 2012, and published online on July 1, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8767210.PN.&OS=PN/8767210RS=PN/8767210

Keywords for this news article include: Aerospace, The United States of America as represented by the Administrator of the National Aeronautics and Sp.

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Source: Defense & Aerospace Week


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