Sohrab Mobasser

Micro Sun sensor

A prototype micro Sun sensor has been developed at the Jet Propulsion Laboratory, California Institute of Technology. It consists of a thin piece of silicon coated with a layer of chrome and a layer of gold with hundreds of small pinholes, placed on top of an Active Pixel Sensor (APS) image detector at a distance of 900 microns. Images of the Sun are formed on the APS image detector when the Sun illuminates the mask. Sun angles are derived by determining the precise location of the Sun images on the detector-just like a sundial. The packaged micro sun sensor has a mass of 11 grams, a volume of 4.2 cm/sup 3/ and a power consumption of 30 mW. The accuracy of the micro sun sensor is better than 1 arcminute and the maximum field of view is 160/spl deg/.

Field test implementation to evaluate a flash LIDAR as a primary sensor for safe lunar landing

From May 2 through May 7 of 2008, the Autonomous Landing and Hazard Avoidance Technology (ALHAT) Exploration Technology Development Program carried out a helicopter field test to assess the use of a flash LIDAR as a primary sensor during lunar landing. The field test data has been used to evaluate the performance of the LIDAR system and of algorithms for LIDAR Hazard Detection and Avoidance, Hazard Relative Navigation, and Passive Optical Terrain Relative Navigation. Reported here is a comprehensive description of the field test hardware, ground infrastructure and trajectory reconstruction methodologies1,2.

Fuzzy image processing in Sun sensor

Sun sensors are widely used in spacecraft attitude determination subsystems to provide a measurement of the Sun vector in spacecraft coordinates. At the Jet Propulsion Laboratory, California Institute of Technology, there is an ongoing research activity to utilize Micro Electro Mechanical Systems (MEMS) processes to develop a smaller and lighter Sun sensor for space applications. A prototype Sun sensor has been designed and constructed. It consists of a piece of silicon coated with a thin layer of chrome, and a layer of gold with hundreds of small pinholes, placed on top of an image detector at a distance of less than a millimeter. Images of the Sun are formed on the detector when the Sun illuminates the assembly. Software algorithms must be able to identify the individual pinholes on the image detector and calculate the angle to the Sun. Fuzzy image processing is utilized in this process. This paper describes how the fuzzy image processing is implemented in the instrument. Also, a camera pin hole model is constructed and used to evaluate the accuracy of the Sun sensor.

Micro sun sensor for spacecraft attitude control

Abstract A micro sun sensor is being developed for use on a Mars rover for the Mars ScienceLaboratory Mission. The micro sun sensor, which is basically a small pinhole camera, consists of a small mask with pinholes, placed on top of an image detector. Images of the sun are formed on the image detector when the sunilluminates the mask. Image processing is performed in the sun sensor that outputs 4 sun centroids, which the rovers main computer converts into sun angles for high gain antenna pointing and heading determination.

Flight qualified micro sun sensor for Mars applications

A flight qualified micro sun sensor is being developed and flight qualified for future Mars missions. The micro sun sensor, which is basically a small pinhole camera, consists of a small mask with pinholes, placed on top of an image detector. Images of the sun are formed on the image detector when the sun illuminates the mask. Image processing is performed in the sun sensor that outputs sun centroids.

Test implementation to evaluate technologies for safe lunar landing

From June 20 through July 7 of 2009, the Autonomous Landing and Hazard Avoidance Technology (ALHAT) Exploration Technology Development Program carried out an aircraft field test over Moon like terrains to assess the use of sensors and algorithms being developed for autonomous, safe lunar landing. The field test data has been used to evaluate the performance of a lidar, a passive optical camera system, and associated algorithms for Terrain Relative Navigation. Reported here is a comprehensive description of the field test hardware, ground infrastructure and trajectory reconstruction methodologies1,2.

Silicon Nanotips Antireflection Surface for Micro Sun Sensor

We have developed a new technique to fabricate antireflection surface using silicon nano-tips for use on a micro sun sensor for Mars rovers. We have achieved randomly distributed nano-tips of radius spanning from 20 nm to 100 nm and aspect ratio of ∼200 using a two-step dry etching process. The 30° specular reflectance at the target wavelength of 1 μm is only about 0.09 %, nearly three orders of magnitude lower than that of bare silicon, and the hemispherical reflectance is ∼8%. By changing the density and aspect ratio of these nanotips, the change in reflectance is demonstrated. Using surfaces covered with these nano-tips, the critical problem of ghost images that are caused by multiple internal reflections in a micro sun sensor was solved.

Raman Scattering From Phonons And Magnons In Magnetic Semiconductor, MnTe

Laser Raman scattering by phonons and two-magnons in antiferromagnetic and paramagnetic phases of manganese telluride has been measured and compared to theoretical prediction. Spectra were obtained experimentally on polished single crystals of. MnTe, using an argon ion laser, a spex double-grating spectromemter, and a photon counting detection system. We report the observation of one E2 phonon, predicted by a group theoretical analysis of the D46h crystal space group, at frequency of 178±1 cm -1, and abroad second order peak at frequency of 360±10 cm-1 which is attributed to a two-magnon scattering. A theoretical calcu-lation of the magnetic spin-wave dispersion was performed, starting from a Heisenberg spin Hamiltonian. A closed form solution of energy versus momentum in three-dimensions was obtained containing three unknown superexchange constants J1, J2, and J3. A Monte Carlo com-puter simulation of two-magnon joint density of states in the full Brillouin zone was used to fit experimental data. The best fit to experiment was obtained for superexchange constants: J1 = -1.440 meV, J2 = +0.220 meV, J3 = -0.024 meV. The spin wave dispersion curves in different symmetry directions were plotted using the obtained superexchange values.

Galileo spacecraft autonomous attitude determination using a V-slit star scanner

Absolute attitude determination and the rotor spin rate of the Galileo spacecraft are derived solely from a two-axis star scanner, (SS) that is sensitive (outside the Jovian magnetosphere) to at least 200 brightest stars. With the 200-star sensitivity, the SS always provides at least three stars for attitude determination anywhere in the spacecraft orbit. Because the SS must function in the intense Jupiter radiation environment, which consists primarily of high-energy electrons and photons, the Galileo SS is designed with radiation hardness as a prime objective. The Galileo SS is described together with an entirely autonomous attitude determination algorithm. The SS analytical model is also presented. The selection process for the models critical parameters which are used by the attitude determination algorithm is explained. Post launch operation of the attitude determination algorithm is discussed and actual flight data are presented.<<ETX>>

A Sun gate for Galileo spacecraft attitude control

The combination of a Sun sensor called a Sun gate (SG) and a digital programmable signal processor on the Galileo spacecraft attitude and articulation control subsystem (AACS) will orient the rotation axis of the spacecraft toward the Sun to satisfy a new requirement imposed by the new spacecraft trajectory. The combination will continuously monitor the pointing direction of the rotation axis, and any off-Sun excursions of more than a preset threshold will be detected, triggering appropriate actions by the flight software to prevent off-Sun cone angles of more than 14 degrees . The design of the SG is described in detail, its principle of operation is given, and the flight software processing of the SG output is discussed.<<ETX>>