Sequence Detection for PPM Optical Communication With ISI

Inter-symbol interference could be reduced.

A method of sequence detection has been proposed to mitigate the effects of inter-slot interference and inter-symbol interference (both denoted “ISI”) in the reception of M-ary pulse-position modulation (PPM) optical signals. The method would make it possible to reduce the error rate for a given slot duration, to use a shorter slot duration (and, hence, to communicate at a higher rate) without exceeding a given error rate, or to use a lower-bandwidth (and, hence, less-expensive) receiver to receive a signal of a given slot width without exceeding a given error rate.

Posted in: Briefs, TSP, Information Sciences, Mathematical models, Architecture, Wireless communication systems

Algorithm for Rapid Searching Among Star-Catalog Entries

Recursive algorithm utilizes precompiled search structure of spherical coverings.

An algorithm searches a star catalog to identify guide stars within the field of view of a telescope or camera. The algorithm is fast: the number of computations needed to perform the search is approximately proportional to the logarithm of the number of stars in the catalog.

Posted in: Briefs, TSP, Information Sciences, Mathematical models, Optics, Identification

Expectation-Based Control of Noise and Chaos

A proposed approach to control of noise and chaos in dynamic systems would supplement conventional methods. The approach is based on fictitious forces composed of expectations governed by Fokker-Planck or Liouville equations that describe the evolution of the probability densities of the controlled parameters. These forces would be utilized as feedback control forces that would suppress the undesired diffusion of the controlled parameters. Examples of dynamic systems in which the approach is expected to prove beneficial include spacecraft, electronic systems, and coupled lasers.

Posted in: Briefs, TSP, Information Sciences, Adaptive control, Electronic equipment, Lasers, Systems engineering, Noise, Spacecraft

Radio Heating of Lunar Soil To Release Gases

A report proposes the development of a system to collect volatile elements and compounds from Lunar soil for use in supporting habitation and processing into rocket fuel. Prior exploratory missions revealed that H2, He, and N2 are present in Lunar soil and there are some indications that water ice may also be present. The proposed system would include a shroud that would be placed on the Lunar surface. Inside the shroud would be a radio antenna aimed downward. The antenna would be excited at a suitably high power and at a frequency chosen to optimize the depth of penetration of radio waves into the soil. The radio waves would heat the soil, thereby releasing volatiles bound to soil particles. The escaping volatiles would be retained by the shroud and collected by condensation in a radiatively cooled vessel connected to the shroud. It has been estimated that through radio-frequency heating at a power of 10 kW for one day, it should be possible to increase the temperature of a soil volume of about 1 m3 by about 200 °C — an amount that should suffice for harvesting a significant quantity of volatile material.

Posted in: Briefs, TSP, Physical Sciences, Antennas, Soils, Spacecraft fuel, Gases, Acoustics, Mining vehicles and equipment

Using Electrostriction To Manipulate Ullage in Microgravity

A report proposes to use electrostriction to manipulate the ullage in a tank containing a dielectric liquid in a microgravitational environment. In the original intended application, the liquid would be a spacecraft propellant and the goal would be to force the ullage (comprising bubbles of noncondensible gas) to coalesce at one end of the tank, to enable use of one of the established means of (1) measuring the position of the gas/liquid interface and (2) inferring the quantity of liquid from the measurement. Electrically insulated wires would be installed in the tank, shaped and positioned so that application of a suitably high potential (e.g., 1 kV) between adjacent wires in successive pairs would give rise to a sufficient electric field gradient along the tank. The resulting electrostriction in the liquid would give rise to a pressure gradient that would force the ullage toward the low-electric-field-magnitude end of the tank. The feasibility of this proposal was demonstrated in an experiment in a tank containing liquid helium aboard an airplane flying a low-gravity arc. The ullage-segregating electrostrictive effect is expected to be considerably greater in other liquids.

Posted in: Briefs, TSP, Physical Sciences, Measurements, Wiring, Spacecraft fuel, Gases, Fuel sensors, Fuel tanks

Equations for Scoring Rules When Data Are Missing

A document presents equations for scoring rules in a diagnostic and/or prognostic artificial-intelligence software system of the rule-based inference- engine type. The equations define a set of metrics that characterize the evaluation of a rule when data required for the antecedence clause(s) of the rule are missing. The metrics include a primary measure denoted the rule completeness metric (RCM) plus a number of subsidiary measures that contribute to the RCM. The RCM is derived from an analysis of a rule with respect to its truth and a measure of the completeness of its input data. The derivation is such that the truth value of an antecedent is independent of the measure of its completeness. The RCM can be used to compare the degree of completeness of two or more rules with respect to a given set of data. Hence, the RCM can be used as a guide to choosing among rules during the rule-selection phase of operation of the artificial-intelligence system.

Posted in: Briefs, TSP, Information Sciences, Mathematical models, Artificial intelligence, Diagnostics, Prognostics

Pseudorandom Switching for Adding Radar to the AFF Sensor

A document describes the proposed addition of a radar function to the Autonomous Formation Flying Sensor, making possible coarse relative-position control to prevent collisions in the event of failure of one of the spacecraft. According to the proposal, in addition to tracking GPS-like one-way ranging signals transmitted by the other normally functioning spacecraft, each spacecraft could simultaneously track the reflection of its own ranging signal from a disabled, non-transmitting spacecraft. From the round-trip travel time, the approximate distance to the disabled spacecraft could be estimated. To prevent jamming of the receiver by the transmitter on the same spacecraft, the receiver would be switched off during transmission.

Posted in: Briefs, TSP, Electronics & Computers, Attitude control, Global positioning systems, Global positioning systems (GPS), Radar, Collision avoidance systems, Spacecraft

Electro-Optical Imaging Fourier-Transform Spectrometer

Size, weight, and vibration are reduced by eliminating moving parts.

An electro-optical (E-O) imaging Fourier- transform spectrometer (IFTS), now under development, is a prototype of improved imaging spectrometers to be used for hyperspectral imaging, especially in the infrared spectral region. Unlike both imaging and non-imaging traditional Fourier- transform spectrometers, the E-O IFTS does not contain any moving parts. Elimination of the moving parts and the associated actuator mechanisms and supporting structures would increase reliability while enabling reductions in size and mass, relative to traditional Fourier-transform spectrometers that offer equivalent capabilities. Elimination of moving parts would also eliminate the vibrations caused by the motions of those parts.

Posted in: Briefs, TSP, Electronics & Computers, Downsizing, Spectroscopy, Product development, Vibration, Reliability

Holographic Plossl Retroreflectors

Lightweight, inexpensive holographic optical elements would be used in place of lenses.

Holographic retroreflectors that function equivalently to Plossl eyepieces have been developed and used in free-space optical communication systems that utilize laser beams. Plossl eyepieces are well known among telescope designers. They have been adopted for use as retroreflectors and as focusing elements (for reception) and collimating elements (for transmission) in optical communication systems. A retroreflector that incorporates a Plossl eyepiece is termed a cat’s eye retroreflector (see figure).

Posted in: Briefs, TSP, ptb catchall, Tech Briefs, Photonics, Lasers, Optics, Telecommunications

High-Speed Laser Scanner Maps a Surface in Three Dimensions

Surface flaws can be scanned automatically and displayed in real time.

A scanning optoelectronic instrument generates the digital equivalent of a three-dimensional (X,Y,Z) map of a surface that spans an area with resolution on the order of 0.005 in. (≈0.125mm). Originally intended for characterizing surface flaws (e.g., pits) on space-shuttle thermal-insulation tiles, the instrument could just as well be used for similar purposes in other settings in which there are requirements to inspect the surfaces of many objects. While many commercial instruments can perform this surface-inspection function, the present instrument offers a unique combination of capabilities not available in commercial instruments.

Posted in: Briefs, TSP, ptb catchall, Tech Briefs, Photonics, Measurements, Lasers, Optics, Inspections

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