A compact optoelectronic sensor unit measures the apparent motion of the Sun across the sky. The data acquired by this chip are processed in an external processor to estimate the relative orientation of the axis of rotation of the Earth. Hence, the combination of this chip and the external processor finds the direction of true North relative to the chip: in other words, the combination acts as a solar compass. If the compass is further combined with a clock, then the combination can be used to establish a three-axis inertial coordinate system. If, in addition, an auxiliary sensor measures the local vertical direction, then the resulting system can determine the geographic position.
This chip and the software used in the processor are based mostly on the same design and operation as those of the unit described in "Micro Sun Sensor for Spacecraft" (NPO-30867) elsewhere in this issue of NASA Tech Briefs. Like the unit described in that article, this unit includes a small multiple-pinhole camera comprising a micromachined mask containing a rectangular array of microscopic pinholes mounted a short distance in front of an image detector of the active-pixel sensor (APS) type (see figure). Further as in the other unit, the digitized output of the APS in this chip is processed to compute the centroids of the pinhole Sun images on the APS. Then the direction to the Sun, relative to the compass chip, is computed from the positions of the centroids (just like a sundial).
In the operation of this chip, one is interested not only in the instantaneous direction to the Sun but also in the apparent path traced out by the direction to the Sun as a result of rotation of the Earth during an observation interval (during which the Sun sensor must remain stationary with respect to the Earth). The apparent path of the Sun across the sky is projected on a sphere. The axis of rotation of the Earth lies at the center of the projected circle on the sphere surface. Hence, true North (not magnetic North), relative to the chip, can be estimated from paths of the Sun images across the APS.
In a test, this solar compass has been found to yield a coarse estimate of the North (within tens of degrees) in an observation time of about ten minutes. As expected, the accuracy was found to increase with observation time: after a few hours, the estimated direction of the rotation axis becomes accurate to within a small fraction of a degree.
This work was done by Carl Christian Liebe of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category. Refer to NPO-30872.
This Brief includes a Technical Support Package (TSP).
Compact Optoelectronic Compass
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Overview
The document discusses the Solar Compass Chip (SCC), a novel technology developed by the Jet Propulsion Laboratory (JPL) under NASA's sponsorship. The SCC is a small, MEMS (Micro-Electro-Mechanical Systems) based photosensitive chip designed to image the Sun, enabling it to determine the Earth's North heading with high accuracy. Unlike traditional compasses that rely on the Earth's magnetic field, the SCC calculates true North based on the Earth's rotation axis, offering a more precise navigation solution.
The chip operates by capturing images of the Sun over a period, typically around half an hour. As the Sun moves across the sky due to the Earth's rotation, the SCC records its position. The data collected allows for the fitting of a circle on a sphere, representing the Sun's trajectory, with the center of this circle corresponding to the Earth's rotation axis. This method provides an accuracy of a fraction of a degree, making it a significant improvement over conventional magnetic compasses.
The document highlights the potential applications of the SCC, particularly in the toy industry, where a decrease in production costs could lead to widespread use in consumer products. The SCC's ability to determine geographic positions when combined with a clock and other sensors positions it as a valuable tool for navigation and positioning systems, similar to GPS.
Additionally, the document outlines the technical aspects of the SCC, including its power consumption of 10 mW at 30 frames per second and its compact size, measuring approximately 10mm x 15mm x 1.5mm and weighing around 0.5 grams. The chip's design and algorithms are discussed, emphasizing its innovative approach to celestial navigation.
In summary, the Solar Compass Chip represents a significant advancement in navigation technology, leveraging solar imaging to provide accurate directional information. Its potential applications extend beyond traditional uses, opening new markets and opportunities, particularly in consumer electronics and toys. The document serves as a technical support package detailing the SCC's development, functionality, and future prospects, showcasing JPL's commitment to innovation in avionics and sensor technology.
