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Browse Prior Art Database

Rotating Satellite Interferometer for Direction Finding

IP.com Disclosure Number: IPCOM000073788D
Original Publication Date: 1971-Feb-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

Raabe, HP: AUTHOR

Abstract

This is an active spinning interferometer radiating a signal into space from which a receiver can derive the angular coordinates of its position with respect to the position and attitude of the interferometer. The interferometer is mounted on a spinning synchronous satellite and the signal received on the ground can be used for navigation. Being an active source, an unlimited number of users can participate. The spinning axis is normal to the orbital plane and the interferometer may thus be a rigid part of a spin-stabilized satellite.

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Rotating Satellite Interferometer for Direction Finding

This is an active spinning interferometer radiating a signal into space from which a receiver can derive the angular coordinates of its position with respect to the position and attitude of the interferometer. The interferometer is mounted on a spinning synchronous satellite and the signal received on the ground can be used for navigation. Being an active source, an unlimited number of users can participate. The spinning axis is normal to the orbital plane and the interferometer may thus be a rigid part of a spin-stabilized satellite.

When the base line of the interferometer is tilted at an arbitrary angle, preferably 45 degrees with respect to the spin axis, the interferometer radiation pattern will generate signals on the ground whose modulation is an unambiguous function of the position of the receiver. The system is especially useful when the satellite is in an equatorial synchronous orbit.

In Fig. A, center body 1 has two arms, 2 and 3, which support radiators 4 and 5 and balancing masses 6 and 7. The arms also incorporate RF transmission lines connecting the radiation to a power source located in the center body. The mass distribution is designed so that the satellite's highest moment of inertia is aligned with the body of axis S-S. The satellite is injected into the synchronous orbit with a spin about axis S-S so that it is normal to the plane of the orbit.

The patterns of radiators 4 and 5 should be identical in field strength and polarization and do not have to cover more than the solid angle subtended by the earth, which is about 17 degrees wide. Because despinning of the radiators is not very practical, biconical horn antennas, generating a doughnut-shaped pattern, are used. The polarization of the radiation is parallel to the spin axis.

When the radiators operate in phase, the interferometer pattern consists of 2n + 1 lobes with n = b/Lambda, where b is the distance between the radiators. The peaks are aligned with as many concentric cones as lobes around the base line axis W-W. Thus, the lobes are separated by 2n nodes where the interferometer field vanishes. If radiators 4 and 5 are fed with out-of-phase signals, 2n lobes are generated. They will also be aligned with as many concentric cones. Thy two sets of lobes will interleave in such a way that the peak of one set will be found in the directions of the nodes of the other set.

As the interferometer rotates with the spinning satellite, the cone patterns cause a F-M periodic amplified modulation of the received RF signal. By geometry each cycle of satellite rotation represents a unique signature of the location P of a receiver on the earth's surface.

The angle Alpha defines the cone pattern with respect to an orthogonal coordinate system established by the spin axis S-S, the line X directed toward the center of the earth and the line VEL which is tangential to the circular synchronous orbit of the satellite.

An...