A Unique Space to Earth Data Transmitting System
- Paper number
IAC-07-B2.3.02
- Author
Mr. V. Sambasiva Rao, Indian Space Research Organisation (ISRO), India
- Coauthor
Dr. Surendra Pal, Indian Space Research Organisation (ISRO), India
- Year
2007
- Abstract
This paper presents a novel architecture of a unique system designed for transmitting high bit rate data with two X-band frequencies to any two pre-designated ground stations simultaneously from an agile satellite, maneuvered with Step and Stare techniques and with spacecraft rotation upto +/- 30 deg in Roll axis. To utilize the advantages of high gain antennas for onboard spacecraft use, the antenna needs to be steered over full hemisphere, to take care of an additional +/-30deg spacecraft rotation in addition to the +/- 60deg normal coverage requirement, to point to a ground station during a visible orbital period. Mechanical steering of the high gain antennas like parabolic dishes to provide hemispherical coverage causes platform disturbances due to the rotating motor and can provide only one beam. Alternative solution is to adopt electronic scanning viz., active phased arrays. The major advantage of phased array antennas is their ability to dynamically reconfigure their radiation pattern to achieve optimal signal transmission in a changing environment. The beam steering is normally carried out by processing the signals going to the distinct radiators that are placed in some spatial planar configuration. The radiating patterns of Planar arrays change heavily when scanned over wide angles, resulting limited angle scanning. To scan a reasonably stable antenna pattern over wide angles, a very large number of spatially controllable radiators is needed resulting in very expensive systems and impractical configuration for spacecraft use. The optimal configuration for an array antenna that needs a true spherical or hemispherical coverage is a spherical active phased array. An electronically scanned space borne spherical phased array antenna that provides hemispherical coverage is designed and developed. The antenna is capable of generating two radiating beams independently scanning +/- 90deg electronically in the azimuth and elevation planes with the help of five bit phase shifters built in the power amplifiers driving the radiating elements. Beam steering unit involving a microprocessor, provides control of phase shift of the amplifiers, selection and switching the amplifiers to electronically steer the two radiating beams to two particular positions. The radiating elements are mounted on a hemispherical dome made of aluminum. A spherical active phased array assembly was built for generating two beams of +20 dBm eirp at X-band. Two X-band carriers QPSK modulated with high bit rate data are routed through Solid State Power Amplifiers which are sized to get the desired eirp. The radiating elements are distributed on a spherical surface of an hemispherical aluminum dome to get near equal directivity for any beam direction. The system is characterized in anechoic chamber and successfully used onboard IRS-TES satellite.
- Abstract document
- Manuscript document
IAC-07-B2.3.02.pdf (🔒 authorized access only).
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