Phased Array
A phased array is an antenna system formed by arranging multiple radiating elements with regular spacing and controlling the relative phase (and often amplitude) of the signals applied to each element. By appropriate phase control, the radiated fields from the elements reinforce in desired directions and cancel in others, producing a beam with increased gain and directivity. Changing the relative phases allows the beam to be steered electronically without physically rotating the antenna structure.
Phased arrays can form beams in off-axis directions by introducing progressive phase shifts between adjacent elements, and the beam can be swept over a range of angles by dynamically adjusting these phase shifts. This capability makes phased arrays particularly attractive for satellite and mobile terminals, where rapid beam steering and the avoidance of mechanical pointing mechanisms are important.
Phased arrays are widely used in satellite communications systems. S-band phased-array antennas on tracking and relay satellites can form multiple simultaneous receive beams to support several spacecraft at once. Modern military and commercial satellites employ large receive and transmit phased arrays operating at X-band, Ka-band, or EHF to generate multiple independently steerable beams with variable shape and size, ranging from narrow spot beams to Earth-coverage beams. In low Earth orbit constellations, phased arrays are commonly used to form multiple fixed or steerable beams to support frequency reuse and seamless handover between satellites.
For example, on TDRSS, S-band phased-array antennas can receive signals from five spacecraft at a time—five receive beams can be formed from a 32-element phased array and one transmit beam from a 15-element array. Each AEHF satellite has a 44-GHz receive phased array and two 20-GHz transmit arrays. A WGS satellite has a 188-element receive phased array and a 312-element transmit array, both providing up to eight beams at X-band on opposite circular polarizations—each beam can have an arbitrary shape and size from 2.2° to Earth coverage. A WGS satellite has a 188-element receive phased array and a 312-element transmit array, both providing up to eight beams at X-band on opposite circular polarizations—each beam can have an arbitrary shape and size with a beamwidth from 2.2° to Earth coverage.
On Iridium satellites, each L-band phased array panel has 100 elements and creates 16 fixed-size beams. Globalstar satellites have transmit S-band phased arrays and a L-band receive phased array. The 16 beams produced by a Globalstar satellite are the same for transmit and receive with a central beam at nadir surrounded by nine beams.