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Anil Pandey : Electronics Design and Simulations

Monopulse Tracking System for Satellite Tracking

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A tracking system in earth station antenna tracks the motion of the satellite. Especially in the case of Low-earth-orbit (LEO) satellites, that appears for the small time window to earth station antenna, it’s necessary to communicate with satellite effectively in that short duration of time and receive or download all necessary data, that is stored in satellite computer to earth station server. There are many other applications of tracking system like to track satellite launch vehicles, missile trajectory path track.

The antenna of the satellite must be pointed accurately toward the earth station antenna with which the satellite is in communication using a high-gain reflector antenna system. In order to achieve this accurate pointing, satellite antenna commonly employ tracking systems to provide signals indicative of the pointing errors in elevation and azimuth relative to the antenna beam of the earth station antenna. In a tracking system, three signals are used for the pointing accuracy of the satellite antenna. These three tracking signals are the azimuth difference signal, elevation difference signal, and the sum signal. The phase and amplitude characteristics of these three signals are utilized in a conventional manner to generate elevation angle error and azimuth angle error signals to control the pointing direction of the satellite antenna.

Typically, the tracking system on the satellite utilizes a monopulse-tracking configuration in which a plurality of antennas, feeding a reflector system, is employed to develop three tracking signals indicative of the pointing accuracy of the satellite antenna. These three tracking signals are the azimuth difference signal, elevation difference signal, and the sum signal. These signals can be generated by multimode monopulse tracking. One disadvantage of conventional monopulse tracking systems is that such systems are designed to operate with cumbersome antenna arrays (4 horns are 5 horns). In such arrays, the plurality of antennas is used to develop the sum and difference signals needed to provide the receiver with the means for developing the elevation and azimuth angle error signals for controlling the tracking system. Such cumbersome plural antenna arrays tend to be larger and heavier than desirable at high frequencies. Depending on the applications, different types of multimode monopulse techniques are feasible but none of these designs operate in dual mode.

The Monopulse multimode tracking feed eliminates all these issues. The constraints of the requirement of single dual-mode tracking feed with communication channel necessitate the development of this new type multi-mode tracking feed.In multi-mode monopulse system, higher order modes of circular waveguide are used for tracking. In this system, when an antenna receives an incident wave, the output level of the communications signal is maximum when the antenna points directly toward a point signal source. On the other hand, higher order modes are excited in the waveguide when the boresight axis of the antenna feed is not in line with the point source.
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