SSR Based Monopulse Tracking System Development

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Dudás Levente
Department of Broadband Infocommunications and Electromagnetic Theory

Secondary surveillance radar systems are tracking aircrafts those have mose A/C or S transponders on it. Monopulse is special beam forming technology that applies simultaneously overlapping beams. Using these beams it obtains sum -and delta channels which helps to track the targets much more accurately than the conventional angle-measurement technologies.

This thesis is about a monopulse tracking system that uses the mode S transponder's signals to detect the aircrafts. Using the proper interface and control system, the reciever antenna array can be rotated in the exact direction of the target.

The development started with the antenna design, that is suited for the transponder's transmit frequency which is 1090 MHz. After the simulation the antenna has been fabricated. Since it's a monopulse system, it needs at least four antennas to obtain the required sum- and delta channels. To avoid manufacture -and material complexities, microstrip rectangular patch antennas were chosen and a patch antenna array with 4 elements has been fabricated. To preserve phase stability we need to process these signals in high frequency, which requires a microstrip hybrid system. The three output channels (sum and two delta) require at least two hybrids that shifts 180° in phase. Ratrace hybrids at this frequency cannot be used unfortunately because they need too big radii. Hence quadrature hybrids will be used with one elongated input, where the lengthening is quarter-lambda. This results is a 180° phase shift. As for the output, we get 3 channels which now can be connected to the printed circuit board (PCB) for further processing. The first step is filtering, then mixing it to a 4 kHz intermediate frequency which already can be digitalized. A Xilinx Spartan 6 FPGA is being used to process the data and to create the control system of the rotator using the proper interface.

Summarizing the job that has been done in this thesis the main parts were antenna -and high frequency signal process design, individual PCB design, programming using Verilog HDL and obtaining a control system for the rotator.


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