Unlike primary radars, Secondary Surveillance Radar (SSR) calculates the range and azimuth of a target, such as an aircraft, using a bidirectional communication link to gather information. Engineers use SSR in both military and civil aviation, with the former incorporating an identifying friend-or-foe system.

Figure 1. Overall architecture of the ATE to test the SSR.
SSR works in different modes to obtain information from the target. The system sends interrogating pulses from the radar in a bidirectional rotating antenna at 1030 MHz. If a target detects interrogation, the transponder of the target replies with a frame of pulses at 1090 MHz. Radar at the ground station generates interrogating pulses and requests information such as identity, altitude, or country code from the target represented as mode-A/3A, mode-C, or mode-S. Based on the interrogation answers, the aircraft replies with a standard reply pulse format. The system calculates range and azimuth based on the speed-to-distance relation and rotary antenna position with respect to north or the heading direction.

Today’s radars need rigorous testing before they are deployed in military or civil aviation. An automated test equipment (ATE) system was developed using NI PXI modular instruments from National Instruments (Austin, TX) to facilitate the functionality tests of the radar and physical parameters test of the receiver (Rx) and transmitter (Tx), including Rx bandwidth, Rx sensitivity, Tx power, and Tx pulse parameters. Functionality tests included a target simulator to the radar at 1090 MHz, video signal detection, and radar scan converter display using synthetic transistor-transistor logic (TTL) video signal and LAN communication. Reply pulses in the target and multitarget simulators were stationary and trajectory motion.

The system was composed of an NI PXI-1042 eight-slot chassis and an NI PXI-8196 embedded controller. The radar was kept either in transmitting mode or receiving mode to test the Tx and Rx functionality. External antenna signals north and azimuth count pulses (ACPs) were generated and simu lated through an FPGA board. Target reply pulses were generated through an NI PXI-5671 vector signal generator (VSG) at 1090 MHz. The system acquired demodulated video signals from the receiver through an oscilloscope card for Rx functionality tests. High-power transmitted RF pulses were acquired through an NI PXI-5661 vector signal analyzer (VSA) to measure Tx signal power and pulse parameters. The synthesized video at the TTL level generated from the radar processing unit was acquired through FPGA digital input, and used for a radar scan converter to display the target on a polar plot with its range and azimuth position, informantion code, altitude, and country code.

Figure 2. The radar scan converter display decoded through the FPGA.
Each trigger and sync pulse was synchronized with the interrogating RF pulse of the SSR. To protect the instruments, the radar transmitter was switched off during the Rx tests because the radar had a built-in TR module. Both Tx and Rx ports shared the same physical port, which connected to an antenna. The VSA and VSG connected to this same physical port, replacing the antenna, and generating and acquiring RF signals at 1090 MHz and 1030 MHz.

Tx out of the radar is connected to the VSA of the ATE with an attenuator.

Rx in the radar is received by the RF pulses generated through the VSG, which was synchronized with a trigger/sync pulse. Each sync pulse was synchronized with the interrogating pulse. After receiving a sync pulse to the trigger port of the VSG and FPGA, the RF pulse out was generated through the VSG. The Rx video out was connected to the oscilloscope card to measure receiver sensitivity, bandwidth, dynamic range, and frequency stability; phase differential; reception chain operational sensitivity; and reception chain side lobe suppression.

In the functional test, the system generated the antennal simulation signals, such as north and ACP. It simulated multiple targets at different azimuths and ranges in both stationary and trajectory motion, and represented the transponder’s azimuth and range in a radar scan conversion application.

Proper functional test of an Rx can be conducted through target simulation using the VSG based on sync pulses. In this case, the ATE acts as a target signal generator coming from the antenna. Each interrogation is synchronized by a trigger pulse connected to both the VSG trigger and the FPGA. Users can configure the range and azimuth to simulate with the target. When a target is ready for simulation, the VSG generates the reply RF pulses of a target after the azimuth count is reached in the FPGA and the next sync trigger is received from the radar. The user can select reply code and mode, and scripted pulses are generated at the specified range and azimuth. Targets are simulated for stationary and trajectory motion. A user configures moving paths at different trajectories. The system can simulate multiple targets at different ranges and azimuths from the same VSG.

A modular, editable sequence of tests was developed in LabVIEW to test total functionality. Users can select either automatic or manual mode for individual parameter test. With a diagnostic pan - el, users can access the individual PXI instruments for loop-back or self-test.

This article was written by Vishwanath Kalkur and Mondeep Duarah of Captronic Systems Pvt Ltd. using National Instruments products. For more information, Click Here .

Aerospace & Defense Technology Magazine

This article first appeared in the December, 2015 issue of Aerospace & Defense Technology Magazine.

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