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Automated Test of Secondary Surveillance Radar Transponders

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Original Authors: Tomasz Marzec, Becker Avionics Polska

Edited by Cyth Systems

SSR transponder technology serving as an air traffic control beacon.
SSR transponder technology serving as an air traffic control beacon.

The Challenge

Creating a flexible, scalable, and automated test system that allows various test scenarios for secondary surveillance radar (SSR) transponder (XPDR), compliant with Automatic Dependent Surveillance–Broadcast (ADS-B) technology, that can perform RF communication test and simulate, monitor, and control real airborne environments, from power supply to all variants of communication buses like ARINC429, CAN, TIA-422.

The Solution

Using NI PXI products to facilitate the functionality tests of the SSR transponder in accordance with RTCA/ICAO required documents to develop the XPDR Test System.

Becker Avionics developed the XPDR Test System using NI PXI products to facilitate the functionality tests of the SSR transponder in accordance with RTCA/ICAO required documents. Functionality tests included simulators of multiple ground stations, onboard navigation systems (real-time trajectory motion), cockpit instruments, displays, and switches (flight control and management system).

We chose the NI PXI system with RIO (Reconfigurable I/O) for fast development of complex RF stimuli generation and RF response analysis. The flexibility of the hardware and software platform (LabVIEW and the LabVIEW FPGA Module) from NI allowed performing even the most complex tests described in DO-181E and DO-260B documents. The RF communication capability of the software we developed was separated and released as the XPDR Communication Library. We qualified this software tool internally according to DO-178C and DO-330.

Introduction to SSR and ADS-B

SSR is a radar system used in air traffic control (ATC) to detect, identify, and measure the position of aircraft. Compared with the primary surveillance radar system that measures only the range and bearing of targets by detecting reflected radio signals, SSR relies on targets equipped with an SSR transponder that replies on each interrogation signal by transmitting a response containing encoded data. The transponder is a radio receiver/transmitter, which receives on 1,030 MHz and transmits on 1,090 MHz.

SSR essentially provides two-way air-to-ground data communication and operates in several modes of interrogation (for example Mode A, Mode C, and Mode S). Each mode produces a different response from the aircraft (identification, altitude, and multipurpose—flight ID, latitude, longitude, altitude). Modes A and C use simple pulse–position modulated interrogations and replies. Mode S uses differential phase shift keying modulation for encoded data in interrogation.

A Dataflow Graph of the SSR’s communication protocol and library.
A Dataflow Graph of the SSR’s communication protocol and library.

Automatic Dependent Surveillance–Broadcast (ADS-B) technology automatically transmits, through data link, position data derived from the onboard navigation system. ADS-B provides real-time surveillance information to ATC (as a replacement for SSR) and to other aircraft for situational awareness and self-separation.

Selected Technical Challenges

Testing SSR transponders is an exceptionally demanding task that requires understanding various complex engineering areas and technologies that are not separate, but depend heavily on one another. Specialized knowledge and experience is crucial for success in such test development and involves:

  • RF generation and analysis—short duration, non-periodic pulses

  • Dynamic range considerations and weak signals of interest in the presence of strong interferers

  • Real-time signal analysis

  • RF multichannel timing and synchronization

  • High bandwidth and low-latency data streaming

We used commercial off-the-shelf (COTS) technologies from NI to reduce our efforts related to the aforementioned topics.

System Bandwidth

Total system bandwidth is one of the most important issues when dealing with RF instruments. The NI PXI Express chassis delivers a high-bandwidth backplane to meet a wide range of high-performance test and measurement application needs. NI peer-to-peer technology ensures high-throughput and low latency (~10 us) module-to-module data transfer.

In the XPDR Test System, we can group the data transfers by the following links:

§ FPGA to VSG peer-to-peer links—100 MS/s, 4 B/S, 2X TX channel gives 800 MB/s

§ VSA to FPGA peer-to-peer links—50 MS/s, 4 B/S, 2X RX channel gives 400 MB/s

§ Host to/from FPGA DMA links—up to 16 DMA channels, depends on system load, average 10 MB/s

Total system bandwidth exceeds 1.2 GB/s.

System Timing and Synchronization

Proper timing and reliable synchronization methods are key elements of a test system. The PXI backplane of the NI PXI Express chassis supports timing, synchronization, and triggers to meet long-term and stable RF measurements. Synchronization is achieved by sharing a PXI 10 MHz reference clock. To ease timing alignment, the NI PXI modules share start triggers. Generations and acquisitions begin at precisely the same time (strictly defined for the rest of the XPDR Test System).

Example Test Procedures

XPDR desensitization and recovery test procedure ( from DO-181E document) checks that the XPDR receiver shall recover sensitivity within 3 dB of the minimum triggering level (MTL approximately 74 dBm), within 15 us after reception of desensitizing pulse having signal strength up to 50 dB above MTL. For example, high power Mode S interrogation signal (such as 50 dB over MTL) is transmitted to the transponder and just after that high-power signal (a few µs later), there is a Mode A interrogation signal arriving at a MTL +3 dB level. The transponder needs to react with defined reply efficiency on such dynamic range RF signals at its inputs.

XPDR maximum susceptibility test procedure is a load test, in which hundreds of interrogations are generated within 4 s (Mode A, Mode C, and Mode S with varying delay, period, power, content, and interferences). After 4 s, the transponder is interrogated for 1 s with almost 2,000 ADS-B In signals. The transponder later should not only generate back RF replies, but also repeat all received ADS-B message stream by Ethernet interface. This test is very important for analyzing the behavior of the system in extreme situations, where tens, if not hundreds, of airplanes are in close proximity of the airport and collision avoidance system.

These two example tests show the power of the solution based on software-defined instruments and NI RF solutions in PXI form factor. The system can schedule large numbers of signal interrogations with highly-customizable delays, powers, and shapes, including the possibility of generating interfering signals.

Business Results and Next Steps

We developed the XPDR Test System across multiple stages of transponder research and development. Thanks to the nature of FPGA, which are easily reconfigured, we could quickly adapt tests set to new functions. The innovative NI approach to test platforms with open, software-defined firmware (FPGA) with commercial off-the-shelf RF hardware, reduced the time needed to prepare and perform complicated test scenarios.

NI delivers completely new technical capabilities to automated test systems. This enables new levels of performance and reliability confirmed by long-term development testing (a full set of tests takes two weeks 24/7, single test set works for months). Any failure of the test system would significantly prolong the test time and consequently could delay further product development.

It is also worth noting that NI supported the project by providing important information, benchmarks, and LabVIEW FPGA examples of streaming capabilities between FlexRIO and RF instruments, which streamlined the process of moving into the NI PXI RF platform.

Original Authors:

Tomasz Marzec, Becker Avionics Polska

Edited by Cyth Systems

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