Automated Test System for SSR Transponder

Author(s):

Tomasz Marzec - Becker Avionics Polska
Michał Kozarzewski - National Instruments Poland

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.

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 Overview

Hardware Platform

The system allows for testing double-antenna (diversity) transponders, which can read the RF signals from both the bottom and top of the airplane, and consequently choose to work with the stronger signals. Such a double-antenna transponder system significantly increases the complexity of the test system.

The PXI part of the XPDR Test System comprises the following equipment:

• NI PXI Express Chassis—The PXI Express backplane handles data transfer, timing, synchronization, and triggering
• NI PXI Express Controller—The host PC with test execution engine handles serial communication with XPDR, ADS-B in message stream over Ethernet, power supply, and RF box control
• 2x NI PXI Express Data Acquisition—XPDR control and monitoring (digital I/O), power supply voltage monitoring (analog input), and audio frequency generator (analog outputs)
• NI PXI CAN—ARINC825/ARINC826 field loadable software (CAN for airborne system for data loading, firmware update, and more)
• AIT PXI ARINC429—Handling ARINC735B (traffic computer TCAS), ARINC718A (ATCRBS), ARINC743A (GNSS sensor), air data computer, inertial reference system, radio altimeter, flight control computer, flight management computer, and more
• 2x NI PXI Express FlexRIO—FPGA target with in-line digital signal processing, streaming to/from controller memory, peer-to-peer data streaming (refer to XPDR Communication Library chapter for details)
• 2x NI FAM—Digital high-rate data I/O (100 MHz), for example it provides monitoring IQ interface of RF input and RF output signals,
• 2x NI PXI Express VSGs—Transmit RF interrogation signals to the transponder under test (custom waveform created by RIO)
• 2x NI PXI Express VSAs—Receive the RF reply signal from the transponder under test

Additionally, the system uses a custom-designed RF box and interface box. An RF box is a USB-controlled device with fixed RF attenuators (transponder output power might reach 57 dBm, which equals 500 W in linear units), RF switching matrix (remote controlled connections between RF devices and XPDR antennas), and circulators. An interface box delivers an easy and protected interconnection between the PXI system and XPDR (signal conditioning).

Figure 1 shows a schematic of the XPDR Test System.

Software Platform

Software related to RF communication was separately released as the XPDR Communication Library. We used LabVIEW and the LabVIEW FPGA Module to develop it and compiled it into .NET assemblies. Other departments can incorporate SSR-related RF communication into existing test frameworks (based on .NET platform) using the XPDR Communication Library.

XPDR Communication Library

The XPDR Communication Library supports SSR signal generation and analysis. It provides a .NET API to perform measurements on SSR signals. Figure 2 shows a high-level dataflow graph for the XPDR Communication Library.

Consecutive stages of data flow can be described as follows:

1. Host PC (NI PXI Express controller) is used primarily for storing and retrieving test parameters from the database and creating test schedules. Requested test stimuli are continuously streamed to the FPGA target (NI PXI Express FlexRIO) using DMA channels.
2. The FPGA target creates a custom interrogation signal, performs simple math operations on the signals, and generates required sequences with selected gain, pulse widths, and intervals. Optionally digital frequency shift could be applied to IQ data. Generated signal (IQ data) is then streamed using peer-to-peer to VSG.
3. The VSG receives IQ Stream and generates an RF signal with desired carrier frequency and power level to the transponder under test. Based on system requirements related to pulse characteristic (rise time in range 50..100 ns), the VSG generates IQ samples with time resolution as short as 10 ns (which is a result of 100 MHz sample rate).
4. The XPDR acquires and analyzes interrogations and sends replies back. The content of replies depends on the XPDR configuration, simulated navigation data (such as GNSS data), altimeter indication, and more.
5. The VSA receives and analyzes the RF reply signal from transponder under test on desired center frequency and reference level. IQ signal analysis is performed using 50 MHz sample rate. IQ samples are streamed through peer-to-peer to the same FPGA target. The VSG and VSA are synchronized to precise measurements of response time from the transponder.
6. The FPGA target performs real-time processing on received data such as:
   • Reply Classification, Timing
   • Reply Power Level and Frequency
   • Reply Data Content
   • Pulse Measurements (start/rise/fall time, pulse duration, pulse power)
   All the information is compiled into a report stream and sent back to the host PC using DMA channels.
7. Host PC receives the data and returns in high-level format to test execution engine as test results.

Software-defined processing of the XPDR Communication Library delivers new technical capabilities to our XPDR Test System. FPGA ensures real-time analysis by means of point-by-point processing with strictly defined clock. In this context it means that:

• all sampled data is processed, process is continuous without any dead time between acquisitions (gap free)
• all DSP algorithms are hardware implemented (like FFT spectrum generation), which results in consistent speed and dedicated logic in silicon for highest reliability
• all measurements are performed with high rates achieved by parallel processing (task parallelism, data parallelism, pipelining)

User-programmable and fully deterministic FPGA enables a new level of performance and flexibility.

The XPDR Communication Library is compatible with any NI PXI RF vector signal analyzers as well as the NI software defined radio (SDR) platform such as the FlexRIO RF transceiver (peer-to-peer streams will be replaced with target-scoped FIFO).

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 (2.4.2.6 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 during 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 interrogation 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 (full set of test takes two weeks 24/7, single test set working 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.

We will use the XPDR Test System to test XPDRs during manufacturing process.

Author Information:

Tomasz Marzec
Becker Avionics Polska
ul. Jana Długosza 60
Wroclaw 51-162
Poland
tomasz.marzec@becker-avionics.pl