Overview

Exigence implemented a real-time, embedded sensor-control system for an upgrade to the US Air Force NRETS (Nose Radome Electrical Test System) for the F-16 fighter. The component of NRETS that Exigence worked on involves measuring the attitude of the radome while it is in place on a testing pedestal.

Problem

The NRETS range was developed in the early 1980’s to provide periodic maintenance to the radome units for the F-16 airframe. As periodic maintenance, radome units are taken to the NRETS range, tested to determine if their physical shape has affected their RF characteristics, and worked back into the correct shape by Air Force maintenance personnel.

The existing NRETS system has been very effective. However, after 20 years of demanding use, it began suffering two problems: age and performance.

Age

Many hardware components in the original NRETS reached the end of their designed lifespan and began causing regular system failures. For example, servo controllers were burning out, data cables were becoming worn to the point of shielding failure, etc.

Performance

The original NRETS used a minicomputer for the core data processing. While state of the art in its day, the minicomputer was stretched to its limit by the data processing demands of the complex analysis involved in the test process.

Previous Redesign Failure

Along with the old age and suboptimal performance, an additional complexity arose because the Air Force had previously contracted a separate redesign of the NRETS (an effort which did not include Exigence).

However, the previous contractor failed to achieve the very high precision and performance requirements of the system and another contractor had been given the chance to implement the NRETS redesign.

Solution

Exigence contributed on the second NRETS redesign, coordinated by the primary contractors TQS and Westenhaver Wizard Works.

TQS and Westenhaver Wizard Works were contracted to salvage as much as possible from the first redesign attempt while implementing an NRETS system capable of meeting the accuracy and timing requirements.

Westenhaver Wizard Works determined the previous redesign suffered from several critical flaws, including imprecise data collection, and turned to Exigence to implement this key data collection requirement.

PCs are “Slow”

The previous redesign attempt used a standard PC configuration with a high-end commodity microprocessor for data collection. In theory, even a typical 1 GHz CPU has more than enough speed to read the data.

However, in reality, the Microsoft Windows NT operating system is simply too imprecise to achieve the required timing precision.

For example, the data collection system needs to read data every 25 milliseconds. However, Windows NT, being a non-real time operating system, might schedule others tasks for 30 milliseconds before it the system would be able to read the data. For a human, this delay is not noticeable—all tasks are still accomplished relatively quickly, without perceptible delay. But in the NRETS environment, this unpredictability in latency is unacceptable.

Real Time System

Exigence built a 44.2 MHz embedded system that performs more precisely than a 1 GHz modern PC.

To address the needs of the NRETS project, Exigence designed and implemented the Beam Deflection Management System (BDMS) with a real-time software core that can guarantee the system is capable of meeting the strict timing requirements.

The embedded hardware system has a 44.2 MHz processor and a mere 512 KB of RAM; however, because the BDMS code is the only thread of code running (there is literally no operating system running in the background), Exigence was able to guarantee that nothing would interrupt the code and cause a failure to achieve the precise timing requirements.

The BDMS has proven to be successful in resolving all the issues faced by the failed original redesign effort due to several critical success factors.

Industry-Standard Network Protocols

After reading sensor data with real-time precision, the BDMS then sends the data onto a PC for data analysis.

While several other NRETS components devised custom communication protocols, Exigence recommended that the previous system’s RS-232 serial communication be replaced with a TCP/IP network over Ethernet.

Using the TCP/IP protocol has several benefits:

TCP/IP is also a proven, open-standards protocol ideal for the long lifespan required for the NRETS installation.

Long Product Lifespan

The new NRETS installation, and Exigence’s BDMS component, is expected to be in use for 20 years or more. Exigence evaluated all BDMS components (such as embedded board selection, programming techniques, and protocols) to ensure this lifespan will be met.

High Reliability

Exigence’s BDMS is capable of completing over 400,000 sensor readings within 60 minutes.

The critical nature of the project, coupled with the long life span of the system, implied that the BDMS must tested to a high standard to be eliminate all possible defects.

To achieve this high quality standard, Exigence combined its hardware and software expertise to develop an automated regression testing system using JUnit. While JUnit is a Java-based unit testing framework, Exigence used JUnit on a PC to drive a custom microcontroller. JUnit and the custom microcontroller then automatically execute a full suite of tests against the BDMS ensure proper operation in a wide variety of scenarios.

Exigence also built this framework at the very start of the project—instead of postponing QA to the very end of the project—and exposed even alpha and beta revisions to frequent, effective testing and QA.

Early Delivery

Exigence’s robust testing framework also allowed the delivery of preview functionality to the NRETS range well before final delivery. Consequently, other system developers could begin interfacing their own systems with the BDMS.

This was a huge win schedule-wise, allowing many integration scenarios to be tested and fixed—something done only after final delivery in many other projects. The early delivery approach used by Exigence avoided risking many delays and cost overruns.

Results

The BDMS project is currently in late-beta phase pending client acceptance review. The system is feature-complete and has no known bugs.

Next Steps

Exigence is continuing to work with TQS and Westenhaver Wizard Works as the BDMS system is integrated into the new NRETS.

The TQS/Westenhaver Wizard Works/Exigence solution may eventually be leveraged for other radome test ranges around the US. If this happens, Exigence will be involved in producing and customizing the BDMS for each range.

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