To fly the fastest, most efficient routes worldwide, air traffic control (ATC) organizations are mandating that aircraft be equipped with sophisticated avionics. Consequently, the U.S. Air Force must update its aircraft through the Global Air Traffic Management (GATM) program to meet new international standards for communications, navigation and surveillance. Otherwise aircraft will be restricted to slower air routes and altitudes that could impede their missions. The service’s C-5 cargo and KC-135 aircraft are the first to go through the GATM program.
Dual-Use Integration Tool
To reduce the risk of integrating commercial off-the-shelf (COTS) air traffic control subsystems into aircraft, the Air Force’s Electronic Systems Center (ESC) and Mitre Corp., Bedford, Mass., developed a special test facility. The Reconfigurable Cockpit and Avionics Testbed, or RCAT (pronounced R-Cat), provides the Air Force with a hands-on demonstration, experimentation, training and test center. Mitre also uses the RCAT for advanced technology demonstrations that show how GATM equipment can be used to improve situational awareness and provide new command and control (C2) opportunities.
The ESC-Global Air Traffic Operations/Mobility Command and Control (GATO/MC2) program office at Hanscom AFB, Mass., uses the RCAT to ensure that avionics in Air Force aircraft are compatible with worldwide ATC systems. "Before you put COTS avionics in a live bird, you need to test its capability first," says GATO/MC2 Program Director Col. Al Moseley. "The RCAT gives us complete end-to-end communications testing, as we go from a voice-based system to a data network."
So far, the RCAT has addressed the security concerns associated with:
The commercial data network infrastructure,
Interoperability issues between different vendors’ equipment,
Man/machine issues in the cockpit, and
Operational and procedural issues of �the new ATC data links.
It also has been used to reduce risks in integrating some avionics systems on the VC-25A, or Air Force One.
"RCAT testing not only helps the aircraft gain access to the airspace, but also improves the safety of people flying in that airspace," says Pauline Froebel, the GATO/MC2 deputy program director. Froebel notes that the RCAT gives ESC the capability to test interoperability of ATM equipment between the Mobility Air Force and the Combat Air Force. "It’s part of our enterprise test capability," she says.
Close to the Real Thing
The ESC/Mitre team conducts RCAT testing in real time and uses real air traffic communication links with either simulated or real controllers or dispatchers, so that end-to-end performance can be observed. RCAT uses commercial data links employed in the GATM program: VHF data link (VDL)–Mode 0/ACARS and Mode 2–high-frequency data link (HFDL) and the Inmarsat Aero-I link. (ACARS stands for aircraft communications addressing and reporting system.)
The RCAT lab is essentially a completely qualified airplane with its own tail number (160414). When properly coordinated, an air traffic controller sitting at a workstation can communicate with the RCAT and view its position. Since the RCAT appears on air traffic control monitors as a real aircraft, actual ATC networks can be used to check communication system availability, integrity, throughput and delay. Other labs that test avionics subsystems by simulating air traffic communications with computers cannot test so completely.
RCAT applications are continually evolving. In 1997, George Borrelli, a project leader at Mitre, conceived the RCAT as a way to certify boxes of subsystems for ATC. "The Air Force had just started a COTS procurement program for air traffic control systems," says Borrelli. "I felt it was important that we become experts in all of the subsystems that we certified."� Mitre recommended the reconfigurable testbed concept, which the Air Force accepted.
The RCAT is based on a cockpit mockup originally built for Federal Aviation Administration testing by Mitre’s Center for Advanced Aviation System Development (CAASD). Avionics systems installed in the RCAT are specific to the GATM program.
Various equipment can be installed in the RCAT’s cockpit, enabling users to simulate a wide range of military aircraft for testing equipment or software in a controlled environment. The RCAT can be reconfigured in minutes for different aircraft that use any combination of HF, VHF, or satellite communications. With the appropriate equipment, the RCAT can mimic just about any aircraft going through the GATM upgrade, such as the KC-135 tanker or the C-5 cargo plane. And since it never leaves the ground, the RCAT doesn’t incur the overhead costs of flight crews and maintenance. "It gives us the ability to plan experiments without large investment in equipping real aircraft," says Moseley.
�Much of the equipment in the RCAT is the exact avionics available through the Global Air Traffic Operations/Mobility, Command and Control e-business Web site (https://igatm.hanscom.af.mil/), and the remainder is industry loans, creating a unique combination. The COTS avionics suite installed in the RCAT cockpit mockup uses the commercial ACARS network. It also includes applications for aeronautical operational control (AOC) and air traffic services (ATS). The VHF and HF equipment is ARINC-certified, and the Inmarsat Aero-I link is approved by Societe Internationale de Telecommunication Aeronautiques (SITA).
A typical GATM-equipped flight deck might include COTS avionics systems from Rockwell Collins, Honeywell and Smiths Aerospace. Data link services are supplied by ARINC or SITA. Ground-based AOC workstations can include an ARINC GLOBALink Gateway and an Allied Global Data Center workstation (AFISCOM). A minimum set of avionics is clustered around a communications management unit (CMU). The set includes a flight management system (FMS), multifunction control display unit (MCDU), printer, data loader and the communications system (see above). The data loader can be a floppy disk drive or optical disks.
"We can help a platform’s system program office (SPO) decide what equipment complement should go on a plane," says Mike Bernock, Mitre’s RCAT testbed manager. Because Mitre manages federally funded research and development centers and has trusted nondisclosure status with avionics makers, they readily provide their equipment for interoperability experiments.
"Boxes from Rockwell, Honeywell and Teledyne are likely to be connected together for interoperability experiments," says Bernock. "For example, we can check to see if a Rockwell display is compatible with a Teledyne router." This process reduces cost and risk since SPOs can check interoperability issues before making an investment. As Mitre is prevented by law from working directly for commercial companies, it is able to maintain objectivity in evaluating commercial products.
The RCAT’s capabilities were showcased in 1999 and 2000 during the Air Force’s Joint Expeditionary Force Experiments (JEFX). For the first time, it demonstrated the use of GATM data links for command and control, using AOC applications. During JEFX-99 the RCAT demonstrated near real-time automated global tracking of aircraft positions and other data, using commercial avionics data links: HFDL, VDL and Inmarsat Aero-I. Ground stations included ARINC, Allied and Honeywell software applications, demonstrating C2, using COTS hardware. This set the stage for the Air Force to add requirements for VDL Mode 2 and data link applications, as part of the GATM upgrades, and to establish a baseline for future experiments.
JEFX-00 demonstrated dynamic aircraft retasking through data link upload of flight plans into an onboard FMS. As a result of the JEFX successes, the Air Mobility Command (AMC) adopted the dual use of GATM communications for aircraft dispatch and flight following operations.
RCAT Firsts
The RCAT also was the first testbed to establish a live data link system integration lab supporting end-to-end AOC and ATS communications. In addition, it was the first military testbed to contract and manage a full-complement data link service provider (DSP), which included three service providers and three radio frequency (RF) links. Other RCAT firsts were the exercising of the ARC-190 HFDL variant in the GlobalLink Network and the demonstration of Fortezza encryption of ACARS messages.
Using RCAT, the ESC/Mitre team showed how communications could be improved by using commercial data links in place of voice between an aircraft and the Tanker Airlift Control Center (TACC). "Sending an aircraft’s position report by voice over an HF frequency might take 100 seconds," says Bernock, "but sending digital data takes only two to three seconds. The exercise proved that C2 message traffic could coexist with air traffic control message traffic without interference. AMC found it could leverage the GATM bandwidth for command and control.
"Part of our role is to figure out new ways to use avionics for better situational awareness and C2," Bernock adds. "For example, we explore how to apply COTS GATM systems for C2 applications. We also investigate how to apply C2 systems for GATM applications and how to apply COTS non-GATM systems to GATM and C2 applications."
Since most of the RCAT staff are not trained pilots, they may uncover glitches resulting in unintended consequences. "We fly two to three missions a week," says Bernock. "Besides testing equipment for performance and interoperability, we can use the RCAT to make the avionics suite do things it wasn’t designed to do and reveal potential problems. For instance, we may have the switches in a wrong mode when we do something. Or we may create a set of conditions that causes a computer reset not previously encountered by the manufacturer.
"But if we could make the system do that, so could somebody else. It could be a condition that was never tested because the vendor thought it would never happen. With this, we help vendors fix problems before they happen."
�The RCAT can test in three modes:
As an aircraft, where the controller can’t discern the cockpit from a real aircraft, except that there is not correlated radar track data;
As the ATC center or dispatcher; and
As both the aircraft and ATC center (or dispatcher) simultaneously for end-to-end communications testing.
End-to-end testing verifies how well messages pass back and forth from pilots to controllers through interfaces such as a keyboard and printers. Sometimes, software simulation of an interface is incomplete or inaccurate, and it’s not always possible to detect a collision or a lost message. RCAT solves that problem.
Another test verifies the accuracy of the interface control document (ICD), which describes how messages are formatted. For example, the ICD defines how longitude, latitude, airspeed, altitude, etc., are separated in a stream of data so that they are placed in their proper fields. Separations are made by delimiters, which can be a symbol such as a slash, space or comma. If the proper delimiter is missing, the bounds of the data can’t be determined and the computer won’t operate correctly.
Regression testing is used to determine whether a software fix has affected anything else. Ideally, the application software is partitioned so that if, for example, an ATC message set is altered, it won’t affect other software modules. Otherwise, it could alter the way remaining fuel is counted or cause some other anomalous condition.
Bugs Have Been Detected
"We have identified and captured several software bugs that have impacted the operating system, the flight management software, and the ATC messaging application," says Bernock. "The first thing we do is try to reproduce the problem to make sure we didn’t cause it ourselves. Then we document it. If there’s any fault data recorded by the equipment, we download the data and send it to the vendor to give it a heads up. In this way we’re able to speed changes to the field, especially if the vendor was unaware of the problem.
"We may have discovered something new, or they may say they’ve already solved that problem with the latest software release, in which case we’ll get the upgrade," continues Bernock. "People say ‘it’s only software,’ but their perception of the flexibility of software always seems to exceed its ability to be flexible."
David Van Cleave is a senior writer/editor at the Mitre Corp.