Faster Than A Speeding Aircraft: Advances in Flight Test Telemetry

From the early days of aviation over a century ago, advances in aerospace technology have depended mostly on tests conducted under actual flight conditions. The wireless transmission and reception of flight test data — known as telemetry — first became widespread in the 1950s and has allowed near-real-time access to engineering data required to reduce risks and costs while incrementally advancing technology.

Telemetry is critical for the aviation industry to deliver new products. It’s a key asset to aerospace research, testing and FAA certification. Higher speeds, altitudes and performance requirements, combined with increasingly complex and integrated systems, have added to the need for instantaneous test data.

Airplanes, helicopters, missiles, commercial space vehicles, unmanned aircraft systems, and electronic vertical takeoff and landing aircraft are all born as “test articles” and must be pushed to their limits in flight tests to succeed. For example, “flutter” tests are performed to stress the wings and control surfaces to the point where aerodynamic, elastic and inertia forces can produce potentially violent oscillations that, once begun, are difficult to control. Likewise, flight envelope-expansion testing, such as stall, spin, and dive tests, requires that aircraft be flown to the limits of their airspeed and load factor design capabilities. All of these tests are inherently dangerous to the pilot and persons on the ground. Even routine tests can involve significant safety risks.

Telemetry allows controllers on the ground to monitor the second-by-second performance of a test article. Controllers can warn a TA pilot to abort maneuvers that threaten a structural failure, out-of-control flight, or other system emergency. And real-time data captured can be analyzed to resolve potential problems that could make or break a new aerospace design, or even aid in reconstructing the cause of a crash.

Drinking from a Data Firehose

An aerospace industry survey conducted in 2014 indicated a fourfold increase in the number of flight tests conducted over a three-year period (2009-2011). Moreover, the number of test measurements is growing exponentially with the increased requirements to fly at higher altitudes and faster speeds. According to a 2004 study conducted by the MITRE Corporation, the amount of data required to be telemetered from aircraft test points had roughly doubled every three to five years since 1972, with few signs that this rise has abated. For example, the FAA certification process for the Boeing 707 airliner in 1954 included the electronic monitoring of 300 data channels. By 1995, certification for the Boeing 777 aircraft included the monitoring of approximately 64,000 channels. In 2011, the flight testing required to certify the Boeing 787 exceeded 200,000 individual data channels.

TmNS represents a paradigm shift from Pulse Code Modulation (PCM) to Ethernet-based telemetry. As shown in the graphic above, TmNS allows “bidirectional” communications for real-time access to current and past measurements of the aircraft in order to “back fill” any data drops during testing. Curtiss-Wright image.

The frequency and precision of measurements have also increased, with current tests requiring over 400 test samples per second that output up to 32 bits per sample. Test aircraft now routinely collect data at a rate of several gigabits per second. However, due to bandwidth limitations, the rate at which that data can be transmitted is only about 5 to 15 megabits per second, forcing test engineers to transmit only the most critical aircraft safety data being collected.

Graphic 2 — Curtiss-Wright recently developed the nXCVR-3140A-2, which is an Internet Protocol (IP) transceiver designed for air-to-ground and ground-to-air wireless TmNS-based communications. The air and ground transceivers work together to perform wireless router functions transparently, connecting two or more radios in a point-to-point or multipoint configuration. Curtiss-Wright image.

Not Your Father’s Telemetry

Because flight test instrumentation has changed so dramatically over the last decade, data acquisition units have transitioned from using protocols like Pulse Code Modulation (PCM) — which are limited by frequency bandwidth availability — to Ethernet. In order for these cutting-edge protocols to be uniformly applied to the industry, standards must be developed. The standards body for flight testing is the Inter-Range Instrumentation Group, and the standard specifically developed for telemetry is published under IRIG 106.

The 19th revision of IRIG 106 (IRIG 106-19) introduces the application of a new tool for flight test telemetry industry known as the Telemetry Network Standard. The system requirements for TmNS touch all the standard flight test equipment, including data acquisition units, switches, recorders, radios, and the ground elements such as the antenna and the ground system software.

TmNS aims to move telemetry into the network age, representing a paradigm shift from unidirectional PCM architectures to something akin to existing Internet-based networks. A fundamental principle of the TmNS approach is to enhance, rather than replace, today’s telemetry systems by providing significant improvements in frequency spectrum efficiency in order to more effectively handle vast quantities of data.

“Ranges in the U.S. are now rolling out TmNS-aware equipment for current flight test programs,” said Paul Cook, director of missile systems for Curtiss-Wright Defense Solutions, with 40 years of experience at the company. “Telemetry has been around since the 1950s. What’s changed is the amount of data. It’s a fun time right now in telemetry.”

Sridhar Kanamaluru agrees. He also works at Curtiss-Wright, as a technical fellow and the chief architect for the telemetry business. “Data is currency in our business,” said Kanamalaru, “and there has been an exponential growth in telemetry. A manufacturer of a TA has to hit test points so they can determine if a test point might need repeated.”

Paul Cook, Director of Missile Systems for Curtiss-Wright Defense Solutions

The TmNS program calls for compatible data acquisition units, radios, radio accessories, switches, recorders, and software as seed products to grow the system. Curtiss-Wright undertook the development of many of these and brought to market its own TmNS transceiver — the nXCVR-3140A-2.

The nXCVR-3140A-2 (graphic 2 previous page) is an IP transceiver designed for air-to-ground and ground-to-air wireless TmNS-based communications. The air and ground transceivers work together to perform wireless router functions transparently, connecting two or more radios in a point-to-point or multipoint configuration. This facilitates higher traffic, increased compatibility, and access to many useful features now available in the commercial space market.

Dewesoft has a variety of data acquisition hardware that can interface with the telemetry market and uses DewesoftX data acquisition software​​​​​​​ that can read and decode the IRIG 106 Chapter 10 data from any telemetry data recorder in real-time over the ethernet, the company says. Dewesoft image.

For example, the nXCVR-3140A-2 solves one of the most protracted problems with traditional telemetry — data dropouts. These gaps in the flight test data can occur at any point during a test flight, and they can prevent the ground controllers from knowing if a test point was completed successfully. With real-time access to data stored in the recorder, a ground engineer can retrieve any past measurements. This data can then be patched back into the stream to fill in any gaps or correct corrupted data, giving the engineer a complete aggregate of the data.

The TmNS radio achieves this through its ability for “bidirectional communications” that allows the retrieval of measurements from the TAs in near-real time that were dropped in the Serial Streaming Telemetry feed (PCM dropouts). Bidirectional communications also allow real-time access to current and past measurements of the TA — both directly from the sensors and from the recorders — and the ability to status, configure, and control TA equipment from the ground station (see image at top of page 46).

The Need for Data

“We can also provide close to real-time telemetry,” explained Kanamaluru. “TmNS also allows multiple TAs to be tested at the same time.” Paul Cook agreed: “Over the past two years, we’ve seen a significant increase in production and orders. Our equipment is very popular on the east and west coast ranges, and we are very successful with the commercial launch industry.”

Cook explained that the industry “needs greater capabilities for vehicles like hypersonic missiles,” and Curtiss-Wright’s nXCVR-3140A-2 radio fits the bill. TmNS provides the ability to share spectrum resources among many concurrent test activities based on instantaneous demand for telemetry resources, and/or the priorities of certain activities or measurements over others. TmNS also provides the ability to seamlessly transition the transmission and receipt of data from TAs from one antenna to another, including antennas in different networks (frequencies) and in other ranges. Finally, TmNS provides the ability to perform “over-the-horizon” telemetry via communication relays to support tests involving large numbers of TAs and long distances.

The bottom line is that “they need the data,” Cook said.

Slovenia-based Dewesoft offers “total solutions” for complete electric and hybrid vehicle development, validation and production, to include electric motor and inverter testing, battery and battery charge testing, and hydrogen testing. Dewesoft image.

Cook also mentioned the potential impact of the recent arrival of 5G. With its frequency band, spatial diversity, and multipath compensation needs, 5G is well suited for the cellphone industry. However, in a telemetry use case, 5G will have its challenges. It uses a higher frequency band where wide channel bandwidth is available. The problem with this is that the transmission distances in these bands are only some 1,500 feet. For in-flight test telemetry, transmission distances are typically about 130 miles and also line-of-sight. 5G relies on its ground infrastructure to relay the information from ground tower to tower to gain distance, so it is possible to extend its range. However, this would add latency as an aircraft moves down range while increasing the risk of possible data loss. Many ranges would also need to install significant numbers of ground towers to gain the required range, which would add significant expense and maintenance overhead.

High Speed and Low Drag: The Future of Flight Test Telemetry

Most of Curtiss-Wright’s sales in flight test operations are for its hardware; however, the company has been developing software and is currently moving into the business of providing services. From a business perspective, a veritable cottage industry of products and services has emerged in order to take advantage of the recent advancements in flight test and telemetry.

For example, AeroTEC, a small independent company based in Washington state, will hire on as a “full service partner” to an aerospace manufacturer from initial engineering and design to prototype manufacturing, testing, and airworthiness certification. Founded in 2003, AeroTEC utilizes in-house instrumentation, software, tools and processes for projects involving winglets, special mission platforms, new aircraft type certifications, and UAS. It advertises that “you don’t need to research or detail what you need done. We’ve got this. We do all the legwork for you so you can focus on your business.”

Another such company is Dewesoft. Based in Slovenia, and with offices in 15 other countries including the U.S., DEWEsoft offers a variety of solutions for aerospace testing, from standard data recording and structural dynamics to rotating machinery analysis, acoustic testing, and ground station telemetry.

Dewesoft provides flexible data acquisition hardware and software that can match OEM requirements for testing airplanes, helicopters, rockets, and satellites in the air, space, or on the ground with wind tunnel, vibration shaker or acoustic chamber (see images above).

Dewesoft hardware and software form a “total solution” for all test and measurement applications.

The systems are modular and can be gradually expanded from one to thousands of channels for any measurement challenge. The company also offers free lifetime software upgrades and no maintenance fees and touts a “plug and play” solution for any device, sensor or signal. It even offers training, despite its claim that its systems are very easy to use.

It may not be glamorous, but telemetry is critical for the future of aviation. Without advances in telemetry equipment, software and services, flight tests would incur reduced safety, higher costs, and long delays, negatively impacting competitiveness.