High-Speed Data Transmission in Military Aircraft

Commercial systems are driving the need to transfer more data at higher speeds. Ethernet at 10 Gigabit is now making its way online. However, high bandwidth applications on networks using video or copious amounts of data are also starting to drive the need for faster data rates as high as 25 and 40 gigabits.

Also, the industry is using trade studies to consider new systems using fiber optics to complete trade studies and comparisons.

We are starting to see more discussion around developing industry standards and best practices. Standards such as SAE AS6070 now offer opportunities for OEMs to consider how to implement the best wiring and cabling solutions during the design phase. Once standards are established, trade studies can consider other important characteristics such as weight and size that will bring additional benefits to the system and platform.

Military 38999 connectors that are not impedance controlled, poor cable assembly, connector-cable termination techniques, and tight cable routing can create reflections that degrade signal integrity and cause errors at higher data rates. Designers using conventional standards such as Ethernet and Fibre Channel may feel comfortable that they will work the same way in an aircraft environment as in an office environment. However, on aircraft, unpredictable EMI sources, fluctuations in power supplies and temperature extremes can dramatically affect receivers, resulting in lower margin. Therefore, signal integrity is an essential characteristic for high-speed interconnects as data rates continue to increase at a rapid pace.

Our conversations with customers having system performance issues often start with finding the right combination of connectors and cables. Connector-cable compatibility and proper termination practices are vital components in finding the right solution and ensuring overall system performance.

For example, a customer planned to use a Fibre Channel standard in a system running at a lower data rate and attempted to design cables around loss parameters. Our application engineers helped them to understand the importance of designing interconnects for future needs as data rates increase, and also helped them find a better solution. In time, the customer realized the connector they originally selected would not work in their devices that eventually needed to connect to higher frequencies. Fortunately, our customer saved cable replacement costs by selecting, designing and installing the right interconnect in their system early in the process to meet higher data rates in the future.

Gore has published best practices for terminating leading high-speed aerospace connectors with our Ethernet cables and related electrical data for connector-cable compatibility available at gore.com/ethernet-cable-connectors.

Yes, we have talked to our customers and performed extensive testing that showed not all Ethernet interconnects are the same.

Recently, NAVAIR approached Gore to help define Ethernet specifications because of the limitations with off-the-shelf commercial cables that were failing in harsh aerospace environments. Understanding the minimum performance, having a clear threshold, and ensuring repeatable performance is a vast improvement for various aircraft systems.

Aircraft maintainers are retrofitting more interconnects in existing airframes with less space to meet higher data rate protocols. They often have to bend interconnects beyond the minimum bend radius (MBR) during routing that can significantly degrade system performance. Different interconnects have different bend radii at the point where functional performance starts to degrade.

Gore has published a series of white papers and related videos around selecting, designing and installing the right Ethernet interconnect to ensure reliable performance in aircraft available at gore.com/aerospace-ethernet-cables.

In the webinar, wiring issues were cited as the cause of 24% of U.S. Air Force Class A Mishaps to RPA's (Remote Piloted Aircraft).

Polytetrafluoroethylene (PTFE) has long been used as the preferred insulation material for military interconnects due to its significant properties such as thermal resistance, low flammability and low-dielectric content. However, there are some concerns with cold flow when used under various types of clamping environments.

Gore has developed proprietary technologies that allow PTFE to be engineered to withstand a wide variety of environmental and mechanical challenges. Expanded PTFE is the core material in many of Gore’s solutions because of its unique characteristics, including a high strength-to-weight ratio, biocompatibility, high thermal resistance and many others. Our cables and cable assemblies have been used in the military for more than 30 years and have been proven on many platforms.

NAVAIR has created MIL-DTL-32546 (Connectors, Electrical, Circular, for High-Speed Data Bus Transmission, Copper Conductor, General Specification).

Gore offers a Fiber Optic, 1.8 mm Simplex cable that supports Cat8 Ethernet requirements. Currently, we do not provide a copper Cat8 cable that has been fully qualified to TIA 568 and IEC 11801 standards. However, we do have the capabilities to manufacturer our Ethernet cables to support applications at 40 gigabits.

The main upgrade for a Cat8 cable is the requirement to specify performance up to 2 GHz beyond the 500 MHz requirement for Cat6a protocol. We currently provide digital interconnects with performance specifications well beyond 2 GHz. Also, we are searching for applications that require 25 and 40 Gigabit Ethernet to provide a Cat8 offering to our customers.

Please contact a Gore application engineer to discuss your specific application requirements and find a solution that meets your needs.

Although there is no exact answer for what length impedance discontinuity is acceptable for a given application, a general rule of thumb is that an impedance discontinuity becomes critical when its length becomes more than one-tenth of a wavelength of the maximum frequency being transmitted.

For example, Cat6a protocol specifies frequencies up to 500 MHz. In the air, the wavelength at 500 MHz is approximately 50 centimeters (cm). In connector-cable materials, the wavelength is reduced by half. Therefore, you can estimate the critical length at approximately 2.5 cm at 500 MHz. For systems operating at 2 GHz, the critical length would be approximately 0.5 to 1.0 cm. In addition to the length of an impedance mismatch, the number of discontinuities in a link and the distance between each discontinuity can also be important factors. Designers typically model connectors with a safety margin to ensure operation at desired frequencies.

It depends on the system and your objectives. A common practice is to perform a trade study to list and consider key typical factors such as weight, electrical performance margin after routing, installed cost, upgradability, and sustainability.

Please contact a Gore application engineer for assistance with these activities. 

CAN Bus protocols do not require low crosstalk, and data rates and frequencies are much lower. However, CAN Bus protocols require a specified characteristic impedance that creates advantages for low-dielectric constant materials to reduce size and weight while improving flexibility.

You can learn more by visiting gore.com/highdatarateaircraftcables.

Download data sheets, catalogs and technical information, watch videos, and more. Or, please contact a Gore application engineer to discuss your specific application needs.