Wireless Sensing — the Road to Future Digital Avionics

A look at the comparative performance of wired and wireless sensors, type of wireless sensors & interfaces, frequency performance, protocols, network topologies, and qualification standards.

The performance of avionics systems is dictated by the timely availability and usage of critical health parameters. Various sensors acquire and communicate the desired parameters. In current scenarios, sensors are hardwired and the number of sensors are growing due to automation, which increases the accuracy of intended aircraft functions.

Pictorial representation of the distribution of various wired sensors all over an aircraft. Although it does not include nearly all sensors, it does give some indication of the amount of wiring required throughout an aircraft, which is substantially high. This wiring contributes to weight of the aircraft and operational inefficiency due to fuel usage.
Sensors are distributed all over the aircraft and they are connected through wired networks for signal processing and communication. Line-replaceable units (LRUs) that integrate various sensors also use a wired approach for communication.

The use of a wired network approach poses challenges in terms of cable routing, stray capacitances, noise, mechanical structure, and added weight to the structure. The weight of hundreds of miles of wires and cables contributes significantly to the overall weight of the aircraft, and, of course, as the weight of aircraft increases, the required fuel quantity also increases. The key driver for airline operational cost is fuel.

Use of wireless sensors in aircraft brings in tremendous advantages in terms of design optimization, flexibility in sensor configuration, and weight optimization. However, even though the avionics industry is trying to adopt wireless sensors, there are some points of concern in deploying wireless sensors and networks across the aircraft. Some of the key factors to be considered for determining the feasibility of wireless technology and sensors are protocol, standards, compliance, and certification. Additional factors are internal and external infrastructure, various topologies for sensor networks, and expandability for the same. Signal integrity and fault detection methods are also key features of signal processing in aerospace applications.

Sensor Network Topologies

Typical wired sensor interface techniques.
Various wireless technologies can be considered for communication inside aircraft, and to take advantage of them it is necessary to address and understand the sensor network topologies that provide the architectural framework.

A peer-to-peer network allows each node to communicate directly with another node without needing to go through a centralized communications hub. Each peer device is able to function as both a “client” and a “server” to the other nodes on the network. This type of network can be used while communicating data from/to proximity sensor units.

Tree networks use a central hub called a root node as the main communications router. One level down from the root node in the hierarchy is a central hub. This lower level then forms a star network. The tree network can be considered a hybrid of both the star and peer-to-peer networking topologies. This type of network can be used while communicating data from/to fuel sensor units. Based on the location of the fuel tank, each of the nodes can transmit the related information to the main node.

Various wireless technologies available and their frequency of operation and modulation scheme.
Mesh networks allow data to “hop” from node to node, allowing the network to be self-healing. Each node is then able to communicate with each other as data is routed from node to node until it reaches the desired location. This type of network is one of the most complex and costs a significant amount of money to deploy properly. It can be used while communicating data from/to various data concentrator units.

Star networks are connected to a centralized communications hub. Nodes cannot communicate directly with one another; all communications must be routed through the centralized hub. Each node is then a “client” while the central hub is the “server”. This type of network can be used while communicating data from/to the central data concentrator unit, which gets connected to the flight-management computer.

Key Factors in Feasibility Consideration

Possible replacement for the wired communication at sensor and line-replaceable unit level.
Besides considering wireless technologies, sensor and LRU interfaces, and sensor network topologies, there are additional factors specific to the avionics industry for deploying wireless sensors.

Data integrity is a key factor in protocols used in the avionics industry. High reliability data communication without loss of data is critical for safety systems. With wireless sensors and LRUs it is important to have the robust protocols that will address the data-integrity needs. Existing wireless protocols may need some tweaks to address the high reliability data communication needed over wireless media.

Selection of wireless standards and technology is important based on the type of the sensor and availability of a license-free spectrum. Wireless sensors are expected to work at specific speeds and most of the time as self-powered devices, so data rate for communication and power management techniques (duty cycle) plays an important role for selection. Location of the sensor (inside or outside of fuselage) and data rate (low or high) dictates wireless standards to some extent. To leverage the chipsets and radio, existing off-the-shelf wireless technology is recommended. Interference with other systems has to be analyzed thoroughly.

Means of compliance and applicability of various aerospace DO standards is mandatory. DO-160 and DO-294C applicability and analysis is expected well in advance. For complex electronics hardware for LRUs, DO-254 is to be followed during the item/component design phase. The same is true for the software via DO-178B.

Applicable wireless certifications for various transmitters (FCC, CE, EN standards, etc.) should be used. Usage of the certified chips and radios for Wi-Fi and Bluetooth helps for wireless certification at the component/item or LRU level, although the FCC mandates component-/item-level certification.

There is a need to check the availability and usage of ISM band while selecting the wireless spectrum. Current scenarios use the 2.4 GHz license-free band for IEEE 802.11 b/g/n (Wi-Fi) and IEEE 802.15.4 (Zigbee). The spectrum should support low data rate as well as high data rate.

Constraints, Challenges, and Guidelines

Typical diagram showing node-to-node communication via a tree network.
Designers need to pay close attention to environmental, EMI/EMC considerations, and tests as per DO-160, “Environmental Conditions and Test Procedures for Airborne Equipment,” for wireless sensors and wireless LRUs. Various sections of DO-160 have an impact on wireless devices.

Whether the wireless sensor is self-powered or aircraft-bus powered will dictate the applicability of Section 16 and 17.

RF emission (conducted and radiated) applicable limit levels need to be checked against the FCC limits for specific wireless technology. If the DO-160 Section 21 limits are more stringent than the FCC limits, design and test should be based on Section 21, which will then in turn satisfy FCC requirements. In some scenarios (based on the location) of the wireless sensor and LRU, FCC limits should be sufficient.

In addition to conducted and radiated emission considerations, the design should also consider conducted and radiated susceptibility. Some design techniques to consider are: specific EMI filters with small-form factors for various I/O's and power lines; impedance matching; and various layout considerations for avoiding current loops and proper isolation of RF. Different isolation techniques like physical isolation on printed circuit boards and digital isolators as well as various layout rules can be used.

For specific wireless technology it is recommended to refer to the DO-294C, “Guidance on Allowing Transmitting Portable Electronic Devices on Aircraft,” characterization matrix for building/developing wireless sensors and LRUs. This will allow meeting characteristics of intended wireless design without violations.

FCC regulations for the intentional and unintentional radiators must be studied for the specific wireless technology as well. Designers and integrators of wireless sensors need to coordinate with the appropriate certification agencies and authorities to determine all applicable wireless regulation standards and decide whether to fully comply or submit any necessary deviations/waivers.

Portable electronic devices (PEDs) that intentionally radiate signals within the aircraft fuselage are potential sources for RF interference to installed aircraft systems. PEDs with intentional radiators (i.e. transmitters) are called TPEDs. All the PEDs will have both intentional and unintentional radiators. Unintentional radiators are also referred to as spurious emissions. Spurious emissions are the ones that fall (wideband and narrowband) outside the nominal range of operating frequency. Intentional radiators operate at the required transmitting frequency for wireless communication of PEDs. DO-294C provides guidance on the process of evaluating the T-PED effect on the aircraft operation.

Challenges and Adverse Effects of Wireless

Due to the RF signal propagation and reflections inside the aircraft environment, there is a possibility of the signal not getting detected by the antenna at receiver end. This is also known as the scattering effect. These reflections create the multiple paths for RF signal propagation. RF path propagation study and analysis for different types of antennas will enable the selection of the best antenna that can detect the RF signal under these multipath phenomena. Analysis, simulation, prototype, and testing of the antenna is the best approach.

There are two types of diversities, referred to as antenna diversity and radio-level diversity. Antenna diversity consists of spatial diversity, in which two slightly offset antennas see different amounts of multipath fading: angle diversity, in which multipath levels are altered thereby changing signal amplitude; and polarization diversity, in which misalignment and multipath cause polarization loss. Dual polarization (two-in-one antenna) provides two polarizations to choose. This is more compact than two antennas and reduces the margin needed in link budget.

Radio-level diversity consists of frequency diversity, which allows the switching of channels to diversify; temporal diversity, which consists of packet-based collision avoidance with guaranteed time slot transmissions; and code diversity, which consists of coding the signal in a unique way to reduce interference and simultaneous transmission of more radios.

The majority of the wireless technologies discussed here support radio-level diversity. Antenna diversity is a major consideration, requiring due diligence as well as multiple antenna designs, simulations, and prototypes. Radiolevel diversity for the selected wireless technology and proper antenna diversity in the design of the wireless sensors and LRU will avoid unintentional jamming due to other wireless devices and radio towers. It will also avoid any catastrophic events due to failures.

Low-power wireless networks will not contribute to conflicts between networks when two aircraft are close to each other. Such low-power networks will not have sufficient energy to propagate and interfere with the nearby aircraft wireless network.

Various wireless consortiums and working groups are engaged in the development of wireless sensing systems. The general approach should be to categorize the non-essential/non-critical sensors and high-critical sensors in various systems over the entire aircraft. It is recommended to complete non-essential/non-critical sensor development first with prototyping and testing, so that the knowledge gained there can be used in developing high-critical sensors.

LRUs/signal processing units that should be considered for the migration to wireless sensing in the near future include the engine control and health management system, proximity sensing system, aircraft structural health control system, lighting and cabin control system, and latch and landing gear sensors.

This article is based on SAE technical paper 2014-01-2132 by Prashant Vadgaonkar, Ullas Janardhan, and Adishesha Sivaramasastry, UTC Aerospace Systems.