A FLEXIBLE AIRBORNE DATALINK SYSTEM ARCHITECTURE FOR CIVIL HELICOPTERS

WANG Yun-sheng, LI YanxiaoCETC Avionics Co., Ltd.

AbstractDue to high operating cost, limited coverage

range, complex installation and additional weight, helicopter and other general aviation aircraft may not have datalink function implemented as portion of its avionics system. However, datalink has great advantages of releasing crowed VHF channel resources, reducing pilot and controller workload, supporting automation application, gaining time efficiency and operation precision. During the pre-flight, in-the-air and post-flight periods, general aviation operators and ground stations always demand data from helicopters, similar to Airline Operation Control (AOC) for air transportation. The automotive control trends and mandatory standards also drive the installation of data communication for the helicopters. By using SysML modeling language, this paper tries to identify the data link communication scenarios and requirements for helicopters. The feasibility of utilizing existing high speed mobile network for helicopters data communication is analyzed and confirmed, which removes the cost obstacles of helicopter datalink applications. Based on the operational concept and requirement analysis, flexible and scalable datalink system architecture for helicopters is proposed, which support both the VHF ACARS and mobile cellular network. Since the weight and size of airborne system for helicopter is severely restricted, besides the data communication radios, all the other router, protocol stacks and applications should be hosted in the modular computing resources and/or multi-function displays. With an integrated data communication radio and the datalink partition hosted applications, the airborne datalink system can support the 4G/LTE, WiFi, VHF ACARS and other

future new communication channels, e.g. broadband satellite communication and AeroMACS system etc.

1. INTRODUCTIONA helicopter is an aircraft which is lifted and

propelled by one or more horizontal rotors. Compared with fixed-wing aircraft, helicopters have the great features of vertical taking-off and landing without runways, extended and steady hover, as well as high maneuverability under low airspeed and low altitude conditions. With these advantages, helicopters are widely used in commercial and military fields, and play a unique role in search and rescue, firefighting, emergency medical services, offshore transportation etc. [1]. Among these scenarios, data communication functionality is required for the air traffic control, operation control (similar to AOC) and mission system. Compared with the traditional voice communication methods, data communication can improve communication efficiency, facilitate data recording and checking, and enable smart applications [2].

Datalink is a generic term of the air-ground data communication system. Through different types of air-ground networks, the airborne datalink system establishes the bidirectional connections and performs data transmitting and receiving between the aircraft and ground [3]. With the rapid increase of aircrafts number in the airspace, datalink systems effectively reduce the workload of pilot and controller, increase airspace capability, and smooth the aircraft traffic in recent years. In addition, some datalink based applications, such as PHM (Prognostics and Health Management) and HUMS (Health and Usage Management System), save a lot of costs and time for aircraft operators, improve

aircraft safety and task efficiency. With the proceeding of the Next Generation Air Transportation System (NextGen) [4] and the Single European Sky ATM Research (SESAR) [5], airborne datalink system has become a necessary portion of avionics system in air transportation, business and regional aircraft, and general aviation (including helicopters).

However, Compared to air transportation and fixed wing aircraft, the structure of helicopter is more compact, and correspondingly the payload and profit are relatively limited. Due to high operating cost, complex installation and additional weight, helicopter and other small general aviation aircraft may not have datalink function implemented as portion of its avionics system.

Nowadays, with the progress of avionics and communication technology, it is achievable and valuable for the development and deployment of the new generation airborne datalink system for helicopters. The Integrated Modular Avionics (IMA) provides the shared resources framework, and makes it possible to further reduce the weight and size of airborne equipment [6]. The mobile communication technologies, such as 4G/LTE, 5G, WiFi and satellite makes the data communication more swift and reliable, and many of these have been applied and adapted into the aviation field. The airworthiness considerations and suggestions were given in [7]. The feasibility of using 4G/LTE technology to aerial vehicles is studied in [8, 9]. The test of low altitude connected drone by cellular network was conducted in [10]. Solutions for future datalink application based on satellite communication were offered by [11].

The remainder of the present paper is organized as follows. Section 2 describes the operational scenarios of helicopter datalink system, and then establishes its SysML use case diagram model. Section 3 analyses and confirms the feasibility of utilizing existing and new mobile communication technologies in helicopter datalink system. Section 4 proposes a new generation helicopter datalink architecture that supports ACARS and mobile

communication networks using the AADL modeling language. Section 5 concludes the feasibility and flexibility of the helicopter datalink architecture.

2. Operational ScenariosAs shown in Figure 1, helicopter has very

plentiful operational scenarios, which include non-airline transportation, low-altitude flight, as well as special terrain operation. Almost all of these scenarios require data communication functions. According to the different operational scenarios of helicopter data communication, datalink functions can be divided into the following six types of applications and operational concepts.

1) Air Traffic Control (ATC) communications provide flight guidance for helicopter to prevent collisions and organize the flow of air traffic. ATC include Automatic Dependent Surveillance (ADS), Controller – Pilot Data Link Communication (CPDLC), Context Manager (CM) and Aircraft Facilities Notification (AFN) [12].

Figure1. The Operational Scenarios and Concepts of Helicopter Datalink Systems

2) Air Traffic Service (ATS) communication provides flight information and advisory service to helicopter, which include Pre-flight Departure Clearance, Digital – Automatic Terminal Information System (D-ATIS) and so on. ATS is also recognized as part of the ATC system.

3) Airline Operational Communications (AOC) involves any kind applications dedicated to airlines

administrative between the helicopter and the airline operational center. Typical AOC applications include OOOI (out of the gate, off the ground, on the ground and into the gate), helicopter fuel information and crew roster. In addition, it also includes Airline Administrative Communications (AAC) applications such as airline passenger communication, helicopter health monitoring report and weather information [13].

4) Command and Control Service (CCS) enables the helicopter to transfer real-time task instructions and data with ground task center and other collaborative task units, such as other helicopters, vehicles, ships and even UAVs. Usually this function is implemented as part of the mission system besides the core avionics system, which is a basic portion of the helicopter platform.

5) Communication Status Application (CSA) is responsible for the route management. It provides pilot with the means to configure the communication strategy and selects links according to the current position, message size, link cost and so on.

SysML is used in the field of system engineering and provides a unified modeling language for system engineers. SysML supports the analysis, design, verification and validation of complex systems [14]. Use case diagrams in SysML can clearly describe the service provided by the system and the relationship among the relevant stakeholders. By using use case diagrams, the datalink communication scenarios and requirements are identified as shown in Figure 2, according to the above analysis.

Figure 2. The Use Case of Helicopter Datalink Systems

3. Technical Feasibility AnalysisIn terms of communication methods, traditional

datalink communication methods include HF，VHF and SATCOM [13]. Since HF and SATCOM communication is much more expensive, VHF channel is the most popular method for air-ground communication with the support from DSPs (data service provider) all over the world, e.g. ARINC, SITA and ADCC. While the mission altitude of helicopter is usually around 300m above surface [15]. Due to low flight altitude and terrain blocking, the transmission effect of traditional VHF methods is less than satisfactory [16]. In recent years, with the increasing coverage of 4G/LTE, the rapid development of 5G and the popularity of WiFi and AeroMACS network, it is possible to implement real-time, high reliable and low-cost air-to-ground datalink network. The data transmission rate can be seen in Figure 3.

Figure 3. Generation of Mobile Communication Systems

In order to utilize the mobile cellular network for helicopter data communication, the concerns of signal coverage and the real-time performance need to be addressed. The altitude of signal coverage is determined by the signal receiving power, quantified as RSRP (Reference Signal Receiving Power), and the real-time performance concern is related to end-to-end latency.

In 2017, CAAC (Civil Aviation Administration of China) conducted a series of tests in several areas within China to study the feasibility of utilizing mobile cellular network for data transmission of low-altitude UAV (Unmanned Aerial Vehicle) and other aviation applications [10]. These tests considered many different scenarios, which represent different station spacing and obstacle conditions etc. The details are presented in Table 1.

Table 1. Low Altitude Coverage Tests Scenarios

Scenarios Station Spacing

Surroundings

Urban Gym

180m Many buildings

Urban Park 300m Open field with lakes and trees

Industrial Park

400m Many low-rise buildings

Suburban School

1000m Open field with a few buildings

Suburb 2000m Open field with mountain forest

The test frequency band covered both TDD-LTE D band (2575~2635M) and F band (1885~1915M). The three important test items are listed as following.

The distribution of RSRP on different altitude, to address the altitude coverage of mobile cellular network;

Real-time Flight Data Reporting Test (downlink test), to measure the latency of data from UAV to ground station, such as position and speed;

Real-time Flight Command Transmitting Test (uplink test), to measure the latency of command data from the ground center to the drone.

The RSRP test results are shown in Figure 4. CAAC concluded that the 4G/LTE mobile cellular network meets the needs of most of scenarios below 120 meters and the safety requirements below 300 meters. And other tests by academia also drew the similar conclusion [17]. CAAC also did similar tests to 5G mobile network and concluded the coverage of 1000 meters and below, as shown in Figure 1.

Figure 4. The Distribution Diagram of RSRP on Different Altitude Level

For real-time performance, the test results show that end-to-end delay of the 4G/LTE network is between 50ms and 300ms, as shown in Figure 5.

Figure 5. End-to-end Delay of the 4G/LTE Network

Since helicopters are routinely operated at 300 meters altitude, the mobile cellular network can provide these helicopters with high-speed data communication capability and acceptable latency according to the tests and analysis above.

4. Datalink System Architecture for helicopter

In terms of equipment weight, the traditional federal datalink systems have a large weight, due to the existence of Multipurpose Control and Display Units (MCDU) and Communication Management Units (CMU). IMA based architectures are state-of-the-art for avionics systems, and it provides the possibility for the reduction of avionics equipment. In IMA architecture, the display and operation interface of datalink can be integrated into the Multi-Function Display (MFD), and datalink applications are hosted in the IMA cabinet with interface to the external peripherals. Even the IMA cabinet can be integrated into displays as a high-performance processing unit.

Figure 6. The Functionality of Airborne Datalink Systems

Considering the above analysis, the functionality of airborne datalink system is shown in Figure 6. Pilots use the human-machine interface to input instructions, obtain messages, and then realize the interaction with the datalink application. The communication management function includes the ACARS and ATN protocol stack, parsing different application messages, and ensuring that the data message is sent and received through the correct communication link.

The avionics system architecture of different types of helicopter may be different from each others. For examples, the avionics could be various from light single engine, light twin engines, to medium and heavy helicopter. However, the development of a family of helicopter avionics system is achievable according the avionics implementation of helicopter avionics from airbus helicopter and avionics vendors, such as Rockwell Collins, Honeywell and Garmin etc.

The avionics system of helicopter has been implemented as core avionics system and customizable avionics system, which support different helicopter applications. Take EC175 as an example, the core avionics of EC175 support more than 40 peripheral plug-in-and-play LRUs from different vendors all over the world. And the avionics system of EC175 could be tailored to meet the special application of the helicopter. The avionics vendors, e.g. Rockwell Collins, Honeywell, and Thales etc., keep moving to that direction for the avionics family to fit all types of helicopters with the same flexible architecture. Lot of the studies has been done in architecture design, LRU implementation and software architecture, such as HELIONIX from Airbus helicopter, Primus and APEX from Honeywell, Topdeck and Avionics 2020 from Thales. The U.S. Army’s Technical Applications Program Office (TAPO) has adopted a product line approach for the avionics software used for the Army’s special operations helicopters. That software is based on Rockwell Collins’ Common Avionics Architecture System (CAAS), which was developed in late 1990s.

The product line has evolved beyond its original scope and is now being adopted to include other Army aviation platforms such as cargo and utility helicopters.

Table 2. Helicopter Platform Supported by CAAS

Platform Platform Variant DescriptionLittle Bird AH-6, MH-6 Boeing 2T light

single engineChinook MH-47D, MH-47E Boeing 22T

heavyBlackhawk MH-60K, MH-60L Sikorsky 10T

mediumThe CAAS product line represents an effort to

reduce development, maintenance, and integration costs for the Army’s fleet of special operations helicopters. The two major organizations involved in the acquisition and development of the CAAS product line were TAPO and Rockwell Collins. The CAAS architecture was to support the three classes of helicopter at the time as shown in Table 2 [18]. CAAS is a great example that a family avionics with flexible architecture can be fit into different helicopter platform, from light to middle and heavy helicopters.

From communication function perspective, the CAAS avionics system can be abstracted to the diagram shown in Figure 4 with AADL modeling language. PFD and MFD are used from display and control. The CPU1 and CPU2 are implemented in either displays or standalone data processing unit to host the various application software components for the avionics system. A DCU (Data Concentrator Units) is used to interface with communication radios, e.g. VHF and SATCOM, through ARINC 429 bus or equivalent interfaces. A high-speed data bus is used to interconnect the displays, CPUs and DCUs. Regarding for the high-speed data link communication provided by the LTE and WiFi, the best implementation within the architecture is to have the LTE/WiFi LRU access to the helicopter databus directly.

Figure 7. Helicopter Airborne Communication System Architecture with LTE/WiFi

Regarding for the datalink function of helicopters, besides the ATC, AOC and CSA application mentioned above, there are another 2 major datalink functions as listed below:

The Communication Management Function (CMF) is to manage the datalink router and protocol stack to ensure the data is transmitted/received through the right radio channel and to/from the right application or peripherals. The major protocol stacks of datalink system include Aircraft Communications Addressing and Reporting System (ACARS) and Aeronautical Telecommunication Network (ATN). When the 3G, 4G/LTE, 5G and WiFi is utilized as a channel for helicopter airborne datalink, the TCP/IP protocol should be implemented for wideband IP based communication as shown in Figure 3.

The Cockpit Display Server (CDS) is to manage the display and communicate with application via a ARINC 661 protocol. The CDS display the widgets and manage the interface to simplify the development of display with the model-based method.

Due to the constraints from size and weight, there will be no dedicated hardware for these datalink functions within the context of helicopter avionics

system. All these applications should be implemented as hosted applications. If we take datalink as a subsystem, the system architecture model with AADL language is shown in Figure 8. Within this architecture, 4G/LTE is a separate LRU with high-speed databus access. The connection from VHF radio, SATCOM, and 4G/LTE to CMF is allocated to the helicopter data bus, and communication with CMF hosted application through ARINC 653 ports.

Figure 8. Helicopter Datalink Subsystem Architecture Model

The ATC, AOC and CSA applications are hosted in the processing CPU as well, as shown in Figure 9. These helicopter ATC, AOC and CSA applications could be different from the applications of airliner aircraft. They are designed for specific helicopter with dedicated functionality.

Figure 9. Hosted applications for helicopter datalink functions

5. ConclusionThe aviation industry demands the datalink

function for helicopters. Helicopters routinely operate

at low space, which make it feasible for using the 3G, 4G/LTE, 5G and WiFi as a supplement to the VHF and SATCOM data communication. It also reduces the data communication cost greatly. However, due to the strict limitation of weight and size, the datalink system architecture for helicopters shall minimize the additional LRU installations. The airborne datalink system for helicopters should take the advantage of partitioned architecture and IMA, which hosted the datalink related CMF, ATC, AOC, CSA functions to be implemented as partition software in the helicopter avionics system. An integrated data communication radio, including 3G, 4G/LTE, 5G, WiFi and AeroMACS channels, is also the key enabler for the helicopter datalink system.

References[1] Federal Aviation Administration, 2000, Rotorcraft Flying Handbook , FAA-H-8083-21.

[2] WANG Yun-sheng, Steven SAVAGE and LEI Hang, 2016, The Architecture of Airborne Datalink System in Distributed Integrated Modular Avionics, Integrated Communications Navigation and Surveillance (ICNS) Conference.

[3] LIU Tian-hua, 2010, Datalink and Communication Management Technology of Civil Aircraft, Telecommunication Engineering, Vol.50(5), pp.84-88.

[4] Federal Aviation Administration, 2013, NextGen Implementation Plan.

[5] European Commission, 2007, State of Process with the Project to Implement the New Generation European Air Traffic Management System(SESAR).

[6] WANG Yun-sheng, LEI Hang and HAN Xuan, 2015, The Stochastic Petri Net based Reliability Analysis for Software Partition Integrated Modular Avionics, IEEE Aerospace and Electronic Systems Magazine, Vol.30(4), pp.30-37.

[7] Liu Tian-hua, 2014, Data Link Applications for Civil Aircraft: Air-Worthiness Requirements and

Implementation Suggestions, Tele communication Engineering, Vol.54(10), pp.1326-1329.

[8] 3GPP TR 22.891, 2016, Feasibility Study on New Services and Markets Technology Enablers, Release 14.

[9] 3GPP TR 36.777, 2018, Study on Enhanced LTE Support for Aerial Vehicles, Release 15.

[10] Civil Aviation Administration of China, 2018, Low Altitude Connected Drone Flight Safety Test Report.

[11] David Fernández, et.al. 2014, Satellite Communication Data Link Solution for Long Term Air Traffic Management, SESAR Innovation Days.

[12] Airlines Electronic Engineering Committee, 2005, ARINC Characteristics 785 - 2: Communication Management Unit (CMU) Mark 2.

[13] Civil Aviation Administration of China, 2008, Standards and Guidelines of Aviation Operators to use the air-ground data communication system.

[14] Object Management Group, 2017, OMG Systems Modeling Language, Version 1.5.

[15] Eric Greenwood, 2017, Helicopter Flight Procedures for Community Noise Reduction, American Helicopter Society Forum.

[16] A A M Mostafizur Rahman, et. al., 2016, Feasibility study of GSM network for tracking low altitude helicopter, International Conference on Electrical Engineering and Information Communication Technology.

[17] Luís Afonso, et. al., 2016, Cellular for the Skies: Exploiting Mobile Network Infrastructure for Low Altitude Air-to-Ground Communications, IEEE Aerospace and Electronic Systems Magazine, Vol. 31(8), pp.4-11.

[18] Paul Clements, John Bergey. The U.S. Army’s Common Avionics Architecture System (CAAS) Product Line: A Case Study[R]. USA:CMU/SEI-2005-TR-019. 2005,9.

Acknowledgements2018Integrated Communications Navigation and

Surveillance (ICNS) ConferenceApril 10-12, 2018