Real-Time Air-to-Ground Data Communication Technology of AeroengineHealthManagement Systemwith Adaptive Rate in the Whole Airspace

To overcome the problem of data transmission of the aeroengine health management system, a multilink communication system combining ultrahigh-frequency communication link, 4G cellular mobile communication link, and BeiDou satellite communication link was proposed. -is system can realize the functions such as data receiving and sending, data encryption, and resuming transfer from the break point based on multiple links. When the flight altitude is not high, the communication distance is short, so the UHF digital transmission radio communication link is adopted, which is highly efficient and stable. When the communication distance is long, the 4G cellular mobile communication link can ensure both the communication distance and the communication rate. In the area where 4G signal cannot be covered in extreme terrain environment, BeiDou satellite communication link is used for data transmission. Besides, in order to ensure the communication rate of the link, a multilink adaptive switching technology was also developed. -e test verified that the system can perform adaptive switching among multiple links, realize air-ground data communication in the whole airspace, and achieve a good communication rate, which has significative value of engineering application.


Introduction
With the gradual expansion of domestic aviation platforms and the rapid growth of the transportation industry, the requirements for real-time, safe, and reliable air-to-ground communication technology have continuously improved. e air-to-ground data link system for engine health management, as the main method of real-time monitoring of engine status, has been extensively applied in civil aviation owing to its fast transmission, strong anti-interference ability, low bit error rate, and high reliability [1,2]. e failure strategy of engine gas circuit performance monitoring, fault diagnosis, and response is an important part of engine health management [3,4]. In the aviation industry, flight accidents are often caused by engine failures. erefore, it is extremely important to strengthen real-time monitoring of engine operating status. Although health management technology is the most effective method to ensure the safe operation of the aeroengine and reduce maintenance costs, the timeliness and accuracy of data sources used by it will directly affect the quality of health assessment. In terms of algorithm theory research on aeroengine health management, typical algorithms include methods based on physical models [5,6], methods based on data [7,8], methods based on experience, and fusion methods based on intelligent algorithms [9][10][11].
During the flight, the air-to-ground data link system realizes real-time monitoring of engine operating status by continuously monitoring the operating status of the aircraft, sending the on-board information to the ground terminal in real time, and using the powerful ground network control processing center to analyze and process it, so as to provide communication of the entire communication system in the whole airspace. Figure 1 shows the air-ground segment data link of the health management system.

Research on Ground-Ground Segment Communication
Technology. During the health data management of the aeroengine, the ground control center needs to process and transmit the obtained health data to multiple departments. To realize the transmission of data to multiple departments, ground-ground segment communication needs to meet the requirements for one-to-many or many-to-many interactive communication, so as to realize data sharing.
To allow the health management data to be accurately and timely distributed to other departments and to realize the sharing of engine's health management data, after research and tests, the ground-ground segment adopted the user datagram protocol (UDP) network communication protocol for communication.
UDP network communication protocol is a connectionless transport layer protocol, with the advantages such as small resource consumption, fast processing, and high real-time performance. In addition, it supports one-tomany and many-to-many interactive communication, which can achieve the function of transmitting health management data to different departments. Figure 2 exhibits the groundground segment data link of the health management system.

Research on the Technology of Compression and Encryption of Key Health Management Information.
As the "heart" of the aircraft, the aeroengine represents the highest level of industrial technology of a country and is hailed as the "Pearl" on the "Crown" of industry. us, its health management data are evidently vital. In the communication system for air-to-ground data of health management, the data need to be processed to prevent it from being stolen, attacked, and destroyed. Before transmitted, the data can be encrypted to reduce threats and ensure data security to the greatest extent.
e security encryption algorithms for information transmission have been extensively studied, and in this system, DES security encryption algorithm was used. DES is a world-recognized standard encryption format, which performs encryption by 64-bit data packets before replacement and iteration [25,26]. e length of the key is 56 bits, and processing such as replacement and cyclic shift is also required. In the initial IP replacement, the 64-bit plaintext is disorganized according to an 8 * 8 replacement table to increase the strength of encryption; subkey is used in 16 iterative computations; the initial ciphertext formed by each packet undergoes the inverse initial replacement again, which is the inverse operation of the initial replacement, to obtain the final ciphertext of the packets. e specific process is displayed in Figure 3.

Research on the Technology of Resuming Transfer from the Break Point of Key Health Management Information.
Resuming transfer from the break point means that, during the data upload or download process, when the connection is reestablished after an interruption, the part that has never been uploaded or downloaded will continue to be transferred without repeating the transmission of the existing content. is technology can effectively save time and improve the efficiency of the system [27,28].
ere are two key issues that need to be paid attention to in resuming transfer from the break point: (1) When the transmission is interrupted, the local download information is saved: a temporary file needs to be created to save the local download information, including the number of downloaded bytes, the file pointer, and the total file size, so that the download information will be stored in a temporary file when the transmission is interrupted, and the file pointer will point to the end of the downloaded file. (2) e local download information needs to be checked when resuming the transfer from the break point: first, it is necessary to determine whether the downloaded file exists locally; if it exists, the download information of the downloaded file is read, and the pointer is moved to the next position of the downloaded bytes at the same time so that the transfer of the file can be resumed from the break point. Figure 4 shows the flowchart of resuming transfer from the break point.

Research on the Technology of Air-to-Ground Data
Communication of Health Management with Adaptive Rate. Different communication links are applicable to different working scenarios, working ranges, and transmission distances. If only one communication link is used for communication, the communication effect will vary among different scenarios where the aircraft performs tasks. e application of multiple communication links can not only take advantage of each communication link to ensure communication quality but also improve the survivability of the entire communication system. erefore, it is necessary to use multiple data links for communication.
When multiple communication links exist in the system, how to coordinate these communication links to allow them to work efficiently is a key issue that needs to be solved. erefore, an adaptive switching mechanism based on three communication links was investigated. Considering the differences in the transmission range and update frequency among UHF digital radio station, 4G cellular mobile communication, and BeiDou satellite communication, the default priority of the communication links (from high to low) was set as follows: UHF digital radio station, 4G cellular mobile communication, and BeiDou satellite communication. Even if the information of several communication links is received simultaneously, the ground control center will only process and distribute the data of the communication link with the highest priority currently.
is mechanism conducts analysis according to the packet loss and transmission delay to judge the communication quality of the

Implementation of Each Component of the Prototype System for Real-Time Air-to-Ground Data Communication of Health Management.
e real-time communication system of air-to-ground data of health management is mainly composed of the on-board subsystem and the ground subsystem. e two subsystems are connected by the onboard terminal and ground terminal of data link.

On-Board Subsystem.
e on-board subsystem is mainly composed of on-board data link terminal, engine simulation equipment, and flight control system.
(1) Engine simulation equipment: the STM32F405 hardware platform with a basic frequency of 168 MHz was used for simulated operation, as shown in Figure 7, so that information such as flight status can be obtained from the on-board flight control system. en, the engine nonlinear on-board realtime model was adopted to calculate the operating data of the simulated engine; finally, these data were sent to the data link on-board terminal, with the remote control command received from the onboard terminal of data link simultaneously. (2) e on-board terminal of data link: the UHF communication module is shown in Figure 8, 4G communication module is shown in Figure 9, and BeiDou module is shown in Figure 10; the three data link onboard terminals are, respectively, connected to the data link comprehensive integrated unit via the serial port, the data link integrated unit is a single-chip microcomputer based on STM32L476RGT6, which mainly completes the functions of channel encoding and decoding, data packets deframing, data dispreading, modulation and demodulation, subsystem built-in test, and fault diagnosis and processing of the data link on-board terminal and then realize data communication with the engine simulation equipment and the ground terminal of data link, as shown in Figure 11, in which USART1 is connected to the 4G core board; USART2 is connected to the RS232 female port; UART4 is connected to the RDSS module; UART5 is connected to the UHF data transfer module; LPUART1 is connected to the engine simulation module.  Figure 12. On this basis, the secondary development was carried out, and a self-defined protocol was used for communication.

Ground Subsystem.
e ground subsystem consists of data link ground terminal, ground control center, and the simulation equipment of outfield PMA, support center, and maintenance center, as shown in Figure 13.

Method of the Engine's Health Management Data
Simulation. Due to the high cost and difficulty of collecting the health management data of real engine, interpolation was used to build a high-confidence real-time on-board engine model to simulate and calculate the working data of the engine. e parameters of each section of the current engine were obtained through the interpolation of the throttle lever command given by the flight control, and then the data of the engine were sent to the on-board terminal of data link through the serial port.

Establishment of a Flight Test Platform for Real-Time Communication and Transmission of Air-to-Ground Data.
Owing to the high cost and great risk of using aircraft and engine for data transmission simulation test, a vertical take-  cruising, long flying distance, long endurance, and no terrain requirements for take-off and landing. e schematic diagram of the test platform is displayed in Figure 15. e outfield PMA, maintenance center, support center, and ground control center are, respectively, integrated on different computers. e outfield PMA, maintenance center, and support center are connected to the ground control center through the network interface, and the ground control center is connected to the ground terminal of data link through serial port to receive the telemetry data from the ground terminal of data link for analysis and processing and forwards the received telemetry data to the outfield PMA, maintenance center, and support center for analysis and processing. In addition to the on-board flight control system, the UAV test platform is also equipped with engine simulation equipment to simulate the real-time working status of the engine and obtain data. After receiving the aircraft status

Simulation Test of Air-to-Ground Data Transmission of
Aircraft Engine, Outfield PMA, Maintenance Center, and Support Center. To verify the technology of real-time air-toground data communication with adaptive rate of the aeroengine health management system in the whole airspace, the on-board engine model based on STM32F405 was mounted on the UAV as the engine simulation equipment for the data link communication with the ground terminal, and then the air-to-ground data transmission simulation was carried out. According to the engine data, UAV data, and the physical characteristics of the communication links, three formats of communication protocols were defined, and three types of data messages were generated. rough the flight test data of real UAV and the engine simulation data, the communication of the air-ground links for engine's health management data was tested, involving the communication tests of the UHF data radio communication, the 4G cellular mobile communication link, and the BeiDou satellite communication link and the test on the adaptive switching among the three links; when the packet loss rate of the current communication link is too high or the transmission delay is too large, it can be manually or automatically switched to other communication links. In addition, with the test on the function of ground-ground segment UDP data     distribution, all tests passed, with the transmission delay within 100 ms and the packet loss rate less than 1%. e entire system can accurately and effectively communicate real-time data in different flight scenarios, which greatly improves the reliability and survivability of the system. Figure 16 shows the communication test of 4G cellular mobile air-to-ground data. Figure 17 displays the UAV/ engine data switching from 4G communication link to UHF data radio communication link and then to BeiDou satellite communication link, and Figure 18 shows the communication between the ground control center and the outfield PMA, maintenance center, and support center.

Switch to BeiDou satellite communication link
Switch to UHF data radio communication link

Conclusion
In this paper, an aeroengine health management system was described, which realized the real-time communication of air-to-ground data in the whole airspace with adaptive rate. e system focuses on the multilink adaptive switching among UHF data transfer link, 4G cellular mobile network link, and BeiDou satellite link and also uses DES symmetric encryption algorithm and the technology of resuming transfer from the break point to ensure the security and integrity of data transmission. e simulation of air-to-ground data transmission showed that this system can realize the monitoring of aircraft and engine data and the adaptive switching among multiple links, which verified the feasibility of the system in practical application.

Data Availability
e data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest
e authors declare that they have no known conflicts of interest or personal relationships that could have appeared to influence the work reported in this paper.