Communication Management and Data Compression Algorithm Design of BeiDou Transparent Transmission Terminal for Argo Buoy

Abstract

The Argo buoy detects marine environmental data by making profile movements in the ocean and transmits the profile detection data to the shore base through the communication terminal. However, due to the large volume of data collected from profile detections and the continuous operation of the terminal, the remote communication of buoys is characterized by lengthy communication times and significant power consumption. A low-power Beidou transparent transmission terminal is designed to solve these problems in this paper. The terminal performs low-power operation management and power management for terminal remote communication. After the end of a communication process, the microcontroller turns the Beidou module power off and enters STOP mode. Before the next communication process begins, the serial port wakes up the microcontroller, which powers up the Beidou module. Before the remote communication, the microcontroller compacts the profile detection data collected by the buoy to reduce the quantity of remote communication data. In this paper, a variety of data compression methods are used to compare the compression rate, and the best compression method is selected according to the format characteristics of the data. The results show that the Beidou transparent transmission terminal of the Argo buoy can realize low power consumption for remote communication in ocean exploration. The terminal reduces power consumption by 77.282% per communication, and the average number of remote communications for each profile detection data are reduced by 55 times. The low-power Beidou transparent transmission terminal improves the battery life and is conducive to the long-term operation of the buoy.

1. Introduction

The ocean is a vast reservoir of resources, rich in oil, natural gas, and other mineral resources [1]. Ocean exploration is an essential means of recognizing the ocean and understanding it. Collecting various data on the ocean through ocean exploration instruments can help us have a comprehensive understanding of the ocean [2,3,4]. Detection instruments can be mounted on platforms such as Argo buoys, underwater gliders, and marine robots, which are operational within the oceanic realm. Among these, Argo buoys are particularly advantageous in ocean profile detection and are extensively used for the acquisition of marine profile data. Argo buoys equipped with conductivity temperature depth (CTD) sensors can collect sectional temperature, salinity, and depth information through ascent and descent movements [5].

With the advancement of satellite communication technology, the acquisition of profile information collected by buoys has improved from the past, when data could only be obtained after the buoys were recovered, to real-time acquisition using remote data transmission [6,7]. Many researchers use Iridium systems, maritime satellite systems, and Argos systems for satellite communications [8,9,10], but these satellites are developed and operated by other countries. All transmitted information is passed through other countries’ satellite systems, causing a high risk of data leakage in our coastal waters. In this context, the BeiDou Satellite Navigation System, an independent development of our nation, offers a secure alternative. It provides high-precision, high-reliability services for positioning, navigation, and timing to users across the globe, in addition to capabilities for short message communication [11,12]. After the buoy floats to the sea surface, it is positioned by the Beidou module (BDM), and profile information and location information are sent to shore via Beidou satellites, achieving remote data transmission and real-time positioning [13].

The buoy needs to perform data processing on the data and transform it into a format that conforms to the Beidou communication to realize the remote transmission, which increases the burden in the main control board of the buoy, the risk of error, and the difficulty of debugging and testing. Therefore, a separate Beidou transparent transmission terminal (BTTT) is designed for the data remote transmission part. Based on the original function, the shore-based remote control function is added to analyze the data and realize the corresponding functions for the remote information sent from the shore. The buoys are far from land and are powered by batteries with only limited power, making low-power design an essential part of the design for satellite communication terminals.

In 2016, Tang Chao [1] of the National University of Defense Technology proposed disconnecting the power supply of satellite communication-related modules and devices during non-satellite over-topping periods to reduce power consumption. The power supply is re-powered during satellite over-topping to restore the normal operating status of these modules and devices. Nonetheless, given the satellite overhead cycle repeating roughly every 90 min for only about 8 min, the inability to transmit the profiling data of a buoy within this brief period necessitates prolonged waits for subsequent satellite passes, leading to substantial waiting times. Men Yabin et al. [8] of Tianjin University addressed power management for Iridium communication modules by employing a real-time clock to schedule power on and off. Since the buoy’s operational timing in ocean exploration can fluctuate due to environmental factors, this power scheduling adversely impacts the success rate of remote communication.

There are fewer studies [1,8] on the low-power design of satellite communication terminals, and many scholars have conducted plenty of research on the low-power design of ocean observation equipment, which is of great reference significance. For instance, Zhang Xi [14] of the National Marine Technology Center conducted low-power operation design and power management for multi-parameter surface drifting buoys. Zhang Qingguo et al. [15] of Kunming Marine Equipment Research and Testing Center carried out a low-power design for a type of underwater navigation body, abandoning the commonly used isolation, switching, and other large-size, high-power components of the method and adopting a small-size, low-power combination of integrated design solutions. Arul Muthiah M. et al. [16] of India reduced the power consumption of a tsunami surface buoy system by configuring distributed batteries.

In summary, although the research [1,8] on low-power design of satellite communication terminals has been carried out, there are still many shortcomings. In this paper, a low-power BTTT is designed by drawing on the research results of previous researchers in the low-power design of satellite communication terminals and ocean exploration equipment. The terminal reduces power consumption from two perspectives: reducing the power consumption of a single communication and decreasing the number of communications. Full consideration is given to the energy consumption generated by the BTTT during the operation of the buoy to realize low power consumption.

The objective of this study is to reduce the power consumption of remote communication for the Argo buoy. In this paper, communication management for the BTTT is addressed, encompassing both low-power operation and power management strategies aimed at reducing the power consumption associated with communication processes. Additionally, a differential coding-Huffman hybrid compression algorithm is utilized to reduce the volume of data transmitted.

2. Methods

The operation process of the Argo buoy is shown in Figure 1. After the buoy runs a profile and floats up to the sea surface, it uploads the collected deep-sea data, positional information, buoy status information, and other information to the satellite through the BTTT. The shore-based terminal receives the information through the satellite and then sends it to the upper computer for handling and display. The upper computer can send remote commands to the shore-based terminal for uploading to the satellite. The terminal receives the information via satellite and transmits it to the buoy to change the parameter settings for the next profile. Accordingly, remote communication occupies a significant role in buoy-based marine detection. Throughout the operational cycle of the buoy, multiple communications are required to be conducted by the BTTT. The direction for the design of low-power terminals is oriented towards shortening the duration of communication and reducing the power consumption of the terminal.

The BTTT is optimized to improve the speed and accuracy of the transmitted data. The original BDM protocol is hidden, requiring users to parse it separately. To facilitate user operation and ensure data transmission speed and accuracy, an extended design is made for the BDM. Each function of the terminal corresponds to a specific command. Users only need to send short commands to achieve complex functions. The system analyzes Beidou communication information, extracts message content, parses the content again, and accurately transmits the information to the user.

The remote communication process of the Argo buoy [17] is shown in Figure 2. The BTTT depends on the Beidou transparent transmission control board, BDM, and antenna to achieve buoy remote communication functions. The design of the terminal is comprised of hardware design, control program design, and data compression algorithm design. Because the buoy belongs to limited energy equipment, the requirements for power consumption are very high. Through many revisions and optimizations, the following measures are taken to realize the ultra-low power consumption design of the terminal: First, selecting ultra-low power microcontrollers and using stop mode to reduce static power consumption; second, designing a reasonable power management process; and finally, compressing data to shorten communication time, improve communication speed, and reduce power consumption.

2.1. The Design of the Hardware

The system architecture of the BTTT, encapsulated in Figure 3, comprises a main control unit, the BDM, an antenna, the power source, and an electric level conversion module. The main control unit is tasked with data management, enabling transparent data transmission, compressing datasets, executing power regulation, and orchestrating system entry and exit from STOP mode for power conservation. In concert, the BDM and its antenna are accountable for pinpointing the buoy’s location and facilitating its data transmissions wirelessly. The power module converts voltage to the levels requisite for other components, while the conversion module transitions between TTL and 232 levels. External interface pins enable power sourcing and connectivity with the buoy’s main control board, permitting seamless interaction between the buoy’s command center and the terminal. The main control unit, power source, and electric level conversion module are welded on the front of the Beidou transparent transmission board, and the Beidou module is placed on the back of the board. The board material is a glass fiber epoxy resin copper-clad plate (FR-4).

Low power consumption in hardware design is mainly achieved through two aspects: on the one hand, the communication interval for civilian Beidou cards is 60 s, and the BDM remains static during this period, so the power supply to the BDM can be cut off, and the VCC to 12 V and VCC to 5 V voltage conversion circuits are disabled, leaving only the 3.3 V power supply. On the other hand, the STM32L073RZT6 ultra-low-power microcontroller features a low-power mode, where the STOP mode stops the core clock while ensuring that SRAM data are not lost. It can be awakened by a low-power UART. Compared to sleep mode, it has a lower power consumption and a UART wake-up function than standby mode. Therefore, choosing the STM32L073RZT6 as the main control chip in the hardware design can further reduce power consumption.

2.1.1. Main Control Unit

The STM32L073RZT6 is selected as the core controller, which is an ultra-low-power 32-bit microcontroller based on the Cortex-M0 core. The chip has four UARTs and one low-power UART. In the circuit design, UART1 connects to the RDSS port of the BDM for communication between the main control chip and the BDM, and UART2 connects to the RNSS port of the BDM to receive positioning information. The low-power UART (LPUART1) connects to the MAX3232ECUE+ chip to convert TTL-232 levels and uses an RS232 interface to connect the BTTTs low-power UART to the buoy main control board, enabling information transmission between them. The buoy main control board controls the enable pin of the power supply chip; when high, the BDM power turns on, while when low, the BDM power turns off. The buoy main control board can wake up the terminal in STOP mode through UART wake-up operation, achieving low power consumption control.

2.1.2. BDM Circuit and Antenna

The BTTT uses the BDM910 module, integrating RDSS radio frequency transceiver chips, power amplifier chips, and dedicated RDSS baseband circuits. The BDM910 module is made of aluminum. And the module supports Beidou RDSS and RNSS functions and can achieve RDSS positioning, communication, and RNSS navigation positioning [18]. The module has a small size, high integration, low power consumption, and simple interfaces, with a transmission power consumption of 36 W and a standby power consumption of 0.9 W. The BDM910 module power interface is divided into VCC and VCCPA, with input voltages of 5 V and 12 V, respectively. The BDM910 module connects to the main control chip’s two UARTs through a connector and processes communication and positioning data through the RNSS and RDSS ports. The module does not require an external LNA and can be used with a passive antenna.

The antenna model TA-011 and GPS are utilized, with a size specification of 52.8 mm by 52.8 mm by 23.3 m and fabricated using dielectric ceramic material. The frequency ranges prescribed for the Beidou antenna are limited to the L-band at 1615.68 MHz ± 4.08 MHz and the S-band at 2491.75 MHz ± 4.08 MHz. It is defined electrically by a standing wave ratio (SWR) that is less than or equal to 2.0, an impedance value of 50 ohms, a normal axial ratio that does not exceed 3 dB, an LS isolation of at least 20 dB, an L-bandwidth exceeding 20 MHz, and an S-bandwidth exceeding 100 MHz. Regarding its gain, the antenna’s normal polarization gain is measured at 2.0 dBic in the L-band and at 4.5 dBic in the S-band. The Beidou module and antenna are produced by Beijing BDStar Navigation Technology Co., Ltd. (Beijing, China).

2.1.3. Power Control Circuit

The Beidou transparent transmission terminal requires a wide voltage input of 9 V to 32 V. Specifically, the main control chip of the Beidou transparent transmission terminal requires a 3.3 V input power supply, which also powers the TTL to 232 conversion circuit. The BDM910 Beidou module requires power inputs of 12 V and 5 V. Thus, the LM5175PWPR for stabilizing to 12 V, the TPS5430DDAR for narrowing down to 5 V, and the HE2210M533R for the 3.3 V supply essential to the main control chip and the TTL to 232 conversion circuits.

The main control chip GPIO interface connects to the enable pins of the LM5175PWPR and TPS5430DDAR chips, controlling the on and off states of the 12 V and 5 V power supplies by adjusting the output level. This strategic power management affords considerable energy savings and extends the operational lifespan of the buoy’s battery charge.

2.2. The Design of the Control Program

BTTT is a passive operation terminal after receiving the command of the upper computer or the buoy main control board. The main function of the terminal in the operation of the buoy is to receive remote commands from the shore base to the buoy to remotely modify the initialization parameters of the buoy, profile motion parameters, hover parameters, sleep parameters, sampling interval parameters, and other information. After completing a profile, the shore base sends a recovery location command to the buoy, and the buoy remotely returns its positioning information. On receiving profile detection data and buoy state information transmitted by the buoy main control board, the terminal remotely sends it to the shore. During the noncommunication phases, the BDM is disconnected from the power and enters STOP mode. Controlled by the buoy’s main control board, it wakes up at the appropriate time through the serial port. Under the control of the main control chip, the BDM power is turned on, and the control program flow chart of the BTTT is shown in Figure 4, and each step is divided by a dotted-lined square.

Once the terminal receives information, it first determines whether it is the RDSS serial port or the USER serial port that receives the information, respectively, corresponding to the information sent by the shore base to the terminal and the information sent by the buoy main control board.

Secondly, the content of the information is judged. If it is a remote command sent from the shore base, the terminal is tasked with forwarding the remote command to the buoy control board for altering operational parameters and providing feedback on the success information to the shore base. In the event that a retrieval and positioning instruction from the shore base is received, the terminal first reads the positioning information continuously transmitted to it by the BDM before dispatching this information to both the buoy control board and the shore base.

When the terminal receives buoy status information from the master control board of the buoy, it processes the received buoy status information for remote transmission to the shore base through a communication request. If it receives profile detection data, the terminal first compresses the data before sending the data packets to the shore base through a communication request.

Thirdly, after the terminal sends the information once, it enters the STOP low power consumption mode and disconnects power to the BDM. After 55 s, the buoy main controller wakes up the terminal through the USER port and powers the BDM.

Finally, determine whether all the position information or profile detection data has been sent. If the information is not sent completely, loop the sending process until the end.

2.3. The Design of the Data Compression Algorithm

After the Argo buoy floats to the surface, the buoy sends the ocean profile data detected during the profile detection process to the shore base through the BTTT. The terminal receives the data transmitted from the main control board of the buoy and judges that it is profile detection data. First, the data are compressed, and then the compressed data are converted into message content, added with other information, and sent to the shore base.

The Argo buoy carries a CTD sensor to detect the conductivity, temperature, and depth information of the ocean profile. The CTD data are represented in ASCII code format, including numbers, punctuation marks, spaces, and carriage returns. Environmental factors such as temperature, salinity, and depth have a certain numerical range and change with increase and decrease. The data composition is shown in

Table 1. Profile detection data table.

Profiling Data (Time, T/°C, D/m, C/µS/cm)
230224071515, 01.3870, 4001.1, 0376
230224073436, 01.3896, 3882.5, 0404
230224074043, 01.3919, 3846.1, 0404
..........................................................
..........................................................
230224173957, 28.8629, 0002.1, 0872
230224174020, 28.8775, 0001.3, 0872

. According to the characteristics of the data, differential encoding algorithms, Huffman algorithms, LZW algorithms, and mixed algorithms can be used to achieve data compression, reduce data volume, reduce the number of profile data transmission times, shorten the communication time of the terminal, and reduce power consumption.

2.3.1. Differential Coding Algorithm

Upon receipt of the profile detection data by the Beidou transparent transmission terminal, it extracts the time, temperature, depth, and salinity information, respectively, preserving the first data and representing the subsequent data as the difference between it and the previous data [19]. Take the time parameter, for example. The difference between the time of the second frame of data and the previous time is 243, which may be used as a substitute for the original value, reducing 9 bytes compared to one frame of data.

2.3.2. LZW Algorithm

The LZW algorithm [20] has to create a dictionary table. An encoding index is created within the dictionary for the first occurrence of each character in the data. Subsequent characters are read and combined with the previous character to form a string. It is then assessed whether this string already exists in the dictionary. If so, it is replaced by its encoding index. If not, a new encoding index is formed within the dictionary. These steps are repeated continuously until all characters in the original string have been read, completing the process as illustrated in Figure 5. As a result, longer repeated sequences within the data can be substituted by shorter codes, thereby achieving data compression.

2.3.3. Huffman Algorithm

The Huffman algorithm assigns encodings based on the probability of characters appearing in the data [21]. A shorter code is assigned to higher probabilities and a longer code to less frequent characters, thus achieving data compression as shown in Figure 6. This algorithm commences with a preliminary data traversal, constructing a Huffman tree in which higher frequency characters are positioned closer to the root. A binary system marks the encoding direction—‘0′ denotes a leftward pathway; while ‘1′ indicates a rightward trajectory. After the shore base receives the compressed data, it is decompressed according to the constructed Huffman tree, revealing the buoy profile detection data.

3. Results and Discussion

In an open outdoor area with no obstructions, the BTTT is tested with the Beidou antenna facing south at 45°, as shown in Figure 7. The shore base communication terminal contains a superior computer and a shipborne Beidou terminal. Communication between the buoy main control board and the terminal is achieved through a serial port connection to the superior computer using serial port debugging software.

3.1. Low Power Consumption Test

With a power source and power meter for power consumption tests, the results are shown in Figure 8. When the buoy is underwater, the buoy main control board controls the BTTT to power off the relay, which is the stopping state W0. The Beidou communication interval is 60 s, during which communication cannot be performed, representing the standby state W1. The microcontroller controls the 12 V and 5 V power supplies of the BDM, retaining only the 3.3 V power supply that maintains basic functions while entering STOP mode to reduce power consumption further, defining this low-power standby state as W2. When the buoy floats to the surface and communicates with the shore base, the terminal receives and sends data, consuming the most power, representing communication state W3. In addition, the single low power consumption state W4 was also tested, which only enters STOP mode without taking measures to disconnect the BDM power.

When the USER initiates the awakening of the system, the corresponding transmission or reception durations are clocked at 0.3 s, interspersed by a 55 s period of low power functionality between communications, followed by a 5 s latency post-activation. The process duration for the terminal to handle incoming data and orchestrate its dispatch is pegged at 1 s. The associated power consumption metrics are as follows:

$$0.3\times W3+1\times W1+0.3\times W3+55\times W2+5\times W1=34.276\mathrm{J}$$

(1)

Upon activation by the RDSS, a power supply to the BDM is necessary to apprehend the arousal signal from the shore base. Post-signal dispatch by the shore base, a 60 s interlude is mandated prior to enabling remote communication. During this interval, the power consumption, sustained in a non-energy-conserving standby state, is tabulated as follows:

$$0.3\times W3+60\times W1+0.3\times W3+1\times W1+0.3\times W3+60\times W4=287.634\mathrm{J}$$

(2)

Absent the implementation of any power-saving protocols, the power draw of the terminal for a single communication session is calculated accordingly:

$$0.3\times W3+1\times W1+0.3\times W3+60\times W1=150.876\mathrm{J}$$

(3)

Consequently, tapping into the USER port through the buoy’s control board to signal awakening within the low power mode of the terminal is proven to be energy-efficient both in consumption and duration. Embracing power-conservation strategies culminates in a reduction of 116.600 J per communication, slashing power usage by 77.282% relative to scenarios devoid of such measures. Additionally, the duration of each communication session is maintained at 61.6 s.

3.2. Data Compression Test

The profile detection data of the buoy contains the following data: time, temperature, depth, and conductivity. The length of a single pack of data are a fixed 32 bytes; the maximum number of bytes received from the buoy main control board by the terminal at one time is 512; and the compression ratio of 50 groups of profile detection data are compared according to data compression algorithms. The compression ratio is expressed as the percentage of the byte length of the compressed data compared to the original data browser length. The test results are shown in Figure 9.

The average compression ratios of the differential encoding, LZW, and Huffman algorithms are 59.1%, 87.4%, and 42.8%, respectively. When the differential encoding is combined with the LZW and Huffman algorithms for double data compression, the compression ratios are 53.3% and 25.9%, respectively. Therefore, differential encoding and Huffman algorithms are used to carry out double data compression on the profile detection data. The maximum number of bytes for a single Beidou civil card communication is 78 bytes; the maximum number of bytes for the telegram content section of a communication application is 60 bytes. After completing a profile run, the buoy transmits the profile exploration data, which has an average byte size of 4463 and contains 134 packets of data. This requires 75 transmission requests to be sent. After the detection data of a profile is employed by double data compression, both differential encoding and Huffman data, it only needs to send 20 communication applications, reducing the number of remote communications and lowering power consumption.

3.3. Discussion

Combining the decrease in single communication static power consumption and the reduction in the number of communications, the single communication power consumption without energy-saving measures is 150.876 J, the number of single-profile communications without data compression algorithm is 75 times, and the total consumption is 11,315.700 J; the single communication power consumption of measures with low energy consumption is 34.276 J, the number of single-profile communications with data compression algorithm measures is 20 times, and the total power consumption is 685.520 J. Therefore, after the energy-saving design of the BTTT, the remote communication energy consumption of a single profile detection data of the buoy is reduced by 10,630.180 J, which is 93.942% lower than the original single profile communication energy consumption.

The differential coding-Huffman data compression method has been found to be more effective at reducing power consumption than implementing communication management for the Beidou short message service terminal. The analysis suggests that this outcome occurs because the date data (year, month, and day) for individual profile detections exhibit a high degree of repetition, and the variability in temperature, salinity, and depth measurements demonstrates strong regularity, which leads to enhanced data compression results.

4. Conclusions

In this study, a Beidou communication terminal is designed to establish remote communication with a shore base during ocean testing executed by an Argo buoy. The terminal is engineered to enable the real-time transmission of short messages, conveying critical data regarding the buoy’s status, location, and environmental profiling, which assists shore-based operators in the adjustment of the buoy’s operational state and the acquisition of its positional data through the issuance of remote commands. Moreover, the Beidou communication terminal integrates the Beidou 4.0 protocol to facilitate transparent data transmission, thereby enhancing the clarity of the data exchanged between the terminal and the buoy’s primary control board.

The design of a BTTT tailored for low-power consumption is presented in this paper. Utilizing a low-power microcontroller and associated components, power consumption is mitigated by controlling the BDMs power states so that it remains inactive during non-communication intervals. The STM32L073RZT6 microcontroller’s low-power serial port, LPUART1, governs the transition into and out of the STOP mode for the Beidou board, further diminishing energy use. Experiments have shown that adopting the energy-saving measures mentioned in the above sections can effectively reduce the power consumption of a single communication by up to 77.282%. The hybrid data compression algorithm uses both differential encoding and Huffman to compress the profile data, which leads to an average reduction of 55 remote communications per profile and greatly reduces power consumption. When these measures are combined—reducing static power consumption and decreasing the number of communications—the system’s power consumption is cut to just 6.058% of its original demand. Such reductions in the power usage of the BTTT are pivotal, promising to prolong the battery life and operational duration of oceanic buoys, which will have a positive impact on the development of ocean exploration instruments.

In recent years, the development of compression algorithms has progressed rapidly. Employing new compression techniques and combining different algorithms for the compression of profile detection data represents an avenue for future work. Extensive testing and research in this domain would be beneficial in enhancing the compression ratio, thereby reducing communication duration and lowering energy consumption. At the same time, we will test and improve the reliability of the Beidou transparent transmission terminal through lake trials in the future.