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Design and Implementation of Container Global Monitoring Device Based on Low-Orbit Satellite Communication Module

Containers are important equipment for global cargo transportation. After the USA 9/11 terrorist attacksand, safety of container transportation has attracted much attention. The real-time monitoring of containers has become an inevitable trend for future container intelligent development. In this paper, current development status and advantage of application on monitoring container of low-orbit satellites are introduced, and circuit design and development solution based on low-orbit satellite module is elaborated, finally the monitoring device is proved that has great practicability by actual test experiment.


The Orbcomm system
Orbcomm system consists of 36 satellites with an orbital height of approximately 825km. It is a commercial low-orbit satellite constellation system specifically designed for short data transmission. It is the first and only two-way short data low satellite Orbit satellite communication system. Since its application in 1995, the system has obtained operation rights in nearly 90 countries around the world. Since the Orbcomm system is specifically designed for short data transmission, the price is lower than other systems. User terminals include handheld, vehicle (machine, ship), fixed and semi-fixed terminals. The short data service of the system is mainly used for information transmission, remote monitoring of infrastructure and automatic data collection and processing. Because the system is very suitable for the military to use in the battlefield environment, intelligence data collection and transmission, and emergency rescue, it has attracted considerable attention.

Design requirements and circuit solutions
According to the special characteristics of container transportation, the design of related circuits should meet the following design requirements, mainly including: environmental adaptability design, equipment explosion-proof design, low-cost design, low power consumption design, reliability design, extended compatibility design.
 Environmentally adaptable design: The working environment of the container is harsh, and the circuit design requires resistance to salt haze, corrosion, etc. At the same time, it is required to work in an environment with an ambient temperature from minus 40 degrees Celsius to 60 degrees Celsius. Therefore, in the circuit design process, the entire electronic components and circuit boards need to be protected from moisture and salt mist. In the case of extreme low temperature, it is also necessary to carry out thermal insulation treatment of the equipment, so that even if the external ambient temperature is -40 degrees Celsius, the working temperature of the battery can be maintained at -20 degrees Celsius. In this way, the problems of fast discharge and easy damage of the battery under extremely low temperature environment can be greatly improved.
 Explosion-proof design: Containers are often stored in shipping or stacked in the yard, so it must be ensured that the equipment installed on the container has good explosion-proof performance, otherwise the consequences will be very serious in the event of an explosion. It is not enough to choose a battery with good explosion-proof performance, and other measures need to be added to strengthen the explosion-proof performance. We choose to use an explosion-proof shell design to protect the main  Real-time design of data transmission: At present, data transmission is mainly achieved through 4G network, but some countries and regions still mainly use 2G network. The data communication based on the low-orbit satellite module is mainly to solve the problem that when the 2G or 4G main network has no signal, the monitoring data can be transmitted to the remote monitoring system through satellite communication. This requires that the device be designed to have both 2G/4G network and low-orbit satellite communication network. When one of the transmission networks fails to transmit data, the data communication management module will automatically switch the network to find available data transmission network. Satellite communication is susceptible to occlusion. If neither mode can communicate, the time stamp and the environmental information and location information collected by the sensor should be temporarily stored in the circuit memory. Once the network is connected, these stored data will be sent to the remote monitoring system together, so that there is no blind spot in the management system.
 Low cost design: the cargoes transported by the container mainly include ordinary goods, high value-added goods, fresh goods, chilled goods and hazardous goods etc. ordinary goods account for a large proportion of container transport, but the value of the cargoes may be not high. Therefore, by reducing the cost of equipment, it can be beneficial to the promotion and application of equipment. This design uses all the commonly used components in the market. For example, the temperature and humidity sensor selects the general SHT1X sensor, and the inclination selects the SCA100T sensor. The monitoring device has lower cost.
 Low power consumption design: The design of the low power consumption of the device is mainly to meet the needs of the device to work for a long time. In the design of low power consumption, in addition to selecting a chip circuit with low power consumption, it is more important to optimize the work flow of the hardware circuit to reduce power consumption. We use a power management module to implement multi-branch power network management, so that the power supply of each functional module of the system is relatively independent, and the working time of each module can be effectively controlled, especially for the communication module, because positioning and communication are very power consuming The working current of general 4G modules and satellite positioning modules can reach tens of milliamperes, and the average working current of satellite communication modules is higher. Therefore, when the device does not work, it enters a low-power standby state. Only when the sensor senses changes of state, such as changes in position, vibration, temperature, etc., the monitoring device will be woken up, and usually a timer can be used for long-term wake-up.
 Power supply design: Since container transportation is a global transportation, the frequency of battery replacement determines whether the container tracking device can be successfully applied to the container. Basically, it is better no to replace the battery for 5 years, and the capacity of the current existing battery is difficult to meet the requirements. Therefore, we design it as a dual power supply mode, one-time lithium battery power supply plus rechargeable solar battery power supply, usually the main power supply is powered by solar cells. When the outside temperature is too low or the solar cell voltage is too low, the battery management module can quickly switch the battery and reduce the frequency of data transmission. At present, it is designed to send out a data every 3 hours. When the container is in a stationary state, the device remains dormant. After the container is detected to start shipping, it will enter a new timing cycle, which can maximize the working time of the device. After calculation, the monitoring equipment can meet the needs of 5-year of use.

Circuit module design and physical realizations
The circuit design of the container monitoring device includes the main control circuit, sensor module, power management module, positioning module, communication module and so on. This circuit integrates temperature and humidity sensor, light sensor, acceleration sensor and power management module. The positioning module selects the Beidou/GPS combination, the communication module adopts GPRS/4G module and low-orbit satellite module, and the battery power supply selects lithium The temperature and humidity sensor, light sensor, acceleration sensor send the collected environmental information to the main controller chip through the IIC bus, Beidou/GPS module sent location information to the main controller chip through SPI bus, and then the main controller sends the monitoring data to the remote management system through the data communication module .

TEST VERIFICATION
In order to better verify the performance of the monitoring equipment, we chose to test on the China-Europe train. There are two routes for testing. The monitoring equipment has to undergo various weather changes along the way, including the severe cold at a minimum temperature of -30 degrees Celsius.
The monitoring equipment is installed in the position of the vent of container as shown in Figure 3.     Monitoring device also recorded location and related information of container. These information includes temperature, humidity, transmission way and battery power.
Through testing, we found that the device can work normally, and the recorded position information and sensor information are completely correct. Of course, we found the low-orbit satellite will replace the 4G/2G network when in the area far away from city, we also found that in some areas, there are failure cases of data communication, mainly because of terrain and other occlusions, resulting in satellite data transmission failure, in response to this situation, we have optimized the monitoring equipment, recorded the collected sensor information, positioning information and collection time in the data storage, once out of the data collection blind area, immediately send the sensor and positioning information with collecting time.

CONCLUTION
The container tracking device using the low-orbit satellite module will be an important option for the monitoring of container transportation in the future. With the increasing number of low-orbit satellites, the data communication capability will have a qualitative leap. From this research, we can draw a basic conclusion that the application of low-orbit satellite modules for the global tracking of containers is completely feasible, but due to the limited number of experiments, there are still uncertain problems in large-scale commercial use. The next step is to plan a larger scale test application and accumulate more practical experience.