Methods for Tracking CHE in Port Terminals and Future Prospects for Technologies
As we mentioned it in our article on 4 Stages of Big Data Exploitation in Container Terminals, the most rudimentary infrastructure required for data transmission and reception in a terminal is network technology and equipment positioning. In this piece, we are going to describe the method for tracking the location of the equipment (ITV, forklift, empty container handler, and reach stacker) that registers a high level of free mobility among container handling equipment (CHE) found in terminals.
Unlike cranes (such as RMG, RTG, and STS), such equipment has access to various spaces of the yard, and for this reason, they are harder to locate accurately, compared to cranes that move along specified paths.
Then, we will take a look at representative methods for positioning the above-mentioned equipment in port terminals.
RFID (Radio Frequency Identification) is composed of tag and reader antenna. When the RFID tag attached to CHE enters an area where a specific antenna is installed, the antenna reads the tag. By calculating the distance to the tag thus read, the two-dimensional position of CHE in the terminal is calculated.
Location-determining algorithms include various methods of TOA, TDOA, AOA, and RSS, of which TOA par exemple calculates the location by using the distance between a tag and three or more antennas.
The performance of this RFID-based RTLS (Real-Time Location Service) varies according to its used tag, antenna, and reader, and generally registers a 3-5m accuracy.
However, the RFID-based positioning has various limitations. To ensure real-time RFID-based positioning, antenna and reader must be installed to cover a wide port terminal, which is bound to add to costs. Moreover, even if enough coverage is secured by installing more antennas, interference won’t be easy to get rid of.
Despite such technical limitations, there have been various attempts to apply it to ports until recently, while for example, YANICT conducted research on some other method for installing antenna and reader in ITV (Ting et al., 2012).
With its large equipment and high price, it used to be employed in certain industries only, but lately, distribution of smartphones and widespread use of vehicle navigators have made it a technology that is highly accessible to ordinary people. A number of ports are also using GPS technology to locate cranes and ITV.
GPS as we know it is the US-operated GNSS (Global Navigation Satellite System), and more systems are available, including Russia-operated GLONASS, Beidou of China, and Galileo of EU.
As the name suggests, satellites are on the move along a specified orbit around the earth. And triangulation is employed to calculate locations by simultaneously receiving a plural number of (minimum 4) satellite signals for a specific location at the current point of time.
In a notable case, GPS has been adopted by PNC Terminal in Busan, where low-priced GPS equipment is installed in ITV, R/S, and ECH to perform real-time
Even such a low-priced GPS receiver can define a location as accurately as about 2m in an open field presenting no signal blockage.
However, in a yard under an STS crane where containers are piled high, an error may run up to several tens of meters. To minimize such errors, PNC is operating map matching, which compares to a map through post-processing.
That is a brief description of RFID/RTLS and GPS, the leading methods for positioning CHE in port terminals. The presence of a great number of containers in a terminal affects positioning with radio-frequency interference, and there is no method that can completely perform positioning without incurring a great cost in such adverse conditions.
I think that the best policy under such circumstances is to choose the best available performance instead of the best possible performance while accepting limitations of different technologies and focusing their good sides.
Difficulty involved in positioning in port terminals
Various positioning solutions have long been developed and advanced in academia and in the industry, but no solution has been established for port terminals.
Most solutions for positioning in a large space are based on RF communications. RFID and GPS, which we discussed earlier, are both RF-based methods, and their biggest drawback is multi-path error.
The error occurs when the antenna which receives signals catches the signals bouncing off those various substances around. The multi-path error is known to become serious, especially when metal is involved (Yi et al., 2012).
Mixing & combination of multiple technologies for improving terminal positioning
Thus, the method for determining location through wireless communications in the specific environment of a port is not doing its job properly because of deterring factors. Then, what is an optimized positioning solution?
An alternative that is drawing spotlight is the positioning method that merges two or more technologies and thereby makes up for one another’s shortcomings, which is up and coming over positioning with one single technology. For example, GPS/INS positioning integrates navigation satellite system, which calculates absolute position, and INS (Inertial Navigation System), which calculates movements based on relative positioning.
CyberLogitec is also working to find a solution in the integrated positioning. In other words, while overall positioning is performed in GPS, INS technology kicks in and starts positioning operation when satellite signal is unstable, has its quality lowered, and doesn’t have enough visible satellites. In locations like tunnels where satellite signal is completely kept out, satellite navigation system can’t calculate positions. Here, by using INS, which combines gyroscope and acceleration sensor, one can guess at the subsequent positions by identifying direction and speed.
Previously used mainly for military systems or to research purpose, INS reached its commercialization when the latest development of MEMS technology promoted its application in cell phone and drone, the unmanned helicopter. While price gap between high-end and low-end products is huge, effective positioning accuracy could be ensured by using low-priced MENS IMU to overcome the limitations and drawbacks of GPS only. CyberLogitec is operating such GPS/INS equipment in Jebel Ali &3, which is operated by NCT and DP World in Saudi Arabia. Positions are 100% calculated on a specific project site with 1.5m accuracy.
Directions for future advancement
Currently used positioning algorithm integrates GPS and INS only. GPS/INS is no perfect solution, however, because INS has the technical disadvantage that increasing duration from its default value adds to its errors.
Considering the characteristically poor conditions of a port terminal, we think that additional integrated navigation method is necessary to ensure maximum accuracy. For example, if one uses vision technology to recognize a specific reference point in an area where satellite signal is blocked and GPS positioning is difficult and then uses it as a value to renew the default value of INS, a more reliable positioning will be possible.
T.-H. Yi, H.-N. Li, M. Gu (2012), “Effect of different construction materials on propagation of GPS monitoring signals”, Measurement 45, p1126-1139.
Joo-sang, Park (2010), “Active RFID-based Real-Time Location Systems”, Korea Internet conference 2010.
Calin, G. and Roda, V (2007), “Real-time disparity map extraction in a dual head stereo vision system”, Latin America Applied Research 37, 21-24.