# Digital Twin in the Maritime Domain: A Review and Emerging Trends
## 2 Inception of the Digital Twin Concept  
The initial conception of having a \"Twin\" was first implemented by NASA in their \"Apollo\" program, which used two identically built satellites in their missions [7]. One would go on the actual voyage in outer space while the other physical twin will remain in the laboratory in a controlled space, thus allowing the mission scientists to analyze the conditions of the launched vehicle by comparing the conditions with the ground twin. The same helped them in parameter monitoring and fault analysis with a minimum data transfer between the two vehicles. A similar concept was utilized in the \"Iron Bird\" (Figure 1) ground-based aircraft system testing platform by Airbus Industries, where a hardware simulator twin was used in system evaluation, design and testing while a simulated cockpit was utilized by the test pilots [8]. The combined usage of the two terms of \"Digital + Twin\" first came up in a white paper related to the design for 3D arterial phantoms in coronary arteries published in the _Radiology_ journal of RSNA (Radiological Society of North America) [9]. Authors have  
Figure 1: Reflection of the Original Conceptual Idea of DT by Dr [PERSON] in 2002.  
developed a realistic model of arteries using stereolithography using a computer-based model and they have referred to the computer-based design as a \"digital twin\". The novel concept of the DT was first proposed in the year 2002 by Dr [PERSON], a leading scientist in the field of advanced manufacturing at the Florida Institute of Technology at a manufacturing engineers conference in Troy, Michigan. His idea was based on constructing digital information on the physical model on his own (Figure 1). This digital information is a replica of the data embedded in the physical system, which will be connected with the physical system in the complete life cycle of a particular system/component [10]. The concept of DT was also brought up by NASA under their \"Modeling, Simulation, Information Technology & Processing Roadmap at Technology Area \(11\)\", where a DT is introduced as a multiscale simulation of a vehicle or system with its own incorporated physics by optimally utilizing its physical data, sensor data, historical data, etc., in the effort of obtaining a real-time image related to the life of its corresponding physical twin in outer space. Since the concept of DT is practical and more realistic in nature, application for a single vehicle or even for interdependent multiple assets can be performed robustly. The anomalies that occurred during the manufacturing stage that could imperil the space missions can also be foreseen during its manipulation. DT can act as the backbone of any high-fidelity physical model, supporting the integrated vehicle health management system in evaluating historical data. With the input dataset, DT will enhance the mission success credibility of the craft with continuous monitoring and evaluation of the operational condition and the remaining running hours ahead. A robust DT model will assist damage mitigation or degradation by real-time data analysis/forecasting and suggest required changes in an ongoing space mission to enhance the life span leading to mission success [11]. Later, USAF came up with the concept of Digital Thread, where each USAF aircraft enters the fleet with its own DT. The DT will help in Structural Health Monitoring (SHM), and maintenance, thus allowing calibration of the craft in its operation state by comparing the sensor readings of flight and DT. It also acts as a digital surrogate to plan the design, production, and acquisition phases of the project [12]. Though it was a clear definition for DT to be made considering the representation fidelity, model simulation capabilities, synchronization techniques, data collection, exchange attributes, etc., it is also stated as a virtual representation of the real-world asset with an exchange of information in a predetermined frequency [13].