# The Big Picture: An Improved Method for Mapping Shipping Activities
## 2 Background
### Spatio-Temporal Data and the Automatic Identification System (AIS)
#### 2.1.1 Data Fitness  
The concept of data quality is somewhat vague, but an effective and commonly used definition for data quality is \"fitness for use\", which is the ability of the data collected to meet user requirements [12]. Many works have focused on addressing such uncertainty in location data related to modelling and representation. Specific applications may allow some imprecision based on their requirements. AIS datasets have long been used for maritime  
Figure 1: In grey is the true trajectory, while in purple the approximated trajectory is captured by a given sensor. density maps and researchers have identified some of the underlying difficulties affecting the data fitness [19; 20; 21; 22; 23; 24]. Although AIS datasets are some of the most important sources of information for maritime traffic, the resulting spatial trajectories may have several missing data points due to several factors, including design features or malicious activity:  
1. Firstly, datasets that have been collected by Satellites and those by Terrestrial stations will have different granularities and resolutions. Earth orbiting satellite collecting AIS messages are easily congested when there is a large number of vessels within their given field of view. AIS is based on the Time Division Multiple Access (TDMA) radio access scheme which ensures that no two ships within radio range of each other are transmitting at the same time. The TDMA defined in the AIS standard creates 4500 available time-slots in each minute but this can be easily overwhelmed by the large satellite reception footprints and the increasing numbers of AIS transceivers, resulting in message collisions, which the satellite receiver cannot process. Schemes such as the TDMA were designed for successful ship-to-ship or ship-to-shore communication, not for ship-to-satellite communication, which heavily degrades their efficiency [25]. However, in the case of the satellite segment of the AIS, the efficiency of the implemented access schemes is heavily degraded due to the high ratio of the AIS packets' collisions. 2. Additionally, according to the AIS specifications, Class A transceivers reserve their time slots for transmission via Self Organized Time Division Multiple Access (SOTDMA). After performing a scan to ascertain which slots have already been reserved by other vessels, they reserve an empty slot. The device lets nearby AIS devices know that it intends to use this slot for future broadcasts. On the other hand, Class B transceivers are permitted to transmit via Carrier Sense Time Division Multiple Access (CSTDMA), where, unlike SOTDMA, slots are not reserved. They instead simply scan for available space and transmit when a free one is determined to be available. Transmission priority is given to Class A transceivers, which use SOTDMA since they reserve time slots. The timing of Class B transmissions via CSTDMA must work around the time slots reserved by Class A transceivers. If a Class B transceiver is unable to find an empty space, their transmissions are delayed. 3. Recently, a different type of Class B transmitter that uses SOTDMA, namely Class B \"SO\" (Self-Organizing), was produced. Class B \"SO\" and Class A transmitters fitted aboard vessels have a critical difference which also affects Satellite reception. According to the International Telecommunications Union specifications, provision should be made for two levels of nominal power (high power and low power), as required by some applications. The default operation of the AIS station should be on the high nominal power level. The two power settings should be 1 W and 12.5 W or 1 W and 5 W for Class B \"SO\". Evidently, the weaker signal of Class B devices means it is more difficult to receive these signals from space. Additionally, SAT-AIS cannot capture the signal of all transmitting vessels at once; several orbits are required in order to capture a representative density sample.