We explain discrepancies between satellite and terrestrial AIS performance in regions with dense vessel traffic.
Classifying a High Traffic Zone
Ship activity in some global regions is significantly more dense than average. These regions are called high traffic zones (HTZs). High traffic zones generally occur near areas with major ports.
Terrestrial Performance within High Traffic Zones
AIS was originally designed for ground-to-ground reception, so terrestrial stations don't experience any significant performance degradation in HTZs within their estimated 80-kilometer radius.
Satellite Performance over High Traffic Zones
As compared to terrestrial stations, our satellites are listening to ships within a 2000-kilometer radius, making them prone to AIS message collisioning - an effect that impacts all satellite AIS providers.
When a satellite passes over a HTZ, it becomes difficult for the satellite to differentiate between the higher number of AIS messages being transmitted at the same time. The end result is actually fewer than usual AIS messages being decoded.
As an analogy, this is similar to the effect of listening to one conversation, as compared to 20 simultaneous conversations. Understanding a single conversation is significantly easier than attempting to follow 20.
Spire has and continues to develop methods of reducing the impact of AIS message collisioning on its satellites.