GPS can be jammed, either unintentionally or intentionally. Signals from the satellites can be interfered with by ice build-up on the vessel’s antennas, by other communication equipment, or by physical obstacles. Submarines cannot usually use the system.
 
Linköping University doctoral student Rickard Karlsson at the Center for Control and Communication describes in his thesis how modern, simulation-based methods of treating signals can be used to monitor and, if necessary, to take over the GPS function on a vessel.
 
This technology, unique in the world, requires no external infrastructure and is not susceptible to interference. Instead, the vessel’s own radar is used to measure the distance to surrounding shores, and this data is then compared with a digital sea chart. In a submarine, information from sonar equipment is compared with a digital depth chart. In combination with data about the movement of the vessel, the correct position can be calculated.
 
The method is based on a mathematical algorithm, a so-called particle filter, which is installed as a program in the vessel’s computer system. There is no need for any further hardware to be installed beyond what is already on board. Preliminary trials show that the method works just as well as GPS in navigating an archipelago.
 
The dissertation Particle Filtering for Positioning and Tracking Applications deals with several other uses of the same principle: positioning industrial robots, tracking vehicles from another vehicle to avoid collisions, and tracking boats and ships from an airplane.