The local refractive index model (LBM) on ships is a computer and sensor system developed at the Swedish Defence Research Agency, the division of NBC Defence (FOI), by order of the Swedish Defence Material Administration (FMV). The model is of use when calculating atmospheric refractive index, ducts and radar wave propagation to provide support in estimations of the radar coverage of ships. The aim of this report is to describe the functionality and use of the meteorological model, the program user interface and the sensor system. Also, it focuses on user guidance; the program user interface is developed to allow users to draw accurate conclusions regarding radar range. The introductory section treats the meteorological model, the local refractive index of radar waves, ducts and ray trace propagation. Presented here is also an overview of the computer program. The program user interface, which displays results from calculations and input data, is described in detail in the following section. With the help of sensors connected to the system, automatic input of air temperature, humidity and water temperature can be measured from the surroundings of the ship. The final section gives guidelines on the installation and maintenance of these sensors.