Last century saw huge improvements in the accuracy and ease by which ships navigated. Prior to the satellite systems that we now use, networks of radio waves were set up, for example by Decca. Ships were fitted with receivers that detected these signals and, by receiving signals from at least two stations, your position could be accurately plotted using these radio waves. These were particularly useful near land where accurate navigation was most important and the signals closer together.
Today we use GPS global positioning systems, which use geostationary satellites hundreds of miles above the Earths surface. Although these are very modern and bear no resemblance to any of the traditional methods, they use the same principles; an accurate measurement of time, the speed equation and fixings from three different locations.
Each GPS device is both a transmitter and receiver. A signal sent from your device hits one satellite which then sends a signal back to you. The signal travels at the speed of light about 300,000km/s. Using distance = speed x time, the distance from the satellite is calculated. By the signal hitting three satellites your position can be accurately plotted.
You can easily buy a GPS device they are not much more expensive than a mobile phone and can be accurate to a few metres. This is a remarkable achievement considering the challenges and costs that our ancestors had to overcome in order to be accurate within a hundred kilometres.