GPS

GPS System

The Navstar Global Positioning System (GPS) is a network of 24 satellites in orbit around the earth that provides users with information about their position and movement. A GPS receiver computes position information by comparing the time it takes for radar signals from three or four different GPS satellites to reach the receiver.

Finding Location with GPS

Global Positioning System (GPS) satellites orbit high above the surface of Earth at precise locations. They allow a user with a GPS receiver to determine latitude, longitude, and altitude. The receiver measures the time it takes for signals sent from the different satellites (A, B, and C) to reach the receiver. From this data, the receiver triangulates an exact position. At any given time there are multiple satellites within the range of any location on Earth. Three satellites are needed to determine latitude and longitude, while a fourth satellite (D) is necessary to determine altitude.

Of major concern to us is the accuracy of the GPS information we read on the monitor. The coordinates from the satellites have to be correlated to a digital map display and there are two styles on the market. Vector charts are virtual, meaning they are digital and an interpretation of a paper chart; raster charts are basically a digital picture of a paper chart. The problem is that all the marine charts in use today date back as far as 300 years and maybe farther. Navigators that sailed the coasts and drew what they saw created the marine charts that are in use today. There have been some modifications over time but we still use the basic chart. All GPS literature related to boats approaching land say to not rely completely on the information provided because of charting errors. The following pictures are the track of the boat as it entered marinas and bays. It’s fairly obvious to see where the red line goes over land. If we had followed the image on the monitor we would have gone aground. That’s why most mariners prefer to make port entries in daylight.