GPS

During the days of Christopher Columbus there was still some debate about whether the world was flat or round. Many people feared that if they sailed to the edge of the earth, they would fall off. I wish I could have been there to ask a question to those believing that world was flat. That is, if the world is flat, WHAT'S HOLDING THE WATER ON? Why don't the oceans drain off and become deserts?

Since the 1400's, mariners have conquered the "flat vs. round" issue and many more. Wouldn't Christopher Columbus be impressed with how far things have come? Not only are you dealing with a "round" world, you have satellites overhead to assist you with navigation. Let's talk about those satellites.

Twenty-four satellites support today's Global Positioning System (GPS). The satellites are in six orbital planes. The system normally operates with twenty-one satellites in service. The remaining three satellites are spares. At all times at least four satellites can be observed from any position on earth. If you have ever used a GPS receiver, you have seen many more than four satellite signals on your screen. Each GPS satellite transmits on two frequencies. 

The GPS device that you carry is either a "hand-held" or a "permanently mounted" electronic receiver. Without going into the engineering "mumbo jumbo" this all means that the GPS unit on your boat receives regular and reliable information. Your receiver interprets the signals received and calculates several things for you.

The most vital calculation that you read on your GPS receiver is your current position (latitude and longitude). With that information, you can plot a course to your next waypoint or destination. Based on a series of positions, your GPS indicates your speed (over the ground), your direction, and even the estimated time of arrival (ETA) to your next waypoint or destination.

All of these calculations are done on the same basis as those done on paper. If you plotted a series of known points on a chart, you could easily calculate your course, speed and ETA. Your GPS is just "infinitely faster" at it than you or I could ever be. Christopher Columbus would love this!

How can you be sure that the satellites are operating properly? There are five monitoring stations on earth. They are located in Colorado, Hawaii, Kwajalein, Diego Garcia, and Ascension Island. All five stations receive signals from the satellites and three of them have "up-link" capabilities.

A Master Control Station in Colorado Springs, Colorado processes the information from the monitoring stations. Based on what is received by the monitoring stations, information is transmitted back to the satellites. Such information keeps the satellites continually updated.

Your GPS receiver makes "time-of-arrival" measurements of the satellite signals. That's how your receiver knows how far it is from each satellite. Your GPS receiver can calculate a three-dimensional position (including altitude) from the signals received. 

For the most part, mariners do not do much with "altitude." But mariners are not always at sea level. For example, Lake Ontario is over 200 feet above sea level. However, as long as your feet are not wet, you are usually satisfied with your altitude. That's how I look at it!

Until next time, I wish you clear skies, fair winds, and calm seas!