# Time Travel

The Global Positioning Systems (GPS) we enjoy today appears to be the product of our new technology, but in fact it is not new. The concept was in use over 300 years ago.

HOW IT WORKS: The GPS system is based on a triangulation and ranging concept. Precise position can be determined by determining the elapsed time it takes a signal to travel from a satellite to GPS receiver and back. By knowing the speed of the signal (the speed of light which is approximately 186,000 miles per second) and the exact broadcast time, the distance traveled by the signal can be computed from the arrival time. Then, by comparing the signal from a series of satellites the position is narrowed down to the exact location. The key to the whole thing is knowing the ‘exact broadcast time.’

IT’S ABOUT TIME: In the early 1700’s sailing ship navigators used the same method, but in those days knowing the exact time was a problem. Most clocks then used a pendulum and weight to operate the clock. 18th Century Problem: A pendulum will not work onboard a ship on a rolling sea. It was only when the ‘sea clock’ (marine chronometer) was invented — using a spring — that time could be properly kept on the ship. Why was keeping time so important? It is the same concept as today’s GPS – If you can tell time, you can determine where you are. Here’s how

• LONGITUDE: When a ship departed Europe, they would do so with one clock set to Greenwich Mean Time, the same “universal time” we use today. After sailing west for several days, the navigator would mark the time when the Sun was directly overhead – High Noon. Then, the navigator would check the clock that remained on GMT. If it were noon on the ship and at the same time it was 5:00 PM GMT, then the navigator could calculate the ship’s longitude. The Earth turns 15 degrees per hour. For a five hour difference between ship time and GMT, the ship would therefore be at 75 degrees west longitude (5 x 15 = 75).
• AND LATITUDE: The ship’s latitude was easy to determine using the date and the Sun’s angle above the horizon or the stars at night. If the Sun was directly overhead on June 20 (June 20th is the longest day of the year, when the Sun is directly over the Tropic of Cancer – determining Latitude) while there was a five hour time differential (determining Longitude), the ship would be at San Salvador. Columbus landed at San Salvador in 1492, but of course he did not really know where he was because he did not have an accurate clock … which explains a lot.

New Rewards: Today, the GPS computer does all those calculations for us. And today, we can get much more accurate location information because our clocks are substantially better. The clocks used today do not use a pendulum or spring; they use electromagnetic waves given off by certain atoms and molecules. Atomic clocks will not lose even a second in 200,000 years so we can depend on them to pinpoint a location.

New Dangers: The biggest fear pertaining to GPS is that pilots will get lazy and depend on them too much leaving them unable to navigate without them. Don’t let GPS make you complacent. Blend the new-world technology and old-world technology so that you can do both, and remember that GPS technology is an ‘old-world’ concept that has just been improved by computers, satellites — and really, really good clocks.