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Letter from Stephanie: Global Positioning System

Dear Fellow Explorers,

As I write to you, I take a peek out the porthole window; the cold Antarctic Ocean stretches as far as the eye can see.

Where are we? This is a common question aboard our vessel (often followed by, "Are we having fun yet?"). And I can figure out where we are by using both traditional and modern tools.

Being able to plot our location at any given moment is very important for our research. In order to be able to make accurate interpretations of our data, we need to know the exact location of any geological features we see or samples we collect. We keep careful navigation records so that we can return to sample locations in the future. Of course we also need to know where we are and where we are going so that we can get home eventually!

Exploration navigation has gone through many changes since the first humans set out over the seas or over the lands surrounding their early communities. The earliest explorers had few navigational tools; they stayed within sight of land if they were on the water, or within sight of landmarks while on land. As human beings learned the patterns of celestial bodies, explorers could use the positions of the Sun or stars to guide them. Consider the advantages–what could the stars in the sky or the position of the Sun offer that features on land could not? And what would happen if weather blocked the view of these heavenly navigation devices? Stars and other celestial bodies were unique and easy to identify; and while landmarks required that explorers stay close to land, celestial bodies could be seen from anywhere on the globe, far out on the high seas. On the other hand, clouds and storms made it impossible to use celestial bodies for navigation; in poor weather, explorers could get lost.

Major advances in navigation technology helped to spur the exploration age, starting with the common use of compasses on European ships in the 1100's. (The Chinese had begun using compasses long before; the first compass was used in China in 200 B.C.; and by 800 A.D., compasses had gained widespread use!) Because a compass needle always points toward the magnetic poles, sailors were finally able to steer a course using a known direction–and could return from that direction–all without landmarks or even the guiding light of stars.

By the 1500's, the navigation technology revolution made another leap, with the advent of the astrolabe and sextant. These instruments measured the position of the Sun or stars relative to the horizon in order to determine precise latitudes. With a compass and a sextant, sailors could determine the direction in which they were heading, and their latitude.

The astrolabe and the sextant helped sailors pin-point their latitudinal position on the globe; but they still needed a way to plot longitude. In other words, they could calculate their position relative to the North and South Poles, but they could not determine their east-west position; and thus, they couldn't determine where they were on the globe.

Then in 1761, an English clock maker named John Harrison solved this conundrum. That's right, the same clock that you take for granted today was a revolutionary navigation tool in the eighteenth century. Harrison's well-made clock could keep accurate time in rough seas or harsh travel; and sailors knew that there were fifteen degrees of longitude for every hour difference between noon at home and noon at any given location. By knowing the time at home, and knowing when it was noon in their new location, navigators could determine their distance from home. This was the last piece of the puzzle–at last explorers could pinpoint their locations accurately during the journey.

For a few hundred years, these navigation tools were enough. With the advent of radio signals, however, ships at sea could plot their locations and navigate a course with even greater accuracy. Ships (and planes) received radio signals from a known location; using the time they received the signal, as well as the pattern of the signal, sailors could calculate their positions with a greater degree of accuracy.

Ships at sea still use all of these methods; but many of them have disadvantages, especially in Antarctica. What might be the disadvantages of the compass, the astrolabe and sextant, and even radio signals in Antarctica?

Well, the compass uses the magnetic pull of the South Pole; but Antarctica is too close to the South Pole to get an accurate compass reading! The extreme conditions of Antarctica make it hard to use the astrolabe and the sextant; it's hard to see the Sun and stars during blizzards! Even reasonably calm days may include constant cloud cover; and don't forget, you can't see the Sun for six months during the Antarctica winter, and you can't see the stars for six months during the Antarctica summer! Finally, Antarctica is huge, and very far from radio transmitters; as a result, radio signals have limited use there.

These disadvantages are eliminated with the latest navigation technology, the Global Positioning System (GPS). The network and technology is still quite new–the system was not up and running until 1994. But today research teams in Antarctica and all over the world use GPS, a network of satellites that allow for the most accurate positioning and navigation, even in the most extreme weather conditions of Antarctica–and even in the darkest, windiest, coldest Antarctic winter. This is good news to anyone working on the continent or in the icy Antarctic waters, where knowing your position can be a matter of life and death!

So how does it work? There are twenty-one working satellites and three spare satellites in the GPS, making a total of twenty-four in the network. Each satellite circles the globe twice a day, at about 20,200 kilometers above Earth's surface; and at any point in time, six of the satellites can be "seen" from almost every location on Earth. Each satellite sends its own set of radio signals back to Earth, and the radio signals are different for each satellite; this way, scientists can tell which satellite is sending the signal, and thus pinpoint the location they are seeking. The radio signals include exact transmission time and location information; they travel very fast, at the speed of light, in fact! That's 186,000 meters per second!

Who receives these signals? You might have received one, if you have GPS in your car! A ship, a car, a plane, or a person can have a GPS receiver, which gathers and translates the signals from three or four satellites. Because the satellite signals have a time stamp (the time when the signal was sent from the satellite), receivers can calculate the exact amount it took to receive the signal. Because the speed of the signal is known, the time from the moment the signal was sent by the satellite to the moment it was received by receiver can be used to calculate the distance between the satellite and the receiver. (That's a simple equation you might have learned in math class–rate times time equals distance!)

Just one satellite wouldn't be enough to calculate an exact position; in fact, the receiver uses calculations from the three or four satellite signals to determine the position. Each satellite sends a signal in all directions. Imagine that the signal is a sphere with the satellite at its center; the sphere intersects the sphere of the Earth in a circular area. You might want to draw a picture of two intersecting spheres to see how that works for yourself.

This signal reaches Earth's surface and is "heard" by the receiver; but the receiver can only determine the area of the circle, not its exact location within that circle. With a second satellite signal, the receiver has two circular areas that define its location; so the location has to be somewhere where these two circles intersect. Try drawing two intersecting circles–they have to intersect at two points. That means that the receiver has figured out two possible points for its location. It needs a third signal to know which of the two locations is the right one. By calculating position based on three or four satellites, the receiver can determine a location to within 100 meters!

GPS is used by the scientists in many ways, such as locating meteorites, mapping the features left by past glaciers, and surveying new regions. Without GPS, we would not be able to monitor tiny changes in the size of sea ice or in the thickness of the ice sheet. If we want to make accurate predictions about how the Antarctic environment may change in the future, it's critical that we can pinpoint these tiny changes.

But right now I just want to pinpoint the location for breakfast–and for that, I don't need any navigation tool but my own nose, guiding me towards the smell of pancakes up in the galley! That's all for now–enjoy your exploration!

American Museum of Natural History

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