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That's where pulsar navigation, an innovation of NASA's Goddard Space Flight Center, comes in. Clearly, a better solution would be to place a navigational instrument on the spacecraft so it could calculate its position independently. Travel even farther, and the accuracy of earthbound tracking systems falls off even more. A radio signal sent by a tracking station would take 5.5 hours to reach you and then another 5.5 hours to travel back (assuming the waves were traveling at the speed of light), making it more difficult to pinpoint your exact location. When your spacecraft reaches Pluto, you'll be 3,673,500,000 miles (5.9 billion kilometers) away from Earth. Now imagine you want to travel to the outer reaches of the solar system. Using this information, ground crews can calculate the position of the rocket in space.
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Those waves bounce off the craft and return to Earth, where instruments measure the time it took the waves to make the journey and the shift in frequency caused by the Doppler effect. When a rocket leaves our planet for Mars, Jupiter or beyond, ground crews beam radio waves from the tracking stations out to the vessel. Current navigational techniques use a network of earthbound tracking stations that look up and out into space. This doesn't mean missions to destinations beyond Earth have to fly blind. Put another way: GPS satellites only transmit down, not up. Venture above LEO, however, and your handy GPS receiver will quickly find itself above the satellite constellation and, as a result, no longer be able to record a signal.
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Flying in low-Earth orbit (LEO)? You're covered. Cruising along the planet's surface? You're good to go. If you're located below the satellites and have a receiver capable of detecting the signals, you can reliably determine your location. That's because the 24 satellites that make up the GPS "constellation" transmit their signals toward Earth. GPS seems like the logical candidate for such endeavors, but the system only works if your travel is limited to Earthly destinations. Spacefaring explorers may face similar tragedies ifthey can't find a reliable method to orient themselves as they travel to distant planets and, perhaps, faraway stars.