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In the immortal words of Buckaroo Bonzai, “Wherever you go, there you are.” But if you want to know precisely where “there” is, you need a GPS device. Let’s examine how this technology operates.
The fundamental idea of a satellite-based navigation system was conceived prior to Word War II, but no one pursued the idea aggressively until the Russians launched Sputnik, the first artificial satellite. Research continued through the 1960s, and the U.S. Department of Defense settled on the first design in 1973.
The first developmental GPS satellite—Navstar 1—was launched in 1978, the first fully operational GPS satellite was put into orbit in 1989, and the system was declared fully operational in 1995. Although GPS remains an indispensable military tool (and is maintained by the U.S. Department of Defense), the technology was made available to consumers in the 1980s and can now be found in relatively inexpensive devices ranging from cellphones and PDAs to dedicated handheld GPS receivers.
The Global Positioning System consists of three segments: a network of satellites (24 in the original system, 31 today) orbiting 12,600 miles above Earth (the space segment), a series of ground stations (the control segment), and individual GPS receivers (the user segment). The satellites are positioned in space so that a GPS receiver anywhere in the world can receive signals from at least four simultaneously (i.e., at least four satellites are above the horizon at any point on the planet). We’ll explain the importance of having access to four satellites at the same time shortly.
Each satellite transmits two coded radio signals, designated L1 and L2, to Earth. The L1 signal operates at a frequency of 1,575.42MHz, and the L2 signal operates at 1,227.60MHz. These signals are of low power (between 20 and 50 watts each), and they travel by line of sight, which means they can pass through clouds, glass, or plastic on their way to a receiver, but they’re obstructed by more solid objects, such as buildings and mountains.
The L1 signal contains two pieces of information: a coarse-acquisition code (a pseudorandom number that identifies a particular satellite) and a navigation message. A pseudorandom number exhibits all the properties of a random sequence, but it’s actually generated by a complex algorithm and can therefore be repeated. The L2 signal contains an encrypted precision code that can be decrypted only by military-grade GPS receivers. The navigation message in the L1 signal contains the date and time the signal originated, information related to the satellite’s status and health, ephemeris data (the satellite’s precise location at a given time, which the receiver uses to calculate the satellite’s exact position based on the speed at which the satellite is traveling and the current time), and almanac data (coarse orbital parameters for all the satellites in the constellation).
Ephemeris data is highly detailed and is considered valid for only four hours after receipt; almanac data is more general and remains valid for 180 days after being downloaded to the receiver. The receiver uses almanac data to determine which satellites it should search for, based on the current time and their last known position (as reported in the almanac).
Each satellite’s flight path is monitored by a network of six U.S. Air Force stations located around the world, which record any deviations in the satellites’ orbits (slight changes are usually caused by the pull of the moon and the sun). Each station forwards the information it receives from the satellites to a master control station located in Colorado Springs. The master control station synchronizes the atomic clocks carried on each satellite and uploads any orbit changes, which are in turn sent to GPS receivers as part of the satellites’ signals.
If a GPS satellite’s orbit ever needs to be adjusted (or if the satellite is otherwise determined to be unreliable), the master controller labels it as “unhealthy,” so GPS receivers won’t use it in their calculations. Once the problem has been resolved (following an orbit correction, for example), the master controller uploads the satellite’s new ephemeris data and tags it as healthy again.
In order to calculate its position, a GPS receiver compares the time the satellites’ signals arrive to the time at which the satellites initiated their transmissions. It then multiplies these differences by the speed of light to determine the distance that each signal has traveled.
Considering the great distances involved, making a precise calculation requires that the clocks on the GPS receiver and each of the satellites be synchronized to the nanosecond, which typically can be achieved only with costly atomic clocks. It’s not feasible to put an atomic clock in a consumer GPS receiver, but there is a clever solution: Since the master control station synchronizes the atomic clocks on all the satellites, the receiver constantly resets its inexpensive quartz clock to match the time that the satellites are reporting.
The receiver uses trilateration to determine its location on a 2D plane. Trilateration is similar to triangulation, but where the latter method uses angle measurements and at least one known reference point to determine the coordinates of a specific location, the former uses the known locations of three reference points and the calculated distance between the object and those known reference points (see diagram). Using a fourth satellite enables the GPS receiver to determine its current altitude.
Once the receiver has determined its exact position on Earth, it translates this information into latitude and longitude and plugs that data into a map file stored in its memory.
If the GPS receiver calculates that it is 13,000 miles from one satellite, it knows that it is located somewhere on an imaginary sphere with a radius of 13,000 miles. The satellite is in the center of this sphere, and the receiver is at the outer edge. The receiver then measures its distance from the other two satellites and generates two more imaginary spheres. The receiver will be located at the precise point at which all three spheres intersect. A GPS receiver able to communicate with a fourth satellite can determine your current altitude.
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