The GPS system is made of 24 NAVSTAR satellites and five ground stations. The ground stations are responsible for keeping the satellites in precise orbit. The DoD placed each of the 24 satellites in a precise orbit at an altitude of 10,900 miles. Each satellite weighs two tons, is 18.5 feet long, and orbits the earth in a little less than 12 hours. Each of the 24 satellites transmits its own unique signal. The GPS unit has stored in it those 24 separate "signatures" and therefor knows the postition of each satellite. By measuring the distance to at least four satellites, each in its distinct orbit, location of the GPS receiver can be pinpointed down to as little as 3 meters. Distance to each satellite is measured simply by the time it takes for a radio wave to reach the GPS unit. To be able to lock onto four signals, a GPS unit needs to have at least four channels. Most units have 12 channels. Calculations were made for the orbits necessary for each of the 24 satellites so that at least five are "visible" to any one point on the earth at one time. GPS can be used in any type of weather, and is used on land, in the air, and for marine applications. Some conditions limit its usefulness. Heavy tree cover and cliffs, steep hills, or tall buildings can interfere, but often in those situations one can move to a better location and still not be too far off the intended route.
For a short video tutorial on how GPS works, click here.
History of GPS
The Global Positioning System (GPS) was developed by the U. S. Department of Defense (DOD), Ivan Getting, and the Massachusetts Institute of Technology (MIT). Originally consisting of 11 orbiting satellites, the GPS--official name NAVSTAR (Navigation System with Timing And Ranging)--was launched by DOD in 1978 strictly for military use. The idea for the system began much, much earlier. In fact the basic idea of GPS navigation can be traced back centuries to the first explorer who asked the question, "Where am I?" The technology of GPS history, however, first emerged in the 1950s.
In 1957, Russia launched a satellite (Sputnik) into space. After the launch, MIT researchers noticed that as Sputnik orbited the planet, its radio signal varied in strength. As it approached their position, the signal strength increased. When the satellite departed the MIT researchers' position, the signal strength decreased. From this increase and decrease of the radio signal, the MIT researchers could determine Sputnik's exact orbit. This recognition that radio signals from a satellite, or "artificial star" could determine distinct positions on the ground was truly the launching pad of the GPS system.
Even before that, however, the seeds for the GPS technology were being planted. In 1951, Dr. Ivan Getting, a graduate of MIT student and a Rhodes Scholar from Oxford, put his Ph.D. in astrophysics to use at Raytheon. The Air Force requested a guidance system for a proposed ICBM traveling via railroad. In response to this request, Dr. Getting developed the first three-dimensional, position-finding system based on time difference of arrival. This system became the basis for the future GPS.
The deployment portion of the GPS history began in 1973 with the decision to develop a satellite navigation system based on existing technology of the U. S. Air Force and the U. S. Navy. The system went through extensive testing during the next three years. In 1977 the first transmitters were installed on the surface of the Earth and tested--even though no satellites have been launched yet. The transmitters were dubbed Pseudolites (pseudo satellites).
Between 1978 and 1985, eleven satellites were launched into space and put into position. In 1979, the decision was made to increase the number of satellites to 18. In 1980, the first Block I satellite was launched. This "bird' had sensors specifically designed to detect atomic explosions, and was placed in orbit as a means of monitoring the Soviet Union's compliance with the 1963 agreement with the United States to refrain from nuclear testing. Also in 1980, the onboard atomic clocks, the most accurate timepieces in the world, were activated. Developed by physicists, atomic clocks measure time by the change in energy levels of electrons. These clocks are stable, continuous, and accurate to a nanosecond, or one-billionth of a second.
In 1983, the GPS ceased being solely a military system and was made available for public use. President Ronald Reagan declassified the GPS system in response to a tragedy involving a civilian airplane. Korean Airline flight 007 got lost over Soviet territory and was shot down by Soviet fighters. In 1986, the Challenger space shuttle tragedy halted space shuttle launches, and thereby delayed the GPS system, because shuttles were supposed to transport the new Block II satellites. Eventually, Delta rockets, the original transport, were put back into use to launch the satellites, and in 1988, the decision was made to increase the number of satellites to 24, because functionality with 18 satellites was still limited.
Though the GPS system was available for public use, the first Gulf War in 1990 saw temporary deactivation of use by the public, because the military needed more receivers. Public use returned in 1993 along with the decision that the GPS system would be available free of charge to the entire world. Full Operational Capacity (FOC) was achieved in July of 1995 with the placement and activation of the last of the 24 satellites. Since 1995, GPS history has seen considerable technological advancement. More satellites have been put into orbit, increasing both availability and accuracy. Previously GPS could locate a subject within 100 meters. That accuracy improved to finding subjects within 10-15 meters, and at present can be measured within centimeters in some cases. Availability improved from utilization in military and the position-required industries of aviation, nautical navigation, and land surveying, to include many personal applications. With the use of a fairly inexpensive receiver, employers use GPS to track their fleet of vehicles, parents use the system to keep track of small children and of disobedient teenagers, and pet lovers use it to keep track of their dogs.
GPS for Runners
The continuing revolution in miniaturization of electonics has allowed GPS technology to evolve to where we can now wear a GPS computer and visually obtain LOTS of amazing information about our running. The interface of these GPS wrist units to computers allows even more amazing things to be done with these little marvels. You can transfer your GPS recorded routes to mapping software that will allow you to do 3D flyovers of your route through the mountains. You can share maps of your routes with friends via collaborative web sites. You can purchase units with heart rate monitors that will track your workout effort, count your calorie expenditure and lots more. It's fun, too!
The first GPS units for runners were introduced by Timex. They required the use of s separate heavy, bulky GPS receiver that had to be worn on the bicep or on a backpack. The unit communicated wirelessly to a wrist monitor that gave you speed, distance and altitude. Garmin then revolutionized the market with the Forerunner 101 - a 3 1/2" rectangular wrist unit that incorporated the receiver in the wrist piece - no more bulky receiver to bounce around on your body. The 101/201/301 (201 and 301 could interface to a computer through a serial port and the 301 had heart rate functionality) were followed quickly by the even smaller 205 and 305. The 205 and 305 used the much easier USB interface to transfer data to the computer. The use of software "agents" on the PC allowed the data transfers to happen automitically once the unit was plugged in to the PC. Just when you thought it could not get any better, Garmin pulled another revolutionary coup for runners with the introduction of the 405. It was even smaller in size (finally something that more or less fit a woman's wrist), had a more powerful GPS receiver that located satellites much more quickly and with greater sensitivity for fewer "dropouts". The 405 also features a touch bezel for most of the menu navigation. Takes a bit of getting used to, but it does make more sense than trying to remember which combo of buttons you need to press to get to the system menu... AND, to make things even easier, the 405 incorporates the new ANT interface which is a low power wireless communication device that makes the 405 communicate AUTOMATICALLY when you are within 10 feet of your PC. A heart rate option is available as well as a foot pod unit for those who work out indoors on a treadmill (GPS won't work here as the treadmill is not moving...) The 405 also functions as a coach, allowing you to structure workouts which the 405 will then "coach" you through intervals, fartlek, hill repeats. You can program your own workouts, or download various kinds of workouts from Garmin's website into the 405 For a cool video intro to the Garmin 405, click here.
So, get into gear - come visit Phidippides and take a look at the cool new 405s!
