A simple guide to how GPS works - Part 1 rocket science

There are many many articles and websites well worth visting about GPS and how it works. Mike Forbes at Electric Compass takes a look at the main elements.The Global Positioning System, or GPS, is a satellite-based navigation system. It was developed by the United States Department of Defense (DOD) for military and government use, but the information it provides is available free for civilian and commercial uses.From satellite-guided bombs used in the war against terrorism to handheld receivers carried by hikers, GPS offers a wide range of applications and uses.The first GPS satellite was launched in 1978. The full constellation of 24 satellites was in place in 1994 and the system was declared fully operational in 1995.Another key date in GPS history is May 1, 2000, the day “Selective Availability” was discontinued, significantly increasing the accuracy of GPS signals.
In simple terms, GPS is a broadcasting (not receiving) system in which satellites transmit information toward Earth.GPS receivers take the transmitted information and use a form of triangulation to calculate the user’s exact location. The basic premise of the technology is that the GPS receiver compares signal transmission time with the signal reception time, and then uses the time difference and the propagation speed to deduce the distance from each of the visible satellites.

Of course, it is not that simple – GPS really is quite literally “rocket science.” The best place to start a review of the Global Positioning System is with the three segments that make it up:The Space Segment, consisting of the GPS satellites orbiting the earth.The Control Segment, consisting of a system of tracking stations located around the world.The User Segment, consisting of GPS receivers and the user community.

Space Segment

The Space Segment consists of a minimum of 24 satellites orbiting 12,600 miles above the earth. Each satellite travels at about 7,000 miles per hour, enabling them to orbit the earth in just under twelve hours; the altitude and orbital inclination are such that each satellite repeats the same ground track in each twelve-hour orbit. The satellites are arranged in six orbital planes, spaced equally at 60 degrees apart, and each inclined at about fifty-five degrees with respect to the equatorial plane.This spacing is intended to ensure that the required four satellites are viewable at any giventime from any spot on Earth, however there are often eight and up to twelve satellites visible. Each satellite weighs approximately 900 Kilo, is approximately 5 meters across, and uses solar panels to power its electronics and transmit the GPS signal. It’s worth noting that at 50 watts or less, the GPS signal is at approximately the same level as the background noise of the universe by the time it reaches Earth.

Control Segment

The Control Segment consists of a network of monitor stations located around the world used totrack the “health” of all of the satellites, as well as one master control facility located at a US Air Force Base in Colorado Springs. The orbital models for each satellite describes the degree to which each SV is on its proper flight path; the monitor stationsmeasure certain signals from the satellites, determine to what degree each satellite is off course,and compute new orbital data and clock corrections. The monitor stations then send the new orbital information (known as ephemeris data) and the clock corrections to the master control station, which then relays the information to the satellites.

User Segment

The User Segment consists of the GPS receivers in the hands of the community of GPS users. GPS receivers convert satellite signals into position and time estimates, and often use this information to calculate other information such as velocity and heading. GPS receivers make positioning, navigation, and time dissemination possible. This information is then used for recreational, educational, commercial, research, and many other applications including Navigation and Tracking.

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