Number 262 - March 2005

Global Positioning System -- Where Exactly Are You?
by Joe Schmitt, Tampa Bay Computer Society, Florida
This is the first of a three article series on the Global Positioning System. The first discusses what the system is and how it works. The two subsequent articles will delve into receivers and uses to civilians.    For centuries, man has relied on the most rudimentary of tools to navigate. Things like a compass or sextant, and later a timepiece, were the best he had. In the past, the most difficult task of a ship's captain was to maintain the location and heading of a ship pounded by winds and currents. As time went on, those tools were perfected and improved. Though they worked well, they were not perfect. It was a time consuming process to mark position; early airplane pilots often navigated using existing landmarks and roads because the speed of their craft made traditional methods cumbersome.    A system called Loran (Long Range Navigation) was developed in the 1940's that utilized radio pulses which were projected on board a ship using a cathode ray tube, similar to your computer monitor. Though accurate to a degree, this system was expensive and offered only a limited area of use.    The global positioning system is a revolution in the way we navigate today. The global positioning system is a set of satellites that just like Loran that use radio signals to determine position. Popularly known as GPS, the system uses a "constellation" of 24 satellites in high orbit to determine position in latitude and longitude. In addition to position, the system also can determine altitude and speed.    GPS was first implemented in the early 1970s as a way for the military to improve its ability to navigate and position vehicles. This system eventually evolved to include civilian as well as military uses. In the early days of GPS the signal was intentionally diffused to deteriorate the accuracy of civilian receivers. This use of selective availability as the Department of Defense (DOD) calls it, rendered civilian receivers accurate only within a hundred yards or so. Imagine having the unit telling you make a turn on the next road after you've passed it. In 2000, President Clinton took a serious look at the system and its potential to the civilian population. Selective availability was disabled on May 2, 2000 making the system accurate to within thirty feet or so.    So how does all this work? Well, as mentioned previously, there are 24 satellites orbiting the earth that broadcast radio signals. To better understand the system, a simple idea of two-dimensional navigation must be explained. Suppose you are in a field with one of those sign posts that point to a bunch of different places with distances. The top arrow of the sign says Montgomery, Alabama - 139 miles. The next arrow down says Savannah, Georgia - 217 miles. The third arrow points to Nashville, Tennessee - 231 miles. Now sit down at your favorite map and draw a circle around Montgomery with a radius of 139 miles. Next draw a circle around Savannah and Nashville with the radius the distance on our imaginary sign post. Where those three circles all intersect is roughly Atlanta, Georgia. Easy enough to understand, huh?    GPS works with the same principal. The receiver determines its location by its distance from the satellites. Hold it one minute! But those aren't	satellites stationary? Well the satellites move on a predictable path and within a consistent time frame. Inside each GPS receiver, is a programmed almanac of the position of each satellite based on time. Each satellite broadcasts a signal which moves at the speed of light. By measuring how long the signal takes to reach the receiver, distance from the satellite is determined. Using three or more satellites the receiver can then triangulate its position very much the same way we did with our imaginary signpost.    These kinds of accurate calculations with radio signals require precise timing, so on each satellite is an atomic clock which is monitored and corrected by ground stations. Using the accuracy of the atomic clock, a code pattern is broadcast from the satellite at a starting point. The receiver knows what time the signal left the satellite based on the portion of the code it is currently receiving. Measuring the time it took for the signal to reach the receiver by comparing the segment of code received with the time in the receiver, the distance to the satellite is determined. The distances from three or more satellites are compared and the position is formulated. That position is then displayed on your receiver as a set of coordinates, or graphically on an electronic map.    Using this pattern the receiver checks and adjusts its internal quartz clock to match that of the atomic clock. So, in addition to knowing where you are, you also know the precise time. Imagine having an atomic clock in your pocket! The receiver also can determine speed by measuring its change in position over time. This is a three dimensional arrangement and so altitude is also calculated.    This is all great and wonderful, but there are some issues. Gravitational pulls from the sun and moon affect the orbits of the satellites. Changing atmospheric conditions also can affect how well the signal travels from the satellite to the receiver. To combat this, the DOD has several ground stations that constantly monitor the signals and make adjustments in the satellites as necessary. In addition, there are two geo-stationary satellites which are in a fixed position in the sky on both sides of the earth. These systems are called WAAS, or Wide Angle Augmentation System. This system's sole purpose is to correct for the issues that affect the accuracy of the broadcast code.    Now with a simple receiver, John Q Public can use a multibillion dollar DOD system. At this time it is estimated that there are approximately 200 civilian users for each military user of the system!    Next month, I'll discuss the different types of receivers available to you as a consumer and some neat things that can be done with them. In the mean time, check out these interesting sites:    Official page for GPS management http://gps.losangeles.af.mil/    NASA page on GPS http://gpshome.ssc.nasa.gov    U.S. Navy GPS Timing Operations http://tycho.usno.navy.mil/gps.html    Block II specifications: http://www.spaceandtech.com/ spacedata/ constellations/ navstar-gps-block2_conspecs.shtml
Number 262 - March 2005