Msg #201638 Type:B Pour:INFO @PNW Exp:N7FSP Date:950112 Subject:How GPS receivers work, an intro. R:950112/1145Z @:VE7KIT.#VANC.BC.CAN.NA #:5030 [YVR] $:57968_N7FSP R:950111/1903Z @:VE7ROB.#VANC.BC.CAN.NOAM #:17272 FBB5.15c $:57968_N7FSP R:950112/1023Z @:VE7DIE.#SVI.BC.CAN.NOAM #:12255 [Victoria, BC] $:57968_N7FSP R:950112/0943Z @:KB7WE.#WWA.WA.USA.NOAM #:8784 [BREMERTON] $:57968_N7FSP R:950112/0819z 57968@N7FSP.#SEA.#WWA.WA.USA.NOAM HOW GPS WORKS: AN INTRODUCTION by Craig Haggart (haggart@slac.stanford.edu) Amazingly precise satellite navigation receivers are now widely available and reasonably priced, thanks to the Global Positioning System (GPS). How do these little wonders figure out exactly where you are? The basic principle behind GPS is simple, and it's one that you may have used if you have done any coastal navigation: if you know where a landmark is located, and you know how far you are from it, you can plot a line of position. (In reality, it's a circle or sphere of position, but it can be treated as a line if the circle is very large.) If you plot two or more lines of position, you know that your location is the point where those lines cross. With GPS, the landmarks are a couple of dozen satellites flying about 11,000 miles above the earth. Although they are moving very rapidly, their positions and orbits are known with great precision at all times. Part of every GPS receiver is a radio listening for the signals being broadcast by these satellites. Each spacecraft continuously sends a data stream that contains its orbit information, equipment status, and the exact time. All of the information is useful, but the exact time is crucial. GPS receivers have computers that can calculate the difference between the time a satellite sends a signal and the time it is received. The computer multiplies this time of signal travel by the speed of travel to get the distance between the GPS receiver and the satellite (TIME x SPEED = DISTANCE), location in space. RECEIVER CLOCK ERROR Even with two lines of position, though, the resulting fix may not be very good due to receiver clock error. The orbiting satellites have extremely accurate (and expensive!) clocks that use the vibrations of an atom as the fundamental unit of time, but if GPS receivers had similar atomic clocks they would be much too expensive for recreational users. Since precise measurement of time is so critical to the system - a clock error of only one thousandth of a second causes a position error of almost 200 miles - the system designers were faced with a dilemma. Geometry to the rescue! It turns out that GPS receivers can use inexpensive quartz clocks (like the ones used in wristwatches) and still come up with extremely accurate position fixes as long as one extra line of position is calculated. How does this work? First, imagine two earthbound landmarks with well-known positions. For example, we'll use two BIG landmarks: the cities of Honolulu and Los Angeles. If we measure the travel time of radio waves from each of these cities to San Francisco, we can use the known speed of the radio waves to compute two lines of position that cross. If our clock is a little fast, those position lines will show us to be closer to both cities than we really are; the lines will cross, but the crossing point might be somewhere out in the ocean southwest of San Francisco. On the other hand, if our clock is too slow, the radio waves will seem to take longer to get to us, and the position lines will show us to be farther away from the landmarks than we really are. The lines might cross near Sacramento, to the northeast. Now, if we get just one more position line - from Seattle, let's say - the three lines would form a triangle, and the center of the area in this triangle is our REAL position. The clock error is the same for all three lines, just in different directions, so moving the lines together until they converge on a point gives us the error. Therefore, it's OK if our GPS receiver's clock is a little off, as long as the clocks on the satellites are keeping exact time and we have a computer that can pinpoint the center of a geometric area and therefore calculate the error. SYSTEM ACCURACY For accurate two-dimensional (latitude and longitude) position fixes, then, we always need to get signals from at least three satellites to get around receiver clock error. Fortunately, there are now enough GPS satellites orbiting the earth to allow even three-dimensional position determination (latitude, longitude, and altitude, which requires signals from at least FOUR satellites) anytime, from anywhere in the world. The more satellites your receiver can "see" at one time, the more accurate your position fix will be, up to the system's standard accuracy limit of a few hundred feet. The U.S. Department of Defense (DoD) is responsible for the GPS system, and it reserves increased accuracy for military users. For this reason, the satellites broadcast a coded signal - the encrypted P-code - that only special military receivers can use, providing positions that are about ten times more accurate than those available with standard receivers. In addition, random errors are put into the satellite clock signals that the civilian GPS receivers use. Not everybody is happy with this intentional degradation of accuracy, though, including the U.S. Coast Guard. DIFFERENTIAL GPS To get around the DoD-imposed accuracy limitation, the Coast Guard is setting up "differential beacons" around the U.S. A differential beacon is a special device positioned at a precisely-surveyed location; it picks up GPS satellite signals and determines the difference between its real position and the computed position based on GPS. The beacon then broadcasts this error information over a radio channel for all nearby differential-equipped receivers to use. With this method, inexpensive GPS receivers can produce position information accurate to within a few inches using the standard, uncoded civilian signal. GPS receivers that can take advantage of this differential broadcast are becoming quite common, although a separate differential beacon receiver usually must be purchased. SINGLE-FREQUENCY OPERATION The way GPS receivers pick up the satellite signals is pretty interesting: all of the satellites broadcast their messages on the same frequency, but they each include a unique identification code. The receiver determines which message is from which satellite by matching the identification code with the codes stored in its memory. This is similar to standing in a room with many people speaking at the same time - you can listen to what only one person is saying among all of the conversations taking place simultaneously, and you can identify a person's voice by its unique sound. In the same way, a GPS receiver picks up signals from all of the satellites in view and matches them with patterns in memory until it figures out which ones are "talking" and what they are saying. This technique allows GPS receivers without backyard-sized dish antennas to reliably use the tiny signals that the satellites transmit toward earth. EVERYONE ON THE DOCK WILL HAVE ONE SOON Ten years ago, it would have been hard to believe that you could buy a device capable of using the most advanced satellite technology to provide your precise location anywhere on the globe, and even harder to believe that it would be smaller than a frozen waffle and cost less than a TV set. In just a few years, though, I suspect that these technological marvels will be just about everywhere, and even cheaper - at this writing (Summer 1994), there are terrific handheld units with basic course plotters selling for under $500, and the prices keep going down.