| How does GPS work ? The global positioning system is a satellite-based navigation system consisting of a network of 24 orbiting satellites that are eleven thousand nautical miles in space and in six different orbital paths. The satellites are constantly moving, making two complete orbits around the Earth in just under 24 hours. This means that each satellite moves about 1.8 miles per second. The orbital paths of these satellites take them between roughly 60 degrees North and 60 degrees South latitudes, virtually ensuring you can receive satellite signals anywhere in the world, at any time. As you move closer to the poles (on your next North Pole expedition!), you will still pick up the GPS satellites One of the biggest benefits over previous land-based navigation systems is GPS works in all weather conditions. No matter what your application is—when you need it the most, when you're most likely to get lost—your GPS receiver will keep right on working. The information does a GPS satellite transmit? The GPS signal contains a 'pseudo-random code', ephemeris (pronounced: ee-fem-er-is) and almanac data. The pseudo-random code identifies which satellite is transmitting—in other words, an I.D. code. We refer to satellites by their PRN (pseudo-random number), from 1 through 32, and this is the number displayed on a GPS receiver to indicate which satellite(s) we are receiving. So why are there more than 24 PRN numbers? This simplifies maintenance of the GPS network. A replacement satellite can be launched, turned on, and used before the satellite it was intended to replace actually fails! They simply use a different number (again from 1 through 32) to identify the new satellite. Ephemeris data is constantly transmitted by each satellite and contains important information such as status of the satellite (healthy or unhealthy), current date, and time. Without this part of the message, your GPS receiver would have no idea what the current time and date are. This part of the signal is essential to determining a position, as we'll see in a moment. The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system. By now the overall picture of how GPS works should be getting much clearer. Each satellite transmits a message which essentially says, "I'm satellite #X, my position is currently Y, and this message was sent at time Z." Of course, this is a gross oversimplification, but you get the idea. Your GPS receiver reads the message and saves the ephemeris and almanac data for continual use. This information can also be used to set (or correct) the clock within the GPS receiver. Now, to determine your position the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received by the GPS receiver. The time difference tells the GPS receiver how far away that particular satellite is. If we add distance measurements from a few more satellites, we can triangulate our position. This is exactly what a GPS receiver does. With a minimum of three or more satellites, your GPS receiver can determine a latitude/longitude position—what's called a 2D position fix. With four or more satellites, a GPS receiver can determine a 3D position which includes latitude, longitude, and altitude. By continuously updating your position, a GPS receiver can also accurately provide speed and direction of travel (referred to as 'ground speed' and 'ground track'). One factor affecting GPS accuracy is satellite geometry. In simple terms, satellite geometry refers to where the satellites are located relative to each other (from the perspective of the GPS receiver). If a GPS receiver is locked onto four satellites and all four of these satellites are in the sky to the north and west of the receiver, satellite geometry is rather poor. In fact, the GPS receiver may be unable to provide a position reading! Why? Because all the distance measurements are from the same general direction. This means triangulation is poor and the common area where these distance measurements intersect is fairly large (i.e., the area where the GPS receiver thinks our position is covers a large space, so pinpoint positioning is not possible). In this scenario, even if the GPS receiver does report a position, accuracy will not be very good (maybe off as much as 300-500 feet).
Satellite geometry also becomes an issue when using a GPS receiver in a vehicle, near tall buildings, or in mountainous or canyon areas. When the GPS signals are blocked from several satellites, the relative position of the remaining satellites will determine how accurate the GPS position will be (and the number of remaining satellites will determine if a position can even be determined). As more and more of the sky is obstructed by buildings or terrain, it becomes increasingly difficult to determine a position. A quality GPS receiver indicates not only which satellites are available for use, but where they are in the sky (azimuth and elevation) so you may determine if the signal of a given satellite is being obstructed. Another source of error is multipath. Simply put, multipath is the result of a radio signal being reflected off an object. Multipath is what causes 'ghost' images on a television set. With GPS, multipath occurs when the signal bounces off a building or terrain before reaching the GPS receiver's antenna. The signal takes longer to reach the receiver than if it travelled a direct path. This added time makes the GPS receiver think the satellite is farther away than it really is, which adds error to the overall position determination. When they occur, multipath errors typically add under 15 feet of error to your overall position. Are there any other sources of error? Propagation delay due to atmospheric effects can affect accuracy. So can internal clock errors. In both cases, the GPS receiver is designed to compensate for these effects and will do so quite efficiently. But, very small errors due to these items can still occur. Propagation delay is the 'slowing down' of the GPS signal as it passes through Earth's ionosphere and troposphere. In space, radio signals travel at the speed of light, but they are significantly slower once they enter our atmosphere. How accurate is GPS, really? A typical civilian GPS receiver provides up to 3 Meter accuracy (at least that's what the manufactures tell us) However as I would not know the difference between 3 meter and 10 meter accuracy (how good is your map reading to tell them apart ?) This accuracy depends on the number of satellites available and the geometry of these satellites. More sophisticated and expensive GPS receivers, costing thousands of pounds can provide accuracies within a centimeter by using more than one GPS frequency. Civilian GPS receiver's accuracy can be improved to under three feet through a process known as Differential GPS (DGPS). DGPS employs a second receiver to compute corrections to the GPS satellite measurements. How are these corrections provided to your GPS receiver? There are a number of free and subscription services available to provide DGPS corrections. The U.S. Coast Guard and U.S. Army Corps and many governments transmit DGPS corrections through marine beacon stations. These beacons operate in the 283.5 - 325.0 kHz frequency range and are free of charge. Your cost to use this service is the purchase of a DGPS Beacon Receiver. This receiver is then coupled to your GPS receiver via a three-wire connection, which relays the corrections in a standard serial data format called 'RTCM SC-104. As with everything the prices vary according to the level of accuracy desired. |
