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GPS Information FAQ

GPS receiver So you are wondering, exactly how does that little device you hold in your hand while Geocaching know exactly where you are? It is easy, Well OK maybe not easy, but not as hard as you would expect either. First you need to understand that, this small device you are holding in your hand ( a GPS receiver) is only a small part of the whole system. Aside from the receiver, the system also includes several satellites and various ground stations as well.

The GPS we use today is a result of the combining of two separate satellite navigation programs which were originally operated by the U.S. Navy and U.S.Air Force. This new combined program later became known as NAVSTAR (Navigation System with Timing And Ranging), or NAVSTAR GPS. Originally designed to give U.S. Military forces an accurate way of navigating anywhere in the world, under all weather conditions, GPS has since evolved to its current state where it now has many civilian uses. Among these are surveying, boat and aircraft navigation, and recreational uses like hiking and most of course, Geocaching.

So let's start from the top down, the satellites. Due to requirements of the receiver (which will be discussed later), GPS requires a constellation of 24 satellites in orbit around the earth. The twenty-fourth satellite was placed in orbit in March of 1994. Now these satellites could not be placed just anywhere. To maintain GPS availability in all areas each satellite maintains an orbit of 20,200 kilometers above the earth. The system is divided into six orbital planes with four satellites in each plane. The placement of the satellites orbit means that at any given time at least four (and usually five to eight) satellites are in view at any given time, from any point on earth. The U.S. Government constantly monitors the positions of these satellites and makes corrections to their orbits as needed.

Once in orbit and operational the satellites that make up the GPS begin transmitting signals that can then be used by the GPS receiver to determine its location. These signals contain a data stream that contains among other things, the satellites identification, the precise time the signal was sent. These signals include the 'C/A' code and 'P' code signals. In the beginning the C/A (Course Acquisition) code, which is intended for civilian use, was deliberately degraded by intentionally injecting errors into the signal at regular intervals. This was done to limit the accuracy of the receiver to within approximately 100 meters or so, mainly for security reasons. The 'P' (Precision) code, which was intended for military use, was left alone thereby affording them a more accurate positioning system (less than 20 meters in most cases). This process of providing the different signal qualities became known as Selective Availability, often referred to as 'SA' . While SA made it possible for civilian navigation, it was only useful in cases where extreme accuracy would not have been critical. The good news for the civilian users came in May of 2000 when under an order from the Clinton Administration, SA was turned off. Within days the first GPS Stash Hunt took place, but that is a whole different story. For now let us just say that the civilian users now could enjoy almost the same accuracy as military users.

Now lets talk about the receiver. The small device that you have on the dashboard of your car, in your boat or carry in your hand is a GPS receiver (GPSr). It is usually referred to simply as a GPS and this is quite acceptable. This magic little box is as simple in many ways as it is complicated. Because the receivers come in all different shapes and sizes with as many different features, we will only cover the basic operation here. Two of the most important features required in any GPSr are an almanac (which contains the names of each of the satellites and their precise location on a given time), and a very accurate quartz clock. What the GPS receiver does is to take the information contained in the coded signals it receives from the satellites, and then use its almanac and clock to calculate its exact location. When you turn your receiver on it will begin to search for signals from visible satellites. Once it has acquired a lock on the signal that contains the data stream it will synchronize its clock with the satellites clock. Once the receiver has locked on to at least three satellites, and the clocks is synchronized it can begin the process of determining where you are.

Now remember, the data stream of the received signal contains the identification for the satellite it originated from. The receiver looks up the satellite in its almanac to determine exactly where that specific satellite is located at that particular moment. OK, so we know where the signal came from now what? Next it checks the Time Stamp that was received with the signal. This tells the receiver precisely when the signal was transmitted. By comparing the time the signal was transmitted with the time it was received the GPSr is able to determine how long it took for the signal to arrive. Since the receiver is programmed with how fast these radio signals travel it now simply multiplies the elapsed time by the known speed to determine how far away the satellite is. Now obviously this alone does not do you much good, because at this point you could be anywhere in a 360 degree radius of the sending satellite. To determine your position, the receiver actually needs the signals from at least three satellites, though four are usually used because you also will need your altitude. So, Using the same process the receiver will determine how far it is from each of the remaining satellites.

Now that the receiver has determined how far it is from each satellite, it is time to calculate your position. Remember the almanac within the receiver knows the position of each of the satellites. Having the position for each satellite as well as the distance from each, allows the receiver to plot your position using a process called Trilateration. This is a geometric process in which three or more spheres are used, each having a radius equal to the distance from the target point (in this case your location). Each spheres center-point in this case would be the location of a satellite from which you are receiving the signal. As we discussed earlier, the distance from one satellite (one sphere) would not be of much help because your location could be anyone of 360 points along that sphere. Now by adding the second satellites data you acquire a second sphere, which intersects with the first at two different points. So you have now narrowed your location down to two possible points. By adding a third satellites sphere you would now have three intersecting spheres, but only one point where all three meet. A fourth satellite and, well you get the idea. This point where ALL spheres meet would be your location. Now because GPS is dealing with a three-dimensional world, a fourth satellite (sphere) is needed to determine altitude. If for some reason however the GPS is only able to lock on three satellites it will still be able to determine your location by using an imaginary sphere to represent the earth. This however would prevent it from determining your altitude.

There are other features available on some GPS receivers that we will not go into here other than to make you aware of their names. One is DGPS (Differential GPS), which adds the use of not only the four satellites used by GPS, but also the signal of a fixed ground station. And the new WAAS (Wide Area Augmentation System) technology being developed by the FAA that also adds the use of ground stations for increased accuracy.

So there you have it, a brief introduction to GPS and how it works. This is by no means a complete description on the service and we recommend you check out our Links section for information on where you can find more detailed information.



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