GPS (Global Positioning system) Explained click here to open a satellite orbiting the earth animated movie clip GPS (Global Positioning system) is a satellite navigation system. It is operated by the U.S. Department of Defence but is available to anyone wanting to use the system. It can be accessed by users on the land, sea or in the air but is not available underwater or underground, such as in a mine. GPS was developed by the Department of Defence primarily for military purposes at a cost of around 10 billion dollars. The system relies on 24 satellites which orbit the earth twice each day. The distribution of these near circular orbits is even, such that they provide a uniform net around the entire surface of the earth. At any location, at any point in time, up to ten of these satellites may be 'visible' to a receiver of GPS signals. GPS is the best navigation system available at the present time. It can provide immediate information regarding position on the earth's surface, altitude, speed, direction of travel and time. GPS is a revolutionary system of navigation because it works anywhere in the world, in any weather conditions, 24 hours a day, every day and has no cost to the user. The system has three components. There are the 24 satellites, commonly called S.V.'s (satellite vehicles), comprising the 'space segment'. Military bases on the earth's surface form the 'control segment' tracking the satellites and checking their performance levels. The 'user segment' refers to anyone using GPS.
How does GPS work? Calculating a Position A GPS receiver calculates its position by a technique called satellite ranging, which involves measuring the distance between the GPS receiver and the GPS satellites it is tracking. The range (the range a receiver calculates is actually a pseudo-range, or an estimate of range rather than a true range) or distance, is measured as elapsed transit time. The position of each satellite is known, and the satellites transmit their positions as part of the "messages" they send via radio waves. The GPS receiver on the ground is the unknown point, and must compute its position based on the information it receives from the satellites. (Click here to open a video clip containing an explanation of how satellites calculate a vehicle position) Measuring Distance to Satellites The first step in measuring the distance between the GPS receiver and a satellite requires measuring the time it takes for the signal to travel from the satellite to the receiver. Once the receiver knows how much time has elapsed, it multiplies the travel time of the signal times the speed of light (because the satellite signals travel at the speed of light, approximately 186,000 miles per second) to compute the distance. Distance measurements to four satellites are required to compute a 3-dimensional (latitude, longitude and altitude) position. In order to measure the travel time of the satellite signal, the receiver has to know when the signal left the satellite and when the signal reached the receiver. Knowing when the signal reaches the receiver is easy, the GPS receiver just "checks" its internal clock when the signal arrives to see what time it is. But how does it "know" when the signal left the satellite? All GPS receivers are synchronized with the satellites so they generate the same digital code at the same time. When the GPS receiver receives a code from a satellite, it can look back in its memory bank and "remember" when it emitted the same code. This little "trick" allows the GPS receiver to determine when the signal left the satellite. Please visit the Resources page for links to more detailed, but accessible explanation of how GPS works. can be found at How accurate is GPS? The accuracy that can be achieved using GPS depends on the type of equipment used, the time of observation, and the positions of the satellites being used to compute positions. In general, recreational and mapping grade receivers using C/A code without differential correction are accurate to between 5 and 15 meters. Many people using recreational grade receivers don't realize they cannot get highly accurate readings using them autonomously (without differential correction). Most mapping and recreational grade receivers with differential correction can provide from about 1 to 5 meter accuracy. Some receivers use what is called "carrier-smoothed code" to increase the accuracy of the C/A code. This involves measuring the distance from the receiver to the satellites by counting the number of waves that carry the C/A code signal. These receivers can achieve 10 cm to 1 meter accuracy with differential correction. Dual frequency survey grade receivers using more advanced network survey techniques can achieve centimeter to millimeter accuracy. Some people wonder why GPS is better than Loran or other systems that use ground-based transmitters. The accuracy of ground-based location systems such as Loran, which uses low frequency radio signals, is affected by signal distortion, varied terrain, local atmospheric disturbances and limited coverage. Since GPS signals come from satellites, the problems common to ground-based systems can be avoided.
What Should I Know Before Purchasing a GPS Receiver? Before investing in GPS equipment, it is important to clearly define your needs in terms of accuracy level required and end results expected. Do you simply want to be able to navigate in the woods, or do you want to map out points, lines and areas that can be differentially corrected and imported into a GIS (a computer mapping system)? Do you need real-time differential GPS for any reason? Is 15 meter accuracy good enough? If so, you don't have to worry about differential correction. If you want to make a map from your data, is 1-5 meter accuracy sufficient, or do you need sub-meter accuracy for your application? Remember that more accurate equipment is more expensive. If you decide you need high accuracy, be sure you can justify the added expense. In addition, consider your needs for durability and weather resistance, and details such as whether or not an external antenna can be connected to the receiver, and its size, weight and suitability for your method of survey (e.g., will it be used in a backpack, mounted on a vehicle, or carried in your hand?). Identifying your requirements ahead of time will help you determine which type of receiver to purchase, and specific features you will need in order to accomplish your objectives. It will help you avoid purchasing a receiver that you will be disappointed with later because it can't perform the way you expect it to. A good strategy is to clearly outline your project requirements and then contact several GPS equipment manufacturers with your specifications. As you research available equipment and ask questions, you will gain an understanding of what kinds of equipment are currently available and will meet your needs.
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