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What is GPS?

 

GPS  (Global Positioning system) is a Satellite Navigation System. Developed 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.

 

satellite orbiting earthGPS was  developed by the United States 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.

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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.

24 satellite constellation image

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)

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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.a_garmin_gps_handheld_screenshot

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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