Opinion Center
GPS Topics Survey
 
 

 
Location Location Location
Please check each of the statements to the right that you believe are true.
GPS satellites transmit low power radio signals -- only about 50 watts. The signals pass through clouds, glass and plastic, but will not go through most solid objects including buildings and mountains.
The US Department of defense created GPS in the early 1980s. The official name for GPS satellites is NAVSTAR.
GPS devices use extremely precise time differential calculations to determine locations.
GPS devices use a process of triangulation and timing to determine where they are located.
GPS receivers that are readily available for less than several hundreds of dollars will let a user determine his location to within 10 feet.
GPS satellites need constant adjustments to keep them in precise orbits.
About this survey:  The points below are intended to provide a basic overview of some fascinating science involving space, geography, and multifunctional aspects present in everyday life. You are invited to review the points below. Try checking each button to the right as you proceed in your understanding. Please do not check the buttons to the right of concepts that are not clear enough for you to understand. OK
1.  The Global Navigation Satellite System (GNSS) is a constellation of satellites, transmitting signals used for several types of navigation and positioning applications, anywhere on the surface of the earth. There are currently two Global Navigation Satellite Systems in operation: the US Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS). A third system, Galileo, is currently being developed in Europe. Galileo will provide a wide range of positioning signals and services including high-reliability service required by safety-of-life-critical applications. High level of integrity will be provided on the global scale and without any additional augmentation system. All the three systems will be interoperable.
Another name for GPS (Global Positioning System) given by its owner, the US Department of Defense, is NAVSTAR (Navigation Satellite Timing and Ranging). GPS is a satellite based navigation system, operational since late 1995. Its constellation consists out of 24 satellites, which transmit ranging signals and navigation data 24 hours a day free of charge. GPS receivers obtain their locations on earth by measuring the distance from the satellites and computing their position relative to the Earth-fixed coordinate frame, WGS-84. GPS is funded and controlled by the U.S. Department of Defense (DoD) and was originally intended for military applications. In the 1980s, the U.S. government made the GPS system available for civilian use. It was the first system widely available to civilian users.		 OK
2.  The following points identify GPS fundamental characteristics.
—  There are 24 satellites in the GPS space segment and they orbit the earth at about 12,000 miles. They make two complete orbits in less than 24 hours. They travel at roughly 7,000 miles an hour.
—  GPS satellites are powered by solar energy. They have backup batteries to keep them running during solar eclipses.
—  Each satellite has small rocket boosters used to adjust their orbits and keep them flying in the correct paths.
—  The first GPS satellite was launched in 1978. A full constellation of 24 satellites was reached in 1994.
—  Each satellite has a life of about 10 years. Replacements are always in construction and launched as needed.
—  A typical GPS satellite weighs approximately 2,000 pounds and is about 17 feet wide with the solar panels extended.
—  Transmitter power is 50 watts or less.		 OK
3.  GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map. A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude.) Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more. OK
4.  Factors that can degrade the GPS signal and thereby affect accuracy include:
—   Ionosphere and troposphere delays:  The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
—   Signal multipath:  This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
—   Receiver clock errors:  A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
—   Orbital errors:  Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
—   Number of satellites visible:  The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
—   Satellite geometry/shading:  This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
—   Intentional degradation of the satellite signal:  Selective Availability, SA, is an intentional degradation of the signal once imposed by the US Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.		 OK
5.  GPS devices calculate where they are using the following process.
Each satellite constantly transmits a long digital pattern called a pseudo-random code, which is an identifying signal unique to each satellite. At the same time, the receiver plays back a stored copy of the pseudo-random code to itself while listening for the satellite’s signal. When the signal reaches the receiver the satellite’s pseudo-random code will be slightly delayed compared to the receiver’s playback of the pattern. This delay is due to the large distance between the satellite and receiver, as well as the signal-slowing effects of the troposphere which the signal has to pass through. The signal travel time is the length of the delay. The receiver merely multiplies this time by the speed of light (GPS signals are electromagnetic waves, which all travel at the speed of light) to find out the distance between the satellite and receiver. When the receiver has determined the distance to four satellites, it is able to calculate it’s three-dimension position of latitude, longitude and elevation by the principle of trilateration.		 OK
6.  The latest GPS receivers include a feature known as Wide Area Augmentation System, WAAS, a capability that can improve accuracy to less than three meters. No additional equipment or fees are needed to take advantage of WAAS. Increased accuracy can be achieved with Differential GPS, DGPS, which corrects GPS signals to within an average of three to five meters. The US Coast Guard operates the most common DGPS correction service. Their system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. Users must have a differential beacon receiver and beacon antenna in addition to their GPS device to receive the corrected signals. OK
7.  This introduction to GPS and its underlying technology presents important terms and concepts. Do you feel more informed after reviewing this survey? Yes
No
Ideas:
8.  I am: Female
Male
9.  My age group is: Under 20
20-29
30-39
40-49
50-59
60-69
70 & older

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