GPS satellites

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C. Nitesh
II B.Tech ECE Ph no. 9985722275

II B.Tech ECE Ph no. 9849086915

II B.Tech ECE Ph no. 9703032124


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GPS (Global Positioning System), a constellation of 24 NAVSTARs (Navigation Satellites for Time and Ranging) in six groups of four owned and operated by the Department Of Defense (DOD), is the first positioning system to offer highly precise location data for any point on the planet, in any weather. Today’s GPS receivers are extremely accurate. WAAS (Wide Area Augmentation System) can give us even better accuracy to an average of up to five times better. We can also get better accuracy with Differential GPS (DGPS) to an accuracy of three to five meters. For example Militaryapproved equipment can pinpoint accuracy with one meter. The signals that are transmitted by GPS satellite are two low power radio signals; designated L1 and L2 .A GPS signal contain information such as which satellite is

phone. When it is switched on it can tell us where we are any where in the world .It will give us a map reference .It also shows our height above sea level, and if we are moving, how fast we are going and in which direction. The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 NAVSTARS placed into orbit by the U.S. Department of Defense. GPS was originally intended for military

applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making to complete orbits in less than 24 hours. These satellites are traveling at speeds of roughly 7,000 miles an hour. GPS satellites are powered by solar energy the have backup batteries onboard to keep them running in the event of solar eclipse, when there’s no solar power. Small rocket boosters on each satellite keep them

transmitting information, current date and time, and orbital information. There are some factors that can degrade the GPS signals, which are explained briefly in the foregoing papers. There are wide ranges of applications of GPS on land, at sea, and in the air .The first and most obvious applications of GPS is the simple determination of a “position “ or location. In future this GPS may become very much accurate as currently it is. These can be explained in a wide view in the following papers.

Sailors and explorers used to find their Travelers, way using a compass, and by measuring the positions of the Sun, Moon and Stars. Today, all we need is a GPS (Global Positioning System). It looks like a mobile

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Reference Frame (ITRF). In addition, an informational summary file is provided to document the computation and to convey relevant information about the observed

satellites, such as maneuvers or maintenance. The orbits generally are available two to six days after the date of observation.




Here are some interesting facts about the GPS satellites:  1978.   A full constellation of 24 satellites was achieved in 1994. Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.  A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.  Transmitter power is only 50 watts or less The first GPS satellite was launched in

GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the frequency of 1575.42MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains. A GPS signal contains three different bits of information-a pseudorandom code, ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies information. Ephemeris data, by each which is which satellite is transmitting

The Transportation's U.S Civil Department GPS Service of has




contains important information about the status of the satellite, current date and time. This part of the signal is essential for determining a position. The almanac data tells the GPS receiver when each GPS satellite should be at any time throughout the day.

designated NOAA to be the federal agency responsible for providing accurate and timely Global Positioning System (GPS) satellite

ephemeredes ("orbits") to the general public. The GPS precise orbits are derived using 24hour data segments from the global GPS network coordinated by the International Geo dynamics GPS Service (IGS). The reference frame used in the computation is the





International Earth Rotation Service Terrestrial

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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 determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more. You will need a Garmin GPS unit to help you find or mark cache location and internet access for finding or sharing the coordinates.12-channel units an accuracy of less than 15 meters (49ft), and less than 3 meters (10ft) on WASS-capable models.

use very precise radar to check each satellite's exact altitude, position and speed. GPS

satellites are so high up their orbits is very predictable. The errors they're checking for are called "ephemeris errors" because they affect the satellite's orbit or "ephemeris." These errors are caused by gravitational pulls from the moon and sun and by the pressure of solar radiation on the satellites. The errors are usually insignificant, but for the most part, if you want to have good accuracy you will want to make sure you have no errors coming in. Once the DOD has measured a satellite's exact position, they relay that information back up to the satellite itself. The satellite then includes this new corrected position information in the timing signals its broadcasting. It also contains a navigation message with ephemeris information as well.

DOD intentionally sends noise into each signal to create inaccuracy. The purpose of this is to ensure no hostile force uses GPS as means to exploit a terrorist agenda. Military personal can use access codes to decrypt the errors; therefore they are still able to use GPS with great accuracy.

The User Segment:
The user segment consists of the several GPS receivers and the users themselves. Navigation is the primary function of GPS. Navigation receivers are made for various ships, vehicles, and people. Time and

The Control Segment:
The GPS consists of ground control stations, with the main control station located in Schriever Air Force Base in Colorado. Each individual monitor station

measures signals from the space vehicles (SV’s) located inside each satellite. The main station in Colorado uploads ephemeral and clock data to the SV’s. The SV’s then send subsets of orbital data to PPS receivers. They

Frequency have also become a popular use for GPS, as laboratory standards can be set to precise time signals, and within well-referenced points. With special purpose GPS receivers, precise positioning is possible using receivers at

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reference locations providing correction and relative positioning data for remote receivers. Examples of this include surveying, geological studies, and plate tectonic studies.

does not require additional receiving equipment, while DGPS does. A WAAS-capable receiver can give you a position accuracy of better than three meters 95% of the time. The Federal Aviation Administration

Today’s extremely accurate, GPS receiver 12 are Garmin’s parallel

(FAA) and the Department of Transportation (DOT) are developing the WASS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA’s navigation requirements for accuracy, integrity, and

channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other source of error can affect the accuracy of GPS receivers. Garmin GPS receivers are accurate to within 15 meters on an average.

availability. WAAS corrects for GPS signal errors caused by ionosphere disturbance,

timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each.

WAAS consists of approximately 25 ground reference stations positioned across the United Newer Garmin GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three meters on average. No additional equipment or fees are required to take advantage of WAAS. Users can also get better accuracy with Differential GPS (DGPS), which corrects GPS signals to within an average of three to five meters. States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction

accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is than broadcast through one of two geo stationary satellites with a fixed position over the equator. The information is compatible





WASS signal reception is ideal for open land marine applications. WASS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WASS is that it

with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal. Currently, WAAS satellite coverage is only available in North America. There are no 5|P age

ground reference stations in South America, so even through GPS users there can receive WAAS, and the signal has not been corrected and thus would not improve the accuracy of their unit. 

universal measurement system capable of positioning things on a very precise scale.

FEATURES THAT CAN DEGRADE THAT GPS SIGNAL: 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.  Other governments are developing similar satellite-based differential systems. In Asia, it’s the Japanese Multi-Functional Satellite Signal multi path-This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surface 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. There fore, it may have very slight timing errors. Users can also get better accuracy with Differential GPS or "DGPS", which corrects GPS signal to within an average of three to five meters. The U.S. Coast Guard operates the most common DGPS   Orbital errors-Also known as ephemeris errors, these are inaccuracies of the satellites 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.  That improved accuracy has a profound effect on the importance of GPS as a resource. With it, GPS becomes more than just a system for navigating boats and planes around the world. It becomes a Satellite geometry/ shading-This refers to the relative position on 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.

Augmentation System (MSAS), while Europe has the Euro Geo stationary Navigation Overlay Service (EGNOS).


correction service. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receivers and beacon antenna in addition to their GPS.

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Remember that GPS receivers use timing signals from at least four satellites to establish a position. Each of those timing signals is going to have some error or delay depending on what place






automobiles as well. Some basic systems are in and provide emergency roadside

assistance at the push of a button (by transmitting your current position to a dispatch center). More sophisticated system that shows our position on street map is also available. Currently these systems allow a driver to keep track of where he or she is and suggest the best route to follow to reach a designated location. ADVANTAGES:  GPS offers highly precise location data for any point on the planet, in any weather.  With precise point-to-point navigation, GPS saves fuel and extend an aircraft’s range by ensuring pilots don’t stray from the most direct routes to their destination. In advanced forms of GPS we can make measurements to better than a centimeter, literally giving each space on earth a specific address. DRAWBACK: Basically, GPS is usable everywhere except where it’s impossible to receive the signal such as inside most buildings, in caves and other subterranean locations, and

sort of perils have befallen it on its trip down to us.

APPLICATIONS OF GPS: GPS has a variety of applications on land, at sea and in the air. The most common  airborne applications are for navigation by general aviation and commercial aircraft. At sea, GPS is also typically used for navigation by recreational boaters, commercial fishermen, and professional mariners. Land-based 

applications are more diverse. The scientific community uses GPS for its precision timing capability and position information. Recreational uses of GPS are almost as varied as the number of recreational sports available. GPS is popular among hikers, hunters, snow mobilers, mountain bikers, and cross-country skiers, just to name a few. Any one who needs to keep track of where he or she is, to find his or her way to specified location, or know what direction and how fast he or she is going can utilize the benefits of the global positioning system.

underwater. CONCLUSIONS: We conclude that the GPS is very useful for finding out the position or location of anything on the planet in any weather with maximum accuracy of one centimeter. By implementing GPS receivers to all cars we may prevent accidents to some extent. Even in remote and lonely places, a GPS satellite can find our location. REFERENCES:

7|P age Satellite Communications - Gulati Satellite Communications- D.Agarwal

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Description: All things about GPS satellites......We presented this paper in many technical college level fests and we won many prizes....