GPS stands for Global Positioning System, a satellite navigation system with twenty-four satellites in orbit. These satellites were put in place by the United States Department of Defense for military uses, and were called NAVSTAR. The first satellite was launched in 1978 and the twenty-fourth and last satellite was finally in place in 1994. In 1980 NAVSTAR was made available to the general public for commercial use.
GPS works 24 hours a day in any weather. The satellites orbit the earth twice a day in a specific orbit that is about 12,000 miles above us. In orbit, the satellites travel as fast as 7,000 miles an hour. As they are orbiting, they transmit information to receivers on earth. The receivers use this information to calculate the user’s location. This calculation is made by determining the difference between the time a transmission was made and when the receiver received it. This is then used to calculate the distance and the position is displayed on the receiver.
For a receiver to calculate a latitude and longitude position is to receive information from three satellites. To calculate latitude, longitude and altitude a receiver must be able to receive information from four or more satellites. After position is known the GPS can then tell the user information about speed, trip distance, the distance to a desired destination, sunrise and sunset times, bearing and other information.
While in orbit, the satellites are powered by solar energy. They also have backup batteries that are used in the event of no solar power such as an eclipse. The energy is used to power small rockets on the satellites that keep them in the proper orbit. At any one time only about 50 watts of power or less is used to transmit information. The satellites are designed to last about ten years, and the U.S. Department of Defense is constantly making and launching replacement satellites. Each satellite is about 2,000 pounds and seventeen feet across when the solar panels are out.
GPS receivers are generally accurate within 15 meters. Other than investing in a receiver there are no fees or other equipment required to access the GPS signal. If very accurate readings are needed, Differential Global Position Systems (DGPS) will provide accuracy within three to five meters. The United States Coast Guard operates the most popular DGPS.
Two power signals are transmitted and are referred to as L1 and L2. The L1 frequency is used for civilian purposes. These signals are relatively low power signals and travel by line of sight, so they can go through clouds, glass, and plastic, but not solid objects like buildings or mountains. In every transmission the satellite sends three types of information, its pseudorandom code, ephemeris data and almanac data. The pseudorandom code is an I.S. code that identifies which satellite the information is being sent from. Ephemeris data tells the receiver where the satellite should be at any time of the day, and almanac data sends information about the status of the satellite, the current date and the time. The almanac data is the part that is essential for determining the user’s position.
By: Chris Simons
Posts Tagged ‘Proper Orbit’
GPS And How It Works
January 17th, 2010Posted in Article
Tags: Altitude Backup Batteries Department Of Defense Global Positioning System Latitude And Longitude Latitude Longitude Proper Orbit Receivers Satellite Navigation System Satellites Small Rockets Solar Energy Solar Panels Solar Power Sunrise And Sunset Sunrise And Sunset Times Sunrise Sunset U S Department United States Department United States Department Of Defense
Ephemeris Error – Is This An Issue With Your GPS?
December 21st, 2009
The clock and ephemeris error is one GPS issue which users might have to contend with. Correcting these errors is a significant challenge to improving GPS position accuracy.
The navigation message from a satellite is sent out only every 12.5 minutes. In reality, the data contained in these messages tend to be out of date by an even larger amount.
When a GPS satellite is boosted back into a proper orbit, for some time following this movement, the receiver’s calculation of the satellite’s position will be incorrect until it receives another ephemeris update.
The onboard clocks are extremely accurate, but they do suffer from some clock drift. This problem tends to be very small but may add up to six feet of inaccuracy. This class of error is more stable than ionospheric problems and tends to change over days or weeks rather than minutes. This makes correction fairly simple by sending out a more accurate almanac on a separate channel.
According to the theory of relativity, due to their constant movement and height relative to the Earth-centered inertial reference of frame, the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity). For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly because they are in a weaker gravitational field than the atomic clocks on the Earth’s surface. On the other hand, special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks.
When combined, the discrepancy is 38 microseconds per day. To account for this, the frequency of the clock on board each satellite is given a rate offset prior to launch so that it will run slightly slower than the desired frequency on Earth.
GPS observation processing must also compensate for another relativistic effect called the Sagnac effect. The GPS time scale is defined in an inertial system, but observations are processed in Earth centered and Earth fixed system which is co-rotating and simultaneity is not uniquely defined.
The Lorentz transformation between the two systems modifies the signal run time – a correction having opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres. Ignoring this effect will produce an east-west error on the order of hundreds of nanoseconds – or tens of meters in position.
The atomic clocks on board the GPS satellites are precisely tuned. This makes the system a practical engineering application of the scientific theory of relativity in a real-world system.
To know more on the solution on problems with GPS system, please visit GPSAutoTracker for more tips on how to maximize the use of your GPS system.
By: Audrey Ly
Posted in Article
Tags: Atomic Clocks Clock Drift General Relativity Gps Accuracy Gps Clock Gps Observation Gps Satellite Gps Satellites Gps Time Gravitational Field Inertial Reference Microseconds Navigation Message Orbital Speeds Position Accuracy Proper Orbit Sagnac Effect Six Feet Special Relativity Theory Of Relativity
