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 In reality, the data contained in these messages tend to be out of date by an even larger amount.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
Posts Tagged ‘Atomic Clocks’
Ephemeris Error – Is This An Issue With Your GPS?
December 21st, 2009Posted 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
How GPS Works
October 4th, 2009
Global Positioning System (GPS) is a navigational aid originally developed for the military. The system simply receives signals. It is the applied technology that gives the GPS its versatility.
If you have ever used map and compass, you will understand a little about how the GPS works. In order to find your position on a map, you need to have three points of reference. The intersecting line from the reference points is where you are. Map and compass work uses triangulation (bearings), GPS uses trilateration (distances) to calculate location. Satellites orbiting the earth emit unique signals that can be received by a GPS. The GPS software interprets the signal, identifying the satellite that it came from, where it was located, and the time that it took for the signal to reach the system. Once the receiver has both time and distance it begins to determine position.
Three satellites provide the intersection point and the fourth is used to check that the positioning is accurate. Accuracy depends upon the synchronization of atomic clocks in the satellites with the clock in the GPS system. Although the clock in the GPS is not
atomic, utilizing the fourth satellite gives it that functionality as the internal clock adjusts itself to correct any discrepancy discovered.
GPS has gone far beyond its initial military application. Drivers can find their way through city streets, long distance trekkers use the technology to cross unfamiliar terrain, mariners and pilots use GPS enhanced data to cross the seas and skies.
In–vehicle GPS can be integrated into the car entertainment system or can be installed as a removable device. These systems need to tell the driver where he/she is and how to reach their destination. The information includes road directions plus relevant features along the way such as rest stops, gas stations, points of interest, etc. Auto GPS uses voice commands so that the driver can concentrate on the road.
Hikers and trekkers use similar technology, but normally without the inclusion of road systems on their devices. Mapping software defines the territory that the hiker will encounter. The user can enter waypoints (points of reference) so they can return using the same route. They can add points of interest such as water sources, possible campsites, and other items of interest on their trail. However, the portability demanded by hikers will also limit the functionality of the system as small screens mean that some detail will be lost.
It is GPS technology that is used to track individuals on home arrest, to trace missing pets, stolen vehicles, and missing people. Small systems can be incorporated into pet collars and wristwatches. As long as the receiver is active, it can be found.
Marine and aviation GPS units are sophisticated and specialized. The principles involved are the same as any standard system; the software is much more highly developed.
Any fisherman, who is using a fish finder on his boat, is using a GPS that is enhanced by sonar and tracking devices. Units have been developed for use on float tubes also -– as GPS technology advances, the systems become more and more compact and their uses
more and more extensive.
If you are considering purchasing a GPS, make sure that it can be updated easily. This is especially true if you buy a multi–function GPS or one that is used where conditions change regularly. An in–vehicle GPS soon loses its usefulness if it is not updated as road systems change.
Updates vary according to the device being used. They can come in CD/DVD packages or as computer downloads. The user can purchase maps specific to the area in which the GPS will be used or a range of maps and routes. These are available from GPS software
companies who will charge proportionally to the sophistication of the software.
GPS units vary in price according to their usefulness. It is possible to buy units for less than one hundred dollars to units costing more than one thousand dollars. What your needs are
will be a factor in the cost of your unit. If you are a backpacker then portability is a major consideration. If you are a trucker, you need to be able to find a delivery point as quickly and conveniently as possible. Whatever device you go for, cost is generally related to quality. Buy the best you can afford.
By: Anne King
Posted in Article
Tags: Atomic Clocks Car Entertainment System City Streets Compass Work Global Positioning System Global Positioning System Gps Gps Software Gps System Internal Clock Intersection Point Military Application Navigational Aid Reference Points Relevant Features Rest Stops Road Directions Time And Distance Triangulation Unfamiliar Terrain Voice Commands
