GPS ionospheric delay is one of the most common issues with the gps navigation systems. Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth’s atmosphere and ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy.
These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the signal is affected for a longer time. Once the receiver’s approximate location is known, a mathematical model can be used to estimate and compensate for these errors.
Because ionospheric delay affects the speed of microwave signals differently based on frequency – a characteristic known as dispersion – both frequency bands can be used to help reduce this error. Some military and expensive survey-grade civilian receivers compare the different delay in the frequencies to measure atmosphere dispersion and apply a more precise correction.
This can be done in civilian GPS receivers without decrypting the P(Y) signal carried on L2 by tracking the carrier wave instead of the modulated code. To do this on lower cost receivers, a new civilian code signal on L2 called L2C was added to the satellites. This new signal allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave.
The effects of the ionosphere generally change slowly and can be averaged over time. The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location.
Several systems send this information over radio or other links to allow L1 only receivers to make corrections. The date is transmitted via satellite system and transmits it on the GPS frequency using a special pseudo-random number so only one antenna and receiver is required.
Humidity also causes a variable delay resulting in errors similar to ionospheric delay but occurring in the troposphere. This effect is more localized and changes more quickly than ionospheric effects and is not frequency dependent. These traits make it much more difficult to make precise measurement and compensation for humidity errors than with the ionospheric effects.
Changes in altitude also change the amount of delay due to the signal passing through less of the atmosphere at higher elevations. Since the GPS receiver computes its approximate altitude, this error is relatively simple to correct.
GPS signals can also be affected by multi-path issues where the radio signals reflect off of surrounding terrain such as buildings, canyon walls, and hard ground. These delayed signals can cause inaccuracy as a well.
To correct these errors, many techniques have been developed. How these techniques work depends on addressing the long delay multi-path or shorter delay multi-path. 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 ‘Gps Signals’
Top Causes of GPS Interruption
February 8th, 2010
One of the first reasons you might suffer from GPS interruption, is the U.S. Government. They perform interruption tests on GPS systems quite often and there is really nothing you can do about it. You might experience interruption in the following places:
China Lake, CA Lax, CA Cedar Rapids, IA White Sands, NM Cape Canaveral, FL Patuxent River, MD Rancho Bernardo, CA Nellis AFB, NV
Another cause of interruption is shutdown of two Wide Angle Augmentation System satellites. This system is used as a no-cost differential correction signal for GPS. They are maintained by the FFA or Federal Aviation Administration, but are also available to civilians. The reason for this error is because some systems require the WAAS correction to operate properly. If you think you are experiencing this problem, you need to update your GPS system software. Most GPS manufacturers have information posted on their websites about firmware updates that fix problems just like this.
One of the most common reasons is objects blocking a GPS signal. An expert was quoted saying, “anything that can block sunlight can block GPS signals.” This could be anything from a building, tree, bridge, or even a vehicle. The only way to solve this problem is to move to an open location that can receive a signal.
Additionally, signals can be interrupted by disturbances in the atmosphere where they must travel, and the ionospheric layer is sensitive to solar disturbances which are currently at a solar maximum of an 11 year cycle. One could certainly expect frequent interruption for several years before a lull in the cycle.
By: Chaz Newman
GPS Auto Tracker – Definition of GPS Systems
October 22nd, 2009
Before the arrival of the General Positioning System (GPS) technology, having a trip was certainly the most daunting experience in our life. For instance, it was perfectly acceptable to see a situation where someone was driving his car, another one was looking at the map to find the right direction, quite funny! Nowadays, this situation is obsolete and ridiculous for GPS systems have drastically changed the way we used to deal with route finding during a trip. Indeed by just installing a GPS device in your car, you can accurately and safely get the right direction.
Usually when someone mentions GPS, he is referring to the GPS receiver even if the GPS architecture is actually more complex than this small device installed in your car. In short, GPS systems are a cohort of 27 satellites rolling around the center of the Earth. Only 24 satellites are fully functional while the remaining ones are just used for storage purposes. The military industry pioneered the technology before being used for general purpose by the public.
Each GPS satellite powered by the Sun rotates around the center of the Earth completes a distance of about 12,000 miles which is equal to 19,300 kilometers. The actual trajectories of these satellites are made so that at least four satellites are available in the space at any time. The GPS receiver is responsible of finding these available satellites, working out its distance to these satellites and finding its position. This process makes use of a mathematical model called trilateration.
The GPS signals are hindered by weather conditions. Nowadays, there are many research projects aiming at finding ways around this issue in order to increase the correctness of GPS systems. Having located the satellites, the triliteration is used to work out its distance and reduce these errors.
Briefly speaking, GPS systems are used for several purposes mainly as guiding aid. The user simply needs to input his final destination and the GPS receiver will do the rest of the job. GPS systems can also be used for tracking purposed as it is noticed in the automobile industry.
The GPS technology is also used for many other purposes. For instance, in the automobile industry, GPS systems are used to deter car thieves. Private investigators are also making use of the GPS technology in order to monitor a cheating wife. The GPS technology is also extensively used in the military industry as it was pioneered by the army. Some military usages include navigation, target tracking, missile and weapon guidance, search and rescue lost soldiers and acknowledgement and map creation. The possibility to keep track of a vehicle position every time is an appealing feature of the GPS technology. Other systems can furthermore keep track of a driver. This feature is of great help to anxious parents who are concerned about their children’s ability to safely drive.
Another use of the GPS for civilian purpose is to help prevent civilian GPS guidance from being used in an enemy’s military or improvised weaponry, the US Government controls the export of civilian receivers. A US-based manufacturer cannot generally export a GPS receiver unless the receiver contains limits restricting it from functioning when it is simultaneously at an altitude above 18 kilometers and traveling at over 515 m/s.
By: John B Mayall
