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 ‘Atmospheric Conditions’
How Accurate is GPS?
January 27th, 2010
The answer to this question keeps changing along with advancing technology as well as United States law. The first consumer GPS receivers on the market were far less accurate than the receivers available today even though the satellites are the same. Also, before the year 2000, the military intentionally introduced inaccuracies into the civilian GPS signal for “security” reasons. They called this intentional signal error “Selective Availability”. In May of 2000, President Clinton made a decision to turn off SA and allow civilians access to increasingly accurate GPS signals. The military still has the ability to turn off or introduce errors into GPS signals at any time and with no notice but this ability is generally only used for specific reasons of national security.
The best consumer GPS technology available as of 2007 achieves an accuracy level of 15 to 30 feet under normal conditions. Garmin, currently the most popular manufacturer of GPS receivers, claims their GPS units are accurate to within 50 feet 95% of the time. Even within the Garmin GPS receiver line, accuracy specs differ because of the different chips used in the design of different receivers.
GPS Receiver Design Affects GPS Accuracy
If you are looking for the most accurate consumer GPS unit, it’s important to pay attention to the unit’s chip type. The latest technology in GPS receiver chips is the SiRFStar III from SiRF. With this chip in your receiver and WAAS enabled you can expect to reach the best GPS accuracy levels available under current conditions.
Environmental Factors Affect GPS Accuracy
Many different environmental factors can also affect the accuracy of your GPS receiver. Heavy tree cover, tall buildings, deep canyons, and any location in which your view of the sky is limited can affect how accurate your GPS is. Atmospheric conditions can also degrade the accuracy of your GPS unit.
As you can see, there is no easy answer to the question “how accurate is GPS?”. However, you now know the main factors that can affect GPS accuracy.
By: Markus Hamilton
