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 ‘Position Accuracy’
GPS Ionospheric Delay – The Problem and Solution
February 21st, 2010Posted in Article
Tags: Approximate Location Atmospheric Conditions Carrier Wave Civilian Gps Receivers Code Signal Dispersion Frequency Bands General Location Geographical Area Gps Navigation Systems Gps Signals Inconsistencies Ionosphere L2c Mathematical Model New Signal Position Accuracy Pseudo Random Number Satellite System Solution Gps
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
