Ephemeris Error: Is This An Issue With Your Gps?

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 In reality, the data contained in these messages tend to be out of date by an even larger amount.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 repidly 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

Another possible problem for GPS systems has to do with interference and jamming. There are tons and tons of GPS receivers out there these days, so interference is probably going to come into play. Plus, jamming can be a problem in overloading the system as well.

Since GPS signals at terrestrial receivers tend to be relatively weak, it’s easy for other sources of electromagnetic radiation to desensitize the receiver. This makes acquiring and tracking the satellite signals difficult or impossible.

One of the sources of interference is a naturally occurring emission is called solar flares and they have the potential to degrade GPS reception. Their impact can affect reception over the half of the Earth facing the sun. GPS signals can also be interfered with by naturally occurring geomagnetic store that are mostly found near the poles of the Earth’s magnetic field.

Man-made interference can also disrupt or jam GPS signals. There was one documented case where an entire harbor was unable to receive GPS signals due to unintentional jamming caused by a malfunctioning television antenna. Intentional jamming is also possible.

Generally stronger signals can interfere with GPS receivers when they are within radio range or line of sight. Jamming a GPS signal can be done even by the layman. In fact, a 2002 article that appeared in the online magazine Phrack gave a detailed description on how to build a short range jammer.

The US government believes that jammers such as these were used occasionally during the war in Afghanistan. The US military also claimed to have destroyed a jammer with a GPS-guided bomb during the Iraq War. Such a jammer is relatively easy to detect and locate making it an attractive target for anti-radiation missiles.

Because of the potential for natural and man-made noise that interferes with GPS signals, there are many techniques being developed to deal with the interference. One obvious technique is to not rely on GPS as a sole source. There should be a fallback plan that should be in place in the event of a GPS malfunction.

In many receivers, there is a feature included called Receiver Autonomous Integrity Monitoring (RAIM). This is designed to provide a warning to the user if jamming or another problem is detected.

The US government has also deployed their Selective Availability Anti-Spoofing Module in their Defense Advanced GPS Receiver. This device is supposed to be able to detect jamming and maintains its lock on the encrypted GPS signals during interference which causes civilian receivers to lose a lock on the signal.

So what is being done to solve some of the problems that can occur with GPS signals both man-made and natural occurrences? Actually, there are a lot of things being done to help with problems like these.

GPS manufacturers are using augmentation methods to improve accuracy of GPS systems. These systems rely on external information being integrated into the calculation process. There are many such systems in place already and they are name or described based on how the GPS sensor receives the information.

Some systems transmit additional information about sources of error like clock drift, ephemeris, or ionospheric delay. Others give direct measurements of how much the signal was off in the past. A third group provides additional navigational or vehicle information that is integrated into the calculation process.

The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways.

The first is called dual frequency monitoring. This method refers to systems that can compare two or more signals like the L1 frequency versus the L2 frequency. Since these are two different frequencies, they are affected in different yet predictable ways by the atmosphere and objects around the receiver. After monitoring these signals, it’s possible to calculate and fix the error.

Receivers that have the correct decryption key can decode the P(Y) code relatively easily. This code is transmitted on both the L1 and L2 to measure the error. Receivers that do not possess the key can still use a processor called “codeless” to compare the encrypted information on L1 and L2 to gain much of the same error information.

The downside is that this technique is currently limited to specialized surveying equipment. Developers hope that in the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies. When these become operational, all users will be able to make the same comparison and directly measure some of the errors.

Another form of precise monitoring is called Carrier Phase Enhancement. The error that this program fixes arises because the pulse transition of the PRN is not instantaneous which makes the satellite-receiver sequence matching operation imperfect.

This approach utilizes the L1 carrier wave which has a period a thousand times smaller than that of the C/A bit period to act as an additional clock signal and resolve the uncertainty

The phase difference error in the normal GPS amounts to between 6 and 10 feet of ambiguity. The Carrier Phase Enhancement monitoring works to within one percent of perfect transition reduces this error to one inch of ambiguity. By eliminating this source of error, Carrier Phase Enhancement coupled with DGPS normally realizes between 8 and 12 inches of absolute accuracy.

Finally, there is another approach for a precise GPS-based positioning system. This is called Relative Kinematic Positioning. In this approach, determination of range signal can be resolved to an accuracy of less than four inches. This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver.

This can be accomplished by using a combination of differential GPS correction data, transmitting GPS signal information and ambiguity resolution techniques via statistical tests. This is actually possibly able to be conducted with processing in real-time as well.

As we’ve said, most people use their GPS system as a navigational aid. That means that, depending on where you are going, you will need to have a map of the place you are visiting. If you are a big traveler, you are going to need a lot of maps then, so let’s take a look at the maps you can get for your GPS receiver.

By: Audrey Ly

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Thinking of buying a GPS system? Or already own one? Learn more about the inner workings of GPS systems by visiting GPSAutoTracker now. Learn about the various GPS system as well as understanding GPS issues and solutions.

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