Interference and jamming can cause GPS signal problems. 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.
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.
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
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.
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