The Global Positioning System (GPS) allows for precise location identification on the earth by measuring the distance between satellites. It allows you to record or create locations from anywhere on the planet and then navigate to and from those locations.
GPS was conceived in 1960 under the auspices of the United States Air Force, but other branches of the United States military joined the effort in 1974. In 1978, the first satellites were launched into space. In April 1995, the System was declared fully operational.
Originally intended only for military applications, the GPS was not made available for civilian use until the 1980s. GPS comprises 24 satellites that circle the globe every 12 hours to provide global position, time, and velocity data.
Position errors in GPS receivers can occur from the following sources:
User mistakes account for most GPS errors
Incorrect data and typographic errors when entering coordinates into a GPS receiver can cause errors of several kilometers. Unknowingly relying on fewer than four satellites to determine position coordinates can result in unreliable position fixes that can be off by more than a mile. Signal interference can be caused by anything, including the human body. Holding a GPS receiver close to the body can cause some satellite signals to be blocked, making accurate positioning difficult. Because most GPS satellites are oriented more in the earth’s southern hemisphere, facing to the south can help alleviate signal blockage caused by the body if a GPS receiver must be handheld without the benefit of an external antenna. A GPS receiver has no way of detecting and correcting user errors.
Satellite clock errors and orbit errors
Satellite clock errors are caused by slight discrepancies in each satellite’s four atomic clocks. These errors are monitored and corrected by the Master Control Station.
Satellite orbit (also known as “satellite ephemeris”) refers to a satellite’s altitude, position, and speed. Satellite orbits change due to gravitational pull and changes in solar pressure. The Master Control Station also monitors and corrects orbital errors.
Ionospheric interference
The ionosphere is the layer of the atmosphere between 50 and 500 km altitude, primarily made up of ionized air. The GPS satellite radio signals are refracted as they pass through the earth’s atmosphere due to ionospheric interference, causing them to slow down or speed up. As a result, GPS receivers on the ground measure their positions incorrectly. Even though satellite signals contain ionospheric interference correction information, they can only remove about half of the possible 70 nanoseconds of delay, potentially leaving a ten-meter horizontal error on the ground. GPS receivers also try to “average” the amount of signal speed reduction brought on by the atmosphere when determining a position fix. However, this only works up to a point. Fortunately, atmospheric conditions usually cause an error of fewer than 10 meters. The Wide Area Augmentation System (WAAS), a space and ground-based augmentation to GPS, has further reduced this source of error.
Tropospheric interference
Water vapor in the troposphere which is the lower layer of the earth’s atmosphere (below 13 km) that changes temperature, pressure, and humidity due to weather changes, contributes significantly to GPS errors. Tropospheric interference has little effect on GPS.
When activated, receiver noise is the electromagnetic field produced by the receiver’s internal electronics. Electromagnetic fields distort radio waves. This affects the time it takes GPS signals to travel before the receiver can process them. Remote antennas can help to reduce noise. The GPS receiver cannot correct this error.
Multipath interference
Multipath interference is caused by reflected radio signals from nearby surfaces, which can either interfere with or be confused with the true signal that follows an uninterrupted path from a satellite. The ghosting image on a TV equipped with rabbit ear antennas is an example of a multipath. Multipath is difficult to detect and, in some cases, impossible to avoid or correct by the user or the receiver. Car bodies, buildings, power lines, and water are all examples of multipath sources. When using GPS in a vehicle, mounting an external antenna on the vehicle’s roof eliminates most of the vehicle’s signal interference. When using a GPS receiver mounted on the dashboard, there will always be some multipath interference.
Selective Availability (S/A)
The intentional degradation of satellite signals by a time-varying bias was known as Selective Availability (S/A). The DOD controls Selective Availability, originally implemented for security reasons to limit accuracy for non-US military and government users. Under pressure from business and the White House, the Pentagon reduced Selective Availability to zero in May 2000. The Pentagon did not disable S/A but reduced signal interference to zero meters, eliminating deliberate position errors. The Pentagon can reactivate S/A without informing non-government GPS users. As a result, it’s critical to understand Selective Availability and be aware that the US military may reactivate it at any time without prior notice.
Number of satellites visible
The greater the number of satellites the receiver can “see,” the greater the accuracy. Buildings, terrain, electronic interference, and dense foliage can all interfere with signal reception. The better the reception, the clearer the view to the receiver.
Satellite geometry
This refers to the satellites’ relative position at any given time. Ideal satellite geometry is achieved when satellites are positioned at large angles to one another. The geometry is poor when satellites are arranged in a straight line or a small group.
