I am sure you already know what a GPS is, but have you ever wondered: How does GPS work? How does it actually calculate your position and what sort of limitations are there are to its accuracy?
In its most basic form, a GPS triangulates your position by measuring how far away it is from at least four different satellites. It knows the position of its satellites, and by drawing an imaginary ring around each one, it calculates its position as the point where the rings intersect.
If you would rather watch a video about how marine GPS works, you can watch the video below.
Development of GPS
The USA developed NAVSTAR (GPS) to provide position fixing accuracy for the military. They launched a series of satellites through the 1970s and 1980s, refining the concept until the system was ready. The complete system comprises 24 satellites, with extras in reserve. This gives at least 4 in view from every position on the surface of the earth.
GPS was only initially used by the military, but it wasn’t long until it was opened up for civilian use. As a direct result of gaining access, the aeronautical and maritime sectors both benefited from improved position fixing accuracy.
Civilian users were denied full positional accuracy with a feature called “Selective Availability”. Basically, it added intentional errors to GPS signals. Errors of up to 50m horizontally and 100m vertically were expected. The US military kept full accuracy with a “key”, allowing them to compensate for these errors. In 2000, all users regained full accuracy when Selective Availability was effectively deactivated.
How is your GPS Position Calculated?
GPS receivers attempt to measure the distance from their antenna to at least four different satellites. Each satellite broadcasts a unique code at precise time intervals. Receivers know the precise time that each code is due to be broadcast, so they can measure the time difference between the time that they receive the code and the time it was supposed to be broadcast. This time difference is simply due to the distance that the signal traveled.
Even after calculating how far the transmission traveled, the receiver is still not able to calculate an accurate range. Initially, it can only calculate a pseudorange. This is because the receiver does not have a clock accurate enough to keep time in the same way the satellites do.
In contrast to the receivers, satellites are able to keep precise time. Each one has multiple atomic clocks on board. Hypothetically, if every receiver also had an atomic clock, the calculated range would be an actual range instead of a pseudorange. Unfortunately, cost and practicality mean that GPS receivers do not contain atomic clocks.
The effect of this time inaccuracy is that GPS receivers need to lock onto four satellites instead of three. Only three would be needed if time was precise. Instead of trying to calculate only latitude, longitude and altitude, the receivers also need to calculate time. We can find all four unknowns using algebra. Algebra allows calculation of four unknown values by solving a set of four simultaneous equations.
Solving all four equations gives latitude, longitude, altitude and time. Aside from giving an accurate position, every GPS receiver produces the precise atomic time as held by the constellation of satellites.
Errors of GPS
No matter how accurate you make the clocks and measurements, errors can (and do) always creep in. These can be broken down into a number of different categories.
These are errors that arise due to signals from satellites slowing down as they pass through different layers of the atmosphere. Different atmospheric layers are made of different elements, creating different densities. As signals pass through these layers, the speed at which they travels reduces slightly. Normally GPS signals travel at the speed of light, but if they slow down in places then they will take longer than expected to reach the receiver. This results in a slightly erroneous measurement of pseudorange to the satellite, resulting in slight inaccuracies to the final calculated position.
GPS can compensate for some propagation errors by transmitting a second signal at a different frequency. Different frequencies are disrupted differently when passing through the layers of the atmosphere. Comparing the second frequency to the original gives enough information to work out a correction to apply.
Multi-path errors are caused by signals from the satellites not taking a direct path to the receiver. Maybe they bounce off a building or a cliff face before reaching the receiver. This results in the signal taking a longer time to reach the receiver than it would have taken had it traveled along the direct route. Again, this results in an erroneous distance being calculated.
Multi-path errors are usually noticed by the receiver because it takes measurements from multiple satellites. It then discards measurements that are clearly the result of multi-path. In terms of marine GPS, these errors are not so common because there are not many structures at sea that result in multi-path errors.
These errors are due to slight variations in the satellite orbits. If the orbit was precise, all the numbers would tie in perfectly. As the orbit of the satellites vary slightly small errors can creep in to the calculated positions. Ephemeris errors are correctable because satellite positions are observed by ground stations. Ground stations calculate the difference between the satellite’s expected position and their actual position. The difference is applied to the receiver’s calculations, effectively eliminating ephemeris errors.
Receiver noise refers to errors resulting from electronic signals within the GPS receiver unit itself. The receiver has a small level of interference, which can affect the signals received and in turn affect the calculated pseudoranges to each of the satellites.
These are errors that result from relativistic effects within the satellites themselves. The theory of relativity itself could fill many articles, but the part we are interested in is the part that describes how time runs at different speeds depending on the relative speed between two observers. This is important for us because satellites orbiting the earth travel so much faster than observers on earth. Time on satellites appears to run differently to time on earth. Relativistic effects were considered when the satellites were built, so their internal clocks actually run at a slightly different rate than they would on earth. From the point of view of the observer on earth though, their time appears to run correctly. Despite this forethought however, small errors can still creep in over time.
Satellites are monitored from stations on the surface, so relativistic errors are monitored. Corrections are passed to compensate for any errors that are detected, but we still need to be aware that it could result in a small decrease in accuracy.
Differential GPS (DGPS)
Differential GPS was a system originally developed to combat Selective Availability. Selective Availability degraded the accuracy of signals randomly. This made positional accuracy less precise for users that were not part of the US Military.
The theory behind DGPS was that you could build a receiver in a fixed location and compare its GPS position to its actual position. The difference between the two was due to Selective Availability. The difference could be broadcast as a correction to all receivers in the area that were equipped to receive DGPS corrections. The correction effectively eliminated selective availability, contributing to its eventual termination.
Today, even though Selective Availability is no longer an issue, DGPS is still used to provide corrections to GPS signals to increase its overall accuracy.
How to tell the Accuracy of your GPS
You can get a rough estimate of the accuracy of the position given by your GPS receiver by looking at its HDOP value. HDOP means “Horizontal Dilution of Precision”. It is a measure of how much error receivers in that area can expect. A crude way to measure precise accuracy is to multiply the expected accuracy (say 1m in this example) by the HDOP value. This means that a HDOP of 1 will give positional accuracy of 1m in our example. As the HDOP value increases the expected accuracy of your position decreases.
Top Alternative Global Navigation Satellite Systems
So far within this article we have discussed NAVSTAR GPS, which is the system operated by the US military. NAVSTAR is just one example of a Global Navigation Satellite System (GNSS). There are others that have been developed by other countries.
GLONASS is probably the most developed alternative system to date. It is operated by Russia, in a slightly lower orbit that NAVSTAR. This results in improved accuracy of position at higher latitudes. Considering Russian territory extends a long way north, this improved accuracy is beneficial to their system.
Galileo is another GNSS currently in development by the European Union. It is due to offer accuracy of around 1m for most users and up to 1cm for premium users. Galileo satellites are planned for a higher orbit than NAVSTAR satellites which will reduce multipath errors in city environments by placing the satellites higher in the sky than current GPS.
Although we have discussed satellite navigation at length, it is important to remember that there are alternatives. Alternatives are important to maintain accuracy of position fixing should GPS signals prove unreliable due to any cause.
eLoran is the most promising alterative in my opinion. This system is similar to GPS in that eLORAN units on vessels receive signals from stations and triangulate position. The difference is that eLORAN stations are land based rather than satellites, and the signals transmitted are high power, low frequency. They complement the low power, high frequency signals of GPS because they still reach vessels in situations where GPS may be subject to interference or unreliable.
Marine GPS is a very effective way of finding your position at sea, and has become an essential piece of navigational kit to be carried on board every vessel. Now that you have seen how it actually works, you will be able to assess its accuracy and reliability for yourself.