Science Behind Sat Navs

How do Sat Navs work?

Research has found that 52% of car drivers now have and use a Sat Nav device in their vehicle. Many now come installed in new cars and vans and are an essential tool for a lot of road users to ensure they do not get lost on long or short journeys.

But how do these pieces of equipment work? Of course, drivers who have one will know exactly how to programme a route and find out where to go, but are probably clueless about how the science and technology behind it works. This post breaks down how a Sat Nav establishes a vehicle’s location, its interaction with satellites and important changes to the technology.

Sat nav science

GPS Satellites

Sat Navs use the Global Positioning System (GPS) to determine the exact position of where you are on earth. GPS is a system which uses a network of 30 satellites which are orbiting the earth at an altitude of around 20,000 km and speed of 7,000 mph. They were originally developed and sent into orbit by the US government for military purposes, but since the 1980s have been available for civilian use.

These satellites circle the world twice a day in the exact same orbit, beaming signal information back down to earth. The information is transmitted at the speed of light and, based on how long it takes each message to arrive, the internal electronics of the Sat Nav converts these signals into the distances between the device and satellites.    

Two low power radio signals are transmitted by the satellites, called the L1 and L2. Civilian GPS use L1 frequency, whereas L2 is for military and other encrypted services. They travel by line of sight, so can easily travel through clouds, glass and plastic but not solid objects like buildings. The signal contains information such as the pseudorandom code, which can be viewed on the GPS’ satellite page, simply showing which satellite it is receiving a signal from.   

Trilateration

Wherever in the world you are, at least four GPS satellites will be visible. Only three are required to generate an accurate location though. GPS receivers use the information sent from three separate satellites as a trilateration to calculate the Sat Nav’s exact destination. At least three overlapping spheres generated by the satellites will meet at different distances from the receiver (Sat Nav) to precisely work out the location.

These distance measurements will then be calculated to display the user’s position on the Sat Nav’s electronic map. When the vehicle moves the three satellites will recalculate the location, and if it moves outside of one sphere then the sphere of another satellite should pick it up. If for some reason the Sat Nav cannot lock onto three satellites, then a position will not be generated.

Signal from at least three satellites is required to form a 2D position (latitude and longitude) and track movement on a Sat Nav. For a 3D position (including altitude as well) the signal from at least four satellites is needed. When all this is in place and there is a decent signal, further information can be calculated, such as speed, distance to destination, trip distance and more.    

GPS Accuracy

The GPS technology has developed a lot since the 1980s to become ever more accurate. Receivers with Wide Area Augmentation System (WAAS) capability are the most accurate. This is an air navigation aid developed to allow all aircraft to rely on GPS through the entirety of any flight. It helps maintain strong locking to satellites, even when the Sat Nav is in an area surrounded by atmospheric factors which can lead to problems, such as high foliage or tall buildings.

Delays and errors can still occur with a GPS signal, affecting Sat Navs due to a few reasons. The signal can reflect off tall objects, increasing its time to reach the receiver and causing time errors, while just passing through the atmosphere can also cause delays. Timing errors can happen if the Sat Nav’s clock is not as accurate as that of the GPS satellite too.

Sat Navs in Society

The use of Sat Navs is now so prevalent that it is being incorporated into the ‘independent driving’ sector of the UK’s driving test, with candidates having to follow directions from a Sat Nav accurately to pass. There is also a multi-million pound project underway, spearheaded by Ordnance Survey, to create a database with information about 200,000 miles of roadways.

This is to provide accurate information about road widths, weight restrictions, bridge heights and more, to prevent lorries and cars getting stuck when following Sat Nav instructions. It could be argued that this is due to an over reliance on the technology, but given that the benefits of Sat Navs outweigh the negatives, its use is only likely to increase.      

Image courtesy of iStock.

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