The Global Positioning System (GPS) is a series of satellites which communicate with GPS receivers, used to find precise locations on the globe.
Originally the GPS was created in the United States of America for military 2 purposes. In 1973 the decision was made to create a satellite navigation system based on the existing systems available at the time. This included TRANSIT, TIMATION and 621B (Of the US Air Force and Navy). Between 1974 and 1979 the system was developed, refined and tested. Tests were performed before any satellites were launched. This was achieved by setting up transmitters on the earth’s surface, these were called Psedolites (Pseudo satellites). Some of the first satellites launched doubled as peacekeepers, containing sensors used to detect atomic explosions all over the world.
Throughout this time GPS signals are publicly available to anyone with a GPS receiver, however, in order to hold military advantage, the US Military throttles and limits usage by civilians, this is called ‘Selective Availability’. During the Gulf War, in 1990, Selective Availability was disabled temporarily due to the limited supply of military GPS receivers, requiring the use of civilian receivers.
In 1995 ‘Full Operational Compatibility was announced. This publicly stated that the GPS systems in place were fully operating to expectations giving accurate information where expected.
In 2000 Selective Availability was disabled, improving accuracy for civilian users from about 100m to 20m. Current technology, including new systems such as differential GPS users are able to achieve an accuracy of 3-5m.
GPS uses high frequency, low power radio signals, which are transverse waves. These waves do not require particles to move and travel at the speed of light. They are produced via finely tunes electromagnetic pulses with predefined amplitudes which will not bombard the Earth with electromagnetic energy but will be enough to penetrate the atmosphere and retain readable data. The frequency of these waves is changed to transmit data (frequency modulation). Power to the satellites is generally provided via solar panels. These waves have two main purposes; the first is to transmit data. GPS satellites transmit down a sequence, which is seemingly random data; a seeded data generator creates a random, but predictable, pattern of data. This is reset at a certain point in time, for example midnight. This data is useless by itself, however, being predictable data, it is possible to calculate the precise distance from a transmitting satellite if it is known exactly what should be transmitted at any one point in time. It is possible to calculate the offset of the wave pattern and therefore to know how far the signal is lagging behind. With the knowledge that radio/electromagnetic waves travel at the speed of light (that is about 300,000 km per second) and using the time that the data has taken to travel from the satellite to the receiver the distance can be calculated. 300,000/time = distance in km.
One issue with this system is the prediction of the pattern. To accurately know what the current pattern is the clock at the receiver must be exactly the same as the clock controlling the computer on the satellite, down to tiny fractions of a second, Impossible without atomic clocks worth between $50,000 and $100,000 each.
It isn’t feasible to charge $50,000 for every phone and GPS unit. The solution is quite a clever one. Satellites themselves all contain an atomic clock which keep perfect time. Receivers have clocks that only keep approximate time while being constantly adjusted by the GPS receiver computer. The exact time is calculated by considering the several different radio waves coming in from GPS satellites and calculating what the precise time must be for the data pattern to line up with the other patterns being received. By receiving multiple signals the receiver can calculate the offset for the time taken to travel the distance.
Now that the distance between satellites can be calculated it is possible to determine location on the globe, this process is called triangulation. Looking at 2D triangulation this is easier to see, if the distance to two known points is given then there are two possible locations that the receiver can be in as shown below.
The receiver can be on either of the two points where the lines cross. If a third known distance to location is introduced then only one possible location remains.
3D Triangulation, the technique used for GPS location is quite similar to this. However, since locations are being made in 3D space there are spheres of known distance instead of circles, this is harder to visualise but means that when two distance spheres are known there is a ring of possible locations where these spheres overlap. Introduce a third and reduces to exactly two possible points.
Having two points is not an issue as only one of these will ever be located within the Earth’s atmosphere. The receiver automatically discards the location in space. This shows that with the knowledge of the distance of 3 satellites it is possible to find a single point of location in 3D space on the Earth.
One thing this requires, which has not been covered yet, is the exact positions of the satellites. Since satellites are continuously moving in orbit around the Earth it is not enough to simply have a known table of locations programmed into the receiver. Helpfully it is possible for physicists to predict quite accurately the position of a satellite at any one point in time throughout its orbit. This data is programmed into each GPS receiver and is called an almanac. Slight adjustments sometimes have to be made due to external factors such as the pull of the moon; these changes are monitored by the U.S. Department of Defense and beamed out to GPS receivers via the signals being sent from the satellites themselves.
Although only 3 satellites are technically required to determine location many GPS receivers will only consider a location valid if it has 4 signals that all line up with the information calculated, this ensures signals that are being corrupted or delayed by the atmosphere do not give faulty results.
GPS is a major part of today’s society; it is used for a huge range of different purposes; from perfectly aiming missiles to helping find a new mate’s house. GPS was originally designed for military uses it has, since being open to the public, become a heavily relied upon means of tracking and locating. While still being used for military purposes these large scale systems are also being used for the benefit of the world. Systems such as physical post and global trade use GPS for locating packages and shipments, Ships, cars and even aeroplanes use GPS. Aeroplanes use GPS as a second, backup altimeter because the GPS system can he used to determine altitude as well as location.
This system isn’t always so adored by the public though, systems such as phone tracking, reliability, constant radio waves and security of transmissions are constantly being questioned. Today, many smartphones have built in GPS receivers alongside a constant Internet connection thanks to HSPA+, 3G and LTE. This allows devices to be found if they are lost by simply logging into a website which will then show the phone’s location on a map. This is often considered a bad thing and many people disable this feature in fear of these websites being ‘hacked’ allowing them to be tracked as they carry their phone around throughout the day. The same goes for signal interceptions, although in reality intercepting signals in order to locate someone is next to impossible it is still feared by many people, sometimes enough to disable GPS on their devices entirely. Finally, as is always complained about at some point, many people considered the constant exposure of radio waves being transmitted all over the Earth is bad for people’s health and damaging to their brains. Again, due to the low energy of these waves there really is no harm.
Overall GPS is a highly beneficial system, making even the street directory obsolete and allowing individuals to track back to their lost phone in the middle of the city. While there are fears and stigmas associated with related technology the world has accepted GPS quickly and it has become a central part of Western civilization. This has impacted sociality in a positive way, pushing globalization and simplifying otherwise impossible tasks.
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