What is GPS? Global Positioning System Explained
You might be wondering, how do phones track our exact location? Why does it know where you are all the time? Is NSA tracking us? While the latter isn’t true, GPS has been very helpful to us. Whether looking for the nearest restaurants or finding your way out of an unknown place, GPS is the reason we always find our way.
What is GPS
GPS or Global Positioning System is a satellite-based navigation system. GPS consists of 24 satellites placed by the US Department of Defense and was originally intended for military applications. In 1980s, the Government made GPS available for public use.
A Brief History of GPS
The US Department of Defense developed the first satellite-based global positioning technology and its first intention was to be used for military applications. In 1980, GPS was made available to the public, but the military had interference to the signals to ensure that their own version was the only one to be used with any precision.
In 2000, and after 4 years of deliberations, President Clinton signed a bill ordering to cease scrambling satellite systems used by civilians. This paved way to a better and more accurate GPS systems that people are free to use.
The GPS Satellite System
There are 24 satellites that take up the GPS space segment which is orbiting the earth 12,000 miles above us. These satellites are constantly moving and makes 2 complete orbits in less than 24 hours. Because of this, theses satellites travel at around 7,000 miles per hour.
GPS satellites are powered by solar energy. They also have batteries onboard to keep them running when they aren’t directed by solar power (ex. solar eclipse). There are also equipped with small rocket boosters to keep them flying in the correct path.
How it Works
Today, there are 24 US-based satellites that orbit the earth. This ensures that at least 3 satellites are available for any device that requests its position anywhere in the globe. Russia has its own positioning system called GLONASS. It has 22 satellites and will one day work hand in hand with GPS for additional accuracy.
GPS finds your location by looking at the time stamp from the number of satellites orbiting the earth, how far each one is from you and how far apart are they from one another.
With that data, the system triangulates your position on the ground. Because of Theory of Relativity, the clocks in the satellites advance slightly faster than the clocks on the surface of the earth.
Not only that, this technology is made possible through atomic clocks. Atomic clocks are probably the most accurate of the timepieces. These clocks are very precise that it is precise only within a billionth of a second.
Atomic clocks are very accurate clocks which are regulated by the vibration of an atomic or molecular system. These clocks were initially created by physicists seeking for answers to questions about nature of the universe and is now used primarily for global positioning.
GPS basically works because of Einstein’s Theory of Relativity. General and special relativity predict the time differences that will appear between these clocks and the clock on earth.
General relativity predicts that time appear slower under stronger gravitational pull. This is why clocks on the satellites run faster than a clock on earth. Special relativity, on the other hand, predicts that because satellites’ clocks move relative to a clock on earth, they appear to run slower. The whole GPS networks run under these relativity.
What is Transmitted by GPS Satellites
Each GPS satellites broadcasts a continuous stream of data broken into segments and transmitted over 30 seconds. Each of these 30 seconds frame has the following information:
- Clock and time data
- The location of the transmitting satellite (Ephemeris Data)
- The orbits of all GPS satellites (Almanac Data)
- Status of the transmitting satellite
GPS satellites transmit these data at 50 bits per second. It may be slower compared to your internet connection but it is fast enough to update your location several times per second.
GPS satellites transmit two low power radio signals. These signals can travel through clouds, glass, plastic and other solid objects, but cannot go through most solid objects like buildings and mountains. The satellite use L1 frequency of 1575.42MHz in UHF bands.
How Accurate is GPS
The accuracy of GPS depends on many factors: the quality of the GPS receiver, the position of the GPS satellite at the time it was recorded, the surroundings (buildings, valleys, tree cover, etc.), weather etc. Also, the accuracy of GPS depend on the following factors:
GPS Receiver Quality
How accurate your location is also depends on the quality of algorithms and circuitry used in processing it. Most consumer grade GPS receivers does a pretty job of getting your position within about 25 feet most of the time.
When accuracy is critical, you’ll have to use GPS systems fit for that purpose.
When moving around
If you are moving around, the set of satellites your GPS receiver is able to use will change as you pass by obstructions like buildings and hills. To solve this issue Ephemeris data is used. In this way, it can quickly switch to using other satellites when the signals shifts out of view.
Most GPS receivers have an accuracy of about 10 to 50 feet, 95% of the time. To improve the accuracy, many receivers have WAAS capability. WAAS or Wide Area Augmentation System is an enhancement to GPS to give a more accurate position.
WAAS uses a network of ground-based stations to compare the precisely known location with the location calculated from the GPS satellite signals. Any differences found can be used to correct data that is broadcast from WAAS Satellites. WAAS only works in North America.
International Degradation of Satellite Signal
Selective Availability (SA) is an international degradation of the satellite signal imposed by the US Department of Defense.
It was intended for use to prevent military adversaries from using accurate GPS signals. SA was turned off in May 2000; thus every GPS signal from around the world are now as accurate as ever.