Introduction to Global Positioning Systems
Have you ever lost your wallet and couldn’t find it or any of your belongings you have missed and were not able to track? This is where a Global Positioning Systems can come out to your escape and make you cheer. Today, GPS is a term that doesn’t needs much introduction. GPS has been a huge part of our daily lives, from searching to travelling, locating to reaching, and from finding to tracking & connecting. Since its inception in 1973 in the United States by the U.S. Department of Defense, GPS has already made its significant place in mankind and has undoubtedly proven its importance for each one of us.
GPS, as its name suggests, Global Positioning System, tells the position of any object globally, i.e. relative to this whole planet, planet Earth with the help of satellite navigation system in which a plethora of satellites which are revolving around the Earth are used to locate an object. It lets you know where a specific object is exactly located currently or was located in the past, its velocity at a time, the whole day, irrespective of the weather conditions, anywhere in the world.
Initially developed for use by the military services of the United States, it was made publicly available at a very low cost. Today, GPS receivers are attached to all types of commercial products like cell phones, smartphones, watches, automobiles, bikes, etc. In fact, GPS has been made so cheap that you can just use it for any tiny object to track it.
Working of Global Positioning Systems (GPS)
GPS uses the mathematics principle of “trilateration” to determine the position of an object and it is a system of more than 30 satellites orbiting the Earth.
The entities involved in the working of GPS are:
- Satellites (the space segment)
- Control system (operated by the U.S. military)
- Signal Receivers (user segment – civilian and military users with their GPS devices with antenna)
- The position of satellites is known from the signals which they broadcast to the receiver.
- These signals are received by the receivers connected in the objects which are to be located.
- The important point here is that minimum 4 satellites are required to get the exact location of the receiver on Earth.
- 3 satellites are used to trace the location place.
- 4th satellite is used to confirm the target location of satellites.
- The GPS receiver takes the information from the satellite and uses the method of triangulation to determine a user’s exact position.
Advantages and Disadvantages of GPS
Given below are advantages and disadvantages of Global Positioning Systems:
- A great tool for military, civil & commercial users
- Driverless cars, Vehicle tracking systems, GPS-based navigation systems provide us with turn by turn & point to point directions
- GPS has a very quick speed of tracking an object
- Accuracy issues in urban areas
- Weak signals (so not of much use in water, indoor, or dense areas)
Errors of Global Positioning Systems
GPS involves satellites, receivers, signal transmission between satellites and receivers. So errors can occur in any of the above mentioned three entities.
Errors in Signal propagation
As we know the signals from the GPS satellites which are orbiting the Earth have to reach the receiver and for that, the signals have to travel through the Earth’s atmosphere. Firstly, the signals go via the Ionosphere (topmost part of the atmosphere), then through the Troposphere (lowest part of the atmosphere). Thus, a GPS signal is affected twice by the atmosphere due to which there is a range of errors from zero to 30 meters in the signals.
The first error is the satellite position error. Since the position of satellites is a prediction computed from the previous observations of the satellites by the GPS control stations, the predictions cannot be made with absolute accuracy, the predictions are inaccurate.
Thus, the positions of satellites contained in the “satellite message”, is not exact due to which the object location calculated is sometimes incorrect.
Secondly, there are satellite clock errors. Since the atomic clocks used in the satellites are not perfect, the measurement of travel time taken by the signals to reach the receivers, and, the range between satellite & receiver is corrupted.
Errors occurring at the Receiver
Four major errors occur at the receiver end or its antenna:
1. Receiver clock error: A receiver also has a clock for comparing with the clock inside the satellite for measurement of the signal travel time. This clock has errors that are termed as “receiver clock errors”.
2. Measurement noise: This depends on the type of “ruler” used for range measurements. GPS signals have 3 rulers
- Coarse acquisition (C/A) code
- Carrier phase
These rulers allow the precision of a few meters, a few decimeters, and a few millimeters.
3. Multipath errors: These occur due to the reflection of signals by buildings or other objects interfering with the direct signals sent by satellites.
4. Antenna movement errors: These occur due to movement of electrical reference point or phase center of the antenna while the signals are being received from different directions.
Below are some limitations of GPS:
1. Reception of signals
Since GPS receiver requires continuous signal reception from at least 4 GPS satellites, if there is water, walls, soil or other obstacles in the travel path of the signals then the signals are not able to penetrate these obstacles. In such cases, the GPS visibility of GPS satellites is very limited. Like in urban areas with huge skyscrapers, the signals are missing for a long period of time.
Any system which involves measurement has a possibility of mistakes. GPS is no different than that. Since a lot of measurement is involved in GPS, there are a lot of possibilities of inaccuracy in providing the exact location of an object.
A receiver requests information from satellites, they send signals known as “satellite messages” to the receiver containing encoded information of distance of the satellite from the receiver, positions of satellites, etc. When a receiver gets a combined measurement from 4 satellites it is able to show its location. But if the satellite position is calculated wrong or range is wrong then this results in an incorrect receiver position. The signals involved in such issues are referred to as “faulty signals”.These signals impact the integrity of the location provided to the user. A user is unaware of the fact whether the position is correct or not. Thus, it is vastly advised for monitoring the integrity of GPS data.
It can be concluded that depending on GPS is good but one should always keep a check on the accuracy of results of GPS. It can also be observed that the accuracy can be augmented but at costs. Despite all the limitations and errors, GPS is still the best positioning system today.
This has been a guide to Global Positioning Systems. Here we discuss the introduction, working, limitations advantages, and disadvantages of GPS. You may also have a look at the following articles to learn more–