Global Positioning System or GPS is one of the technologies around us that we take for granted.
It is one of the most underrated technologies right now, without which our lives would have been a lot harder, considering our dependence on digital gadgets these days.
It involves a set of complicated procedures which might make you wonder in awe when known about in detail.
In this article, we bring you the basic facts about GPS and how this technology works behind the scene.
We are pretty sure that you will enjoy this article, knowing the innards of GPS and will end up appreciating the technology.
The article has been written in a way to make you feel comfortable and right at home with all the theoretical descriptions explained in a lucid manner.
So sit back, relax and read along!
What is GPS?
GPS is a satellite-based radio navigation system fully owned by US government and operated by the US Department of Defence. This technology make use of a cluster of satellites specifically send up to the space for tracking and navigation purpose.
The GPS service is available free of cost for all civilian users around the world and the only requirement for reception is a GPS receiver in hand or better a gadget with dedicated GPS chipset such as a smartphone.
History of GPS
The idea of sending up a group of satellites to the space for location tracking was first met with success in 1973, by the Department of Defence, United States of America.
Way back then, this technology was solely meant for military purposes. The civilians at the time neither had a GPS receiver nor the access to these signals for personal usage.
The seventies saw elevated tensions rising between America and Russia, as part of the infamous cold war.
In this era, these two countries were busy innovating technologies for claiming the upper hand in military and scientific fields, which ironically became a blessing in disguise.
Despite the tensions, the innovations and inventions made during this period, paved way for much of the technologies we use today on a regular basis, such as cryptography, satellite communications to name a few.
After the disastrous air accident in which a Korean Air plane was shot down by the Russian military allegedly after the plane had accidentally crossed into the restricted Russian airspace, then US president Ronald Reagan decided to open the GPS signals for civilian location tracking.
By mid 1980s, the US government had made GPS technology accessible to the common public around the world, but the price and availability of GPS receivers made it hard for common people to use the technology.
Structure of GPS
The GPS technology is comprised of three different layers namely the space segment, the control segment and the user segment.
Space segment
The space segment of GPS involves the satellite clusters which relay information back to the GPS receiver on the ground.
The GPS system at present consists of a constellation comprising of 31 satellites, orbiting earth at a height of about 22,000 km, 55 degrees inclined to the equator.
The arrangement of these satellites is staged in such a way that, in a given instant at least 6 satellites will be within the range of a GPS receiver anywhere on earth.
Control segment
The GPS not only comprises of satellite constellations up in the space. There are several ground stations around the globe in charge of tuning the atomic clock inside these satellites from time to time.
These ground relay stations also send regular almanac and ephemeris data to the satellites, about which I will be detailing in the coming sections.
User segment
This is the customer premise equipment or CPE which we use to receive the GPS signals from the satellites.
The CPE can be a dedicated sat-nav device, a smartphone with a dedicated GPS chip or even a smartwatch with GPS functionality.
Almanac and Ephemeris
Almanac and Ephemeris are the two data forms that GPS satellites in space use to transmit location data back to the receivers on the ground.
The almanac consists of rough orbital parameters of the satellites which are accessible by the receiver. The almanac data is just for the signal lock and it can’t provide tracking on its own without the ephemeris data.
Each satellite will have almanac data about itself and all other satellites in the constellation and this data gets updated once in a month or once in three months.
When you turn on the receiver, the first duty it performs is checking whether a valid almanac data is already available on the device.
If found, the device will further search for ephemeris data. If the almanac data is out of date, the device will then download new almanac data.
Ephemeris data comprises of precise orbital coordinates for the satellites and the clock correction data which syncs up the time on your receiver with that of the atomic clock on the satellite.
The Ephemeris data is very precise and updates once every 30 minutes in the GPS receiver.
Cold start
When a GPS receiver remains turned off for days and then turned back on again, it has to download the whole new ephemeris data. This is termed as a cold start.
Warm start
When the GPS receiver is periodically turned on and off and if this interval is less than 30 minutes, the device will lock on to the satellites quickly without having to wait for the data from the satellites, provided the receiver remains static at the time of requesting the connection.
Assisted GPS (A-GPS)
Assisted GPS is commonly found on GPS enabled smartphones.
When the weather is bad or if you are inside a building, it might be tough for the GPS chip to get the almanacs and ephemeris directly from the satellite, delaying the lock-in with them.
To solve this, the smartphones will collect almanac and ephemeris data over the internet for the GPS chips to get a faster fix and precise location.
Working Principle of GPS
The GPS constellation consists of 31 satellites placed at 20,000 km above earth, positioned at 55 degrees inclined to the equator.
This arrangement ensures that a GPS receiver will always be in the line of sight of at least four satellites at any given time.
These satellites transmit the almanac and ephemeris at regular intervals, which is then caught by the receiver.
With this data, the receiver calculates the time for the signal to reach from the satellite to the receiver.
Combining data from multiple satellites at once, the receiver can then triangulate the current position by a process called trilateration.
Operation Modes
The GPS, owned by the American government has two operational modes separately for civilian and military use.
The PPS (Precise Positioning System) reserved for military purposes have much better accuracy than the conventional SPS (Standard Positioning System) civilians uses.
Errors in GPS calculations
The GPS is not necessarily almost always accurate in location tracking.
Since it’s a long distance radio-based satellite navigation technology, the chance of errors creeping into the calculations still exists even though its very slim.
Some of the factors which decide the accuracy of the GPS are listed below:
Satellite atomic clock issues
The atomic clock on board the GPS satellite is very precise and has a rated time skip of just 1 second per 300 years.
These clocks are made using Rubidium and Caesium elements and have the possibility of malfunctioning if something unexpected happens.
There are even situations where atomic clocks on board the navigation satellites got stalled and the data from these became useless.
Troposphere and Ionosphere delays
The GPS satellite constellations are placed about 22,000 km above the earth and the signal has to pass through different layers of the atmosphere, slightly altering the time to reach the receiver.
These delays cause a GPS receiver to display the location off up to 2m.
Satellite geometry
High rise buildings and walls might block the satellite signals from reaching the target GPS receiver.
Similarly, the accuracy of the data reaching the receiver might vary from time to time and place to place and cause an error margin of up to 10m.
Human and Software errors
This is one of the most obvious types of errors occurring in position calculation by a GPS module.
GPS receivers might behave haywire when placed in the interference field like the presence of magnets and such.
The software running on the receiver too might contribute to the position anomaly by a significant margin.
Uses of GPS
The primary function of the GPS technology is location tracking but it is not limited to this alone.
Some of the areas where GPS technology is being effectively used are as follows:
- Tracking – tracking the current position of an object, person or a landmark.
- Mapping the path of travel – the transit from point A to point B can be marked on a map.
- Navigation – Plotting the path to reach easily between two places without having any previous familiarity with the place.
- Timing – GPS helps in calculating the time of travel between two points.
Alternatives to GPS
Since GPS was the first locating tracking technology that entered the masses, many people still think that it is the only system for location tracking. That isn’t the case.
GPS is one of the location tracking method in the world and it is developed by the American government. There are some other alternatives doing the same, but with varying levels of precision.
GLONASS is the Russian equivalent of GPS with an array of 24 satellites located 19,100 km from the Earth.
These Russian satellites are inclined at an angle of 64.5 degrees to the equator and orbits the earth once every 11.15 hours.
Beidou is the Chinese version of the GPS which is a work in progress mission.
At present, the Beidou system consists of 9 satellites in its constellation and plans to increase the number to 35 by the year 2020.
Galileo is the European version of the GPS with 18 operational satellites in orbit. The Galileo programme was started way back in 2011.
Galileo offers a paid high precision location tracking service for commercial use and a free service for civilian use albeit with less accuracy.
IRNSS is an ambitious project by the Indian government for building up an indigenously developed location tracking service.
At the time of Kargil war, America had blocked access for the Indian military to use the GPS. Then and there the policymakers in the Government of India think tank decided to build a system from ground up for future uninterrupted location-based services.
Running under the code name of NAVIC, IRNSS now has 7 operational satellites up in the sky which covers the entire Indian subcontinent and an area of 1,500 km around it.
There is a standard positioning system for civilian use as well as an encrypted high precision military positioning system for defence use.
The future plans of IRNSS mission state that the number of satellites will be increased to 24 and they will be eventually placed at a height of 24,000 km from Earth.
Bottom Line
GPS is an amazing feat of technology that impact our daily life. It feels almost unreal to know that a tiny chip inside our smartphone is making direct contact with the satellites up above in the sky for tracking our location in real time.
And with our own IRNSS nearing completion, we can expect the chips to support the service for a more localized and precise location service as well.
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