From aiding military intelligence for the US army in the 1970’s, to driving Intelligent Navigation Systems of today, the Global Positioning System (GPS) tracking technology has come a long way.
Today, It’s an all-pervasive technology that not only helps us find our way and locate missing vehicles and people, but also is an invaluable tool in disaster relief, law enforcement, agriculture, and environmental conservation.
Here is a step by step understanding on how GPS tracking works:
1.GPS is basically a Global Navigation Satellite System (GNSS), that uses a constellation of 27 satellites effectively covering the entire earth.
It was the first such system initiated by the United States Department of Defense, in the 1970’s. A number of other such systems now exist, notable among them being GLONASS (Russia), Galileo (EU), BeiDou (China), IRNSS (India) and QZSS (Japan).
2.The satellites orbit the earth at a height of over 20,000 kms, and are continuously beaming radio signals down to Earth. These signals contain information about the satellite’s location and orbit parameters, its current status, and a time stamp based on high precision atomic clocks on-board.
3.The signal frequency is approximately 1.5GHz, essentially higher than FM radio, but lower than microwave frequencies. The signals travel at the speed of light, but by the time they pass through layers of the earth’s atmosphere and reach the ground, they are very weak, and have to be effectively ‘picked up’ by the user device.
4.The GPS user device receives signals from multiple satellites, and, for each satellite, ascertains the time of transmission. Next, based on its own internal clock, it deduces the time taken for the signal to reach it.
5.Since the location/ orbit parameters of the satellites are known, the distance between a satellite and user device is calculated using the equation: Distance = Speed x Time taken. However, for an accurate estimation, one must have at least 4 navigational satellites in the line of sight of the receiver / user equipment.
6.The user location can now be calculated using a mathematical technique called Trilateration. For each satellite, a distance range can be calculated around the satellite in the form of a sphere, and the receiver could be located anywhere on the outside border of the sphere.
This process is then repeated for the other three satellites, and location can be determined based on the intersection of these spheres.
Generally, 2 spheres would intersect on a circle, and a third sphere would intersect them on 2 points, only one of which would be on the Earth’s surface.
7.For an accurate estimation, the fourth reading is taken to mitigate the time difference between the high precision atomic clocks of the satellites and the not- so-precise clocks used by equipment on the earth.
This process should ideally give us an estimation of location with an error range of about 15 meters.
8.Error Sources: There are potentially a host of factors affecting accuracy, that needs to be understood and addressed as far as possible, especially for activities and businesses that needs a higher level of accuracy.
a.Propagation: Signals get delayed due to air and weather conditions affecting density in the Ionosphere and Stratosphere.
b.Multipath: Signals bounce off man made structures like buildings or topographic occurrences such as hills and forest-covers, which also cause delays.
c.Ephemeris/ Orbital errors: Orbital paths tends to vary slightly over time, due to earth’s gravitational pull, and solar pressure. This error can be corrected by ground stations that constantly track the satellites.
d.Receiver Noise: Internal resistance of the receivers hinders the signal reception. High quality receivers, though expensive, have lower resistance and can be calibrated to pick up very low signals.
e.Relativity Errors: These are errors based on Einstein’s special theory of relativity, and general theory of relativity that essentially say that time is relative and passes slower for faster moving objects; and that space-time is warped around mass and expands near a large mass.
Taken together, this means satellite clocks run faster by 38 microseconds per day. Hence, the atomic clocks though incredibly accurate, have to be readjusted to counter these effects.
9.Enhanced GPS Technologies: Many such technologies are being used to reduce errors and greatly improve the accuracy:
a.Multi frequency receivers: Since higher frequency signals can travel faster through the atmosphere even in adverse weather conditions, using multiple frequencies can mitigate propagation error.
b.Precise Point Positioning: This involves the usage of globally available corrections received from a network of known and referenced base receivers, to achieve very high levels of accuracy.
c.Real Time Kinematic: Instead of using the signal data, this technology focuses on measuring the phase of the carrier wave of the signal.
The range to a satellite is calculated by multiplying the carrier wavelength by the number of whole cycles upto the satellite, and the error in estimation is mitigated by collected phase measurements from a fixed pre-surveyed base station, or a number of such stations.
d.Satellite based Augmentation systems: Typically, these systems monitor signal strength of satellites, send out alerts, calculate corrections and relay them over a wide area via satellite or cellular technologies. Examples of such systems are SBAS, WAAS, GAGAN, GBAS, MSAS etc.
10.Integrated Technologies: Nowadays GPS technology is increasingly integrated with other software and hardware to provide more comprehensible solutions for businesses in various industries.
Notable among these are GPS Vehicle Tracking Systems, that use specialized tracking devices, global mapping technology as well as fleet management software to track vehicle locations, plus a whole lot more: speed, fuel amount, engine temperature, tire pressure, battery status, cumulative idling, trigger events such as ignition on/ off or door open/ closed etc., to name a few.
GPS Vehicle tracking devices can be “passive” or “active”.
Passive devices will store location and vehicle data, which can later be retrieved by downloading the data for analysis.
Active devices collect the same data but transmit it in real time to a computer or data center over a cellular or satellite network.
