GPS or Global Positioning System is one of the five satellite-based navigation systems currently being used. US Department of Defence started developing the technology in 1973 for military use, which became fully functional in 1993.
GPS is a 27 slot constellation of satellites, out of which 24 are active at once — with three in reserve to replace any of the active ones in case of failure — working and placed in six orbital planes.
The satellites placed in these six equally spaced orbital planes have four slots each. The four slots are occupied by baseline satellites. This placement allows the user to view a minimum of four satellites at any point on Earth. Further, this placement allows pinpointing the location accurately.
The satellites orbit the Earth, along a nearly circular trajectory, in 11 hours and 58 minutes which is half of a sidereal day. The satellites revolve in a Medium Earth Orbit (MEO) at a distance of approximately 20,200 km. A Master Station near Colorado Springs, Colorado, manages the whole system. Similarly, six other monitoring stations are present at the following locations.
- Ascension Island
- Diego Garcia
- Cape Canaveral
- Colorado Springs
Most of the monitoring stations are not crewed and are situated near the equator. The stations forward the pseudo-range measurements to the main station at Colorado. Pseudo-range is the pseudo distance between the satellite and the receiver. The Master Station, in turn, corrects the data and sends it back to the satellites.
Other navigation systems are as follows.
- GLONASS (Globalnaya Navigazionnaya Sputnikovaya Sistema), developed by Russia.
- IRNSS (Indian Regional Navigation Satellite System), developed by India.
- BDS (BeiDou Navigation Satellite System), developed by China.
- Galileo, developed by the European Union.
While GPS and GLONASS are global systems, the rest are area-specific.
How a GPS works: Trilateration
GPS receivers require at least four satellites to pinpoint the location. While one satellite is required to correct the time on the receiver and align it with the time of the atomic clock onboard the satellite, the other three satellites are needed to locate the position of the receiver.
The satellites send data to the receivers at the speed of light. The receivers can be our mobile phones, devices used in vehicles or any other GPS-enabled device. The data contains a timestamp and the satellite position
A receiver compares the time difference between the signal sent and the signal received. This gives the location of the satellite. The difference is then multiplied with the speed of light to get the distance between the receiver and satellite.
The receiver gets a vague idea about its location, which is on the outer border of the sphere. The radius of the sphere is the distance between the satellite and the receiver.
Similarly, the other two satellites also form a sphere. A common meeting point of all spheres gives us the exact location of the receiver.
Four factors that affect GPS connection
The Ionosphere is the region of the upper atmosphere present at altitudes between 60 to 1000km. It comprises Thermosphere and parts of Mesosphere as well as the Exosphere. The Solar Radiation ionises the atoms and molecules in this layer, and a layer of free electrons is formed.
The free electrons cause the electromagnetic waves to deflect, which can cause an error in the measurement of the location. The error depends on the location of the satellite. Closer the satellite, smaller the error.
The atomic clocks are exact and thus even a minute error of 1 nanosecond in the clocks can lead to a variation of 1 foot in the distance. The atomic clocks can slow up to a millisecond at a time. The monitoring station at Colorado corrects these errors.
The lower atmosphere comprises of Troposphere, Tropopause and the Stratosphere. Signals relayed by satellites closer to the horizon have to cover more of the lower atmosphere and thus can cause an error.
A multipath error occurs when apart from the satellite signals, many other signals are picked up by the receivers. This type of error is common in the hills, forests, and narrow lanes of cities.
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