GPS
The Global Positioning System (GPS), originally
Navstar GPS,
A satellite-based radio navigation system owned by the United States government and
operated by the United States Space Force.
It is one of the global navigation satellite systems (GNSS) that provides geo-
location and time information to a GPS receiver anywhere on or near the Earth where
there is an unobstructed line of sight to four or more GPS satellites.
Obstacles such as mountains and buildings can block the relatively weak GPS signals.
How GPS works?
GPS (Global Positioning System) is a satellite-based navigation system. It provides time
and location-based information to a GPS receiver, located anywhere on or near the earth surface.
GPS works in all weather conditions, provided there is an unobstructed line of sight
communication with 4 or more GPS satellites. GPS is managed by the US Air Force.
A GPS operates independently of the user’s internet connection or telephone signal.
However, their presence increases the effectiveness of GPS positioning. GPS was initially
developed by the US government for military purpose, but currently, anyone with a GPS receiver
can receive radio signals from GPS satellites.
Note:
Initially when GPS was developed for military use, there were 24 GPS satellites orbiting the
earth every 12 hours at a height of 20, 180 km.
4 GPS satellites were located in each of the 6 orbits with 60 degree orientation between each
other. These orbital planes do not rotate with respect to any star.
Localization of any GPS receiver is done through time of flight measurement.
The greater the number of satellite in line of sight to a GPS receiver, the greater is the
accuracy in determining the position of the receiver.
How GPS works?
Any instant of time, there are at least 4 GPS satellites in line of sight to a receiver on the
earth. Each of these GPS satellites sends information about its position and the current time to
the GPS receiver at fixed regular instants of time. This information is transmitted to the receiver
in the form of signal which is then intercepted by the receiver devices. These signals are radio
signals that travel with the speed of light. The distance between a GPS receiver and the satellite
is calculated by finding the difference between the time the signal was sent from GPS satellite
and the time the GPS receiver received the signal.
Once the receiver receives the signal from at least three satellites, the receiver then points
its location using trilateration process. A GPS requires at least 3 satellites to calculate 2-D
position (latitude and longitude on a map). In this case, the GPS receiver assumes that it is
located at mean sea level. However, it requires at least 4 satellites to find receivers 3-D position
(latitude, longitude, and altitude).
What is trilateration?
Trilateration is the process of determining your position based on the intersection of
spheres. When a receiver receives a signal from one of the satellite, it calculates its distance from
the satellite considering a 3-D sphere with the satellite located at the center of the sphere. Once
the receiver does the same with 3 other GPS satellites, the receiver then proceeds to find the
intersection point of the 3 spheres to calculate it’s location.
Once the position of a receiver is calculated, the GPS device can then easily calculate:
Time of sunrise and sunset
Speed
Track
distance to destination of the GPS receiver.
Technical challenges face by GPS:
Time synchronization between individual satellites and the GPS receiver
Real time update of the exact location of the GPS satellite
Precise measurement of time of flight
Interference with other signals
Time synchronization:
Each of the GPS satellites are equipped with an atomic clock to keep the time updated and
accurate. In order to update and provide precise timing on the receiver, the receiver uses the
fourth GPS satellite to keeps it’s timing accurate. With the timing with the receiver and GPS
satellites maintained by atomic clocks, the exact time difference can be calculated by the
receiver.
The GPS satellites are constantly managed from ground-based stations to resolve time
synchronization.
Significance of ultra precision of time synchronization:
Electromagnetic radiation propagate with the speed of light. The accuracy in position is
directly proportional to the time measurement, as speed * time = distance, therefore, even a
slight deviation in time calculation can create huge difference in distance due to large speed
of light.
Real time update of exact location of the satellite.
This is done by monitoring the satellites from a number of widely distributed ground based
system.
Master station analyses all the measurements and transmits the actual position of each
satellite.
Differential GPS [DGPS]
DGPS is an improvement over GPS that provides enhanced location accuracy
Working:
The GPS receiver also called as the base station must be set up at a precisely known location
The base station receiver calculates it’s position based on the satellite signals and compares
it’s location to the known location
The difference between the two locations is applied to the data recorded by the GPS receiver
Provides position accuracy in sub-meter to cm range
DGPS uses a network of fixed ground-based stations to broadcast the difference between the
positions indicated by the GPS satellite systems and the known fixed positions. These stations
broadcast the difference between the measured satellite pseudo-ranges and actual pseudo-ranges
and the receiver stations may correct their pseudo-ranges by the same amount.
GNSS:
Global Navigation Satellite System is a general term describing any satellite
constellation that provides positioning, navigation, and timing (PNT)
Errors in GPS
As GPS signals travel down to the Earth from space, the layers of the atmosphere
refracts and slightly delays the signals, particularly within the ionosphere. This delay
interferes with the range solutions from the GPS receiver on the ground to the satellite,
resulting in positional errors of several meters.
Satellite signal blockage due to buildings, bridges, trees, etc. Indoor or underground use.
Regular GPS cannot pinpoint locations to greater than 3-m accuracy
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