In this specific article, we are going to learn about the working of GPS. In the upcoming articles, we will be dealing with the interfacing of the GPS module with the Arduino. I feel that it’s important to know the working of the module which we use in our project instead of just learning about ‘how to make the module throw its data values at us”
GPS stands for Global Positioning System and is mainly used to locate the exact location of the receiver with the help of data which it gives (out of which most important is Longitude and Latitude coordinates)
How does GPS locate a position?
The working of the GPS module is based on the communication between the satellites and the GPS receiver module. For locating the position of a place on earth, we need several parameters like Longitude and Latitude (2-D) and an extra Altitude (for 3-D)
To locate the position, in GPS, we have something known as Trilateration.
In Two-Dimensional Space:
we need a total of 2 satellites (say S1 and S2 here). We are suppose located at point O and we need our location through GPS.
Text version of the above flowchart
Step-1: The distance between the satellite and the GPS receiver (referred as O) is to be calculated. So we get that O is at a distance of d1 from S1.
Step 2: But still, O can be anywhere on the circle-1 with center S1 and radius as d1.
Step 3: Also, the same thing goes with S2, i.e., O can be anywhere on circle 2 as well.
This implies that, O is somewhere on the region common to circle-1 and circle-2. This means it lies in the points of intersection of circle 1 & 2 (here O and P).
To decide between O and P, we take into account the circle-3 which is the earth surface itself. All the 3 circles intersect at O (therefore, P is eliminated) and hence we obtain the position of a GPS receiver in 2-D.
In Three-Dimensional Space:
We need a total of 3 satellites for locating position of GPS receiver
Here, we need to consider just the spheres instead of circle.
For quick overview,
Text version of the above flowchart
Step 1: We get 2 spheres of radius d1 and d2. The intersection gives a circle (say C-1). So ‘O’ must lie on the C-1.
Step 2: Now, we have a third satellite, S3; it measures the distance from O as d3. This implies that O lies somewhere on the sphere (of radius d3).
Step 3: So O lies on C-1 as well as on the sphere of radius d3. This means O must be one of the two points of intersection between the sphere of radius d3 and the circle C-1.
Step 4: For the final answer, we take the fourth sphere – Earth itself. The intersection removes the ambiguity and gives the final point as ‘O’,
which is the actual location of the GPS receiver.
But, there exists a problem of time delay, since the satellites have accurate atomic clocks while the GPS receivers uses the clocks which are installed in mobile phones.
But since all the satellites use the same specifications for atomic clock, the ‘time offset’ is the same. Even error of microseconds can give an error in kilometers !! Hence we use fourth satellite (S4).
As we discussed earlier, that we need the distance ‘d’ for locating the positions of satellites.
How do We Exactly Determine ‘d’?
The ‘radio signal’ which is sent, it carries 2 information:
- Exact time when it was transmitted (t1)
- Position of satellite
Now the receiver receives the signal at time t2 (say) :
where c is the speed of light
What happens inside the GPS receiver
Components involved in GPS (based on the above flowchart):
Antenna: Receives the signal
Filter: It removes the extra signals that are not needed and only keeps the one that has the GPS-related information.
Decoder: Takes out the information from the signal
Output Display: Displays the position on the device
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