First, let�s understand what Global Positioning Systems are - A GPS system is essentially a receiver that
measures the incoming phase of satellite signals.
Now, there is a network of about 30 satellites orbiting the Earth at an altitude of 20,000 km.
Wherever you are on the planet, at least four GPS satellites are �visible� at any time.
That is you are in direct line of sight of at least 4 of those satellites wherever you are on the globe
Now each satellite is continuously transmitting information about its position and the current time.
These signals, travelling at the speed of light, are intercepted by your GPS receiver, which calculates how far
away each satellite is based on how long it took for the messages to arrive.
Once it has information on how far away at least three satellites are, your GPS receiver can pinpoint
your location using a process called trilateration.
Let's see how it works.
Imagine you are standing on a flat surface and there is a satellite at some distance resting on that flat surface.
If you know how far you are from that satellite, you will know you will be anywhere in this circle.
Now consider this-- 2 circles can meet only at one point or maximum at 2 points.
They cannot meet at more than 2 points.
Similarly 3 circles can meet only at one single point in a plane
There will not be any other point where all the three intersect.
Now consider that there are 3 satellites that are resting on that surface at various distances away.
If you do the same for the other two satellites, you can work out your location by seeing where the three circles intersect
This is your accurate positionm that is determined by 3 satellites.
This is just what your GPS receiver does!
Now we live on our globe and our world is in 3 dimensions.
So similar to circles, ... in 3 dimensions
if you know the distance to the satellite, you would know that you are somewhere on the surface of the sphere.
If there are 4 satellites, we can easily find your position accurately where the 4 spheres intersect.
However as the satellite signals propagate from satellites to receivers they pass and are affected by the atmosphere.
The atmosphere layers that influence the incoming signal are the ionosphere and troposphere
Disturbances and transmission delays cause degradation in the accuracy of the observations.
The precision also depends on factors like the number of satellites tracked, satellite geometry, observation
time, ephemeris accuracy,
multi path, resolved ambiguities, etc
Obstruction from objects like trees and buildings can make it impossible to get accurate outputs from GPS systems.
Hence, for the purpose of acquiring survey grade accuracies, a differential method of observation is done
The most basic way to do this is to observe a short baseline with two GPS receivers.
Since the factors affecting the precision of the two receivers are almost identical,
they can be subtracted out to get a more accurate reading of the coordinates.
The shorter the baseline is, the more the effects will be removed,
as the atmosphere through which the signal passes to the two receivers will be almost identical.
Another more sophisticated and faster differential GPS system is called Real Time Kinematics (RTK).
This uses a base station and a rover receiver. The base station is stationary and the rover
moves across different points.
Real time kinematics data collection uses differential GPS corrections broadcast by a base
station to solve for coordinates at a rover receiver in real time. The real time transmission of data between the base
station aand rover receivers is via radio communication.
The accuracy of the coordinate measurements depends on the number of satellites in view, resolved
ambiguities, satellite geometry, etc
RTK mode for geodetic measurements is a very fast method for surveying and results are available immediately.
There is no need for additional data processing afterwards since corrections are made from the base
station during the measurement through radio communication.
An alternative to RTK is Post Process Kinematics (PPK)
This method of GCP capture is similar to RTK except for one critical aspect -
there is no communication between the base station and the rover receiver.
The correction in the rover receiver coordinates is done using different GPS post processing tools on your PC
There are several advantages that a PPK system has over an RTK system.
One advantage in is that there is no dependence on communication between the base station and the rover.
This allows for the rover to travel to greater distances away from the base station without having to worry about
radio communication links.
Another advantage is that you can process data with different processing algorithms to check which
one is most accurate.
And finally, in PPK the processing is more thorough as it traces back and forth through the data
multiple times to give more comprehensive results.
GPS systems based on RTK and PPK methods of capture can reach several centimeters accuracy depending
on the equipment, the area, and the country.