44:005: Foundations of GIS
Lab 8: Intro to Global Positioning System (GPS)
GPS refers to the Global Positioning System and the navigation receivers that make use of it.
Positioning System is a constellation of satellites that orbit the earth twice a day, transmitting precise time and
With a GPS receiver, users can determine their location anywhere on Earth.
The GPS is a
location system based on a constellation of about 30 satellites orbiting the Earth at altitudes of approximately
The basis of GPS technology is precise time and position information.
Using atomic clocks (accurate to
within one second every 70,000 years) and location data, each satellite continuously broadcasts the time and its
A GPS receiver receives these signals, listening to three or more satellites at once, to determine the
user's position on Earth.
By measuring the time interval between the transmission and the reception of a
satellite signal, the GPS receiver calculates the distance between the user and each satellite.
Using the distance
measurements of at least three satellites in an algorithm computation, the GPS receiver performs a triangulation
procedure to arrive at an accurate position fix.
Many would argue that GPS has found its greatest utility in the field of Geographic Information Systems
With some consideration for error, GPS can provide any point on Earth with a unique address (its
A GIS is basically a descriptive database of the Earth (or a specific part of the Earth).
tells you that you are at point X, Y, Z while GIS tells you that X, Y, Z is an oak tree, or a spot in a stream with a
pH level of 5.4.
GPS tells us the "where,” GIS tells us the "what."
GPS/GIS are reshaping the way we locate,
organize, analyze and map our resources.
This is why GPS is an important tool in geography and
Computing the Distance between Your Position and the GPS Satellites
GPS determines distance between a GPS satellite and a GPS receiver by measuring the amount of time it
takes a radio signal (the GPS signal) to travel from the satellite to the receiver.
Radio waves travel at the speed
of light, which is about 186,000 miles per second.
So, if the amount of time it takes for the signal to travel from
the satellite to the receiver is known, the distance from the satellite to the receiver
(distance = speed x time)
can be determined.
If the exact time when the signal was transmitted and the exact time when it was received
are known, the signal's travel time can be determined.
The GPS Error Budget
The GPS system has been designed to be as nearly accurate as possible.
However, there are still errors.
few significant sources of error include: interference from the ionosphere or troposphere, clock drift, satellite
geometry, or poor line of sight.
Added together, these errors can cause a deviation of
+/- 50 -100
the actual GPS receiver position.
Signals are not generally affected by clouds or bad weather.