GPS, which stands for Global Positioning System, is the only system today able to show you your exact position on the Earth anytime, in any weather, anywhere.
The three parts of GPS are:
Satellites
Receivers
Software
Satellites
There are quite a number of satellites out there in space. They are used for a wide range of purposes: satellite TV, cellular phones, military purposes and etc. Satellites can also be used by GPS receivers.
GPS Satellites
The GPS Operational Constellation consists of 24 satellites that orbit the Earth in very precise orbits twice a day. GPS satellites emit continuous navigation signals.
Receivers and Satellites
GPS units are made to communicate with GPS satellites (which have a much better view of the Earth) to find out exactly where they are on the global scale of things.
GPS Signals
Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant.
Physically the signal is just a complicated digital code, or in other words, a complicated sequence of “on” and “off” pulses.
Time Difference
The GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is.
Calculating Distance
Velocity x Time = Distance
Radio waves travel at the speed of light, roughly 186,000 miles per second (mps)
If it took 0.06 seconds to receive a signal transmitted by a satellite floating directly overhead, use this formula to find your distance from the satellite.
186,000 mps x 0.06 seconds = 11,160 miles
Line of Sight Transmissions
Line of sight is the ability to draw a straight line between two objects without any other objects getting in the way. GPS transmission are line-of-sight transmissions.
Obstructions such as trees, buildings, or natural formations may prevent clear line of sight.
Light Refraction
Sometimes the GPS signal from the satellite doesn’t follow a straight line.
Refraction is the bending of light as it travels through one media to another.
Signal Refraction
Signals from satellites can be like light. When they hit some interference (air patterns in the atmosphere, uneven geography, etc.) they sometimes bend a little.
Signal Interference
Sometimes the signals bounce off things before they hit the receivers.
Satellite Distribution
When the satellites are all in the same part of the sky, readings will be less accurate.
PDOP
All of this combines to make the signal less accurate, and gives it what we call a high “PDOP.”
PDOP = Positional Dilution of Precision
11,000 miles
11,000 miles
11,000 miles
11,000 miles
A PDOP of <4 is excellent
A PDOP of 4-8 is good
A PDOP of >8 is poor
Differential Correction
Differential correction is a technique that greatly increases the accuracy of the collected GPS data. It involves using a receiver at a known location - the "base station“- and comparing that data with GPS positions collected from unknown locations with "roving receivers."
ISU Base Station - http://134.50.65.125/
Postprocessing / Real-time
Before
After
Latitude and Longitude
Latitude and Longitude are spherical coordinates on the surface of the earth. Latitude is measured North or South of the Equator. Longitude is measured East or West of Greenwich. GPS uses Latitudes and Longitudes to reference locations.
Waypoints
Waypoints are locations or landmarks that can be stored in your GPS. Waypoints may be defined and stored in the unit manually by inputting latitude and longitude from a map or other reference.
Latitude and Longitude
Your location
Direction of waypoint
Date and
Time
Waypoint
Or more usually, waypoints may be entered directly by taking a reading with the unit at the location itself, giving it a name, and then saving the point.
Data Dictionary
GPS units collect data in:
Points
Lines
Areas
These are called features.
A data dictionary is a means by which we collect specific information about a data feature.
