GPS Classroom
What is GPS: Global Position System (GPS) is developed and operated by the Department of Defense (DOD) of the United States, on which the accuracy and maintenance of this system fully depends. Any change made by the authorities might influence the accuracy and performance of the GPS equipments.
How GPS works: GPS provides satellite signals which are specially coded for the computation in a GPS receiver to produce the position, velocity and time. Usually it requires four GPS satellite signals to computer correctly the position in three dimensions and the offset time of the GPS receiver’s clock.
Limitations on GPS reception: Initial or any use after a longer interval over four hours takes a few minutes for a successful location. Any obstruction above or around the receiver, such as high buildings in the neighborhood, or bad reception location, such as in a tunnel or in the building, will influence the time needed for a successful GPS location.
Cold Start: Cold start of the GPS device refers to the state of the tracker when time and position are known to within some limits, the almanac known, and the ephemeris unknown.
Example: If the GPS device has been off for a few hours, and the ephemeris data is known for at least three satellites, the start up will be a warm start and fix the positional in 10 - 20 seconds. Therefore, if ephemeris data for only 2 or less satellites is known it's a cold start and acquisition will take as much as a few minutes.
Warm Start: Warm start of the GPS device refers to the state of the device when time and position are known to within some limits, the almanac known, and at least 3 satellite ephemeris are known from previous operation.
Example: If the GPS device has been off for only a few minutes, the ephemeris data for all the satellites will be known and therefore the GPS device will fix the positional in a matter of seconds.
Note: The almanac data is an estimated (computed) data and can be valid for months while the ephemeris is only valid for 3 - 6 hours.
Note: Actual acquisition time depends on the terrain and satellite coverage.