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World Geographic Reference System (GEOREF)
- The World Geographic Reference System is used for aircraft navigation.
- GEOREF is based on latitude and longitude.
- The globe is divided into twelve bands of latitude and twenty-four zones of longitude, each 15 degrees in extent.
World Geographic Reference System Index
- These 15 degree areas are further divided into one degree units identified by 15 characters.
- Two numeric characters designate the integer number of minutes of longitude east of the one degree quadrangle boundary longitude.
- Two additional numeric characters designate the number of minutes of latitude north of the one degree quadrangle boundary latitude.
GEOREF Example
NAD-83 Lattitude N 30:16:28.82 Longitude W 97:44:25.19
World Geographic Reference System
FJHA1516
- The World Geographic Reference System can be extended to refer to larger areas of operation.
- A larger East-West area can be designated by adding an "S" and the number of of nautical miles to the east and west sides of the referenced point.
- A larger north-south area can be designated by adding an "X" and the number of nautical miles to the north and south.
- A circular area can be designated by adding an "R" and the radius of the circle in nautical miles.
- An altitude zone can be defined by adding an "H" and a value of altitude. The number of digits indicates the precision of the value. Five digits implies units in feet. Four digits implies tens of feet, three digits, hundreds of feet, and two digits, thousands of feet.
GPS is a Satellite Navigation System
- GPS is funded by and controlled by the U. S. Department of Defense (DOD). While there are many thousands of civil users of GPS world-wide, the system was designed for and is operated by the U. S. military.
- GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time.
- Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock.
Position and Time from Four GPS Satellite Signals
Space Segment
- The Space Segment of the system consists of the GPS satellites. These space vehicles (SVs) send radio signals from space.
- The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 hours. There are often more than 24 operational satellites as new ones are launched to replace older satellites. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight SVs visible from any point on the earth.
GPS Constellation
GPS Satellites and Ground Tracks
GPS Nominal Orbit Planes
Control Segment
The Control Segment consists of a system of tracking stations
located around the world.
GPS Master Control and Monitor
Network
The Master Control facility is located at Schriever Air Force Base (formerly
Falcon AFB) in Colorado. These monitor stations measure signals from the
SVs which are incorporated into orbital models for each satellites. The
models compute precise orbital data (ephemeris) and SV clock corrections
for each satellite. The Master Control station uploads ephemeris and clock
data to the SVs. The SVs then send subsets of the orbital ephemeris data
to GPS receivers over radio signals.
GPS Control Monitor
User Segment
The GPS User Segment consists of the GPS receivers and the
user community. GPS receivers convert SV signals into position, velocity,
and time estimates. Four satellites are required to compute the four dimensions
of X, Y, Z (position) and Time. GPS receivers are used for navigation, positioning,
time dissemination, and other research.
Navigation in three dimensions is the primary function of GPS. Navigation
receivers are made for aircraft, ships, ground vehicles, and for hand carrying
by individuals.
GPS Navigation
Precise positioning is possible using GPS receivers at reference locations
providing corrections and relative positioning data for remote receivers.
Surveying, geodetic control, and plate tectonic studies are examples.
Time and frequency dissemination, based on the precise clocks on board the
SVs and controlled by the monitor stations, is another use for GPS. Astronomical
observatories, telecommunications facilities, and laboratory standards can
be set to precise time signals or controlled to accurate frequencies by
special purpose GPS receivers.
Research projects have used GPS signals to measure atmospheric parameters.
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