How GPS Works, an Introduction to GPS
The Global Positioning System consists of a set of 24
operational satellites and land-based control stations. The system was designed for and is operated by the U. S. military, owned by the US Department of Defense (DoD). But it is available on a worldwide basis to all civil GPS users with no direct charge.
GPS determines distance between a GPS satellite and a GPS receiver by measuring the amount of time it takes a radio signal to travel from the satellite to the receiver
(figure 1). Radio waves travel at the speed of light, which is about 300,000 kilometer 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.
Fig. 1: Principle of the location estimation via GPS
In order to do this, the satellites and the receivers use very accurate clocks which are
synchronized so that they generate the same code at exactly the same time. The code received from the satellite can be compared with the code generated by the receiver. By comparing the
codes, the time difference between when the satellite generated the code and when the receiver generated the code can be determined. This interval is the travel time of the code.
Multiplying this travel time, in seconds, by 300,000 kilometer per second gives the distance from the receiver position to the satellite in miles.
Each satellite transmits a coded signal on two carrier frequencies that says, amongst other things, where it is in the sky. In its simplest form a GPS receiver receives the signal from the
GPS satellite and uses the code to determine its distance from the satellite. If the distance to four satellites can be measured at once then the receiver can calculate its own position in real
time. A position calculated in this way is accurate to better than 15 metres.
GPS System Segments:
The GPS consists of three major segments: SPACE, CONTROL and USER.
The space segment
consists of 24 satellites, each in its own orbit 20,200 kilometers above the Earth. 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 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 satellites in each, equally spaced (60 degrees apart). This constellation provides the user with between five and eight satelites visible from any point on the earth.
The control segment: The Control Segment consists of a system of tracking stations located
around the world (figure 2): the GPS Master Control station and several monitor stations.
The Master Control unit, located at Schriever Air Force Base in Colorado, is responsible for
overall management of the remote monitoring and transmission sites.
Fig. 2: Location of GPS monitor- and control stations
It measure signals from the NAVSTAR satelites and it calculates any position or clock errors for
each individual satellite based on information received from the monitor stations. The “new” corrected data are uploaded by the Master Control station. And at least the NAVSTAR satellites
send the new data over radio signals to the GPS receiver back to earth.
The 4 Monitor Stations are unmanned stations located around the world (Hawaii and Kwajalein in the Pacific Ocean; Diego Garcia in the Indian Ocean; Ascension Island in the
Atlantic Ocean. They track up to 11 satellites twice a day.
The user segment consists of GPS-receivers, located in cars, planes or even in GPS collars for wildlife. They can be as small as a mobile phone.
Differential GPS (DGPS)
The accuracy of GPS localizations averages 15 meters. This “inaccuracy” results from the purposeful interference of the GPS systems operator (DoD) with the satellite signals as well as
from systems error. Such systems errors include, for example, the inaccuracies built into the GPS receivers’ clocks as well as errors caused by deflection of electromagnetic signals
transmitted by the satellites when they enter the atmosphere (ionspheric error, troposphereic error).
With the aid of correctional methods it is possible to improve the accuracy of GPS determined
positions. This method is the Differential GPS method. Here correction signals are sent to DGPS receivers which then correct the position errors.
Two methods are used for correction: terrestrial and satellite supported DGPS:
In the terrestrial method
the correction of position error occurs via exactly surveyed ground stations, so called reference stations, which continually ascertain their position with the signals
from the GPS satellites, compare these signals to their actually known position, and from this calculate a corrected value. These corrected values can be transmitted via UKW or LW
transmitter to the mobile DGPS receivers at their interface (RTCM format). These in turn can then correct their own position coordinates. The accuracy obtained by this method ranges
between 1 – 3 meters, whereby the accuracy depends on the distance to the correctional data transmitter and the signal quality (Fig. 3).
Fig. 3: Principle of the differential GPS for the terrestrial method
Satellite supported methods such as the WAAS (Wide Area Augmentation System) or EGNOS
(European Geostationary Navigation Overlay System) send the corrected data via geostationary satellite and thus reach a greater reception area than the terrestrial methods.
DOP = Dilution of Precision is a value of probability for the geometrical effect on GPS accuracy.
Several external sources introduce errors into a GPS position estimated by a GPS receiver. One important factor in determining positional accuracy is the constellation, or geometry, of the
group of satellites from which signals are being received. DOP only depends on the position of the satellites: how many satellites you can see, how high they are in the sky, and the bearing
towards them. This is often refered to as the geometry. An indicator of the quality of the geometry of the satellite constellation is the Dilution of Precision or DOP. The computed position
can vary depending on which satellites are used for the measurement. Different satellite geometries can magnify or lessen the position error. A greater angle between the satellites
lowers the DOP, and provides a better measurement. A higher DOP indicates poor satellite geometry, and an inferior measurement cofiguration, or in other words: the lower the value the
greater the confidence in the solution.
Good Dilution of Precison
Poor Dilution of Precision
DOP is often divided up into components. These componets are used because the accuracy of the GPS system varies. The satellites move, so the geometry varies with time, but it is very predictable.
GDOP is computed from the geometric relationships between the receiver position and the positions of the satellites the receiver is using for navigation.GDOP Components are:
PDOP = Position Dilution of Precision (3-D), sometimes the Spherical DOP.
HDOP = Horizontal Dilution of Precision (Latitude, Longitude).
VDOP = Vertical Dilution of Precision (Height).
TDOP = Time Dilution of Precision (Time).
While each of these GDOP terms can be individually computed, they are formed from covariances and so are not independent of each other. A high TDOP (time dilution of precision),
for example, will cause receiver clock errors which will eventually result in increased position errors.
Good GDOP, a small value representing a large unit-vector-volume, results when angles from
receiver to satellites are different. Wheras poor GDOP, a large value representing a small unit vector-volume, results when angles from receiver to the set of SVs used are similar.
Clustered satellites give
poor GDOP values.
The higher the DOP, the weaker the geometry.
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