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Global Positioning System - An overview
Introduction
GPS is a satellite-based navigation system operated by the US Department of Defence (DOD). The present NAVigation System with Timing And Ranging (NAVSTAR) GPS was conceived as a ranging system, from known positions of satellites in space to unknown positions on land, sea, in air and space. It is 24 hour, all weather, space based navigation system to accurately determine position, velocity and time in a common reference frame, anywhere on or near the earth on a continuous basis. In the recent times, apart from NAVSTAR GPS satellites, other GPS satellite system, viz., Global Navigation Satellite System (GLONASS) by Russia Federation, Ministry of Defence and GALELLIO by the European community, has also been placed. In the present text, GPS satellite means the NAVSTAR satellites, unless otherwise specified. The satellites or Space Vehicles (SVs) emit signals that can be tracked by receivers for positioning and navigational purposes. The positioning accuracy of GPS ranges from ±100m (using pseudoranges (PR) derived from code measurements) to a few mm (with measurement of carrier phase PR), depending upon the type of receivers, type of surveying method and techniques in post-processing of data. The position computed is referenced to a mathematical ellipsoid, the WGS-84. The GPS system consists of three major segments viz., space segment, control segment and user segment.
Space Segment
Space segment consits of the all weather global system of 24 satellites, orbiting the earth every 12 hours, in six orbital planes, at an altitude of 20,200km inclined at 550 to the equator in a sun-synchronous orbit. Figure-1 shows the nominal constellation of satellites. There are often more than 24 operational satellites as new ones are launched to replace older satellites. 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). The satellites are orientated in such a way that from any place on the earth, at any time, at least four SVs are available for navigational purposes.
fig. 1
Control Segment
Control segment consists of a group of four ground based monitor stations, three upload stations and a master control station. The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado. Monitor station tracks the satellite continuously and provides data to the master control station. The monitor stations measure signals from the SVs, which are incorporated into orbital models for each satellites. The master control station calculates satellite ephemeris and clock correction coefficients and forwards them to an upload station. The upload stations transmit the data to each satellite at least once a day. The SVs then send subsets of the orbital ephemeris to GPS receivers over radio signals
User Segment
GPS errors are a combination of noise, bias, blunders.
fig. 2: Three interacting components of GPS
Calculating Locations
A GPS receiver determines its position by using the signals that it observes from different satellites. Since the receiver must solve for its position (X,Y,Z) and the clock error (x), four SVs are required to solve receiver's position using the following four equations:
R12 = (X - X1)2 + (Y - y1)2 + (Z - z1)2 + x2
R22 = (X - X2)2 + (Y - y2)2 + (Z - z2)2 + x2 (3)
R32 = (X - X3)2 + (Y - y3)2 + (Z - z3)2 + x2
R42 = (X - X4)2 + (Y - y4)2 + (Z - z4)2 + x2
where (x1,y1) (x2,y2) (x3,y3) and (x4, y4) stand for the location of satellites and R1, R2, R3, R4 are the distances of satellites from the receiver position (Figure-3). Hence solving the four equations for four unknowns X,Y, Z and x, the position or location of the station is calculated.
Differential GPS
In order to achieve on-line positioning with high accuracies, Differential GPS (DGPS) is used. Differential positioning user the point position derived from satellite signals and applies correction to that position. These corrections, difference of determined position and the known position, are generated by a reference receiver, whose position is known and is fed to the instrument, and are used by the second receiver to correct its internally generated position. This is known as Differential GPS (Figure-4). It is assumed that the two receivers suffer from approximately the same magnitude of geometry and timing errors and the most of the common errors cancel out using this correction technique. To remove Selective Availability (SA) and other bias errors, differential corrections are computed at the reference station and applied at the remote receiver at an update rate that is less than the correlation time of SA, which is usually less than twenty seconds. The differential positioning accuracy is of order of 1-5 m.
fig. 3: Calculating locations using GPS
Surveying Techniques
There are different surveying techniques, depending upon the application and accuracy requirements. A few of the surveying techniques are described below:
Standard Static
It involves setting up two receivers, one at a reference point and the other on the station to be determined, and observing them simultaneously for at least 1 hr during a single survey session.
Dynamic Observation Techniques
Dynamic surveying implies some sort of motion. It allows one or more receiver(s) to move during the survey session to collect more point or baselines (the distance between two GPS sets) while other receiver is kept stationary at a base station. The basic difference between various dynamic techniques is that how quickly one can move from one point to the other. The dynamic techniques are further classified as,
The baseline processor utilises the precise pseudoranges and carrier-phase observable to resolve the baseline more efficiently than static GPS processing and more reliably than kinematic.
Kinematic
It provides the highest potential productivity. It speeds up the data collection portion of survey but there are few restrictions. Main restriction is that, during a survey, both receivers must maintain lock on the same satellites all the times. There must be continuous tracking of at least four satellites. If either receiver drops below four satellites, the rover must be returned to a previously surveyed point or some other point of known position. It must be initialised from a known point. It is valid for small area(radius<10km).
fig. 4: Differential GPS
Uses of GPS GPS receivers are used for navigation, positioning, time dissemination, and other research.
GPS is a satellite-based navigation system operated by the US Department of Defence (DOD). The present NAVigation System with Timing And Ranging (NAVSTAR) GPS was conceived as a ranging system, from known positions of satellites in space to unknown positions on land, sea, in air and space. It is 24 hour, all weather, space based navigation system to accurately determine position, velocity and time in a common reference frame, anywhere on or near the earth on a continuous basis. In the recent times, apart from NAVSTAR GPS satellites, other GPS satellite system, viz., Global Navigation Satellite System (GLONASS) by Russia Federation, Ministry of Defence and GALELLIO by the European community, has also been placed. In the present text, GPS satellite means the NAVSTAR satellites, unless otherwise specified. The satellites or Space Vehicles (SVs) emit signals that can be tracked by receivers for positioning and navigational purposes. The positioning accuracy of GPS ranges from ±100m (using pseudoranges (PR) derived from code measurements) to a few mm (with measurement of carrier phase PR), depending upon the type of receivers, type of surveying method and techniques in post-processing of data. The position computed is referenced to a mathematical ellipsoid, the WGS-84. The GPS system consists of three major segments viz., space segment, control segment and user segment.
Space Segment
Space segment consits of the all weather global system of 24 satellites, orbiting the earth every 12 hours, in six orbital planes, at an altitude of 20,200km inclined at 550 to the equator in a sun-synchronous orbit. Figure-1 shows the nominal constellation of satellites. There are often more than 24 operational satellites as new ones are launched to replace older satellites. 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). The satellites are orientated in such a way that from any place on the earth, at any time, at least four SVs are available for navigational purposes.
fig. 1
Control Segment
Control segment consists of a group of four ground based monitor stations, three upload stations and a master control station. The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado. Monitor station tracks the satellite continuously and provides data to the master control station. The monitor stations measure signals from the SVs, which are incorporated into orbital models for each satellites. The master control station calculates satellite ephemeris and clock correction coefficients and forwards them to an upload station. The upload stations transmit the data to each satellite at least once a day. The SVs then send subsets of the orbital ephemeris to GPS receivers over radio signals
User Segment
- GPS User Segment consists of the GPS receivers and the user community. GPS receivers convert SV signals into position, velocity, and time estimates. A minimum of four satellites are required to compute the four dimensions of X, Y, Z (position) and Time.
GPS errors are a combination of noise, bias, blunders.
- Noise errors are the combined effect of code noise (around 1 meter) and noise within the receiver noise (around 1 meter). Bias errors result from Selective Availability and other factors
- Selective Availability (SA): SA is the intentional degradation of the GPS signals by a time varying bias. SA is controlled by the DOD to limit accuracy for non-U. S. military and government users. However, SA has been removed as on 2nd May, 2000 and this has increased the locational accuracy by 10 times.
- Other Bias Error sources include SV clock errors, errors due to atmospheric effects
- Blunders can result in errors of hundred of kilometers and can be the cause of control segment mistakes, human mistake or receiver errors from software or hardware failures can cause blunder errors of any size.
Noise and bias errors combine, resulting in typical ranging errors of around fifteen meters for each satellite used in the position solution.
fig. 2: Three interacting components of GPS
Calculating Locations
A GPS receiver determines its position by using the signals that it observes from different satellites. Since the receiver must solve for its position (X,Y,Z) and the clock error (x), four SVs are required to solve receiver's position using the following four equations:
R12 = (X - X1)2 + (Y - y1)2 + (Z - z1)2 + x2
R22 = (X - X2)2 + (Y - y2)2 + (Z - z2)2 + x2 (3)
R32 = (X - X3)2 + (Y - y3)2 + (Z - z3)2 + x2
R42 = (X - X4)2 + (Y - y4)2 + (Z - z4)2 + x2
where (x1,y1) (x2,y2) (x3,y3) and (x4, y4) stand for the location of satellites and R1, R2, R3, R4 are the distances of satellites from the receiver position (Figure-3). Hence solving the four equations for four unknowns X,Y, Z and x, the position or location of the station is calculated.
Differential GPS
In order to achieve on-line positioning with high accuracies, Differential GPS (DGPS) is used. Differential positioning user the point position derived from satellite signals and applies correction to that position. These corrections, difference of determined position and the known position, are generated by a reference receiver, whose position is known and is fed to the instrument, and are used by the second receiver to correct its internally generated position. This is known as Differential GPS (Figure-4). It is assumed that the two receivers suffer from approximately the same magnitude of geometry and timing errors and the most of the common errors cancel out using this correction technique. To remove Selective Availability (SA) and other bias errors, differential corrections are computed at the reference station and applied at the remote receiver at an update rate that is less than the correlation time of SA, which is usually less than twenty seconds. The differential positioning accuracy is of order of 1-5 m.
fig. 3: Calculating locations using GPS
Surveying Techniques
There are different surveying techniques, depending upon the application and accuracy requirements. A few of the surveying techniques are described below:
Standard Static
It involves setting up two receivers, one at a reference point and the other on the station to be determined, and observing them simultaneously for at least 1 hr during a single survey session.
Dynamic Observation Techniques
Dynamic surveying implies some sort of motion. It allows one or more receiver(s) to move during the survey session to collect more point or baselines (the distance between two GPS sets) while other receiver is kept stationary at a base station. The basic difference between various dynamic techniques is that how quickly one can move from one point to the other. The dynamic techniques are further classified as,
- Pseudostatic: It involves using the "best parts" of a static observation period without having to occupy the baseline for 1 hr. It permits to observe a station for 10 min at either end of the static observation and then leave to observe other stations. After 1hr or more, receivers are returned to the first station and again observed for 10 min. By this the receiver views the change in geometry of the satellites, to compute the integer ambiguities, without waiting for 1 hr on the station. It requires at least four satellites during station observations.
- Fast Static: This survey requires one occupation of a baseline, usually running 5-20 minutes in duration. The observation time for fast static baseline is highly dependent on baseline length and satellite geometry. Following time estimates consider only satellite geometry, assuming short (<20 km) baselines (Trimble Navigation, 1994)< /li>
| No.of SVs | Observation Time (minutes) |
|
4 5-6 6+ |
20+ 10-20 5-10 |
The baseline processor utilises the precise pseudoranges and carrier-phase observable to resolve the baseline more efficiently than static GPS processing and more reliably than kinematic.
Kinematic
It provides the highest potential productivity. It speeds up the data collection portion of survey but there are few restrictions. Main restriction is that, during a survey, both receivers must maintain lock on the same satellites all the times. There must be continuous tracking of at least four satellites. If either receiver drops below four satellites, the rover must be returned to a previously surveyed point or some other point of known position. It must be initialised from a known point. It is valid for small area(radius<10km).
fig. 4: Differential GPS
Uses of GPS GPS receivers are used for navigation, positioning, time dissemination, and other research.
- Navigation in three dimensions is the primary function of GPS.
- 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.
- Research projects have used GPS signals to measure atmospheric parameters.
- Georeferencing: that is assigning correct latitude and longitude to the control points of satellite imageries and topographic maps.