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Chapter: 11th 12th std standard Geography earth space Higher secondary school College Notes

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Global Positioning System (GPS) is a Satellite Navigation System

GPS is funded by, and also 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.

Global positioning systems (GPS)

Learning Objectives

 

Students learn to understand the usefulness of the global positioning system within geography and its applications elsewhere and in ways useful to human life and comforts

 

Global Positioning System (GPS) is a Satellite Navigation System

 

GPS is funded by, and also 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.

 

GPS Satellites

 

There are 24 GPS Satellites orbiting the entire globe, transmitting positioning and timing data day and night in all weather conditions, all courtesy of the U.S. Government 

 

GPS Segments

 

There are three major segments to the GPS and they are as follows:

1.    Space Segment: The Space Segment of the system consists of the GPS satellites. These space vehicles (SVs) send radio signals from space

 

2.    Control Segment: The Control Segment consists of a system of tracking stations located around the world. 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 satellite. 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.

 

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 T (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. 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.

 

GPS Error Sources

GPS errors are a combination of noise, bias and blunders.

 

Noise, Bias, and Blunders

 

Noise errors are the combined effect of PRN code noise (around 1 metre) and noise within the receiver noise (around 1 metre). Bias errors result from Selective Availability and other factors.

 

Selective Availability (SA)

 

SA is the intentional degradation of the SPS signals by a time varying bias. SA is controlled by the DOD to limit accuracy for non-U. S. military and government users. The potential accuracy of the C/A code of around 30 metres is reduced to 100 metres (two standard deviations). The SA bias on each satellite signal is different, and so the resulting position solution is a function of the combined SA bias from each SV used in the navigation solution. Because SA is a changing bias with low frequency terms in excess of a few hours, position solutions or individual SV pseudo-ranges cannot be effectively averaged over periods shorter than a few hours. Differential corrections must be updated at a rate less than the correlation time of SA (and other bias errors).

 

Other Bias Error Sources: SV clock errors uncorrected by Control Segment can result in one meter errors: Ephemeris data errors: 1 metre and Tropospheric delays: 1 metre. The troposphere is the lower part (ground level to from 8 to 13 km) of the atmosphere that experiences the changes in temperature, pressure, and humidity associated with weather changes. Complex models of tropospheric delay require estimates or measurements of these parameters.

 

Unmodelled ionospheric delays: 10 metres. The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air. The transmitted model can only remove about half of the possible 70 ions of delay leaving a ten metre un-modelled residual.

 

Multipath: 0.5 metres. Multipath is caused by reflected signals from surfaces near the receiver that can either interfere with or be mistaken for the signal that follows the straight line path from the satellite. Multipath is difficult to detect and sometime hard to avoid.

 

Blunders: They can result in errors of hundred of kilometres. Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of kilometres. User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to hundreds of metres.

 

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 metres for each satellite used in the position solution.

 

Carrier Phase Tracking (Surveying): An Application of GPS

 

Carrier-phase tracking of GPS signals has resulted in a revolution in land surveying. A line of sight along the ground is no longer necessary for precise positioning. Positions can be measured up to 30 km from reference point without intermediate points. This use of GPS requires specially equipped carrier tracking receivers.

 

The L1 and/or L2 carrier signals are used in carrier phase surveying. L1 carrier cycles have a wavelength of 19 centimetres. If tracked and measured these carrier signals can provide ranging measurements with relative accuracies of millimetres under special circumstances.

 

Tracking carrier phase signals provides no time for transmission information. The carrier signals, while modulated with time tagged binary codes, carry no time-tags that distinguish one cycle from another. The measurements used in carrier phase tracking are differences in carrier phase cycles and fractions of cycles over time. At least two receivers track carrier signals at the same time. Ionospheric delay differences at the two receivers must be small enough to insure that carrier phase cycles are properly accounted for. This usually requires that the two receivers be within about 30 km of each other.

 

Carrier phase is tracked at both receivers and the changes in tracked phase are recorded over time in both receivers. All carrier-phase tracking is differential, requiring both a reference and remote receiver tracking carrier phases at the same time. Unless the reference and remote receivers use L1-L2 differences to measure the ionospheric delay, they must be close enough to insure that the ionospheric delay difference is less than a carrier wavelength. Using L1-L2 ionospheric measurements and long measurement averaging periods, relative positions of fixed sites can be determined over baselines of hundreds of kilometres.

Phase difference changes in the two receivers are reduced using software to differences in three position dimensions between the reference station and the remote receiver. High accuracy range difference measurements with sub-centimetre accuracy are possible. Problems result from the difficulty of tracking carrier signals in noise or while the receiver moves. Two receivers and one SV over time result in single difference.

 

Automatic Vehicle Location, A Tracking Technology Example

 

Automatic Vehicle Location (AVL) is a technology used for track-ing vehicles, vessels, and mobile assets such as trailers, containers, and equipment. Each mobile unit has a GPS receiver that reports its posi-tion to the base station over a communications network. This allows the base station to monitor the entire fleet and manage the mobile as-sets.

 

Mobile GPS Unit

 

In each vehicle you need a GPS receiver to track the satellites and calculate your position. But actually Trimble's mobile GPS units do a lot more than just that. Altogether they:

1. Receive GPS satellite signals.

 

2. Calculate your position, speed, heading and altitude.

 

3. Make adjustments for Differential GPS and/or Dead Reckoning.

 

4. Communicate with the base station - using either built-in communications or interfacing with an external radio.

Use the IQ Event Engine to decide when to report.

Log data.

Receive the precise time (the satellites use atomic clocks).


Communications Network

 

You need some sort of communications network so that the vehicle can transmit its position and other information to the base station. The communication goes both ways so that the base station can check the status of its vehicles and perhaps send new instructions for the IQ Event Engine.

 

Base Station Software

 

The base station needs a computer system and software to handle all the position reports and communications. Altogether it:

 

1.  Manages communications over the communications network.

 

2.  Processes position & status reports from all the vehicles.

 

3.  Displays the vehicles on a map in real time.

 

4.  Logs data, which can later be replayed and analyzed.

 

5.  Sends instructions to vehicles for the IQ Event Engine.

 

Interfaces with 3rd party software for extended functionality.


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