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GPS and DGPS :

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GPS and DGPS :

GPS is a Satellite Navigation System:

GPS is founded by and controlled by the U. S. Department of

Defense (DOD).

Four GPS satellite signals are used to compute positions in

three dimensions and the time offset in the receiver clock.

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GPS and DGPS :

SPACE SEGMENT:

Figure 2[Space Satellite]

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.

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GPS and DGPS :

Figure 3[Satellite Inclination]

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.

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GPS and DGPS :

CONTROL SEGMENT:

The Control Segment consists of a system of tracking stations locatedaround the world.

The Master Control facility is located at Schriever Air Force Base

(formerly Falcon AFB) in Colorado

USER SEGMENT :

Figure 4[Gps Navigation]

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

Navigation receivers are made for aircraft, ships, ground vehicles, and

for hand carrying by individuals.

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GPS and DGPS :

GPS POSITIONING SERVICES:

Precise Positioning Service (PPS)

Standard Positioning Service (SPS)

GPS DATA:

The GPS Navigation Message consists of time-tagged data bits

marking the time of transmission of each sub frame at the time they

are transmitted by the SV.

Data frames (1500 bits) are sent every thirty seconds. Each frame

consists of five sub frames.

Other system parameters and flags are sent that characterize

details of the system

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GPS and DGPS :

POSITION AND TIME FROM GPS :

Figure.5 [GPS receiver]

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GPS and DGPS :

GPS ERROR SOURCES :

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

Bias errors result from Selective Availability and other factors

Figure.6[ position estimation]

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GPS and DGPS :

GEOMETRIC DILUTION OF PRECISION (GDOP) AND

VISIBILITY:

Poor GDOP, a large value representing a small unit vector-volume, resultswhen angles from receiver to the set of SVs used are similar.

Good GDOP, a small value representing a large unit-vector-volume,

results when angles from receiver to SVs are different.

Figure.7 [GDOP]

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GPS and DGPS :

DIFFERENTIAL GPS (DGPS) TECHNIQUES:

The idea behind all differential positioning is to correct bias errors at onelocation with measured bias errors at a known position.

A reference receiver, or base station, computes corrections for each satellite

signal.

Figure.8[ DGPS ]

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GPS and DGPS :

DIFFERENTIAL CODE GPS (NAVIGATION) :

Differential corrections may be used in real-time or later, with post-

processing techniques.

DGPS removes common-mode errors, those errors common to both the

reference and remote receivers (not multipath or receiver noise)

Figure.9[differential code GPS]

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GPS and DGPS :

DIFFERENTIAL CARRIER GPS (SURVEY):

All carrier-phase tracking is differential, requiring both areference and remote receiver tracking carrier phases at the

same time.

In order to correctly estimate the number of carrier wavelengthsat the reference and remote receivers, they must be close

enough to insure that the ionospheric delay difference is less

than a carrier wavelength.

Carrier-phase GPS measurements must be taken with a remote

and reference station within about 30 kilometers of each other

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GPS and DGPS :

GPS TECHNIQUES AND PROJECT COSTS :

Receiver costs vary depending on capabilities.

Project tasks can often be categorized by required accuracies which

will determine equipment cost.

Low-cost, single-receiver SPS projects (100 meter accuracy)Medium-cost, differential SPS code Positioning(1-10 meter accuracyHigh-cost, single-receiver PPS projects (20 meter accuracy)High-cost, differential carrier phase surveys (1 mm to 1 cm accuracy)

Figure.10[GPS KIT]

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GPS and DGPS :

GPS APPLICATION:

GPS has a variety of applications on land, at sea and in the air

The scientific community uses GPS for its precision timing capability and

position information.

Surveyors use GPS for an increasing portion of their work

GPS is now commonplace in automobiles as well. Some basic systems are i

place and provide emergency roadside assistance at the push of a button (by

transmitting your current position to a dispatch center)

Recreational uses of GPS are almost as varied as the number of recreationalsports available. GPS is popular among hikers, hunters, snowmobilers,

mountain bikers, and cross-country skiers, just to name a few

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GPS and DGPS :

The Global Positioning System (GPS) is a worldwide radio-navigation

system formed from a constellation of 24 satellites and their ground stations.

GPS uses these "man-made stars" as reference points to calculate positions

accurate to a matter of meters. In fact, with advanced forms of GPS[DGPS]you can make measurements to better than a centimeter! In a sense it's like

giving every square meter on the planet a unique address. GPS receivers

have been miniaturized to just a few integrated circuits and so are becoming

very economical. And that makes the technology accessible to virtually

everyone.

Conclusion :

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GPS and DGPS :