UNIT 3 GPS

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Unit 3GPS Surveying

Introduction:

Satellite navigation is a leading-edge technology which allows anyone with a receiver todetermine their position very accurately at any time by picking up signals from a constellation ofseveral satellites. Currently, the United States lobal !ositioning System "!S# and the $ussian%&'(SS system are the only operational Satellite navigation systems. )urope has begun thedevelopment of a third independent global system, known as *alileo+.

Satellite navigation &verview:

he following system overview uses eamples based on !S, however the principles

apply to all satellite navigation system. !S is a satellite navigation system capable of providinga highly accurate, continuous, global navigation service independent of other positioning aids!S provides / hour, all-weather, worldwide coverage with position, velocity and timinginformation.

he system uses / operational satellites to provide a receiver with at least si satellites inview at all times. ( minimum of four satellites in view are needed to allow the receiver to computeits current latitude, longitude, altitude and time. 0ith this information the user+s receiver can alsocalculate other parameters such as its velocity and acceleration.

Prof. Sachin Patel (91-9179883194)

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Components of !S:

(ny satellite navigation system has three parts:he Space segmenthe Control segmenthe User segment

(ll these parts operate together to provide accurate three-dimensional positioning, timing andvelocity data to users worldwide.

he Space Segment:

he !S system constellation has / satellites in si 112 orbital planes, with four satellitesin each plane, with room for spares. he orbit period of each satellite is approimately 3 hours atan altitude of 4,356 kilometers. 0ith this constellation, a user receiver has at least si satellitesin view from any point on earth. &ther systems use satellites in different orbits and orbital periods.

he satellite broadcast signal contains data which identifies the satellite and provides thepositioning, timing, ranging data, satellite status and corrected orbit parameters of the satellite.!S satellites transmit on two fre7uencies8 one centered at 3191./ ;

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(pplications of Satellite 'avigation:

Satellite navigation applications are almost limitless, but some typical ones include:

? air traffic navigation and control and their related accuracy and integrity8 enhancementinfrastructure8

? management and tracking of ship and land vehicle fleets8

? rental and personal car navigation systems8

? automation of container location and tracking to increase the efficiency of ports8

? navigation systems for remotely piloted air, land and water vehicles8

? road and rail traffic monitoring8

? dispatch and monitoring of emergency services8

? automated car and truck guidance systems8

? automated guidance of agricultural e7uipment for efficiency improvements in crop sprayingand harvesting

? recreational guidance for hikers, boaters, cyclists and eplorers8? aerial, seismic, and land surveying8

? large structure monitoring "such as dams, bridges, buildings, etc#8

? accurate timing systems for communications and commerce8 and

? earth7uake and tsunami detection and warning systems.

ap @atums:

0ell-defined coordinate systems are re7uired for positioning points in @ or 6@ space on

surface of earth. ;owever, one needs to represent or ideali

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(s the actual earth surface is highly undulating, defining position on this surface is 7uitedifficult. 0e use the concept of datum which is a mathematical model of the earth we use tocalculate the coordinates "@ or 6@# on any map, chart, or survey system. he datum can bevertical to define vertical position "F# with respect to a reference surface or hori

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ellipsoids are taken to be concentric with their coordinate system, geocentric or near-geocentricwith the ais of revolution coinciding with the 644.5439

eoidal undulation 4 meters

(

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. Carrier phase receivers

hese receiver types can be further subdivided as

3. C>( code receivers

. !-code receivers

6. Codeless receivers/. Single and dual-fre7uency receivers

1. $eceivers using cross-correlation or s7uaring or !-0 techni7ues

Code dependent or code phase receivers:

hese are also called code correlating receivers since they need access to the satellitenavigation message of the !- or C>(-code signal for operation. Aollowing are the characteristics8

Use almanac data from satellite navigation message for operation and signal processing

!rovides real-time navigation data

;ave anywhere-fi capability because it can synchroni

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Cheaper than dual fre7uency receivers

Used effectively to relative positioning mode for accurate baselines of less than 14 km or

where ionosphere effects can generally be ignored.

"b# @ual fre7uency receiver racks both %3 and % fre7uency signal

ore epensive than a single fre7uency receiver Can more effectively resolve longer baselines of more than 14 km where ionosphere

effects have a larger impact. )liminate almost all ionosphere effects by combining %3 and % observations.

Comparison of single and double fre7uency receivers:

Single Frequency Double frequency

(ccess to %3 only (ccess to %3 and %

ostly civilian users ostly military users

uch cheaper Lery epensive

odulated with C>( and ! codes It may not be possible for civilian users once H code isthere.

Corrupted by ionospheric delay (lmost independent of ionospheric delay

Used for short base lines Used for both long and short base lines

ost receivers are coded ost receivers with dual fre7uency are codeless

ost dual-fre7uency receivers utili

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eodetic receivers:

hese receiver are essentially used for geodetic>surveying applications with the followingcharacteristics "Seeber, 446#:

carrier phase data as observables

availability of both fre7uencies "%3, % # access to the ! code, at least for larger distances, and in geographical region with strong

ionospheric disturbances "low and high latitudes#.

Aollowing factors should be kept in mind for such receivers "Seeber, 446#:

racking all signals from each visible satellite at any time "!S only system re7uires 3

dual fre7uency channels8 !SE%&'(SS system needs 4 dual fre7uency channels#

Noth fre7uencies should be available

%ow phase and code noise

;igh data rate " O 34 ;

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radio modem

@!S and $ capability "eplained later#

operate over difficult meteorological conditions

ease in interfacing to other systems and from other manufacturer

ease and fleibility of use "multi purpose applications#

fleible set up "tripod, pole, pillar, vehicle#

Considerations in selection of !S receivers:

Structure of !S receiver:

Aunctionally, there are two groups of !S receiver structures:

o (pplication processing

o Signal processing

Application processing

ime and fre7uency transfer

Static and kinematic surveying 'avigation

Ionospheric otal )lectron Content ")C# monitoring

&peration as differential !S "@!S# reference station


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!S signal integrity monitoring

Signal processing

Splitting of incoming signal into multiple satellite signals

eneration of reference carrier

eneration of reference !$' code (c7uisition of satellite signal

racking of code and carrier

@emodulation and system data etraction

)traction of code phase measurements

)traction of carrier fre7uency and carrier phase

)traction of satellite Signal to 'oise $atio "S'$# information

$elationship of !S system time

Components of !S receiver:

(ntenna with preamplifier

$adio fre7uency "$A# and intermediate fre7uency "IA# Aront end section

Signal tracker and Code co-relator section

$eference oscillator

icroprocessor "navigational solution unit#

&ther parts: memory, power supply, display and control

(i) Antenna and preamplifier


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@etects the electromagnetic waves arriving from the satellite, converts the wave energy

into an electrical current, amplifies the signal strength and passes on the signal receiverelectronics.

!S signal structure re7uires that all !S antennas are right-handed circularly polariIA section. his is done by

combining the $A signal with a sinusoidal signal generated by the local referenceoscillator.

IA signal contains all code and data signals from the original $A signal with low carrier

fre7uency.

(iii) Signal tracker and code correlator

IA signal from all satellites is passed on to this section. ;ere signals are isolated, identified

by their codes and assigned to a particular channel.

$eceiver channel is main electronic unit of !S receiver.

Larious channel types: parallel, se7uencing, and multipleing

(iv) Reference oscillator

Inepensive 7uart< oscillators are used.

Some receivers can also accept eternal high precision signal from atomic fre7uencystandards with less noise.

(v) icroprocessor (navigational solution unit)

Controls the operation, including signal ac7uisition, signal processing, and decoding of

broadcast message.

Computation of on-line positions and velocity, conversion into a given local datum, @!S

correction (ccepts commands from the user, display of information, and data flow through

communication port.

(vi) !t"er parts# memory$ po%er supply$ command and display


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Aor post processing data is stored on internal or eternal memory devices, odern

receivers have internal solid state "$(# memories or removable memory cards. @ata canalso be transferred to laptops by using $S6 or e7uivalent communication port.

$eceivers use low voltage @C power with low energy consumption and generally userechargeable nickel-cadmium or lithium batteries.

;andheld keypads are used for display and communication between user and receiver.

!ost-processing software from the vendor can be used for further processing of !S data.

(dvantages of !S:

Unlike conventional surveying procedures, there is no need for inter-visibility betweenstations.

Independent of weather conditions as a result of using radio fre7uencies to transmit the

signals. Use of same field and data reduction procedures results in position accuracy which are

independent of network shape or geometry and are primarily a function of inter-station

distance. !S surveying provides generally homogeneous accuracy. ;ence, geodetic network

planning in the classical sense is no longer relevant. he points can be establishedwherever they are re7uired and need not be located at evenly distributed sites atopmountains to satisfy inter-visibility, or network geometry criteria.

!S surveying is more efficient, more fleible and less time consuming positioningtechni7ue than using conventional terrestrial survey technologies.

!S can be used to obtain high accuracy three dimensional "6@# information, anywhereand any time with relatively little effort on a global datum .

he !S instrumentation and the data processing software do not radically change even if

very high or moderately high accuracies are re7uired "from 3 part in 34 /to 3 part in 34=#.

Current %imitations of !S:

!S re7uires that there is clear opening to sky without any obstruction to the signals by

overhanging branches or structures "though the antenna can be raised above theobstruction#. ;ence, underground usage is not possible. Aurther, there may be limitedapplications in densely settled urban areas .

&ne needs careful advanced planning to reali

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!S results are, in general, more accurate than the surrounding control marks established

by terrestrial techni7ues over time. Comparison of !S and terrestrial results will be thesource of confusion, controversy and conflict for many years to come .

Since !S vertical information is not available in universally acceptable geoid based

height system, !S heights have to be reduced to a sea level datum by suitabletransformation.

he !S instrumentation is still comparatively epensive. (lthough the price of one

receiver is likely to soon match that of a theodolite-)@ instrument, generally a minimumof two are re7uired for most survey works.

Necause of comple procedures for planning, data reduction and post-processing, !S

surveys re7uire skilled personnel for operations.


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UNIT 3 GPS