Ch-4 Satellite Communications III

7/30/2019 Ch-4 Satellite Communications III

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Satellite Communications-III

Dr. Nasir D. Gohar

http://electronics.howstuffworks.com/gps.htm

http://www.colorado.edu/geography/gcraft/notes/gps/gps_ftoc.html


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WHAT IS SATELLITE NAVIGATION? Navigation- art or science of plotting, ascertaining, ordirecting of movements (knowing your whereabouts and being

able to find your way around)

Celestial NavigationDirection and distances determined with timed

sighting of starsWanderingTechnique used by most of us while at new place

PilotingFixing position and direction wrt familiar and significantlandmarks

Radio / Electronic NavigationPosition is determined by measuringthe travel time of radio wave as it moves from Tx to Rx

Terrestrial Systemssuch as Decca, Omega, Loran etc.

Satellite Systems such as LEO based Navy Transit GPS, MEObased Navstar GPS and Russian Counter Part,.


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LORANLOng RAnge Navigation Most Effective, Reliable, and Accurate Terrestrial System

LORAN-A, Developed during World WarII

LORAN-C, developed in 1980s and used for recreational aircrafts and ships

Principle:The elapsed time of coded signals from four land-based Txs,

whose locations are known, at any Rx determines the position of the Rx basedon Tri-lateration

Problems &

Limitations:

Atmospheric

Conditions and MultipathTransmission

No Global Coverage

U r here.


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Navstar GPS

Navigation System with Time and Ranging &Global Positioning SystemSatellite based Navigation, 3D positioning, and Time-Distribution System

Owned by USA DoD (maintained by US Air Force), 1994 (formally declared 1995)

Provides continuous, highly precise position, velocity, and time information to anyuser with a GPS Rx, at any time, at any place (land, sea, air, space) in all weatherconditions


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Navstar GPS

Navigation System with Time and Ranging &Global Positioning System

Navstar GPS ServicesTwo level service or accuracy

Standard Positioning Service

Civil users worldwide use the SPS without charge or restrictions. Most receivers arecapable of receiving and using the SPS signal. The SPS accuracy is intentionallydegraded by the DOD by the use ofSelective Availability.

SPS Predictable Accuracy 100 meter horizontal accuracy

156 meter vertical accuracy

340 nanoseconds time accuracy Precise Positioning Service

Authorized users with cryptographic equipment and keys and specially equippedreceivers use the Precise Positioning System. U. S. and Allied military, certain U. S.Government agencies, and selected civil users specifically approved by the U. S.Government, can use the PPS.

PPS Predictable Accuracy 22 meter Horizontal accuracy

27.7 meter vertical accuracy

200 nanosecond time (UTC) accuracy
http://localhost/var/www/apps/conversion/GPS/GPS%20-%20An%20Overview.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20-%20An%20Overview.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20-%20An%20Overview.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20-%20An%20Overview.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20-%20An%20Overview.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20-%20An%20Overview.htm


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Navstar GPS Segments

Space Segment-1The Space Segment of the system consists of the24 GPS satellites (21 in Operation, 3 as spare)

These space vehicles (SVs) send radio signalsfrom space

GPS Satellites orbit the earth in 12 hours

The satellite orbits repeat almost the sameground track (as the earth turns beneath them)once each day

The orbit altitude (20, 200 km) is such that thesatellites repeat the same track and configurationover any point approximately each 12 hours (4minutes earlier each day)

Six orbital planes (with nominally four SVs ineach), equally spaced (60 degrees apart), andinclined at about fifty-five (55) degrees withrespect to the equatorial plane

Five and eight SVs are visible from any point onthe earth


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Space Segment-2 Satellite Relative Positions


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Space Segment-3The Mercator Projection of Navstar GPS Satellite Orbits: 3 GPS satellites providehorizontal (two-dimensional) location of a GPS Rx where as four GPS satellites provide its 3D

position (including altitude)


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Control SegmentThe Control Segment consists of a system of tracking stations located around the world The 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 foreach satellites

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


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User Segment Navigation in three dimensions is the primaryfunction of GPS

GPS User Segment consists of the GPS receiversand the user community such as aircrafts, ships,ground vehicles, and for hand carrying by individuals

GPS receivers convert SV signals into position,velocity, and time estimates

Four satellites are required to compute the fourdimensions of X, Y, Z (position) and Time

GPS receivers are used for navigation, positioning,time dissemination, and other research projects

Precise positioning is possible using GPS receiversat reference locations providing corrections andrelative positioning data for remote receivers -Surveying, geodetic control, and plate tectonic

studies are examples Time and frequency dissemination, based on theprecise clocks on board the SVs and controlled bythe monitor stations, is another use for GPS -Astronomical observatories, telecommunicationsfacilities, and laboratory standards can be set toprecise time signals or controlled to accuratefrequencies by special purpose GPS receivers

Research projects have used GPS signals tomeasure atmospheric parameters


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GPS Satellite Signals

The SVs transmit two MW carrier signals:-The L1 frequency (1575.42 MHz) carries the navigation message and the SPScode signals

The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay byPPS equipped receivers

Three binary codes shift the L1 and/or L2 carrier phase :-The C/A Code (Coarse Acquisition) modulates the L1 carrier phase

The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code

This noise-like code modulates the L1 carrier signal, "spreading" the spectrum over a 1

MHz bandwidth

The C/A code repeats every 1023 bits (one millisecond)

There is a different C/A code PRN for each SV. GPS satellites are often identified by

their PRN number, the unique identifier for each pseudo-random-noise code

The C/A code that modulates the L1 carrier is the basis for the civil SPS

The P-Code (Precise) modulates both the L1 and L2 carrier phasesThe P-Code is a very long (seven days) 10 MHz PRN code

In the Anti-Spoofing (AS) mode of operation, the P-Code is encrypted into the Y-Code

The encrypted Y-Code requires a classified AS Module for each receiver channel and is

for use only by authorized users with cryptographic keys

The P (Y)-Code is the basis for the PPS

The Navigation Message also modulates the L1-C/A code signal -The Navigation Messageis a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections,

and other system parameters.


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Back

GPS Satellite Signals


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GPS Satellite Data and its Format


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GPS Satellite Data and its Format

The GPS Navigation Message consists of time-tagged data bits marking the time oftransmission of each subframe at the time they are transmitted by the SV

A data bit frame consists of 1500 bits divided into five sub-frames each carrying 300 bits

Data bit sub-frames (300 bits transmitted over six seconds) contain parity bits that allow for data checking

and limited error correction

Three six-second sub-frames contain orbital and clock data

SV Clock corrections are sent in sub-frame one

Precise SV orbital data sets (ephemeris data parameters)for the transmitting SV are sent in sub-frames two and three

Sub-frames four and five are used to transmit different pages of system data

A data frame is transmitted every thirty seconds

An entire set of twenty-five frames (125 sub-frames) makes up the complete NavigationMessage that is sent over a 12.5 minute period

Clock dataparameters describe the SV clock and its relationship to GPS time (Clock Algorithm)

Ephemeris data parameters describe SV orbits for short sections of the satellite orbits

Normally, a receiver gathers new ephemeris data each hour, but can use old data for up to four hours

without much error

The ephemeris parameters are used with an algorithm that computes the SV position for any time withinthe period of the orbit described by the ephemeris parameter set
http://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Ephemeris%20and%20Clock%20Data%20Samples.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Ephemeris%20and%20Clock%20Data%20Samples.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Ephemeris%20and%20Clock%20Data%20Samples.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Ephemeris%20and%20Clock%20Data%20Samples.htmhttp://localhost/var/www/apps/conversion/GPS/Clock%20Parameters%20to%20Clock%20Correction%20Code.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Ephemeris%20and%20Clock%20Data%20Samples.htmhttp://localhost/var/www/apps/conversion/GPS/Ephemeris%20Parameters%20to%20XYZ%20Algorithm.htmhttp://localhost/var/www/apps/conversion/GPS/Ephemeris%20Parameters%20to%20XYZ%20Algorithm.htmhttp://localhost/var/www/apps/conversion/GPS/Ephemeris%20Parameters%20to%20XYZ%20Algorithm.htmhttp://localhost/var/www/apps/conversion/GPS/Ephemeris%20Parameters%20to%20XYZ%20Algorithm.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Ephemeris%20and%20Clock%20Data%20Samples.htmhttp://localhost/var/www/apps/conversion/GPS/Clock%20Parameters%20to%20Clock%20Correction%20Code.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Ephemeris%20and%20Clock%20Data%20Samples.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Ephemeris%20and%20Clock%20Data%20Samples.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Ephemeris%20and%20Clock%20Data%20Samples.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Ephemeris%20and%20Clock%20Data%20Samples.htm


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GPS Satellite Astronomical Almanac

Almanacs are approximate orbital data parameters for all SVs The ten-parameter almanacs describe SV orbits over extended periods of time (useful formonths in some cases) and a set for all SVs is sent by each SV over a period of 12.5 minutes(at least)

Signal acquisition time on receiver start-up can be significantly aided by the availability ofcurrent almanacs

The approximate orbital data is used to preset the receiver with the approximate position

and carrier Doppler frequency (the frequency shift caused by the rate of change in range tothe moving SV) of each SV in the constellation
http://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Almnac%20Parameters.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Almnac%20Parameters.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Almnac%20Parameters.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Almnac%20Parameters.htm


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GPS Satellite Astronomical Almanac

Almanacs are approximate orbital data parameters for all SVs The ten-parameter almanacs describe SV orbits over extended periods of time (useful formonths in some cases) and a set for all SVs is sent by each SV over a period of 12.5 minutes(at least)

Signal acquisition time on receiver start-up can be significantly aided by the availability ofcurrent almanacs

The approximate orbital data is used to preset the receiver with the approximate position

and carrier Doppler frequency (the frequency shift caused by the rate of change in range tothe moving SV) of each SV in the constellation

Phase Delay due to Ionosphere - Each complete SV data set includes an ionosphericmodel that is used in the receiver to approximates the phase delay through the ionosphere atany location and time

GPS Time Offset from Universal Coordinated Time (UTC) - Each SV sends the amount towhich GPS Time is offset from Universal Coordinated Time. This correction can be used bythe receiver to set UTC to within 100 ns
http://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Almnac%20Parameters.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Almnac%20Parameters.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20data%20Ionospheric%20Model%20Parameter%20Sample.htmhttp://localhost/var/www/apps/conversion/GPS/UTC%20Parameter%20Sample.htmhttp://localhost/var/www/apps/conversion/GPS/UTC%20Parameter%20Sample.htmhttp://localhost/var/www/apps/conversion/GPS/UTC%20Parameter%20Sample.htmhttp://localhost/var/www/apps/conversion/GPS/UTC%20Parameter%20Sample.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20data%20Ionospheric%20Model%20Parameter%20Sample.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20data%20Ionospheric%20Model%20Parameter%20Sample.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20data%20Ionospheric%20Model%20Parameter%20Sample.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Almnac%20Parameters.htmhttp://localhost/var/www/apps/conversion/GPS/GPS%20Satellite%20Almnac%20Parameters.htm


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GPS Satellite Grouping

Three Distinct Groups and one Sub-group of Navstar GPS satellites 11 Block-I Group satellites were prototypes and just for testing purpose

Block-II Group satellites were first set of fully functional satellites with cesium atomic clocks

Can detect certain errors and provide alarms using coded messages

Can operate for about 3.5 days between receiving updates and corrections from Control Segment

Block IIa satellites are more intelligent and can go for 180 days between uploads

Block IIR satellites are similar to Block-IIa satellites except having autonomous navigationcapabilities

GPS Satellite Identification Three Identifying Numbers

Navstar Number identifying the specific satellite onboard HW

SV Number is space vehicle number assigned in the order of vehicle launch

PRN Code Number is a unique integer number used for encrypting the signal from satellite
http://localhost/var/www/apps/conversion/GPS/Howstuffworks%20How%20Atomic%20Clocks%20Work.htmhttp://localhost/var/www/apps/conversion/GPS/Howstuffworks%20How%20Atomic%20Clocks%20Work.htm


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GPS Satellite Receiver-1

The GPS receiver produces replicas of the C/A and/or P (Y)-Code Each PRN code is a noise-like, but pre-determined, unique series of bits

The receiver produces the C/A code sequence for a specific SV with some form of a C/Acode generator

Modern receivers usually store a complete set of pre-computed C/A code chips in memory,but a hardware, shift register, implementation can also be used


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GPS Satellite Receiver-2 A GPS receiver uses the detected signal power in the correlated signal to align the C/A code in the receiver with the code in the

SV signal Usually a late version of the code is compared with an early version to insure that the correlation peak is tracked.

A phase locked loop that can lock to either a positive or negative half-cycle (a bi-phase lock loop) is used to demodulate the 50HZ navigation message from the GPS carrier signal

The same loop can be used to measure and track the carrier frequency (Doppler shift) and by keeping track of the changes to thenumerically controlled oscillator, carrier frequency phase can be tracked and measured

The receiver PRN code start position at the time of full correlation is the time of arrival (TOA) of the SV PRN at receiver

This TOA is a measure of the range to SV offset by the amount to which the receiver clock is offset from GPS time

This TOA is called the pseudo-range

Data Bit Demodulation and C/A Code Control


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GPS Satellite Receiver-2 The C/A Code Generator

The C/A code generator produces a different 1023 chip sequence for each phase tap setting

In a shift register implementation the code chips are shifted in time by slewing the clock that controls the shiftregisters

In a memory lookup scheme the required code chips are retrieved from memory

C/A Code Phase Assignments

The C/A code generator repeats the same 1023-chip PRN-code sequence every millisecond

PRN codes are defined for 32 satellite identification numbers

C/A Code PRN Chips

The receiver slides a replica of the code in time until there is correlation with the SV code.

Correlation Animation (250k)

Back
http://www.colorado.edu/geography/gcraft/notes/gps/gif/cacodes.gifhttp://www.colorado.edu/geography/gcraft/notes/gps/gif/codebits.gifhttp://www.colorado.edu/geography/gcraft/notes/gps/gif/bitsanim.gifhttp://www.colorado.edu/geography/gcraft/notes/gps/gif/bitsanim.gifhttp://www.colorado.edu/geography/gcraft/notes/gps/gif/bitsanim.gifhttp://www.colorado.edu/geography/gcraft/notes/gps/gif/codebits.gifhttp://www.colorado.edu/geography/gcraft/notes/gps/gif/codebits.gifhttp://www.colorado.edu/geography/gcraft/notes/gps/gif/cacodes.gifhttp://www.colorado.edu/geography/gcraft/notes/gps/gif/cacodes.gif


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GPS Satellite Ranging-1 The GPS Pseudo Ranging and Rx Clock Bias

Position is determined from multiple pseudo-range measurements at a single measurement epoch

The pseudo range measurements are used together with SV position estimates based on the precise orbital

elements (the ephemeris data) sent by each SV

This orbital data allows the receiver to compute the SV positions in three dimensions at the instant that they

sent their respective signals

Four satellites (normal navigation) can be used to determine three position dimensions and time

Position dimensions are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ)

coordinatesTime is used to correct the offset in the receiver clock, allowing the use of an inexpensive receiver clock

SV Position in XYZ is computed from four SV pseudo-ranges and the clock correction and ephemeris data


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GPS Satellite Ranging-2 The GPS Pseudo Ranging and Rx Clock Bias

Receiver position iscomputed from the SVpositions, the measuredpseudo-ranges (correctedfor SV clock offsets, iono-spheric delays, andrelativistic effects), and a

receiver position estimate(usually the last computed

receiver position)


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GPS Satellite Ranging-3 The GPS Rx 3D Position Calculation


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GPS Sources of Errors

GPS errors are a combination of noise, bias, blunders

Noise errors are the combined effect of PRN code noise (around 1 meter) and

noise within the receiver noise (around 1 meter)

Noise and bias errors combine, resulting in typical ranging errors of around

fifteen meters for each satellite used in the position solution


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Differential GPS - 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.

Because individual pseudo-ranges must be corrected prior to the formation of a navigation solution,

DGPS implementations require software in the reference receiver that can track all SVs in view and form

individual pseudo-range corrections for each SV.

These corrections are passed to the remote, or rover, receiver which must be capable of applying these

individual pseudo-range corrections to each SV used in the navigation solution.

Applying a simple position correction from the reference receiver to the remote receiver has limited effect

at useful ranges because both receivers would have to be using the same set of SVs in their navigationsolutions and have identical GDOP terms (not possible at different locations) to be identically affected by

bias errors


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Blunders

Blunders can result in errors of hundred of kilometers

Control segment mistakes due to computer or human error cancause errors from one meter to hundreds of kilometers

User mistakes, including incorrect geodetic datum selection,

can cause errors from 1 to hundreds of meters

Receiver errors from software or hardware failures can cause

blunder errors of any size

Ch-4 Satellite Communications III

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