Ch-4 Satellite Communications III
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Transcript of 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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Navstar GPS Segments
Space Segment-2 Satellite Relative Positions
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Navstar GPS Segments
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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Navstar GPS Segments
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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Navstar GPS Segments
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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Satellite Communications-III
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