Satellite Communications-III Satellite Radio Navigation and GPS

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Satellite Communications-IIISatellite Radio
Navigation and GPS

• Dr. Nasir D. Gohar

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Satellite Communications-III

• WHAT IS SATELLITE NAVIGATION?
• Navigation- art or science of plotting,
  ascertaining, or directing of movements (knowing
  your whereabouts and being able to find your way
  around)
• Celestial Navigation Direction and distances
  determined with timed sighting of stars
• Wandering Technique used by most of us while
  at new place
• Piloting Fixing position and direction wrt
  familiar and significant landmarks
• Radio / Electronic Navigation Position is
  determined by measuring the travel time of radio
  wave as it moves from Tx to Rx
• Terrestrial Systems such as Decca, Omega, Loran
  etc.
• Satellite Systems such as LEO based Navy Transit
  GPS, MEO based Navstar GPS and Russian Counter
  Part,.

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Satellite Communications-III

• LORAN LOng RAnge Navigation
• Most Effective, Reliable, and Accurate
  Terrestrial System
• LORAN-A, Developed during World War II
• 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 based on Tri-lateration

• Problems Limitations
• Atmospheric Conditions and Multipath
  Transmission
• No Global Coverage

U r here.
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Satellite Communications-III

• Navstar GPS
• Navigation System with Time and Ranging
• Global Positioning System
• Satellite 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 any user with a
  GPS Rx, at any time, at any place (land, sea,
  air, space) in all weather conditions

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Satellite Communications-III

• Navstar GPS
• Navigation System with Time and Ranging
• Global Positioning System
• Navstar GPS Services Two level service or
  accuracy
• Standard Positioning Service
• Civil users worldwide use the SPS without charge
  or restrictions. Most receivers are capable of
  receiving and using the SPS signal. The SPS
  accuracy is intentionally degraded by the DOD by
  the use of Selective 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 equipped receivers 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

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Satellite Communications-III

• Navstar GPS Segments
• Space Segment-1
• The Space Segment of the system consists of the
  24 GPS satellites (21 in Operation, 3 as spare)
• These space vehicles (SVs) send radio signals
  from space
• GPS Satellites orbit the earth in 12 hours
• The satellite orbits repeat almost the same
  ground track (as the earth turns beneath them)
  once each day
• The orbit altitude (20, 200 km) is such that the
  satellites repeat the same track and
  configuration over any point approximately each
  12 hours (4 minutes earlier each day)
• Six orbital planes (with nominally four SVs in
  each), equally spaced (60 degrees apart), and
  inclined at about fifty-five (55) degrees with
  respect to the equatorial plane
• Five and eight SVs are visible from any point on
  the earth

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Satellite Communications-III

• Navstar GPS Segments
• Space Segment-2
• Satellite Relative Positions

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Satellite Communications-III

• Navstar GPS Segments
• Space Segment-3
• The Mercator Projection of Navstar GPS Satellite
  Orbits 3 GPS satellites provide horizontal
  (two-dimensional) location of a GPS Rx where as
  four GPS satellites provide its 3D position
  (including altitude)

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Satellite Communications-III

• Navstar GPS Segments
• Control Segment
• The 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
  for each 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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Satellite Communications-III

• Navstar GPS Segments
• User Segment
• Navigation in three dimensions is the primary
  function of GPS
• GPS User Segment consists of the GPS receivers
  and 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 four
  dimensions 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
  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

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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 SPS code signals
• The L2 frequency (1227.60 MHz) is used to measure
  the ionospheric delay by PPS 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 phases
• The 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 Message is a 50 Hz
  signal consisting of data bits that describe the
  GPS satellite orbits, clock corrections, and
  other system parameters.

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Satellite Communications-III

• GPS Satellite Signals

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Satellite Communications-III

• GPS Satellite Data and its Format

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Satellite Communications-III

• GPS Satellite Data and its Format
• The GPS Navigation Message consists of
  time-tagged data bits marking the time of
  transmission 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 Navigation
  Message that is sent over a 12.5 minute period
• Clock data parameters 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 within the period of the orbit described by
  the ephemeris parameter set

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Satellite Communications-III

• 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 for months
  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 of
  current 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 to the
  moving SV) of each SV in the constellation

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Satellite Communications-III

• 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 for months
  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 of
  current 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 to the
  moving SV) of each SV in the constellation
• Phase Delay due to Ionosphere - Each complete SV
  data set includes an ionospheric model that is
  used in the receiver to approximates the phase
  delay through the ionosphere at any location and
  time
• GPS Time Offset from Universal Coordinated Time
  (UTC) - Each SV sends the amount to which GPS
  Time is offset from Universal Coordinated Time.
  This correction can be used by the receiver to
  set UTC to within 100 ns

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Satellite Communications-III

• 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 navigation
  capabilities
• 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

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Satellite Communications-III

• 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/A code
  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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Satellite Communications-III

• 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 50 HZ 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 the numerically
  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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Satellite Communications-III

• 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 shift registers
• 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)

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Satellite Communications-III

• 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)
  coordinates
• Time 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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Satellite Communications-III

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

• Receiver position is computed from the SV
  positions, the measured pseudo-ranges (corrected
  for SV clock offsets, iono-spheric delays, and
  relativistic effects), and a receiver position
  estimate (usually the last computed receiver
  position)

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Satellite Communications-III

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

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Satellite Communications-III

• 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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Satellite Communications-III

• Differential GPS - The idea behind all
  differential positioning is to correct bias
  errors at one location 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 navigation solutions and have
  identical GDOP terms (not possible at different
  locations) to be identically affected by bias
  errors

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Bias Errors

• 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 meters is
  reduced to 100 meters (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 meter
• Tropospheric delays 1 meter. 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.
• Unmodeled ionosphere delays 10 meters. 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 ns of delay leaving a ten meter
  un-modeled residual.
• Multipath 0.5 meters. 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.

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Blunders

• Blunders can result in errors of hundred of
  kilometers
• Control segment mistakes due to computer or human
  error can cause 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