GLOBAL NAVIGATION SATELLITE SYSTEM

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GLOBAL NAVIGATION SATELLITE SYSTEM

• GNSS

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WHAT IS IT?

• GNSS is a series of medium earth orbit (MEO
  approx. 23,000 km) navigation satellites in
  conjunction with ground based stations.
• The GNSS system is a combination of the American
  Global Positioning System (GPS) and the Russian
  Global Navigation Satellite System (GLONASS).
• The European Union is developing a third system
  called Galileo which is expected to be aviation
  friendly by 2010.
• To date only GNSS based on GPS is approved for
  aviation use.
• GNSS is approved for IFR en-route, terminal, and
  approach phases of flight by Transport Canada.

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ADVANTAGES

• GNSS navigation shares the advantages of all area
  navigation systems (RNAV)
• Increased fuel efficiency
• Improved airspace utilization
• Reduced flight times
• Some other advantages unique to GNSS are
• VNAV precision approach capability
• Economical (INS)
• Transoceanic navigation (INS)

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GPS

• The American GPS consists of a constellation of
  24 BLOCK ll (plus 4 standby on the ground)
  satellites and their ground based stations.
• The BLOCK ll satellites orbit the earth once
  every 12 hours on six orbital planes angled 55
  from the equatorial plane.
• Life expectancy of these satellites is 7.5 years.
• Ground station locations are Hawaii, Ascension
  Island, Diego Garcia, Kwajalein, and Colorado
  Springs.

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DELTA6000/7000 Launch Vehicle USA
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BLOCK ll GPS satellite
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GPS constellation
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GLONASS

• The Russian GLONASS consists of a constellation
  of 24 (21 active and 3 spares) KOSMOS satellites
  and their ground based stations.
• The KOSMOS satellites orbit the earth once every
  11hours and 15 minutes on three orbital planes
  separated by 120.
• Life expectancy of these satellites is 3-5 years.
  Next generation satellites are being developed
  with an expected service life of 10 years.
• All ground based stations are located within
  former Soviet Union territory.

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Polar Satellite Launch Vehicle (PSLV) India
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KOSMOS GLONASS satellite
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GLONASS constellation
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GALILEO

• The European Union Galileo will consist of a
  constellation of 30 GSTB-V2 satellites (27 active
  and 3 spares) and their ground based stations.
• The GSTB-V2 satellites orbit the earth once every
  14 hours on three orbital planes angled 56 from
  the equatorial plane.
• Life expectancy of the satellites is yet to be
  determined.
• Ground based stations will be located throughout
  Europe.

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Soyuz Launch Vehicle Russia
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GSTB-V2 GALILEO Satellite
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GALILEO constellation
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HOW GNSS WORKS

• Each navigation satellite transmits a unique UHF
  signal.
• The receiver pairs itself to this transmission
  and determines the time difference between the
  satellite clock and the receiver clock.
• The time difference multiplied by the speed of
  light gives the receiver distance from the
  satellite.
• Signals from multiple satellites are used to fix
  the receivers position in space.

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DETERMINING POSITION

• One satellite will narrow receiver position to
  any point on a sphere surrounding the satellite.

• Two satellites will narrow position to anywhere
  on a circle where the two spheres intersect.

• Three satellites will narrow position to two
  points. One point is usually either too far from
  the earth or moving at an impossible velocity and
  can be eliminated.

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DETERMINING DISTANCE

• The time it takes for a signal to travel from
  the satellite to the receiver gives a direct
  indication of distance. This distance from the
  satellite is used by the receiver to fix position.

• The satellite transmits a complex digital
  combination of on and off codes (Pseudo Random
  Code) along with current time. This code is
  matched by the receiver and the time delay
  between the two signals indicates the time it
  takes the satellite signal to reach the receiver.
  Each satellite transmits a unique PRC which
  creates its own unique signature.

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TIMING IS EVERYTHING

• Accurate time is a critical component of GNSS.
• If the satellite was directly overhead it would
  take 0.06 sec. for the signal to reach the
  receiver.
• An error of one thousandth of a second translates
  into an error of 200 miles.
• Satellites incorporate atomic clocks to provide
  the precision timing necessary.

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TIMEPIECE OF CHOICE

• Atomic clocks measure the oscillations of atoms
  as a method of measuring the passage of time.
• All atoms of the same type oscillate at the same
  frequency.
• Cesium atoms are used in most atomic clocks.
• These atomic clocks are accurate to one second in
  316,000 years.

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NOT GOOD ENOUGH

• New clocks have been developed with greater
  accuracy than conventional atomic clocks.
• Hydrogen Maser Clocks measure the radiation of
  hydrogen atoms as a method of precise time
  keeping.
• These clocks have proven accuracy of one second
  in 1.7 million years and show promise for
  accuracy of one second in 300 million years.
• The Galileo navigation satellites will use a
  combination of atomic and Hydrogen Maser clocks.

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REDUNDANCY

• Each GPS satellite carries four atomic clocks on
  board. Two are operational while the two reserve
  clocks take over in the event of a discrepancy or
  failure.
• The GALILEO satellites contain two hydrogen maser
  clocks and two atomic clocks. The hydrogen maser
  acts as the master clock while an atomic clock is
  powered in standby mode to take over
  instantaneously in the event of a failure. The
  reserve maser and atomic clocks will power up at
  this point and take over timing duties.

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ATOMIC CLOCK (50 to 100K )
HYDROGEN MASER CLOCK (Lots )
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RECEIVER CLOCKS

• Due to the hefty price tag associated with atomic
  clocks aircraft receivers are stuck with quartz
  based clocks.
• Since GNSS is so time sensitive the errors
  associated with such a timepiece are
  unacceptable.
• This time correction is applied by the use of a
  fourth satellite.
• The receiver position is further narrowed by this
  fourth satellite which corrects for any time
  discrepancies.

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If the receivers clock was precise all the
spheres would intersect at a single point. Any
inaccuracy in the receiver which translates to an
out of sync condition with satellite (accurate)
time will not allow the lines to intersect at a
single point. Instead the lines will intersect to
form a triangle with the receivers position
somewhere inside. The addition of distance
information from a fourth satellite allows the
receiver to recognize a time discrepancy and
compute a correction which when applied equally
to each sphere will result in all the spheres
intersecting at a single point.
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The dashed lines represent the intersection of
the three spheres the grey stripes represent the
area of uncertainty due to receiver clock
inaccuracies.
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The solid lines depict where the receiver thinks
the spheres are located. Inaccuracies in the
receivers clock mean the spheres do not intersect
at a single point. Instead they intersect to form
a triangle.
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With information from a fourth satellite the
receiver can recognize and compute a correction
which when applied equally to all spheres will
allow the lines to intersect at a single point.
In this way the receiver knows the amount of time
error that exists within its internal clock and
can adjust it accordingly. (essentially matching
its own timepiece to the atomic clocks on board
the satellites).
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SATELLITE POSITION

• So far we have seen how the receiver calculates
  distance from the satellite and corrects for any
  time discrepancies.
• The receiver also needs to know the exact
  position of the satellite in order to create an
  accurate fix.
• Each receiver is programmed with an almanac which
  provides the orbital position of each satellite
  at any given time. (ephemeris)
• However the satellites orbital position is
  subject to errors.

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GROUND STATIONS

• Ground stations continually monitor the
  navigation satellites position and operational
  health.
• Although the satellites are injected into a
  precise orbital pattern they are subject to
  trajectory changes.
• These ephemeris errors or orbit errors are caused
  by
• gravitational pull from the moon and sun.
• pressure from solar radiation.
• If the ground station detects an ephemeris error
  it transmits the exact position to the satellite.
• The satellite includes this corrected position
  information in its transmissions.
• Time discrepancies are also monitored by ground
  stations and corrections are transmitted to the
  satellites.
• If the ground station detects an operational
  fault in a satellite it instructs the satellite
  to take itself offline.

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Ground stations use precise radar to constantly
monitor satellites for ephemeris and time errors
and transmit corrections.
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ERRORS

• There are a few other variables that the receiver
  needs to take into account to ensure accuracy
• Atmospheric error
• Multipath error
• Satellite geometry
• Selective Availability ( SA was the method the US
  military used to degrade civilian GPS accuracy,
  SA was turned off in 2000 and is no longer
  applicable)

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ATMOSPERIC ERROR
As the satellite signal travels through the
atmosphere it is affected by the ionosphere and
the water vapour in the troposphere. The signal
is refracted slightly as it travels through these
mediums which delays its arrival time at the
receiver.
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ATMOSPHERIC CORRECTIONS

• The errors caused by the atmosphere change
  depending on the time of day and current weather
  conditions.
• A mathematical model correction of the average
  atmosphere could be applied but it would include
  inaccuracies caused by the ever changing
  atmosphere.
• The receiver also has to apply corrections
  depending on the angle of the satellites position
  (the more time spent traveling through the
  atmosphere the greater the effect).
• A more accurate method is called dual frequency
  measuring.
• Dual frequency measuring utilizes the fact that
  low frequency signals refract more than high
  frequency signals.
• The satellite actually transmits two signals on
  slightly different frequencies.
• Advanced receivers compare the delay between the
  two signals and are able to compute the
  atmospheric error and correct accordingly.

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MULTIPATH ERROR
In terrain sensitive areas the signal will
reflect from objects and cause ghosting when
multiple signals reach the receiver.
Sophisticated receivers eliminate ghost signals
by rejecting all but the direct signal which is
the first received.
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SATELLITE GEOMETRY
The angle of the satellite in relation to the
receiver has an affect on accuracy. Satellites
directly overhead result in a less accurate fix
than satellites which are widely spaced.
Receivers know the position of the satellites and
selectively use the ones with the most beneficial
geometry.
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AUGMENTATION SYSTEMS

• Even with all the errors previously discussed
  GNSS is still an extremely accurate navigation
  system.
• GPS currently produces accuracies of 6m
  horizontal and 8m vertical 95 of the time.
• GALILEO is expected to have a guaranteed accuracy
  of 4m with enhanced services producing 10cm
  accuracy.
• The stringent accuracy, integrity, continuity,
  and availability requirements of aviation have
  produced the emergence of augmentation systems.
• There are currently three types of augmentation
  systems
• aircraft-based augmentation system (ABAS)
• satellite-based augmentation system (SBAS)
• ground-based augmentation system (GBAS)

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AIRCRAFT-BASED AUGMENTATION SYSTEM(ABAS)

• All IFR certified receivers incorporate a RAIM
  (receiver autonomous integrity monitoring)
  function.
• RAIM is a fault detection scheme which monitors
  and analyzes satellite position, geometry, and
  signal integrity.
• To do this RAIM requires a fifth satellite in
  order to compare information.
• If RAIM detects a satellite position or geometry
  which doesnt support limitations for that phase
  of flight a RAIM alert will be displayed to the
  pilot. (2nm en-route, 1nm terminal, 0.3nm
  non-precision approach)
• In this case GNSS navigation should only be used
  in an emergency.
• If RAIM detects a range error, typically caused
  by a satellite malfunction that may cause an
  accuracy degradation exceeding limits for the
  phase of flight the receiver will discontinue
  supplying navigation information and flag the HSI
  or CDI accordingly.
• GNSS navigation is not possible until the
  satellite is flagged unhealthy by the ground
  station or normal satellite operation is
  restored.

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• Some receivers incorporate a FDE (fault detection
  and exclusion) function.
• FDE has the ability to recognize and isolate
  faulty satellites to allow continued and
  uninterrupted GNSS navigation.
• FDE requires a minimum of six satellites with
  good geometry in order to function.

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• RAIM availability is based on an analysis of
  satellite geometry and can be predicted by the
  receiver.
• Some receivers accept altimeter information from
  the aircraft altitude encoder.
• This baro-aiding reduces the required number of
  satellites by one which increases the
  availability of RAIM and enables VNAV capability.
• IRS (inertial reference systems) can be
  integrated into the GNSS receiver to allow it to
  coast through periods of low availability.

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SATELLITE-BASED AUGMENTATION SYSTEM(SBAS)

• SBAS uses geostationary earth orbit (GEO)
  satellites to supply receivers with corrections.
• Ground-based reference stations monitor satellite
  signals and asses their validity.
• Any corrections are sent to the GEO satellites
  which in turn broadcast to the receivers.
• The receivers are supplied with integrity
  information which takes the place of RAIM and
  reduces the amount of satellites necessary.
• Range corrections are also sent to receivers
  which compensate for common errors. (ephemeris,
  clock, atmospheric)

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• SBAS allows for the accuracy required for lateral
  navigation similar to a localizer and vertical
  performance somewhat better than BARO VNAV
  without the need for temperature correction or a
  field altimeter setting.
• SBAS has the potential to meet CAT I approach
  standards as the next generation of GPS
  satellites emerge.
• WAAS (wide area augmentation system) was the
  first SBAS to emerge and has been approved for
  en-route, terminal, and non-precision approach in
  Canada since 2003.
• WAAS receivers increase the availability of
  non-precision approaches to virtually 100.
  Accuracy is increased to lt3m.
• WAAS supported VNAV is currently available in
  most of the western provinces and a planned
  increase in ground reference systems will improve
  coverage.

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GROUND-BASED AUGMENTATION SYSTEM(GBAS)

• GBAS uses ground-based stations located locally
  (within 30nm of the airport) to provide integrity
  broadcasts to the aircraft receiver.
• Also known as LAAS (local area augmentation
  system) it holds promise to support all
  categories of precision approach and surface
  movement guidance.
• GBAS is currently not available in Canada.

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AUGMENTATION SYSTEM BENEFITS

• These augmentation systems all provide the level
  of navigation accuracy and integrity needed to
  support precision approaches with VNAV
  capability.
• The nature of GNSS approaches allows for designs
  which take advantage of existing terrain and
  variable approach paths.
• GNSS based approaches have the ability to reduce
  minimums in areas of difficult terrain, and
  replace existing ILS approaches.
• GNSS eliminates the need for a procedure turn and
  a straight in approach can be flown to most
  runways.

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RNP (required navigation performance) company
approach based on GNSS.
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GNSS APPROVAL

• Currently GPS and GPS augmented by WAAS is
  approved in Canada for en-route, terminal, and
  non-precision approaches.
• Approaches with vertical guidance classified as
  LPV (localizer performance with vertical
  guidance) and lateral navigation/vertical
  navigation (LNAV/VNAV) approaches are approved
  using WAAS.
• LNAV/VNAV approaches may be flown using GPS for
  LNAV and BARO VNAV.
• See AIM COM 3.16.5

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The following table lists the capability required
for each phase of flight
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Classification of GNSS-based RNAV Approaches

• Approaches served by traditional navaids are
  classified as
• NPA (Non-precision approach) lateral guidance
  only
• PA (Precision approach) lateral and vertical
  guidance
• The emergence of GNSS-based approaches has
  spawned some new classifications
• APV (Approach and landing operations with
  vertical guidance) GNSS-based lateral and
  vertical guidance

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GNSS-based RNAV Approaches

• Typical RNAV (GPS) and RNAV (GNSS) approaches
  will typically have three sets of minima
• LPV (localizer performance with vertical
  guidance-APV)
• LNAV/VNAV (lateral/vertical navigation-APV)
• LNAV (lateral navigation only-NPA)

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NEXT GENERATION GNSS

• The future of GNSS is bright with GALILEO
  expected to be operational by 2010 and a
  modernized GPS constellation should be
  operational by 2015.
• The two systems will be compatible and
  complimentary.
• This next generation of navigation satellite will
  transmit higher power signals on at least two
  frequencies.
• This will allow new avionics to compensate for
  atmospheric error.
• This should allow SBAS to support CAT l
  approaches.

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REQUIRED NAVIGATION PERFORMANCE (RNP)

• Typically to date aircraft have been required to
  carry specific minimum avionics for IFR flight.
• RNP will dictate the performance required to
  operate in a defined airspace rather than
  specific avionics.
• It will be up to the operator to determine if an
  aircraft is equipped to meet these performance
  specifications and that the required accuracy
  will be available throughout the phase of flight.
• Example in order to operate on a defined route
  the RNP might be lateral accuracy of 1nm. It is
  up to the operator to ensure the equipment on
  board will support this accuracy for the duration
  of the flight within the airspace.
• Aircraft performance factors will also be
  considered.
• Example aircraft with strong single-engine climb
  performance will be permitted to use lower
  minimums as the missed approach will be tailored
  to specific aircraft performance.