Satellite Positioning System

1
Satellite Positioning System

• in late 1980s, US Department of Defense (DoD)
  began to implement a second generation guidance
  system
• Navigation Satellite Timing And Ranging (NAVSTAR)
  Global Positioning System (GPS)

2
Satellite Positioning System

• this guidance system has tremendous potential for
  control surveys
• prior to NAVSTAR, precise positioning was
  determined from
• low-altitude satellites or
• inertial guidance systems

3
TRANSIT - 1st generation satellite

• consists of 5 satellites in polar orbit at an
  altitude of only 1000 kms
• positioning accuracy from 0.2 to 0.3 m using
  translocation techniques
• 1 receiver occupied a positioning of known
  coordinates while another occupied a point of
  unknown position
• data received at the known position was used to
  reduce transmission and orbital errors, thus
  permitting more precise results

4
INERTIAL SURVEYING SYSTEM (ISS)

• required a vehicle, truck or helicopter to occupy
  a point of known coordinates (X, Y and Z)
• as the vehicle moved, its location was constantly
  updated by the use of 3 computer-controlled
  accelerometers, each aligned to a north-south,
  east-west and vertical axis
• the accelerometer platform was also oriented
  towards the 3 directions by means of 3
  computer-controlled gyroscopes, each of which
  aligned to one of the three axes

5
INERTIAL SURVEYING SYSTEM (ISS)

• analysis of acceleration data gives rectangular
  (latitude and longitude) displacement factors for
  horizontal movement, in addition to vertical
  displacement
• replaced by GPS techniques for above ground
  positioning because of high cost of both the ISS
  equipment and its operation

6
GPS SYSTEM

• current system is based on accurate ephemeris
  data on the real-time location of each satellite
  and on a very precisely kept time
• uses satellite signals, accurate time, and
  computer programs to triangulate positions
  anywhere on earth
• the system consists of 24 satellite (3 spares)
• orbits have been designed so that positioning can
  be determined at any location on earth at any
  time of the day or night

7
GPS SYSTEM

• minimum of 4 satellites must be tracked to solve
  the positioning intersection equations
• the system, originally designed for military
  guidance, has quickly attracted a wide variety of
  proposed civilian users

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Applications

• commercial aviation
• boating and shipping navigation
• trucking and rail car inventory positioning
• emergency routing
• dashboard-mounted monitors displaying trip
  progress and destination maps in automobiles
• wide variety of surveying applications

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General Applications of GPS
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General Applications of GPS

• Surveying and Mapping
• on land, at sea and from the air
• applications are of relatively high accuracy, for
  positioning in both the stationary and moving
  mode
• includes geophysical and resource surveys, GIS
  data capture surveys, etc.

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General Applications of GPS

• Land, Sea and Air Navigation
• including enroute as well as precision
  navigation, cargo monitoring, vehicle tracking,
  etc.

12
General Applications of GPS

• Search and Rescue Operations
• including collision avoidance and rendezvous
  functions.
• Spacecraft Operations.
• Military Applications.

13
General Applications of GPS

• Recreational Uses
• on land, at sea and in the air.

14
General Applications of GPS

• Other specialised uses, such as time transfer,
  attitude determination, automatic operation, etc.

15
Differential GPS

• High-precision surveying receivers can determine
  positions
• to within a few metres when used alone
  (autonomously), and
• to one centimetre (or less) when used in
  differential mode
• one receiver occupies a station of known
  coordinates while other receivers are placed at
  stations requiring coordination

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Differential Mode

• after 30 to 60 minutes of observation, enough
  data is received to enable computation of
  coordinates (X, Y, and Z) to within one
  centimetre

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GPS in Survey Control

• Advantages
• distances and directions between points that are
  not intervisible can be precisely determined
• measurements can be performed in any weather and
  at any time of the day or night
• Accuracy of control is independent of the
  geometry of the network.
• Some receivers can be turned on and off remotely
  a valuable asset in deformation studies.

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GPS Satellites

• orbit the earth at about 20,200 km in a period of
  12 hours
• transmit at 2 L-band frequencies
• L1 at 1,575.42 MHz (? at about 19 cm)
• L2 at 1,227.6 MHz
• (? at 24 cm)

GPS Block II satellite 1st launched in 1989 (last
one 94)
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GPS Satellites

• L1 signal is modulated with 2 codes and a
  navigation message
• Coarse Acquisition (C/A) code
• Precise (P) code
• The message contains clock corrections and
  predicted orbital parameters, which are used in
  computer programs to assist in positioning
  solutions

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Selective Availability

• the C/A code is available to the public
• the P code is designed for military use
• only the P code is modulated on the L2 band
• in times of national emergency, DoD can degrade
  the satellite signals
• this degradation, called Selective Availability
  (SA), will occur on the P code and possibly on
  the C/A code as well.

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Positioning

• the key dimension in positioning is the parameter
  of time.
• time is kept onboard the satellites by atomic
  clocks with a precision of 1 nano second
  (0.0000000001s)
• the ground receivers are equipped with
  less-precise quartz clocks.
• uncertainties caused by these quartz clocks are
  resolved when observing the signals from 4
  satellites instead of the basic 3-satellite
  configuration required for rough positioning.

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Multipath Error

• similar to the ghosting effect seen on TV
• some signals are received directly and others are
  received after they have been reflected off
  adjacent features.

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Ionospheric and Atmospheric Refraction

• signals are slowed as they travel through these
  earth-centered layers

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Geometric Dilution of Precision (GDOP)

• the geometric strength of the figures that are
  developed by tracing the four-satellite signal
  intersections.
• GDOP can be optimized if many satellites are
  tracked and then the strongest four selected for
  computations

25
Poor GDOP

• when the satellites are close together or in a
  straight line, a low-accuracy fix is obtained

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Good GDOP

• When the satellites are wide apart, almost
  forming a square, a high accuracy is obtainable

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GDOP

• the satellite configuration with respect to the
  ground station is called GDOP
• GDOP number
• small good configuration
• large poor configuration

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Other DOP Parameters
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GDOP

• Observations should be avoided when large DOP
  values prevail
• 50 of the time
• HDOP ? 1.4 VDOP ? 2.0
• 90 of the time
• HDOP 1.7 VDOP 2.8
• GPS receiver searches for and uses the best GDOP
  satellites during observation

30
DOP values

• ?p ?DOP x ?R
• where
• ?p standard deviation of positional accuracy
• ?R standard deviation of the range
• For a VDOP 2.0, HDOP 1.5 and ?R ? 5m then
• ?p ? 10 m for the vertical position and
• ? 7.5 m for the horizontal

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Static GPS

• most of the above errors and the denial of access
  by the DoD can be surmounted by using
  differential surveying techniques.
• the net errors in the satellite transmission can
  be identified by the receiver placed at a point
  of known coordinates.

32
Static GPS

• The corrections
• can be applied in later post-processing, or
• can be broadcast from the base receiver to the
  rover receivers, with corrections being processed
  on site.
• As the satellites are so high, it can be safely
  assumed that many of the errors at one receiver
  (base) will be the same as errors at the other
  receivers.

33
Static GPS

• The technique of differential GPS positioning
  where one base receiver is placed over a point of
  known coordinates, while others are placed over
  points to be located, is known as Static GPS.
• This techniques requires 30 to 60 minutes of
  observations, some of which must be simultaneous
  between the base station and the surveyed
  station.

34
Kinematic GPS

• This technique begins with the base receiver and
  the river receiver occupying 2 known points on a
  short (usually) baseline
• After the initialization, the rover receiver is
  moved to all survey points requiring coordination
• Reading time at each station is quite short (2 to
  3 minutes)
• The trick is not to lose any of the four required
  tracking signals as the receiver and its antenna
  are moved from point to point

35
Kinematic GPS

• The travel can be by foot or by vehicle, with the
  antenna attached to a referenced external mount
• If the signals to 4 satellites are interrupted,
  e.g. due to underpass, tree cover, tall building
  interference, the rover receiver must return to
  one of the previously surveyed points for
  re-initialization
• If more than 4 satellites are originally tracked,
  a safety factor is created that can save repeat
  work
• Much mapping work has already been completed
  using this method.

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Pseudo-kinematic

• combination of static and kinematic techniques
• requiring the roving receiver to reoccupy each
  survey point several times so that readings can
  be received from the tracked satellites at all
  significantly different geometric views of the
  constellation
• more time-consuming than kinematic GPS

37
Pseudo-kinematic

• a benefit that the satellites do not have to be
  continuously tracked, in fact, receivers could be
  turned off between stations
• ideal for use in urban and wooded areas, where
  kinematic techniques may not be realistically
  employed because of signal interference

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Other Satellite Positioning System

• GLONASS
• Galileo
• Beidou Satellites

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GLONASS

• Global Navigation Satellite System designed by
  Soviets
• similar to GPS, full network includes 24
  satellites - 21 operational and 3 spares
• transmit identical codes but at different
  frequencies (reverse of the scheme used for GPS

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GLONASS
41
GLONASS

• orbits are at an altitude of 19,100 km slightly
  lower than GPS satellites
• satellites are placed in 3 orbital planes
  (inclination of 64.8º), each containing 8
  satellites
• each satellite complete an orbit in 11 hrs 15
  mins
• location accuracy capabilities roughly similar to
  those of GPS
• does not impose selective availability (SA) on
  civilian users

42
GLONASS

• Although in operation since 1983, full
  constellation has never been implemented due to
  the troubled economic circumstances in Russia
• as of mid 2001, only 8 are in operational but the
  Russians hope to have 12 working in orbit by
  early 2002

43
GLONASS

• there has been some work in building receivers
  that can obtain signals from both GPS and
  GLONASS, providing substantially greater accuracy
  than would be possible from either by itself
• use of two satellite systems also allows users a
  continued operational capability if one of the
  systems is shut down

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Problem in Combining GLONASS GPS

• they use different global coordinate systems
• GPS uses WGS-84 in which the precise location of
  the North Pole is fixed at its location in 1984
• GLONASS uses PZ-90 in which the precise location
  of the North Pole is given as an average of its
  position from 1900 to 1905
• linking the 2 coordinate systems has proven
  difficult since GLONASS has fewer receivers than
  GPS receivers and performing calibrations between
  the two systems has been troublesome

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Galileo

• European Community is now implementing the
  Global Navigation Satellite System 1 (GNSS-1)
• GNSS-1 will integrate services from GPS, GLONASS,
  WAAS, MTSAT and EGNOS augmentation networks
• stepping stone to a completely independent
  European GNSS-2

47
GALILEO

• GNSS-2 or Gailieo will be based on an entirely
  new satellite system
• a constellation of 21 or 36 satellites that will
  also be integrated with ground augmentation
  networks
• unlike GPS, Galileo will be under complete
  civilian control
• European military forces have expressed interest
  in making use of Galileo, but have not offered to
  help with funding

48
GALILEO

• positioning services will be offered free but the
  system may include paid-access services, such as
  navigation-related telecommunications channels,
  to help defray costs
• tax on receivers is also being considered
• expected to begin operation no earlier than 2005
• Russians and the Japanese may also join effort
• at present, the scheme remains bogged down in
  negotiations and bureaucracy

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GALILEO
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Satellite Positioning Systems
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Beidou Satellites

• China is experimenting with her own satellite
  navigation system
• Beidou-1 Navigation Test Satellite was launched
  by a Chinese Long March 3M booster on 31 Oct.
  2000 into geostationary orbit slot at 140? E
  Longitude to the east of China

52
Beidou Satellites

• companion Beidou-2 satellite may be put into
  geostationary orbit at 70? E Longitude to the
  west of China
• the 2 satellites will provide navigational
  coverage over the entire country

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GPS Upgrade

• GPS modernization programme
• removal of Selective Availability
• increase in number of operational satellites
• introduce a third frequency (close to L1)

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Pseudolites

• overcome problem of masking
• include activities in tunnels and mines, very
  heavy tree canopies, major built up areas and
  inside buildings
• pseudolites - small devices that can be connected
  to a GPS antenna to transmit GPS look-alike signal

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Pseudolites

• enormous potential inside buildings and other
  places that current GPS signals cannot be reached
• (Source Cross, P.A. (1999) Summary of Keynote
  Speech, the 1st Hong Kong Symposium on Satellite
  Positioning System Application 99.)

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Hong Kong GPS Network

• links GPS measurement to Hong Kong Spatial
  Reference System
• defines reference frame for GPS positioning
• Hong Kong Active Control System

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Hong Kong Active Control System

• collects GPS data continuously from multiple
  reference stations and delivers quality-checked
  data to the users
• provide cm-level accuracy within short periods of
  time
• reduces both labour cost and equipment investment

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GPS Network

• 1991
• jointly conducted by British forces, H K
  Government and the Macau Government
• adjustment carries out by 512 Specialist Team
  Royal engineers (STRE)
• known as STRE91 reference frame
• 2000
• densified network consists of 46 points
• average station spacing is about 10 km
• coordinates values published in the year 2000
• average relative accuracy is 0.2 ppm

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Kau Yi Chau Permanent GPS Reference Station
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Kau Yi Chau Permanent GPS Reference Station
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Tiu Keng Leng RTK Reference Station