satellitepositioningsystemhc2001: 1 satellite positioning system in late 1980s, us department of...
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SatellitePositioningSystemHC2001: 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)
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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
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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
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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
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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
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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
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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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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.
General Applications of GPS
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Land, Sea and Air Navigation
including enroute as well as precision navigation, cargo monitoring, vehicle tracking, etc.
General Applications of GPS
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Search and Rescue Operations including collision avoidance and rendezvous functions.
Spacecraft Operations.
Military Applications.
General Applications of GPS
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Recreational Uses on land, at sea and in the air.
General Applications of GPS
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General Applications of GPS
Other specialised uses, such as time transfer, attitude determination, automatic operation, etc.
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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 Block II satellite 1st launched in 1989 (last one 94)
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)
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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
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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
Parameters Description Dimension(s)VDOP Vertical vector oneHDOP Horizontal vector twoPDOP Position vector threeTDOP Time vector -GDOP Geometric position and time vector four
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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 = 2.8GPS receiver searches for and uses the best GDOP
satellites during observation
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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.
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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.
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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.
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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
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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
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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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GLONASS Galileo Beidou Satellites
Other Satellite Positioning System
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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
GLONASS
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GLONASS
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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
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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
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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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Table Comparing GPS and GLONASS ( IVANON & SALISTCHEV, 1991 )
Parameter GLONASS GPS
Ephemeris information presentationmethod
9 parameters of s/c motion in thegecentric rectangular rotated coordinatesystem
Interpolation coefficientsof satelite orbits
Geodesic coordinate system SGS 85 WGS 84
Referencing of the ranging signalphases
To the timer of GLONASS systemTo the timer of GPSsystem
System time corrections relative to theuniversal coordinates time ( UTC )
UTC ( SU ) UTC ( USNO )
Duration of the almanac transmission 2.5 min 12.5 min
Number of satelites in the fulloperational system
21 + 3 apares 21 + 3 spares
Number of orbital planes 3 6
Inclination 64.8 55
Orbit altitude 19.100 km 20.180 km
Orbital period 11 h 15 min 12 h
Satelite signal division method Frequency division Code division
frequency band allocated1602.5625-1615.5
0.5 MHz1575.42 1 MHz
Type of ranging code PRN-sequence of maximal length Gold code
Number of code elements 511 1023
Timing frequency of code 0.511 MHz 1.023 MHz
Crosstalk level between twoneighboring channels
- 48 dB - 21.6 dB
Synchrocode repetition period 2 sec 6 sec
Symbol number in the synchrocode 30 8
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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”
Galileo
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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
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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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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
Beidou Satellites
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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
Beidou Satellites
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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