SatellitePositioningSystemHC2001: 1 Satellite Positioning System in late 1980s, US Department of...

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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