GNSS an introduction - ULisboa · satellite's internal orbital model, and internal clock ......
Transcript of GNSS an introduction - ULisboa · satellite's internal orbital model, and internal clock ......
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GNSS
an introduction
“ Imagine, accurate positioning dropping right out of the sky at
the touch of a button!”
Jeff Hurn for Trimble Navigation, 1993
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The use of out-of-Earth references was always a natural
approach to the problem of positioning. The use of artificial
satellites – the GPS is the most used of such systems – is in
this evolution path: it allows to easily get the coordinates of a
point with low-cost devices.
Using it for topographic or geodetical purposes requires a
distinct methodology and equipment from the simple
navigation tasks most users perform.
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Spatial positioning
Uraniborg Obs.
McDonald Obs. –Texas Univ. Ale
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Astronomical positioning
Radar and distancemeter – WWII
TRANSIT
Navigation system (low precision)
Doppler process allowing submetric precision
1970
VLBI Very Long Baseline Interferometry
SLR Sattelite Laser Ranging
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VERY LONG BASELINE INTERFEROMETRY
Observations of a sender (object) that are made simultaneously by a set of receivers (telescopes) to be combined: emulates a large receiver
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Goddard Geophysical and Astronomical Observatory. LAGEOS
SATELLITE LASER RANGING
LUNAR LASER RANGING
Laser Ranging Station
at McDonald Observatory
Satellites with
retroreflectors and
measurements of
millimeter level
precision, useful for
geodynamic studies
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1978 – 1st satellite block I
1989 – 1st satellite block II
1985 – 10 satellite block I
1994 – Operational
2000 – S/A discontinued (selective availability) allowing users to receive a non-degraded signal
1973 Navigation System with Timing and Ranging Global Positioning System (DoD)
1981 – 1º receiver code/phase
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GPS
Block II satellite
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WHAT IS GNSS FOR? Get the position
Get the velocity
Get [precise] time
BASIC PRINCIPLE Measure the receiver-to-satellite distances
GNSS APPLICATIONS
Navigation (sea, earth, air)
Geodesy and geodynamics
Topography and cartography
GIS
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SPACE SEGMENT
CONTROL SEGMENT
USER SEGMENT
GPS SYSTEM COMPONENTS
GPS is the most used GNSS
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Constellation of 24 satellites:
8 per orbital plane
Altitude: 20200 km
Period: 11h58m
Constellation period: 23h56m
Inclination: 50o (tilt relative to equator)
SPACE SEGMENT
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CONTROL SEGMENT
1 Control Station (Schriever Air Force Base - Colorado Springs) adjust the ephemeris of each
satellite's internal orbital model, and internal
clock
4 Monitoring Stations
(Hawaii and Kwajalein, Pacific; Diego Garcia, Indian and Ascension Island,
Atlantic) for information retrieval on each
satellite’s behavior, and communication with
Colorado Springs
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USER SEGMENT
“CIVIL” : SPS (10m)
“MILITARY – USA”: PPS (1m)
SA - SELECTIVE AVAILABILITY
(currently disabled)
AS – “Anti Spoofing”
(encrypted, preventing access to code P)
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New satellites (Block IIR-M, IIF)
Change of signal: L1 and L2 (increased signal power)
New signal (L5)
New codes have been created
- L2C (Civilian L2)
- M (Military)
Next generation - GPS III: NASA has requested that Block III
satellites carry laser retro-reflectors: that makes tracking the orbits of the
satellites independent of the radio signals, which allows satellite clock
errors to be disentangled from errors of the ephemeris.
System evolution
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Other GNSS
Galileo satellite (ESA)
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GLONASS – Global Orbiting
Navigation Satellite System
Conceived by:
Russian Federation Dept. of Defense
Space segment: 21 satellites in 3 orbital planes, + 3 spare; orbit
19100 km, period 11h15m, system period 8 days
Completed constellation in 1995: completion, decay, modernization
Send both standard precision (SP) and an obfuscated high precision
(HP) signals
GPS and GLONASS use different coordinate systems
As of 2008, GLONASS availability: Russia 66.2% whole Earth 56.0%
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EGNOS- European Geostationary Navigation Overlay
Service ( perspective: increase the positional precision from 20m to 2m)
•SPATIAL SEGMENT
•Three geostationary satellites (A geostationary orbit (GEO) is a
geosynchronous orbit directly above the Earth's equator)
•Inmarsat III Atlantic Ocean region – East (15.5ºW)
•ESA ARTEMIS (21.5ºE)
•Inmarsat III F5 (25ºE)
And:
Precise information on each GPS
satellite’s position;
Information from on board clocks;
Ionosphere parameters
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CONTROL SEGMENT
34 additional stations Main control stations
USER SEGMENT
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WAAS – Wide Area Augmentation System
Federal Aviation Administration (FAA) and Department of Transportation
(DOT)
OPERATION:
25 stations in USA; 2 control stations
Receive GPS information, calculate and
broadcast corrections to apply to GPS data
(orbital drifts, clock errors, ionosphere and
troposphere delays)
USE IN: North America and West Europe
Operational Sept- 2002 : position precision: horizontal 1-2 m
vertical 2-3 m
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Coverage areas for WAAS, EGNOS and
MSAS
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SPACE SEGMENT
30 satellites (orbit 24 000 km, ~ 4 000km more
than GPS) in 3 orbital planes, inclination 56º.
9+1 satellites per orbital plane.
CONTROL SEGMENT
2 main centers in Central Europe
GALILEO – European Satellite Navigation System
European Commission and ESA
In 2007 the 27 EU transportation
ministers anounced operationality by
2013
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Four future different navigation services:
•Open Service (OS), free access. Accuracy of <4 m
(horiz.); <8 m (vert.) in simultaneous use both OS bands;
Single band <15 m (h.) / <35 m (v.)
•Commercial Service (CS), encrypted, available for a
fee. Accuracy < 1 m. If complemented by ground stations:
accuracy <10 cm.
•Public Regulated Service (PRS) and Safety of Life
Service (SoL), both encrypted, similar accuracy to OS.
Robustness against jamming and reliable detection of
problems within 10 seconds. Exclusive use by police,
military and safety-critical transport applications (air-traffic
control, automated aircraft landing, etc.), respectively.
GALILEO – European Satellite Navigation System
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OPERATION
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6 seconds
4 seconds
AB
In a perfect world
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6 seconds4 seconds
5 seconds(wrong time)
7 seconds(wrong time)
AB
hypothetical 1s delay in receiver’s clock A
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6 seconds4 seconds
8 seconds
AB
C
again the perfect world
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5 seconds(wrong time)
7 seconds(wrong time)
9 seconds(wrong time)
BA
C
hypothetical 1s delay in receiver’s clock
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OPERATION
•The basis of GPS is triangulation from satellites: to
triangulate, a GPS receiver measures the distance
using the travel time of radio signals
•To measure travel time, GPS needs very accurate
timing which it achieves with some tricks
•Along with distance, the satellites’ position must be
known. High orbits and careful monitoring are critical
•The correction of any delays the signal experiences as
it travels through the atmosphere, must be accounted
for
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Fo=10,23 MHz
L1= 154xFo = 1575.42 MHz (19,0cm wavelength)*
L2= 120xFo = 1227.60 MHz (24.4cm)
C/A (Coarse/Acquisition) 1.023 MHz
P (Precise) 10.23 MHz
GPS SIGNAL
(oscillation of the carrier wave)
(Carrier Phase)
Code
* this makes the carrier signal act as a
much more accurate reference than
the pseudo random code by itself
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r r
r
r
1
2 3
4 t o
t 1 r 4 = t 1 t o v ( - )
PSEUDO-DISTANCE (Pseudo-Range)
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.)()()(
,)()()(
,)()()(
,)()()(
2424244
2323233
2222222
2121211
kkkkk
kkkkk
kkkkk
kkkkk
cdtZZYYXX
cdtZZYYXX
cdtZZYYXX
cdtZZYYXX
(Xi,Yi,Zi) – Satellitei position (Xk,Yk,Zk) – Receiver position dtk – Receiver clock status
CALCULATION OF COORDINATES BASED ON PSEUDO-DISTANCES
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D t
USE OF CODE
Receiver-generated signal
Received satellite signal
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Complete and integer number of cycles from the
emission instant to the reception instant
USE OF CARRIER WAVE SIGNAL
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Epoch (0)
Ambiguity
Epoch (i)
Ambiguity
Counted cycles
Measured phase
It is necessary to measure the ambiguity of the
carrier wave phase, in order to use its information
as a distance measuring procedure along with time
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METHODS FOR SOLVING THE
AMBIGUITIES
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OPERATION A
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0,50 m 3-5 min 1 sec.
< 40 km
ABSOLUTE POSITIONING
DGPS
2,00 m 3-5 min 1 sec.
< 300 km
WADGPS 4 m
SPS 5 to 15 m
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Single frequency Double frequency
C/A + L1 C/A + L1,L2
Static (Conventional)
Static (Rapid)
Kinematic
0,02 m 30 min to 10 km
+ 3 min/km 5 to 10 s < 20 km
0,01 m 20 min to 10 Km
+ 2 min/km 5 to 15 s < 20 km
0,005 m 20 min to 10 km
+ 2 min/km 5 to 30 s < 20 Km
0,01 m 10 min to 10 km
+ 1 min/km 5 to 10 s < 20 km
0,01 m (RTK) < 20 km -
RELATIVE POSITIONING
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Navigation mode
SPS (C/A) : 10m
PPS (P) : 1m
post-processing
real time
ABSOLUTE POSITIONING
WADGPS: regional differential correction
DGPS: local differential correction
0,5m-2m
4m
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Reference station at Instituto Superior Técnico (http://websig.civil.ist.utl.pt/gps)
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Rinex format
Receiver Independent Exchange Format Version
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Differential correction broadcast
Coast stations (Beacon Stations)
Geostationary (Racal/OmniStar)
GSM antennas
Other (via radio)
RTCM – Radio Technical Commission for Maritime Services
(Standard for differential correction transmissions)
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SPS receivers
Most future mass market receivers, such as these, will process
both the GPS C/A and the Galileo OS signals, for maximum
coverage.
automotive navigation systems recreational navigation systems
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navigation system
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DGPS receivers
(real time or post-processing)
Border demarcation in
Timor-Leste / Indonesia
Trimble Geo-Explorer 2005
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DGPS receivers
(real time or post-processing)
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AFTER DGPS CORRECTION
BEFORE DGPS CORRECTION
EXAMPLE
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Static
RELATIVE POSITIONING
Conventional
Rapid Static
Kinematic
Real Time Kinematic
(RTK)
Post-processing
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B A
RELATIVE
POSITIONING
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RENEP
REDE NACIONAL DE ESTAÇÕES PERMANENTES
(INSTITUTO GEOGRÁFICO PORTUGUÊS)
Cascais
Ponta Delgada
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Accuracy (m)
1.00
0.10
0.01
Conventional static 0 120
Rapid static
0 2 5
Time (min)
Unsolved ambiguities
Solved ambiguities
When the ambiguities (number of integer cycles) are
solved, accuracy will not increase significantly
COLLECTING TIME
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For large bases (>20 km), where the greatest possible precision is required Useful for implementing geodetical networks Useful for large areas
For bases up to 20 km Short occupying times Great productivity in most applications
Static mode
Static-rapid
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For moving objects (simulation of continuous measurements of the position) Useful for road mapping and related applications
KINEMATIC POSITIONING
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Post-processing is not needed Availability of results right on occasion Several applications in navigation
RTK – Real-Time Kinematic
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Mono frequency receivers
Base measurement with accuracy up to 2 cm + 1 ppm
(rms) Use carrier wave L1
Topographical applications with base length up to 15 km Inexpensive solution, but difficult to achieve the above
mentioned precision
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Base measurement with accuracy up to 3 mm + 0.5 ppm (rms) Use in geodesy and topography :
Geodetic networks, Geodynamics, Network densification, Photogrammetric control, Topographic detail
Double frequency receivers
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MEASUREMENTS
With Selective Availability
on
2001
2006
http://www.mar-it.de/NavGen/navgen_r.htm
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Multi-path
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Ionospheric delay
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Re
ce
ive
r n
ois
e
Mu
lti-
pa
th
Sa
tel. c
locks
Tro
po
sp
he
re
Ep
he
me
ris
Ion
osp
he
re 0
10
20
30
40
m
Relative importance of error sources A
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Error propagation
PDOP – Position Dilution of Precision
HDOP – Horizontal Dilution of Precision
VDOP – Vertical Dilution of Precision