school of engineering gps (introduction)rumc/msewirecom/gps/msewirecom gps.pdf · . chapter 1.1:...
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School of
Engineering
MSE, Rumc, GPS, 1
References
[1] Jean-Marie Zogg [HTW Chur], „GPS, Essentials of Satellite Navigation, Compendium“,
Document: GPS-X-02007-D, February 2009, http://www.u-blox.com/de/tutorials-links-gps.html.
Chapter 1.1: The principle of measuring signal transit time
Chapter 2.3.4: WGS-84
Chapter 4: GNSS technology: the GPS example
Chapter 7.2: Sources of GPS error
Chapter 8.2: Data interfaces
[2] GPS SPS Signal Specification, 2nd Edition (June 2, 1995),
http://www.navcen.uscg.gov/pubs/gps/sigspec/default.htm
[3] beautiful visualisation of the satellites‘ positions by HSR / ICOM
http://icom4u.hsr.ch/giove_a/index.htm
[4] Parkinson, Spilker, „Global Positioning System: Theory and Applications“, Volume I/II,
Progress in Astronautics and Aeronautics, Volume 163/164, 1996.
Terms
NAVSTAR GPS („Navigational Satellite Timing and Ranging - Global Positioning System)
is a GNSS (Global Navigation Satellite System), developed by the US-DoD in 197x and
fully operational since 1993.
Other GNSS under „development“: Glonass (Ru), Galileo (EU), Beidou/Compass (China)
GPS (Introduction)
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School of
Engineering GPS-Principle
Assumptions
1. distance A between Tx is known.
2. Tx transmit synchronously,
Rx can only measure TDOA
(time difference of arrival).
Determination of positions via Time-of-Fly measurements
Conclusions
x-position (and time) with 2 Tx and
x,y,z-positions (and time) with 4 Tx
determinable!
Source: [1]
MSE, Rumc, GPS, 2
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School of
Engineering
TDOA measurement by code correlation
Tx1 Tx2 Rx
D = (Δt∙c+A)/2
A
Code s1 with N chips
Tx1
Tx2
Rx
t
t
t
DSSS-modulation
(small peak-power
supports CDMA)
after correlation
with code s1
with code s2
∆τ
Tchip
Tchip
∆τ2
∆τ1
N chips
N chips
GPS-Principle MSE, Rumc, GPS, 3
Code s2 with N chips
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School of
Engineering
Worldwide Reference Ellipsoid WGS-84
Ellipsoid approximates true (complex) shape of the earth
there are many different reference systems
GPS works with geocentric WGS-84 reference system
Source: [1]
cartesian coordinates
ellipsoidal coordinates (longitude, latitude, altitude) used for further processing
1° Grad = 60’ Bogenminuten.
1’ Bogenminute Breite = 1 Seemeile bzw. 1 nautischen Meile (NM) = 1.852 km.
1’ Bogenminute Länge = 1.852 km mal cos(Breitengrad).
conversion into CH-1903
coordinates required
[1]
GPS-Principle MSE, Rumc, GPS, 4
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School of
Engineering
Basic equations
x,y,z,t coordinates and time of user
xi,yi,zi,ti coordinates and time of 4 satellites
(x1-x)2 + (y1-y)2 + (z1-z)2 = [c·(t1-t)]2
(x2-x)2 + (y2-y)2 + (z2-z)2 = [c·(t2-t)]2
(x3-x)2 + (y3-y)2 + (z3-z)2 = [c·(t3-t)]2
(x4-x)2 + (y4-y)2 + (z4-z)2 = [c·(t4-t)]2
4 equations (c: speed of light) and 4 unknowns
GPS-Principle
Source: [1]
MSE, Rumc, GPS, 5
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School of
Engineering GPS-Subsystems
(orbital data)
1 Master Control Station (Colorado)
5 Monitor Stations world wide
3 Ground Control Stations
(with Satellite Uplink)
Source: [1]
MSE, Rumc, GPS, 6
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School of
Engineering GPS-Space Segment
24 to 32 Satellites
55°
• at a height of 20‘180 km
• 6 different orbital planes
(4-5 satellites per plane)
• time of circulation ≈ 12 h
• always ≥ 4 satellites
visible everywhere on
earth
MSE, Rumc, GPS, 7
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Engineering
[1]
coverage area
GPS-Space Segment
Orbit and coverage area
MSE, Rumc, GPS, 8
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School of
Engineering GPS-Space Segment
Link budget
25119 km (border of coverage area)
L1 (1575.42 MHz) Coarse/Acquisition (C/A-) Code for civil use
min. sensitivity
specified in [2]
[1]
MSE, Rumc, GPS, 9
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Engineering
Spectral power density of received signal and (thermal) noise floor
MSE, Rumc, GPS, 10
Link Budget
<= -130 dBm / MHz
-
source
bandwidth
1 MHz ≈ 1/Tchip
[1]
-174
signal before
despreading
-160
+ 14 dB
signal after
despreading
f – fL1
<= thermal noise + noise figure F
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School of
Engineering Satellite-Signal
1575.42 MHz
Tchip ≈ 1 / Bandwidth
Source: [1]
MSE, Rumc, GPS, 11
t / ms 1 2 20
C/A-code C/A-code C/A-code
Tbit
1023 Tchip
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School of
Engineering
32 Gold- / PRN-codes with N = 1023 chips
Generation with 2 LFSR, chip rate 1.023 Mchip/s
satellite identified by PRN-number
=> CDMA
GPS-Coarse/Acquisition-Codes MSE, Rumc, GPS, 12
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School of
Engineering GPS User Segment
Correlation receiver Source [1]
(Doppler-Shift ± 5000 Hz)
Process-Gain 10·log10(1023) ≈ 30 dB
SNR = -16 dB before despreading => SNR = +14 dB after despreading
correlation time for data demodulation is 20 times longer
Gain
MSE, Rumc, GPS, 13
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School of
Engineering GPS Navigation Message
Source: [1]
MSE, Rumc, GPS, 14
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School of
Engineering
Navigation message contains 25 frames and lasts 12.5 minutes
a GPS-frame has 5 x 300 = 1500 bits and lasts 30 s
Subframes 1-3 are identical for all the 25 frames
subframe 1 contains clock data of transmitting satellite
subframes 2 and 3 contain ephemeris data of transmitting satellite
ephemeris data are highly accurate orbital data
a receiver has the complete clock values and ephemeris
data from the transmitting satellite every 30 seconds
Time-To-First-Fix (cold start autonomous) at least 18-36 s
=> slow start-up is a system-inherent limitation of GPS
Subframe 4-5 are different for all the 25 frames
subframe 5 contains almanac data of first 24 satellites plus health
almanac data are less accurate than ephemeris data
subframe 4 contains almanac data of satellites 25-32
and difference between GPS and UTC time
GPS Navigation Message MSE, Rumc, GPS, 15
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School of
Engineering Accuracy without Selective Availability
Source: [1]
95%- or 2σ-accuracy: 100 m 95%- or 2σ-accuracy: 13 m
Deactivation of SA in the year 2000
68% or σ-accuracy: 6.5 m
MSE, Rumc, GPS, 16
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School of
Engineering Improved GPS
Accuracy
90% < 10 m, artifical degradation switched off since 2000
Differential GPS
Main sources of GPS errors
effect of the ionosphere (counter measure: two frequency receiver)
multipath (mainly in urban areas)
effect of the satellite constellation (DOPs [Dilution of Precision])
transmission of
correction factors Source: [1]
MSE, Rumc, GPS, 17
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School of
Engineering
EGNOS (European Geostationary Navigation Overlay System)
34 ground stations calculate correction signals (à la DGPS)
for GPS correction in a radius of about 200 km around the reference station
broadcast of correction signals via 3 geostationary satellites (C/A-Codes >32)
1-3 m accuracy
Improved GPS
Source: [1]
MSE, Rumc, GPS, 18
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School of
Engineering Improved GPS
Achievable accuracy with DGPS and SBAS
SBAS: satellite based augmentation systems
[1]
MSE, Rumc, GPS, 19
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School of
Engineering Improved GPS
Some Location Based Services are based on satellite navigation
GPS-Rx not always „on“, e.g. because of current consumption
time to first fix (cold start): 18-36 s (missing orbital data)
Assisted GPS (A-GPS)
delivery of missing orbital data via „fast“ channel, e.g. GSM/GPRS
[1]
MSE, Rumc, GPS, 20
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School of
Engineering Data Interface to Peripherals
NMEA-0183 data interface
standardized by National Marine Electronics Association (NMEA)
data telegram for serial interface
Example: GGA data set (GPS fix data)
$GPGGA,130305.0,4717.115,N,00833.912,E,1,08,0.94,00499,M,047,M,,*58<CR><LF>
MSE, Rumc, GPS, 21
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School of
Engineering Time Pulse
Most GPS-Rx generate 1- 4 time pulses per s
time puls is synchronized to UTC-time
Accuracy 5 - 60 ns
[1]
MSE, Rumc, GPS, 22
GPS-time-pulse is often used to synchronize devices
in a «large» area as e.g. base stations, gliders, …
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School of
Engineering Performance Data of a GPS-Rx MSE, Rumc, GPS, 23
NEO-M8 series:
12.2 x 16.0 x 2.4 mm
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School of
Engineering Modernization: BOC-Modulation
Advantages higher interference robustness and bandwidth efficiency
[1]
MSE, Rumc, GPS, 24
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School of
Engineering Modernization: BOC-Modulation
BOC(1,1) and BPSK(1) have minimal impact on each other
Source: [1]
MSE, Rumc, GPS, 25
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School of
Engineering GPS-Modernization
2. and 3. frequency for civil applications
compensation of ionosphere errors!
after 2013
integrity-signals, Search-and-Rescue-Functions
Source: [1]
MSE, Rumc, GPS, 26
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School of
Engineering GPS-Simulator: An Example MSE, Rumc, GPS, 27
GPSG-1000 from Aeroflex / Cobham
• validation and test of GPS receivers
as well as navigation and tracking systems
• 3D position may be user entered
or 3D position may be dynamically simulated
• simultaneous GPS/Galileo simulations
antenna coupler
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School of
Engineering
GNSS-Update: Frequency Bands see Navipedia http://www.navipedia.net/index.php/Main_Page
and some comments, https://www.zhaw.ch/~rumc/MSEwirecom.html
T. Kouwenhoven, "Gnss navigational frequency bands.png",, Jan 2011, also available at
http://www.navipedia.net/index.php/File:GNSS_navigational_frequency_bands.png
MSE, Rumc, GPS, 28
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School of
Engineering Availability of GPS civil signals (Sep 2016) MSE, Rumc, GPS, 29
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School of
Engineering Availability of Galileo civil signals (Sep 2016) MSE, Rumc, GPS, 30
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School of
Engineering GNSS-Update: Signal-Spectra MSE, Rumc, GPS, 31
Source: Stefan Wallner, http://www.navipedia.net/index.php/GNSS_signal
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School of
Engineering GNSS-Update: Signal-Spectra MSE, Rumc, GPS, 32
Source: Stefan Wallner
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School of
Engineering Correlation Matrices of GPS-Satellite 9
PRN periode = 20 ms
∆f = 3100 Hz ∆f = 2400 Hz
doppler shift ∆f = 2300 Hz correlations show expected coherence
regarding the doppler shifts (∆f is
proportional to carrier frequency fc)
MSE, Rumc, GPS, 33
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School of
Engineering
MSE, Rumc, GPS, 34
Real Time Kinematics (RTK)
• is a differential GNSS technique
• provides cm-level positioning performance in the vicinity of a base station
• carrier-based (rather than code-based) positioning
• see also: http://www.novatel.com/an-introduction-to-gnss/chapter-5-
resolving-errors/real-time-kinematic-rtk/
GNSS-Update: RTK
complicated process
“ambiguity resolution”
is needed to determine
the number of whole cycles.
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School of
Engineering
u-blox, „u-blox bringt GNSS-Technologie mit zentimetergenauer Präzision
für den Massenmarkt“, https://www.u-blox.com/de/press-release/u-blox-
brings-centimeter-level-precision-gnss-technology-mass-market
Example: GNSS RTK module from uBlox
RTCM protocol
MSE, Rumc, GPS, 35
NEO-M8P (1-frequency Rx)
faster with multi-frequency GNSS-Rx
some m to 1-10 km