gps: everything you wanted to know, but were afraid to ask andria bilich national geodetic survey
TRANSCRIPT
GPS:Everything you wanted
to know, but were afraid to ask
Andria BilichNational Geodetic Survey
Outline
• What is GPS?• History of GPS and its components• GPS signals• GPS observables• Errors on GPS signals• Science using GPS ‘errors’
What is GPS?
• GPS = Global Positioning System
• Satellite navigation system– Radio signals– Worldwide
coverage• System = satellites
+ ground stations + receivers
Drawing: DoD
How does GPS work?
• Trilateration • Distance from
objects (satellites) allows determination of position
A
rA B
rB
C
rC
How well does GPS work?
Surveying: single-freq, differential = cm to
meters
Handheld: single-freq, real-time = several meters
Geodesy: dual-freq, post-processing, relative positioning = mm to cm
Uses of GPS
• Position• Navigation• Mapping• Land surveying• Cell phones• Science• Synchronization and time transfer
GPS History
• Mid-70s: designed by the Dept of Defense• 1978 = first satellite launch• Early 80s = launched ~10 experimental satellites for
validation of concept (Block I)• 1989 = Block II launches began• 1995 = completed constellation (24 satellites)
declared fully operational• 2000 = “selective availability” turned off• 2005 = first launch of next generation satellites (L2C)
GPS System Segments
Space segment(satellites) Control
segment(operations)
User segment(receivers)
Space Segment
• 24+ satellites• Orbit
– 26K km radius– 12 hour period– Stationary ground
tracks– 6 orbital planes
Control Segment
• Monitor stations
• Master Control Station
• Ground antennas
Predict and upload satellite clock and orbit parameters
Steer satellitesMonitor healthMaintain GPS time
User Segment
• That’s us!• Actually, GPS receivers, antennas, and
processors• Receive GPS signals and use them to
compute position, velocity, and/or time
GPS signal (1)
Receiver takes in…• 4-12 satellites (in view)• 2 L-band (1-2 GHz) frequencies
– L1 = 1572.42 MHz– L2 = 1227.60 MHz
• Signal components– Carrier (sinusoidal signal)– PRN code (data bits for satellite ID and ranging)– Navigation message (satellite position/velocity
info)• Timing information
GPS signal (2)PRN codes
• C/A – “Coarse acquisition”
code– Civil use– Chip = 1s = 300m
wavelength– Range +/-30m
• P(Y)– “Precision” code– Military use– Chip = 0.1 s = 30m
wavelength– Range +/-3m– Encrypted (Y code)
to limit access = anti-spoofing3 separate signals:
•On L1 = C/A and P(Y)•On L2 = P(Y) only
Future Improvements
What?• New signals
– L2C (civilian L2) - now– L1C (new civilian L1) - 2013– L5 (aviation) - 2008
• New satellites
Why?• Dual frequency =
remove ionosphere• More options:
– Signal power– Center frequency– Code chip rate– Correlation properties– Interference protection
• Play well with other GNSS…
GNSSGlobal Navigation Satellite Systems
System Who runs it # Satellites(design/in use)
When
GPS US DoD 24 / ~30 now
GLONASS Russia & India 24 / ~10 usable
2011
Galileo EU & ESA 30 / 1 test 2012
All are L-band radio systems (~ 1100 -1600 MHz)
Mostly free signals
GPS Observables
Two ways to determine satellite-receiver distance:
• Pseudorange– From the PRN codes– Local code aligned
with incoming signal– Time delay between
local and incoming signal * speed of light = pseudorange
– Precision• C/A: 30 m• P: 3 m
• Phase– From the carrier signal– Local carrier
determines fractional phase
– Integer # cycles + fractional phase = phase
– Precision• L1: 1.9 mm• L2: 2.4 mm
Observable Models
€
Rrs = ρ − cδ s + cδr + ρ trop + ρ ion + ρmulti + ρ rel + ε
€
φλ =ρ−cδ s + cδr + ρ trop − ρ ion + ρmulti + ρ rel + Nλ + ε
Pseudorange
Carrierphase
One Person’s Noise is Another’s Signal
• Ionosphere = space weather
• Multipath = ground water
•Troposphere = water vapor (atmospheric weather)
Ionospherewith US-TEC
• TEC = total electron content
• ~ 100 real-time GPS stations
• Input = GPS phase and PR observations
• Output = vertical and slant TEC for continental U.S.
www.sec.noaa.gov/ustec
US-TEC
midnight 6 am
6 pmnoon www.sec.noaa.gov/ustec
Troposphere (Water Vapor)with GPS-MET
TOTALATMOSPHERICDELAY
IONOSPHERICDELAY
NEUTRALDELAY
WETDELAY
HYDROSTATICDELAY
PW = P • ZWD
whereP = ( , f physical constants Tm)
Estimate from dual frequency observations and known dispersion relations
Estimate during geodetic inversion
Estimate from surface pressure measurement
= - Wet neutral hydrostatic
Wet delay is nearly proportional to PW
• Input = dual-freq GPS observations + pressure
• Output = ZTD … integrated precipitable water values (IPW)
gpsmet.noaa.gov
GPS-MET
GPS-IPW: ~400 GPS stationsEvery 30min for each station
gpsmet.noaa.gov
Soil Moisturefrom Multipath
• Multipath = signal travels indirect path = range error
• Ground reflections– Amplitude of
received signal = attenuation at ground
– Soil moisture affects attenuation (reflection coefficient)
Conclusions
• GPS/GNSS have complicated signals … lots of opportunity for science!
• New signals and systems = more complicated = more science