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