GPS: Everything you wanted to know, but were afraid to ask Andria Bilich National 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 rArA B rBrB C rCrC

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 satellites Monitor health Maintain 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 = MHz –L2 = 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 = 1s = 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-spoofing 3 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) –L5 (aviation) 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…

GNSS Global Navigation Satellite Systems SystemWho runs it# Satellites (design/in use) When GPSUS DoD24 / ~30now GLONASSRussia & India24 / ~10 usable 2011 GalileoEU & ESA30 / 1 test2012 All are L-band radio systems (~ 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 Pseudorange Carrier phase

One Person’s Noise is Another’s Signal Ionosphere = space weather Multipath = ground water Troposphere = water vapor (atmospheric weather)

Ionosphere with 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.

US-TEC midnight 6 am 6 pm noon

Troposphere (Water Vapor) with GPS-MET Input = dual- freq GPS observations + pressure Output = ZTD … integrated precipitable water values (IPW) gpsmet.noaa.gov

GPS-MET GPS-IPW: ~400 GPS stations Every 30min for each station gpsmet.noaa.gov

Soil Moisture from 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