Mapping the GPS Multipath Environment Using the Signal-to- Noise Ratio (SNR) Andria Bilich*, Kristine M. Larson + * Geosciences Research Division, National Geodetic Survey + Department of Aerospace Engineering Sciences, University of Colorado, Boulder

National Geodetic Survey National Oceanic and Atmospheric Administration Overview GPS system basics Motivation: multipath with GPS signals Why do we care? What do we know? Measurement: SNR Technique: power spectral maps

National Geodetic Survey National Oceanic and Atmospheric Administration Global Positioning System (GPS) Radio navigation system L-band MHz (L1) MHz (L2) 28+ satellites CDMA Global coverage 4-10 in view at any instant courtesy of Dept. of Defense

National Geodetic Survey National Oceanic and Atmospheric Administration Positioning with GPS Trilateration using distance to satellites Must have accurate satellite- receiver range A rArA B rBrB C rCrC

National Geodetic Survey National Oceanic and Atmospheric Administration Multipath with GPS Multipath Range error = positioning error Systematic (quasi-sinusoidal) Large magnitude Site-specific

National Geodetic Survey National Oceanic and Atmospheric Administration Signal-to-Noise Ratio (SNR) SNR = strength of composite signal Phase relationship changes with satellite motion direct multipath composite Multipath strength

National Geodetic Survey National Oceanic and Atmospheric Administration Multipath Oscillations in SNR Parameters affecting multipath frequency: Reflector distance h Reflection angle  GPS wavelength Multipath frequency

National Geodetic Survey National Oceanic and Atmospheric Administration Ground Distance vs. Multipath Period Fast MP = far away Slow MP = nearby For a fixed reflector, satellite motion generates time- varying signature

National Geodetic Survey National Oceanic and Atmospheric Administration Multipath and SNR: putting it all together Frequency = distance to reflector Amplitude = multipath strength Satellite position: Azimuth/elevation = location of reflections relative to antenna Rate of elevation change = impact on frequency and height

National Geodetic Survey National Oceanic and Atmospheric Administration SNR Data Total SNR = direct plus reflected signal(s) Direct amplitude = dominant trend Multipath signal = superimposed on direct

National Geodetic Survey National Oceanic and Atmospheric Administration Power Spectral Maps Wavelet spectra of detrended SNR Assign frequency and power to satellite azimuth & elevation Plot all points on a grid (sky map)

National Geodetic Survey National Oceanic and Atmospheric Administration Multipath from Nearby Structure: TRO1 Antenna on a mast: 4.09 m above ground surface 1.3 m above flat tar-paper roof Roof to S of antenna

National Geodetic Survey National Oceanic and Atmospheric Administration TRO1 Power Maps High power at long periods = close-in reflector

National Geodetic Survey National Oceanic and Atmospheric Administration Multipath from Distant Topography: MKEA Mauna Kea (MKEA), Hawaii

National Geodetic Survey National Oceanic and Atmospheric Administration MKEA Power Maps Frequency (distance to reflector) changes with satellite position High power returns from cinder cones 60-90s 30-60s10-30s

National Geodetic Survey National Oceanic and Atmospheric Administration Combined Multipath Environments: KYVW Standard GPS monument ~ 1.8m above ground Nearfield: sandy, flat ground Farfield: gentle hillsides to NW and E

National Geodetic Survey National Oceanic and Atmospheric Administration KYVW Power Maps Long periods (L1) Short periods (L2) Ground reflections Reflections from hillsides

National Geodetic Survey National Oceanic and Atmospheric Administration Summary

National Geodetic Survey National Oceanic and Atmospheric Administration Acknowledgements Tools: Torrence and Compo wavelet toolbox: Generic Mapping Tools (GMT) IGS, CORS, SOPAC, UNAVCO, JPL NSF grants and fellowships Bilich, A., K.M. Larson (2007) Mapping the GPS multipath environment using the signal-to- noise ratio (SNR), Radio Science, 42, RS6003.

National Geodetic Survey National Oceanic and Atmospheric Administration Multipath Assessment: Power Spectral Maps Idea: frequency and power content of SNR  multipath environment Method: Wavelet spectra of detrended SNR Assign frequency and power to satellite azimuth/elevatio n Plot all points on a grid (sky map)

National Geodetic Survey National Oceanic and Atmospheric Administration Mauna Kea (MKEA)

National Geodetic Survey National Oceanic and Atmospheric Administration What do these equations tell us? Oscillations in SNR, phase MP, and pseudorange MP all have common frequency MP frequency Key to determining  Function of Reflector distance h Reflection angle  GPS wavelength Fast MP = far away Slow MP = nearby For a fixed reflector, satellite motion generates time- varying signature

National Geodetic Survey National Oceanic and Atmospheric Administration Time-evolving Multipath

National Geodetic Survey National Oceanic and Atmospheric Administration Understanding Multipath: Power Spectral Maps Idea: frequency and power content of SNR  multipath environment Method: Wavelet spectra of detrended SNR Assign frequency and power to satellite azimuth/elevation Plot all points on a grid (sky map)

National Geodetic Survey National Oceanic and Atmospheric Administration Dual-Frequency Power Spectral Maps S1S2 Reflection from distant object (building?) Reflection from nearby object (rock outcrops?)

National Geodetic Survey National Oceanic and Atmospheric Administration Simplified Multipath Model and SNR Recorded SNR = direct + multipath signal Carrier phase error: Code (pseudorange) error (short delay) : multipath direct composite  MP direct multipath composite

National Geodetic Survey National Oceanic and Atmospheric Administration

National Geodetic Survey National Oceanic and Atmospheric Administration Take-home lessons: Environmental Imaging Assess multipath environment Frequency: distance to object Amplitude: magnitude of errors due to object Consider position errors at different frequencies (think high-rate GPS positioning) No new equipment SNR routinely recorded … but need precise and accurate SNR related to multipath model (not always possible)

National Geodetic Survey National Oceanic and Atmospheric Administration Summary Existing CGPS networks extended to unforeseen science applications Sensing soil moisture Understanding reflections and potential sources of error Measuring displacements from short-period, transient phenomena

National Geodetic Survey National Oceanic and Atmospheric Administration Space Segment 24+ satellites Orbit 26K km radius 12 hour period Stationary ground tracks 6 orbital planes

National Geodetic Survey National Oceanic and Atmospheric Administration 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

National Geodetic Survey National Oceanic and Atmospheric Administration 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

National Geodetic Survey National Oceanic and Atmospheric Administration GNSS Global Navigation Satellite Systems SystemWho runs it# Satellites (design/in use) When GPSUS DoD24 / ~30now GLONASSRussia & India 24 / ~10 usable 2011 GalileoEU & ESA30 / 1 test2012 All are L-band radio systems (~ MHz) Mostly free signals

National Geodetic Survey National Oceanic and Atmospheric Administration What is multipath? Multipath introduces range error -> position error Why is multipath such an issue? Difficult to prevent Difficult to model Systematic error Problematic for high-rate applications How can we understand, characterize, or remove multipath?

National Geodetic Survey National Oceanic and Atmospheric Administration Multipath Geometry amplitudesangles A d direct signal amplitude A m multipath signal amplitude h reflector distance  angle of reflection  satellite elevation angle  path delay