Bistatic GPS Measurement of Ocean Currents Arian Lalezari 1

1
Bistatic GPS Measurement of Ocean Currents

• Arian Lalezari
• 19 November 2004

2
Overview

• Motivation
• Introduction to GPS
• Key principles
• Other applications
• Conclusions

3
Motivation

• Why measure ocean currents?
• Weather
• Heat transport by ocean currents moves weather
• Water temperature affects air pressure
• Air pressure results in wind
• Current recording techniques are clunky
• Timing
• Processing
• Ocean Forecast
• Freak waves
• Other ocean storms and currents

4
Introduction to GPS

• What is GPS?
• Global Positioning System
• Signals from Satellites
• Received on ground to determine position
• Owned by Department of Defense

• How will we use it?
• Measure height of ocean surface
• Relative height of waves
• Monitor change over time
• Novel use of GPS to track ocean current

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Introduction to GPS

• Constellation of 24 active satellites
• Six orbital planes around the earth
• Four active satellites each plus one spare
• Twelve-hour orbits (not geo-synchronous)
• Each satellite broadcasts a signal onto the earth
  below
• Satellite illumination is designed to guarantee
  visibility of four satellites mostly

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Introduction to GPS

• Position solution
• x,y,z,t or equivalent
• Four unknowns
• Four systems of equations from four visible
  satellites
• Distance to each satellite determined by time
  delay to transmit signal called pseudorange
• Four pseudoranges used to determine position
• Accuracy of position can be determined for space
  and time

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Introduction to GPS

• Broadcast signal
• Low power
• Same carrier frequency
• Message
• Coded Identification Number
• Known by receiver
• Ephemeris
• Almanac

8
Key GPS Principles

• Reading the Signal
• Listen for all satellites
• Decipher signals from noise using correlation
• A receiver can also ID which satellite it is
  specifically hearing
• This ID acts like a header
• Signals can be read despite low transmit power
  and congestion on GPS band

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Key GPS Principles

• Multipath
• Signals may reflect off objects
• A reflected signal may look like a direct signal
• When a signal bounces, the satellite looks
  farther away
• Difference in pseudorange due to reflection
  changes position
• Multipath is usually undesirable, but can be
  exploited by observing both signals bistatically

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Key GPS Principles

• Multipath

Bad Multipath
Good Multipath
Building
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Key GPS Principles

• Multipath
• GPS signals are right-hand circularly polarized
  (RHCP)
• To listen to direct signal, use RHCP antenna
• Reflection reverses polarization to left-hand
  circularly polarized (LHCP)
• To listen to reflected signal, use LHCP antenna
• A bistatic receiver uses both an RHCP and LHCP
  antenna simultaneously

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Application of Multipath

• If we know the difference in time between direct
  and reflected signals as well as the position of
  the satellite, we can use simple geometry to
  determine height of reflecting surface

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Application of Multipath

• Single Ray Geometry

Direct path from ship antenna to aircraft antenna

• in AHO OA h / sin(a)
• in ABO cos(2a) AB / OA
• s.t. AB (h / sin(a)) cos(2a)
• D OA AB 2h sin(a)
• h D/2sin(a)

B
Multi-path from ship antenna to aircraft antenna
a
A
a
h
a
a
H
O
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Application of Multipath

• How do we integrate over many reflected signals
  to get the big picture?
• Because of satellites motion, signals have
  Doppler effect
• There are rings where Doppler effect is roughly
  constant
• There are also rings where the code range is also
  constant

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Application of Multipath

• Using the intersections of these rings, we can
  grid up the ocean
• This allows us to identify from where on the
  ocean surface reflection sources emanate
• This allows us to place altitude data on a grid

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Synergy

• Using any bistatic receiver, we can grid up the
  altitude of the oceans surface
• Aircraft
• Satellites
• By watching the changes in surface height over
  time, we can determine motion of ocean currents
• Can combine with thermal data for more thorough
  results

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Conclusions

• Cheaper
• Existing satellites
• Less manual post-processing
• Faster
• Can report data virtually in real-time
• Observable
• Can easily watch large regions for extended
  periods of time

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Other Applications

• General altimetry
• Topology and terrain mapping
• Soil moisture
• Ice cap movement
• Avalanches
• Earthquakes
• Military applications
• (Dont ask Ill have to kill you)

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Resources

• http//ccar.colorado.edu/dmr/ocean/
• http//gps.csr.utexas.edu/reflect/
• http//www.etl.noaa.gov/vzavorotny/
• http//esto.nasa.gov/files/1999/Zuffada.pdf
• http//gps.faa.gov/FAQ/faq-gps.htm
• http//tycho.usno.navy.mil/gpscurr.html
• http//sealevel.jpl.nasa.gov/science/invest-emery.
  html
• http//www.oceanweather.com/