Applications for Precision GPS: Seismology, Volcanic Eruptions, Ice Sheet Dynamics,

and Soil Moisture

Kristine M. LarsonDept. of Aerospace Engineering Sciences

University of Coloradokristinem.larson@gmail.com

Outline

• Traditional solid Earth geophysical applications of GPS

• Space Weather (briefly)• Ice Sheets • Earthquakes • Volcanoes • Soil Moisture

Start with 24-hour averaged positions using dual-frequency receivers

And then you wait.

Plate Boundary Deformation

GSI

ITRF2005 Altamimi et al., 2007

Global Plate Motions

Space Weather Implications

• Ambiguity resolution is critical. To the extent that better knowledge of TEC can aid ambiguity resolution, solid Earth geophysicists care about space weather.

• Some new applications described in this talk have obvious real-time potential - for which ambiguity resolution is even more important (and challenging).

Space Weather Implications 2

• Many GPS receivers have been installed to catch “once in a career” geophysical signals. Receiver failure because of space weather is always a concern.

• For example, my dissertation data were collected in campaigns during June 1986, September 1987, March 1988, and March 1989.

Using GPS at time scales less than a day

• Ice sheet speeds, ~100 m/yr (1-2 cm/hr); are they linear?

• Earthquakes, 1-100 cm/sec; large accelerations.

• Volcanoes, 1-2 cm/hr; not linear, but low accelerations.

• Soil moisture, non-traditional GPS application.

Ice sheet velocities in Greenland

• Install poles; measure position.

• Return following year; remeasure position; compute velocity.

Thomas et al., 2000

Installed a GPS receiver on the ice

Number of days where temperature was above freezing.

Zwally et al., 2002.

Earthquakes

• Global plate velocities are based on ~10 million measurements per site.

• Each Greenland (12-hr) ice sheet velocity is based on ~1000 GPS measurements.

• For seismic applications, each position is based on 6-10 measurements.

Denali, Alaska Earthquake, November 2002

Geodetic Challenges• Multipath (reflected signals) is important (and doesn’t

difference out).

Oscillations in position time series reflect different multipath environments

Good news: multipath looks the “same” from day to day

The GPS orbital period is 1/2 sidereal day, shift time should be 1 sidereal day (or one day minus 236 seconds). Is it?

Repeat Times Vary by Satellite

Choi et al., 2004

Repeat Times Vary by Day of Year

See the GPS Tool Box for code to calculate repeat times.

Does it Really Matter?

multipath

Badly corrected multipath

GPS Seismograms

Miyazaki et al., 2004; Emore et al., BSSA, 2007.

East

Advantages of GPS seismology

Another advantage of subdaily GPS

Earthquake

Postseismic

Each point is a 24-hr average

Great earthquakes are often more complicated than this

M8

M7.4

Past two years:55 cm extension

25 cm uplift

Dike intrusionDeflation

Larson and Miklius, in preparation

cm

Multipath/Soil Moisture

• The frequency of ground multipath (reflections) is determined by the antenna height.

• The amplitude of ground multipath is determined by ground reflectance, which can be related to soil moisture content.

Soil Moisture

day of year

QuickTime™ and a decompressor

are needed to see this picture.

160 210 260

Kurc and Small, 2004

PBO Site Marshall, CO

Larson et al., GPS Solutions, 2007

The data are free.

And many other networks

Conclusions

• There are lots of geophysical problems that benefit from high-precision GPS measurements at sub-daily time intervals.

• Constellation asymmetry, the troposphere and multipath are currently the limiting error sources.

Acknowledgements

• Co-authors• NSF, NASA, JSPS• IGS & ITRF• UNAVCO,

NEHRP, NGS, USGS, CORS, GEONET, NRCAN, SOPAC, CDDIS, IRIS, EUREF, SCIGN, GIPSY.