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Introduction to GPS and Absolute Gravity at Tide Gauges. Philip L. Woodworth Permanent Service for Mean Sea Level With an enormous amount of assistance from Norman Teferle and Richard Bingley University of Nottingham, UK Simon Williams and others Proudman Oceanographic Laboratory.

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introduction to gps and absolute gravity at tide gauges

Introduction to GPS and Absolute Gravity at Tide Gauges

Philip L. Woodworth

Permanent Service for Mean Sea Level

With an enormous amount of assistance from

Norman Teferle and Richard Bingley

University of Nottingham, UK

Simon Williams and others

Proudman Oceanographic Laboratory

gps at tide gauges contents
GPS at Tide Gauges: Contents
  • Sea level and vertical land movements
  • Meetings
  • Initiatives
  • Recommendations
  • Projects
global sea level
Global Sea Level
  • IPCC (1990, 1995) estimated rise in global sea level of 10 to 20 cm over the last century

IPCC (2001) predicted rise in global sea level of 9 to 88 cm over the period for 1990 to 2100

Sea Level Rise (IPCC 2001)

absolute relative mean sea level

Tide

Gauge

Vertical

Land

Movement

‘Absolute

MSL’

‘Relative

MSL’

‘Absolute’/’Relative’ Mean Sea Level
relative sea level rise
Relative Sea Level Rise

British Geological Survey

using gps at tide gauges

Tide

Gauge

GPS

Vertical

Land

Movement

‘Absolute

MSL’

‘Relative

MSL’

Using GPS at Tide Gauges
woods hole meeting
‘Woods Hole Meeting’
  • IAPSO Commission on Mean Sea Level and Tides
  • Woods Hole Oceanographic Institute, USA, 1988
  • Carter et al (1989) report“TGBM’s should be connected to the International Terrestrial Reference Frame (ITRF) and monitored using GPS”
tide gauge benchmark fixing
Tide Gauge Benchmark Fixing

GPS

satellite

Tide Gauge

Precise

Level

GPS Station

close to

Tide Gauge

(TGGS)

GPS Station

co-located

with ITRF Station

TGBM

MSL

advances in gps technology
Advances in GPS Technology
  • Cheaper and more reliable GPS receivers
  • Completion of the GPS satellite constellation
  • Establishment of the International GPS Service
surrey meeting
‘Surrey Meeting’
  • IAPSO Commission on Mean Sea Level and Tides
  • Institute of Oceanographic Sciences, UK, 1993
  • Carter (1993) report“Continuous GPS (CGPS) stations should be installed at about 100 tide gauges world-wide to form a ‘core network’ of a global absolute sea level monitoring system”
the development of cgps
The Development of CGPS
  • Essential to the success of the IGS
    • Improved orbit determination
    • Automated processing on a daily basis
    • Delivery of products within reasonable time scales
  • Demonstrated on a regional scale, eg BIFROST in Fennoscandia
jpl meeting
‘JPL Meeting’
  • IGS and PSMSL
  • Jet Propulsion Laboratory, USA, 1997
  • Neilan et al (1997) report“tide gauge benchmark monitoring”and“altimeter calibration”
first experiences
First experiences
  • Solomons Island in the Chesapeake Bay
    • Operational since April 1994
    • Part of the BAYONET project
    • Nerem et al (1997)
  • Porto Corsini in the Mediterranean
    • Operational since July 1996
    • Part of the SELF II project
    • Zerbini et al (1997)
  • Sheerness in the UK
    • Operational since March 1997
    • Part of the UKGAUGE project
    • Ashkenazi et al (1997)
cgps@tg working group
CGPS@TG Working Group
  • Site the CGPS station as close as possible to the tide gauge
    • vertical land movements included in the relative MSL trends are exactly those that are measured by GPS
  • Periodically measure the vertical tie between the CGPS station, the tide gauge sensor and a series of TGBM’s using precise spirit levelling
  • Monumentation
  • Instrumentation
eoss working group 1
EOSS Working Group 1
  • Dual-CGPS station concept
  • First CPS station located as close as possible to the tide gauge
    • vertical land movements included in the relative MSL trends are exactly those that are measured by GPS
    • vertical tie between CGPS station and tide gauge = constant
  • Second CGPS station located on ‘stable rock’ within a few kilometres of the tide gauge
    • underlying geophysical land movements measured by GPS
    • vertical tie between the 2nd and 1st CGPS stations (and tide gauge) measured continuously
example cgps coordinate time series
Example CGPS Coordinate Time Series

JPL, 2003 (http://sideshow.jpl.nasa.gov/mbh/series.html)

research questions being addressed by tiga etc
Research Questions being Addressed by TIGA etc
  • How good are CGPS time series and vertical station velocities ?
    • CGPS data processing
    • CGPS coordinate time series analysis
    • CGPS station velocity estimation
    • CGPS station velocity uncertainties
  • How well do CGPS time series represent vertical land movements ?
    • Regional, local or very local movements
    • Long term or short term movements
gps conclusions
GPS Conclusions
  • CGPS@TG is more feasible now than a few years ago thanks to reduction in cost of receivers
  • A community of CGPS@TG people exists which is ready to share experiences. Regular (typically every 2 years) meetings take place within the CGPS@TG group.
  • An international IGS-sponsored programme exists (TIGA) within which GPS from Chile (for example) can be processed and analysed and lessons learned collaboratively.
introduction to a second technique absolute gravity
Introduction to a Second Technique: Absolute Gravity
  • The “Carter reports” on sea level monitoring recommended the use of Absolute Gravity (AG) as a complimentary technique to CGPS for VLM monitoring.
  • Here results from AG measurements at UK tide gauges are presented
  • For this work it was decided to concentrate on 3 UK tide gauges, considered to be the core UK gauges, Newlyn, Lerwick and Aberdeen
absolute gravity
Absolute Gravity

How do we Measure “g”?

Using free fall methods.

G = Gravitational Constant = 6.67x10-11 m3kg-1 s-2

Me = Mass of the Earth

Re = Radius of the Earth

Measure x and t of a mass in free fall (in a vacuum) and use the above equation to get g.

absolute gravity29
Absolute Gravity

This machine has a precision and accuracy of about 1-2mgal(1mgal=10-8 ms-2)

absolute gravity30
Absolute Gravity

So how does measuring g give us changes in vertical land movement?

If additional mass is involved then

This assumes there is no additional mass : “Free Air Model”

This is the “Bouger Model”

methodology
Methodology
  • Measure for at least 3 days at each site, at least once per year (hopefully!)
  • Data from each day are processed separately and correction made for solid-earth tides, ocean-loading effects, atmospheric pressure, polar motion and comparator response.
  • Single admittance factor and local pressure data are used to correct for atmospheric pressure.
  • Gravity gradients for Newlyn and Aberdeen determined using a relative spring gravimeter
conclusions
Conclusions
  • AG measurement spanning 5-6 years can be used to measure vertical land movements at tide gauges.
  • Absolute Gravity is a useful complimentary technique to CGPS.
  • Good site selection is essential to AG measurement success.
  • Given a few more years of measurement, results can be used to test competing GIA models.