Transcript No Slide Title

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
www.pol.ac.uk
GPS at Tide Gauges: Contents
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Sea level and vertical land movements
Meetings
Initiatives
Recommendations
Projects
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)
European Mean Sea Level Data
Measured by Tide Gauges
2000
Stockholm
Aberdeen
1000
Newlyn
500
Brest
Cascais
1980
1950
1920
1890
0
1860
MSL (mm)
1500
Year
[Woodworth et al, 1990]
‘Absolute’/’Relative’ Mean Sea Level
Tide
Gauge
Vertical
Land
Movement
‘Relative
MSL’
‘Absolute
MSL’
Relative Sea Level Rise
British Geological Survey
Using GPS at Tide Gauges
Tide
Gauge
GPS
Vertical
Land
Movement
‘Relative
MSL’
‘Absolute
MSL’
‘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
GPS
satellite
Precise
Level
GPS Station
co-located
with ITRF Station
GPS Station
close to
Tide Gauge
(TGGS)
Tide Gauge
TGBM
MSL
The SELF I Project
Difference (mm)
(1994 - 1993)
0
-5
-10
-15
-20
LGOS
CCAS
LXOS
ALIC
CORU
VIGO
MARS
SJLZ
BRES
DIEP
DUNK
NEWL
PORT
DOVE
STOR
LWIK
The EUROGAUGE Project
20
15
10
5
Advances in GPS Technology
• Cheaper and more reliable GPS receivers
• Completion of the GPS satellite constellation
• Establishment of the International GPS Service
‘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
• 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
Global IGS Tracking Network
‘JPL Meeting’
• IGS and PSMSL
• Jet Propulsion Laboratory, USA, 1997
• Neilan et al (1997) report
“tide gauge benchmark monitoring”
and
“altimeter calibration”
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)
Recommendations: IGS/PSMSL
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
Recommendations: EOSS
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
New Initiatives 2001: IGS TIGA-PP
New Initiatives 2001: ESEAS
Example CGPS Coordinate Time
Series
JPL, 2003 (http://sideshow.jpl.nasa.gov/mbh/series.html)
Research Questions being Addressed by TIGA etc
• How good are CGPS time series and vertical
station velocities ?
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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
• 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
• 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
GmM
F  ma 
2
R
GM e
2
g
 9.8ms
2
Re
G = Gravitational Constant
= 6.67x10-11 m3kg-1 s-2
Me = Mass of the Earth
Re = Radius of the Earth
How do we Measure “g”?
Using free fall methods.
x  g
integratin g twice with respect to time gives
the equation of motion
1
x  x0  v0t  gt 2
2
Measure x and t of a mass in
free fall (in a vacuum) and use
the above equation to get g.
Absolute Gravity
This machine has a precision and accuracy of about 1-2
mgal (1 mgal =10-8 ms-2)
Absolute Gravity
So how does measuring g give
us changes in vertical land
movement?
Differenti ate the basic equation
with respect to distance Re
 2GM e  2 g
g
 g 

3
Re
Re
Re
 3.1e 6 ms -2 per m
This assumes there is no
additional mass : “Free Air Model”
If additional mass is involved
then
 2g
g 
 2G
Re
where  is the density of the mass
involved. (   1600 - 3300 kg m -3 )
 2.4e 6  g  1.7e 6
This is the “Bouger Model”
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
Intercomparison
Results
Post-glacial rebound
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.