Satellite Altimetry- possibilities and limitations

by

Per KnudsenKort & MatrikelstyrelsenGeodetic DepartmentRentemestervej 8DK-2400 Copenhagen September 2001

Contents:• The system:

• Principle of altimetry• Sampling characteristics

• Applications:• Mean sea surface• Sea level variability• Sea level changes

• Operational issues• Data services• In Europe

The System:Principle of altimetry

– A space borne tide

gauge:

S: The distance is measured by the onboard altimeter,

H: The position and height of the satellite is computed,

SSH = H – S: The sea surface height above a reference ellipsoid is obtained.

Altimeter Satellites:Satellites: Launch:Skylab 1973

GEOS 3 1974

SeaSat 1978

GEOSAT 1984

ERS-1 1991

TOPEX/POSEIDON 1992ERS-2 1995GFO 1998JASON 2001ENVISAT 2001

The Altimeter:The pulse and its

return:The pulse is generated

by a 13.5 GigaHz oscillator, transmitted at 1000 Hz, and averaged at 1 sec.s

The return of the pulse depends on

• Distance• Surface roughness• Significant wave

height

The Altimeter:

The waveform:

The waveform depends on

• Travel time: Distance / height

• Surface roughness sigma_0 - Windspeed

• The slope - Significant waveheight

Sampling of the Sea Surface

Altimeter data are collected along the satellites ground track with a spacing of 7 km – 1 sec averages.

Sampling of the Sea Surface

One revolution of the satellites orbit takes about 100 - 115 min.s depending on the altitude of the satellite (800 – 1350 km).

Hence, the satellite completes 13–14 revolutions per day.

Sampling of the Sea Surface

The number of tracks, i.e. the track density, depends on the repeat period, e.g. 3 days.

The coverage depends on the inclination of the orbit plane.

Orbit Parameters

Satellite

Repeat Period

Track spacing

Inclination

Geosat 17 days 163 km 108°

ERS 35 days 80 km 98°

T/P 10 days 315 km 66.5°

The actual coverage of the sea surface depends on the orbit parameters such as the inclination of the orbit plane and repeat preiod.

Applications 1

Mean sea surface: • Reference surface• Geo-centric• Consistent with GPS / GNSS

Example: Mean sea surface from a merge of about 10 years of altimetric sea surface heights.

Mean Sea Surface from multi-mission altimetry

Applications 2

Sea surface variability: • Statistics of variability• Sea level anomalies

Periodic signals Non-periodic signals

Examples:1. Variability (RMS) of sea surface w/o

tides2. Daily signals: Ocean tides3. Meso-scale: El niño 1997 event4. Seasonal: Annual cycle

Sea level variability

M2 ocean tides model

M2 loop

Seasonal Cycle

El Niño - 1997

El Niño - 1997

Applications 3

Sea level changes: • Global coverage – open ocean• Uniform Geocentric reference

• About 10 years of data Spatial characteristics

• Calibration needed at tide gauges

ERS

Altimeter data services

Satellite altimetry is made available through the space agencies:• ESA• NASA, CNES, • US-navy

Upgraded / value added products are available through• Space agencies ( NASA/JPL,

CNES/CLS )• Research institutions ( CSR, DEOS,

KMS, GFZ )

Operational issues

Satellite altimetry is becoming available in near-real time to contribute to the monitoring of present changes in sea level to contribute to programmes such as• GOOS, and• El Niño forecasting

Problems:• Not real-time (4-6 hours)• Insufficient sampling locally – more

satellites

Operational issues

Potential data products:• Mean sea surface• Statistics of variability, extreme

sea level• Ocean tides• Seasonal cycles• Sea level anomalies• Sea level trends – decadal

variations

ConclusionsSatellite altimetry has proven its value in studies

of• Mean sea surface• Ocean dynamics

Satellite altimetry has shown its potential in studies of• Sea level changes

Satellite altimetry has, furthermore, a potential in• Hydrography / surveying – Bathymetry - GPS• Operational sea level services