HOW IT WORKS
ALTIMETRY : DATA FLOW
o Altimeter measurements
o Geophysical corrections
o Instrumental errors
o Reference surfaces
Data Qualification: ”Calval”
o Calibration DATA phase
o Data exploitation
Topex / Poseidon satellite
A technique for measuring height.
Measures the time taken by a radar
pulse to travel from the satellite
antenna to the surface and back to
the satellite receiver.
Emits a radar wave signals
at high frequencies
(over1700pulse per sec) and
analyses the return of the
Analysis includes SURFACE
,WAVEFORM of the signal
Surface height (corrected) =
Altitude - ( corrected) Range
For oceans ,the sea surface height
integrates effects such as:
• GEOID : sea surface height exists
with out the influence of any
factors ( wind , currents , tides
• DYNAMIC TOPOGRAPHY or ocean
circulation : permanent circulation
linked to Earth's rotation,
Step 0 – Data acquisition : data from the
satellites are downlinked to the ground
Step 1 – Row telemetry ( level 0) and
level 1data : row telemetry is forwarded
to quality control and processing units
and processed to obtain level 1 data( ie
timed and located data with appropriate
units, and checked for quality)
Step 2 – Level 1 data and level 2
geophysical data: corrections for
different errors are done , level 1data
for instrumental and atmospheric
errors. Precise orbit determination also
performed for accuracy .
Step 3 – Data validation and
qualification: geo physical data validated
for quality assurance . Validation involves
precise quality control and monitoring the
Step 4 – Level 3 and level 4 data :”
value added products”: level 3 data
validated, cross calibration between the
missions done .level 4 are multi satellite
Data acquisition from the altimetry
satellites is performed by each agency.
Several ground stations around the
world (wallops for NASA, Sweden for
Collects raw data from satellites and
send them to the control and processing
Altimetry data processing performed by
ground segment includes :
i. Altimetry measurements
ii. Orbit Determination
iii. Geophysical corrections
iv. Instrumental errors
v. Reference surface
vii. High-level processing
Altimetric range -
Significant wave height –is computed
from the slope of the return radar pulse.
Backscatter coefficient – from the
power of the altimeters return pulse.
Wind speed – backscatter coefficient
can be related to wind speed. Emperical
models have established relation ship
b\w wind speed ,back scatter and
significant wave height.
Following corrections are computed:
Geophysical corrections :
• Ocean tides
• Solid earth tides
• Ionosphere : correction for path delay in the
radar signal due to the atmospheres electron
• Wet troposphere : correction due to liquid water
• Dry troposphere : due to the presence of dry gas
component in the atmosphere.
• Inverse barometer: correction for variations in
sea surface height due to atmospheric pressure
• Electromagnetic bias : due to errors formed by
varying reflectivity of wave crust and troughs.
USO correction ( Ultra – stable
Oscillator): correction for drift of
onboard oscillators used for sending
Centre of gravity: due to the variation of
satellite’s Centre of gravity due to fuel
consumption solar panel orientation .
Wave form : corr for effects due to
filters used to eliminate certain
frequencies in the return of radar signal.
Reference ellipsoid : is a rough
approximation of the Earths shape,
Geoid : is the shape of sea surface in the
absence of perturbing forces.
Mean dynamic topography: is the
permanent stationary component of ocean
• Used to indicate some data such as –
what type of surface beneath the
satellite ,which instruments are on ,
which frequencies are used.
• Retracking altimetry data is done by
computing the departure of the
waveform's leading edge from the
altimeter tracking gate and correcting
the satellite range measurement
• Calval (CALibration / VALidation ) is what
determines data quality.
• Involves a series of quality controls designed
to ensure a continuous supply of data.
• Calval operations consist in determining data
quality and keeping a check on instrument
• Quality control is achieved through statistical
data analysis, analyses at crossover points, and
comparisons between orbit cycles.
• Data are also compared with other missions.
• Absolute calibration is the comparison
of the engineering measurement with an
independent measurement (e.g. buoy,
tide gauge) of the same parameter
(range, sea surface height, etc.).
• Absolute calibration provides one
reference point for the complete
altimetric time series
• In relative calibration, two altimetry
systems are compared through their
global geophysical data products. Due to
the huge number of globally-distributed
measurements processed, relative
calibration is significantly more precise
than local absolute calibration
• Ensure consistency.
• After the Calibration Phase, the Routine
Exploitation phase begins, and continues
until the end of the altimetry mission.
• The commissioning phase group is then
required to formulate further
recommendations at the completion of the
• During the routine phase these
recommendations are taken on board, and
new instrument or processing anomalies are
investigated and solved.
Selecting a proper
inclination, we should taken
into account that the
precession of satellite orbit
should not coincide with the
harmonics of tidal cycle.
As a result TOPEX /
POSEDON AND JASON – 1
satellites are formed, have
altitude 1300km and
inclinaton 66 degree.
Topex/posedon is based
on three earlier earth –
orbiting satellite missions.
It’s primary instrument
for measuring ocean
topography is an
A second instrument
called a radiometer is
used to correct the
influence of water in
spacecraft uses a high –
altimeter to take
measurements of sea –
surface height over 90%
of the world’s ice – free
Approximately 10 days
to complete 127 orbits.
has most accurate orbit
Its altimeter has a
approximately 2.5 cm
resulting sea level is
accurate to 5 cm.
1. GEODESY & GEOPHYSICS-
4. LAND APPLICATIONS –
Precise satellite altimetry missions have transformed the
way we view Earth and its oceans. Highly accurate
altimetry measurements give us the ability to observe
sea surface height systematically.
A wealth of applications are possible using radar
altimetry measurements, involving most geoscience
fields and practised by more than a thousand teams
of users around the world.
From the 'historical' applications (geodesy, general
ocean circulation) to the developing ones (solid
Earth and coastal applications, etc.) and the ones
that have become classic (ocean variability, ice
topography, hydrology), altimetry has shown over
and over that it is a very productive technique.
1. Satellite Altimetry, The Marine Geoid, And
The Oceanic General Circulation : Carl
Wunsch And Detlef Stammer Department Of
Earth, Atmospheric, And Planetary Sciences,
Massachusetts Institute Of Technology,
Cambridge Geophysical Applications of
Satellite Altimetry David t . Sandwell
Geological Research Division, Scripps
Institution of Oceanography, La Jolla, CA
92093 3.Bruce C. Douglas (1997). "Global
Sea Rise: A Redetermination". Surveys In
Geophysics 18: 279–292.
2. Bruce C. Douglas (1997). "Global Sea Rise:
A Redetermination". Surveys In Geophysics 18
3. .Radar Altimetry TutorialApril 2009
V. Rosmorduc, J. Benveniste, O. Lauret,
C. Maheu, M. Milagro, N. Picot
4. Recent Progress In The Application Of
Satellite Altimetry To Observing
The Mesoscale Variability And General
Circulation Of The Oceans Jet
Propulsion Laboratory, California Institute
Of Technology Lee-Lueng Fu.