The Role of Altimetry in Coastal Observing Systems

Abstract

The last few years have witnessed the foundation and
development of a new discipline, coastal altimetry, and
the coalescence of an active community of researchers
who are now enthusiastically developing the topic. In
the present community white paper, we summarize the
technical challenges that satellite altimetry faces in the
coastal zone, and the research that is currently being
carried out to overcome those challenges. We introduce
the new coastal altimetry data that are becoming
available, and describe how we can calibrate/validate
those data. Then we show several of the possible
applications of coastal altimetry and conclude by
looking at the future of the discipline, and at how we
can build capacity in coastal altimetry.

Full text

THE ROLE OF ALTIMETRY IN COASTAL OBSERVING SYSTEMS
Paolo Cipollini
(1)
, Jérôme Benveniste
(2)
, Jérôme Bouffard
(3)
, William Emery
(4)
, Luciana Fenoglio-Marc
(5)
,
Christine Gommenginger
(1)
, David Griffin
(6)
, Jacob Høyer
(7)
, Alexandre Kurapov
(8)
, Kristine Madsen
(9)
,
Franck Mercier
(10)
, Laury Miller
(11)
, Ananda Pascual
(3)
, Muthalagu Ravichandran
(12)
, Frank Shillington
(13)
,
Helen Snaith
(1)
, P. Ted Strub
(8)
, Doug Vandemark
(14)
, Stefano Vignudelli
(15)
, John Wilkin
(16)
,
Philip Woodworth
(17)
, Javier Zavala-Garay
(16)
(1)
National Oceanography Centre, Waterfront Campus, European Way, Southampton SO14 3ZH, United Kingdom
cipo@noc.ac.uk; cg1@noc.ac.uk; hms@noc.ac.uk
(2)
European Space Agency/ESRIN (European Space Research Institute), Via Galileo Galilei, Casella Postale 64,
I-00044 Frascati, Italy, Jerome.Benveniste@esa.int
(3)
Institut Mediterrani d´Estudis Avançats, C/Miquel Marquès, 21, 07190 Esporles, Spain, jerome.bouffard@uib.es;
ananda.pascual@uib.es
(4)
University of Colorado, Boulder, CO 80309-0429, USA, emery@colorado.edu
(5)
Darmstadt Univ. of Technology, Petersenstrasse 13, D-64287 Darmstadt, Germany, fenoglio@ipg.tu-darmstadt.de
(6)
CSIRO (Commonwealth Scientific and Industrial Research Organisation) Marine and Atmospheric Research,
Castray Esplanade, Hobart Tas 7000, Australia, David.Griffin@csiro.au
(7)
Danish Meteorological Institute, Lyngbyvej 100, 2100 København, Denmark, jlh@dmi.dk
(8)
COAS (College of Oceanic and Atmospheric Sciences), Oregon State University, 104 COAS Admin. Bldg, Corvallis,
OR 97331-5503, USA, kurapov@coas.oregonstate.edu; tstrub@coas.oregonstate.edu
(9)
Univ. Copenhagen & Danish Meteorological Institute, Lyngbyvej 100, 2100 København, Denmark, ksm@fys.ku.dk
(10)
Collecte Localisation Satellites, Rue Hermès, Parc Tech. du Canal, 31520 Ramonville St. Agne, France,
fmercier@cls.fr
(11)
Laboratory for Satellite Altimetry, NOAA (National Oceanic and Atmospheric Administration), 1335 East West
Hwy, Silver Spring MD 20910-3226, Laury.Miller@noaa.gov
(12)
Indian National Centre for Ocean Info. Services, P.B. 21, IDA Jeedimetla P.O, Hyderabad, 500 055 India,
ravi@incois.gov.in
(13)
Dept. Oceanography, Univ. Cape Town, Priv. Bag X3, Rondebosch, Cape Town, 770, South Africa,
Frank.Shillington@uct.ac.za
(14)
OPAL (Ocean Process Analysis Laboratory), University of New Hampshire, 39 College Rd., Durham, NH 03824,
USA, doug.vandemark@unh.edu
(15)
Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Via Moruzzi 1, I-56100 Pisa, Italy, vignudelli@pi.ibf.cnr.it
(16)
Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901, USA, wilkin@marine.rutgers.edu;
jzavala@marine.rutgers.edu
(17)
National Oceanography Centre, J. Proudman Bldg, 6 Brownlow St., Liverpool L3 5DA, United Kingdom,
plw@pol.ac.uk
ABSTRACT
The last few years have witnessed the foundation and
development of a new discipline, coastal altimetry, and
the coalescence of an active community of researchers
who are now enthusiastically developing the topic. In
the present community white paper, we summarize the
technical challenges that satellite altimetry faces in the
coastal zone, and the research that is currently being
carried out to overcome those challenges. We introduce
the new coastal altimetry data that are becoming
available, and describe how we can calibrate/validate
those data. Then we show several of the possible
applications of coastal altimetry and conclude by
looking at the future of the discipline, and at how we
can build capacity in coastal altimetry.
1. INTRODUCTION
Satellite altimetry over the open ocean is a mature
discipline, and data are routinely assimilated for
operational applications. In contrast, global altimetry
data collected over the coastal ocean remain largely
unexploited in the data archives, simply because
intrinsic difficulties in the corrections (especially the
wet tropospheric component, the high-frequency
atmospheric and oceanic signals and the tides) and
issues of land contamination in the footprint have so far
resulted in systematic flagging and rejection of these
data, leaving „gaps‟ in the coastal zone. In the last
couple of years, considerable research has been carried
out into overcoming these problems and extending the
capabilities of current and future altimeters as close as
possible to the coast, with the ultimate aim to integrate
the altimeter-derived measurements of sea level, wind
speed and significant wave height into coastal ocean
observing systems. At the same time, the major Space
Agencies have recognized the importance of the topic
and are sustaining coastal altimetry research through
dedicated projects. This new “coastal altimetry”

community, inherently interdisciplinary, has already had
four well-attended international workshops (see
http://www.coastalaltimetry.org), and produced a book
on the subject [1]. The first custom-processed coastal
altimetry data are now available, and many more data
from Jason-1, Jason-2 and Envisat will become
available in the near future. Eighteen years of coastal
zone data from various past and present altimetric
missions are waiting to be reprocessed, we could almost
say rediscovered, to describe the status of the entire
world‟s coastal zone, a region of vital interest for human
society and sustainable development, including many
stretches where a space-borne altimeter provides the
only measurement device.
Filling the aforementioned gaps in the coastal zone
would provide current and wave observations for
erosion and sediment transport studies, ship routing and
coastal defence design and operation, and would help to
monitor surges and to measure long-term coastal sea
level change. Other applications include fisheries,
search and rescue, and the movement of hazardous
spills, pollution and harmful algal blooms. In all of
these cases, the availability of reprocessed altimeter data
and their assimilation into models alongside sea surface
temperature (SST) and data from research vessels and
ships of opportunity, moorings, wave buoys, tide
gauges, gliders and HF (High Frequency) radars has the
potential to increase the realism and accuracy of the
predictions.
In this community white paper, after a brief review of
the challenges in coastal altimetry and description of the
new products, we illustrate (with the aid of some
application examples) how the new products can be
used, and we discuss the plans for further integration of
this novel data source into the coastal observing
systems.
2. CHALLENGES IN COASTAL ALTIMETRY
A number of challenges remain in the processing of the
altimetric waveforms in the coastal zone and in the
correction of the measurements for path delay and
geophysical effects (tides and atmospheric). These were
first identified and described in the summary of the first
Coastal Altimeter Workshop [2] and the improvements
were reviewed at the second Altimetry Workshop [3]. In
summary:
- Retracking is needed to recover the sea surface
height (SSH) signal in the „last 10 km‟ next to the
coast. Specific coastal retrackers, subject of ongoing
research, should give better accuracy & precision
than generic deep ocean retrackers. Contamination
effects on waveforms due to land and bright targets
might be predicted in some places using detailed
coastlines and digital elevation models (DEMs).
- Farther from the coast, the wet tropospheric
correction is a main source of error. Strong gradients
in water vapour across atmospheric fronts near land
produce changes in path delay equivalent to several
cm over 20-50 km, which must be corrected. Several
methods are being developed, with encouraging
results: (I) dynamic extrapolation methods, using
high-resolution atmospheric models; (II) GNSS
(Global Navigation Satellite Systems) measurements
of ZTD (Zenith Total Delay) (and meteorological
correction to ZWD – Zenith Wet Delay); (III)
decontamination of the land effects, based on
detailed DEMs and/or statistical approaches relying
on inherent inter-channel correlation for mixed
ocean/land pixels; (IV) retrieval of integrated water
vapour from infrared imagery, with inherent 1-km
resolution.
- For those applications needing the removal of tidal
and atmospheric signals, large errors in tidal models
and in the models used to obtain HF and inverse
barometer corrections remain a problem. These
models are improving, but require detailed coastal
bathymetry with horizontal resolutions of at least 1
km (preferably 200 m), from the 200 m isobaths to
the coast.
- The ionospheric delay correction is affected when
the C-band (or S-band) footprint of the altimeter
“sees” the coast (prior to the Ku-band).
- The sea state bias correction is also a concern,
although not the greatest error source, and a
thorough comparison against coastal wave
measurements and models is needed.
For all those applications requiring the retrieval of
absolute dynamic topography and absolute geostrophic
currents from altimetric data in coastal areas, there is an
additional crucial challenge, i.e. the estimation of an
accurate and high resolution coastal Mean Dynamic
Topography (MDT).
3. INITIATIVES AND NEW PRODUCTS
The importance of coastal altimetry has been recognized
by the major Space Agencies, which are supporting
research and development in the field. The European
Space Agency (ESA) is funding the COASTALT
(Coastal Altimetry) Project (http://www.coastalt.eu)
aiming at defining and developing a prototype software
processor for Envisat and testing it over a few pilot
areas surrounding Europe. Ultimately, the plans are for
ESA to routinely generate and distribute these new
Envisat coastal altimetry products globally. In France,
the Centre National d'Études Spatiales (CNES) is
funding the PISTACH (Prototype Innovant de Système
de Traitement pour les Applications Côtières et
l'Hydrologie) Project for the reprocessing of the global
Jason-2 altimeter records in the coastal zone. This

project takes advantage of the improved
performances of Jason-2 near the coasts to give access
to high-resolution (20 Hz i.e. ~350 m) along-track
altimetric measurements, with an ensured continuity
from the open ocean up to the shoreline. These data,
now freely accessible via FTP from
ftp://ftpsedr.cls.fr/pub/oceano/pistach/ allow a finer
description of short scale (5-20 km) coastal
phenomenon such as river plumes, coastal upwelling
and circulation, with the most significant gain with
respect to classical 1-Hz Level-2 altimetry products
expected to be a better representation of local gradients
and hence currents. Higher-level along-track altimetry
products (e.g. regional MERSEA (Marine Environment
and Security for the European Area) product over the
Gulf of Mexico or North-East Atlantic) will also
directly benefit from the PISTACH products if they are
used as input. CNES and the U.S. National Aeronautics
and Space Administration (NASA) are also funding a
number of projects related to coastal altimetry within
the OST-ST (Ocean Surface Topography Science Team)
framework.
The development of coastal altimetry and its application
to current and future missions adds to the increasing
need for harmonised products across all available data.
Modern computing resources have allowed less-
compact formats to be introduced, like NetCDF
(Network Common Data Form), which is self-
describing, allowing products to be very flexible in
content. As this format becomes more accepted across
the climate community, standards have evolved for
variable naming and links between measurement data
and their associated errors and quality control
information. The introduction of these standards allows
development of more generic software solutions, giving
access to the data for non–specialists. Coastal altimetry
does not, of itself, impose any additional constraints on
the development of data products. However, the coastal
application of data has two key drivers that should
reinforce existing developments. The first is that the
wide range of potential corrections, auxiliary data and
processing required for the extended range of potential
applications and more complex geophysical coastal
processes make the inclusion of more complete
metadata a higher priority. This is being addressed by
new coastal products with their full gamut of descriptive
metadata included in the NetCDF format. The second
driver is that users will be less reliant on altimetry as
their primary data source, and hence be more reluctant
to invest large amounts of time in developing their
systems to incorporate these, potentially complex, new
products. This is increasing the move away from fixed
products towards tools and services to generate
customized products, on demand. These services can
tailor products to the required application and region, at
the time of generation, increasing the uptake of
altimetry by operation users, important for continuation
of altimetry missions.
4. CALIBRATION AND VALIDATION
The challenges outlined above call for a comprehensive
validation and intercalibration of both coastal altimetry
and other observational data dedicated to coastal ocean
studies. The first step consists in implementing the
existing technological advances in satellite altimetry in
the coastal area. This is part of an ongoing work carried
out by several groups, as in the aforementioned
COASTALT and PISTACH projects. In a second step,
improved altimetry measurements must be compared
with independent observing systems at several temporal
and spatial scales. There is consensus that the so far
unexploited capabilities arising from the merging of
existing in situ (glider, tide gauges, drifters) data
sources with remote sensing data (SST, altimetry) have
to be developed in a calibration-validation environment,
where the understanding of the physical contents of
each sensor is thoroughly investigated by comparing
altimetry with other kinds of data. As an example, Fig. 1
shows some results from an intensive observational
program in the Western Mediterranean carried out by
the Institut Mediterrani d'Estudis Avançats (IMEDEA)
by running coastal glider missions along selected
Envisat and Jason-1/2 altimeter tracks. The goal of this
experiment is threefold: i) to investigate the limitations
and potential improvements of altimetry data in the
coastal area ii) to test the feasibility of new technologies
to study coastal dynamics and iii) to understand the
physical content of each dataset. The first results show
good agreements and are encouraging.

Figure 1. Along track absolute dynamic topography from JASON-1 and JASON-2 altimeters and dynamic height from
glider data for the Alboran Sea [4] and Balearic Sea [5] in the Western Mediterranean. For the Balearic Sea, both the
onward (left) and return (right) legs of the glider are shown. Absolute dynamic topography is computed by adding to
the along track SSH anomalies a Mean Dynamic Topography [6]. Dynamic height is obtained from the glider pressure,
temperature, and salinity profiles with a reference level of 180 m.
5. SYNERGY OF COASTAL ALTIMETRY AND
OTHER DATASETS
Dynamics along the continental slopes, over the shelf
and in the coastal areas are difficult to observe given the
wide spectrum of temporal and spatial variability of the
various physical processes (coastal currents, eddies,
meanders, filaments, small-scale convection, coastal
trapped waves, etc.). Studying such complex dynamics
requires the development of synergistic approaches
through the combined use of modelling and observing
systems at several spatial/temporal sampling levels.
Satellite altimeters provide sustained observations of
phenomena often unachievable by other means. But
after the along-track SSH data have been successfully
retrieved, a second set of issues relate to how the
information is used. Does one use the more basic and
higher resolution SSH data along the tracks? Or does
one attempt to produce two-dimensional horizontal
maps of SSH? Both space and time scales decrease in
the coastal ocean as one approaches land. Given the
distance between tracks and the time between repeats,
traditional nadir altimeter data will never resolve the
scales (10+ km and 1+ day) of the variability in coastal
currents and SSH fields through even the most
sophisticated “gridding” techniques. This remains true
for multiple (up to 4) altimeters and even for proposed
swath altimeters, which will have high spatial resolution
but infrequent (5-days or longer) repeat coverage. The
result is a need to combine the information from
altimeters with that from other satellites (individual or
sequences of SST and colour fields, when clouds allow),
land-based radars (for surface currents within 50-150
km of land) and in situ measurements (moorings,
gliders, drifters, ship surveys and tide gauges). Even if
altimeters were capable of providing continuous high-
resolution fields of SSH, other means would be needed
to observe the highly baroclinic subsurface fields. Thus,
altimetry must be considered as one part of Integrated
Ocean Observing Systems (IOOS), which should also
include regional hydrodynamic modelling of shelves
and coastal circulation. This new approach is being
increasingly adopted. For instance, for the coastal ocean
bordering European seas we can cite several studies:
- combining altimetry and models, for example as in
[7] and [8] through comparison experiments
respectively over the Mediterranean and Atlantic
coastal zone;
- combining altimetry and in-situ data, for example as
in [9], [10] and [11]; studies using both mooring and
altimetry data to monitor a narrow coastal current;
monitoring of coastal dynamics by using gliders and
altimetric data [4] and [5];
- combining altimetry and SST like in [12] who show
the feasibility of monitoring coastal processes such
as current intrusion over the continental shelf of the
Gulf of Lion; or like in [13] who investigated eddy
generation;
- using in situ, remote-sensing and modelling
together: reference [14] recently demonstrated the
possibility of monitoring of deep water formation
from space in this way.
For the applications requiring absolute currents and
therefore a coastal MDT, the synergy must include
gravity missions as GRACE (Gravity Recovery And

Climate Experiment) and GOCE (Gravity and Steady
State Ocean Circulation Explorer). However, as the
resolution from these missions is not sufficient per se, a
strong synergy is also expected between, altimetry,
GRACE and GOCE data and in-situ gravimetric
measurements (shipborne/airborne) to enhance the
resolution of the geoid, and therefore the MDT
regionally.
6. APPLICATIONS OF COASTAL ALTIMETRY
Some of the uses of coastal altimetry have already been
mentioned, and current and future applications have also
been indicated by the user community in a recent survey
carried out by PISTACH and COASTALT. At the most
general level, we can say that the SSH, wave and wind
information about the coastal and shelf region can help
monitoring surges, long term coastal sea level
variations, erosion and sediment transport studies, ship
routing, fisheries, search and rescue, and the movement
of hazardous spills, pollutants and harmful algae
blooms. Here we provide more details on the full range
of possible applications, illustrating them with examples
where possible.
6.1. Sea Level and Tides
The quintessential most immediate application of
coastal altimetry is to monitor coastal sea level. This has
three interrelated aspects: short term sea level variations
(leading to monitoring of surges), long-term sea level
change due to climate change, and tides.
A good example of a statistical model which combines
satellite and tide gauge observations through a
multivariate regression analysis is the one set up to
study the near real time SSH variability in the North Sea
and Baltic Sea, with focus on storm surges in coastal
regions [15]. The model has been run for test cases
(Fig. 2) and shows a performance comparable to state-
of-the-art hydrodynamic models when estimating the
near real time SSH. It is planned to implement it
operationally for real-time storm surge predictions,
including Jason-2 observations.
This type of modelling will greatly benefit from quality-
controlled coastal altimetry, because it will allow
resolving near coastal processes. For the North Sea area
this is particularly important, as the SSH signals tend to
travel as Kelvin waves along the coast, with
exponentially decreasing amplitude away from the
coast. For island filled areas, such as the Inner Danish
Waters, a high return of good quality data is essential
for the use of the satellite altimetry. Even if one
considers the difficulties of assimilation (getting good
quality near-real time data, determining covariances,
handling the space-time sampling) which might hamper
the full adoption of coastal altimetry for near-real time
surge studies, the new products should at least provide
an independent data set that can be used for offline
surge model validation, in combination with an accurate
coastal tide model.
Figure 2. An example of the model performance for the
storm surge of December 7, 2003 in the Inner Danish
Waters. The dots indicate the predictions of the
statistical model. Observations from tide gauges used in
the model are marked by circles, observations from tide
gauges not used in the model are marked by squares,
and can be used for model validation.
A long record of quality-controlled coastal data is also
of great value at a time of increasing focus on mean sea
level (MSL) and the impact of sea level change. A high
quality global coastal dataset will allow for local sea
level trend determination. There are two particular
instances in which MSL studies could benefit from
coastal altimetry. One is where there are coastal
dynamical effects resulting in different signatures of
MSL variability and trends at the coast and farther
offshore. The availability of altimetry closer to the coast
will provide data sets with which to connect the coastal
tide gauge data to the offshore altimeter data. Another
application is in altimeter calibration by means of tide
gauges, an important method used to calibrate
TOPEX/Poseidon, Jason-1 and -2 and ESA missions.
By providing data closer to the gauge, concerns about
tidal and other variability in the intervening distance
from ground track to tide gauge will be reduced, as this
variability can be better estimated.
The liaison between altimetry and tidal science is a
good example of give and take; if on one side altimetry
is calibrated with tide gauges, on the other it provides
crucial data for tidal models. Reference [16] describe
well the difficulties in deriving accurate coastal tide
models given the present space-time sampling of
altimetry, but with the new coastal altimetry products,
the next generation of altimeters (including delay-
Doppler and wide swath instruments) and advanced
assimilation techniques, significant improvements in
coastal models will be possible. A key role that coastal
IOOS may play to aid coastal altimetry would be in the
deployment of a limited number of long-time series
bottom pressure measurements along the shelf. These

data would support the refinement of shallow sea and
coastal margin tidal models, making it possible to more
accurately de-tide the coastal altimeter SSH.
6.2. Assimilation in 3-D circulation models
Perhaps the most ambitious application of the surface
dynamic topography from coastal altimetry is to
estimate and forecast the three-dimensional ocean state
through data assimilation. A good case in point is the
work by the Ocean Modeling group at Rutgers
University on variational data assimilation in the
Regional Ocean Modeling System (ROMS).
Observations assimilated are satellite along-track SSH
anomalies, SST, HF radar surface currents, and
subsurface temperature and salinity from ships,
autonomous underwater vehicles and/or profiling floats,
as part of the Mid-Atlantic Regional Coastal Ocean
Observing System (MARCOOS). The dominant
circulation in the Mid-Atlantic Bight (MAB) has short,
anisotropic scales due to the shelf-slope front along the
continental shelf-break, a significant Slope Sea gyre
circulation, and Gulf Stream (GS) warm core ring
interactions with the shelf-slope front that drive episodic
exchanges of coastal and deep ocean waters. The
variational approach allows combining data from
multiple satellites and in situ observing systems with the
model physics, therefore introducing dynamic and
kinematic constraints into the analysis. The resulting
analyses will then be used for diagnostic studies of the
area and for predictability studies. Work is also
underway to incorporate real time data in the
assimilation system for operational prediction.
Realizing that satellite data is the only source of
information in real time in many parts of the world
ocean, including some parts of the US east coast, the
Rutgers group has developed methodologies that
correctly exploit the information content in remotely
sensed observations. The hydrographic in-situ
observations are then used to evaluate the quality of the
inversion. In the deep ocean (GS and Slope Sea area)
the two main sources of satellite information, namely
SST and SSH, are found to be highly complementary
and therefore both need to be assimilated in order to
obtain the correct 3-dimensional structure of the ocean.
The assimilation system can also correct for biases in
the model estimate due mainly to strong biases present
in the global ocean models that are used to force
ROMS. Future work on the analysis/forecast system
will be to evaluate a reprocessed coastal altimeter data
stream derived using de-aliasing corrections and other
required near-coast data filtering and error-correction
approaches. The resulting analyses will then be
compared with the current assimilation system to
evaluate to what extent reprocessed altimeter data can
constrain and improve the observed dynamics. We
anticipate the corrected SSH anomalies to be a valuable
data set for constraining dynamics in the MAB (and
many other coastal regions). When applied in the full
domain (coastal zone and adjacent deep ocean) the
altimeter data could help to constrain the along- and
across-shelf pressure gradients that are fundamental to
the shelf circulation, shelf/slope front, and the remotely
generated forcing associated with GS ring interactions.
On the U.S. west coast, modellers at Oregon State
University (OSU) are also developing methods to
assimilate along-track SSH anomalies into coastal
circulation (non-linear ROMS) models. Their data
assimilation system uses variational representer-based
methods [17], incorporated into tangent linear and
adjoint AVRORA codes developed at OSU. The present
OSU pilot forecast model provides daily updates of 3-
day forecasts of SST and surface current fields,
available freely over the web (http://www.nanoos.org/).
An example application for the forecast fields is their
use by tuna fishers to identify frontal regions between
the cooler water next to the coast and warmer offshore
waters. This is preferred tuna fish habitat, with
temperatures around 14°C. The fishers then plan their
cruises to reach the frontal regions using the shortest
time and least fuel.
Figure 3. SST and altimeter tracks. (Left) Model SST
hindcast without altimeter assimilation; (Middle) SST
hindcast with altimeter assimilation; (Right) GOES
(Global Earth Observation System) satellite SST field
for the same day. The assimilation of the altimeter data
reduces the offshore extent of the SST filament
separation at 43°-44°N (1) and improves the location of
an anticyclonic eddy south of the filament (2), in better
agreement with the satellite data.
Although altimeter data are not routinely assimilated
into the forecasts, experimental hindcasts (using the
methods described above) have shown that the positions
of the SST fronts are improved in comparison to
satellite SST fields when along-track SSH data are
assimilated (Fig. 3). Since the satellite SST fields are
often obscured by clouds, the altimeter data assimilation
offers the most reliable source of improvements to the
model SST fields. Making improved coastal along-track
altimeter data available in near real time (over the past
2-6 days) for assimilation into the models would
increase the realism of the predictions.

6.3. Coastally Trapped Dynamics
One of the prospective applications of a global coastal
altimetry dataset is to look at those peculiar
manifestations of the ocean dynamics that are „trapped‟
along the coast or at the shelf edge. One example are
propagating coastal trapped waves identified in
altimeter data along the continental slope [18], whose
study could be extended over the shelf and in coastal
areas. Similarly, coastal altimetry will contribute to the
investigation of the dissipation (and/or reflection)
mechanisms of mesoscale eddies and westward-
propagating planetary waves once they arrive on the
shelf and in coastal waters.
6.4. Wind and Waves
Finally, there are interesting applications for the wind
and wave coastal altimetry products. For instance, at the
Danish Meteorological Institute, open ocean SWH
(Significant Wave Height) data are currently used for
validation of an operational wave model. An improved
data set of significant wave height in coastal areas
would allow extending the validation to those areas,
which are heavy with ship traffic and vulnerable to oil
spill accidents. In the meantime, one of the most
advanced applications of the wind and wave data is
taking place at NOAA's National Center for
Environmental Prediction (NCEP). NCEP is
consistently increasing its use of Jason-1, Jason-2 and
Envisat altimeter data in both maritime forecasting and
predictive models used for forecast guidance. A
technology transfer effort funded by NASA is being
used to assure that NCEP Ocean Prediction Center
(OPC) forecasters now have these data displayed within
1 to 3 hours of satellite measurements. Real time
altimeter wave height and wind speed data are now, or
will soon be, used in sea state forecasting and analysis
product generation for the US coasts and within the
OPC's offshore coastal margin areas of responsibility.
The NCEP Environmental Modelling Center is now
assimilating all near real time SWH data into the
WAVEWATCH III ocean wave model used by OPC
and all US National Weather Service offices for
nearshore, offshore and high sea forecast guidance.
Moreover the altimeter wind speed data (because they
are not assimilated into surface wind analysis products)
are used as a critical independent validation of the
global and coastal ocean wind speed data used to force
WAVEWATCH III. Near term goals at NCEP are to
provide increasingly refined coastal forecasts and model
guidance, thus an emphasis on coastal altimeter SWH
and wind speed data quality and validation is desirable.
The present interleaved operation of the Jason-1 and
Jason-2 orbits is highly attractive for all these activities.
7. CAPACITY BUILDING IN COASTAL
ALTIMETRY
The recent development of coastal altimetry is having
an additional beneficial effect in that, being related to
coastal management and sustainable development, it
links directly to the work of the local scientists and
administrators in many of the world‟s regions and
therefore contributes to capacity building for altimetry
as a whole. Two examples of capacity building potential
(in India and the Southern African countries) are
illustrated below. Another notable example is in
Thailand via the EU-funded GEO2TECDI project [19].
In India, coastal altimetry studies have been initiated as
a joint project between the National Institute of
Oceanography (NIO) and the Indian National Centre for
Ocean Information Services (INCOIS), called
ALTICORE-India (ALTImetry for COastal REsearch)
and forming a larger collaborative effort with the
European Union group ALTICORE-EU (see
http://www.alticore.eu). Investigations carried under the
above project are based on improvements of
atmospheric and tide related correction terms along
select ground tracks for various satellites, off the west
coast of India, with filtering and reconstruction
involving additional interpolation steps, similar to those
carried out in the Corsica Channel in the Mediterranean
Sea [20]. The Indian Coast lends itself well to coastal
altimetry studies: reference [21] has estimated boundary
currents from altimeter for the East India Coastal
Current (EICC) at the western boundary of the Bay of
Bengal, using experimental reprocessed
TOPEX/Poseidon data for the coastal zone. Their
methodology reveals the full spectrum of along-shore
current, ranging from intra-seasonal to inter-annual time
scales. Furthermore, it has been demonstrated using
coastal altimetry that the seasonal cycle dominates the
variability, but the non-annual timescales have similar
energy levels all along the EICC path [22]. Following
on these encouraging results, NIO and INCOIS are now
planning to reprocess all the along track profiles for the
Indian region with regional corrections. This new
dataset, once validated with other data sets including
tide gauge observations, will be assimilated into ocean
circulation models for operational forecasting system of
the Indian coastal seas. Furthermore, INCOIS provides
operational ocean state forecast for the Indian seas using
WAM/WAVEWATCH models for the coast and the
open ocean. Once the coastal altimetry-derived SWH
data are available, they will be assimilated for the
improved forecast. The future Indian/French
SARAL/Altika (Satellite with ARgos and ALtiKa)
mission [23], due for launch in 2012, holds great
promise to further improve the accuracy and coverage
of the altimeter-derived products.

Southern Africa is uniquely placed in the world, with an
extremely energetic, warm western boundary current,
the Agulhas Current System on the east coast, which
can interact with an extremely energetic eastern
boundary upwelling system, the Benguela Upwelling
System, on the west coast. This interaction takes place
over the very wide Agulhas Bank Shelf region. It should
be noted that over 40% of Africa's population derives its
livelihood from coastal and marine ecosystems and
resources. The continent's reliance on coastal and
marine resources is set to increase substantially, as
coastal populations are expected to double by 2025.
Understanding the dynamics of coastal and shelf seas is
thus essential for managing vulnerable coastal
ecosystems, protecting lives and livelihoods, and
planning sustainable development. Remote sensing in
general and coastal altimetry in particular offers a
powerful and cost effective means of observing the
marine and coastal environment.
Five main areas of southern Africa (Fig. 4) have already
been identified as good locations to make use of coastal
altimetry advances [24]. These are (from west to east)
the Angola-Benguela Frontal Zone (for tracking and
evolution of Benguela Niños), the southern Benguela
Upwelling region (for monitoring the upwelling region
under climate change), the Agulhas Bank south of
Africa (an important spawning region for anchovy and
pelagic fish), the east coast Agulhas Current, which is
an giant wave-current interaction region, and the
Mozambique Channel for the impact of rising sea level
and tropical storms on impoverished communities.
Numerical modelling of the coastal ocean circulation
around southern Africa is at an early stage, but there
have been notable successes on the west coast, and the
Agulhas Bank. Advances in coastal altimetry will serve
as an important set of measurements to calibrate the
models (like ROMS) for current variability, and will
complement significant improvements of the in situ
measurement capability in the Agulhas current system
[25]. Wind, SST and ocean colour are regularly being
obtained from satellites at sufficiently fine scales, and
being used to either force or verify numerical model
studies in the coastal zone.
8. FUTURE PLANS FOR COASTAL
ALTIMETRY
When the existing 18-year archive of altimetry data is
reprocessed to enable reliable estimates of sea level and
its gradient over the continental shelf, an immediate
application will be the construction of a global atlas of
the statistics of sea level and surface current variability
over the continental shelves of the world. In situ
observations are lacking or inadequate for most parts of
the world for the construction of such an atlas.
As marine industries move into deeper and deeper
water, interest in the statistics of currents over the outer
shelf and the continental slope is increasing. But if some
new industry suddenly expresses an interest in some
particular location, they do not want to wait many years
for a targeted observational programme to be conducted.
The only option in such cases is to rely on modelling
and analysis of existing observations, which, in many
cases, will comprise coastal altimetry and the
trajectories of a limited number of drifting buoys.
Figure 4. Pilot areas for the development of coastal altimetry around Southern Africa, with an indication of the main
motivations in each area. From [24], updated.

The energy sector is one with an increasing interest in
ocean currents. For oil or gas extraction, currents
represent a major threat to operational safety and they
are one of the hazards, which need to be accurately
evaluated, of operating in deeper water. But strong deep
ocean currents can also be viewed as a potential energy
resource, as tidal currents are in shallower water. The
East Australian Current, for example, has core speeds
estimated to be 1±0.5m/s in places (Fig. 5), according to
ocean models [26][27]. An assessment of whether it
might be economically feasible to exploit this energy
resource will require detailed assessment of in situ data,
augmented by an analysis of the reprocessed altimetry
data. Knowledge of the statistics of the coastal wave
field from long-term coastal altimetry can also be a
valuable asset in this scenario.
While the widespread adoption of coastal altimetry
relies on maximising the impact of the existing, 18-year
long dataset in applications like those described above,
the future evolution of the field appears to be tightly
linked with the advent of future missions, both nadir-
viewing and wide-swath, which should improve both
quantity and quality of coastal altimetry data. Nadir-
viewing altimetry is expected to make a quantum leap
with the arrival of delay-Doppler instruments like those
on CryoSat (launched in April 2010) and, crucially, on
Sentinel-3, the operational successor of Envisat in
ESA‟s oceanographic mission line-up.
The delay-Doppler technique [28] is a more efficient
alternative to conventional altimetry, exploiting a larger
share of the power in the echo returns, thus resulting in
better accuracy and along-track resolution (a very useful
attribute to sample short-scale features in the coastal
zone). Wide swath altimetry, with missions like the
Surface Water and Ocean Topography (SWOT), relies
on radar interferometry principles to carry out 2-D SSH
measurements at resolutions of the order of 1 km [29],
and a large amount of research can be anticipated on
how to take advantage of this technique in the coastal
environment, including the benefit of deriving a high-
resolution slope map. Finally, improvements in the
coastal geoid, expected from the merging of GRACE
and GOCE data with in situ gravimetric measurements,
will undoubtedly be crucial for all those applications
where an absolute dynamic topography is needed.
Figure 5. Mean and standard dev., over 10 years, of the
near-surface (0-50m) current speed off east Australia,
as estimated by an ocean model of 0.1º × 0.1º spatial
resolution. Re-processed altimetry will be used to assess
the accuracy of this model, and to drive improvements.
9. CONCLUSIONS
This paper has examined the recent development of
coastal altimetry and looked at the challenges and
perspectives of this novel field. Improvements have
been and are being made to processing of altimeter data
and the various corrections, to bring this measurement
closer to the coast. Water vapour and tidal corrections
are particularly crucial, and there is a real need to
increase their precision in the coastal zone. Tidally
forced currents can become non-linear and difficult to
forecast, so more accurate maps of detailed bottom

bathymetry are needed. Then we need the best possible
calibration/validation systems to validate the new data
in the context of dramatic space/time variability in the
coastal zone. Efforts in this direction are being carried
out by a lively community within a number of
initiatives, including the PISTACH and COASTALT
projects. The new generation altimeters are designed to
take up coastal dynamic topography and wave
measurements that would help integrate the sea level
and wind speed into coastal ocean observing systems.
Ultimately, most coastal oceanographers expect that all
of the available measurements will be assimilated into
operational models of the circulation and sea state (and
eventually the ecosystem) to produce nowcasts and
short-term (several days to several weeks) forecasts. For
altimetry to play this important part in operational
coastal observing/modelling systems, however, we must
first overcome the present obstacles in retrieving the
along-track SSH data, as described above. Data
assimilation methods need both SSH values and
estimates of the errors or uncertainties in those values.
The research reviewed above serves as a prelude and
context for the more detailed and focused efforts needed
to move forward, both by promoting the long existing
datasets (a real asset), and by exploiting the several
forthcoming missions. There is no doubt that the new
coastal altimetry products will constitute a crucial input
for coastal observing systems: the experience gained in
optimizing existing coastal altimetric data will guide the
design of the future instruments, so that the multi-
decadal record is not only continued, but also improved
in quality and quantity.
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