Radar Altimeters
Description
Radar altimeters are active sensors which
use the ranging capability of radar to
measure the surface topography profile along
the satellite track. They provide precise
measurements of a satellite’s height above
the ocean and, if appropriately designed,
over land/ice surfaces by measuring the time
interval between the transmission and
reception of very short electromagnetic
pulses.
To date, most spaceborne radar altimeters
have been non-imaging, wide-beam
(pulse-limited) systems operating from low
orbits. Such altimeters are useful for
relatively smooth surfaces such as oceans
and low-relief land surfaces, but are less
effective over high-relief continental
terrain as a result of their large radar
footprint (of the order of 25 km).
Jason-2, also known as the Ocean Surface
Topography Mission (OSTM), extends the
series of satellites that collect global
ocean surface data on a continuous basis for
several decades. Its aims are to measure the
global sea-surface height to an accuracy of
a few centimetres every 10 days, and to
determine ocean circulation and the mean
sea-level trend in support of weather
forecasting, climate monitoring and
operational oceanography. Launched in June
2008, Jason-2 continues to operate in
parallel with the Jason-1 mission. Jason-3
is being developed for launch in the first
half of 2015, with the aim of securing the
continuity of high-accuracy satellite
altimetry observations. The Jason-CS
(Continuity of Service) is under study to
prolong the Jason series observations well
into the 2020s.
Successful exploitation of the height data
is dependent upon precise determination of
the satellite’s orbit. A number of precision
radar altimetry ‘packages’ are available
which contain:
— A high-precision radar altimeter (with
basic measurement accuracy in the range 2 cm
to 4 cm), with dual-frequency capability to
correct height measurements from ionospheric
delays;
— A means of correcting errors induced in
the height measurements by variations in the
amount of water vapour along the path (for
example, by means of a microwave atmospheric
sounder or radiometer);
— A high-precision orbit determination
system (typically based on the GPS, the
DORIS beacon/satellite receiver system
and/or a lidar tracking system).
Radar altimeters have been flown on a number
of satellites. Seasat was the first
ocean-oriented mission carrying an altimeter
package (including a precise orbit
determination system) for the measurement of
ocean circulation. A satellite altimetry
revolution happened with the launch in 1992
of the US–French Topex/Poseidon mission.
Carrying two high-precision altimeters, a
multichannel microwave radiometer and
several precise orbit determination devices
on a dedicated, high-altitude (1336 km),
low-inclination (66°), non-Sun-synchronous
orbit, it enabled the large-scale ocean
circulation to be accurately measured. The
European ERS-1 (from 1991) and ERS-2 (ending
July 2011) also provided long time-series of
complementary altimetric observations from a
Sun-synchronous polar orbit. These
observations were continued with Jason-1
(launched in 2001), Envisat (launched in
2002), and Jason-2 (launched June 2008).
Requirements for satellite altimetry in
support of the Essential Climate Variable
Sea Level include: one high-precision
altimeter mission operating at all times in
the Topex/Poseidon medium-inclination
reference orbit, with planned extensive
overlaps between successive missions for
continuity of global sea-level rise
(continuity with Jason-CS and beyond is a
goal of primary importance to establish a
long-term, climate-related sea-level
record); and two additional – equally
precise – altimeter missions with
high-inclination orbits, which would provide
needed sampling for the mesoscale and in
polar regions.
The French–Indian mission SARAL, carrying an
innovative Ka-band altimeter (AltiKa)
launched in 2012 into the same orbit as
Envisat, to ensure the continuity of
altimetric observations from a
high-inclination orbit.
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Current & planned instruments
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ALT
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Poseidon-3B
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AltiKa
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SIRAL
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Ka-band Radar INterferometer
(KaRIN)
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SARAL
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Poseidon-3
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Applications
A variety of parameters may be inferred using
the information from radar altimeter
measurements. These include: time-varying
sea-surface height (ocean topography), the
lateral extent of sea ice and the altitude of
large icebergs above sea level, as well as the
topography of land and ice sheets, and even
that of the sea floor. Topographical maps of
the structure of the Arctic sea floor have not
only revealed new mineral deposits, but they
also provide new insights into how a large
part of the ocean basin was formed about 100
million years ago.
Observations by current and future radar
altimeters of trends in the ice masses of
Earth are of principal importance in testing
the predicted thinning of Arctic sea ice due
to global warming. They also help to quantify
the extent to which the Antarctic and
Greenland ice sheets have contributed to the
global rise in sea level. New-generation radar
altimeters, such as RA-2 on Envisat, have
provided useful information for the monitoring
of inland waters (river and lake levels).
Satellite altimetry also provides information
that is used in mapping sea-surface wind
speeds and significant wave heights. Precision
ocean altimetry applications for sea-level
monitoring and ocean circulation studies
require more accurate, independent
measurements of the geoid, derived from the
instruments described in the ‘gravity field’
category.
ESA’s CryoSat-2 mission provides an instrument
for studying the topography of areas such as
ice-sheet interiors and margins, and sea-ice
with three-mode operation: conventional
pulse-limited operation for the ice-sheet
interiors (and oceans if desired); synthetic
aperture operation for sea ice; and
dual-channel interferometric synthetic
aperture operation for ice-sheet margins.
The new generation of instruments will provide
more frequent data coverage and faster access
to observations for incorporation into ocean
circulation and wave forecast models that are
used to generate marine information products.
New concepts of altimeter packages are being
developed, such as the wide-swath altimeters,
proposed by the SWOT mission, capable of
providing an imaging capability.
Radar altimeters measure the distance
between the satellite and the sea surface.
The distance between the satellite and the
reference ellipsoid is derived by using the
Doppler effect associated with signals
emitted from marker points on Earth’s
surface as the satellite orbits overhead.
Variations in sea-surface height are caused
by the combined effect of the geoid and
ocean circulation (dynamic topography).
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