Journal of Global Positioning Systems (2002)
Vol. 1, No. 1: 64-65
Ocean Remote Sensing with GPS
Cinzia Zuffada
Jet Propulsion Laboratory California Institute of Technology Pasadena, California 91109
Received: 5 May 2002 / Accepted: 16 June 2002
Bibliography
Cinzia Zuffada, Senior Member Technical Staff in the
Earth Orbiter Systems Group, Jet Propulsion Laboratory
(JPL). Her work at JPL has been focused on the use of
GPS for remote sensing of the atmosphere and the ocean.
She has been managing the research in GPS altimetry at
JPL for the past three years.
Mesoscale ocean eddies, analogous to atmospheric
storms, are thought to play a very significant role in the
transport of momentum, heat, salt, nutrients, and other
chemical properties of the ocean. At present, quantifying
the role of mesoscale eddies in the ocean circulation and
therefore climate variability cannot be done simply
because their spatio-temporal structures are not resolved
by the existing remote-sensing techniques. Current
generation altimeters have repeat cycles of at least ten
days and cross-track separation of up to 300 Km.
Recent research efforts have begun using GPS signals
scattered off the ocean and sensed by an air- or space-
borne receiver in a bistatic radar geometry, as a means of
doing altimetry and scatterometry. Upon impinging on
the ocean surface, the GPS signal is reflected primarily in
the specular (forward) direction, in an amount dependent
on surface roughness and angle of incidence. An airborne
or space-borne receiver, connected to a down-looking
antenna, could collect such scattered signals. One such
receiver, and the 24 transmitters, form a multistatic radar
system, capable of intercepting reflections from several
areas of the ocean simultaneously. Because of the multi-
static nature of the GPS observations, they will improve
our current capability of global sea surface measurements
in two important ways: improved spatio - temporal
resolution and coverage.
A GPS receiver in low-Earth orbit (LEO) with an antenna
pointed toward the Earth’s surface can, in principle, track
about 10 GPS reflections simultaneously, therefore
providing a coverage that is an order of magnitude denser
than nadir-viewing altimeters. For example, the reflection
ground tracks of a satellite at the altitude of 400 km
would cover the Earth nearly uniformly in just 1 day,
with at most about 75 km across-track separation. Such
dense coverage can be translated into a higher temporal
and spatial resolution, thereby providing the ability to
recover certain ocean topography features or processes
that are precluded with traditional altimeters. Such
system would require a high gain, multi beam antenna,
with the ability to steer the beams to track up to ten
moveable reflection points over the ocean.
To demonstrate the feasibility of this measurement, GPS
reflections were collected from airplane experiments over
the ocean off the coast of California. The precision in the
sea surface height obtained thus far is 5 cm over 1 sec,
with 5 km spatial resolution, using two satellites.
Additionally, an experiment with a fixed receiver at a
lakeside indicated a precision of 2 cm, again using 2
satellites. Such precision is suitable for eddy monitoring
where sea level signals are usually 10 cm or more.
The R&D effort carried out at JPL during the past four
years has demonstrated that GPS altimetry is feasible
from moving platforms, and its precision is useful for
scientific applications. The next steps involve the
development of GPS receivers which can track reflections
and process the signals on board and, eventually, the
integration with suitable antenna systems which can
capture the wealth of reflections available over the ocean
at all times.
References
C. Zuffada, T. Elfouhaily and S. Lowe: Deriving Near-Surface
Wind Vector With Ocean Reflected GPS Signals:
Simulations And Measurements, submitted for

Zuffada: Ocean Remote Sensing with GPS 65
publication in Remote Sensing of the Environment, March
2001.
A.K. Fung, C. Zuffada and C. Y. Hsieh, Incoherent Bistatic
Scattering from the Sea Surface at L-Band, IEEE
Transactions on Geoscience and Remote Sensing, Vol. 39,
no. 5, pp. 1006-1012, May 2001.
Lowe, S. T., C. Zuffada, Y. Chao, P. Kroger, J. L. LaBrecque,
L. E. Young, Five-cm-Precision Aircraft Ocean Altimetry
Using GPS Reflections, accepted for publication in
Geophys. Res. Lett., February 02.
R.Treuhaft, S. Lowe, C. Zuffada, and Y. Chao: 2-Cm GPS
Altimetry Over Crater Lake, Geophys. Res. Lett., 28, 23,
p. 4343-4346, December 2001.