Rapp. Comm. int. Mer Médit., 37,2004
109
LIGHT GPS BUOYS FOR THE ENVISAT SATELLITE ALTIMETER CALIBRATION
IN THE NW MEDITERRANEAN
Agustí Julià
1 *
, Josep Torrobella
2
and the GRAC IEEC-ICM Team
1
ICM. Institut de Ciències del Mar C.S.I.C. Passeig Marítim 37-49, 08003 Barcelona, Spain - * ajulia@icm.csic.es
2
IEEC. Institut d’Estudis Espacials de Catalunya. C/ Gran Capità, 2-4. Ed. Nexus, 204. 08034 Barcelona, Spain - badia@ieec.fcr.es
Abstract
A new system is described for the precise measurement of absolute sea level at open sea. It was designed to compare with observations
by satellite radar altimeters, and has been used in the ENVISAT absolute range calibration. The system consists on two attached light buoys
containing GPS antennas connected by coaxial cables to their receivers in a nearby small boat. Form April to October of 2002, 45 short
cruises were performed along 400 km of the NW Mediterranean coast in simultaneity with ENVISAT overpasses.
Keywords: ENVISAT, Satellite altimetry, sea level, GPS, drifting buoys
We describe a newly designed system employed in the calibration
of the ENVISAT radar altimeter, using a double GPS receiving system
?oating freely on the sea surface under the track of the satellite at the
moment of the overpass. The sea level is measured at very high
precision with reference to the WGS84 ellipsoid, and later compared
with the satellite altimetry data. This was part of the joint participation
of Institut d’Estudis Espacials de Catalunya and Institut de Ciències
del Mar (Barcelona) in the activities coordinated by the European
Space Agency during the ENVISAT calibration phase, under ESA
contract No. 15349/01/NL/SF.
We designed a two-buoy rigid system with one GPS choke ring
antenna each, attached to their corresponding receivers using
impermeable low loss coaxial cable and a sustaining rope. The
distance to the boat was around 40 meters to avoid possible re?ections
of the incoming signal on the boat structure, but not longer to avoid
excessive signal loss along the cable.
The system was left to freely drift during 3 hours within 1 nautical
mile of the nominal point, centred in the ENVISAT overpass, and
acquiring two-frequency GPS data at 1 Hz. The boat was also drifting
but keeping the possibility of a manoeuvre to avoid tension on the
cable that could prevent the two-buoy system to exactly follow the
free surface motion. The system oriented spontaneously parallel to the
wave front, and its size allowed the antenna centres (the point exactly
positioned by GPS) to be always at the same distance from the free
sea surface. Careful measurements under controlled conditions (small
pond and harbour) were used to precisely determine this distance
(some 3.75 cm). Simultaneously to the open sea measurements, on
land and near the shore, another fixed system equipped with one
antenna and receiver, recorded reference GPS data in order to estimate
the tropospheric delay. To avoid the error introduced on the re?ected
signal due to the differences between the scattering coefficients of the
land and the sea surface, that could be both present in the foot print
(1), the measuring points at sea were chosen at a 10 miles distance
from the coast, under the satellite track.
The strategy was to cover 10 ENVISAT tracks (6 ascending night
passes and 4 descending day passes) along 400 km of the NW
Mediterranen Catalan coast, between 40º and 42º N. We selected 6
harbours and 10 ground sites for the land station, and hired small
boats (between 7 and 12 meter long) to reach the measuring points.
All were equipped with good navigation facilities and a cabin, where
the receivers were safely installed and campaign notes could be taken.
During the six months calibration phase, from April to October
2002 starting one month after ENVISAT launch, 53 satellite
overpasses took place in the study area, and 45 simultaneous GPS data
acquisitions were achieved (85% success). The failures were due to
equipment malfunctioning, logistic problems and mainly to bad
weather conditions. 18 different people were involved in the
operations, usually three on the boat and one operating the ground
station.
The tight activity schedule, with one operation at sea every 3-4 days
without interruption during 6 months, required a constant and careful
maintenance at ICM. Work at sea, assembling and disassembling, and
transportation by road can produce important stress to the different
elements of the two-buoy system, the ground station and the GPS
receivers. During all the experiment, we have maintained the system
under two main scopes: a general and complete check before each
mission, including the buoy water tightness, batteries charge, cables
inspection and connectors condition. The second one included repair
of the defective components or elements. It was necessary to do 18
repairing interventions, 6 of them on the buoy water tightness and 8
on the cable connections, meaning that 78 % of the maintenance had
to be done on the elements of the system subject to the physical effort
on the sea. In few occasions the GPS receivers had to be substituted
due to malfunctioning.
All the data acquired in the 45 cruises were processed at IEEC,
including the Precise Point Positioning technique (2), GIPSY/OASIS
software and JPL products, and delivered to ESA that used them for
the ENVISAT RA-2 range calibration within the so called regional
approach in the whole NW Mediterranean, including different kinds
of coastal and open sea measurements and modelling techniques (3).
References
1-Chelton D. B, Ries J., Haines B., Fu L-L. and Callahan P. 2001.
Satellite Altimetry. Pp. 4-7. In:Lee-Lueng Fu and Anny Cazenave (ed.),
Satellite Altimetry and Earth Sciences. International Geophysics Series.
vol. 4, chap. 1. Academic Press. San Diego
2-Zumberge J.F., He?in M.B., Jefferson D.C. and Watkins M. M 1997.
Precise point positioning for the efficient and robust analyses of GPS data
from large networks. J. of Geophysical Research., 102: 5005-5017.
3-Francis R. and Roca M., 1997. RA-2 in-orbit calibration plan: Range,
ESA-ESTEC Internal Document, PO-PL-ESA-RA-00xx, 48 p.
