Geoscience Reference
In-Depth Information
of the water. The signals are transmitted permanently in the high frequency domain
(about 14GHz) and the received echo from the sea surface is used for deriving the
round-trip time between the satellite and the sea. The satellite-to-ocean range is
obtained by multiplication of the traveling time of the electromagnetic waves with
the speed of light and averaging the estimates over a second (Todorova
2008
).
Satellite Altimetry Missions
The first satellite-borne altimeter missions were the US SKYLAB, consisting of
three satellites launched in the period of 1973-1974, GEOS-3 launched in 1975,
followed by SEASAT in 1978 and GEOSAT in 1985. As part of several international
oceanographic and meteorological programmes a number of satellite altimetry mis-
sions were launched in the nineties: ERS-1 (1991-1996), Topex/Poseidon (1992) and
ERS-2 (1995). The Jason-1 mission, which was the follow-on to Topex/Poseidon,
was launched in 2001 at the same orbit. On the contrary to the ERS-1 and ERS-2
missions, Topex/Poseidon and Jason-1 carried two-frequency altimeters, which gave
the opportunity to measure the electron density along the ray path. The latest satellite
altimetry mission Jason-2, which is also known as the Ocean Surface Topography
Mission (OSTM) was launched in June 2008.
The Topex/Poseidon was a joint project between NASA and the French space
agency (CNES) with the objective of observing and understanding the ocean circu-
lation (AVISO
2007
). The satellitewas equippedwith two radar altimeters and precise
orbit determination systems, including the DORIS system. The follow-on mission
Jason-1 was the first satellite of a series designed to ensure continuous observation of
the oceans for several decades. It had received itsmain features like orbit, instruments,
measurement accuracy, and others from its predecessor Topex/Poseidon. The orbit
altitude of the two missions was 1,336km with an inclination of 66
◦
, known as the
repeat orbit, causing the satellite pass over the same ground position every 10days.
Jason-1 was followed by Jason-2 as a cooperative mission of CNES, European Orga-
nization for the Exploitation of Meteorological Satellites (Eumetsat), NASA, and the
National Oceanic and Atmospheric Administration (NOAA). It continued monitor-
ing global ocean circulation, discovering the relation between the oceans and the
atmosphere, improving the global climate predictions, and monitoring events such
as El Nino conditions and ocean eddies (ILRS
2011
). Jason-2 carries nearly the
same payload as Jason-1 including the next generation of Poseidon altimeter, the
Poseidon-3. The Poseidon-3 altimeter is a two-frequency solid-state sensor, measur-
ing range with accurate ionospheric corrections. Poseidon-3 has the same general
characteristics of Poseidon-2, which was onboard Jason-1, but with a lower instru-
mental noise. The accuracy is expected to be about 1 cm on the altimeter and also
on the orbit measurements (Dumont et al.
2009
). For more details about the Jason-2
mission refer to CNES (
2011
).
