[CINC] Jason-2 Satellite

[Susie Williams]

Key ocean mission goes into orbit 

By Jonathan Amos 
Science reporter, BBC News 

How Jason-2 will probe the oceans

A space mission that will be critical to our
understanding of 
climate change has launched from California. 

The Jason-2 satellite will become the primary means of
measuring the 
shape of the world's oceans, taking readings with an
accuracy of 
better than 4cm. 

Its data will track not only sea level rise but reveal
how the great 
mass of waters are moving around the globe. 

This information will be fundamental in helping
weather and climate 
agencies make better forecasts. 

The satellite left Earth at 0746 GMT atop a Delta-2
rocket from the 
Vandenberg Air Force Base. 

The spacecraft, built by Thales Alenia Space,
represents the joint 
efforts of the US and French space agencies (Nasa and
CNES), and the 
US and European organisations dedicated to studying
weather and 
climate from orbit (Noaa and Eumetsat). 

Down below 

Jason-2 will provide a topographic map of 95% of the
Earth's ice-
free oceans every 10 days. Although we think of our
seas as being 
flat, they are actually marked by "hills" and
"valleys", where the 
highs and lows may be as much as two metres apart. 

Elevation is a key parameter for oceanographers. Just
as surface air 
pressure reveals what the atmosphere is doing above,
so ocean height 
will betray details about the behaviour of water down
below. 

The data gives clues to temperature and salinity. When
combined with 
gravity information, it will also indicate current
direction and 
speed. 

The oceans store vast amounts of heat from the Sun;
and how they 
move that energy around the globe and interact with
the atmosphere 
are what drive our climate system. 

"The ocean constitutes the long-term memory of the
climate system; 
the time-scales over which the ocean is changing are
the climatic 
timescales," explained Mikael Rattenborg, the director
of operations 
at Eumetsat. 

"In order to understand climate, in order to be able
to predict the 
evolution of the atmosphere over months, years, and
decades even, 
you need to understand the ocean." 

Number one 

Jason-2 is a continuation of a programme that started
in 1992 with 
the Topex/Poseidon mission and is currently maintained
by the Jason-
1 satellite launched in 2001. JASON-2 SPACECRAFT 

1. Advance Microwave Radiometer - measures signal
delay caused by 
water vapour
2. GPS antennas - ensures knowledge of precise orbit
path
3. Poseidon-3 altimeter- measures sea level
4. Doris antenna - tracking and positioning control
5. Laser Retroreflector Array (LRA) - tracks and
calibrates 
measurements
Satellite mass: 525kg (1,155lb) Power generation: 511
watts
Satellite height: 3m (9ft 8in) Orbit: 1,338km (831
miles)
(Source: Eumetsat, Cnes, Nasa) 

The project provides the global reference data for
satellite-
measured ocean height. 

Although other spacecraft in service today can acquire
similar data 
sets, none can match the precision achieved by
Jason-1; and Jason-2, 
when in service, will be the benchmark against which
all other 
spacecraft will be judged and calibrated. 

At the heart of the latest mission is the Poseidon 3
solid-state 
altimeter. The instrument constantly bounces microwave
pulses off 
the sea surface. By timing how long the signal takes
to make the 
return trip, it can determine sea surface height. 

Additionally, the signal can indicate the height of
waves and wind 
speed. 

"It is not a revolution between Jason-1 and Jason-2;
it is an 
evolution, because the main objective is to ensure
continuity," 
explains Francois Parisot, the Jason-2 project chief
at Eumetsat. 

"Nevertheless, there are some improvements in the
instruments. We 
hope to make better measurements closer to the coast
[and over 
inland waters and rivers]; and also, we will deliver
near-realtime 
products - products that will be available within
three hours of the 
measurements. " 

Whale watching 

The latter will be particularly useful in storm
prediction. Jason 
will see the surface waters rise as warm eddies fuel
hurricanes. The 
data will tell meteorologists how a storm is likely to
intensify and 
allow them to issue better, more timely warnings. 

Jason-2 data will have many other uses that may not be
immediately 
obvious. Industry will take the information to make
decisions about 
when conditions are most suitable for undersea
drilling or cable 
laying. 

Jason can help identify where wreckage or pollution
will drift; and 
the satellite will assist marine biologists as they
track whales by 
pinpointing waters with the potential to be prime
feeding and 
breeding grounds. 

One very important use will be in maritime navigation.


"Now that the fuel price is going up, saving fuel for
the companies 
that run ships has become very sensitive; and knowing
the currents, 
you can select your route so that you go faster and
save fuel," said 
Philippe Escudier, a space oceanography at CLS
(Collecte 
Localisation Satellites), Toulouse, France. 

"You can save up to 5% on fuel consumption by making
best use of the 
currents." 

Formation flying 

Jason-2 will spend its first few months flying a
"tandem mission" 
with Jason-1. 

The two spacecraft will be positioned so that they
sweep around the 
Earth, one following the other, with a separation of
just 60 
seconds. 

This will enable, essentially, the two satellites to
measure the 
same patch of ocean surface at very nearly the same
time. 
Changes in ocean height can be a key indicator of
climate cycles

More details


Scientists will use this opportunity to
cross-calibrate the 
instruments so that when Jason-1 is retired (or
fails), the future 
data collected by its successor will be directly
comparable with 
past records. 

This continuity of information will be critical in
recognising long-
term trends in ocean behaviour. It is the data which
underpins the 
observation that global sea level is rising by about
three 
millimetres per year. 

Once the tandem phase is completed, Jason-1 will be
moved to the 
side, doubling the return of data. The importance of
the Jason 
programme means both spacecraft will almost certainly
be run for as 
long as they are serviceable. 

Discussions are already in progress on a Jason-3
satellite. Given 
Europe's role in the project, there is a compelling
case for the 
next mission to be included in the GMES (Global
Monitoring for 
Environment and Security) programme. This would
attract significant EU money.