The Satellites

In 1992, NASA of the U.S.A. and CNES of France embarked upon The Ocean Surface Topography Mission, with an aim of studying the troughs and hills of ocean surfaces [1]. The Mission is hailed as one of the most successful uses of satellite observation, the data from which heavily effected the aforementioned 2009 predictions of sea level rise.

The three Satellites that have been used are:

TOPEX/ Poseidon (figure 1): Launched in 1992, with a predicted life span of five years, TOPEX/ Poseidon was actually decommissioned in 2005 [2]. The Satellite was positioned 830 miles above earth, at an angle of 66 degrees to the equator, allowing the observation of almost all of the Earths oceans [3]. With an orbit of 112 minutes, a complete image of the earth was produced every 10 days, allowing regular recordings of the Earths ocean heights.

Figure 1: Artist impression of TOPEX/ Poseidon Satellite orbiting the earth [4]

Jason 1 (figure 2): Made to eventually replace the TOPEX/ Poseidon satellite, Jason 1 was launched in 2001 and operated alongside the original satellite, until becoming solo in 2005. Jason 1 contained the same radar measuring system as TOPEX/ Poseidon, but was designed to also directly measure climate-change through the observation of year to year changes in global sea level [2]. 

Figure 2: Artists impression of Jason 1, orbiting the Earth [2]

Jason 2 (figure 3): Launched in 2008 to allow the continuation of the sea surface topography observation and data set creation, the readings of Jason 2 are now used in weather forecasts, as well as ocean behaviour data collection [5].

Figure 3: Artists impression of Jason 2 orbiting Earth [5]

Further information on the launch and operation of Jason 2 can be found here: Measuring sea level rise from space By Mark Kinver [6]

Satellite Observation and Sea Level

The Ocean Surface Topography Mission built on ideas of satellite observation of sea level, first attempted with the GEOS-3, Seasat and Geosal satellites in 1975, 1978 and 1985 respectively [3].

Radar altimetry is a system of measurement that fires pulses of radar from the satellite, to the ocean surface and measures the time taken for said pulse to return to the antennae. The raw time set is then adjusted, according to the height of the satellite, which is calculated using GPS to give the satellites distance from the centre of the Earth and any resistance the radar signal may have encountered when travelling, such as water vapour in the atmosphere [3], (figure 4).

1. Jason-2 satellite: From its 1,338km-high circular orbit, the craft maps 95% of the world's ice-free oceans' topography every 10 days

2, GPS satellites: The system is used to track Jason-2's position, ensuring very precise sea level height measurements

3. Sea height measurement via Poseidon-3 altimeter: The dual frequency radar signals are able to measure sea level height, wave height and surface wind speed

4. Sea surface topography: Variations in the height of the sea surface, when combined with measurements from other satellites and in-situ instruments, will allow scientists to improve weather and climate system models

5. Doppler Orbitography and Radiopositioning Integrated by Satellite (Doris) and laser ranging beacon: Ground stations ensure the precise positioning of Jason-2, which enables researchers to gather meaningful data from the satellite

Figure 4: Ocean surface measurement system of Jason 2 [6]

After adjustment, the data sets can be used to study the effect of increased heat in the atmosphere, comparison to past changes and projected to give estimates of future climate change and sea level rise. It is this information that forced the 2009 predictions of an accelerated sea level rise, the specifics of which will be covered in the next post.

For more information of the operation of Jason 2, please see the following video.

References

[1] Sullivan, R., (2006) TOPEX/ Poseidon Sails Off in to the Sunset, [online]. Available at: http://www.jpl.nasa.gov/news/features.cfm?feature=973[28.2.2010]

[2] NASA, n/d Jet Propulsion Labaratory, Jason 1, [online]. Available at: http://www.nasa.gov/centers/jpl/missions/jason.html [28.2.2010]

[3] NASA/ CNES, (1992) Mission to Plannet Earth, TOPEX/ Poseidon [online]. Available at: http://www.jpl.nasa.gov/news/press_kits/topex_poseidon.pdf [28.2.2010]

[4] NASA, (2001) Ocean Mission Celebrates Ninth Year [online]. Available at: http://www.jpl.nasa.gov/news/features.cfm?feature=421 [28.2.2010]

[5] NASA, n/d Ocean Surface Topography Mission/ Jason-2 [online]. Available at: http://www.nasa.gov/mission_pages/ostm/overview/index.html [28.2.2010]

[6]  Kniver, M. (2007) Measuring Sea Level Rise From Space [online]. Available at: http://news.bbc.co.uk/1/hi/sci/tech/6922312.stm [28.2.2010]