Silver Anniversary for Ocean Altimetry Space Mission

Artist rendering of Jason-3 satellite over the Amazon.
Image Courtesy NASA/JPL-Caltech.

August 10th 1992 marked the launch of the TOPEX/Poseidon satellite, the first major oceanographic focussed mission. Twenty five years, and three successor satellites, later the dataset begun by TOPEX/Poseidon is going strong providing sea surface height measurements.

TOPEX/Poseidon was a joint mission between NASA and France’s CNES space agency, with the aim of mapping ocean surface topography to improve our understanding of ocean currents and global climate forecasting. It measured ninety five percent of the world’s ice free oceans within each ten day revisit cycle. The satellite carried two instruments: a single-frequency Ku-band solid-state altimeter and a dual-frequency C- and Ku-band altimeter sending out pulses at 13.6 GHz and 5.3 GHz respectively. The two bands were selected due to atmospheric sensitivity, as the difference between them provides estimates of the ionospheric delay caused by the charged particles in the upper atmosphere that can delay the returned signal. The altimeter sends radio pulses towards the earth and measures the characteristics of the returned echo.

When TOPEX/Poseidon altimetry data is combined with other information from the satellite, it was able to calculate sea surface heights to an accuracy of 4.2 cm. In addition, the strength and shape of the return signal also allow the determination of wave height and wind speed. Despite TOPEX/Poseidon being planned as a three year mission, it was actually active for thirteen years, until January 2006.

The value in the sea level height measurements resulted in a succeeding mission, Jason-1, launched on December 7th 2001. It was put into a co-ordinated orbit with TOPEX/Poseidon and they both took measurements for three years, which allowed both increased data frequency and the opportunity for cross calibration of the instruments. Jason-1 carried a CNES Poseidon-2 Altimeter using the same C- and Ku-bands, and following the same methodology it had the ability to measure sea-surface height to an improved accuracy of 3.3 cm. It made observations for 12 years, and was also overlapped by its successor Jason-2.

Jason-2 was launched on the 20 June 2008. This satellite carried a CNES Poseidon-3 Altimeter with C- and Ku-bands with the intention of measuring sea height to within 2.5cm. With Jason-2, National Oceanic and Atmospheric Administration (NOAA) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) took over the management of the data. The satellite is still active, however due to suspected radiation damage its orbit was lowered by 27 km, enabling it to produce an improved, high-resolution estimate of Earth’s average sea surface height, which in turn will help improve the quality of maps of the ocean floor.

Following the established pattern, Jason-3 was launched on the 17th January 2016. It’s carrying a Poseidon-3B radar altimeter, again using the same C and Ku bands and on a ten day revisit cycle.

Together these missions have provided a 25 year dataset on sea surface height, which has been used for applications such as:

  • El Niño and La Niña forecasting
  • Extreme weather forecasting for hurricanes, floods and droughts
  • Ocean circulation modelling for seasons and how this affects climate through by moving heat around the globe
  • Tidal forecasting and showing how this energy plays an important role in mixing water within the oceans
  • Measurement of inland water levels – at Pixalytics we have a product that we have used to measure river levels in the Congo and is part of the work we are doing on our International Partnership Programme work in Uganda.

In the future, the dataset will be taken forward by the Jason Continuity of Service (Jason-CS) on the Sentinel-6 ocean mission which is expected to be launched in 2020.

Overall, altimetry data from this series of missions is a fantastic resource for operational oceanography and inland water applications, and we look forward to its next twenty five years!

World Oceans Day

Phytoplankton Bloom off South West England. Acquired by MODIS on 12th June 2003. Data courtesy of NASA.

June 8th is World Oceans Day. This is an annual global celebration of the oceans, their importance and how they can be protected for the future.

The idea of a World Ocean Day was originally proposed by the Canadian Government at the Earth Summit in Rio in 1992. In December 2008 a resolution was passed by United Nations General Assembly which officially declared that June 8th would be World Oceans Day. The annual celebration is co-ordinated by the Ocean Project organisation, and is growing from strength to strength with over 100 countries having participated last year.

There is a different theme each year and for 2017 it’s “Our Oceans, Our Future”, with a focus on preventing plastic pollution of the ocean and cleaning marine litter.

Why The Oceans Are Important?

  • The oceans cover over 71% of the planet and account for 96% of the water on Earth.
  • Half of all the oxygen in the atmosphere is released by phytoplankton through photosynthesis. Phytoplankton blooms are of huge interest to us at Pixalytics as despite their miniscule size, in large enough quantities, phytoplankton can be seen from space.
  • They help regulate climate by absorbing around 25% of the CO2 human activities release into the atmosphere.
  • Between 50% and 80% of all life on the planet is found in the oceans.
  • Less than 10% of the oceans have been explored by humans. More people have stood on the moon than the deepest point of the oceans – the Mariana Trench in the Pacific Ocean at around 11 km deep.
  • Fish accounted for about 17% of the global population’s intake of animal protein in 2013.

Why This Year’s Theme Is Important?

The pollution of the oceans by plastic is something which affects us all. From bags and containers washed up on beaches to the plastic filled garbage gyres that circulate within the Atlantic, Pacific and Indian Oceans, human activity is polluting the oceans with plastic and waste. The United Nations believe that as many as 51 trillion particles of microplastic are in the oceans, which is a huge environmental problem.

Everyone will have seen images of dolphins, turtles or birds either eating or being trapped by plastic waste. However, recently Dr Richard Kirby – a friend of Pixalytics – was able to film plastic microfibre being eaten by plankton. As plankton are, in turn, eaten by many marine creatures, this is one example of how waste plastic is entering the food chain. The video can seen here on a BBC report.

Dr Kirby also runs the Secchi Disk project which is a citizen science project to study phytoplankton across the globe and receives data from every ocean.

Get Involved With World Oceans Day

The world oceans are critical to the health of the planet and us! They help regulate climate, generate most of the oxygen we breathe and provide a variety of food and sources of medicines. So everyone should want to help protect and conserve these natural environments. They are a number of ways you can get involved:

  • Participate: There are events planned all across the world. You can have a look here and see if any are close to you.
  • Look: The Ocean Project website has a fantastic set of resources available.
  • Think: Can you reduce your use, or reliance on plastic?
  • Promote: Talk about World Oceans Day, Oceans and their importance.

Monitoring ocean acidification from space

Enhanced pseudo-true colour composite of the United Kingdom showing coccolithophore blooms in light blue. Image acquired by MODIS-Aqua on 24th May 2016. Data courtesy of NASA.

Enhanced pseudo-true colour composite of the United Kingdom showing coccolithophore blooms in light blue. Image acquired by MODIS-Aqua on 24th May 2016. Data courtesy of NASA.

What is ocean acidification?
Since the industrial revolution the oceans have absorbed approximately 50% of the CO2 produced by human activities (The Royal Society, 2005). Scientists previously saw this oceanic absorption as advantageous, however ocean observations in recent decades have shown it has caused a profound change in the ocean chemistry – resulting in ocean acidification (OA); as CO2 dissolves into the oceans it forms carbonic acid, lowering the pH and moving the oceans into a more acidic state. According to the National Oceanic Atmospheric Administration (NOAA) ocean pH has already decreased by about 30% and some studies suggest that if no changes are made, by 2100, ocean pH will decrease by 150%.

Impacts of OA
It’s anticipated OA will impact many marine species. For example, it’s expected it will have a harmful effect on some calcifying species such as corals, oysters, crustaceans, and calcareous plankton e.g. coccolithophores.

OA can significantly reduce the ability of reef-building corals to produce their skeletons and can cause the dissolution of oyster’s and crustacean’s protective shells, making them more susceptible to predation and death. This in turn would affect the entire food web, the wider environment and would have many socio-economic impacts.

Calcifying phytoplankton, such as coccolithophores, are thought to be especially vulnerable to OA. They are the most abundant type of calcifying phytoplankton in the ocean, and are important for the global biogeochemical cycling of carbon and are the base of many marine food webs. It’s projected that OA may disrupt the formation and/or dissolution of coccolithophores, calcium carbonate (CaCO3) shells, impacting future populations. Thus, changes in their abundance due to OA could have far-reaching effects.

Unlike other phytoplankton, coccolithophores are highly effective light scatterers relative to their surroundings due to their production of highly reflective calcium carbonate plates. This allows them to be easily seen on satellite imagery. The figure at the top of this page shows multiple coccolithophore blooms, in light blue, off the coast of the United Kingdom on 24th March 2016.

Current OA monitoring methods
Presently, the monitoring of OA and its effects are predominantly carried out by in situ observations from ships and moorings using buoys and wave gliders for example. Although vital, in situ data is notoriously spatially sparse as it is difficult to take measurements in certain areas of the world, especially in hostile regions (e.g. Polar Oceans). On their own they do not provide a comprehensive and cost-effective way to monitor OA globally. Consequently, this has driven the development of satellite-based sensors.

How can OA be monitored from space?
Although it is difficult to directly monitor changes in ocean pH using remote sensing, satellites can measure sea surface temperature and salinity (SST & SSS) and surface chlorophyll-a, from which ocean pH can be estimated using empirical relationships derived from in situ data. Although surface measurements may not be representative of deeper biological processes, surface observations are important for OA because the change in pH occurs at the surface first.

In 2015 researchers at the University of Exeter, UK became the first scientists to use remote sensing to develop a worldwide map of the ocean’s acidity using satellite imagery from the European Space Agency’s Soil Moisture and Ocean Salinity (SMOS) satellite that was launched in 2009 and NASA’s Aquarius satellite that was launched in 2011; both are still currently in operation. Thermal mounted sensors on the satellites measure the SST while the microwave sensors measure SSS; there are also microwave SST sensors, but they have a coarse spatial resolution.

Future Opportunities – The Copernicus Program
The European Union’s Copernicus Programme is in the process of launching a series of satellites, known as Sentinel satellites, which will improve understanding of large scale global dynamics and climate change. Of all the Sentinel satellite types, Sentinels 2 and 3 are most appropriate for assessment of the marine carbonate system. The Sentinel-3 satellite was launched in February this year andwill be mainly focussing on ocean measurements, including SST, ocean colour and chlorophyll-a.

Overall, OA is a relatively new field of research, with most of the studies being conducted over the last decade. It’s certain that remote sensing will have an exciting and important role to play in the future monitoring of this issue and its effects on the marine environment.

Blog written by Charlie Leaman, BSc, University of Bath during work placement at Pixalytics.

Jason-3 Sets Sail for the Oceanographic Golden Fleece

Artist rendering of Jason-3 satellite over the Amazon. Image Courtesy NASA/JPL-Caltech.

Artist rendering of Jason-3 satellite over the Amazon.
Image Courtesy NASA/JPL-Caltech.

The Jason-3 oceanographic satellite is planned to launch on Sunday 17th January from Vandenberg Air Force Base in California, aboard the Space-X Falcon 9 rocket. Named after the Greek hero Jason, of the Argonauts fame, Jason-3 is actually the fourth in a series of joint US-European missions to measure ocean surface height. The series began with the TOPEX/Poseidon satellite launched in 1992, followed by Jason-1 and Jason-2 which were launched in 2001 and 2008 respectively.

Jason-3 should provide a global map of sea surface height every ten days, which will be invaluable to scientists investigating circulation patterns and climate change.

The primary instrument is the Poseidon-3B radar altimeter, which will measure the time it takes an emitted radar pulse to bounce off the ocean’s surface and return to the satellite’s sensor. Pulses will be emitted at two frequencies: 13.6 GHz in the Ku band and 5.3 GHz in the C band. These bands are used in combination due to atmospheric sensitivity, as the difference between the two frequencies helps to provide estimates of the ionospheric delay caused by the charged particles in the upper atmosphere that can time delay the return.

Once the satellite has received the signal reflected back, it will be able to use its other internal location focussed instruments to provide a highly accurate measurement of sea surface height. Initially the satellite will be able to determine heights to within 3.3cm, although the long-term goal is to reduce this accuracy down to 2.5cm. In addition, the strength and shape of the return signal also allows the determination of wave height and wind speed which are used in ocean models to calculate the speed and direction of ocean currents together the amount and location of heat stored in the ocean.

In addition, Jason-3 carries an Advanced Microwave radiometer (AMR) which measures altimeter signal path delay due to tropospheric water vapour.

The three location focused instruments aboard Jason-3 are:

  • DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) – Uses a ground network of 60 orbitography beacons around the globe to derive the satellite’s speed and therefore allowing it’s precise position in orbit to be determined to within three centimetres.
  • Laser Retroreflector Array (LRA) – An array of mirrors that provide a target for laser tracking measurements from the ground. By analysing the round-trip time of the laser beam, the satellite’s location can be determined.
  • Global Positioning System – Using triangulation from three GPS satellites the satellites exact position can be determined.

The importance of extending the twenty-year time series of sea surface measurements cannot be underestimated, given the huge influence the ocean has on our atmosphere, weather and climate change. For example, increasing our knowledge of the variations in ocean temperature in the Pacific Ocean that result in the El Niño effect – which have caused coral bleaching, droughts, wet weather and movements in the jet stream in 2015, and are expected to continue into this year – will be hugely beneficial.

This type of understanding is what Jason-3 is setting sail to discover.