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!

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.