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!

Earth Observation goes Back to the Future

Typhoon Atsani over the Pacific Ocean on 25 August 2015. Image from Himawari-8. Copyright 2015 EUMETSAT.

Typhoon Atsani over the Pacific Ocean on 25 August 2015. Image from Himawari-8. Copyright 2015 EUMETSAT.

Today is Back to the Future Day! Or more precisely, October 21st 2015 is the date that Marty McFly and Doc Brown travel back to in the second of the Back to the Future (BTTF) films. We’ve seen a few recent articles comparing the imagined 2015 with the actual year, and we decided this week’s blog will examine how Earth observation technology compares to the film’s predictions.

You might be reading this thinking you don’t remember any Earth observation data in the BTTF film? Well that is not strictly true! Whilst there might not have been any reference to pure satellite remote sensing such as Landsat, precision weather forecasting was present.

After arriving in 2015 in the film, Marty doesn’t want to get out of the DeLorean as it is pouring with rain. Doc looks at his watch and tells Marty to wait for five seconds, at which point the rain stops and the sun comes out. Now admittedly, getting such precision timing from a watch is stretching reality a bit, but we’re not that far away. In terms of the device, an Apple Watch with a weather forecasting app is the most obvious equivalent. Although, all smartphones have weather apps and are not that dissimilar; interestingly, the development of mobile technology was something completely missed by BTTF.

On the accuracy of the predictions, regular readers of this blog will know we are Formula One fans and we even sponsored a car last year. On the commentary of those races you will hear the teams using their rain radar maps to give their drivers weather updates such as ‘rain is predicted in ten minutes, it will last five minutes and expected to be heavy’. Accurate predictions are getting closer, although it may be some time until we know the second the rain will stop.

The other major link to Earth observation within the film is the examples of drone technology. The first example is the use of ‘hovercams’ to provide video of breaking news events; whilst again this is something not widely used by news agencies, the concept of using drones to take videos or collect data is something that is very much used within the remote sensing community. There is a second example with the shot of a drone walking a dog, and it looks very similar to drones currently being used. Not quite sure that a drone could walk a dog yet though, despite the videos on the internet!

However, the potential for drones to become more commonplace was recognised this week by the US Transport Secretary who called for a national register and drones and owners. The number of drones flown by the general population is expected to grow rapidly. It’s likely that some form of development of the legal or regulatory framework will occur to ensure these are operated in a manner that does not undermine safety and privacy.

Earth observation and remote sensing technology was part of the 1989 BTTF film. If we look forward 26 years from today to 2041, anyone want to predict what will be the rising technology in remote sensing? Tweet us your ideas!