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.

El Niño causing Coral Bleaching

Variations in Pacific Ocean sea surface height compared to a long term average for the 2015 and 1997/98 El Niño events. Courtesy NASA/JPL-Caltech

Variations in Pacific Ocean sea surface height compared to a long term average for the 2015 and 1997/98 El Niño events. Courtesy NASA/JPL-Caltech

Coral reefs are currently undergoing their third worldwide bleaching event linked to the El Niño effect in the last 20 years, and scientists fear this one could be the worst.

El Niño is the warm phase of the El Niño Southern Oscillation and is associated with a band of warm water that develops in the central and east-central equatorial Pacific. This means waters in the Pacific Ocean are nutrient-poor, and are accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. This causes disruption to weather patterns worldwide including: droughts in Indonesia and Australia, and altering the path of the atmospheric jet stream over America.

Coral reefs cover less than 1% of the earth’s surface, but are some of the most valuable, diverse and vulnerable ecosystems on the planet. Reef building corals thrive in water temperatures between 73° and 84° Fahrenheit, but struggle outside of this range. Climate change is providing a challenge, and when you add on a warming effect like El Niño, the danger for coral reef ecosystems is clear. Worldwide coral bleaching events have occurred in 1997/98 and 2009/10, both of which were El Nino years; and the fear is that this year’s event, which will extend into 2016, could be significant.

Warmer water stresses the coral causing them to expel the photosynthetic algae, called zooxanthellae, living in their tissues. This turns them completely white, hence the term bleaching. Although this does not kill the coral immediately, it does put them at greater risk of dying. For example, half the coral reefs in the Caribbean were lost following a local bleaching event in 2005 – a weak El Niño year.

Satellite data has provided a valuable source of data to monitor the changes in coral reefs. For example, the French Centre National d’etudes Spatiales (CNES) and the USA’s National Aeronautics and Space Administration (NASA), began a mission in 1987 to monitor global ocean changes including measuring sea height by radar altimetry. It began with the TOPEX/Poseidon mission launched in 1992, which provided major data on the way the El Niño effect operated. This was followed Jason-1, launched in 2001, and Jason-2 in 2008. The value of this type of data to monitoring effects like El Niño can be seen at the top of the blog that shows side by side the variations in the Pacific Ocean sea surface height compared to a long term average for the 2015 El Niño and the strong event of 1997/98, with data collected by TOPEX/Poseidon for 1997 and the OSTM/Jason-2 for 2015. Further images and animations from NASA/JPL-Caltech can be found here.

Coral reef ecosystems are a source of food, protection against coastal erosion and provide spawning and nursing grounds for fish. They also provide jobs through fishing and tourism, and are estimated to contribute $29.8 billion to the global economy every year. However, scientists estimate that between 40 and 50% of corals worldwide have been destroyed or lost in the last 50 years. They expect this decline to continue, which could have significant consequences to the human and marine populations that are dependent on them.

How to Measure Heights From Space?

Combining two Sentinel-1A radar scans from 17 and 29 April 2015, this interferogram shows changes on the ground that occurred during the 25 April earthquake that struck Nepal. Contains Copernicus data (2015)/ESA/Norut/PPO.labs/COMET–ESA SEOM INSARAP study

Combining two Sentinel-1A radar scans from 17 and 29 April 2015, this interferogram shows changes on the ground that occurred during the 25 April earthquake that struck Nepal. Contains Copernicus data (2015)/ESA/Norut/PPO.labs/COMET–ESA SEOM INSARAP study

Accurately measuring the height of buildings, mountains or water bodies is possible from space. Active satellite sensors send out pulses of energy towards the Earth, and measure the strength and origin of the energy received back enabling them to determine of the heights of objects struck by the pulse energy on Earth.

This measurement of the time it takes an energy pulse to return to the sensor, can be used for both optical and microwave data. Optical techniques such as Lidar send out a laser pulse; however within this blog we’re going to focus on techniques using microwave energy, which operate within the Ku, C, S and Ka frequency bands.

Altimetry is a traditional technique for measuring heights. This type of technique is termed Low Resolution Mode, as it sends out a pulse of energy that return as a wide footprint on the Earth’s surface. Therefore, care needs to be taken with variable surfaces as the energy reflected back to the sensor gives measurements from different surfaces. The signal also needs to be corrected for speed of travel through the atmosphere and small changes in the orbit of the satellite, before it can be used to calculate a height to centimetre accuracy. Satellites that use this type of methodology include Jason-2, which operates at the Ku frequency, and Saral/AltiKa operating in the Ka band. Pixalytics has been working on a technique to measure river and flood water heights using this type of satellite data. This would have a wide range of applications in both remote area monitoring, early warning systems, disaster relief, and as shown in the paper ‘Challenges for GIS remain around the uncertainty and availability of data’ by Tina Thomson, offers potential for the insurance and risk industries.

A second methodology for measuring heights using microwave data is Interferometric Synthetic Aperture Radar (InSAR), which uses phase measurements from two or more successive satellite SAR images to determine the Earth’s shape and topography. It can calculate millimetre scale changes in heights and can be used to monitor natural hazards and subsidence. InSAR is useful with relatively static surfaces, such as buildings, as the successive satellite images can be accurately compared. However, where you have dynamic surfaces, such as water, the technique is much more difficult to use as the surface will have naturally changed between images. Both ESA’s Sentinel-1 and the CryoSat-2 carry instruments where this technique can be applied.

The image at the top of the blog is an interferogram using data collected by Sentinel-1 in the aftermath of the recent earthquake in Nepal. The colours on the image reflect the movement of ground between the before, and after, image; and initial investigations from scientists indicates that Mount Everest has shrunk by 2.8 cm (1 inch) following the quake; although this needs further research to confirm the height change.

From the largest mountain to the smallest changes, satellite data can help measure heights across the world.

Measuring Water Heights, upcoming presentation at GEO-Business

Freshwater is integral to our survival on earth; whether it’s for drinking, growing food, sanitation or energy production. However, water is also a finite natural resource controlled by the complex and evolving water cycle. Many people know that 97% of the world’s water is salt water, but of the remaining freshwater 70% is locked in ice caps and of what remains only 1% is readily accessible.

The bodies of UN Water and estimate that 85% of the world’s population live in the driest half of the planet; taking a five-minute shower uses more water than the average person in a developing country uses for an entire day and more people in the world have access to a mobile phone than a toilet. Global demand for water is forecast to increase by 55% in the next 40 years, added to which climate evolution is going to change the distribution and availability of freshwater across the world. Last winter’s weather in the UK demonstrated how important it’s going to be to for us to adapt to new water patterns.

Satellite remote sensing has an important role to play in helping the world monitor and manage this natural resource. From the identification and mapping of water bodies by optical remote sensing, through the monitoring of hydrologic variables (like rainfall, soil moisture and water quality) to real time flood monitoring and disaster relief. Remote sensing applications are offering real value to the world and with launch of Sentinel-1 the European Copernicus data stream has started to come online; this week I’m at the Sentinel-2 for Science Workshop. Sentinel-2 is a high resolution optical mission due to launch in early 2015.

Water height calculation in the Congo using Jason 2

Water height calculation in the Congo using Jason 2

Over the last year I’ve developed a system to determine water heights in estuaries, rivers and lakes using satellite optical and altimetry data. Radar altimeters emit short bursts of microwave energy towards the earth’s surface, and the time delay of the return of those pulses gives a height. It becomes complicated over inland water bodies, especially those that are relatively small (not large inland seas) and varying river banks and general land topography; however there are improved approaches and new data coming on-stream.

Testing my altimetry based height determination has given positive results, when compared to in situ data taken for the Congo; the first study site. By using this approach I was able to provide the customer with water heights without them needing to get data from a water gauge. The other major advantage was the generation of a historical time series for several sites of interest where water gauges had never been installed.

Wednesday next week, 28th May, I will be giving a presentation on my work at the 2014 Geo-Business Conference in London and I’ll give you more details in a future blog. If you’re at Geo-Business, come up and say hello, otherwise come back to the blog for more details.