Living Planet Is Really Buzzing!

Living planet rotating global in the exhibition area, photo: S Lavender

Living planet rotating global in the exhibition area, photo: S Lavender

This week I’m at the 2016 European Space Agency’s Living Planet Symposium taking place in sunny Prague. I didn’t arrive until lunchtime on Monday and with the event already underway I hurried to the venue. First port of call was the European Association of Remote Sensing Companies (EARSC) stand as we’ve got copies of flyers and leaflets on their stand. Why not pop along and have look!

The current excitement and interest in Earth observation (EO) was obvious when I made my way towards the final sessions of the day. The Sentinel-2 and Landsat-8 synergy presentations were packed out, all seats taken and people were crowding the door to watch!

I started with the Thematic Exploitation Platforms session. For a long time the remote sensing community has wanted more data, and now we’re receiving it in ever larger quantities e.g., the current Copernicus missions are generating terabytes of data daily. With the storage requirements this generates there is a lot of interest in the use of online platforms to hold data, and then you upload your code to it, or use tools provided by the platform, rather than everyone trying to download their own individual copies. It was interesting to compare and contrast the approaches taken with hydrology, polar, coastal, forestry and urban EO data.

Tuesday was always going to be my busiest day of the Symposium as I was chairing two sessions and giving a presentation. I had an early start as the 0800 session on Coastal Zones I was co-chairing alongside Bob Brewin –a former PhD student of mine! It was great to see people presenting their results using Sentinel-2. The spatial resolution, 10m for the highest resolution wavebands, allows us to see the detail of suspended sediment resuspension events and the 705 nm waveband can be used for phytoplankton; but we’d still like an ocean colour sensor at this spatial resolution!

In the afternoon I headed into European Climate Data Records, where there was an interesting presentation on a long time-series AVHRR above-land aerosol dataset where the AVHRR data is being vicariously calibrated using the SeaWiFS ocean colour sensor. Great to see innovation within the industry where sensors launched one set of applications can be reused in others. One thing that was emphasised by presenters in both this session, and the Coastal Zone one earlier, was the need to reprocess datasets to create improved data records.

My last session of the day was on Virtual Research, where I was both co-chairing and presenting. It returned to the theme of handling large datasets, and the presentations focused on building resources that make using EO data easier. This ranged from bringing in-situ and EO data together by standardising the formatting and metadata of the in-situ data, through community datasets for algorithm performance evaluation, to data cubes that bring all the data needed to answer specific questions together into a three- (or higher) dimensional array that means you don’t spend all your time trying to read different datasets versus ask questions of them. My own presentation focused on our involvement with the ESA funded E-Collaboration for Earth Observation (E-CEO) project, which developed a collaborative platform  where challenges can be initiated and evaluated; allowing participants to upload their code and have it evaluated against a range of metrics. We’d run an example challenge focused on the comparison of atmospheric correction processors for ocean colour data that, once setup, could easily be rerun.

I’ve already realised that there too many interesting parallel sessions here, as I missed the ocean colour presentations which I’ve heard were great. The good news for me is that these sessions were recorded. So if you haven’t be able to make to Prague in person, or like me you are here but haven’t seen everything you wanted there are going to be selection of sessions to view on ESA’s site, for example, you can see the opening session here.

Not only do events like this gives you to a fantastic chance learn about what’s happening across the EO community, but they also give you the opportunity to catch up with old friends. I am looking forward to the rest of the week!

Ocean Colour Cubes

August 2009 Monthly Chlorophyll-a Composite; data courtesy of the ESA Ocean Colour Climate Change Initiative project

August 2009 Monthly Chlorophyll-a Composite; data courtesy of the ESA Ocean Colour Climate Change Initiative project

It’s an exciting time to be in ocean colour! A couple of weeks ago we highlighted the new US partnership using ocean colour as an early warning system for harmful freshwater algae blooms, and last week a new ocean colour CubeSat development was announced.

Ocean colour is something very close to our heart; it was the basis of Sam’s PhD and a field of research she is highly active in today. When Sam began studying her PhD, Coastal Zone Color Scanner (CZCS) was the main source of satellite ocean colour data, until it was superseded by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) that became the focus of her role at Plymouth Marine Laboratory.

Currently, there are a number ocean colour instruments in orbit:

  • NASA’s twin MODIS instruments on the Terra and Aqua satellites
  • NOAA’s Visible Infrared Imager Radiometer Suite (VIIRS)
  • China’s Medium Resolution Spectral Imager (MERSI), Chinese Ocean Colour and Temperature Scanner (COCTS) and Coastal Zone Imager (CZI) onboard several satellites
  • South Korea’s Geostationary Ocean Color Imager (GOCI)
  • India’s Ocean Colour Monitor on-board Oceansat-2

Despite having these instruments in orbit, there is very limited global ocean colour data available for research applications. This is because the Chinese data is not easily accessible outside China, Oceansat-2 data isn’t of sufficient quality for climate research and GOCI is a geostationary satellite so the data is only for a limited geographical area focussed on South Korea. With MODIS, the Terra satellite has limited ocean colour applications due to issues with its mirror and hence calibration; and recently the calibration on Aqua has also become unstable due to its age. Therefore, the ocean colour community is just left with VIIRS; and the data from this instrument has only been recently proved.

With limited good quality ocean colour data, there is significant concern over the potential loss of continuity in this valuable dataset. The next planned instrument to provide a global dataset will be OLCI onboard ESA’s Sentinel 3A, due to be launched in November 2015; with everyone having their fingers crossed that MODIS will hang on until then.

Launching a satellite takes time and money, and satellites carrying ocean colour sensors have generally been big, for example, Sentinel 3A weighs 1250 kg and MODIS 228.7 kg. This is why the project was announced last week to build two Ocean Colour CubeSats is so exciting; they are planned to weigh only 4 kg which reduces both the expense and the launch lead time.

The project, called SOCON (Sustained Ocean Observation from Nanosatellites), will see Clyde Space, from Glasgow in the UK, will build an initial two prototype SeaHawk CubeSats with HawkEye Ocean Colour Sensors, with a ground resolution of between 75 m and 150 m per pixel to be launched in early 2017. The project consortium includes the University of North Carolina, NASA’s Goddard Space Flight Centre, Hawk Institute for Space Sciences and Cloudland Instruments. The eventual aim is to have constellations of CubeSats providing a global view of both ocean and inland waters.

There are a number of other planned ocean colour satellite launches in the next ten years including following on missions such as Oceansat-3, two missions from China, GOCI 2, and a second VIIRS mission.

With new missions, new data applications and miniaturised technology, we could be entering a purple patch for ocean colour data – although purple in ocean colour usually represents a Chlorophyll-a concentration of around 0.01 mg/m3 on the standard SeaWiFS colour palette as shown on the image at the top of the page.

We’re truly excited and looking forward to research, products and services this golden age may offer.

Goodbye HICO, Hello PACE – Ocean Colour’s Satellite Symmetry

HICO™ Data, image of Hong Kong from the Oregon State University HICO Sample Image Gallery, provided by the Naval Research Laboratory

HICO™ Data, image of Hong Kong from the Oregon State University HICO Sample Image Gallery, provided by the Naval Research Laboratory

Ocean colour is the acorn from which Pixalytics eventually grew, and so we were delighted to see last week’s NASA announcement that one of their next generation ocean colour satellites is now more secure with a scheduled launched for 2022.

Unsurprisingly the term ocean colour refers to the study of the colour of the ocean, although in reality it’s a name that includes a suite of different products, with the central one for the open oceans being the concentration of phytoplankton. Ocean colour is determined by the how much of the sun’s energy the ocean scatters and absorbs, which in turn is dependent on the water itself alongside substances within the water that include phytoplankton and suspended sediments together with dissolves substances and chemicals. Phytoplankton can be used a barometer of the health of the oceans; in that phytoplankton are found where nutrient levels are high and oceans with low nutrients have little phytoplankton. Sam’s PhD involved the measurement of suspended sediment coming out of the Humber estuary back in 1995, and it’s remained an active field of her research for the last 20 years.

Satellite ocean colour remote sensing began with the launch of NASA’s Coastal Zone Colour Scanner (CZCS) on the 24th October 1978. It had six spectral bands, four of which were devoted to ocean colour, and a spatial resolution of around 800m. Despite only having an anticipated lifespan of one year, it operated until the 22nd June 1986 and has been used as a key dataset ever since. Sadly, CZCS’s demise marked the start of a decade gap in NASA’s ocean colour data archive.

Although there were some intermediate ocean colour missions, it was the launch of the Sea-viewing Wide Field-of-view (SeaWiFS) satellite that brought the next significant archive of ocean colour data. SeaWiFS had 8 spectral bands optimized for ocean colour and operated at a 1 km spatial resolution. One of Sam’s first jobs was developing a SeaWiFS data processor, and the satellite collected data until the end of its mission in December 2010.

Currently, global ocean colour data primarily comes from either NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) on-board the twin Aqua and Terra satellites, or the Visible Infrared Imaging Radiometer Suite (VIIRS) which is on a joint NOAA / NASA satellite called Suomi NPP. MODIS has 36 spectral bands and spatial resolution ranging from 250 to 1000 m; whilst VIIRS has twenty two spectral bands and a resolution of 375 to 750 m.

Until recently, there was also the ONR / NRL / NASA Hyperspectral Imager for the Coastal Ocean (HICO) mission on-board the International Space Station. It collected selected coastal region data with a spectral resolution range of 380 to 960nm and 90m spatial resolution. It was designed to collect only one scene per orbit and has acquired over 10,000 such scenes since its launch. However, unfortunately it suffered during a solar storm in September 2014. Its retirement was officially announced a few days ago with the confirmation that it wasn’t possible to repair the damage.

In the same week we wave goodbye to HICO, NASA announced the 2022 launch of the Pre-Aerosol and ocean Ecosystem (PACE) mission in a form of ocean colour symmetry. PACE is part of the next generation of ocean colour satellites, and it’s intended to have an ocean ecosystem spectrometer/radiometer called built by NASA’s Goddard Space Flight Centre and will measure spectral wavebands from ultraviolet to near infrared. It will also have an aerosol/cloud polarimeter to help improve our understanding of the flow, and role, of aerosols in the environment.

PACE will be preceded by several other missions with an ocean colour focus including the European Sentinel-3 mission within the next year; it will have an Ocean and Land Colour Instrument with 21 spectral bands and 300 m spatial resolution, and will be building on Envisat’s Medium Resolution Imaging Spectrometer (MERIS) instrument. Sentinel-3 will also carry a Sea and Land Surface Temperature Radiometer and a polarimeter for mapping aerosols and clouds. It should help to significantly improve the quality of the ocean colour data by supporting the improvement of atmospheric correction.

Knowledge the global phytoplankton biomass is critical to understanding the health of the oceans, which in turn impacts on the planet’s carbon cycle and in turn affects the evolution of our planet’s climate. A continuous ocean colour time series data is critical to this, and so we are already looking forward to the data from Sentinel-3 and PACE.

Looking Deeper At Phytoplankton from Space

NASA is currently in the middle of a joint airborne and sea campaign to study the ocean and atmosphere in preparation for developing instruments for future spaceborne missions. The Ship-Aircraft Bio-Optical Research (SABOR) campaign has brought together experts from a variety of disciples to focus on the issue of the polarization of light in the ocean; it runs from 17th July to 7th August and will co-ordinate ocean measurements with overflights.

One of the instruments on SABOR is an airborne Lidar-Polarimeter aimed at overcoming the limitation of vertically integrated surface measurements as captured by many existing Earth Observation satellites. These traditional satellites measure the water-leaving radiance, which is the signal returned from an area of water; the problem is that the signal is returned from a variety of different depths and it’s then aggregated to provide a single vertically integrated measurement for that area.

Diffuse attenuation depth at 490 nm, Kd(490), created from the SeaWiFS mission climatological data; data products retrieved from http://oceancolor.gsfc.nasa.gov/

Diffuse attenuation depth at 490 nm, Kd(490), created from the SeaWiFS mission climatological data; data products retrieved from http://oceancolor.gsfc.nasa.gov/

In effect, this means that a phytoplankon bloom at the surface will show up as a strong concentration on an image, however the same bloom at a deeper depth will show as having lower concentrations. The figure on the right shows the diffuse attenuation depth at 490 nm, blue light, created from the SeaWiFS mission climatological data collected between 1997 and 2010; the higher the value the shallower the depth of maximum passive light penetration. So, in summary, the light penetrates further within the open ocean than in many coastal waters that are more turbid.

The SABOR Lidar is based on lasers and will provide depth-resolved profiles, so instead of having a single value for an area of water, the measurements will be separable for different depths; expected to penetrate to around 50m. This will enable a much more detailed analysis of what’s happening within the water column. Satellite Lidar measurements have already been used to provide initial insights into the scattering of light resulting from phytoplankton through the CALIPSO satellite, an atmospheric focused Lidar mission launched in 2006.

In addition, the polarimeter element of SABOR will improve the quantification of the in-water constituents, such as the concentration of Chlorophyll-a (the primary pigment in most phytoplankton as well as land based plants) plus an understanding of the marine aerosols and clouds. Polarimeters have been launched before with the POLDER/PARASOL missions being examples.

The SABOR campaign will provide valuable information to support a proposal to have an Ocean Profiling Atmospheric Lidar (OPAL) deployed from the International Space Station (ISS) in 2015. If successful, it will join the existing Earth Observation mission on the ISS, called the Hyperspectral Imager for the Coastal Ocean (HICO), which I discussed in an earlier blog.

The potential offered by depth profiled oceanic measurements is exciting and will offer much more granularity beyond the ocean’s surface. I’m looking forward to the campaign’s results.