Footprints in Remote Sensing

Plymouth Sound on 25th July 2014 from Landsat 8: Image courtesy of USGS/NASA Landsat

Plymouth Sound on 25th July 2014 from Landsat 8: Image courtesy of USGS/NASA Landsat

I’ve just finished my summer with Pixalytics! As I wrote a blog when I first arrived, I thought it would be nice symmetry to finish my ERASMUS+ placement with a second one.

When I started my internship, I had very little real-world experience. I was really excited and really nervous, but this internship has been a huge eye opener for me. I spent the first week understanding and reviewing the practicals within Pixalytics’ forthcoming book ‘The Practical Handbook of Remote Sensing’ to check for any errors prior to publication, which gave me a good understanding of the basics of remote sensing.

Over the next few weeks I applied my new knowledge to finding and downloading Landsat data for a commercial client. I then downloaded additional Landsat datasats and compared them to altimetry datasets to look for patterns between the two sources for the NovaSAR project. My other main job was processing Landsat 8 data to create a UK-wide vegetation mosaic. This needed cloud free images which is really difficult because the weather in UK is always cloudy, even in summer!

Plymouth is a deeply captivating city with astonishingly magnificent views and landscapes. You get the urban city, fantastic scenery and all around Plymouth are nice beaches, cities and the Dartmoor National Park which is always worth a visit. It’s a safe quiet place where everything is so close together that you can walk everywhere. The people are generally friendly and warm-hearted, and the experience of living in the Plymouth for two months has helped me to gain a more fluent level of English and a better understanding of the British culture – I now know why they constantly talk about the weather!

Overall, I’ve learnt a lot from the internship including practical skills that I will be able to carry with me to my next position. Needless to say, I will miss Pixalytics and Plymouth very dearly, and I’m thankful for the chance to work and live there. ERASMUS+ is an great opportunity that everyone should try to be part of, and I totally recommend going abroad because is an experience that stays with you to rest of your life.

Bye Plymouth, Bye Pixalytics!

Selin

Blog by Selin Cakaloglu, Erasmus+ Intern at Pixalytics

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 Water.org 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.

Vienna!

Last week I was in Vienna, Austria, attending the 2014 European Geophysical Union (EGU) General Assembly. It was a scientific smorgasbord laid in front of over 12,000 people from 106 countries. Over 4,800 oral presentations were given and 9,500 posters displayed, this was coupled with a variety of other sessions and an exhibition; which created a varied programme. I really liked the plan to create smart umbrellas to collect rain data, which has already received press coverage.

My EGU experience began with a poster summary session on the Thursday morning; these are short three minute presentations giving delegates a flavour of the posters being displayed to encourage people to come and see them. I then moved onto watching presentations and visiting the posters.

Two presentations really caught my eye. The first was about NASA’s upcoming mission Cyclone Global Navigation Satellite System (CYGNSS) which will be studying ocean surface winds using reflected Global Navigation Satellite System (GNSS) signals that are primarily used for positioning, such as within your mobile phone, and timing measurements. This technique, often called GNSS reflectometry, was previously demonstrated on the SSTL’s UK-DMC-1 mission.

The second one focussed on using the altimeter SARAL/AltiKa to study storm Xaver that impacted the southern North Sea / northern Europe with hurricane force winds and a tidal surge at the beginning of December 2013. Launched in February 2013, SARAL/AltiKa is a new collaboration between the French Space Agency (CNES) and Indian Space Research Organization (ISRO) filling a gap left by the loss of ESA’s Envisat as it has the same ground track; while we wait for the Copernicus Sentinel-3 mission that will include altimetry, ocean colour and sea surface temperature instruments.

On the Friday I presented a poster on an ESA project I’m involved with titled E-Collaboration for Earth Observation (E-CEO), which addresses the technologies and architectures needed to provide a collaborative research platform for automating data mining and information extraction experiments. Our aim is to run Earth Observation challenges akin to those used to solve computing tasks, and the poster presented the first of the challenges – focusing on the atmospheric correction of ocean colour imagery.