Our Spatial Planet

With the advent of easy 64 bit computing and multi-core CPU’s time has come for every company to have its own super-computer, not just research groups in Universities. The nature of data processing in Remote sensing lends itself easily to parallelization. Most of the imagery data is multi-band 2-dimensional rasters, 3-dimensional matrices from a mathematicians viewpoint. From a computational task the major processes of geometry correction, spectral correction, collation of frames and compression for transmission can be done on a pixel by pixel or block by block basis allowing segmentation of the tasks to multiple processors.

With the uptake of more projects single machine based processing became an issue at Apogee and solutions were sought for continuously running general processing on large high resolution datasets. The processing chains have been automated and set up on a beowulf server farm with quad-core CPU’s and identical diskless systems to run in parallel using a Message Passing Interface (MPI) or Parallel Python. This solution enables us to quickly finish larger projects, serve more clients and develop more elaborate processing, since computational complexity is not a barrier any more. The processes are also more fault tolerant due to the use of a more stable and syncronized operating environment with regular checkpointing for major outage recovery.

Beowulf Cluster System Diagram

Following up to TerraSAR-X captures detailed radar image of Grafton Floods. Apogee and Infoterra have tasked the TerraSAR-X satellite to acquire a ScanSAR image over the Northern New South Wales coast on the 28th May, to complement the StripMap image acquired on the 24th May. The additional ScanSAR image enhances the information that was available from the single StripMap image. Allowing Apogee to extract more information about the extent of the flooding and how the floods have receded between the two acquisition dates.

This composite below combines the TerraSAR-X StripMap acquired on the 24th of May with the TerraSAR-X ScanSAR image acquired on the 28th of May. The flooding can clearly be seen, where the Red areas show flood level on the 24th of May and the Dark areas show where there is still standing water. Below the composite the seperate StripMap and ScanSAR images show that in this area the water has receded almost completely.

Below the composite TerraSAR-X image shows the flooding extent in Grafton Northern NSW. Red areas in the composite show where there was flooding on the 24th of May, while Dark areas show where there is still flooding. The composite clearly shows that while flooding in the Grafton area has receded, there is still a significant amount of flooding around Grafton. This information is valuable for both managing recovery response during flooding emergency and planning for the emergency response for similar future disasters.

Current state-of-art in commercial and research based SAR Systems.

Air-Borne Systems

Commercial – Very few purely commercial players exist in this field

1.     Intermap IFSAR – Operational X-Band single pass Interferometric System with proven track record and very large archive of proven quality data (All of USA, Europe, Britain has been mapped as well as part of Asia and Australia). Long wavelength system for foliage penetration is currently available as repeat-pass system with multi-frequency single-pass interferometric system in development.

2.     Fugro-EarthData GeoSAR – Newly operational for X-Band and P-band single pass interferometry. Available data archive is limited and data validation is not wide-spread. Theoretically should produce good quality DEM’s using P-Band but this may conflict with the X-Band results, needing reconciliation. The system is ex-NASA. The accuracy in the system is achieved by redundancy/repeat flights. A good set of samples can be obtained at the NOAA site.

3.     Orbisat InSAR – A Brazilian system with InSAR capability in X-Band and  P-band. No validation available

Research – A number of research systems exist, operated by space agencies and educational institutions. The data from these systems has limited availability and is based on research campaigns. A suitable summary is on the POLSARPRO site.

1.     AIRSAR(NASA/JPL) – The elder statesman of air-borne systems, last known campaign was in 2004.

2.     EMISAR(DCRS) – Technical University of Denmark dual-band(L/C) fully polarimetric system.

3.     ESAR(DLR) – Quad-Band(X/C/L/P) fully polarimetric system with very high quality data used for Insar, Polsar and Polinsar research. This system served as a template for the TerraSAR-X sensor.

4.     Pi-SAR(NASDA-CRL) – JAXA Airborne L-Band system, the inspiration behind JERS and ALOS-PALSAR.

5.     RAMESES/SETHI(ONERA) – Someone in France must be obsessed with Egyptian history and pharaohs, or may be it is related to the sand penetration experiments with these systems.

6.     SAR-Convair(CCRS) – Polarimetic X/C-Band system used as a test-bed for Radardat 1 and 2 sensors by the Canadians. Mainly used for ship detection research, and ocean monitoring.

Space-Borne Systems – Recent years have seen the launch of numerous SAR sensors, both civilian and military.

The following SAR satellites are those that have readily accessible data, are currently operational or will be in the near future (which can mean anytime in the next 5 years given the nature of the space industry – you can really feel the relativistic time dilation, we must be near a black hole).  Among the military ones, we can mention SARLupe-1 and 2(Germany) , YaoGan(Chinese), and many more.

Currently In-Orbit Systems – These are either old die-hard systems, long past their scheduled expiry date or recently launched top-of-the-line sensors.

1.     RADARSAT-1 – The long lived Canadian SAR system operating in C-Band HH.

2.     ENVISAT-ASAR – SAR sensor on the multi-sensor Envisat bus. The data from this sensor is accessible for research from a rolling archive over the last 15days. The sensor can operate in alternate polarization mode.

3.     ALOS-PALSAR – The first fully polarimetric L-Band space borne sensor. The data from this sensor is heavily consumed by the Kyoto and Carbon project for global forest monitoring. It collects on a fixed schedule over all land-mass. The data is highly affordable and of good quality.

4.     TerraSAR-X – Newly launched poster child of the SAR world, first commercial SAR sensor to provide up to 1m resolution. Alternate polarization mode is operational, full-polarimetry and along track interferometry are some of the research modes available.

5.     RADARSAT-2 – After long delay, it is the first fully polarimetric C-band spaceborne system and provides data to 3m resolution.

6.     Cosmo-Skymed  - 3 out of 4 satellites are currently in orbit. With a very short revisit time, this new X-band polarimetric SAR constellation is a real advantage for monitoring applications.

Planned/To-be-launched-soon systems – These are the bad boys, getting to school late or the toddlers which show great promise. Not yet in orbit but will be nice to have data from them.

1.  Sentinel-1 – Follow on to the aging ENVISAT system mentioned above, with upgrades with new technology in C-band. Unlike its predecessor, it will be a smaller and dedicated SAR bus, other optical sensors will have to find their own rides on Sentinel 2 and 3. It is due for launch in 2011

2.     TerraSAR-L(Cartwheel) and Tandem-X – The novel concept in SAR systems is a constellation, this will allow single pass along-track and cross-track interferometry.

3.     MAPSAR – An L-band joint program between INPE(Brazil) and DLR, due some time the next decade.

4.     RADARSAT Constellation – Another program due next decade or after that is designed to provide daily global coverage using SAR.

There are probably more exotic sensors, both for research and military purposes and any comments on those sensors are more than welcome.

RapidEye is a privately funded provider of satellite-derived information and services. With the release of the first public image, Earth observation is entering a new era. The  constellation of 5 identical satellites allows up to 4 million km2 to be imaged at high resolution in a daily basis.

Each satellite system can acquire data in five spectral band. It is the first commercial satellite to offer a Red-Edge band  to identify and measure unique change in the health of green vegetation.

The constellation opens up new opportunities in areas such as Agribusiness, Emergency management, Forestry, Oil & Gas, Environmental Monitoring, Defense and other markets where reliable and repetitive monitoring are required.

RapidEye Constellation

This video clip shows 36cm resolution aerial imagery draped over an IFSAR 1m z absolute accuracy Digital elevation model over Murray River, South Australia. The clip has been made with Apogee’s NEXTIMAGE.

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In today’s world where everyone is looking for ways of improving efficiencies, Digital Elevation Models have many applications including Mining exploration, flood modelling, city planning, river and aquifer mapping, vegetation(density, height) mapping and many others in area such as road, rail, pipeline planning.

So you may be asking “What exactly is a Digital Elevation Model?”
Well, a Digital Elevation Model is a way of digitally representing the elevation at a given geographic coordinate. For every point in the DEM there are 3 values; the normal X and Y values represent the coordinates, and the Z value represents the relative height. Using these three points, we can accurately plot terrain into a visual format that makes it easier to use, while retaining the underlying data and allowing the user to extract accurate height data. The images below show 3D render of a DEM over the Coorong and part of Hindmash Island in South Australia beside a satellite image of the same area.

Coorong 50cm Z accuracy IFSAR DEM

So “Are all Digital Elevation Models the same?”
The short answer is no. There are 2 forms of DEM, DTM and DSM. DSM stands for Digital Surface Model and includes the vegetation and building elevation with the ground elevation. DTM stands for Digital Terrain Model. In a DTM the vegetation and building data have been artificially removed from the DEM to provide an elevation model of the underlying terrain.

How Digital Elevation Models are acquired?
DEM can be generated using different techniques and sensors:

• Radar Interferometry (InSAR): a SAR instrument sends microwave radiation and then record the strength and time delay of the returning signal to produce images of the ground. Elevation information can be extracted from the time delay difference between 2 SAR images over the same area. The Shuttle Radar Topography Mission (SRTM) is the most famous example of a global Digital elevation model based on space borne InSAR.  InSAR DEM can also be produced using Aerial IFSAR radar imagery to achieve better resolution and accuracy than space borne sensors. Intermap technologies provides this kind of digital elevation model worldwide.

• Aerial and Satellite photogrammetry: Two or more images are used to extract the height of any pixels on image stereo pairs utilising acquisition parameters such as focal length, principal points, platform location etc. (example ALOS PRISM DEM, SPOT DEM, EROS DEM)

• Direct coordinate acquisition: It can be achieved with surveying techniques and GPS measurement where 3 dimensional object positions are accurately determined and associated with positions on the surface the earth.

• LiDAR: LiDAR is a remote sensing mapping technique which uses a laser scanner to measure the distance between the sensor and the surfaces. Mounted on an aircraft of helicopter, millions of x,y,z positions are acquired and form a surface map. Ground LiDAR are used to measure features along profiles.

What can DEMs be used for?
When it comes to DEMs the uses are unlimited. They are essential for things such as urban planning, construction of pipelines and other infrastructure, hydrological modelling from flood prevention to tsunami predictions – remotely sensed elevation maps are invaluable to accurately map the terrain over large areas where the same data using conventional surveying methods would be prohibitively expensive. Accurate DEMs are used in flood mapping, physical modelisation,  where accuracy of the terrain mapping determines the accuracy of the resulting flood map. DEMs also have applications for precision agriculture and many other scientific and commercial areas.

Flood level using Adelaide IFSAR DEM

Interesting Related Links:
NEXTMap USA

We at Apogee are proud to present Our Spatial Planet; Apogee’s new blog about all things Spatial. With regular articles, images and videos keeping you up to date about new and interesting things happening with Aerial and Satellite Sensors.