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Earth Observation

 

 

Earth Observation in the frame of EO-MINERS - Overview of remote sensing methods, sensors and applications

Forthcoming system - a short review

EnMAP - The Environmental Mapping and Analysis Program (EnMAP) is a German hyperspectral mission, scheduled for launch in 2015. The primary goal of EnMAP is to offer accurate, diagnostic information on the state and evolution of terrestrial ecosystems on a timely and frequent basis, and to allow for a detailed analysis of surface parameters with regard to the characterization of vegetation canopies, rock/soil targets and coastal waters on a global scale. EnMAP is designed to record bio-physical, bio-chemical and geo-chemical variables to increase our understanding of biospheric /geospheric processes and to ensure the sustainability of our resources. EnMAP will monitor the Earth's surface with a ground sampling distance (GSD) of 30 m x 30 m (30 km x 5000 km per day) measuring in the 420-2450 nm range by means of two separate spectrometers covering the visible to near-infrared (VNIR) and short-wave infrared (SWIR) spectral regions with 244 contiguous bands. The mean spectral sampling distance and resolution is of 6.5 nm at the VNIR, and of 10 nm at the SWIR. Accurate radiometric and spectral responses are guaranteed by a defined signal-to-noise ratio (SNR) of ≥ 400:1 in the VNIR and ≥ 170:1 in the SWIR, radiometric calibration accuracy better than 5% and a spectral calibration uncertainty of 0.5 in the VNIR and 1 nm in the SWIR. An off-nadir pointing capability of up to 30° enables a target revisit time of 4 days.

EnMap future satellite illustration
EnMap future satellite illustration (http://www.gfz-potsdam.de/)

In this program the GFZ Potsdam has the scientific lead, Kayser-Threde is the industrial prime and OHB Systems provides the bus. The German Space Agency is managing the project and the German Aerospace Establishment is responsible for the ground segment.

 

HyspIRI - The Hyperspectral Infrared Imager or HyspIRI mission will study the world’s ecosystems and provide critical information on natural disasters such as volcanoes, wildfires and drought. HyspIRI will be able to identify the type of vegetation that is present and whether the vegetation is healthy. The mission will provide a benchmark on the state of the worlds ecosystems against which future changes can be assessed. The mission will also assess the pre-eruptive behaviour of volcanoes and the likelihood of future eruptions as well as the carbon and other gases released from wildfires. The HyspIRI mission includes two instruments mounted on a satellite in Low Earth Orbit. There is an imaging spectrometer measuring from the visible to short wave infrared (VSWIR: 380 nm - 2500 nm) in 10 nm contiguous bands and a multispectral imager measuring from 3 to 12 um in the mid and thermal infrared (TIR). The VSWIR and TIR instruments both have a spatial resolution of 60 m at nadir. The VSWIR will have a revisit of 19 days and the TIR will have a revisit of 5 days. HyspIRI also includes an Intelligent Payload Module (IPM) which will enable direct broadcast of a subset of the data.

HyspIRI future satellite illustration
HyspIRI future satellite illustration (http://hyspiri.jpl.nasa.gov/)

The data from HyspIRI will be used for a wide variety of studies primarily in the Carbon Cycle and Ecosystem and Earth Surface and Interior focus areas. The mission was recommended in the recent National Research Council Decadal Survey requested by NASA, NOAA, and USGS. The mission is currently at the study stage and this website is being provided as a focal point for information on the mission and to keep the community informed on the mission activities.

Sentinel - ESA is developing five new missions called Sentinels specifically for the operational needs of the GMES program.
The Sentinel missions are based on a constellation of two satellites to fulfil revisit and coverage requirements, providing robust datasets for GMES Services.

Sentinel future satellite illustration
Sentinel future satellite illustration (http://www.esa.int/esaLP/SEM097EH1TF_LPgmes_1.html)

 

WorldView-3  - DigitalGlobe's next satellite WorldView-3 is in a phased development process for an advanced fourth-generation satellite scheduled to launch in mid-2014 and will offer 0.31 meter resolution panchromatic, eight-band VNIR multispectral imagery and eight band SWIR multispectral imagery. WorldView-3 was recently licensed by the National Oceanic and Atmospheric Administration (NOAA) to collect eight-band short-wave infrared (SWIR) imagery. WorldView-3 will be the first multi-payload, super-spectral, high-resolution commercial satellite operating at an expected altitude of 617 km. WorldView-3 provides 31 cm panchromatic resolution, 1.24 m multispectral resolution, and 3.7 m short wave infrared resolution. WorldView-3 has an average revisit time of <1 day and is capable of collecting up to 680,000 km2 per day.

WorldView-3 will bear a strong resemblance to WorldView-2 launched on October 8, 2009 in terms of its performance characteristics. The WorldView-3 satellite sensor will benefit from significant improvements including cost savings, risk reduction, and faster delivery for its customers.
WorldView-3 is currently being constructed by Ball Aerospace, which has designed and built all of DigitalGlobe's operational satellites. The imaging instruments, including the SWIR sensor and optics, were engineered and are being built by ITT Exelis.

Future satellite illustration
(http://www.satimagingcorp.com/satellite-sensors/worldview-3.html)

 

WV3 spectral configuration (http://www.digitalglobe.com/content/worldview3/)

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Thermal Sensors - These sensors can be on the ground, airborne or spaceborne. They are a good tool to detect environmental impacts related to temperature and emissivity signals. As thermal images are a function of earths’ emission, they can be taken at day or at night. The amount of thermal energy arriving at the sensor is in most cases low, as a result, pixel size must be increased and the spatial resolution of most thermal images is low. Hyperspectral thermal sensors are still not common, although they can produce invaluable and added information which the Vis-NIR-SWIR region cannot give. This is due to the technical problems to account for very narrow bands in a spectral region where emitted energy is low. One of the most used airborne thermal sensors is AHS. It is a semi – multispectral sensor that has a relatively high number of bands but still wider. 

AHS (thermal and optical multispectral/hyperspectral airborne scanner)

AHS characteristics

Optical design

scan mirror plus reflective optics with a single IFOV determining field stop (Pfund assembly)

FOV (Field Of View) / IFOV (Instantaneous Field Of View)

90º / 2.5 mrad

GSD (Ground Sampling Distance)

2.1 mrad (0.12 degrees)

Scan rates

12.5, 18.75, 25, 35 r.p.s., with corresponding ground sampling distances from 7 to 2 meters.

Digitization precision

12 bits to sample the analogue signal, with gain level from x0.25 to x10.

Samples per scan line:

750 pixels/line

Reference sources

two controllable thermal black bodies within the field of view, set to a temperature range from -15ºC to +25ºC with respect to scan head heat sink temperature

Spectrometer:

four dichroic filters to split radiation in four optical ports, and diffraction gratings within each port

Detectors:

Si array for VIS/NIR port; InSb and MCT arrays, cooled in N2 dewars, for SWIR, MIR and TIR ports.

Spectral bands:

continuous coverage in four spectral regions + single band at 1.5 micrometers

PORT 1

coverage(micrometers)- 0.43 -> 1.03, bandwidth (FWHM)- 28 nm, nº of bands- 20

PORT 2A

coverage(micrometers)- 1.55 -> 1.75, bandwidth (FWHM)- 200 nm, nº of bands- 1

PORT 2

coverage(micrometers)- 2.0 -> 2.54, bandwidth (FWHM)- 13 nm, nº of bands 42

PORT 3

coverage(micrometers)- 3.3 -> 5.4, bandwidth (FWHM)- 30 nm, nº of bands-  7

PORT 4

coverage(micrometers)- 8.2 -> 12.7, bandwidth (FWHM)- 40-50 nm, nº of bands- 10

 

AHS night thermal image of Sokolov mine site
AHS night thermal image of Sokolov mine site (red colour represents high temperatures) (source: EO-MINERS, TAU)

 

RADAR - Radio Detection and Ranging employs ground-based, airborne and spaceborne radar instruments. Since wavelength used by RADAR are usually less dependent to weather conditions, they are usually considered as a good alternative to optical imaging which is heavily affected by meteorological changes (clouds, …). Since microwaves can penetrate the surface (to a certain extent, depending on the wavelength used), RADAR can be used to map subsurface characteristics. At last, microwaves are commonly used to detect metallic structures and humidity content.
From a physical point of view, RADAR detects changes of back-scattered radiations. They can efficiently be used to map changes in roughness or geometric properties of the surfaces. Since RADAR is basically measuring distances, they are especially successful in creating elevation models and surface deformations (Interferometric RADAR).

RADAR-mapping diagram
RADAR-mapping (from www-star.stanford.edu).

Radar variants are SAR (synthetic Aperture RADAR) that allows the use of smaller antennas and Doppler RADAR systems that are also used in meteorology to map clouds, rain drops and hail. Various configurations or acquisition modes exist, please refer, for example to any recent edition of the ‘Radar Handbook’ by Merrill Skolnik.

Terra-SAR-X (spaceborne X-band radar sensor with a range of different modes of operation)

Terra-SAR-X Characteristics

Radar frequency

9.65 Gigahertz

Power consumption

800 watt (on average)

Resolution

1 metre, 3 metres, or 16 metres
(depending on the image size)

Launch vehicle

Dnepr 1 (converted SS-18)

Launch date

15 June 2007, 4:14 CEST

Orbital altitude

514 kilometres

Angle of inclination with respect to the equator

97.4 degrees (Sun synchronous)

Mission life time

at least 5 years

 

TerraSAR-X image
TerraSAR-X image of the Strait of Gibraltar (source: DLR)

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LiDAR Sensors - Light Detection And Ranging (LADAR) is an optical or thermal remote sensing technology that measures the returning radiations from an emitted LASER pulse. Applications range from  Digital Elevation Models (DEM) generation to atmospheric analysis. Common variants of LiDAR are Differential Absorption LIDAR (DIAL), Doppler LIDAR and Rayleigh Doppler LIDAR and are meeting specific needs, usually in atmosphere analysis. There currently exits well establish ground and air-born LiDAR systems; satellite-born LiDAR start be available (IceSat, Calipso).

LiDAR Image
Rendering of LiDAR data from an altitude of 1000 m over downtown Manhattan after the attack on the World Trade Center on 9 September 2001 (from www.noaanews.noaa.gov).

An example of LiDAR system specifications (source: NASA http://ngom.usgs.gov/dsp/tech/eaarl/index.php)

NASA EAARL System Specifications

Total system weight:

250 lbs.

Maximum power requirement:

28 VDC at 24 amps

Nominal surveying altitude:

300 m AGL

Raster scan rate:

20 scanlines per second

Laser sample per raster:

120 rasters/second

Swath width at 300 m altitude:    

240 m

Sample spacing:

Swath center = 2 x 2 m
Swath edges = 2 x 4 m

Area surveyed per hour: (300 m altitude, 50 m/s)    

43 km2 per hour

Nominal power required:

400 Watts

Illuminated laser spot diameter on the surface:

20 cm

Nominal ranging accuracy:

3 - 5 cm

Nominal horizontal positioning accuracy:

< 1 m

Digitizer temporal resolution:

1 nanosecond (13.9 cm in air, 11.3 cm in water)

Minimum water depth:

0 m

Maximum measurable water depth:

26 m

 

Satellite Navigation Systems - Triangulation as a method to determine a location relative to known points is a techniques that dates back to antiquity. While it is strictly speaking not an (imaging) remote sensing method, the underlying principles can be used to develop 2D- and 3D-images of the world. While in history stars or other visible known points have been used as reference points, radio-beacons (e.g. DECCA, LORAN) and more recently satellites have been employed: e.g. the US American Global Position System (GPS) or its future competitor GALILEO. However, they can be used to locate relatively precisely (GPS: 10 m horizontal, GALILEO: 5 m horizontal) sampling locations and other points of interest.

GPS Calculation diagram GPS Calculation diagram
The principle of position calculation with GPS (from www.gsm-modem.de)

Differential GPS (DGPS) employs two receivers of which one is installed at a precisely known location. This base station receiver calculates its position based on the satellite signals and compares his calculated location to the known location. The resulting correction is applied to the GPS data recorded by the second, roving GPS receiver.

GPS and DGPS can be used in conjunction with tilt-, gradio- or inclinometers to precisely map the surface topography and slope inclinations. Inclinometers usually compare the tilt of the surface of interest with reference to the vertical component of the Earth's gravity field. There is a wide variety of constructions employing pendulums, spirit level-type or communicating pipes-type arrangements etc.

Principles of differential GPS diagram
The principle of differential GPS.

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