Radar Satellite Data

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Simply put, radar data are produced when:

  • a radar transmitter emits a beam of micro or millimeter waves
  • waves reflect from the surfaces they strike
  • backscattered radiation is detected by the radar system’s receiving antenna, which is tuned to the frequency of the transmitted waves

The resultant radar data can be used to produce radar images.

SHARED Tip There are many sensor-specific importers and direct read capabilities within ERDAS IMAGINE for most types of radar data. Use SAR Metadata Editor in IMAGINE Radar to attach SAR image metadata to SAR images including creating or editing the radar ephemeris.

A radar system can be airborne, spaceborne, or ground-based. Airborne radar systems have typically been mounted on civilian and military aircraft, but in 1978, the radar satellite Seasat-1 was launched. The radar data from that mission and subsequent spaceborne radar systems have been a valuable addition to the data available for use in GIS processing. Researchers are finding that a combination of the characteristics of radar data, visible and infrared data is providing a more complete picture of the Earth. In the last decade, the importance and applications of radar have grown rapidly.

Advantages of Using Radar Data

Radar data have several advantages over other types of remotely sensed imagery:

  • Radar microwaves can penetrate the atmosphere day or night under virtually all weather conditions, providing data even in the presence of haze, light rain, snow, clouds, or smoke.
  • Under certain circumstances, radar can partially penetrate arid and hyperarid surfaces, revealing subsurface features of the Earth.
  • Although radar does not penetrate standing water, it can reflect the surface action of oceans, lakes, and other bodies of water. Surface eddies, swells, and waves are greatly affected by the bottom features of the water body, and a careful study of surface action can provide accurate details about the bottom features.

Radar Sensor Types

Radar images are generated by two different types of sensors:

  • SLAR (Side-looking Airborne Radar)—uses an antenna which is fixed below an aircraft and pointed to the side to transmit and receive the radar signal. (See the figure below.)
  • SAR—uses a side-looking, fixed antenna to create a synthetic aperture. SAR sensors are mounted on satellites and the NASA Space Shuttle. The sensor transmits and receives as it is moving. The signals received over a time interval are combined to create the image.

Both SLAR and SAR systems use side-looking geometry. The figure below shows a representation of an airborne SLAR system.

SLAR Radar

slardiag

Source: Lillesand and Kiefer, 1987.

The figure below shows a graph of data received from the radiation transmitted in the figure above Notice how the data correspond to the terrain in the figure above. These data can be used to produce a radar image of the target area. A target is any object or feature that is the subject of the radar scan.

Received Radar Signal

radar_received_radar_signal_strength_graph

Active and Passive Sensors

An active radar sensor gives off a burst of coherent radiation that reflects from the target, unlike a passive microwave sensor which simply receives the low-level radiation naturally emitted by targets.

Like coherent light from a laser, waves emitted by active sensors travel in phase and interact minimally on their way to the target area. After interaction with the target area, these waves are out of phase, interfering with each other and producing speckle noise. This is due to the different distances they travel from different targets, or single versus multiple bounce scattering.

Radar Reflection from Different Sources and Distances

radar_reflection_from_different_sources_diagram

Source: Lillesand and Kiefer, 1987.

These radar datasets, including their operational wavelengths and polarizations, are most commonly available:

Dataset

Polarization

Wavelength

Revisit Time

Almaz-1

ALOS PALSAR

L-band dual-pol, quad-pol

23.6 cm

46 days

COSMO-SkyMed

X-band dual-pol

3 cm

Envisat ASAR

C-band dual-pol

5.6 cm

35 days

ERS-1

ERS-2

C-VV

5.7 cm

35 days
(tandem: 24 hrs)

JERS-1

L-HH

23 cm

46 days

KOMPSAT5

X-band dual-pol

3 cm

Radarsat-1

C-HH

5.6 cm

24 days

Radarsat-2

C-band dual-pol, quad-pol

5.6 cm

24 days

RISAT-1

C-band

5.6 cm

RISAT-2

X-band

3.1 cm

SIR-A, SIR-B, SIR-C

TanDEM-X

X-band dual-pol, quad-pol

3.1 cm

bistatic

TerraSAR-X

X-band dual-pol, quad-pol

3.1 cm

11 days

Radar bands were named arbitrarily when radar was first developed by the military. The letter designations have no special meaning.

The C band overlaps the X band. Wavelength ranges may vary slightly among sensors.

Speckle Noise

Once out of phase, radar waves can interfere constructively or destructively to produce light and dark pixels known as speckle noise. Speckle noise in radar data must be reduced before the data can be utilized. However, the radar image processing programs used to reduce speckle noise also produce changes to the image. This consideration, combined with the fact that different applications and sensor outputs necessitate different speckle removal models, has lead Hexagon Geospatial to offer several speckle reduction algorithms.

When processing radar data, the order in which the image processing programs are implemented is crucial. This is especially true when considering removal of speckle noise. Since any image processing done before removal of the speckle results in the noise being incorporated into and degrading the image, do not rectify or in any way resample the pixel values before removing speckle noise. A rotation using nearest neighbor might be permissible.

Use IMAGINE Radar utilities to:

  • import radar data into the GIS as a stand-alone source or as an additional layer with other imagery sources
  • remove speckle noise
  • enhance edges
  • perform texture analysis
  • perform radiometric correction

IMAGINE OrthoRadar is designed to orthorectify radar imagery.

IMAGINE InSAR module is designed to generate DEMs from SAR data using interferometric techniques.

IMAGINE StereoSAR DEM module is designed to generate DEMs from SAR data using stereoscopic techniques.

See Enhancement and Radar Concepts for more information on radar imagery enhancement.

Applications for Radar Data

Radar data can be used independently in GIS applications or combined with other satellite data, such as Landsat, SPOT, or AVHRR. Possible GIS applications for radar data include:

  • Geology—radar’s ability to partially penetrate land cover and sensitivity to micro relief makes radar data useful in geologic mapping, mineral exploration, and archeology.
  • Classification—a radar scene can be merged with visible and infrared data as one or more additional layers in vegetation classification for timber mapping, crop monitoring, and so forth.
  • Glaciology—radar imagery of ocean and ice phenomena is used for monitoring climatic change through polar ice variation.
  • Oceanography—radar is used for wind and wave measurement, sea-state and weather forecasting, and monitoring ocean circulation, tides, and polar oceans.
  • Hydrology—radar data are proving useful for measuring soil moisture content and mapping snow distribution and water content.
  • Ship monitoring—the ability to provide day and night all-weather imaging, as well as detect ships and associated wakes, makes radar a tool that can be used for ship navigation through frozen ocean areas such as the Arctic or North Atlantic Passage.
  • Offshore oil activities—radar data are used to provide ice updates for offshore drilling rigs, determining weather and sea conditions for drilling and installation operations, and detecting oil spills.
  • Pollution monitoring—radar can detect oil on the surface of water and can be used to track the spread of an oil spill.

Radar Sensors

Almaz-1

Almaz-1 was launched in 1991 and operated for 18 months before being deorbited in October 1992.

Almaz-T was launched by the Soviet Union in 1987 and functioned for two years. The SAR operated with a single frequency SAR, which was attached to a spacecraft. Almaz-1 provided optically-processed data. The Almaz mission was largely kept secret.

Source: Russian Space Web, 2002.

Almaz-1 provided S-band information. It included a "single polarization SAR as well as a sounding radiometric scanner (RMS) system and several infrared bands" (Atlantis Scientific, Inc., 1997).

Swath width of Almaz-1 was 20-45 km, range resolution was 15-30 m, and azimuth resolution was 15 m.

Source: National Aeronautics and Space Administration, 1996; Atlantis Scientific, Inc., 1997.

ALOS PALSAR

PALSAR, Phased Array type L-band Synthetic Aperture Radar, is an active microwave sensor on board ALOS satellite mission, launched in 2006. It provides fine resolution mode and ScanSAR mode, which allows a swath three to five times wider than conventional SAR images.

PALSAR Characteristics

Fine Mode

ScanSAR Mode

Center Frequency

1270 MHz (L-band)

1270 MHz (L-band)

Polarization

HH or VV

HH+HV or VV+VH

HH or VV

Range Resolution

7 to 44 m
14 to 88 m

100m (multilook)

Observation Swath

40 to 70 km

250 to 350 km

Bit Length

5 bits

5 bits

Source: Japan Aerospace Exploration Agency, 2003b.

COSMO-SkyMed

COSMO-SkyMed mission is a group of four satellites equipped with radar sensors for Earth observation for civil and defense use. The mission was developed by the Italian Space Agency (Agenzia Spaziale Italiana) and Telespazio. The program supplies data for emergency management services, environmental resources management, earth topographic mapping, maritime management, natural resources monitoring, surveillance, interferometric products and digital elevation models.

COSMO 1 and COSMO 2 were launched in 2007, COSMO 3 launched in 2008, and COSMO 4 launched in 2010.

The sensors operate in various wide field and narrow field modes, with multi-polarmetric and multi-temporal capabilities.

COSMO-SkyMed Characteristics

Mode

Swath

Resolution

ScanSAR

Hugeregion

200 km x 200 km

100 m pixel

ScanSAR

Wideregion

100 km x 100 km

30 m pixel

Stripmap

HImage

40 km x 40 km

3 - 15 m pixel

Stripmap

Pingpong

30 km x 30 km

15 m pixel

Spotlight 1

Classified

Classified

Spotlight2

10 km x 10 km

1 m pixel

Sources: Telespazio, 2008; e-GEOS, 2008; Agenzia Spaziale Italiana, 2013.

Envisat

In 2002, European Space Agency launched Envisat (ENVIronmental SATellite), an advanced polar-orbiting Earth observation satellite which provides measurements of the atmosphere, ocean, land, and ice. Envisat mission provides for continuity of the observations started with the ERS-1 and ERS-2 satellite missions, notably atmospheric chemistry, ocean studies and ice studies. Envisat mission ended in April 2012.

Envisat flew in a sun-synchronous polar orbit at about 800 km altitude, with a repeat cycle of 35 days.

Envisat was equipped with these instruments:

  • ASAR - Advanced Synthetic Aperture Radar operating at C-band.
  • MERIS - Programmable, medium-spectral resolution spectrometer operating in 390 nm to 1040 nm spectral range.
  • AATSR - Advanced Along Track Scanning Radiometer continues collection of ATSR-1 and ATSR-2 sea surface temperature data sets.
  • RA-2 - Radar Altimeter 2 determines the two-way delay of the radar echo from the Earth’s surface to a very high precision. Also measures the power and shape of reflected radar pulses.
  • MWR - Microwave radiometer measures the integrated atmospheric water vapor column and cloud liquid water content, as correction terms for the radar altimeter signal.
  • GOMOS - Medium resolution spectrometer measures atmospheric constituents in the spectral bands between 250 nm to 675 nm, 756 nm to 773 nm, and 926 nm to 952 nm. It includes two photometers measuring in the spectral bands between 470 nm to 520 nm and 650 nm to 700 nm.
  • MIPAS - Michelson Interferometer for Passive Atmospheric Sounding is a Fourier transform spectrometer for measuring gaseous emission spectra in the near to mid infrared range.
  • SCIAMACHY - Imaging spectrometer measures trace gases in the troposphere and stratosphere.
  • DORIS - Doppler Orbitography and Radio-positioning Integrated by Satellite instrument is a tracking system to determine the precise location of Envisat satellite.
  • LRR - Laser Retro-Reflector tracks orbit determination and range measurement calibration.

Source: European Space Agency, 2008b; European Space Agency, 2013.

ERS-1

ERS-1, a radar satellite, was launched by ESA in July of 1991. ESA, European Space Agency, announced the end of the ERS-1 mission in March 2000.

ERS-1 was ESA’s first sun-synchronous polar-orbiting mission, acquiring more than 1.5 million Synthetic Aperture Radar scenes. The measurements of sea surface temperatures made by ERS-1 Along-Track Scanning Radiometer are the most accurate ever from space. These and other critical measurements are continued by Envisat.

Source: European Space Agency, 2008a.

One of its primary instruments was Along-Track Scanning Radiometer (ATSR). ATSR monitors changes in vegetation of the Earth’s surface.

Instruments aboard ERS-1 include: SAR Image Mode, SAR Wave Mode, Wind Scatterometer, Radar Altimeter, and Along Track Scanning Radiometer-1 (European Space Agency, 1997).

Some of the information obtained from ERS-1 and ERS-2 missions include:

  • maps of the surface of the Earth through clouds
  • physical ocean features and atmospheric phenomena
  • maps and ice patterns of polar regions
  • database information for use in modeling
  • surface elevation changes

According to ESA,

. . .ERS-1 provides both global and regional views of the Earth, regardless of cloud coverage and sunlight conditions. An operational near-real-time capability for data acquisition, processing and dissemination, offering global data sets within three hours of observation, has allowed the development of time-critical applications particularly in weather, marine and ice forecasting, which are of great importance for many industrial activities (European Space Agency, 1995).

Source: European Space Agency, 1995.

ERS-2

ERS-2, a radar satellite, was launched in 1995 to study the atmosphere, oceans, polar ice, and land in conjunction with ERS-1. ESA, European Space Agency, announced the end of ERS-2 mission in September 2011.

In addition to the same instruments carried aboard ERS-1, ERS-2 also carried Global Ozone Monitoring Experiment (GOME) to measure atmospheric chemistry and Microwave Radiometer (MWR) to measure snow, ice, and sea ice. GOME was one of the longest serving ozone monitors in the world.

Data obtained from ERS-2 used in conjunction with that from ERS-1 enables you to perform interferometric tasks. Using the data from the two sensors, ground movement from earthquakes can be monitored, and DEMs can be created.

For information about ERDAS IMAGINE interferometric software, InSAR, see Radar Interferometry User Guide.

Source: European Space Agency, 1995; European Space Agency, 2011.

JERS-1

Japanese Earth Resources Satellite (JERS) obtained data from 1992 to 1998, and has been superseded by the ALOS mission.

See ALOS for information about Advanced Land Observing Satellite (ALOS).

Source: Japan Aerospace Exploration Agency, 2007.

JERS-1 satellite was launched in February of 1992, with an SAR instrument and a 4-band optical sensor aboard. SAR sensor’s ground resolution was 18 m, and optical sensor’s ground resolution was roughly 18 m across-track and 24 m along-track. Revisit time was every 44 days. The satellite travelled at an altitude of 568 km, at an inclination of 97.67 degrees.

JERS-1 Characteristics

Band

Wavelength

(micrometres)

1

0.52 to 0.60

2

0.63 to 0.69

3

0.76 to 0.86

4

0.76 to 0.86

5

1.60 to 1.71

6

2.01 to 2.12

7

2.13 to 2.25

8

2.27 to 2.40

Band 4 had viewing 15.3 degrees forward

Source: Earth Remote Sensing Data Analysis Center, 2000.

JERS-1 data comes in two different formats: European and Worldwide. European data format consists mainly of coverage for Europe and Antarctica. Worldwide data format has images that were acquired from stations around the globe. According to NASA, "a reduction in transmitter power has limited the use of JERS-1 data" (National Aeronautics and Space Administration, 1996).

Source: Eurimage, 1998; National Aeronautics and Space Administration, 1996.

KOMPSAT5

KOMPSAT-5 is part of the Korean National Development Plan of MEST (Ministry of Education, Science and Technology) which started in 2005.

The project is being developed and managed by KARI (Korea Aerospace Research Institute). The primary mission objective is to develop, launch and operate an Earth observation SAR(Synthetic Aperture Radar) satellite system to provide imagery for geographic information applications and to monitor environmental disasters.

RADARSAT-1

RADARSAT satellite was developed by the Canadian Space Agency and launched in 1995. With the development of RADARSAT-2, the original RADARSAT is also known as RADARSAT-1.

RADARSAT satellite carries SARs, which are capable of transmitting signals that can be received through clouds and during nighttime hours. RADARSAT satellite has multiple imaging modes for collecting data, which include Fine, Standard, Wide, ScanSAR Narrow, ScanSAR Wide, Extended (H), and Extended (L). Resolution and swath width varies with each one of these modes, but in general, Fine offers the best resolution: 8 m.

RADARSAT Beam Mode Resolution

Beam Mode

Resolution

Fine Beam Mode

8 m

Standard Beam Mode

25 m

Wide Beam Mode

30 m

ScanSAR Narrow Beam Mode

50 m

ScanSAR Wide Beam Mode

100 m

Extended High Beam Mode

25 m

Low Beam Mode

35 m

Types of RADARSAT image products include: Single Data, Single Look Complex, Path Image, Path Image Plus, Map Image, Precision Map Image, and Orthorectified.

RADARSAT satellite uses a single frequency, C-band. Altitude of the satellite is 496 miles, or 798 km. The satellite is able to image the entire Earth, and its path is repeated every 24 days. Swath width is 500 km. Daily coverage is available of the Arctic, and any area of Canada can be obtained within three days.

Source: RADARSAT, 1999; Space Imaging, 1999c.

RADARSAT-2

RADARSAT-2, launched in 2007, is a SAR satellite developed by the Canadian Space Agency and MacDonald, Dettwiler, and Associates, Ltd. (MDA). The satellite advancements include 3 meter high-resolution imaging, flexibility in polarization selection, left and right-looking imaging options, and superior data storage.

In addition to RADARSAT-1 beam modes, RADARSAT-2 offers Ultra-Fine, Multi-Look Fine, Fine Quad-Pol, and Standard Quad-Pol beam modes. Quadrature-polarization means that four images are acquired simultaneously; two co-polarized images (HH and VV) and two cross-polarized images (HV and VH).

RADARSAT-2 Characteristics

Geometry of orbit

near-polar, sun-synchronous

Orbit Altitude

798 km

Orbit Inclination

98.6 degrees

Orbit repeat cycle

24 days

Frequency Band

C-band (5.405 GHz)

Channel Bandwidth

11.6, 17.3, 30, 50, 100 MHz

Channel Polarization

HH, HV, VH, VV

Spatial Resolution

3 meters to 100 meters

Source: RADARSAT-2, 2008.

RISAT

RISAT are a series of Indian radar imaging reconnaissance satellites built by the Indian Space Research Organisation (ISRO). These satellites provide all-weather surveillance using synthetic aperture radars (SAR) and are used for numerous applications spanning agriculture planning, management of natural disasters, and defense.

Source: Indian Space Research Organisation (ISRO), 2014.

RISAT-1

Radar Satellite-1 (RISAT-1) is a state of the art Microwave Satellite launched in April 2012 carrying a Synthetic Aperture Radar (SAR) sensor operating in C-band (5.35 GHz).

Active Microwave Remote Sensing provides cloud penetration and day-night imaging capability. These unique characteristics of C-band (5.35GHz) Synthetic Aperture Radar enable applications in agriculture, particularly paddy monitoring in monsoon season and management of natural disasters like flood and cyclone.

Source: Indian Space Research Organisation (ISRO), 2014.

RISAT-2

Radar Satellite-2 (RISAT-2) was launched in January 21, 2008 carrying a Synthetic Aperture Radar (SAR) sensor operating in X-band (9.59 GHz).

Source: Indian Space Research Organisation (ISRO), 2014.

SIR-A

SIR stands for Spaceborne Imaging Radar. SIR-A was launched and collected data in 1981. SIR-A mission built on the Seasat SAR mission that preceded it by increasing the incidence angle with which it captured images. The goal of the SIR-A mission was to collect geological information. This information did not have as pronounced a layover effect as previous imagery.

An important achievement of SIR-A data is that it was capable of penetrating surfaces to obtain information. For example, NASA says that the L-band capability of SIR-A enabled the discovery of dry river beds in the Sahara Desert.

SIR-1 used L-band, had a swath width of 50 km, a range resolution of 40 m, and an azimuth resolution of 40 m (Atlantis Scientific, Inc., 1997).

For information on OrthoRadar, which reduces layover effect, see OrthoRadar Theory.

Source: National Aeronautics and Space Administration, 1995a; National Aeronautics and Space Administration, 1996; Atlantis Scientific, Inc., 1997.

SIR-B

SIR-B was launched and collected data in 1984. SIR-B improved over SIR-A by using an articulating antenna, permitting the incidence angle to range between 15 and 60 degrees. This enabled the mapping of surface features using "multiple-incidence angle backscatter signatures" (National Aeronautics and Space Administration, 1996).

SIR-B used L-band, had a swath width of 10-60 km, a range resolution of 60-10 m, and an azimuth resolution of 25 m (Atlantis Scientific, Inc., 1997).

Source: National Aeronautics and Space Administration, 1995a, National Aeronautics and Space Administration, 1996; Atlantis Scientific, Inc., 1997.

SIR-C

SIR-C sensor was flown onboard two separate NASA Space Shuttle flights in 1994. Flight 1 was notable for a fully polarimetric spaceborne SAR, multi-frequency, X-band, and demonstrated ScanSAR for wide swath array. Flight 2 was notable for the first SAR to re-fly, targeted repeat-pass interferometry, and also demonstrated ScanSAR for wide swath array.

Source: National Aeronautics and Space Administration, 2006.

SIR-C is part of a radar system, SIR-C/X-SAR, which flew in 1994. The system is able to ". . .measure, from space, the radar signature of the surface at three different wavelengths, and to make measurements for different polarizations at two of those wavelengths" (National Aeronautics and Space Administration, 1997). Moreover, it can supply ". . .images of the magnitude of radar backscatter for four polarization combinations" (National Aeronautics and Space Administration, 1995a).

Data provided by SIR-C/X-SAR can be used to measure the following:

  • vegetation type, extent, and deforestation
  • soil moisture content
  • ocean dynamics, wave and surface wind speeds and directions
  • volcanism and tectonic activity
  • soil erosion and desertification

The antenna of the system was composed of three antennas: one at L-band, one at C-band, and one at X-band. The antenna was assembled by the Jet Propulsion Laboratory. The acquisition of data at three different wavelengths makes SIR-C/X-SAR data very useful. SIR-C and X-SAR do not have to be operated together: they can also be operated independent of one another.

SIR-C/X-SAR data come in resolutions from 10 to 200 m. Swath width of the sensor varies from 15 to 90 km, which depends on the direction the antenna is pointing. The system orbited the Earth at 225 km above the surface.

SIR-C SAR Characteristics

Bands

Wavelength

L-Band

0.235 m

C-Band

0.058 m

X-Band

0.031 m

Source: National Aeronautics and Space Administration, 1995a; National Aeronautics and Space Administration, 1997.

TanDEM-X

TanDEM-X mission is to fly in tandem with TerraSAR-X, recording data synchronously to acquire the data basis for a global DEM from uniform sources.

TanDEM-X, launched in 2010, is nearly identical to TerraSAR-X. Both are collecting data simultaneously from different angles. The images are processed into accurate elevation maps with 12 meter resolution and vertical accuracy better than 2 meters. Over the three-year mission, images of the Earth’s entire land surface will be collected several times. Notably, TanDEM-X and TerraSAR-X satellites together are the first configurable synthetic aperture radar interferometer in space.

TanDEM-X is a German satellite manufactured in a public private partnership between the German Aerospace Center (DLR) and Astrium GmbH.

Source: DLR (German Aerospace Center). 2011.

TerraSAR-X

TerraSAR-X, launched in 2007, is a German satellite manufactured in a public private partnership between the German Aerospace Center (DLR), Astrium GmbH, and German Ministry of Education and Science (BMBF).

TerraSAR-X carries a high frequency X-band SAR instrument based on an active phased array antenna technology. Satellite orbit is sun-synchronous at 514 km altitude at 98 degrees inclination and 11 days repeat cycle.

The satellite sensor operates in several modes; Spotlight, high Resolution Spotlight, Stripmap, and ScanSAR, at varying geometrical resolutions between 1 and 16 meters. It provides single or dual polarization data.

TerraSAR-X Characteristics

Mode

Swath

Resolution

Spotlight (SL)

10 x 10 km scene

1 - 3 meters

High Resolution Spotlight (HS)

5 km x 10 km scene

1 - 2 meters

Stripmap (SM)

30 km strip

3 - 6 meters

ScanSAR (SC)

100 km strip

16 meters

Source: DLR (German Aerospace Center). 2008.