ERS (European Remote Sensing Satellite), European Space Agency

The ERS-1, 2 satellites are intended for global measurements of sea wind and waves, ocean and ice monitoring, coastal studies and land sensing using active and passive microwave remote sensing systems. ERS-1 was launched in July 1991 and ERS-2 in April 1995. ERS-1 uses a synthetic aperture radar (SAR) instrument to acquire images of ocean, ice and land regardless of cloud and sunlight conditions. Other microwave instruments measure sea state, sea surface winds, ocean circulation, sea and ice levels, as well as the sea's surface temperature. The near polar sun-synchronous orbit of the satellite enables a global mission providing world-wide geographical and repetitive coverage, primarily oriented towards ocean and ice monitoring, but with an all-weather high resolution microwave imaging capability over land and coastal zones.

ERS-2 is practically identical to ERS-1, with the addition of the GOME sensor for global ozone monitoring. The orbits of ERS-1 and 2 are such that ERS-2 follows the same ground track as ERS-1, except for a 1-day delay. This provides an opportunity to obtain tandem interferometric data of an area using the synthetic aperture radar on the two satellites. The tandem data has better coherence property than the data obtained from 35-day repeat passes of a single satellite. The tandem mission was operational from April to June, 1996 after the launch of ERS-2. ERS-1 has outlived its planned operational life and was deactivated after the tandem mission. Currently, only ERS-2 remains in active operation.

Sensors

• AMI (Active Microwave Instrument): The AMI operating at a frequency of 5.3 GHz (C-band, VV-polarised) combines the functions of a Synthetic Aperture Radar (SAR) and a Wind Scatterometer (WNS). Through its set of four antennae (three for the Scatterometer and one for the SAR - see the figure above), the Earth's surface is illuminated and the backscattered energy is received to produce data on wind fields and wave spectra (WNS mode), and to produce high resolution images (SAR mode) of the Earth's surface.
• RA-1 (Radar Altimeter): The Radar Altimeter measures variations in the satellite's height above sea-level.
• ATSR (Along Track Scanning Radiometer): This passive instrument monitors the thermal emission of the seas and oceans, from which the global sea surface temperature is derived.
• GOME, on ERS-2 (Global Ozone Monitoring Experiment): The GOME is a passive spectrometer for monitoring the ozone content of the atmosphere.

JERS-1 (Japanese Earth Resource Satellite), Japan

JERS-1 Orbit

Sensors

• SAR (Synthetic Aperture Radar)
• OPS (Optical Sensor)
  

JERS-1 SAR Instrument Characteristics

  RADARSAT, Canada

RADARSAT is a Canadian satellite operated by the Canadian Space Agency (CSA)/Canadian Center for Remote Sensing (CCRS) for gathering global data on ice conditions, crops, forests, oceans and geology. The satellite was launched in November 1995, with the launch service provided by NASA, USA.

With its creation in 1989, the Canadian Space Agency took over the implementation and operation of the RADARSAT project as a key component of the Canadian space program. CCRS continues to participate in the RADARSAT project through the Radar Data Development Program (RDDP) and is responsible for data collection and archiving.

RADARSAT, with a planned lifetime of five years, is equipped with a Synthetic Aperture Radar (SAR). Using a single frequency (C-Band), the RADARSAT SAR has the unique ability to shape and steer its radar beam over a 500 kilometre range. Users can have access to a variety of beam selections that can image swath from 35 kilometres to 500 kilometres with resolutions from 10 metres to 100 metres respectively. Incidence angles range from less than 20 degrees to more than 50 degrees.

Sensor

• SAR (Synthetic Aperture Radar): The SAR is able to operate in several beam modes:
  • Standard: Seven beam modes with incidence angle ranging from 20 to 49 deg nominal, 100 km swath width and 25 m resolution.
  • Wide: Three beam modes with varying incidence angles, 150 km swath width.
  • Fine: Five beam modes with 50 km swath width and resolution better than 10 m.
  • Scansar: Wide swath width (300 - 500 km) with a coarser resolution of 50 to 100 m.
  • Extended mode.

RADARSAT Operating Modes

1. Nominal; ground range resolution varies with range
2. Nominal; range and processor dependent
3. Incidence angle depends on sub-mode

ASAR - Advanced Synthetic Aperture Radar (on ENVISAT, European Space Agency)

Envisat was successfully launched on 1 March 2002!

The ESA's ENVISAT satellite, expected to be launched in November 2001, carries the Advanced SAR (ASAR) instrument. The ASAR, operated at C-band (5.331 GHz), can be regarded as an advanced version of the SAR instruments on board the ERS1 and ERS2 satellites. Its beam elevation steerage allow the selection of different swaths at different incidence angles, providing a swath coverage of over 400-km wide using ScanSAR techniques. In alternating polarisation mode, transmit and receive polarisation can be selected allowing scenes to be imaged simultaneously in two polarisations.

In the image mode, ASAR operates in one of seven predetermined swaths (100 km swath width) with either vertically or horizontally polarised radiation; the same polarisation is used for transmit and receive (i.e., HH or VV). The ground resolution is about 30 m (three looks), sampled at pixel separation of 12.5 m.

In the alternating polarisation mode (in one of seven possible swaths), two images in two polarization modes (HH & VV, or HH & HV, or VV & VH) are acquired. The ground resolution is about 30 m (1.8 equivalent number of looks), sampled at 12.5 m spacing.

When operating in the ScanSAR mode, a wide swath of > 400 km can be achieved, at a ground resolution of about 150 m (11.5 equivalent number of looks), sampled at 75 m pixel spacing.

  NASA SIR-C/X-SAR Mission

The multifrequency, multipolarization Synthetic Aperture Radar (SAR) system allows measurement of radar backscattering signature of the Earth's surface from space, at three different frequencies (L, C and X-bands) and at different polarizations.

The SIR-C instrument is a two-frequency radar including L-band (23 cm wavelength) and C-band (6 cm wavelength) with four polarizations (HH, HV, VH, VV). The X-SAR instrument is a single-frequency radar with X-band (3 cm wavelength) and vertical polarization (VV).

  SRTM - Shuttle Radar Topography Mission

The SRTM - Shuttle Radar Topography Mission is an international project of the National Aeronautics and Space Administration (NASA), National Imagery and Mapping Agency (NIMA) of the United States, the German Aerospace Center (DLR) and the Italian Space Agency (ASI). Its objective is to obtain the most complete high-resolution digital topographic database of the Earth.

The SRTM payload onboard the Space Shuttle Endeavour was launched on February 11, 2000. The mission lasted 11 days. The radar instruments are similar to those deployed for the SIR-C/X-SAR mission. To acquire topographic (elevation) data, the SRTM payload was outfitted with two synthetic aperture radar antennas. One antenna was located in the Shuttle's payload bay, the other on the end of a 60 meter mast that extended from the payload bay once the Shuttle was in space. SRTM made use of a technique called radar interferometry to acquire topographic information. In radar interferometry, two radar images are taken from slightly different locations. Differences between these images allow for the calculation of surface elevation, or change. In SRTM, the two radar images required for performing interferometry are acquired simultaneously by the two antennas.

SRTM was launched into an orbit with an altitude of 233 km (nominal), and an inclination of 57 degrees. This allowed most the Earth's land surface that lies between 60 degrees north and 56 degrees south latitude to be covered by the SRTM radars. This is about 80 percent of the Earth's land mass.

There are two types of antenna panels: C-Band and X-Band. The C-band radar data processed at the Jet Propulsion Laboratory (JPL) provide near global digital elevation models (DEMs). Higher resolution DEMs (but not with global coverage) are produced from the X-Band radar data, processed and distributed by the German Aerospace Center, DLR.

Processing of the C-Band data is expected to take two years. The C-band data will be distributed through the United States Geological Survey's EROS Data Center. The final archive is expected to be delivered to USGS in October 2002. Ultimately, the final released SRTM DEM will be at 30 meters for the U.S. and at 90 meters for the rest of the world, although NASA and NIMA are still discussing these issues.

Source : http://www.crisp.nus.edu.sg