Sensor Description... RADARSAT - 1
Background... The RADARSAT - 1 program originated as an ambitious program undertaken by the Canadian Space Agency to develop and launch the world’s first civilian operational radar-equipped satellite. RADARSAT -1 marks Canada’s entry into remote sensing satellites and was the beginning of a new Canadian legacy; RADARSAT - 2 is the next generation Canadian radar satellite and is scheduled for launch in late 2003.
In addition to serving the national needs of government partners, RADARSAT - 1 was designed to generate additional data for use in the commercial marketplace. With its high resolution and broad swath, and the fact that the data can be used in conjunction with imagery from other satellites, RADARSAT - 1 imagery has significant utilization and commercial value.
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At the core of the RADARSAT - 1 program is a commitment to providing a new look at worldwide resource and environmental management information. RADARSAT - 1 SAR penetrates cloud and darkness to obtain reliable, quality images of the Earth’s surface. This provides a definite advantage in viewing such areas as polar regions and equatorial and coastal areas where clouds and foggy conditions preclude visual satellite observation or even the operation of aircraft.
RADARSAT - 1 offers selectable configuration to view smaller areas in greater detail or to cover large areas in lesser detail. The coverage available through ScanSAR provides more frequent coverage, ranging from daily in the polar regions to coverage every six days at the equator. The system is designed to be responsive to user needs in that downlinking systems to ground receiving stations combine the capability of providing near real-time processing of the data. The on-board recorders will allow gathering of data anywhere on the Earth’s surface.
RADARSAT is equipped with a state-of-the-art Synthetic Aperture Radar (SAR) that has the following features:
- Multiple image modes for optimized detection capabilities
- ScanSAR capability for wide area coverage
- Fine beam capability for high resolution uses
- Cloud-free images of the earth
- Archived data since 1996
- Global data availability
- Near real time processing
- Electronic data delivery
- Frequent revisit for monitoring and emergency response
- Programming for emergencies and priorities
- Data calibration for change detection studies
- The RADARSAT SAR can be steered to collect data over a 1,175 km swath using 7 beam modes. This provides users with superb flexibility in acquiring images with a range of resolutions, incidence angles, and coverage areas.
- C-Band wavelength (5.6cm)
- HH polarization (Horizontally transmitted, Horizontally received)
- Resolution: 8 - 100 m
- 35 Incidence Angles: 10 - 59º range
- Right-looking, steerable antenna
- Swath Width: 50 - 500 km range
- 7 beam modes, 35 beam positions
- Repeat Cycle: 24 days
- Stereoscopic Viewing
The RADARSAT SAR instrument consists of a radar transmitter, a radar receiver and a data downlink transmitter. The radar transmitter and receiver operate through an electrically steerable antenna that directs the transmitted energy in a narrow beam normal to the satellite track. The elevation angle and the elevation profile of the beam (beam positions) can be adjusted so that the beam intercepts the earth's surface over the desired range of incidence angles. The ability to choose the beam and position is important because image characteristics vary with the incidence angle associated with each beam. In addition, by varying the characteristics of the transmitted pulses and the receiver timing, different resolution and coverage can be achieved. The beam modes are each characterized by a specific beam elevation angle and profile, as shown below.
Beam Mode Descriptions
Within each RADARSAT beam mode, a number of incidence angle positions are available. These are called beam positions. For example, Standard beam mode, which covers a 100 x 100 km area, has seven beam positions. By specifying a beam position, one of seven 100 x 100 km images within a 500 km accessible swath will be collected.
RADARSAT Beam Modes
The RADARSAT can be steered to image over a 500 km swath using a number of imaging modes. This provides users with superb flexibility in acquiring images with a range of resolutions, incidence angles, and coverage areas.
For any given beam mode, the same beam angle and profile is used for both transmit and receive. The receiver detects the echo resulting from backscattering of the transmitted signal from the earth's surface. The detected signal is then digitized and encoded prior to transmission to the on-ground data reception facility. Data transmission may occur in real time as the data is collected, or the data may be stored on the on-board tape recorders (OBR) for later transmission.
Subsequent processing of the signal allows the formation of high resolution radar images of the earth's surface. Further processing of this data permits extraction of a variety of geophysical data.
The SAR instrument may be operated in one of two modes:
1. Single Beam
Single Beam Mode
- In Single Beam Mode operation, the beam elevation and profile are maintained constant throughout the data collection period. A beam is characterized by its:
- nominal incidence angle
- nominal swath width
- nominal spatial resolution
Single Beam Operational Modes
- Fine Beam - The Fine beam mode is intended for applications which require the best spatial resolution available from the RADARSAT system. The azimuth resolution is 8.4 m, with range resolution 9.1 m to 7.8 m from F1 to F5. Since the radar operates with a higher sampling rate in this mode than in any of the other beam modes, the ground swath coverage has to be reduced to a nominal 50 km in order to keep the downlink signal within its allocated bandwidth. Originally, five Fine beam positions, F1 to F5, were available to cover the far range of the swath, the incidence angle range from 37 to 47 degrees. By modifying timing parameters, 10 new positions have been added with offset ground coverage. Each original Fine beam position can either be shifted closer to or further away from Nadir. The resulting positions are denoted by either an N (Near) or F (Far). For example, F1 is now complemented by F1N and F1F.
- Standard Beam -
Standard beam mode operates with any one of seven beam positions, referred to as S1 to S7. The nominal incidence angle range covered by the full set of Standard beam is from 20 degrees (at the inner edge of S1) to 49 degrees (at the outer edge of S7). The spatial resolution is 27 m in azimuth and 26 m (near) to 20 m (far) in range direction. Each individual beam covers a minimum ground swath of 100 km within the total 500 km accessibility swath of the full set of Standard beams. The nominal spatial resolution in the range direction is 26 m for S1 at near range to 20 m for S7 at far range. The azimuth resolution is the same, 27 m, for all beam positions.
Standard beam modes allow imaging over a wide range of incidence angles with optimum system image quality.
- Wide Beam - Wide beams are similar to the Standard beams except that the swath width achieved by this beam is 150 km rather than 100 km. As a result, only three Wide beams, W1, W2 and W3 are necessary to provide coverage of almost all of the 500 km swath range. They provide the comparable resolution as the standard beam mode although the increased ground swath coverage is obtained at the expense of a slight reduction in overall image quality. W1 and W2 are available for single beam products. W3 is used as one of the beams to form SWA product but not recommended for individual image because it contains a nadir ambiguity (narrow white vertical strip in the image).
- Extended High Beam -
Six Extended High beam positions, EH1 to EH6, are available for collection of data in the 49 to 60 degree incidence angle range. Since this beam mode operates outside the optimum scan angle range of the SAR antenna, some minor degradation of image quality can be expected when compared with the Standard beam mode. Swath widths are restricted to a nominal 80 km for the inner three positions, and 70 km for the outer three positions. The operational beam positions on satellite are EH1, EH3, EH4 and EH6, which can cover the complete Extended High beam swath.
- Extended Low Beam -
A single Extended Low beam position, EL1, is provided for imaging in the incidence angle range 10 to 23 degrees with a nominal ground swath coverage of 170 km. As with the Extended High beam mode, some minor degradation of image quality can be expected due to operation of the antenna beyond its optimum elevation angle range.
In ScanSAR operation either two, three or four single beams are used during data collection. Each beam is selected sequentially so that data is collected from a wider swath than is possible with a single beam. The beam switching rates are chosen to ensure at least one "look" at the earth's surface for each beam within the along track illumination time or dwell time of the antenna beam. In practice, the radar beam switching is adjusted to provide two looks per beam.
The beam multiplexing inherent in ScanSAR operation reduces the effective sampling rate within each of the component beams, hence the increased swath coverage is obtained at the expense of spatial resolution.
RADARSAT-1 is in a near-polar, sun-synchronous orbit 798 km above the Earth. It has a dawn-to-dusk orbit, meaning it crosses the equator at dawn and dusk, and is rarely in eclipse or darkness. The orbit characteristics are:
Geometry circular, sun-synchronous (dawn-dusk)
Altitude 798 km
Period 100.7 minutes
Repeat Cycle 24 days
Orbits Per Day 14
Moving at 7 km per second, RADARSAT circles the Earth 14 times per day in its near-polar orbit. Its trajectory traces an elliptical pathway at an inclination of 98.6 degrees to the equatorial plane. This dawn-dusk orbit allows RADARSAT's solar arrays to continuously draw energy from the sun.
RADARSAT is a right-looking sensor, facing east during the ascending orbit and west during the descending orbit. The look direction can have a significant influence on the appearance of radar imagery especially when features have an organized linear structure (e.g., agricultural surfaces or geological structures).
To some extent, RADARSAT's orbit characteristics will determine the specific date when an acquisition can be made and the number of days between subsequent acquisitions. The revisit schedule for a specific location will depend on:
- the latitude of the location,
- your desired beam mode, and
- the flexibility in changing the beam position.
RADARSAT-1 requires 24 days to return to its original orbit path. This means that for most geographic regions, it will take 24 days to obtain exactly the same image (same beam mode, position, and geographic coverage). However, RADARSAT's imaging flexibility allows images to be acquired on a more frequent basis.
Because RADARSAT's ScanSAR beam mode covers such a large area, a location can be viewed as frequently as once a day in high latitudes and less than once every five days at the equator. For other beam modes, the beam position can be changed so a location can be imaged more frequently.
Additional information about RADARSAT-1 can also be found at