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Слайды и текст этой презентации

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Radar and Satellite Remote Sensing Chris Allen, Associate Director – Technology Center for Remote Sensing of Ice Sheets The University of Kansas

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Outline Background – ice sheet characterization Radar overview Radar basics Radar depth-sounding of ice sheets Example of capabilities of modern radars Synthetic-aperture radar (SAR) Satellite sensing Spaceborne radars Satellite radar data products Future directions

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Background Sea-level rise resulting from the changing global climate is expected to directly impact many millions of people living in low-lying coastal regions. Accelerated discharge from polar outlet glaciers is unpredictable and represents a significant threat. Predictive models of ice sheet behavior require knowledge of the bed conditions, specifically basal topography and whether the bed is frozen or wet. The NSF established CReSIS (Center for Remote Sensing of Ice Sheets) to better understand and predict the role of polar ice sheets in sea-level change.

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CReSIS technology requirements: Radar Technology requirements are driven by science, specifically the data needed by glaciologists to improve our understanding of ice dynamics. The radar sensor system shall: measure the ice thickness with 5-m accuracy to 5-km depths detect and measure the depth of shallow internal layers (depths < 100 m) with 10-cm accuracy measure the depth to internal reflection layers with 5-m accuracy detect and, if present, map the extent of water layers and water channels at the basal surface with 10-m spatial resolution when the depth of the water layer is at least 1 cm provide backscatter data that enables bed roughness characterization with 10-m spatial resolution and roughness characterized at a 1-m scale

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CReSIS technology requirements: Radar The radar sensor system shall: detect and, if present, measure the anisotropic orientation angle within the ice as a function of depth with 25° angular resolution measure ice attenuation with 100-m depth resolution and radiometric accuracy sufficient to estimate englacial temperature to an accuracy of 1 °C detect and, if present, map the structure and extent of englacial moulins

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A brief overview of radar Radar – radio detection and ranging Developed in the early 1900s (pre-World War II) 1904 Europeans demonstrated use for detecting ships in fog 1922 U.S. Navy Research Laboratory (NRL) detected wooden ship on Potomac River 1930 NRL engineers detected an aircraft with simple radar system World War II accelerated radar’s development Radar had a significant impact militarily Called “The Invention That Changed The World” in two books by Robert Buderi Radar’s has deep military roots It continues to be important militarily Growing number of civil applications Objects often called ‘targets’ even civil applications

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A brief overview of radar Uses electromagnetic (EM) waves Frequencies in the MHz, GHz, THz Shares spectrum with FM, TV, GPS, cell phones, wireless technologies, satellite communications Governed by Maxwell’s equations Signals propagate at the speed of light Antennas or optics used to launch/receive waves Related technologies use acoustic waves Ultrasound, seismics, sonar Microphones, accelerometers, hydrophones used as transducers

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A brief overview of radar Active sensor Provides its own illumination Operates in day and night Largely immune to smoke, haze, fog, rain, snow, … Involves both a transmitter and a receiver Related technologies are purely passive Radio astronomy, radiometers Configurations Monostatic transmitter and receiver co-located Bistatic transmitter and receiver separated Multistatic multiple transmitters and/or receivers Passive exploits non-cooperative illuminator

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A brief overview of radar Various classes of operation Pulsed vs. continuous wave (CW) Coherent vs. incoherent Measurement capabilities Detection, Ranging Position (range and direction), Radial velocity (Doppler) Target characteristics (radar cross section – RCS) Mapping, Change detection

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Radar basics

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Radar basics Range resolution The ability to resolve discrete targets based on their range is range resolution, R.

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Radar basics Doppler frequency shift and velocity Time rate of change of target range produces Doppler shift.

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Radar basics

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Synthetic-aperture radar (SAR) concept

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SAR image perception

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Recent field campaigns: Greenland 2007

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Illustration of the airborne depth-sounding radar operation

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Surface clutter For airborne (or spaceborne) radar configurations, radar echoes from the surface of the ice and mask the desired internal layer echoes or even the echo from the ice bed. These unwanted echoes are called clutter. Clutter refers to actual radar echoes returned from targets which are by definition uninteresting to the radar operators. System geometry determines the regions whose clutter echo coincide with the echoes of interest.

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Wide bandwidth depthsounder

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Accumulation radar system

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Radar depth sounding of polar ice Multi-Channel Radar Depth Sounder (MCRDS) Platforms: P-3 Orion Twin Otter Transmit power: 400 W Center frequency: 150 MHz Pulse duration: 3 or 10 s Pulse bandwidth: 20 MHz PRF: 10 kHz Rx noise figure: 3.9 dB Tx antenna array: 5 elements Rx antenna array: 5 elements Element type: /4 dipole folded dipole Element gain: 4.8 dBi Loop sensitivity: 218 dB Provides excellent sensitivity for mapping ice thickness and internal layers along the ground track.

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Multichannel SAR To provide wide-area coverage, a ground-based side-looking synthetic-aperture radar (SAR) was developed to image swaths of the ice-bed interface. Key system parameters Center frequency: 210 MHz Bandwidth: 180 MHz Transmit power: 800 W Pulse duration: 1 and 10 s Noise figure: 2 dB PRF: 6.9 kHz Rx antenna array: 8 elements Tx antenna array: 4 elements Antenna type: TEM horn Element gain: ~ 1 dBi Loop sensitivity: 220 dB Dynamic range: 130 dB # of Tx channels: 2 # of Rx channels: 8 A/D sample frequency: 720 MHz # of A/D converter channels: 2

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Depthsounder data The slower platform speed of a ground-based radar, its increased antenna array size, and improved sensitivity and range resolution enhance the radar’s off-nadir signal detection ability. This essential for mapping the bed over a swath. Frequency-wavenumber (f-k) migration processing is applied to provide fine along-track resolution. Using a 600-m aperture length provides about 5-m along-track resolution at a 3-km depth.

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SAR image mosaic First SAR map of the bed produced through a thick ice sheet. SAR image mosaics of the bed terrain beneath the 3-km ice sheet are shown for the 120-to-200-MHz band and the 210-to-290-MHz band (next slide). These mosaics were produced by piecing together the 1-km-wide swaths from the east-west traverses.

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SAR interferometry – how does it work?

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Satellite sensing

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SAR image of Gibraltar

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SAR imagery of Venus

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SAR imaging characteristics Range Res ~ pulse width Azimuth = L /  m resolution with 3 looks)

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Single-pass interferometry

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Topographic map of North America

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Multipass interferometric SAR (InSAR)

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Digital elevation mapping with InSAR

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Surface velocity mapping with InSAR Multipass InSAR mapping of horizontal displacement provides surface velocities.

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Future directions System refinements Eight-channel digitizer (no more time-multiplexing) (6 dB improvement) Reduced bandwidth from 180 MHz to 80 MHz (140 to 220 MHz) to avoid spectrum use issues. Signal processing Produce more accurate DEM using interferometry. Produce 3-D SAR maps showing topography and backscattering. Platforms Migrate system to airborne platforms (Twin Otter, UAV). Meridian UAV Take-off weight: 1080 lbs Wingspan: 26.4 ft Range: 1750 km Endurance: 13 hrs Payload: 55 kg