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Rutgers University MS 552 - Active microwave systems

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Remote Sensing:John WilkinActive microwave systems (3)more Altimeter …ScatterometerSynthetic Aperture Radar (SAR)[email protected] Building Room 214C732-932-6555 ext 251• Cryosat (ESA)• Altimeter dedicated to polar observation• High inclination orbit 92o , 710 km altitude • 3½ -year mission to determine variations in the thickness of the continental ice sheets and marine ice cover• Test the predictions of thinning arctic ice due to global warming • Low resolution nadir altimeter– operates in SAR mode over ice for increased spatial resolution• Launch 8 October 2005 (oops!)• Cryosat 2 should launch in 2009Future of AltimetryRockot launch vehicle is a modified SS-19ICBM1- 1502 GMT: Cryosatlaunches from Plesetsk, northern Russia2 - 1504: First stage separation 3 - 1508: Second stage separation due - but scientists believe that a software error meant this did not happen. The rocket plunged back to Earth when its fuel ran out. 8 October 2005Plesetsk Cosmodrome, RussiaIceSat (US)• High inclination orbit 94o• Geoscience Laser Altimeter System (GLAS), a space-based LIDAR.– a combined precision surface LIDAR and dual-wavelength cloud and aerosol LIDAR– infrared and visible laser pulses at 1064 and 532 nm wavelengths. • GLAS produces a series of ~70 m diameter laser spots separated by 170 m along ground track.Future of AltimetryOcean Surface Topography Mission (OSTM) will be a follow-on to the Jason mission. It is scheduled to launch June 15, 2008. • same orbit of Topex/Jason, extending the time series of precision sea level height data that began Sept 1992 • Poseidon-3 altimeter• research modes integrated with DORIS and onboard digital terrain model for more precise tracking over inland waters and coasts•POD• LRA (NAS)• DORIS (CNES)• new NASA Global Positioning System Payload (GPSP)• WSOA: the Wide Swath Ocean Altimeter– An altimeter/interferometer project• Several altimeters mounted on masts will acquire measurements simultaneously, providing continuous wide-area coverage. • WSOA is based on a technique combining altimeter and interferometer measurements. It is a wide-field radar altimeter able to measure sea-surface height across a swath centered on the satellite ground track. • The satellite payload will include: – dual-frequency, nadir-looking radar altimeter in Ku and C bands– to provide ionospheric corrections– acquire measurements as accurate as Topex and the Jason– A three-channel radiometer – GPS, Doris and laser reflector precise orbit determination – WSOA, comprising two interferometers mounted on a mast, with a baseline of 6.4 m each covering a swath of 15 to 100 kmFuture of AltimetryWhere is Jason now?WSOA on a Jason-type bus Wide-Swath Ocean AltimeterThe interferometer principle measures the relative delay between reflected signals at 2 antennas separated by a “baseline distance”. The range measurements from the 2 antennas and the baseline form a triangle used for determining the location of the target in the observation plane. The triangle is made up of the baseline B, and the ranges from the target to the two antennas, r1 and r2. The baseline is known from the design of the instrument and the spacecraft attitude. Range r1 is determined by system timing measurements. The range difference between r1 and r2 is determined by measuring the relative phase shift between the two signals, related to the range difference ΔrbyWide-Swath Ocean AltimeterThree factors underlying measurement uncertainty:• Measurement noise, which depends on the antenna baseline (longer baseline = less noise). With an antenna baseline of 6.4 m the raw noise is 5.2 cm• Ionospheric, tropospheric and sea-state bias effects (estimated at 1 to 2 cm)• Errors from satellite roll and pitch steering which impact measurement geometryComparison of T/P+Jason-1 measurements and simulated WSOA data (with Topex/Poseidon shifted into an orbit parallel to Jason-1). This mosaic offers a huge advantage in terms of describing the dynamic topography at high resolution:• It allows a measure of sea surface gradient between pixels and,therefore, geostrophic velocity• Simulations based on realistic model data yield an error of 4.7 cm/s rmson the zonal velocity and 5.9 cm/s on meridional velocity.• Ka-band altimeter (500 MHz) can be flown on a microsatellite or as an auxiliary• Signal frequencies in the Ka-band will enable better observation of ice, rain, coastal zones,land masses (forests, etc.), and wave heights– Sensitivity to water vapor in troposphere is high, so data are lost when rain rate is > 1.5 mm hour-1• Part of plans to develop an operational satellite altimetry system• Light weight and low power consumption. Better signal-to-noise ratio than Poseidon 2 (in Ku band), reducing noise to < 1 cm• Antenna lobe is narrower and the ground footprint smaller. With a higher pulse repetition frequency, this allows more precise measurements near sea/land boundaries ( 5 km from the shoreline) and over inland water bodies.• Low ionospheric attenuation, eliminating the need for a dual-frequency altimeter.• Better description of sea surface roughness than in Ku. The 8 mm wavelength is better suited to describing the slopes of small facets on the sea surface (capillary waves, etc.).• Lower penetration of snow/ ice (1 cm compared to 5 m in Ku). Improves snow data with respect to aging of ice in the surface layers. Ice grain size would also be measurable.Future of Altimetry - AltiKa• Scatterometers– satellite borne– ocean surface vectors winds• Incorporated into ECMWF meteorological analysis• Synthetic Aperture Radar (SAR) – satellite and aircraft– high spatial resolution (tens of meters)– image ocean surface wave field and, by inference, processes thatmodulate the surface wavesAll these systems exploit the resonant “Bragg” scattering of centimeter to decameter wavelength microwave radiation from ocean surface roughness due to short waves.Active microwave systems (3)Scatterometers and SARBragg Scattering from Water SurfacesWind creates small waves on the ocean surface (capillary waves) which in the absence of wind quickly die outIf wind continues, waves will grow in size and increase in wavelength and height to become ultra-gravity waves and eventually gravity wavesA water surface affected by wind will have a spectrum of surface waves, e.g., multiple wavelengths and heightsMicrowave EM energy


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Rutgers University MS 552 - Active microwave systems

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