Remote Sensing: John Wilkin wilkin@marine.rutgers.edu IMCS Building Room 211C 732-932-6555 ext 251 Active microwave systems (1) Satellite Altimetry
Active microwave instruments Scatterometer (scattering from surface roughness) ocean vector winds Synthetic Aperture Radar (SAR) sea ice high resolution wind speed, surface roughness CODAR coastal ocean surface vector currents
Active microwave instruments Altimeters (nadir pointing radar) sea surface height (long wavelengths ~50 km) mesoscale currents, eddies, fronts thermal expansion significant wave height wind speed gravity and bathymetry ice sheets
Microwave energy is largely unaffected by the atmosphere: It has almost 100% transmission
Radar systems operate in the microwave region of the EM spectrum
Key Components of any Radar System Microwave transmitter electronic device used to generate the microwave EM energy transmitted by the radar Microwave receiver electronic device used to detect the microwave pulse that is reflected by the area being imaged by the radar Antenna electronic component through which microwave pulses are transmitted and received
The Radar Equation
The relationship between power received P and power transmitted P T is given by the radar equation P G R T P = 2 2 4π 4π (1) (2) (3) (1) Power of EM wave at range R. G = gain of antenna (2) Radiant intensity in the direction of the radar produced by scatter from a surface with a scattering cross-section σ (3) A e is antenna effective area σ R 1/(4πR 2 ) is isotropic spreading over range R A e
Satellite Altimeters altimeters are nadir-pointing satellite-based radars used to measure the height of the surface of the Earth transmit a radar pulse that is reflected from the Earth s surface measure the time it takes for the pulse to travel to Earth and back, t c = 3 x 10 8 m/s satellite altitude ~ 1200 km t = 2R/c = 0.008 s = 8 milliseconds Poseidon uses 1700 pulses per second range from satellite to surface is R = ½ ct where c = speed of light Precision Orbit Determination (POD) systems measure the altitude of the satellite above a reference ellipsoid
History of Altimetry Skylab 1973-1974 Seasat 1978 Geosat 1985-1990 Jason-1 2001 - GFO 1998 - Topex/ Poseidon 1992 - Envisat 2002 -
Altimetry: How it works Reference ellipsoid Satellite position is determined relative to an arbitrary reference surface, an ellipsoid. This reference ellipsoid is a raw approximation of Earth's surface, a sphere flattened at the poles. The altitude of Jason above the reference ellipsoid, distance S, is measured to within 3 cm.
Sea surface HEIGHT (SSH) Sea Surface Height is satellite altitude minus range It comprises two contributions: geoid and dynamic topography Geoid: The sea surface height that would exist without any motion. This surface is due to gravity variations around the planet due to mass and density differences on the seafloor Major bathymetric features deform sea level by tens of meters and are visible as a hill on the geoid Dynamic topography The ocean circulation comprises a permanent mean component linked to Earth's rotation, mean winds, and density patterns and a highly variable component (wind variability, tides, seasonal heating, eddies)
Sea surface HEIGHT (SSH) Sea Surface Height is satellite altitude minus range Geoid and dynamic topography: To derive the dynamic topography, D, the easiest way would be to subtract the geoid HEIGHT, G, from SSH In practice, the geoid is not yet known accurately enough for all applications and mean sea level is commonly subtracted instead. This yields the variable part of the ocean signal.
The slope of the sea surface relative to the geoid is directly related to the geostrophic current that balances the pressure gradient (due to the sea surface gradient) and the Coriolis force
Changes in sea surface topography Phenomenon Typical Surface Expression Period of Variability Comments Western boundary currents (Gulf Stream, Kuroshio) 130 cm/100 km Days to years Variability in position, and 25% variability in transport Large gyres 50 cm/ 3000 km One to many years Eastern boundary currents 25% variability expected 30 cm/100 km Days to years 100% variability expected, possible direction reversals Mesoscale eddies 25 cm/100 km 100 days 100% variability Rings 100 cm/100 km Weeks to years 100% variability, growth and decay Equatorial currents 30 cm/5000 km Months to years 100% variability Tides 100cm/5000 km Hours to years Aliased to low frequency
Jason satellite AVISO Web site http://www.jason.oceanobs.com/html/missions/jason/welcome_uk.html
Jason launch movies
Satellite orbit and tracking The critical orbital parameters for satellite altimeter missions are altitude, inclination and period Topex/Poseidon and Jason satellites (same orbit) altitude 1336 km relatively high: less drag and more stable orbit inclination of 66 to Earth's polar axis it can "see" only up to 66 North and South the satellite repeats the same ground track every 9.9156 days the ground-tracks are 315 km apart at the equator track repeat precision is about 1km ground scanning velocity is 5.8 km/s, orbit velocity 7.2 km/s
Where is Topex now? Where is Jason now?
Geostrophic current computed from altimeter sea surface height gives only the component perpendicular to the ground-track. To get surface geostrophic current vectors we need to map the SSH field in two dimensions. The high alongtrack resolution (20km) is then lost because of the large separation of the ground-tracks (315 km at Equator) Where is Jason now?
(a) (b) (c) Grid of sea surface height measurements by T/P, ERS-2 and GFO in the Northeast Atlantic over (a) 10 days, (b) 7 days, and (c) and 3 days. There are gaps in coverage of 200 km and more over 3 days. Combining data from all three missions increases coverage. => Multiple satellites are required to resolve mesoscale current patterns
Altimetry: How it works For altimeter observations to be useful for oceanography, range accuracy of order 2 cm is required. Where is Jason now?
The challenges to achieving 2 cm accuracy are: computing the satellite position accurately range corrections for the atmosphere density of atmosphere, water vapor accounting for the aliasing of tides knowing the shape of a reference gravitational potential surface, or geoid, that defines a surface along which gravity is constant (and therefore dynamically level )
Precision Orbit Determination The Jason satellite is tracked in 3 ways 1. Turbo-Rogue Space Receiver (TRSR) continuously tracks up to 16 GPS satellites measures phase of carrier signals and pseudo-range (time) to estimates position to better than 20 m and time to 100 nanoseconds 2. Laser Retroflector Array (LRA) an array of mirrors on the satellite that provide a target for lasertracking measurements from ground stations round-trip time of the laser is another range measurement accuracy is a few mm, but only 10 to 15 stations are in operation 3. DORIS receivers on the satellite measure Doppler shift of signal from groundstation beacons (2 frequencies) gives satellite velocity a dynamic orbit model integrates the velocity and position data, drag, solar forces on satellite, to continuously compute the satellite trajectory Where is Jason now?
Sea state affects the radar reflection
Corrections that must be applied in the range calculation: (1) Sea state bias Electromagnetic bias difference between height of mean sea level and mean scattering surface backscatter from small wave facet is proportional to curvature of long wave part of spectrum ocean troughs have a larger radius of curvature than wave crests greater reflection from wave troughs than wave crests induces an EM sea level bias toward wave troughs Skewness bias non-gaussian distribution of the sea surface height shifts the median from the mean sea level toward wave troughs adding to the EM bias towards wave troughs Where is Jason now?
Corrections that must be applied in the range calculation: (2) Index of refraction (speed of light through atmosphere) Ionospheric correction variation in the number of free electrons present in the sub-satellite ionosphere electron content varies from day to night (few free electrons at night), from summer to winter (fewer during summer), and as a function of the solar cycle (fewer during the solar minimum) Tropospheric correction water vapor and other gases present in the subsatellite troposhere the dry troposphere correction can be modeled via surface pressure measurements the wet correction uses measurements from an onboard radiometer the dry term includes the weight of the water molecules while the wet term accounts for their influence on the index of refraction.
Where is Jason now?
Applications http://sealevel.jpl.nasa.gov/science/investigations.html
Applications http://sealevel.jpl.nasa.gov/science/applications.html
Future of Altimetry Cryosat (ESA) Altimeter dedicated to polar observation High inclination orbit 92 o, 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 can operate in SAR mode Launch July 2005 Where is Jason now?
Future of Altimetry 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 seasurface 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 km Where is Jason now?
WSOA on Jason-2 Three 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 geometry
Comparison 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 rms on the zonal velocity and 5.9 cm/s on meridional velocity.