Remote Sensing John Wilkin Active microwave systems 1 Satellite Altimetry jwilkin rutgers edu IMCS Building Room 214C 732 932 6555 ext 251 Active microwave instruments Scatterometer scattering from surface roughness ocean vector winds Synthetic Aperture Radar SAR sea ice high resolution wind speed land mapping surface roughness and 3 D terrain CODAR coastal ocean surface vector currents Altimeter 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 http www aviso oceanobs com http topex www jpl nasa gov http earth esa int brat html general overview en html Microwave energy is largely unaffected by the atmosphere It has almost 100 transmission Radar systems operate in the microwave region of the EM spectrum Ku band 13 6 GHz C band 5 3 GHz Poseidon dual frequency altimeter http topex www jpl nasa gov technology instrument altimeter html 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 or received usually shared on satellite systems The relationship between power received P and power transmitted PT is given by the radar equation PT G P Ae 2 2 4 R 4 R 1 2 1 Power of EM wave at range R 3 G gain of antenna 2 Radiant intensity in the direction of the radar produced by scatter from a surface with a scattering cross section which depends on area of target fraction of incident radar pulse absorbed and scattered 3 Ae is antenna effective area 1 4 R2 is isotropic spreading over range R in both transmitted and received signal 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 c 3 x 108 m s satellite altitude 1200 km measure the time it takes for the pulse to travel to Earth and back t t 2R c 0 008 s 8 milliseconds Poseidon uses 1700 pulses per second range from satellite to surface is R ct where c speed of light Precision Orbit Determination POD systems measure the altitude of the satellite above a reference ellipsoid Skylab 1973 1974 ERS 2 1995 Seasat 1978 Geosat 1985 1990 History of Altimetry ERS 1 1991 2000 Envisat 2002 GFO 1998 Topex Poseidon 1992 1 5 2006 Jason 1 2001 OSTM Jason 2 2008 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 not flat because of gravity variations around the planet due to mass and density differences associated with the seafloor The geoid is a geopotential surface Major bathymetric features deform sea level by tens of meters and are visible as hills and valleys of 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 mesoscale eddies Sea surface HEIGHT SSH Sea Surface Height is ssh altitude minus range Geoid and dynamic topography To derive the dynamic topography D the easiest way would be to subtract the geoid HEIGHT from SSH In practice the geoid is not yet known accurately enough for many applications and mean sea level is commonly subtracted instead This yields the variable part of the ocean signal Geoid height meters 80 80 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 The long term mean ocean circulation has an associated mean dynamic topography that is a permanent component of the time mean orbit altitude as a function of position Jason 1 satellite AVISO Web site http www aviso oceanobs com en missions current missions index 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 Jason 1 Research Ground segment OSTM Jason 2 Operational Ground segment Tracking Operational real time products ground station redundancy archive Delayed mode reanalysis for research quality datasets 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 Tropical Cyclonic Heat Potential computed from altimetry on 28 August 2005 with Hurricane Katrina s trajectory and intensity overlaid Katrina s intensification seems to coincide with its crossing over the Loop Current Altimetry data in combination with historical hydrographic observations are currently used to estimate synthetic upper ocean temperature profiles These profiles are then used to compute the integrated vertical temperature from the sea surface down to the 26 C isotherm the temperature needed to sustain a