Slide 1Active microwave instrumentsSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Sea surface HEIGHT (SSH)Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Satellite orbit and trackingSlide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Precision Orbit DeterminationSlide 29Slide 30Slide 31Remote Sensing: John WilkinActive microwave systems(1) Satellite [email protected] Building Room 214C732-932-6555 ext 251Active 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•AltimeterActive 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 sheetshttp://topex-www.jpl.nasa.govhttp://www.aviso.oceanobs.comhttp://earth.esa.int/brat/html/general/overview_en.htmlMicrowave energy is largely unaffected by the atmosphere: It has almost 100% transmissionRadar systems operate in the microwave region of the EM spectrum Ku-band 13.6 GHz C-band 5.3 GHzPoseidon dual-frequency altimeter http://topex-www.jpl.nasa.gov/technology/instrument-altimeter.htmlKey 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 (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  (which depends on area of target, fraction of incident radar pulse absorbed and scattered)(3) Ae is antenna effective area1/(4R2) is isotropic spreading over range R in both transmittedand received signaleTARRGPP2244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• 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• c = 3 x 108 m/s• satellite altitude ~ 1200 km• t = 2R/c = 0.008 s = 8 milliseconds• Poseidon uses 1700 pulses per secondHistory of AltimetrySkylab 1973-1974Seasat 1978 Geosat 1985-1990Topex/Poseidon1992 – 1/5/2006Jason-1 2001 -GFO 1998 -Envisat 2002 -ERS-2 1995 -OSTM/Jason-2 2008 -ERS-1 1991-2000Reference ellipsoidSatellite 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.Altimetry: How it worksSea 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 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.Sea surface HEIGHT (SSH)Geoid height (meters)80-80The 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 forceThe 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 satelliteAVISO Web sitehttp://www.aviso.oceanobs.com/en/missions/current-missions/index.htmlJason launch moviesSatellite 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/sWhere is Jason now?Where is Topex now?Jason-1: ResearchGround segmentOSTM/Jason-2: OperationalGround segmentOperational real-time products; ground station redundancy; archiveDelayed-mode reanalysis for research quality datasetsTrackingGeostrophic 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?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