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
Microwave energy is largely unaffected by the atmosphere: It has almost 100% transmission
Radar systems operate in the microwave region of the EM spectrum K u -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 Radar Equation
The relationship between: power received P and power transmitted P T is given by the radar equation = P G R σ R T P 2 2 4π 4π A e (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 (which depends on area of target, fraction of incident radar pulse absorbed and scattered) (3) A e is antenna effective area 1/(4πR 2 ) 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 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 - ERS-1 1991-2000 ERS-2 1995 - GFO 1998 - Envisat 2002 - Topex/Poseidon 1992 1/5/2006
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.
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-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
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 making 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) 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 (at 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?