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MR3522MR3522 - Remote Sensing of the Atmosphere and Ocean - Winter Winter 19991999

Active Microwave RadarActive Microwave Radar

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Operation

= 0

transmit a very short pulse (t = 3 ns or 1 m), therefore the pulse has some thickness and width (determined by gain, GT)

Signal Processing

Time delay gives satellite-surface distance

Consider the earth covered with water, no atmosphere, and no currents

then water surface is an equipotential surface (equal gravitational force)

variations in mass concentration in the earth will distort the surface.

*

*

*

an atmosphere will distort the surface

H

L

also tides will distort the surface

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Definitions

geop: equipotential surface geoid: geop at mean sea level (approximately an ellipsoid) geoid undulation: difference between reference ellipsoid and geoid (~60m) sea surface topography: difference between sea surface and geoid

contributions to sea surface topography:

tides - 1 meter

currents - 1m/100km

atmospheric pressure - few cm

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The Geoid provides information about the ocean bottom topography

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The Sea Surface Topography provides information about ocean currents

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Errors in measurement of ocean height

orbit: 0.3-0.5 m (dominant), laser tracking can reduce this for short times coordinate systems: < 10-20 cm, includes: tracking network, geoid description

inertial reference frame, time and position of earth ionosphere: ~0.5 cm atmospheric gases: Airmass ~ 2.3 m (correctable to ~0.7 cm),

water vapor - 6-30 cm (correctable to ~1 cm) ocean waves: troughs reflect better than crests (1-2 cm bias) rain: failure if rain rate> 5mm/hr (avoidable) calibration/instrument errors: ~7 cm

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Scattering cross Section, o

There are two primary mechanisms which contribute to o

(1) Specular Reflection

Near vertical incidence

reflection of mirror-like facets

Important for< 20° (almost no wave slopes > 25°)

0

0 10 20 30

increasedwind speed

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The Received Power W(t) provides information about:

Significant Wave Height (SWH)Wind Speed

Greater SWH

Greater t from first photon received to peak

Greater Wind Speed

Less total power received

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Standard Error in Altimeter Wind Speed estimates is about 2 m/s

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During August 1992, TOPEX/Poseidon was launched into Earth's orbit by an Ariane 42P rocket from the European Space Agency's Space Center located in Kourou, French Guiana -- the first launch of a NASA payload from this site. From its orbit 1,336 kilometers (830 miles) above the Earth's surface, TOPEX/Poseidon measures sea level along the same path every 10 days using the dual frequency altimeter developed by NASA and the CNES single frequency solid-state altimeter. This information is used to relate changes in ocean currents with atmospheric and climate patterns. Measurements from NASA's Microwave Radiometer provide estimates of the total water-vapor content in the atmosphere, which is used to correct errors in the altimeter measurements. These combined measurements allow scientists to chart the height of the seas across ocean basins with an accuracy of less than 13 centimeters (5 inches)!

TOPEX/PoseidonTOPEX/Poseidon

The dual-frequency NASA radar altimeter is the primary instrument aboard the spacecraft. It works by sending radio pulses at 13.6 GHz and 5.3 GHz toward the earth and measuring the characteristics of the echo. By combining this measurement with data from the microwave radiometer and with other information from the spacecraft and the ground, scientists can calculate the height of the sea surface to within 4.3 centimeters.

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Mass: <= 96 kg Antenna diameter: 1.2 m DC power: <=134.5 W Data rate: <= 15 kbit/sec RF frequency: 13.8 GHz (Ku band) Bandwidth: ocean mode : 330 MHz ice mode : 82.5 MHz Pulse repetition frequency: 1020 Hz RF transmit power: 50 W Pulse length: 20 micro-s chirp Altitude measurement: 10 cm (1s, SWH = 16 m) Significant wave height: 0.5 m or 10% (1s) whichever is smaller Backscatter coefficient: 0.7 dB (1s) Echo waveform samples: 64 x 16 bits at 20 Hz Beam width: 1.3deg. Foot Print: 16 to 20 km (depending on sea state)

ERS-2 Altimeter

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The 300-kg spacecraft is approximately 3-m long and supports the following payload

Radar Altimeter - single frequency (13.5 GHz) with 3.5-cm height precision.

Water Vapor Radiometer - dual frequency (22 and 37 GHz) nadir-looking with a path correction accuracy of 1.9 cm rms.

GPS Receivers - redundant receivers allow precision orbit determination with an rms accuracy in the radial component of 10 cm on wavelengths less than 40,000 km.

Doppler Beacon - GEOSAT performance-stable oscillators and doppler beacons will allow operational orbits to be determined with 1.8-cm rms radial orbit error for mesoscale oceanography (after tilt and bias removal along a 3000-km arc-filter length.

NAVY GEOSAT NAVY GEOSAT FOLLOW-ON (GFO)FOLLOW-ON (GFO)

ALTIMETRY ALTIMETRY MISSIONMISSION

Satellite Launched 10 February

1998

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NRL MODAS

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NRL MODAS 2.0

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Jason is a oceanography mission to monitor global ocean circulation, discover the tie between the oceans and atmosphere, improve global climate predictions, and monitor events such as El Niño conditions and ocean eddies.

The Jason-1 satellite carries a radar altimeter and it is a follow-on mission to the highly successful TOPEX/Poseidon mission. It is joint mission between France and USA. The satellite will be launched in May 2000.

JASONJASON

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Interesting Altimeter sites:

Cal/Val data

Altimeter Missions