1. OC3522Summer 2001 OC3522 - Remote Sensing of the Atmosphere and Ocean - Summer 2001 Active Microwave Radar

2. What can we measure? Satellite height Significant wave height Wind Speed

3. Altimeter Schematic-I

4. Altimeter Schematic-II

5. 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

6. Scattering cross Section,  o (1) Specular Reflection Near vertical incidence reflection off mirror-like facets Important for  25°) 00 0 102030  increased wind speed

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

8. http://www.satobsys.co.uk/WWWaves/

10. Definitions geoid: equipotential surface 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

11.    = H - H measured -( H geoid + SSH mean ) - SSH errors – SSH geophysical_corrections Reference Ellipsoid Geoid Undulations Dynamic Topography Barometric EffectTidesSWH Center of Mass Instantaneous Sea Level Orbit Height = H Mean Sea Level = SSH mean Geoid = H geoid Sensor Geometry Ionosphere Dry Troposphere Wet Troposphere H measured

12. SSH errors SSH geophysical corrections    = H - H measured -( H geoid + SSH mean ) - SSH errors – SSH geophysical_correctionsç ( H geoid + SSH mean ) H

13. Aliasing The altimeter measures sea level once every repeat period (except at cross-over points) If we have a high frequency signal which we measure at a low frequency we get aliasing Because of the way the altimeter lays down its tracks these aliased signals appear to move

14. Aliasing Periods

15. The Geoid provides information about the ocean bottom topography

16. Gravity anomalies from altimetry

17. http://www-ocean.tamu.edu/education/common/notes/Images/chap10/Fig10-3.htm

18. The Sea Surface Topography provides information about ocean currents

19. 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/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.

20. 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

21. 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 FOLLOW-ON (GFO) ALTIMETRY MISSION Satellite Launched 10 February 1998

22. Tides

23. Rossby Waves

24. Meso-scale variability

25. NRL MODAS

26. NRL MODAS 2.0

27. 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. JASON

28. Interesting Altimeter sites: Cal/Val data