1. Active microwave systems (1) Satellite Altimetry
Remote Sensing of the Atmosphere and Ocean
Active microwave systems (1) Satellite Altimetry
John Wilkin
IMCS Building Room 214C (g-voice)

2. Active microwave instruments
Scatterometer (scattering from surface roughness)
ocean vector winds
Synthetic Aperture Radar (SAR)
sea ice
high resolution wind speed over water
land mapping: surface roughness and 3-D terrain
surface currents and swell
CODAR
coastal ocean surface vector currents
Altimeter

3. 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

4. Microwave energy is largely unaffected by the atmosphere: It has almost 100% transmission

5. 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 on Topex, Jason-1 and Jason-2

6. 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)

7. The relationship between: power received P and power transmitted PT is given by the radar equation
(1) (2) (3)
Power of EM wave at range R G = gain of antenna
Radiant intensity in the direction of the radar produced by scatter from a surface with a scattering cross-section s (which depends on area of target, fraction of incident radar pulse absorbed and scattered)
Ae is antenna effective area 1/(4pR2) is isotropic spreading over range R in both transmitted and received signal

8. transmit a radar pulse that is reflected from the Earth’s surface
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 = s = 8 milliseconds
Poseidon uses pulses per second
C = 3x10^8 m/s satellite altitude of 1200 km implies travel time of about s or 8 milliseconds

9. History of Altimetry Skylab 1973-1974 Seasat 1978 Geosat 1985-1990
ERS
History of Altimetry
Topex/Poseidon 1992 – 1/5/2006
GFO
OSTM/Jason
Envisat
Cryosat-2 launched on a Russian/Ukrainian Dnepr based on SS-18 intercontinental ballistic missile from
Cryosat-2 April 8,
ERS
Jason

10. History of Altimetry Skylab 1973-1974 Seasat 1978 Geosat 1985-1990
ERS
History of Altimetry
Topex/Poseidon 1992 – 1/5/2006
GFO
OSTM/Jason
Cryosat-2 launched on a Russian/Ukrainian Dnepr based on SS-18 intercontinental ballistic missile from
Envisat
Cryosat-2 April 8,
ERS
Jason

11. 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)

12. Altimetry: How it works
How altimetry works

13. Altimetry: How it works
Precision orbit determination
Corrected radar range
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 is measured to within 3 cm.
How altimetry works

14. Geoid height (meters) 80 -80
Altimeter range observation is dominated by the gravity field induced shape of the mean sea surface.
Picture shows mean sea surface, computed from 10 years of altimetric data
-80

15. 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 D = ssh – geoid = alt – range – geoid
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 (but removes the mean ocean circulation)

17. 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 range as a function of position.

18. Jason-1 satellite AVISO Web site

19. Jason launch movies

20. 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 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

22. Where is Topex now? Where is Jason now?
Orbital period is approximately 110 minutes 2*pi*R(km)/315(km) 2*pi*(R+altitude)/10 days
Where is Topex now?
Where is Jason now?

23. Jason-1: Research Ground segment

24. OSTM/Jason-2: Operational Ground segment
Tracking
Operational real-time products; ground station redundancy; archive
Delayed-mode reanalysis for research quality datasets

25. 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?

26. (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

28. 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 (empirically) compute the integrated vertical temperature from the sea surface down to the 26°C isotherm.
This quantity is referred to as the Hurricane or Tropical Cyclone Heat Potential (TCHP) [Leipper and Volgenau, 1972] and represents the amount of heat in the upper ocean available for tropical cyclone intensification.
Leipper, D. and D. Volgenau. Hurricane heat potential of the Gulf of Mexico, J. Phys. Oceanogr., 2, , 1972.

29. Altimetry: How it works
How altimetry works
For altimeter observations to be useful for
oceanography, range accuracy of order 2 cm is required.
Where is Jason now?

30. The challenges to achieving 2 cm accuracy are:
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”)
computing the satellite position accurately

31. Precision Orbit Determination
The Jason-2 satellite is tracked in 3 ways
GPS Payload (GPSP)
continuously tracks up to 16 GPS satellites
dual-frequency receiver
estimates position to better than 50 m and time to 50 nanoseconds
Laser Retroflector Array (LRA)
an array of mirrors on the satellite that provide a target for laser-tracking 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
DORIS
receivers on the satellite measure Doppler shift of signal from ground-station 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-1 now?
Where is Jason-2 now?

33. Doris beacons transmit signals at two separate frequencies (2036
Doris beacons transmit signals at two separate frequencies (  MHz and  MHz) to the satellite. The receiver onboard the satellite analyzes the received signal frequencies to calculate its velocity relative to Earth.
This velocity is fed into orbit determination models to derive the satellite's position on orbit to within 2 cm on the radial component.
Doris receiving antenna under the satellite. It receives signals from the terrestrial beacons network (its length is 42 cm).

34. The LRA is a totally passive reflector designed to reflect laser pulses back to their point of origin on Earth. It is used for the calibration of the Precise Orbit Determination system on the spacecraft.                                          
LRA is an array of mirrors that provide a target for laser tracking measurements from the ground. By analyzing the round-trip time of the laser beam, we can locate very precisely where the satellite is on its orbit. The LRA is used to calibrate the other location systems on the satellite (Doris, GPSP) with a very high degree of precision.