FAQ - Frequently Asked Questions
This TOPEX/Poseidon FAQ was created by scientists from NASA's Jet
Propulsion Laboratory (JPL) and the Center
for Space Research . If your question is not answered here, please e-mailMargaret Srinivasan.
Educator Questions
Q1: How do I order my own copy of the Educational
TOPEX/Poseidon CD-ROM?
Q2: I am having problems with the Educational
TOPEX/Poseidon CD-ROM, who do I contact?
Q3: How do I order my own copy of the Informational
TOPEX/Poseidon CD-ROM?
Q4: Where can I get educational or informational
materials about satellite altimetry and TOPEX/Poseidon?
Q5: I am teaching an "Oceanography for teachers"
summer course, do you have educational or informational materials about
satellite altimetry and TOPEX/Poseidon?
General and Science questions
Q6: Can I use your images and
if so what should the credit line read?
Q7: Where can I find bathymetric charts?
Q5: What is the difference between warm and cold
El Niño phases?
Q6: Is there any equation to correlate sea surface temperature to sea
surface topography?
Q7: How is the bathymetry of the ocean floor accounted for in the sea
surface topography?
Q8: What is the difference between an El Niño
and a La Niña?
Q9: How can I find the altitude of my town?
Q10: Is there an equation to equate sea surface temperature to sea surface
topography?
Q11: How is the bathymetry of the ocean floor accounted for in the sea
surface topography?
Q12: What is a geoid?
Q13: What is the reference ellipsoid, and what does this mean physically?
Q14: What is the sensitivity and accuracy of the
instrument regarding windspeed?
Q15: What is the repeat time for the TOPEX/Poseidon
satellite?
Q16: What is the swath width of the windspeed
imagery?
Q17: Where I could find information (images) of
the ocean floor?
Q18: What is the horizontal resolution of the
TOPEX/Poseidon altimeters?
Q19: When we have meteor showers, can anything
be done to protect the TOPEX/Poseidon satellite?
Educator Questions
Q1: How do I order my own copy of the
Educational
TOPEX/Poseidon CD?
The NASA award-winning educational CD-ROM
"Visit to an Ocean Planet" can be ordered on-line at
NASA CORE
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Q2: I am having problems with the Visit
to an Ocean Planet CD-ROM, who do I contact?
Check the Frequently Asked Questions for the CD-ROM.
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Q3: How do I order my own copy of the
Informational TOPEX/Poseidon CD?
The JPL informational CD-ROM "Perspectives
on an Ocean Planet" can be ordered on-line from Margaret Srinivasan.
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Q4: Where can I get educational or
informational materials about satellite altimetry and TOPEX/Poseidon?
For information on posters, brochures, and CD-ROMs e-mail Margaret Srinivasan.
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Q5: I am teaching an "Oceanography
for teachers" summer course, do you have educational or informational
materials about satellite altimetry and TOPEX/Poseidon?
Please e-mail Margaret Srinivasan
for posters and brochures and order the CD-ROMs through the sites listed
above. We have started an Envoy program to provide materials and news
to peer teachers. If you are interested in being an Envoy, please contact
Margaret Srinivasan.
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General and Science questions
Q6: Can I use your images and if
so what should the credit line read?
We are very happy to have our images used for educational purposes. We
have an image
gallery specifically for this purpose. Also, if you are looking for
images that you think we should have but can't find, please contact .
JPL's image release policy
is located at
http://www.jpl.nasa.gov/pictures/imageuse.html . The credit line should
read 'Courtesy of NASA/JPL/Caltech'. We would also really appreciate copies
of publications where you use our imagery.
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Q7: Where can I find bathymetric charts?
The National Ocean Service's
(NOS) MapFinder Web Service provides direct Internet access to NOS
imagery and data holdings for coastal photography, nautical charts, historical
maps, coastal survey maps, environmental sensitivity index maps, hydrographic
survey outlines, water level stations, estuarine bathymetry, and geodetic
control points. These are on-line as directly usable products, or as previews
of higher quality products that can be ordered from NOS. Target users
of this service are resource managers (municipal planners, protected area
managers, etc.), but these products will also benefit a wider public audience.
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Q8: What is the difference between
an El Niño and a La Niña?
An El Niño has warmer than normal
surface waters in the eastern Pacific and cooler than normal surface waters
in the western Pacific. This is generally associated with weaker than
normal winds blowing from east to west (the Trade winds).
A La Niña occurs when the waters in the eastern Pacific are much
colder than normal, while the waters in the west are much warmer than
normal. During a La Niña, the Trade winds are stronger than normal.
For more information see our El
Niño/ La Niña pages .
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Q9:
How can I find out the altitude of my town?.
There are several approaches. You can look in an atlas, quite often the
altitude is listed. If you have an airport, the authorities there will
have a very precise measurement. Otherwise look on a topographic map.
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Q10: Is there any equation to correlate
sea surface temperature to sea surface topography?
No, there is no direct relationship
to convert sea surface temperature to sea surface topography, or vice
versa. The sea surface temperature reflects the temperature in the top
centimeter or so of water, whereas the sea surface height reflects the
heat stored in the ocean. In general a 1°C temperature change over 50m
gives a 1cm change, however changes in sea surface height are a result
of many factors in addition to heating and cooling.
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Q11: How is the bathymetry of the ocean
floor accounted for in the sea surface topography?
The bathymetry effects are removed by removing the geoid. The bathymetry
(mass concentrations/holes) cause different gravity signals at the ocean
surface, which cause a permanent deformation in the sea surface that is
part of the geoid signal.
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Q12: What is a geoid?
The geoid is the shape the ocean surface
would have if it were covered with water at relative rest to a rotating
earth. The geoid will be pulled away from a perfect sphere due to mass
concentrations. Note also that the rotation rate of the Earth affects
the geoid. The geoid is the sum of gravity effects and rotational effects.
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Q13: What is the reference ellipsoid,
and what does this mean physically?
The reference ellipsoid is basically a convenience so that we don't have to work with larger numbers, and so that we can get more precision in our calculations. The reference ellipsoid is the best fitting ellipsoid to the geoid. An ellipsoid is basically a sphere with a bulge at the
equator. To first order, this accounts for over 90% of the geoid. Also, sea surface height measurements from the center of the Earth are on the order of 6000 km. By removing a reference surface, the heights relative to the ellipsoid are on the order of 100 meters. Thus, one can gain several digits of accuracy in numerical calculations.
In fact, any reference surface can be used. A sphere would work, but
sea surface height differences from this surface could be as large as
20 km, thus one would loose the precision that is possible by using an
ellipsoid.
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Q14. What is the sensitivity and accuracy
of the instrument regarding windspeed?
Wind speed is determined from the radar backscatter cross section (sigma0) measurement and an empirical relationship of backscatter to wind ("model function"). Sigma0 is reported with a precision of 0.25 dB which translates to about 0.5 m/s for typical wind speeds. Comparisons to buoys show a variance of about 1.5 m/s. Sigma0 calibration is maintained to about 0.1 dB.
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Q15. What is the repeat time for the
TOPEX/Poseidon satellite
TOPEX/Poseidon is in a "10 day" (9.9155 d) exact repeat at an inclination of 66 degrees. The ground tracks are about 315 km apart at the equator and the orbit period is 112 minutes.
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Q16. What is the swath width of the
windspeed imagery?
The exact footprint size of the altimeter
depends on the significant wave height, but it is 3-5 km in diameter for
typical wave heights. Measurements are taken approximately 1/sec along
track giving a spacing of about 6 km.
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Q17. Where I could find information
(images) of the ocean floor?
Check out our page http://topex-www.jpl.nasa.gov/discover/Bathymetry.html
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Q18: What is the horizontal resolution
of the TOPEX/Poseidon altimeters?
The measurement geometry and operation principles of the TOPEX
Altimeter are very well described in D. B. Chelton, et als, "Pulse compression and sea level tracking in satellite altimetry", J. Atmospheric and Oceanic Tech., Vol 6, #3, June 1989.
The altimeter footprint for ocean applications (or other very flat surfaces, relief less than 100 m) is determined by the compressed pulse length (3.125 nsec), the satellite altitude, and the significant wave height. At the TOPEX altitude of 1335 km, the footprint diameter varies from 2.0 km at 0 m SWH (not very realistic) to 5.5 km at 3 m SWH (typical of open ocean) to 11.7 km at 15 m SWH (about the highest observed during the TOPEX mission).
Measurement values are reported at approximately 1 per second, although high rate data at 10/sec are also given. In 1 sec the satellite nadir point moves about 6 km along the Earth's surface.
For land or ice applications, the beam width of the antenna may be important
as returns may come from anywhere within it. It has a diameter of 25 km.
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Q19. When
we have meteor showers, can anything be done to protect the TOPEX/Poseidon
satellite?
TOPEX/Poseidon rotates it solar array during large meteor showers to
minimize the cross section panel exposure to the incoming meteoroids.
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