TOPEX/POSEIDON Frequently Asked QuestionsThis TOPEX/POSEIDON FAQ was created by scientists from the Center for Space Research and NASA's Jet Propulsion Laboratory (JPL) in an effort to provide a basic understanding of TOPEX/POSEIDON data and how the data is computed.
Do you have a question about TOPEX/POSEIDON?
Q1: How do I order my own copy of the TOPEX/POSEIDON CD? The JPL informational CD-ROM's "Perspectives on an Ocean Planet" and "Visit to an Ocean Planet" can be ordered on-line (and cost-free) at http://podaac.jpl.nasa.gov/tecd.html
Q2: What is the difference between warm and cold El Niño phases? A warm phase 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 cold phase El Niño (also known as 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.
However, some argue that there are few La Niña events. They feel that the shift is just the return to normal conditions, and that the warm phase El Niño is abnormal. However, because the equatorial Pacific oscillates between these two conditions, without ever reaching an intermediate, mean state, neither condition should be considered normal or abnormal. Hence, the usage of the warm/cold phase of the cycle.
The sea surface temperature reflects the temperature in the top inch or so of water, and the temperature can change dramatically with depth in some cases. The sea surface measured by altimetry is related to the temperature at ALL depths, as well as other parameters, such as the water salinity and ocean currents.
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 precision than by using an ellipsoid.
Q8. 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.
Q9. 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.
Q10. 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.
Q11. What is the correlation of sea height to water temperature? I notice that the images don't show a color change for the seawater near the poles. Isn't the polar water colder than in the the mid-Pacific? I'd like to know how you determined the relationship of sea level to temperature. The color coding you see in our graphics is not consistent because each image is showing a different aspect of the data.
For example, the link that shows a series of El Niño images shows sea height RELATIVE TO AVERAGE. You're right, water temperatures near the equator are normally warmer than near the poles. However, the TOPEX/POSEIDON satellite measures SEA SURFACE HEIGHT (and not temperature). Thus it can detect CHANGES in sea level due to thermal expansion and thereby infer changes in temperature relative to normal (higher = hotter; lower = colder). It does not detect the "normal" difference in temperature of ocean water between low and high latitudes.
In the El Niño images, higher-than-average sea levels are red and white; below-average are blue and purple; average is green. In these maps, sea level differs from average because of anomalous warming (El Niño signal near No. and So. America) and cooling (loss of warm water near New Guinea and Australia). In these images the "El Niño" warm water (red and white) are about 3 - 5 degrees Fahrenheit above normal. (This is determined by the "simple" equations for the volumetric expansion of seawater and actual in-water measurements from the Pacific).
HOWEVER, not all changes in sea level are due to heating. If you look at TOPEX/POSEIDON data for different seasons you'll notice that in the tropics (especially in non-El Niño times) that sea level is primarily controlled by wind. Sea level changes from season-to-season in the Indian Ocean shows this very nicely.
Ocean circulation also affects sea level. Ocean currents raise and lower our seas up to 2 meters over the globe. The strongest currents occur at the western edges of ocean basins (Atlantic -> Gulf Stream; Pacific -> Kuroshio Current). These raise sea level up to 1 meter above average.
Note that, in general, the movement of currents has a bigger effect on sea level (+/- 1 meter) than heating and/or winds (+/- 12 centimeters).
Gravity has the biggest effect on sea level deflecting it up to 150 meters. Check out JPL's Tutorial on Earth's Geoid to learn more.
Q12. Where I could find information (images) of the ocean floor? The folks at University of California, San Diego's Scripps Institute of Oceanography have a great map of the seafloor online.
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.
Please e-mail email@example.com for posters and brochures. CD-ROMs can be ordered on-line from http://podaac.jpl.nasa.gov/tecd.html. A soon-to-be-released educational CD-ROM can be ordered from http://topex-www.jpl.nasa.gov/education/cdrom.html.
CSR/TSGC Team Web