TOPEX Outreach

TOPEX/POSEIDON Frequently Asked Questions

This 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

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.

Q3: Why was there an El Niño event in June '93? I thought they occured around Christmas time?

El Niño events generally begin about Christmas time, although this is not always the case. For instance, a more recent, small event began in August, 1994, but ended before Christmas. El Niño events last for several months, and conditions generally return to normal (La Niña conditions) around March to April. However, it is not unprecedented for an El Niño to last beyond March. They can last for more than a year. The El Niño which began near Jan. 1, 1993 lasted until the beginning of July.

Q4: 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. Although a CHANGE in sea surface temperature will cause a CHANGE in the topography, and this can be computed approximately via an equation, one can't compute the total topography from the temperature. If this was possible, no one would be interested in satellite altimetry.

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.

Q5: 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. At the moment, we believe that we can estimate the geoid at distances as small as 600 km to a height accuracy of 20 cm. However, many bathymetry features, such as ridges, change rapidly over 600 km. This will cause large errors in the sea surface topography computed at distances smaller than 600 km. To compensate, the topography is smoothed over larger distances. Generally, these distances are even larger than 600 km. At the moment, we believe that the sea surface topography can be computed accurately over distances of about 2000 km.

Q6: 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 effects the geoid. The geoid is the sum of gravity effects and rotaional effects.

Q7: 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 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.

Q13: 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.

Q14: Where can I get educational or informational materials about satellite altimetry and TOPEX/POSEIDON?

For posters and brochures, e-mail CD-ROMs can be ordered on-line from

Q15: 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 for posters and brochures. CD-ROMs can be ordered on-line from A soon-to-be-released educational CD-ROM can be ordered from


Last Modified: Thu July 15, 1999