1997 Student Paper Competition Winner


Radar Altimetry of the TOPEX/Poseidon Satellite
by Travis Litman

The TOPEX/Poseidon oceanographic satellite is a joint program between NASA and the Centre Nationale d'Etudes Spatiales (CNES). TOPEX/Poseidon, which orbits at an altitude of 1336 kilometers above the earth at a 66 degree inclination, revolves around the planet 127 times every ten days to complete its cycle. The satellite uses radar altimetry to determine sea level measurements (1). In order to gain an appreciation and fundamental knowledge of radar, it is important to understand its brief history. The foundation for radar was developed in the 1860s when the British mathematician and physicist James Clerk Maxwell hypothesized that undiscovered types of electromagnetic waves existed. With the work of German physicist Heinrich R. Hertz in the 1880s, Maxwell's conjectures were substantiated, and consequently, widespread efforts towards research and development of modern radios and radar were started. However, it was not until the 1920s, with the advent of new radio technology that the first radar applications were used. In 1922, naval scientists A.H. Taylor and L.E. Young first detected the passage of a sea vessel in a river using high frequency radio transmissions (2). Shortly thereafter, in 1925, the American physicists Gregory Breitand and Merle A. Tuve bounced short radio pulses off of the ionosphere and determined its height by measuring the length of time it took for the signals to return (3). The Scottish physicist Sir Robert Watson Watt was given credit for the first radar system in 1935 when he refined the pulse techniques to locate aircraft in the ionosphere (2). Throughout World War II advances in radar technology, most notably the magnetron ( a vacuum tube), led to expanded radar use which became vital in the war effort (3). Throughout the 1950s and 1960s, enhanced by computer innovations, radar technology and applications further evolved to include optical radars which utilize high frequency laser light and more advanced microwave radars. In the last three decades radar technology has undergone further refinement leading to extremely accurate radar and remote sensing systems.

Although radar systems are often different in purpose and design, they operate under the same fundamental precepts. Radars produce and transmit signals in the form of electromagnetic or in the case of optival radar, light waves. Upon striking an object the waves are reflected, some of the reflected waves or echoes return along the path from which they were sent. Thus, calculations involving distances, speeds, and the nature of the object can be made from the resulting echoes. The waves transmitted by the radar all possess a definite frequency measured in units of megahertz; in remote sensing applications, common wavebands are usually limited to 1-37.5 GHz. This is predominantly due to the fact that such frequencies are so long that they are not effected by relatively small cloud particles, and, thus, radar at these frequencies can penetrate clouds enabling it to 'see the ground' (5). Radars can also be broken into two general groups, pulse radar and continuous-wave radar. Remote-sensing altimetric radars are generally pulse radars, which send out signals in powerful bursts. After the pulse, lasting only a few millionths of a second, is sent in a narrow beam through an antenna, the object within the area of the beam that it strikes will reflect the waves back to the antenna (4). The radar system is then able to determine the distance from the object and the object's direction. One of the first satellites to use such technology in the measurement of sea-levels was the Seasat 1978-64A. Although lasting for only a period of five months, the Seasat carried a radar altimeter capable of measuring the ocean surface to approximately 10cm and was an important precursor to consequent satellites (6). In the 1990s with more complex and advanced imaging and altimetric systems, radar remote sensing has become a more significant tool and has been utilized by the ERS-1 and Geos-3 satellites extensively (5,7). The most advanced and recent altimetric radar system was launched on August 10, 1992 on the TOPEX/Poseidon satellite which utilizes altimetry systems that are state of the art in achieving unparalleled accuracy in the measurement of sea-surface height. This has been vital in achieving the TOPEX/Poseidon's primary objective of furthering the understanding of global ocean circulation (7).

The TOPEX/Poseidon satellite uses two main techniques to determine the sea level, radar altimetry and precise orbit determination. The altimetric radar data from the TOPEX/Poseidon satellite is derived from the measurement of the satellite's altitude above the ocean surface, whereas the precise orbit determination comes from the satellite's geocentric measurement. The mathematical difference between the precise orbit determination and the radar altimetry determines the sea level or height of the ocean surface relative to the center of the Earth (7). Similar to other radar systems the TOPEX /Poseidon's radar altimeter (located on the bottom or nadir side of the satellite) bounces a microwave pulse off of the ocean surface (1). By measuring the time it takes the echo to return, the altimeter can resolve the distance between the satellite and the ocean surface. In addition, through measurement of the strength and shape of the altimeter's return pulse, ocean wave height and wind speed can be determined (7).

The satellite employs a dual frequency radar altimeter as its primary instrument. The altimeter sends radio pulses at frequencies of 13.6GHz and 5.6GHz through a 1.5 meter (diameter) dish antenna, and then measures the characteristics of the echo. TOPEX/Poseidon also carries an experimental, single frequency, solid state CNES altimeter which has exhibited a precision highly comparable to the NASA dual frequency altimeter (1). Unfortunately, there are several potential sources of error present in the TOPEX/Poseidon's altimetric measurements. These sources of error can include: instrument error, propagation medium error, geoid modeling error, temporal variation error, and spacecraft orbit determination error (8). Thus, effective calibration of the altimeter data is imperative for the resultant measurements tobe useful. Several methods of independent calibration and verification of the satellite's data have been adopted including: comparison with SLR, GPS, and tide gauge measurements from two oil platforms, comparison with measurements from over 70 tide gauge sites throughout the world, and onboard altimeter calibration (9). In addition, through such instruments onboard the satellite as the microwave radiometer, which corrects for signal distortions created by the presence of water in the atmosphere errors are kept at a minimum; a high level of precision and accuracy is thus maintained (1). With the precision of the radar altimeter and supporting instruments as well as the intense calibration, the TOPEX/Poseidon is able to measure the sea-surface height to within an unprecedented 4.3 centimeters (10).

Through the altimetric data measurements and precise orbit determination information, detailed maps of the ocean's topography are made. From such maps of ocean topography, ocean circulation and tidal patterns can be determined, which are especially important to the understanding of large-scale weather patterns (11). Additionally, "TOPEX/Poseidon's sea-surface height data are essential to understanding the role oceans play in regulating the global climate, one of the least understood areas of climate research" (12). Thus, without the high level of precision in the measurements from the radar altimetry system, pursuit of the main objective of the TOPEX/Poseidon oceanography mission would be impossible.

The TOPEX/Poseidon satellite is a key tool aiding in the discovery of knowledge concerning the earth and its oceans, drastically benefiting modern society and civilization. Without comprehension and knowledge about the oceanic circulation and its global effects progress and innovation, especially in regard to oceanic endeavors, cannot reach its full potential.

The TOPEX/Poseidon satellite has led to new discoveries and understandings of the earth's oceans. For instance, as a result of data from the TOPEX/Poseidon satellite, oceanographers have been able to track the remnants of a 1982-3 El Nino event. An El Nino is "an irregularly occurring flow of unusually warm surface water along the western coast of South America that is accompanied by abnormally high rainfall in usually arid areas, and that prevents upwelling of nutrient rich cold deep water causing a decline in the regional fish population" (13). This data suggests that these remnants move across the Pacific ocean may still be affecting weather in the region. Additionally, as Dr. Gregg Jacobs of the Naval Research Laboratory (NRL) commented,

The fact that we are seeing this wave (El Nino) 10 years later is an amazing discovery in and of itself... The ability to observe changes in global ocean circulation as we have seen over the Kuroshio region is a demonstration that the TOPEX/Poseidon satellite is the most valuable global ocean-observing tool we have. We wouldn't have been able to do this work without the TOPEX/Poseidon (11).

Furthermore, by measuring ninety-percent of the global oceans, the TOPEX/Poseidon satellite is also able to measure expansion and contraction of ocean volume leading to a possible relationship between global warming or cooling and those effects on total ocean volume (7). In addition, as a result of the TOPEX/Poseidon's accuracy, tidal effects, which can include: sea floor depression, distortion in gravitational measurements, changes in artificial satellite orbits, etc. can be studied in a greater detail. The knowledge of ocean eddies, (currents that run contrary to the main current) which play a role in ocean circulation, the earth's climate, and biogeochemical systems, has also been enhanced by the TOPEX/Poseidon satellite (10). Finally, in undertaking to meet its ultimate objective the TOPEX/Poseidon has permitted scientists to work on determining ocean circulation and variability; learning to understand the wind's role in circulation; developing descriptions of the nature of ocean dynamics; contributing to the understanding of the transport of heat, mass, nutrients, and salt through the oceans; determining geocentric ocean tides; investigating the interaction of currents with waves; improving our knowledge of the marine geoid; and increasing our understanding of lithospheric and mantle processes (1).

The history of radar and altimetry is in many ways a study in persistence, innovation, and human accomplishment which has led to its modern pinnacle in the TOPEX/Poseidon oceanographic satellite. In a world in which knowledge is of key importance to every aspect of society it is vital to understand or attempt to understand the natural world. Thus, the TOPEX/Poseidon satellite which has led to an improved understanding of the earth's oceans, which greatly effect society, has proven itself to be an exceptionally beneficial and advantageous undertaking. Perhaps the benefit and utility of the TOPEX/Poseidon can best be described by John Locke who stated, "The only fence against the world is a thorough knowledge of it" (14).



REFERENCES

  1. Perspectives on an Ocean Planet: TOPEX/Poseidon. CD-ROM. Pasedena: California Institute of Technology, 1995.
  2. Campbell, James B. Introduction to Remote Sensing. New York: The Guilford Press, 1987; 161-3.
  3. The World Book Encyclopedia. Vol. 16. Chicago: World Book Inc., 1995; 68-9.
  4. Hough, Harold. Satellite Surveillance. Port Townsend: Loompanics Unlimited, 1991; 49-53.
  5. Harris, Ray. Satellite Remote Sensing. London: Routledge & Kegan Paul, 1987; 27.
  6. King-Hele, Desmond. Observing Earth Satellites. New York: Van Nostrand Reinhold Company, 1983; 36.
  7. "The Sensors of the TOPEX/Poseidon Satellite." Online. http: quest.arc.nasa.gov, 25 March 1997.
  8. "Error Sources in Altimeter Measurements." Online. http: www.csr.utexas.edu, 25 March 1997.
  9. "Altimeter Calibration." Online. http: www.csr.utexas.edu, 25 March 1997.
  10. "Online 'Tutorial'." Online. http: topex-www.jpl.nasa.gov, 25 March 1997.
  11. "Understanding Our Environment." Online. http: www.csr.utexas.edu, 25 March 1997.
  12. "Topex/Poseidon In the News -1994 Articles from the Jet Propulsion Laboratory's Newspaper Universe." Online. http: quest.arc.nasa.gov, 25 March 1997.
  13. Merriam Webster's Collegiate Dictionary. Springfield: Merriam Webster Inc., 1993; 375.
  14. Bartlett, John. Familiar Quotations. Boston: Little, Brown and Company, 1955; 283.